Jérôme Aléon
email: jerome.aleon@mnhn.fr

Closed system oxygen isotope redistribution in igneous CAIs upon spinel dissolution

In several Calcium-Aluminum-rich Inclusions (CAIs) from the CV3 chondrites Allende and Efremovka, representative of the most common igneous CAI types (type A, type B and Fractionated with Unknown Nuclear isotopic anomalies, FUN), the relationship between 16 O-excesses and TiO2 content in pyroxene indicates that the latter commonly begins to crystallize with a near-terrestrial 16 O-poor composition and becomes 16 Oenriched during crystallization, reaching a near-solar composition. Mass balance calculations were performed to investigate the contribution of spinel to this 16 Oenrichment. It is found that a back-reaction of early-crystallized 16 O-rich spinel with a silicate partial melt having undergone a 16 O-depletion is consistent with the O isotopic evolution of CAI minerals during magmatic crystallization. Dissolution of spinel explains the O isotopic composition ( 16 O-excess and extent of mass fractionation) of pyroxene as well as that of primary anorthite/dmisteinbergite and possibly that of the last melilite crystallizing immediately before pyroxene. It requires that igneous CAIs behaved as closed-systems relative to oxygen from nebular gas during a significant fraction of their cooling history, contrary to the common assumption that CAI partial melts constantly equilibrated with gas. The mineralogical control on O isotopes in igneous CAIs is thus simply explained by a single 16 O-depletion during magmatic crystallization. This 16 Odepletion occurred in an early stage of the thermal history, after the crystallization of spinel, i.e. in the temperature range for melilite crystallization/partial melting and did not require multiple, complex or late isotope exchange. More experimental work is however required to deduce the protoplanetary disk conditions associated with this 16 Odepletion.

Introduction

Meteoritic Calcium-Aluminum-rich inclusions (CAIs) are the oldest solar system rocks. Igneous CAIs from CV chondrites initially formed in a 16 O-rich environment of near-solar isotopic composition [START_REF] Mckeegan | The oxygen isotopic composition of the Sun inferred from captured solar wind[END_REF] and underwent a 16 O-depletion later in their history resulting in a near-terrestrial composition. Understanding the conditions under which the CAI isotopic composition changed is essential to explaining the origin of the isotopic difference between the Sun and the rocky planetary materials.

The extent of 16 O-enrichment is mineral dependent with spinel and Ca, Al, Ti-rich clinopyroxene being the two most 16 O-rich minerals and melilite and anorthite being the most 16 O-poor (e.g. [START_REF] Clayton | Distribution of the pre-solar component in Allende and other carbonaceous chondrites[END_REF][START_REF] Mckeegan | Stable isotope variations in extraterrestrial materials[END_REF]. This mineralogical control of the 16 O-content is not easily understood and many explanations have been proposed. A key issue is to explain how melilite can be 16 Odepleted, while pyroxene is 16 O-enriched. It has been proposed to result from (1) gassolid equilibration by solid-state diffusion [START_REF] Clayton | Distribution of the pre-solar component in Allende and other carbonaceous chondrites[END_REF][START_REF] Simon | Oxygen isotopic variations in the outer margins and Wark-Lovering rims of refractory inclusions[END_REF], (2) gas-melt equilibration during partial melting and crystallization [START_REF] Aléon | Oxygen isotope heterogeneities in the earliest protosolar gas recorded in a meteoritic calcium-aluminium-rich inclusion[END_REF][START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF][START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF][START_REF] Thrane | Discovery of a New FUN CAI from a CV Carbonaceous Chondrite: Evidence for Multistage Thermal Processing in the Protoplanetary Disk[END_REF][START_REF] Yoshitake | Correlations between oxygen-isotopic composition and petrologic setting in a coarse-grained Ca, Al-rich inclusion[END_REF][START_REF] Yurimoto | Oxygen isotope exchange between refractory inclusion in Allende and solar nebula gas[END_REF] possibly associated with chondrule formation (e.g. Krot et al., 2008), (3) aggregation of solids condensed in different environments (e.g. [START_REF] Kawasaki | Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca-Al-rich inclusion of Efremovka CV3 chondrite[END_REF] or (4) fluid-rock equilibration on a parent-body [START_REF] Aléon | Finegrained, spinel-rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions[END_REF][START_REF] Bodénan | An oxygen isotope study of Wark-Lovering rims on type A CAIs in primitive carbonaceous chondrites[END_REF][START_REF] Krot | Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism[END_REF][START_REF] Wasson | 16 O-rich melilite in CO3.0 chondrites: Possible formation of common, 16 O-poor melilite by aqueous alteration[END_REF].

While some explanations work for given CAI types (e.g. type A CAIs or type C CAIs), none explain satisfactorily observations on type B CAIs. The fluid-rock model explains the isotopic heterogeneity in fine-grained CAI material such as Wark Lovering rims [START_REF] Yoshitake | Correlations between oxygen-isotopic composition and petrologic setting in a coarse-grained Ca, Al-rich inclusion[END_REF], fine-grained or small CAIs (e.g. [START_REF] Aléon | Finegrained, spinel-rich inclusions from the reduced CV chondrite Efremovka: II. Oxygen isotopic compositions[END_REF][START_REF] Wasson | 16 O-rich melilite in CO3.0 chondrites: Possible formation of common, 16 O-poor melilite by aqueous alteration[END_REF], but there is no hypothesis that explains the mineralogical control on all igneous CAIs. The partial melting hypothesis does not match the crystallization sequence of type B CAIs [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF][START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature[END_REF] and requires late crystallization of melilite, (e.g. [START_REF] Yurimoto | Oxygen isotope exchange between refractory inclusion in Allende and solar nebula gas[END_REF], the solid state diffusion model does not match quantitatively type B CAIs observations [START_REF] Ryerson | Determination of oxygen self-diffusion in akermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions[END_REF] and the condensation model requires isolated formation of individual crystals unlike type B CAIs (e.g. [START_REF] Kawasaki | Oxygen isotopic and chemical zoning of melilite crystals in a type A Ca-Al-rich inclusion of Efremovka CV3 chondrite[END_REF]. As these hypotheses have very different implications for the distribution of oxygen isotopes in the protoplanetary disk either with distance from the protosun, height in the disk or with disk age, it is important to unravel the physico-chemical process at the origin of the mineralogical control of O isotopes in CAIs and to evaluate if it works for all igneous CAI types and at the very least for the most enigmatic type B CAIs.

The O isotopes -TiO2 content relationship in pyroxene, the last phase that crystallized in many igneous CAIs, shows that it became 16 O-enriched during its crystallization [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF][START_REF] Macpherson | Primary reverse oxygen-isotope evolution of pyroxene in Compact Type A CAIs from the Efremovka and NWA 3118 CV3 chondrites: insights into internal CAI mixing lines[END_REF]. Because the self-diffusion of O is sluggish in pyroxene [START_REF] Ryerson | Determination of oxygen self-diffusion in akermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions[END_REF], it faithfully records the O isotopic composition of the melt from which it crystallizes thus indicating 16 O-enrichment of the melt during crystallization of pyroxene. This 16 Oenrichment is however at odds with the O isotopic composition of FUN (Fractionated with Unknown Nuclear isotopic anomalies) CAIs [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF], for which isotopic evolution of a mass fractionated 16 O-rich melt by equilibration with an unfractionated 16 O-poor reservoir during crystallization is commonly advocated [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF][START_REF] Thrane | Discovery of a New FUN CAI from a CV Carbonaceous Chondrite: Evidence for Multistage Thermal Processing in the Protoplanetary Disk[END_REF][START_REF] Williams | Thermal and chemical evolution in the early solar system as recorded by FUN CAIs: Part I -Petrology, mineral chemistry, and isotopic composition of Allende FUN CAI CMS-1[END_REF]. Reconciling both observations thus requires alternative interpretations.

A plateau in the abundance of spinel during the experimental crystallization of type B CAI liquids led [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF] to infer the existence of a reaction, previously demonstrated during crystallization of Ca, Mg, Al, Si (CMAS) melts (e.g. [START_REF] Onuma | Study of the system CaMgSi2O6-CaFe3+AlSiO6-CaAl2SiO6-CaTiAl2O6: II. The join CaMgSi2O6-CaAl2SiO6-CaTiAl2O6 and its bearing on the Ca-Al-rich inclusions in carbonaceous chondrite[END_REF][START_REF] Yang | Phase equilibria of the join akermaniteanorthite-forsterite in the system CaO-MgO-Al2O3-SiO2 at atmospheric pressure[END_REF], involving spinel + melt when clinopyroxene begins to crystallize. Here I compile and compare O isotope data in major minerals from selected igneous CAIs representative of the various CAI types including type A, type B, intermediate pyroxene-rich type A and FUN CAIs. FUN CAIs are included here because the mineralogical control on the 16 O-excess is common to FUN and non-FUN CAIs, thus suggesting a possibly common process, and because a continuum of thermal histories and isotopic anomalies has been evidenced between FUN and non-FUN CAIs (e.g. [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]. I notably examine the relationship between O isotopes and TiO2 content in pyroxene, a tracer of crystallization progress [START_REF] Simon | Fassaite composition trends during crystallization of Allende Type B refractory inclusion melts[END_REF]. I subsequently use mass balance considerations in a model involving spinel resorption to investigate to what extent this process can explain the isotopic composition of pyroxene in different CAI types. [START_REF] Clayton | The oxygen isotope record in Murchison and other carbonaceous chondrites[END_REF]. Samples not deviating from the CCAM line are considered to be unfractionated.

Samples and method

Isotopic

The Ti content of the pyroxene is reported here as TiO2 (wt%) with all Ti assumed to be tetravalent.

Samples

Allende USNM 3529-Z. 3529-Z is a type B1 CAI from the CV3 chondrite Allende ~ 2 cm in diameter. It is unfractionated and dominantly composed of melilite (mel), spinel (sp), Al-Ti-rich clinopyroxene (cpx) and anorthite (an) (e.g. [START_REF] Podosek | Correlated study of initial 87Sr/86Sr and Al-Mg isotopic systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite[END_REF].

Melilite is present as 16 O-poor chemically zoned mm-sized blades with Al-rich cores and Mg-rich periphery [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF]. Spinel is 16 O-rich and is poikilitically enclosed in melilite, anorthite and pyroxene. It is among the most documented CAIs for both O isotopes and pyroxene chemical composition [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Mckeegan | Stable isotope variations in extraterrestrial materials[END_REF], Table 1). The Al-Ti-rich cpx has 16 O excesses inversely related with Ti content.

Allende CMS-1. CMS-1 is a new FUN CAI from Allende [START_REF] Williams | Thermal and chemical evolution in the early solar system as recorded by FUN CAIs: Part I -Petrology, mineral chemistry, and isotopic composition of Allende FUN CAI CMS-1[END_REF]. It is irregular, ~4 mm in size, and is mostly composed of melilite, Al-Ti-rich cpx, and spinel. 16 O-rich spinel is poikilitically enclosed by Al-Ti-rich cpx and 16 O-poor melilite (Table 1).

Pyroxene has variable 16 O excesses and TiO2 content (Williams et al., 2017, Table 1).

Spinel is mass fractionated to various extents and melilite is unfractionated (Table 1).

The most 16 O-poor cpx is the least fractionated and conversely (Table 1).

Allende STP-1. STP-1 is a type B2 FUN CAI dominantly composed of melilite (Åk ≤ 39 mol%), spinel, Al-Ti-cpx and dmisteinbergite (dmis) [START_REF] Holst | Hf-182-W-182 age dating of a Al-26-poor inclusion and implications for the origin of short-lived radioisotopes in the early Solar System[END_REF][START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]. Pyroxene is igneously zoned and contains variable TiO2 and 16 O excesses [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]. Spinel is 16 O-rich and variably mass fractionated (Table 1); it is poikilitically enclosed in all other minerals. STP-1 has an O isotopic composition intermediate between that of 3529-Z and CMS-1. Melilite is variably 16 O-rich and mass fractionated (Table 1). Dmisteinbergite, the hexagonal polymorph of CaAl2Si2O8, has the isotopic composition of the most 16 O-rich pyroxene.

Allende 7R-19-1. 7R-19-1 is an igneous CAI from Allende intermediate between type A and type B CAIs. It is dominated by gehlenitic melilite (mostly Ak0-40, [START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF] with spinel, but contains coarse pyroxene grains with igneous zoning [START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF][START_REF] Yurimoto | Oxygen isotope exchange between refractory inclusion in Allende and solar nebula gas[END_REF]. Cpx has variable TiO2 content and O isotopic composition.

Isotopic weighted means were calculated along homogeneous profiles in zoned cpx (Table 1). Other O isotopes data in pyroxene showing a larger range of variation are considered here for comparison. Spinel is 16 O-rich and melilite has a large range of 16 Oexcesses (Table 1).

Efremovka 101,3. E101,3 is a compact type A CAI from the reduced CV3 chondrite Efremovka [START_REF] Aléon | Coupled O and Mg Isotopic Investigations of Compact Type A Refractory Inclusions[END_REF]. It is dominated by melilite and contains small spinel and perovskite grains. Rare interstitial pyroxene has the habit of the last crystallizing phase observed in igneous type A CAIs [START_REF] Simon | Origin of compact type A refractory inclusions from CV3 carbonaceous chondrites[END_REF] and is 16 O-poor (Table 1). It is extremely Al-Ti-rich and Mg-Si-poor with up to 25.0 wt% Al2O3, up to 21.8 wt% TiO2, down to 24.6 wt% SiO2, down to 2.8 wt% MgO and ~24 wt% CaO. Spinel is 16 O-rich.

Melilite is 16 O-poor in the CAI interior (Table 1), when associated with pyroxene, and becomes 16 O-enriched toward the CAI periphery [START_REF] Aléon | Coupled O and Mg Isotopic Investigations of Compact Type A Refractory Inclusions[END_REF].

Method

In order to evaluate the possible contribution of spinel to the oxygen isotopic evolution of CAI melts, I calculate here the O isotopic evolution of a system, in which spinel dissolves in the residual melt during crystallization of lower temperature phases, such as pyroxene, without interaction with an external reservoir. At each crystallization step, the spinel + liquid reaction is considered to produce an intermediate melt having a fraction of its O from spinel and a fraction of its O from the initial melt. Partial crystallization of this intermediate melt yields clinopyroxene and a residual melt, which becomes the new initial melt and in turn reacts with remaining spinel. In temperature and cooling rate conditions representative of cpx crystallization in type B CAIs (1500 K; 5 K/h), the diffusion length of O in diopside [START_REF] Ryerson | Determination of oxygen self-diffusion in akermanite, anorthite, diopside, and spinel: Implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions[END_REF] is 55 nm.

Owing to this low O self-diffusivity, cpx is considered to record the composition of the intermediate melt and is no longer considered in the system. For each CAI, the stoichiometry and extent of this reaction depend on many poorly determined quantities, such as the bulk chemical composition, the extent of initial crystallization, the temperature of reaction, or the kinetics of spinel dissolution. The stoichiometry of the spinel + liquid reaction is therefore taken into account using the fraction of O from the initial melt involved in the reaction as a free parameter denoted fM. The oxygen isotopic composition is then calculated by mass balance. The calculation is stopped arbitrarily after the composition of pyroxene has stabilized.

Intricate sector zoning and growth zoning in cpx probably blur the correlation between TiO2 content and O isotopes (e.g. in CAI TS34, [START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF] but line profiles in zoned pyroxene grains in 3529-Z [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF] indicate that this relationship is preserved and approximately linear within single sectors. The variation of TiO2 content during pyroxene crystallization is thus assumed here to proceed linearly with time. It is scaled to a maximal content close to the maximum TiO2 content in cpx, assumed to be representative of the first pyroxene crystallized and to the minimal cpx TiO2 content, assumed to be representative of the final pyroxene.

The parameters of the model are (i) the initial and final O isotopic composition and Ti content, (ii) the extent of mass fractionation and the proportion of mass fractionated spinel involved in FUN CAIs and (iii) the atomic fraction of O from melt and spinel, i.e. the stoichiometry of the spinel + melt reaction (Table 2). The initial and final compositions are dictated by the observed compositions of the samples. Spinels systematically have ∆ 17 O values ~ -24 ‰ and the residual melt starting ∆ 17 O values are between -0.9‰ and -4.1‰, corresponding to those of the most 16 O-poor melilite in the corresponding CAIs. The initial and final TiO2 contents are also dictated by the composition of the most and least Ti-rich pyroxene in the CAIs considered here. Given that individual cpx crystals have variable maximum TiO2 content within a single CAI [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF][START_REF] Simon | Fassaite composition trends during crystallization of Allende Type B refractory inclusion melts[END_REF], a small range of initial TiO2 contents is used to account for local chemical variations in melt pockets at the origin of individual pyroxene crystals as well as for sector zoning. Although a large range of mass fractionation is observed in spinel, only two endmember compositions are considered for the mass balance calculations. MFsp denotes the mixing fraction of the most fractionated spinel. Conversely, (1-MFsp) is the proportion of unfractionated spinel. It directly depends on the observed composition. As a result, fM, the fraction of oxygen atoms from the melt, is the only parameter that is not fixed by observations and can be adjusted for each CAI.

Scanning electron microscopy of spinel textures in 3529-Z was done using the TESCAN SEM at the Museum National d'Histoire Naturelle in Paris.

Results

Comparison between CAIs

In CMS-1, the most Ti-rich pyroxene is also the most 16 O-poor and the relationship between TiO2 and ∆ 17 O is somewhat hyperbolic and close to that observed in 3529-Z (Fig. 1a). The ∆ 17 O value of cpx drops as TiO2 decreases during crystallization more rapidly in CMS-1 than in 3529-Z (Fig. 1b). A decrease of the ∆ 17 O value during pyroxene crystallization has also been found in the type B1 CAI TS34 [START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF]. Compared to type B CAIs, cpx is both more Ti-rich and more 16 O-poor in the type A CAIs 7R-19-1 and E101,3 with the most Ti-rich cpx being also the most 16 O-depleted.

Interstitial pyroxene in E101,3 has the highest Ti content observed in CAIs and the very 16 O-poor composition usually characteristic of melilite (Fig. 1c). The ∆ 17 O-TiO2 relationship thus defines a sequence, where the most refractory samples have abundant 2). In CMS-1, it intersects the spinel mass fractionation (MF) line at the most fractionated end (Fig. 3) and in STP-1 it intersects the spinel MF line at an intermediate MF value (Fig. 4). In spite of comparable ranges of ∆ 17 O values, cpx in 3529-Z, CMS-1 and STP-1 displays linear trends with an extent of MF in the 16 Orich endmember specific to each individual CAI and seemingly related to the MF in spinel.

Model results

3529-Z

The relationship between ∆ 17 O and TiO2 in cpx in 3529-Z is well reproduced using a fraction of oxygen from the melt (fM) of 0.8 in the sp dissolution reaction (Table 2). The full compositional range is encompassed if the initial TiO2 content is varied between 11 wt% and 15 wt% (Fig. 1). Upon spinel dissolution, the O isotopic composition of the melt first matches the O isotopic composition of anorthite (Fig. 2), a mineral that appears commonly before pyroxene in the crystallization sequence of type B CAIs [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF]. It reaches rapidly the composition of pyroxene (after the third step), once the ∆ 17 O value is lower than -12.8 ‰. Subsequent pyroxene grains crystallize with ∆ 17 O values between -12.8 ‰ and the final value of spinel. As the ∆ 17 O value drops in an asymptotic way, rapidly first and more and more slowly as crystallization proceeds, the proportion of 16 O-rich cpx is increasing as its ∆ 17 O value approaches that of spinel, in agreement with observations.

CMS-1

In CMS-1, the relationship between ∆ 17 O and TiO2 in cpx is satisfactorily reproduced with fM = 0.6, lower than in 3529-Z (Table 2). As in 3529-Z, the full compositional range is encompassed if a variable initial TiO2 content is considered (between 9 wt% and 12 wt%, Fig. 1). When the three O isotopes are considered, the composition of pyroxene is well reproduced if all spinel involved in the spinel + melt reaction is the most mass fractionated spinel (Fig. 3). The O isotopic composition of the melt drops rapidly upon dissolution of the first spinel and converges gradually toward the composition of spinel. As a result, the first pyroxene crystallizes with a ∆ 17 O of -11.6 ‰ in agreement with the most 16 O-poor cpx observed in CMS-1 (∆ 17 O = -12.45 ‰). As in 3529-Z, subsequent pyroxene grains crystallize in increasing proportion as ∆ 17 O approaches that of spinel, again in agreement with observations.

STP-1

The O isotopic composition of pyroxene in STP-1 is correctly reproduced if fM is 0.8 as in 3529-Z and if the spinel dissolving into the melt is a mixture of approximately 50% fractionated spinel and 50% unfractionated spinel (Table 2). As in 3529-Z, the first dissolution steps (4 steps here) yield O isotopic compositions agreeing with those of a mineral that crystallizes before pyroxene, melilite in that case (Fig. 4). The model O isotopic composition matches that of pyroxene once the ∆ 17 O value is inferior to -16.8 ‰. The ∆ 17 O and ∆ 18 O values of dmisteinbergite [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF] are reached when the most 16 O-rich pyroxene crystallizes.

Type A CAIs

Many fewer data are available to compare the composition of the two type A CAIs 7R-19-1 and E101,3 with the model. Still the few O isotopic and TiO2 compositions available are reproduced using (i) a 16 O-poor melt with ∆ 17 O ~ -1‰, corresponding to the most 16 O-poor melilite in the E101,3 core, (ii) a TiO2 content varying between 22 wt% and 11 wt%, i.e. the observed TiO2 range in the two CAIs, and (iii) a fM value of 0.9, higher than for type B CAIs (Fig. 1, Table 2). Compared to the above cases, the distribution of oxygen isotopes in crystallizing pyroxene is more evenly spread between the 16 O-rich and 16 O-poor endmember compositions, with a much higher fraction of 16 Opoor pyroxene (Fig. 5). This agrees well with the isotopic composition of pyroxene in 7R-19-1, which encompasses the full O isotope range and is dominantly 16 O-depleted (Fig. 5). This full range of O isotopic composition in pyroxene from type A CAIs is also in line with the composition of 4 type A CAIs from Efremovka and from the NWA 3118 chondrite [START_REF] Macpherson | Primary reverse oxygen-isotope evolution of pyroxene in Compact Type A CAIs from the Efremovka and NWA 3118 CV3 chondrites: insights into internal CAI mixing lines[END_REF]. The 16 O-poor composition of interstitial pyroxene in E101,3 (∆ 17 O = -1.8 ‰) is intermediate between the initial composition and that reached at the first dissolution step.

Spinel textures in 3529-Z

Spinel enclosed in melilite commonly shows reactional textures, often associated with cpx (Fig. 6a-d). Isolated grains in melilite laths are rounded to subhedral (Fig. 6a).

Reactional textures are uncommon for spinel enclosed in pyroxene, where spinels are mostly subhedral to euhedral (Fig. 6e). However, spinels with sharp angles are rare (Fig. 6e). Small spinels with rounded or anhedral shape are present in all cpx grains (Fig. 6i-l).

Euhedral spinels tend to be associated with 16 O-rich pyroxene, while 16 O-poor cpx cores are often devoid of spinel. Melilite-pyroxene interfaces are commonly scalloped and associated with spinel grains having reactional textures and meniscus at the interface (Fig. 6f-h).

Discussion

16 O-enrichment of pyroxene due to spinel back-reaction with the melt

Because TiO2 is an indicator of pyroxene crystallization in igneous CAIs and TiO2 content in cpx decreases as crystallization progresses [START_REF] Simon | Fassaite composition trends during crystallization of Allende Type B refractory inclusion melts[END_REF], the relationship between TiO2 and ∆ 17 O in CMS-1 indicates that pyroxene in CMS-1 begins to crystallize with a 16 O-depleted composition and becomes progressively enriched in 16 O during crystallization. The O isotopes vs Ti content relationship and range of composition in CMS-1 are very close to those in 3529-Z (Fig. 1), for which such a conclusion has been reached not only from chemical composition but also from spatial distribution within individual crystals having cores enriched in TiO2 and depleted in 16 O [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF]. When type A CAIs are considered this relationship extends to very large TiO2 content and very 16 O-poor composition, but the composition of cpx in 7R-19-1 and E101,3 does not simply extend the trend observed in CMS-1 and 3529-Z to higher values. It rather represents a similar but separated trend (Fig. 1c). Therefore, this hyperbolic 16 O enrichment with decreasing TiO2 content is observed in all CAIs for which data are available, indicating that pyroxene probably starts crystallizing with 16 Odepleted composition in most CAIs. This agrees with the 1N-B type A CAI from NWA 3118 and the type B1 CAI TS34 from Allende, where pyroxene crystals have cores enriched in Ti and depleted in 16 O [START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF][START_REF] Macpherson | Primary reverse oxygen-isotope evolution of pyroxene in Compact Type A CAIs from the Efremovka and NWA 3118 CV3 chondrites: insights into internal CAI mixing lines[END_REF]. The initial and final compositions seem to depend on CAI chemistry with the most refractory CAIs having the most Ti-rich, 16 O-poor pyroxene. While the 16 O-poor pyroxene is rare in type B CAIs and restricted to crystal cores [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF] it is dominant in type A CAIs. These chemical and isotopic similarities between type A / type B CAIs and FUN / non-FUN CAIs are strongly suggestive of a single process controlling the isotopic composition of pyroxene in most types of igneous CAIs, the extent of which being controlled by individual CAI chemistry.

The variations of fM between 0.6 in CMS-1 and 0.9 in 7R-19-1 and E101,3 indicate that in all cases most oxygen atoms are coming from the silicate melt, although in CMS-1 the contribution of spinel is important. In the two type A CAIs, fM is 0.9 indicating a small contribution of spinel. The intermediate fM = 0.8 for the two type B CAIs 3529-Z and STP-1 indicates a dominant contribution of the melt but a still significant contribution of spinel. The contribution of spinel is found to be the smallest for the two CAIs that are independently described from petrographic observations to contain the lowest amount of spinel among the CAIs considered in the present study [START_REF] Aléon | Coupled O and Mg Isotopic Investigations of Compact Type A Refractory Inclusions[END_REF][START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF]. Examination of the bulk chemical composition of CMS-1 [START_REF] Williams | Thermal and chemical evolution in the early solar system as recorded by FUN CAIs: Part I -Petrology, mineral chemistry, and isotopic composition of Allende FUN CAI CMS-1[END_REF] and comparison with a representative range of type A and B CAIs [START_REF] Grossman | Primordial compositions of refractory inclusions[END_REF] and references within) shows that its MgO content ~13 wt% is in the upper range of type B CAIs, while its SiO2 content ~25 wt% is more typical of type A CAIs and in the lower range of type B CAIs. The resulting MgO/SiO2 ratio is among the highest in igneous CAIs. This suggests that spinel is abundant in CMS-1 (modal abundance of 24 wt%, [START_REF] Williams | Thermal and chemical evolution in the early solar system as recorded by FUN CAIs: Part I -Petrology, mineral chemistry, and isotopic composition of Allende FUN CAI CMS-1[END_REF], probably more than in typical type B CAIs such as 3529-Z and STP-1. To summarize, fM, adjusted solely to match the O isotopic composition of pyroxene in a CAI, appears to correlate to a first approximation with the proportion of spinel, with spinel contributing all the more to the pyroxene composition as the CAI is spinel-rich.

Similarly to fM, the proportion of mass fractionated spinel involved in the reaction is also specific to each CAI. Intermediate situations analogous to STP-1 (Fig. 4) with both unfractionated and mass fractionated spinel dissolving into the melt appear to be more common and are found in the C1, EK1-4-1 and B7F6 FUN CAIs [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF].

However, the case where the pyroxene O isotope compositional trend intersects only the most mass fractionated spinel (Fig. 3) is also observed in the KT-1 FUN inclusion [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]. A possible explanation is the preferential dissolution of the last spinel crystallized from an evaporating melt either because it has a smaller grain size or because it represents the outer part of coarser grained spinel, grown on early crystallizing nuclei. Interestingly, the pyroxene isotopic trend in the forsterite-bearing FUN CAIs CG-14 and BG82DH8 [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF] seems more closely related to forsterite (fo) than spinel. Fo-bearing regions and sp-bearing regions in these CAIs are almost systematically mutually exclusive so that the petrological relationships between fo, sp and cpx are probably different from those investigated here. Phase equilibria in comparable CMAS systems enriched in fo [START_REF] Yang | Phase equilibria of the join akermaniteanorthite-forsterite in the system CaO-MgO-Al2O3-SiO2 at atmospheric pressure[END_REF] indicate early crystallization and dissolution of forsterite. Olivine may thus behave somewhat like spinel albeit with a more restricted stability field, so that it may contribute O to the melt in the most fo-rich normative compositions. Olivine resorption has indeed been noticed in the fo-rich FUN CAI Vigarano 1623-5 [START_REF] Davis | Melt solidification and late-stage evaporation in the evolution of a FUN inclusion from the Vigarano C3V chondrite[END_REF]. In any case, the mass fractionated composition of cpx in FUN CAIs, testifies that its 16 O-enrichment is driven by contribution of an interior component, rather than an interaction with an outer reservoir.

A possible caveat to the spinel dissolution model is the lack of obvious textural dissolution features in spinel embedded in cpx, which are commonly subhedral to euhedral in igneous CAIs (e.g. [START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF]. With the reasonable assumption that 3529-Z is a regular, typical type B1 CAI, insights can be gained from the observation of its spinel textures (Fig. 6). Whether the reactional textures observed for spinel [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF]. Finally, anhedral spinels with meniscus at the interface between melilite and pyroxene (Fig 6f-h) support spinel dissolution in the partial melt at the beginning of cpx crystallization, when the spinel contribution to the O isotopic change should be major. The detailed interpretation of spinel textures remains ambiguous but somewhat agrees with spinel dissolution. A more extensive study is required to conclude.

Although much less constrained, the composition of CaAl2Si2O8 (mostly anorthite) from type B CAIs is also matched by the present model. Laboratory crystallization experiments indicate that it crystallizes immediately before, simultaneously with or after pyroxene (e.g. [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF][START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature[END_REF] depending on details of the thermal history or CAI chemistry. O isotopic variations in anorthite have been proposed to result either from partial melting in 16 O-poor conditions and preservation of relict 16 O-rich anorthite (e.g. [START_REF] Nagashima | Micron scale oxygen isotope heterogeneity in anorthite of a forsterite-bearing type B CAI E60 from Efremovka[END_REF] or from isotope exchange during parent body metasomatism (e.g. [START_REF] Krot | Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism[END_REF]. In the present model CaAl2Si2O8 is expected to have an isotopic composition corresponding to the first steps of spinel dissolution if it crystallizes immediately before pyroxene or simultaneously with the most 16 O-poor pyroxene. This is observed for anorthite in 3529-Z (Fig. 2). By contrast, dmisteinbergite in STP-1 has the composition of the final crystallizing pyroxene (Fig. 4), requiring crystallization after pyroxene or simultaneously with the last pyroxene. Such a late crystallization is consistent with the metastable nature of dmisteinbergite suggesting rapid cooling [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF] and with experiments at the highest cooling rate showing that the nucleation of CaAl2Si2O8 is delayed and that crystallization should take place after that of pyroxene [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF][START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: The effects of cooling rate and maximum temperature[END_REF]. The spinel dissolution model is thus consistent with the O isotopic composition of anorthite and is an alternative to previous interpretations. Dissolution of spinel crystallized in an early stage of CAI petrogenesis by back reaction with the silicate melt is thus a process consistent with many observations in CAIs. Considering the right range of isotopic and chemical composition in individual CAIs, such a spinel + melt reaction, long inferred from experimental petrology [START_REF] Stolper | Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study[END_REF], explains (i) the 16 O-enrichment of cpx during crystallization and the observed trend with TiO2 content, (ii) the mixing trend between unfractionated 16 O-poor phases and 16 O-rich mass fractionated minerals (spinel) in FUN CAIs and (iii) its parameterization is consistent with chemical and mineralogical properties of the considered CAIs. The very 16 O-poor interstitial cpx in type A CAIs with texture of a residual melt [START_REF] Simon | Origin of compact type A refractory inclusions from CV3 carbonaceous chondrites[END_REF], observed here in E101,3 [START_REF] Aléon | Coupled O and Mg Isotopic Investigations of Compact Type A Refractory Inclusions[END_REF], is an endmember crystallized almost directly from the 16 O-poor melt, when the CAI reached the solidus after most of the CAI crystallized as melilite, with barely any contribution of spinel.

Closed-system behavior during crystallization of pyroxene

To explain the 16 O-enrichment of pyroxene upon crystallization in the type B CAIs 3529-Z and TS34, it has been proposed that these CAIs experienced a 16 O-depletion in a nebular reservoir during their formation and were subsequently exposed to a 16 O-rich gas [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Kawasaki | Crystal growth and disequilibrium distribution of oxygen isotopes in an igneous Ca-Al-rich inclusion from the Allende carbonaceous chondrite[END_REF]. If the partial melt was equilibrating with the ambiant gas during pyroxene crystallization, for instance due to rapid self-diffusion of O in silicate melts (e.g. [START_REF] Oishi | Oxygen diffusion in liquid silicates and relation to their viscosity[END_REF], the isotopic effect of spinel dissolution would be overprinted by exchange with nebular gas and pyroxene would rapidly reach a single Chemical isolation with absence of interaction between partially molten CAIs and O in the gas is therefore required. In a canonical solar nebula, in the temperature range at which CAI partial melting takes place (~ 1000 -1600 °C) and in thermodynamic equilibrium, O in the gas is dominantly bound into (1) CO and (2) H2O in roughly comparable proportions (Boesenberg et al., 2016 and references within). Experiments of O isotope exchange between CO or H2O gas and chondritic melts [START_REF] Boesenberg | Evaporation and the absence of oxygen isotopic exchange between silicate melt and carbon monoxide gas at nebular pressures[END_REF][START_REF] Boesenberg | Oxygen Isotopic Exchange Between Carbon Monoxide Gas and Silicate Melt: Implications for the Early Solar Nebula[END_REF]DiRocco and Pack, 2015;[START_REF] Yu | Experimental study of high temperature oxygen isotope exchange during chondrule formation[END_REF] showed that isotope exchange is rapid near 1500 °C and near complete isotopic equilibration is achieved in a few hours at 1 bar (CO, [START_REF] Boesenberg | Oxygen Isotopic Exchange Between Carbon Monoxide Gas and Silicate Melt: Implications for the Early Solar Nebula[END_REF] or at 1 bar with a volume dilution (i.e. with lower partial pressure, H2O, DiRocco and Pack, 2015). Exchange rates have been determined for water vapor (DiRocco and Pack, 2015;[START_REF] Yu | Experimental study of high temperature oxygen isotope exchange during chondrule formation[END_REF] but data are still lacking for CO [START_REF] Boesenberg | Oxygen Isotopic Exchange Between Carbon Monoxide Gas and Silicate Melt: Implications for the Early Solar Nebula[END_REF]. Still, approximately 50% to 100% isotope exchange is reached for both gases at 1 bar in a few hours to less than 24 hours, a comparable timescale for astrophysical processes. It is therefore considered here that isotope exchange rates are comparable for H2O and CO. Volume dilution experiments have shown that the isotope exchange efficiency decreases if the partial pressure of H2O decreases in the gas (DiRocco and Pack, 2015;[START_REF] Yu | Experimental study of high temperature oxygen isotope exchange during chondrule formation[END_REF], so that extrapolation to nebular conditions (10 -3 to 10 -6 bars) yield exchange half-lives (i.e. the time at which 50% exchange is reached) between 10 and 16 hours (DiRocco and Pack, 2015). Experiments performed with CO at nebular pressure (4 × 10 -6 bar, [START_REF] Boesenberg | Evaporation and the absence of oxygen isotopic exchange between silicate melt and carbon monoxide gas at nebular pressures[END_REF] have in addition shown an absence of isotope exchange in less than ~2 hours.

The closed system behavior of CAI partial melts during crystallization of pyroxene thus requires that the partial pressure of O-bearing species (CO or H2O) is low enough to prevent efficient exchange reaction at the melt surface or that partial melting and subsequent crystallization is rapid. The result of the CO exchange experiment at 4 × 10 -6 bar [START_REF] Boesenberg | Evaporation and the absence of oxygen isotopic exchange between silicate melt and carbon monoxide gas at nebular pressures[END_REF] suggests that if the partial melting last no longer than a few hours, O isotope equilibration with the ambient gas may remain limited in the solar nebula. However, if the partial pressure of O-bearing species is too low during partial melting, mass fractionation effects are to be expected [START_REF] Nagahara | The role of exchange reactions in oxygen isotope fractionation during CAI and chondrule formation[END_REF]. Such small amounts of mass fractionation are indeed indicated by [START_REF] Boesenberg | Evaporation and the absence of oxygen isotopic exchange between silicate melt and carbon monoxide gas at nebular pressures[END_REF].

O isotopic evolution of igneous CAIs

The lack of isotopic equilibration between CAIs and nebular gas during crystallization of pyroxene implies that CAIs record the isotopic composition of the gas only on limited occasions during their formation. The O isotopic evolution of igneous CAIs can be divided in 5 stages (Fig. 7).

Stage 1. CAIs initially formed from precursors condensed from a gas of near-solar composition at temperatures between 1300 K and 1700 K (e.g. [START_REF] Ebel | Condensation in dust-enriched systems[END_REF]. During condensation, O atoms from the ambient 16 O-rich gas enter directly the mineral structure. This 16 O-rich initial composition is recorded in CAIs that escaped significant remelting (e.g. [START_REF] Liu | Isotopic records in CM hibonites: Implications for timescales of mixing of isotope reservoirs in the solar nebula[END_REF][START_REF] Ushikubo | A long duration of the 16 O-rich reservoir in the solar nebula, as recorded in fine-grained refractory inclusions from the least metamorphosed carbonaceous chondrites[END_REF].

Stage 2. A first partial melting event near the liquidus temperature is required for the crystallization of igneous spinel. The systematic 16 O-rich composition of spinel in CAIs indicates that this partial melting either took place in a 16 O-rich environment or that these first partial melts did not exchange efficiently O isotopes with the surrounding gas. The variable extent of mass fractionation observed in spinel in FUN CAIs (e.g. [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF] indicates different degrees of evaporation for individual CAIs during this first partial melting and subsequent spinel crystallization. Experimental and theoretical considerations (e.g. [START_REF] Mendybaev | Experimental evaporation of mg-and Si-rich melts: Implications for the origin and evolution of FUN CAIs[END_REF][START_REF] Nagahara | The role of exchange reactions in oxygen isotope fractionation during CAI and chondrule formation[END_REF] suggest that these different degrees of evaporation result from variable gas densities (i.e. total pressure) in the ambient medium. Unfractionated CAIs would melt at relatively high pressure (e.g. canonical nebular pressure), while FUN CAIs would melt at lower pressure. Similar enrichments of heavy Mg isotopes in spinel, melilite, and pyroxene in 3535-1, a type B1 CAI from the Leoville CV3 chondrite [START_REF] Kita | Internal 26AL-26Mg isotope systematics of a Type B CAI: Remelting of refractory precursor solids[END_REF], suggest that this evaporation occurred before or at the onset of CAI crystallization.

Stage 3. The relationship between O isotopes and TiO2 content in cpx excludes secondary isotope exchange by solid-state diffusion or by fluid circulations on the parent-body and unmelted 16 O-rich spinel preserves the initial isotopic composition of the melt because the self-diffusion of O in spinel is extremely slow. This implies that the 16 O-depletion occurred after crystallization of spinel and before crystallization of pyroxene, i.e. during crystallization of melilite. Either igneous CAIs experienced an abrupt O isotope change at this point (plain line in Fig. 7) or they experienced a partial remelting event associated with a 16 O-depletion (dashed line in Fig. 7). The existence of 16 O-poor Mg-rich melilite overgrowths around 16 O-rich gehlenitic melilite in 7R-19-1 [START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF][START_REF] Yurimoto | Oxygen isotope exchange between refractory inclusion in Allende and solar nebula gas[END_REF] favors a partial remelting hypothesis. The preservation of 16 O-rich spinel indicates that such a second partial melting event was not intense enough to melt spinel completely.

Stage 4. After the last partial melting event, igneous CAIs crystallized as closedsystem isolated from nebular gas for O isotopes during cooling. The O isotopic composition of pyroxene and anorthite/dmisteinbergite in 3529-Z and STP-1 is well accounted for by dissolution of spinel into the partial melt.

Stage 5. Finally, igneous CAIs underwent a last heating event that upon cooling resulted in Wark-Lovering rim (WLR) formation. The conditions of WLR formation and record of the isotopic composition of the gas are unclear (e.g. [START_REF] Bodénan | An oxygen isotope study of Wark-Lovering rims on type A CAIs in primitive carbonaceous chondrites[END_REF][START_REF] Simon | Oxygen isotopic variations in the outer margins and Wark-Lovering rims of refractory inclusions[END_REF][START_REF] Yoshitake | Correlations between oxygen-isotopic composition and petrologic setting in a coarse-grained Ca, Al-rich inclusion[END_REF]. Many observations favor formation of WLR in a 16 O-rich gas thus requiring a backward isotopic evolution after the previous 16 Odepletion.

According to this scenario, spinel in the interior of igneous CAIs is always 16 Orich, pyroxene has an isotopic composition evolving from 16 O-poor in the Ti-rich cores to 16 O-rich in the Ti-poor crystal edges and anorthite/dmisteinbergite can have a range of composition following the pyroxene trend, from 16 O-poor if it crystallized before pyroxene to 16 O-rich if it crystallized after pyroxene. The case of melilite is more complex as its crystallization/partial melting corresponds to the point at which the O isotopic composition of the partial melt changes from the initial 16 O-rich composition to the pre-pyroxene 16 O-poor composition. Most melilite is thus expected to be 16 O-poor as commonly observed in igneous CAIs. Nevertheless, if the apparition of melilite and the O isotopic re-equilibration of the melt are not completely synchronous, variable isotopic compositions can be produced. Any melilite crystallized before the O isotopic change (i.e. before stage 3) will be 16 O-rich, thus producing 16 O-rich cores/relicts as observed in 7R-19-1 [START_REF] Ito | Oxygen isotopic SIMS analysis in Allende CAI: Details of the very early thermal history of the solar system[END_REF][START_REF] Yurimoto | Oxygen isotope exchange between refractory inclusion in Allende and solar nebula gas[END_REF]. By contrast, if melilite continued to crystallize once the system closed and spinel started to dissolve (i.e. during stage 4), it should be progressively enriched in 16 O along the pyroxene mixing trend as observed for STP-1 [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]. Early or protracted crystallization of melilite in different CAIs is likely due to differing bulk chemical compositions. A variable preservation of O isotope heterogeneity in melilite is possible depending on cooling rates, as solid-state self-diffusion of O within individual crystals could potentially blur or erase the initial variations. This distribution of O isotopes with mineralogy is in excellent agreement with the observations in all igneous CAIs including FUN inclusions and accounts for both the typical composition and the isotopic variability of melilite, anorthite and pyroxene.

Implications for the proto-planetary disk setting

In order to shed light on the O isotopic difference between the 16 O-rich Sun and the 16 O-poor planets, it is essential to understand the conditions under which CAI partial melts became 16 O-poor, i.e. under what conditions the isotopic equilibration was efficient during stage 3. Gas-melt equilibration may result from (1) enhanced O abundance in the gas, e.g. higher H2O, higher dust/gas ratios such as those expected during chondrule formation, for which isotope exchange has been shown to be efficient (DiRocco and Pack, 2015); (2) prolonged heating during more than 10 hours at 1500°C. Such a heating duration is still short enough to match the petrographic observations that imply heating events shorter than a few tens of hours (e.g. [START_REF] Connolly | On type B CAI formation: Experimental constraints on fO2 variations in spinel minor element partitioning and reequilibration effects[END_REF].

An alternative to gas-melt equilibration could be a more efficient O evolution mechanism: DiRocco and Pack (2015) indicate an O isotopic evolution faster than gasmelt isotope exchange by assimilation of FeO from the oxidized iron wire of their experimental device. Assimilation of 16 O-poor silicate/oxide dust following capture by CAI partial melts may thus contribute efficiently to a rapid 16 O-depletion. Such a mechanism may be at work if the 16 O-poor carrier is refractory interstellar dust carrying nucleosynthetic anomalies [START_REF] Krot | Oxygen isotopic composition of the Sun and mean oxygen isotopic composition of the protosolar silicate dust: evidence from refractory inclusions[END_REF]. Shock waves are expected to reproduce adequately the range of temperatures and cooling rates experienced by chondrules and type B CAIs based on textures and chemical composition [START_REF] Desch | A model of the thermal processing of particles in solar nebula shocks: Application to the cooling rates of chondrules[END_REF] i.e. peak heating temperatures in the 1400-1600°C range and cooling rates between a few °C per hour and 50°C per hour in the post-shock regime between 1000 and 1400°C. Di [START_REF] Di Rocco | Triple oxygen isotope exchange between chondrule melt and water vapor: An experimental study[END_REF] showed that isotope exchange between H2O vapor and silicate melt is likely to occur in these conditions due to appropriate temperature-time conditions and elevated dust/gas ratios. It seems thus possible that igneous CAIs experiencing such a shock wave would first partially melt melilite while preserving spinel, subsequently experience significant isotope exchange due to enhanced O abundance in the gas during the peak heating associated with melilite melting and recrystallization and would finally continue to crystallize anorthite and pyroxene at lower cooling rate and lower temperature without isotope exchange with the gas after passage of the shock front.

Whether partial melting and O isotope exchange occurred in a shock or not, they require that the surrounding gas is 16 O-depleted with a near terrestrial isotopic composition. Such a 16 O-poor reservoir is usually proposed to result from (1) the isotope selective photodissociation of CO upon self-shielding, either in the outer regions of the disk [START_REF] Lyons | CO self-shielding as the origin of oxygen isotope anomalies in the early solar nebula[END_REF] or in the presolar molecular cloud [START_REF] Yurimoto | Molecular cloud origin for the oxygen isotope heterogeneity in the solar system[END_REF], (2) non-mass-dependant isotopic fractionation in chemical reactions yet to be firmly identified (e.g. [START_REF] Thiemens | History and applications of mass-independent isotope effects[END_REF], (3) accretion of 16 O-poor interstellar dust [START_REF] Krot | Oxygen isotopic composition of the Sun and mean oxygen isotopic composition of the protosolar silicate dust: evidence from refractory inclusions[END_REF]. These models have very different implications for the distribution of O isotopes in the protoplanetary disk. The self-shielding models produce a horizontal O isotope zoning with a 16 O-enriched disk interior and 16 O-depleted planetary regions. A vertical zoning with a 16 O-poor surface and a 16 O-rich interior has also been proposed to match CAI observations [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF]. While the closed-system evolution during pyroxene crystallization no longer requires such a vertical zoning, the latter still provides a natural explanation for the 16 O-rich composition of the WLR, which occurred in the last environment that CAI met during their formation, possibly the dense disk midplane. Neither the diffuse regions inward the gas disk nor the disk surface are neutral environments at thermodynamical equilibrium. Rather, they are ionized regions where radiation-driven chemistry and kinetics are likely to play a major role. Detailed investigations of the chemistry and physics of these regions with regard to oxygen are required to identify the regions and conditions of 16 O-depletion in igneous CAIs.

Conclusions

Mass balance calculations indicate that the distribution of O isotopes between different CAI minerals is well accounted for by dissolution of early-crystallized 16 O-rich spinels into a 16 O-poor partial melt. The mineral suite crystallized during CAI cooling, that is notably pyroxene, but also anorthite/dmisteinbergite co-crystallized with pyroxene and possibly the last fraction of melilite crystals, became variably 16 O-enriched due to this back-reaction of early spinels with the CAI partial melt. Contrary to the usual assumption that CAIs constantly equilibrated with the ambient nebular gas, it is shown here that igneous CAIs underwent successions of open-system and closed-system behavior. They were notably isolated from the gas for O isotopes during their final stages of crystallization but an open-system behavior is required before that to account Comparison between literature data [START_REF] Aléon | Oxygen isotopes in the early protoplanetary disk inferred from pyroxene in a classical type B CAI[END_REF][START_REF] Mckeegan | Stable isotope variations in extraterrestrial materials[END_REF] Comparison between literature data [START_REF] Krot | Calcium-aluminum-rich inclusions with fractionation and unknown nuclear effects (FUN CAIs): I. Mineralogy, petrology and oxygen isotopic compositions[END_REF]

  compositions are reported as ‰ deviations from the terrestrial oceans reference (Standard Mean Ocean Water, SMOW) with δ 18 O = ( 18 O/ 16 Osample -18 O/ 16 OSMOW) / ( 18 O/ 16 OSMOW) × 1000; similarly δ 17 O for the 17 O/ 16 O ratios. 16 O enrichments are calculated using ∆ 17 O = δ 17 O -0.52 × δ 18 O. Mass fractionation effects are discussed using ∆ 18 O = δ 18 O -(δ 17 O + 4.5)/0.94, i.e. the δ 18 O deviation from the Carbonaceous Chondrite Anhydrous Minerals mixing line (CCAM,

  16 O-poor cpx and the least refractory have abundant 16 O-rich cpx. When the three O isotopes are considered (Figs. 2-4), the compositions of cpx in 3529-Z, STP-1 and CMS-1 define linear arrays with a comparable range of ∆ 17 O values in pyroxene. The most 16 O-rich pyroxenes are comparable with spinel (∆ 17 O ≤ -23‰) and the most 16 O-poor pyroxenes have intermediate ∆ 17 O values between ~ -12‰ and ~-16‰. However, they have different degrees of mass fractionation, with differently mass fractionated 16 O-rich cpx. In 3529-Z the linear array of pyroxene data plots along the CCAM line, as spinel does (Fig.

  embedded in melilite (Fig 6a-d) indicate a co-crystallization of spinel and cpx or crystallization of cpx at the expense of spinel is unclear. In cpx, the contrasting sub-to euhedral texture of spinel grains (Fig 6e) may be in line with (i) spinel dissolution preferentially along the crystallographic structure and/or (ii) recrystallization of spinel from the melt enriched in 16 O after a dissolution phase. The presence of rounded angles and rounded grains (Fig 6i-l) supports the first hypothesis, while the lack of spinel grains in the 16 O-poor cpx cores may agree with the second hypothesis. Late recrystallization agrees with relict spinel having cores enriched in Ti 4+ and rims rich in Ti 3+ in TS34 (Paque et al., 2013) and possibly with the plateau in the abundance of spinel during crystallization of type B CAI melts

  isotopic composition common to all CAIs. As shown above, the O isotopic composition of pyroxene from individual FUN CAIs define a mixing line between a 16 O-poor component, possibly common to all CAIs, and a 16 O-rich component with an extent of mass fractionation specific to each CAI. The 16 O-rich endmember pyroxene composition is thus controlled by a component indigenous to each CAI, i.e. the dissolution of spinel in the present model. This requires that igneous CAIs did not exchange O isotopes with nebular gas during spinel dissolution/pyroxene crystallization and agrees with the present results reproducing observations without need for additional exchange. An important implication is that igneous CAIs behave as isolated closed-system with regard to O isotopes during crystallization of pyroxene and anorthite/dmisteinbergite.Physical isolation of partially molten CAIs from ambient nebular gas is impossible as it would require CAIs to have an outer shell hermetic to O diffusion and partial melting would have to start from the CAI interior, with complete absence of partial melt at the contact with the gas. Even in a CAI with a fully crystallized mantle, the diffusion of O along grain boundaries is likely to maintain the interior of the CAI in contact with the gas.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. Three oxygen isotopes diagram of the type B1 CAI Allende 3529-Z.

  and the present model. Error bars are omitted for clarity. TFL -Terrestrial mass Fractionation Line; CCAM -Carbonaceous Chondrites Anhydrous Minerals mixing line.

Figure 3 .

 3 Figure 3. Three oxygen isotopes diagram of the FUN CAI Allende CMS-1. Comparison

Figure 4 .

 4 Figure 4. Three oxygen isotopes diagram of the type B2 FUN CAI Allende STP-1.

  and the present model. Error bars are omitted for clarity. TFL -Terrestrial mass Fractionation Line; CCAM -Carbonaceous Chondrites Anhydrous Minerals mixing line; SFL -Spinel mass Fractionation Line.

Figure 5 .

 5 Figure 5. Three oxygen isotopes diagram of the type A CAIs Allende 7R-19-1 and

Figure 6 .

 6 Figure 6. Spinel textures in the type B1 CAI Allende 3529-Z. Backscattered electron

Figure 7 .

 7 Figure 7. Temperature-time description of the five stages thermal-isotopic history of

Table 1 :

 1 Summary of the isotopic properties of the considered CAIs

		Allende 3529--Z	Allende CMS--1	Allende STP--1	Allende 7R--19--1	Efremovka E101.3
	CAI type	B1	FUN	B2, FUN	A, cpx--rich	A
	pyroxene TiO2 range	TiO2 range	TiO2 range	TiO2 range	TiO2 content
		2.3 wt% to 10.8 wt%	4.3 wt% to 10.5 wt%	0.03 to 8.7 wt%	~11 wt% to ~21 wt%.	~21 wt% TiO2
		Ti--rich 16 O--poor	∆ 17 O range	correlated ∆ 17 O and ∆ 18 O	Profiles	(up to 21.8 wt%)
		--20.5‰ ≤ ∆ 17 O ≤ --15.0‰	--24.0 ‰ to --12.5 ‰	∆ 18 O = 2.3‰	TiO2 = 20 wt%;	∆ 17 O ~ --2‰
		for TiO2 ≥ 9 wt%	correlated ∆ 17 O and ∆ 18 O	for ∆ 17 O = --16.8‰	∆ 17 Oprofile1 = --10.7‰;
		Ti--poor 16 O--rich	∆ 17 O ~ --12.5‰	∆ 18 O = 4.6‰	∆ 17 Oprofile2= --9.0‰
		--24.3‰ ≤ ∆ 17 O ≤ --21‰	for ∆ 18 O = 6.7‰	for ∆ 17 O ≤ --24‰	∆ 17 O range
		for TiO2 < 6 wt%	∆ 17 O ~--24.0‰		--18.9 ± 3.6‰ to
			for ∆ 18 O ~14.5‰		1.3 ± 3.5‰
	spinel	∆ 17 O ≤ --23.1‰	∆ 17 O ≤ --23.5‰	∆ 17 O ≤ --23.7‰	average ∆ 17 O = --20.6 ±	∆ 17 O ≤ --21‰
			2.5‰ ≤ ∆ 18 O ≤ 14.0‰	1.7‰ ≤ ∆ 18 O ≤ 8.2‰	3.3‰
	melilite	∆ 17 O range	∆ 17 O ~ --3.5‰	correlated ∆ 17 O and ∆ 18 O	∆ 17 O range	16 O--poor in the
		--3.5‰ ≤ ∆ 17 O ≤ --2.0‰	∆ 18 O ≤ 2‰	∆ 18 O ~ 2‰	--25.2 ± 3.3‰ ≤ ∆ 17 O ≤ 7.3	interior, notably
				for ∆ 17 O ≤ --16‰	± 3.8‰	when associated
				∆ 18 O ~ 0‰		with pyroxene
				when ∆ 17 O ≥ --10‰		(∆ 17 O ~ --1‰)
						16 O--enriched
						toward the
						periphery
	CaAl2S2iO8 anorthite		dmisteinbergite
		--12.7‰ ≤ ∆ 17 O ≤ --1.8‰		∆ 17 O ≤ --24.2‰
				4.7‰ ≤ ∆ 18 O ≤ 6.3‰

Table 2 :

 2 Parameters of the model that match igneous CAIs .a. : not available; * calculated from δ 17 O and δ 18 O best matching the samples

	Parameter	3529--Z	CMS--1	STP--1	Type A
	∆ 17 O sp (‰)*	--24.1	--24.0	--24.0	--24.1
	∆ 17 Oi MELT (‰)*	--2.1	--3.3	--4.1	--0.9
	fM	0.8	0.6	0.8	0.9
	TiO2 i (wt%)	13	10.5	n.a.	22
	TiO2 f (wt%)	2	2	n.a.	13
	MFsp	0	1	0.5	0

n

Acknowledgements

Careful reading of an early draft, comments and suggestions by Roger Hewins and Alice Aléon-Toppani and discussions with Mathieu Roskosz were appreciated. Sylvain Pont helped with the MNHN SEM. Frédéric Moynier is thanked for editorial work. Noriyuki Kawasaki and Jean-David Bodénan are thanked for their reviews raising the issue of textures. This work was supported by the PNP-INSU French National Program of Planetology.

for the 16 O-depletion of an initially 16 O-rich melt. More work is clearly required to unravel the exchange rates and conditions of this 16 O-depletion under realistic nebular conditions. This includes investigation of O isotope exchange between CAI melts and H2O and CO vapor, kinetics of mineral dissolution in CAI melts and kinetics of crystallization of CAI partial melts. Only such detailed experimental data can be used for comparison with protoplanetary disk models.


Agata Daszkowska-Golec
email: agata.daszkowska@us.edu.pl

Alain Gojon
email: alain.gojon@inrae.fr

Alix Boulouis
email: boulouis@ibpc.fr

Anne Krapp
email: anne.krapp@inrae.fr

Professor, Andreas P M Weber
email: aweber@hhu.de

Professor, Antonio Molina
email: antonio.molina@upm.es

Bernd Müller-Röber

Ben Field
email: ben.field@univ-amu.fr

Catherine Walshe
email: c.walsh4@lancaster.ac.uk

Crisanto Gutierrez
email: cgutierrez@cbm.csic.es

Chris Bowler
email: cbowler@biologie.ens.fr

PhD Damian Boer
email: damian.boer@wur.nl

Professor, Detlef Weigel
email: detlef.weigel@tuebingen.mpg.de

Professor, Dorothea Bartels
email: dbartels@uni-bonn.de

Edith Heard
email: edith.heard@embl.org

EGFV Eric Gomès
email: eric.gomes@inra.fr

Eva María Gómez Álvarez

Fabien Chardon
email: fabien.chardon@inrae.fr

Hatem Rouached
email: rouached@msu.edu

Hannes Kollist
email: hannes.kollist@ut.ee21

Ive De Smet
email: ive.desmet@psb.vib-ugent.be22

Ido Nir
email: idonir@stanford.edu

Jonathan Jones
email: jonathan.jones@tsl.ac.uk

José Pardo-Tomás

Xenie Johnson
email: xenie.johnson@cea.fr

Jose R Dinneny
email: dinneny@stanford.edu

Laurent Laplaze
email: laurent.laplaze@ird.fr28

Livia Merendino
email: livia.merendino@u-psud.fr

Malcolm Bennett
email: malcolm.bennett@nottingham.ac.uk

IPSiM Martin Antoine
email: antoine.martin@cnrs.fr

Michel Havaux
email: michel.havaux@cea.fr

Michael Hodges
email: michael.hodges@universite-paris-saclay.fr

Michaël Nicolas
email: michael.nicolas@wur.nl

Michael Wrzaczek
email: michael.wrzaczek@umbr.cas.cz

Nathalie Leonhard
email: nathalie.leonhardt@cea.fr

Pedro Luis

Rodriguez Egea
email: prodriguez@ibmcp.upv.es37

Professor, Peter Schlögelhofer
email: peter.schloegelhofer@univie.ac.at38

Jean-Philippe Reichheld

Sophie Brunel-Muguet
email: sophie.brunel-muguet@inrae.fr

Stanislav Kopriva
email: skopriva@uni-koeln.de

Scott Hayes

Vincent Colot

Wigge Philip

Alice Stra

Amal Khalaf

Catherine H Gardener

Clara Stanschewski

Izamar Olivas Orduna

Jose L Moreno Ramirez

Katja Froehlich

Kirti A Singh

Kyle J Lauersen

Maged M Saad

Manuel Aranda

Marilia Almeida-Trapp
email: marilia.trapp@kaust.edu.sa

Hydrology Matthew F Mccabe
email: matthew.mccabe@kaust.edu.sa

Mauricio Lopez-Portillo
email: mauricio.lopezportillomasson@kaust.edu.sa00

Masson

Mark Tester
email: mark.tester@kaust.edu.sa

Brande Wulff
email: brande.wulff@kaust.edu.sa

Ikram Blilou
email: ikram.blilou@kaust.edu.sa

Salim Al-Babili
email: salim.babili@kaust.edu.sa

Jean Colcombet
email: jean.colcombet@universite-paris-saclay.fr

Iain M Young
email: iain.young@kaust.edu.sa

Heribert Hirt
email: heribert.hirt@kaust.edu.sa

PlantACT! -How to tackle the climate crisis 1

Greenhouse gas (GHG) emissions have created a global climate crisis which requires immediate interventions to mitigate the negative effects on all aspects of life on this planet.

As current agriculture and land use contributes to up to 25 % of total GHG emissions, plant scientists are at center stage to find possible solutions to a transition to sustainable agriculture and land use. In this article, the PlantACT! initiative of plant scientists lays out a road map in which areas and how plant scientists can contribute to find immediate, midterm and long-term solutions and what changes are necessary in the way to work out these solutions at the personal, institutional and funding level.

I. The climate emergency

Humanity is facing an unprecedented challenge from climate change [START_REF]IPCC 2022 Climate Change 2022: Impacts, Adaptation and Vulnerability[END_REF]. The CO2 concentration in the atmosphere has dramatically increased from 280 ppm (pre-industrial) to 420 ppm within 150 years. As a consequence, the global average temperature has increased by 1.5°C. This anthropogenic climate change is associated with altered rainfall patterns, extreme weather events and less predictable weather patterns. This presents a major challenge to crop production and food security and thus threatens the foundations of human civilization.

The International Panel on Climate Change (IPCC) had set the goal of limiting global warming to less than 1.5°C. [START_REF] Ourbak | Changing the game: the Paris Agreement and the role of scientific communities[END_REF] Although the goal of 1.5°C is probably not possible any more, achieving climate neutrality is more important than ever, by reducing net CO2 emissions to zero through a 45% reduction in emissions within 10 years [START_REF]IPCC 2022 Climate Change 2022: Impacts, Adaptation and Vulnerability[END_REF]. This represents a disruptive goal which demands new thinking, solutions and commitments.

The atmospheric temperature increase caused by rising carbon dioxide concentrations will not decrease significantly even after zero carbon emissions (peak carbon) have been achieved [START_REF] Solomon | Irreversible climate change due to carbon dioxide emissions[END_REF]. The climate effects of atmospheric CO2 at peak carbon will remain irreversible for at least 1,000 years, if not counteracted by a net reduction in atmospheric CO2. In reality, anthropogenic climate change is irreversible over the next 10 generations at least, unless rapid measures are taken to sequester carbon dioxide from the atmosphere [START_REF] Solomon | Irreversible climate change due to carbon dioxide emissions[END_REF].

The global carbon cycle describes the dynamic cycling of carbon between the atmosphere and marine as well as terrestrial ecosystems (Figure 1). Overall, terrestrial and aquatic net primary production is in the range of 130 Gt C per year. The vast majority of this assimilated carbon is returned to the atmospheric CO2 pool via respiration. Hence, the natural global carbon cycle (not considering anthropogenic emissions) is nearly balanced [START_REF] Friedlingstein | Global Carbon Budget 2022[END_REF]. However, human activities perturb the global carbon cycle, leading to a continuous increase of atmospheric CO2 concentration. Net anthropogenic annual carbon emissions are leading to an estimated 5.2 Gt C increase in atmospheric CO2 in 2022 [START_REF] Friedlingstein | Global Carbon Budget 2022[END_REF] [Figure 1). All paths towards the 1.5°C goal depend on a rapid reduction of the carbon footprint of agriculture, forestry and land use, combined with the use of bioenergy with carbon capture and storage [START_REF] Fajardy | BECCS deployment: a reality check[END_REF][START_REF] Searchinger | Assessing the efficiency of changes in land use for mitigating climate change[END_REF][START_REF] Leifeld | The underappreciated potential of peatlands in global climate change mitigation strategies[END_REF].

II. Agriculture as a Contributor to Climate Change

Agriculture is both a victim and culprit of global climate change as 20-25% of GHGs are released through agricultural activities. Apart from CO2, significant amounts of methane and nitrous oxide are emitted from agriculture which represent more potent greenhouse gases than CO2 (>30 and 300 times respectively). Methane is produced by rice paddy fields, livestock (via enteric fermentation and manure) and organic waste in landfills [START_REF] Smith | Agricultural methane emissions and the potential for mitigationPhil[END_REF].

Nitrous oxide emissions are an indirect product of organic and mineral nitrogen fertilizer use. However, both gases have a shorter lifespan than CO2: methane and N2O remain in the atmosphere for 12 and 114 years compared to 300-1,000 years for CO2 [START_REF] Forster | Changes in atmospheric constituents and in radiative forcing[END_REF]. Hence, unlike CO2, reductions in both of these other greenhouse gases would deliver rapid benefits (Box 1).

The N fertilizer supply chain currently contributes >2% greenhouse gas (GHG) emissions [START_REF] Menegat | Greenhouse gas emissions from global production and use of nitrogen synthetic fertilisers in agriculture[END_REF]. Global use of synthetic N fertilizers is predicted to increase 50% by 2050 [START_REF] Fao | The future of food and agriculture-Alternative pathways to[END_REF]. When N fertilizers are applied, significant amounts of N20 are generated through microbial conversion in the soil [START_REF] Menegat | Greenhouse gas emissions from global production and use of nitrogen synthetic fertilisers in agriculture[END_REF]. In the short term, the most effective strategy is reducing the amount of N applied [START_REF] Sutton | Our Nutrient World: The challenge to produce more food and energy with less pollution[END_REF] to avoid over-fertilization through improvements in agronomy, extension advice and management practices. In the short to medium term, a switch to agro-systems utilizing legume crops able to naturally fix nitrogen represents an urgent priority [START_REF] Stagnari | Multiple benefits of legumes for agriculture sustainability: an overview[END_REF]. In the medium to longer term, improvements in nitrogen use efficiency in cereal crops (currently <50%) through breeding for key traits such as root architecture would also provide major gains but might also carry the danger of inducing rebound effects [START_REF] Hamont | Plant scientists can't ignore Jevons paradox anymore[END_REF]. These plant-based solutions are not reliant on major scientific breakthroughs but exploit existing knowledge that collectively act to reduce fertilizer-related production, usage and emissions.

The majority of CO2 generated by agriculture arises from changes in land use, particularly deforestation for fodder and grazing [START_REF] Shukla | Climate Change and Land: an IPCC special report on climate change, desertification, land degradation, sustainable land management, food security, and[END_REF]. Livestock and fodder production each generate more than 3 billion tons of CO2 equivalent. Changes in food and dietary choice will help to reduce GHG emissions [START_REF] Aleksandrowicz | The Impacts of Dietary Change on Greenhouse Gas Emissions, Land Use, Water Use, and Health: A Systematic Review[END_REF]. For example, currently 10-30 kg plant proteins are required to produce 1 kg of beef. Increasingly shifting away from animal to alternative protein sources would provide major benefits [START_REF] Gaillac | The carbon footprint of meat and dairy proteins: A practical perspective to guide low carbon footprint dietary choices[END_REF]. In the short term, reducing demand for soya-based animal feed would have major benefits through decreased land conversion [START_REF] Boerema | Soybean Trade: Balancing Environmental and Socio-Economic Impacts of an Intercontinental Market[END_REF]. In the mid to long term, adopting plant-based diets remains an efficient option. Plant scientists could contribute to the development of alternative plant-based protein sources by working with food and social scientists.

Given the central importance of food, a reduction of greenhouse-gas emissions from agriculture is a major challenge and will require the implementation of a range of techniques and tools, from capturing or reducing methane emissions at the source, more efficient use of fertilizers, and improved efficiency in meat, dairy and cereal production.

Overall, these measures should be part of a circular agricultural system, integrating crop improvements, mixed crops, field rotations and social interactions with local farming communities.

III. Challenges for Future Global Food Production

Growing global populations, shifting dietary patterns towards greater meat consumption, and increased food waste at both the consumer and supply chain levels, are major factors impacting global food systems. It is unclear how an increase of 70-100% in food production to meet global demands can be achieved in either a sustainable or equitable manner. Given the widespread degradation of terrestrial systems, there is no major surplus of arable lands on which to cultivate new crops. Likewise, any further conversion of forests into agricultural land via deforestation threatens biodiversity, contributing a major source of CO2 emissions and further jeopardizes planetary health. To increase food production using current agricultural practices would require more chemical fertilizers and pesticides, with major negative environmental, climate and human health related impacts.

With most of the land suitable for agriculture already in use, fertile agricultural land is increasingly becoming the preserve of wealthy nations and/or industry, heightening economic disparities between the global North and South.

Plants require sunlight, nutrient rich soils and water for optimal growth. Although mildly higher temperatures can prolong the growing seasons in some regions, extreme temperatures inhibit crop growth and impact yields through decreased fertility.

Furthermore, changing weather patterns alter the timing of rainfall as well as the distribution of pests and diseases. To cope with these challenges, short term agronomic solutions include changing farming practices, such as rotating crops to match water availability and/or adjusting sowing dates to temperature and rainfall patterns (Table 1).

Plant scientists can also contribute by identifying microbes and plant traits for generating (in the medium to longer term) crop varieties (Table 1) showing increased heat-and drought-resistance, enhanced water-use efficiency (Box 2) and, in general, improved resilience to the changes in environmental conditions.

Tackling climate change requires the use of cropping systems, either already available but not broadly used or novel ones to be developed, as well as the development of crop varieties suitable for these new agrosystems. Introducing adaptable and new crop systems could lead to diversification of agricultural production, with positive effects on ecosystems and biodiversity. This strategy promises to enhance crop resilience to biotic and abiotic stresses, but can also improve carbon sequestration and storage. In addition, plant breeding can provide better climate change-adapted crops. The development of new plant species and varieties that are commercially sustainable and resistant to different risks involves the preservation of multiple varieties, landraces, rare breeds and closely related wild relatives of domesticated species.

The current focus of crop adaptation is the expression of traits related to resistance to drought, heat, salinity and flooding. Different regions need crops adapted to different stressors: in some regions, crops that are resilient to drought and/or extreme temperatures are required, while in others, flooding or disease resistance is the priority. Moreover, breeding efforts should consider the need for more diverse and resilient agroecosystems and should benefit from local knowledge related to the adaptation and selection process. Crop varieties that meet these conditions could contribute to efficient adaptation strategies to cope with climate change. In this context, the PlantACT! initiative (Plants for climate ACTion!) will alert, engage and work on solutions to reduce agriculturebased GHG emissions and facilitate a more equitable and sustainable global food production system.

IV. Plants, soil and microbes as actors for mitigation

Soil was long considered solely as an inert growth substrate. If the chemical and physical properties were not suitable, herbicides, fertilizers and pesticides had to be added to soil to provide stable yields. This notion has now changed following recognition that besides the physical and chemical structure of soils, a diverse living community of soil organisms is essential for crop production. Soil microorganisms form beneficial symbiotic associations with plants and help plant roots in nutrient uptake and control of diseases.

Soil microorganisms also play a role in soil water and nutrient holding capacity and can contribute to mitigating climate change by maintaining or increasing soil carbon content.

In the future, holistic approaches of the soil-plant-microbe ecosystem must be considered to achieve sustainable solutions related to climate change [START_REF] Kell | Large-scale sequestration of atmospheric carbon via plant roots in natural and agricultural ecosystems: why and how[END_REF][START_REF] Schweitzer | Innovating carbon-capture biotechnologies through ecosysteminspired solutions[END_REF]. In this context, agriculture is not the only target of this approach, but landscaping and land restoration of unused land could provide novel solutions to climate change (Box3). PlantACT! supports the idea that soil restoration could play a key role in improving agriculture and carbon capture as well as long term carbon sequestration.

V. Conclusions

Given the complexity of the effects of climate change at all levels of planetary life, it is highly unlikely that exclusive disciplinary thinking will provide solutions that will hold up to their promises. Current thinking needs to be readjusted both at the institutional, funding, as well as subject levels to enable multidisciplinary scientific approaches. The presentday scientific culture of exclusive scientific exchange in specific fields needs to be broken down and new forms of interdisciplinary conferences and communication need to be established (e.g. ideas labs, workshops, grass root level proposals that compete with each other for prizes). Information access to farmers, scientists and decision makers via open access platforms is needed to find and evaluate different approaches and solutions.

Solutions must be fact-checked not only in terms of global carbon but also in terms of social and societal impact. The time constraints for proposed solutions (e.g. launching breeding programs for crop adaptations, introducing genes into elite crop varieties takes a decade) have to be considered and weighed against immediate solutions (e.g. changes in agricultural practices, ready microbe-induced crop resilience). Overall, one solution for all will not be possible. Solutions will need to be shaped and targeted differently to reflect geographical and local needs and contexts and will have to be continuously assessed for their impact. For example, solutions need to be targeted differently to the EU and US compared to Sub-Saharan Africa where population growth will be highest this century. Moreover, land in many countries is limited, but less in Africa, where agriculture suffers from low yield and hence supporting intensification in a sustainable manner could have an immediate impact. Overall, if we want to preserve a livable planet, we must leave our well-trodden disciplinary paths and search for novel inter-disciplinary solutions and approaches. Moreover, not only national but overarching transnational funding programs need to be implemented to develop or adapt solutions to local specificities. PlantACT! aims to urgently accelerate these new inter-disciplinary interactions and solutions by stimulating new forms of working and funding (Box 4). Organization of the United Nations, https://www.fao.org/3/cb3808en/cb3808en.pdf). The FAO data includes greenhouse gases other than CO2, converted to CO2 equivalents. Adapted from [START_REF] Jansson | Crops for Carbon Farming[END_REF] with data from [START_REF] Friedlingstein | Global Carbon Budget 2022[END_REF] and [START_REF] Fao | The future of food and agriculture-Alternative pathways to[END_REF] (created with BioRender.com).

Box 1. Reducing methane emissions from rice production.

Methane is the second most important greenhouse gas after CO2 and is >20 times more potent than CO2. Rice paddy fields emit 10g CH4/m 2 [21] and this forms 15-20% of anthropogenic methane emissions. Methane arises from the decomposition of organic matter in anoxic conditions by soil methanogenic archaea. Changes in agronomical practices are already available to significantly reduce methane production in rice agrosystems (short term solution). This includes water management practices such as alternate wetting and drying or aerobic rice that act to conserve water. However, transitioning from irrigated rice systems often leads to a yield penalty and greater interannual yield variability because of reduced access to water, weed competition and changes in nutrient availability [START_REF] Van Oort | Impacts of climate change on rice production in Africa and causes of simulated yield changes[END_REF]. To tackle this, plant scientists (working together with agronomist, hydrologist, microbial ecologist and agro-socio-economists) could contribute by developing (medium term) solutions that include new crop varieties for water-saving and low methane rice agrosystems. Traits include early vigor to deal with weed competition and root traits to improve water and nutrient acquisition in aerobic conditions [START_REF] Ndoye | Root traits for low input agroecosystems in Africa: lessons from three case studies[END_REF] but also the use of perennial rice varieties.

Box 2. Enhancing water use efficiency and carbon capture

Carbon gain in photosynthesis is a water consuming process as fixing one molecule of CO2 requires hundreds of molecules of H2O lost by transpiration. However, there is substantial natural variation of water use efficiency (WUE) among plant species, and this holds great potential to improve this trait in crops. Improved WUE can be achieved by using microbes collected from plants able to cope with extremely low water availability and contributing to this phenotype (short-term solution) and by breeding water-saving crops (mid-term solution). Reducing water loss by narrowed stomatal aperture can lead to decreased CO2 concentration inside the leaves and hence increased photorespiration, in particular at higher temperatures. To avoid a possible penalty on growth in WUE crops, carbon capture efficiency could be improved. Potential advantages of C4 plants (and/or C3-C4 intermediates) can perform photosynthesis at lower stomatal aperture. Examples from breeding for WUE has pointed to genes involved in stomatal patterning, abscisic acid homoeostasis and CO2 signaling [START_REF] Bailey-Serres | Genetic strategies for improving crop yields[END_REF].

Box 3. Building up soil inorganic carbon (SIC) in arid regions

Soil organic carbon (SOC) represents a major form of terrestrial C storage (Figure 1). The importance of SIC is less appreciated. Oxalogenic plants that secrete oxalate and associate with microbes in the soil show great promise for capturing CO2 in an inorganic form that is highly stable. Fungi and bacteria associated to these plants (called oxalotrophs) can use oxalate as their sole carbon and energy sources. In a soil that is rich in Ca 2+ or Mg 2+ , these microbes can produce Ca 2+ -or Mg 2+ -carbonates which thereby increase the soil inorganic carbon (SIC) content [START_REF] Syed | Oxalate Carbonate Pathway-Conversion and Fixation of Soil Carbon-A Potential Scenario for Sustainability[END_REF]. These natural CO2 trapping systems that are primarily found in arid and hyper-arid regions could provide novel and important C sequestration alternatives. Such systems do not compete with agricultural land and can fix carbon in the soil for decades to centuries.

Box 4. Re-designing the way plant-based climate solutions are funded

The time required to develop plant-based climate solutions is rapidly running out. One major challenge is the research grant funding systems currently operating in many countries which impose delays of up to 12 months between submission of an idea to eventually starting a project. There is an urgent need to re-design and accelerate the way plant-based climate solutions are assessed, initially tested and then rolled out. New formats to catalyze trans-disciplinary research solutions are also urgently needed. The Belmont Forum (https://belmontforum.org) provides an example for how such a change can be designed, which involves funding organizations, international science councils, and regional consortia committed to International transdisciplinary research to provide knowledge for understanding, mitigating and adapting to global environmental change.

PlantACT! aims to urgently accelerate new trans-disciplinary interactions and solutions by stimulating new forms of working and funding.

FiguresFigure 1 .

 1 Figures

Table 1 .

 1 Strategies to avoid adverse impact of agriculture on climate change, adapt to the consequences of climate change, and to mitigate climate change. Colors indicate estimated timeframes to implementation: ■ short-term (within a decade), ■ mid-term (one to several decades), ■ long-term (centennial).

Acknowledgements

The work of HH was supported by baseline grant BAS/1/1062-01-01 from King Abdullah University of Science and Technology, Thuwal, KSA. APMW acknowledges funding under Germany's Excellence Strategy EXC-2048/1, Project ID 390686111 and the European Union H2020 project 862087-GAIN4CROPS