Alexis Godefroy
email: alexis.godefroy@promes.cnrs.fr

Maxime Périer-Muzet

Pierre Neveu

Nathalie Mazet

Hybrid thermochemical cycles for low-grade heat storage and conversion into cold and / or power

Keywords: Low grade heat recovery, Cogeneration of power and refrigeration, Thermochemical energy storage, Thermodynamic analysis, Organic Rankine Cycle

In order to recover low grade heat (available at temperatures under 250 °C) provided for instance by an industrial plant or solar energy, innovative thermodynamic cycles are investigated. These cycles are based on the hybridization of a solid/gas chemical sorption cycle (thermochemical cycle) with a power cycle (Organic Rankine Cycle (ORC)), they enable converting a low grade heat input into cold and / or power while providing an intrinsic energy storage feature. Four hybrid configurations are considered, a thermodynamic analysis allows comparing their performances with those of existing ORC through several criteria: energy and exergy efficiencies, specific exergy output and power production ratio. For each of the five systems, potential applications, advantages and weaknesses are summarized.

Introduction

As regards both energy demand and resources, the management of variability is a decisive issue; this topic covers variability in energy form (which can be addressed through developing multi-purpose systems) and time-variability (which can be addressed through integrating energy storage systems). In this context, an interesting option consists in pooling devices of two thermodynamic cycles, in order to combine their functionalities and build a so-called "hybrid" system. This may lead to a substantial improvement in flexibility and efficiency. To that end, the most relevant cycles are:

1. Three-temperatures cycles involving a liquid / gas sorption process (absorption). Using evaporation / condensation of a vapor flow and its absorption / desorption in a liquid solution (typically H2O / LiBr), they take advantage of the related thermal effects (either exo-or endothermal effect, according to the component). Since the liquid solution flows between vapor desorber and absorber, a continuous cold (at the evaporator) or heat (at the absorber) production is achieved. These systems are characterized by quite good COP (from 0.7 to 1). 2. Three-temperatures cycles involving a solid / gas sorption process (thermochemical).

Although the operating mode is similar to liquid / gas absorption cycles, it is based on a reversible solid / gas chemical reaction (exothermal synthesis / endothermal decomposition).

Hence, these cycles are discontinuous: an intrinsic storage feature is thereby provided, with high energy densities [START_REF] Aydin | The latest advancements on thermochemical heat storage systems[END_REF]. Moreover, they can operate in a wide range of operating conditions (T, P) depending on the reactive pair [START_REF] Cabeza | Review on sorption materials and technologies for heat pumps and thermal energy storage[END_REF]. 3. Two-temperatures power cycles: Organic Rankine Cycles (ORC) provide an efficient mechanical energy production from a low grade heat source. Adjusting the working fluid and components allows optimizing the mechanical work production. Both of these three cycles involve a working fluid undergoing state changes in vapor generators and absorbers (evaporator / condenser, desorber / absorber, sorption reactors). Consequently, coupling and hybridizing them will consist in coupling these common components. Considering the three-temperatures liquid / gas absorption cycle and the two-temperatures power cycle, several hybrid configurations were proposed: firstly, Kalina cycle (power cycle developed from the late 1970s) involves a binary mixture as working fluid (typically NH3 / H2O) in order to decrease thermodynamic irreversibility caused by temperature mismatch during heat transfer. With a hot source temperature of 399 °C, its energy efficiency ranges from 0.15 to 0.20 and exergy efficiency is approximately 0.52 [START_REF]Combined cycle and waste heat recovery power systems based on a novel thermodynamic energy cycle utilizing low-temperature heat for power generation[END_REF]. Later, Goswami et al. [START_REF] Xu | A combined power / cooling cycle[END_REF][START_REF] Hasan | First and second law analysis of a new power and refrigeration thermodynamic cycle using a solar heat source[END_REF] proposed an absorption power cycle providing additional cold production. For this system, providing cold at -10 °C using a 140 °C hot source temperature, energy efficiency ranges from 0.17 to 0.24 while exergy efficiency ranges from 0.49 to 0.65. Concerning ORC, Kalina and Goswami cycles, a review was proposed by Karimi et al. [START_REF] Karimi | A review of Organic Rankine, Kalina and Goswami cycle[END_REF]. Considering thermochemical cycle and two-temperatures power cycle, very few hybrid configurations have been studied despite the interesting storage feature of thermochemical cycles. In this area, the main investigations have been carried out on resorption cycles: ▪ For power production only, Jiang et al. [START_REF] Jiang | Exploration of ammonia resorption cycle for power generation by using novel composite sorbent[END_REF] obtained energy and exergy efficiencies ranging respectively from 0.07 to 0.12 and 0.40 to 0.74, with a hot source temperature below 110 °C. ▪ For power & cold cogeneration, one of the first resorption cycles was proposed by Wang et al. [START_REF] Wang | A resorption cycle for the cogeneration of electricity and refrigeration[END_REF], and later improved by Lu et al. [START_REF] Lu | Analysis of an optimal resorption cogeneration using mass and heat recovery processes[END_REF]. Providing cold at 10 °C with a hot source temperature of 110 °C, its COP ranges from 0.83 to 0.93 while exergy efficiency ranges from 0.32 to 0.41. The theoretical and experimental works of Jiang et al. [START_REF] Jiang | Performance prediction on a resorption cogeneration cycle for power and refrigeration with energy storage[END_REF][START_REF] Jiang | Experimental study on a resorption system for power and refrigeration cogeneration[END_REF] lead to energy and exergy efficiencies in the ranges [0.29; 0.42] and [0.12; 0.16] respectively (hot source between 120 and 170 °C, cold production at 10 °C). Finally, an innovative hybrid cycle based on solid / gas chemical sorption was proposed by Bao et al. [START_REF] Bao | Modelling of a chemisorption refrigeration and power cogeneration system[END_REF] for the cogeneration of power and cold. COP and exergy efficiencies are respectively in the ranges [0.34; 0.57] and [0.20; 0.62] (hot source between 85 and 255 °C, cold production at -10 °C). In this paper, innovative hybrid thermochemical cycles for cold and / or power production are investigated using a new methodology, which underlines the sensitivity of the performances to a wide variety of reactive salts. Firstly, the working principle of hybrid thermochemical cycles is presented (components and thermodynamic path), which leads to identify four operating modes for these cycles; then, the framework of the thermodynamic study (assumptions, methodology, performance criteria) is depicted. The following section gathers performance results of both hybrid cycles and ORC, for hot source temperatures under 250 °C. Finally, conclusions are drawn on advantages, weaknesses and possible applications of the five systems in the field of low grade heat recovery.

Hybrid cycles for cold and / or power production

Working principle: general overview

Hybridizing a thermochemical refrigeration cycle with a power cycle leads to the general scheme of Fig. 1. Since it uses a solid sorbent, this system is intrinsically discontinuous: this characteristic provides the storage function of the cycle. Its operation is divided into two steps: ▪ The charging step (step 1) occurs when a hot source is available (high pressure levels). ▪ The discharging step (step 2) is performed when a useful effect is needed (low pressure levels). Each of these two operating steps comprises two main components: ▪ A vapor generator, where an endothermal process (decomposition reaction of a solid sorbent or evaporation of the liquid working fluid) takes place. ▪ A vapor absorber, where an exothermal process (synthesis reaction of a solid sorbent or condensation of the gaseous working fluid) takes place. Regarding heat flows, a heat input (Qin) is required for the high pressure endothermal process (charging step), while a cold production function (Qcold) may be provided by the low pressure endothermal process (discharging step). Furthermore, heat is rejected at ambient (Qamb) and midtemperature (Qm) by respectively, the high and low pressure exothermal processes. Regarding mass flows, in each step, an expander is located between the vapor generator and absorber to take advantage of the reactive vapor flow leaving the vapor generator: it can be either actuated (leading to a non-isobaric step) or bypassed (leading to an isobaric step) as illustrated by the valves on Fig. 1: this provides the power production function (W1 and W2) of the cycle.

Based on Fig. 1, four hybrid cycles (operating modes) are thus considered:

▪ Separated power & cold mode: the expander is actuated in charging step only (isobaric discharging step, W2 = 0). Therefore, cold and power productions are time-shifted: power is generated in charging step while cold is produced in discharging step. ▪ Simultaneous power & cold mode: the expander is actuated in discharging step only (isobaric charging step, W1 = 0). Therefore, both cold and power are produced in discharging step.

▪ Combined power & cold mode: the expander is actuated in both of the two steps of operation (non-isobaric steps, W1 ≠ 0 and W2 ≠ 0). This enables increasing the total power output.

▪ Power mode with auto-thermal discharging step: the expander is actuated in discharging step only (isobaric charging step, W1 = 0). Moreover, in this step, the heat Qm (from the vapor absorber) is transferred to the vapor generator. Thus, the vapor generator pressure and temperature increase: cold production is removed, in favor of an increased power production.

Thermodynamic path

In connection with Fig. 1 (working principle with components), Fig. 2 shows the thermodynamic cycle followed by the working fluid, ammonia. This is the Clausius-Clapeyron diagram (ln(P/P 0 ) as a function of -1/T), well known for thermochemical systems. As highlighted by this diagram, hybrid thermochemical cycle operation is based on the existence of two thermodynamic equilibrium lines: ▪ The High Temperature equilibrium is associated to High Temperature Material (HTM), which is a solid reactive salt. This is a chemical reaction equilibrium.

▪ The Low Temperature equilibrium relates to Low Temperature Material (LTM), which is: ▫ Either ammonia. In this case, named single sorption cycle, the Low Temperature equilibrium is a liquid / vapor phase change equilibrium. ▫ Or a solid reactive salt. In this case, named resorption cycle, the Low Temperature equilibrium is a chemical reaction equilibrium. Based on Fig. 2, the operation of the hybrid cycle is illustrated for the combined power & cold mode: ▪ During the charging step, heat supply (Qin) drives the endothermal desorption of ammonia, which is released close to the HTM equilibrium conditions (point 1). The reactive fluid is then expanded (between points 1 and 2) and finally condensed or absorbed (point 3). ▪ During the discharging step, the endothermal evaporation or desorption of ammonia at a lower pressure (point 4) provides the cooling effect (Qcold); ammonia vapor is superheated (between points 4 and 5), then expanded (between points 5 and 6) and finally absorbed (point 7). More details on the operation of thermochemical cycles can be found in [START_REF] Stitou | Experimental investigation of a solid/gas thermochemical storage process for solar air-conditioning[END_REF] and [START_REF] Fitó | Definition and performance simulations of a novel solar-driven hybrid absorption-thermochemical refrigeration system[END_REF]. Note that ammonia liquid-vapor equilibrium is modelled by (1), while chemical reaction equilibrium is modelled by [START_REF] Cabeza | Review on sorption materials and technologies for heat pumps and thermal energy storage[END_REF]. Those equations are represented by straight lines on Fig. 2.

𝑙𝑛 ( 𝑃 𝑃 0 ) = - 𝐿 𝑣𝑎𝑝 𝑅. 𝑇 + Δ𝒮 𝑣𝑎𝑝 𝑅 (1)
𝑙𝑛 ( 𝑃 𝑃 0 ) = - Δ 𝑟 𝐻 0 𝑅. 𝑇 + Δ 𝑟 𝑆 0 𝑅 (2)
The definition of HTM is a decisive factor for the hot source temperature; moreover, depending on whether charging and discharging steps are isobaric or not, the three cogeneration modes (separated, simultaneous and combined power & cold modes) clearly arise on Fig. 2.

In power mode with auto-thermal discharging step, exothermal synthesis heat Qm (released at point

Thermodynamic study

Operating conditions and main assumptions

The framework of the thermodynamic study is defined by the operating temperatures Tcold = 0 °C and Tamb = 20 °C; Thot,max = 250 °C is set as the maximal hot source temperature since a focus is made on low grade heat sources. Moreover, the following thermodynamic assumptions are used:

▪ An isentropic efficiency ηis = 0.8 is set for expanders. ▪ The temperature pinch for (liquid / liquid) or (liquid / vapor) heat exchange is ΔTHX1 = 5 K. ▪ The temperature pinch for (vapor / vapor) heat exchange is ΔTHX2 = 10 K.

▪ The temperature deviation from thermodynamic equilibrium which is required for a chemical reaction to proceed is ΔTr-eq = 20 K. ▪ Reference temperature for exergy calculations is chosen as T 0 = Tamb = 20 °C.

▪ The pressure drops are neglected. ▪ There are no heat exchanges between components and the surroundings. ▪ A maximal volumetric ratio Rv,max = 10 is assumed for expanders, to comply with their nominal operating range (scroll expanders are particularly targeted, as they are suitable for small facilities). To give an example, Rv is defined as Rv = v6/v5 in discharging step on Fig.

Several expansion stages (number of expansion devices) are added to comply with this constraint.

Finally, this study is restricted to ammonia-salt reactive pairs in the thermochemical reactor. Thus, the working fluid of the whole hybrid thermochemical cycle is ammonia.

Methodology

All calculation processes described hereafter have been carried out with EES software [START_REF] Software | Engineering Software EES: Engineering Equation Solver[END_REF].

Iterative algorithm: screening over the panel of reactive salts

This thermodynamic study is based on energy calculations. All extensive quantities related to each cycle (Qin, Qcold, masses, volumes, …) are computed for a given electrical production (W = 1 kWh).

Specific values are then reported to the cycled mass of working fluid. This thermodynamic study is aimed at exploring the potential of hybrid thermochemical cycles presented in §2. for different solid reactive salts, and to retrieve the most promising ones. A database containing thermochemical quantities (reaction enthalpy ΔrH 0 and reaction entropy ΔrS 0 ) for 103 reactive ammonia salts is used. The content of this database comes from values collected and computed by Touzain [START_REF] Touzain | Thermodynamic values of ammonia salts reactions for chemical sorption heat pump[END_REF] and CNRS-PROMES knowledge. Among the gathered salts are mainly metallic chlorides, bromides and iodides, for instance: CaCl2, MnCl2, FeBr2, SrI2, … The methodology of this study is divided into two main steps: ▪ Firstly, the LT equilibrium is set, being either a chemical reaction equilibrium (for resorption cycles), or an ammonia liquidvapor equilibrium (for single sorption cycles).

▪ Secondly, for each one of the 103 reactive salts, its data (ΔrH 0 , ΔrS 0 ) are used to define the HT equilibrium, and the thermodynamic path (i.e. pressure, temperature, specific enthalpy and entropy at each key point of the cycle) is computed. Note that, in the case of resorption cycles, a LTM has to be chosen in the first step. BaCl2 (8/0) is chosen as the LTM for all resorption cycles in the next sections, since it enables cold production at Tcold with a decomposition reaction pressure (Pdec,L = P4 on Fig. 2) at 0.09 bar, which is considered as acceptable despite the boundary value Pmin = 0.1 bar.

Finally, a similar methodology was applied to realize a thermodynamic analysis of an ORC, considering 15 organic fluids (isobutene, n-pentane, R124, R134a, …).

Definition of relevant performance criteria

The LT and HT thermodynamic equilibria being known, and given that W = 1 kWh, the following quantities are computed at each iteration: ▪ Hot source temperature, Thot (depending on the HTM and temperature pinches). ▪ Volumetric expansion ratio(s), Rv (Rv,1, Rv,2, … if there are several expansion stages). ▪ Heat consumption, Qin. ▪ Cooling effect, Qcold. Note that a residual cold production Qcold may be observed in power mode with auto-thermal step (due to the cold outlet of the expander in discharging step): even if this is not the prioritized useful effect of this operating mode, it enables improving the cycle performances. In order to pick out the most relevant reactive salts, 4 performance criteria are defined and computed for each HTM: ▪ Energy efficiency,

𝜂 𝐼 = 𝑊 + 𝑄 𝑐𝑜𝑙𝑑 𝑄 𝑖𝑛 (3)
▪ Exergy efficiency,

𝜂 𝑒𝑥 = 𝑊 + 𝐸𝑥 𝑐𝑜𝑙𝑑 𝐸𝑥 𝑖𝑛 (4)
▪ Specific exergy output,

𝑤 + 𝑒𝑥 𝑐𝑜𝑙𝑑 = 𝑊 + 𝐸𝑥 𝑐𝑜𝑙𝑑 𝑚 𝑤𝑓 (5)
▪ Power production ratio,

𝜏 𝑤 = 𝑊 𝑊 + 𝑄 𝑐𝑜𝑙𝑑 (6)
, with

𝐸𝑥 𝑖𝑛 = 𝑄 𝑖𝑛 . (1 - 𝑇 0 𝑇 ℎ𝑜𝑡 ) (7)
and

𝐸𝑥 𝑐𝑜𝑙𝑑 = 𝑄 𝑐𝑜𝑙𝑑 . ( 𝑇 0 𝑇 𝑐𝑜𝑙𝑑 -1) (8)
These performance criteria were chosen because they seemed to be the most relevant: without being redundant, they provide enough information to give a fair comparison between the cogeneration and power cycles presented in this study. Indeed:

▪ The energy efficiency ηI, defined in (3), is the ratio of useful energy to input energy. This is a cogeneration efficiency, which is suitable for comparison with other cogeneration cycles working under the same temperatures (Tcold, Tamb, Thot).

▪ The exergy efficiency ηex defined by (4), which is the ratio of useful exergy to input exergy, accounts for the fact that mechanical work and heat do not have the same "quality", through the Carnot weighting factors related to Thot (according to [START_REF] Jiang | Exploration of ammonia resorption cycle for power generation by using novel composite sorbent[END_REF]) and Tcold (according to ( 8)). ▪ Specific exergy output w+excold, as defined by [START_REF] Hasan | First and second law analysis of a new power and refrigeration thermodynamic cycle using a solar heat source[END_REF], should be read alongside the two previous criteria, since it provides a quantitative assessment of both useful effects: respectively work generation and useful exergy are reported to mass unit of the working fluid. ▪ Finally, power production ratio τw, defined in ( 6), gives the missing information to determine specific work output w (see [START_REF] Lu | Analysis of an optimal resorption cogeneration using mass and heat recovery processes[END_REF]). It should be read alongside ηI, whose order of magnitude greatly depends on the useful effects (cold and / or power). Finally, a slightly different selection process was used for ORC: for each of the hot source temperatures obtained in the case of hybrid thermochemical cycles, the organic fluids having the best performances (over the criteria ηI, ηex and w) were retrieved.

𝑤

As mentioned in §3.1., several expansion stages have to be implemented in some cases in order to fulfill the boundary value Rv,max. Only slight differences were observed between the performances of one-to three-expansion cycles. Consequently, only the configurations with the highest number of expansion stages (leading to the best exergy efficiencies) were retrieved in §4. ▪ Finally, ORC (black circles) can operate over the whole temperature range [81; 250 °C]. These differences can be explained using Fig. 2: ▪ Pressure Pdec,H is higher in separated and combined modes (W1 ≠ 0), thus temperature T1 (and, as a consequence, Thot) is higher. ▪ The constraint W1 ≠ 0 does not apply in simultaneous power & cold mode, which reduces the minimal required hot source temperature. However, because of the constraint over P6, all HTMs are not usable to achieve both cold and power production. ▪ In the case of power mode with auto-thermal step, more HTMs can be used, which extends the minimal regeneration temperature. Note that for a given operating mode, resorption cycles (filled triangles) require higher hot source temperatures than single sorption cycles to operate: ▪ In power mode with auto-thermal step, the minimal usable regeneration temperature is Thot = 130 °C in resorption case, against 81 °C for single sorption case. ▪ In separated power & cold mode, the minimal usable regeneration temperature is Thot = 212 °C in resorption case, against 138 °C for single sorption case. This difference is caused by the closeness of LT and HT equilibrium lines, which makes resorption cycle unfeasible for a lot of HTMs.

Results and discussions

Power production ratio τw (Fig. 3d) supports the following analyses, especially about ηI and w+excold.

As regards energy efficiencies:

▪ All cogeneration cycles (separated, simultaneous and combined modes) show good performances: ηI ranges from 0.24 to 0.61, against 0.08 to 0.18 for ORC and 0.01 to 0.20 for power cycle with auto-thermal step. These differences originate in different proportions of power and cold in useful effects: τw ranges from 2 to 30 % for cogeneration cycles, against 58 to 100 % for power mode with auto-thermal step and ORC. A higher proportion of cold production is always beneficial to energy efficiency, but a cooling effect is not as valuable as a power output due to the low related Carnot factor (see the exergy efficiency analysis). ▪ The best values are reached with resorption cycles (filled blue triangles), because their cold production is higher (it originates in the decomposition enthalpy of an ammonia salt, which is higher than the vaporization enthalpy of ammonia: ΔrH 0 ≈ 2.Lvap): τw does not exceed 14 %.

Considering exergy efficiencies: ▪ The most performant cycle is the ORC (ηex ranges from 0.40 to 0.52) because it provides a full power output (τw = 100 %, more exergy content than a cold output) with relatively low requirements in heat supply (the main part of Qin consists in a vaporization enthalpy, whereas it is a decomposition enthalpy for hybrid thermochemical cycles). ▪ Among hybrid thermochemical cycles, combined power & cold mode shows the best values in each of the three temperature areas: ηex ranges from 0.20 to 0.41. This is due to the fact that power production is maximized in comparison with separated and simultaneous modes.

Concerning power mode with auto-thermal step, ηex increases with Thot (because power production increases with pressure P6) and would probably exceed exergy performances of the combined mode if higher hot source temperatures than Thot,max were considered. For all of the five systems, specific exergy output increases when Thot rises, which is directly related to the increase in expansion ratios. Moreover: ▪ Combined power & cold mode exhibits the highest values, over the whole range of hot source temperatures where it is feasible: w+excold ranges from 282 to 639 kJ/kgNH3. ▪ For simultaneous power & cold mode, w+excold increases in the area [107; 195 °C], then remains almost constant between 440 and 485 kJ/kgNH3. This is due to the fact that Psynth,H = P6 reaches the boundary value Pmin when hot source temperature is around 195 °C, then remains constant so that expansion ratio does not increase anymore for Thot > 195 °C. ▪ w ranges from 90 to 162 kJ/kgwf for ORC, which is quite lower than the previous values.

However, since ORC does not provide cold, a fair comparison should be done using [START_REF] Lu | Analysis of an optimal resorption cogeneration using mass and heat recovery processes[END_REF] to compute the mass density of mechanical work in the case of cogeneration cycles. ▪ Finally, power mode with auto-thermal step shows quite good specific exergy outputs: ▫ It reaches 596 kJ/kgNH3 in resorption case. Since resorption cycle uses a solid reactive salt instead of ammonia as the LTM, no additional cold production may be provided at the outlet of the expander (T9 is not low enough on Fig. 2), thus τw = 100 %.

▫ w+excold ranges between 40 and 575 kJ/kgNH3 in single sorption case. This configuration is characterized by lower temperatures T5, T6 and Tm than in resorption case: therefore, an additional cold production is provided at the expander outlet (τw ranges from 58 to 87 %). Finally, the most performant systems are retrieved for each temperature area on Fig. 3 However, the high values obtained for ηI and w+excold in cogeneration modes should be moderated because of the relatively low values of τw.

Conclusion

In addition to the discussions of §4. on performances, a key factor in choosing the most adequate system for a given application is complexity. ORC are typically easier to implement than thermochemical cycles: chemical sorption processes imply heat and / or mass transfer limitations. However, this increase in complexity comes with an interesting storage feature.

Using an innovative methodology to evaluate performances for a large variety of reactive salts, it was shown that hybrid thermochemical cycles can be of great interest to recover low grade heat from a hot source under 250 °C while providing an intrinsic energy storage feature: ▪ If a cold storage and production function is primarily needed, with low power requirements, separated power & cold mode is the most interesting option. ▪ If a power & cold storage and cogeneration function is desired, combined power & cold mode is the most performant system; however, although slightly less performant, simultaneous mode requires only one expansion device and can operate at lower hot source temperatures. ▪ If only power output is desired, ORC is a relevant option, reaching high exergy efficiencies;

however, it provides no storage function, contrary to the power mode with auto-thermal step. The range of energy efficiencies is [0.01; 0.61] for the proposed hybrid thermochemical cycles (with much higher values for cogeneration modes), against [0.08; 0.18] for ORC. Their range of exergy efficiencies is [0.01; 0.41] (highly depending on hot source temperature and mode), below that of ORC: [0.40; 0.52]. Finally, exergy densities increase with hot source temperature and reach particularly high values for the innovative hybrid cycles (640 kJ/kgNH3, against 162 kJ/kgwf for ORC). Fig. 4 summarizes the strengths and weaknesses of each studied cycle. It offers a tool for choosing the most adequate system to recover low grade heat from a given source, according to the needs.

Figure 1 .

 1 Figure 1. Working principle of hybrid thermochemical cycles: components, mass and energy flows in each step of operation (Charge / Discharge).

7 )

 7 is used to achieve endothermal vapor generation (point 4) upstream of the expander in discharging step. Therefore, pressure Pdec,L (or Pevap, according to the nature of the LTM) increases and cold production is replaced by an increased power production.

Figure 2 .

 2 Figure 2. Thermodynamic path followed by the working fluid of a hybrid thermochemical cycle: Clausius-Clapeyron diagram.

  Beside, several technological boundary values are set: ▪ Working fluid pressure is bounded by Pmin = 0.1 bar and Pmax = 30 bar, for the sake of technical feasibility with acceptable costs. ▪ A minimal vapor quality xmin = 0.8 is required at the outlet of the expander.

Fig. 3

 3 Fig. 3 gathers the results for hybrid thermochemical cycles (4 operating modes) and ORC: for each system, the performance criteria of §3.2.2. are displayed as a function of hot source temperature Thot. On these plots, vertical black lines delimit three areas corresponding to the following hot source temperature ranges: [100; 150 °C], [150; 200 °C] and [200; 250 °C]. Concerning the required hot source temperatures: ▪ Power mode with auto-thermal step (red triangles) operates at Thot > 81 °C. ▪ Simultaneous power & cold mode (green squares) operates at Thot > 107 °C. ▪ Separated (blue triangles) and combined (pink circles) modes work at Thot > 138 °C.

Figure 3 .

 3 Figure 3. Performance results of the 5 cycles: hybrid thermochemical cycles (4 operating modes) and ORC. Energy (a) and exergy (b) efficiency, specific exergy output (c) and power production ratio (d).

  : ▪ As regards energy efficiency, combined power & cold mode exhibits the best performance in areas [100; 150 °C] (ηI = 0.51) and [150; 200 °C] (ηI = 0.51), but is overcome in area [200; 250 °C] by the separated power & cold mode in resorption case (ηI = 0.61). ▪ ORC presents the highest exergy efficiencies in both of the three areas (with maximal values of ηex = 0.52, 0.50 and 0.46 respectively). ▪ The highest specific exergy output is reached by combined power & cold mode in each of the three areas (w+excold = 325 kJ/kgNH3, 543 kJ/kgNH3 and 640 kJ/kgNH3 respectively).

Figure 4 .

 4 Figure 4. Summary of the advantages and weaknesses of the five systems.

  Once the computations have been realized for all reactive salts, a first selection is made: thermodynamic cycles that cannot meet technological boundary values Rv,max, xmin, Pmin, Pmax or Thot,max are excluded. The remaining salts are ranked according to each of the previous 4 criteria and, for each of these criteria, only the 10 best salts are retained. At the end of this process, about 15 to 30 solid salts are selected for each cycle configuration (see §4.).

	=	𝜂 𝐼 . 𝜏 𝑤 . (𝑤 + 𝑒𝑥 𝑐𝑜𝑙𝑑 ) 𝜂 𝑒𝑥 . (1 -𝑇 0 𝑇 ℎ𝑜𝑡 )	(9)

Acknowledgments

Alexis Godefroy receives a PhD grant from the Ministry of Education (doctoral contract n° 2017-09-ED.305).


Lionel G Vacher
email: l.vacher@wustl.edu

Maxime Piralla

Matthieu Gounelle

Martin Bizzarro

Yves Marrocchi

Thermal evolution of hydrated asteroids inferred from oxygen isotopes

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Introduction

Dark C-type asteroids dominate the main-belt asteroid population and are genetically related to hydrous primitive CI and CM carbonaceous chondrites (Vilas & Gaffey 1989;Hiroi et al. 1996;Vilas 1994;Burbine et al. 2002;Lauretta et al. 2019). CM chondrites are the most common water-rich meteorites, and CM-like matter represent an important fraction of exogenic clasts reported in other groups of meteorites, implying that CM parent bodies are widespread in the asteroid belt (Briani et al. 2012). CM chondrites are complex aggregates of high-temperature components formed in the disk and low-temperature secondary minerals formed during subsequent parent-body fluid circulations. The latter provide key constraints on the origin of water accreted by asteroids (Vacher et al. 2016;Piani et al. 2018) as well as their accretion and evolution histories (Young et al. 2003;Verdier-Paoletti et al. 2017;Vacher et al. 2017;Fujiya et al. 2015). CM chondrites are therefore important samples because they show varying degrees of alteration that can be easily estimated by their chemical alteration index (Rubin et al. 2007;Marrocchi et al. 2014;Vacher et al. 2018). Among secondary minerals, carbonates are of primary importance as they represent direct proxies of the asteroidal fluids from which they formed and can, in theory, be used to decipher their thermal evolution (Clayton & Mayeda 1984). However, determining carbonate precipitation temperatures requires knowledge of the O isotopic compositions of their parental fluids, which itself requires knowledge of the carbonate precipitation temperatures, leading to a seemingly circular problem.

The temperature of CM carbonate precipitation remains largely underconstrained and proposed values cover a large range of temperatures. More generally, the sequence of formation of the different secondary phases (carbonates, serpentine) is poorly understood.

Based on 'clumped-isotopes' and oxygen isotopic analyses, it has been proposed that CM carbonates could have precipitated at both low and medium temperatures, in the range of 0-75°C (Clayton & Mayeda 1984;Benedix et al. 2003;Guo & Eiler 2007) and 50-350°C (Verdier-Paoletti et al. 2017;Alexander et al. 2015). However, these two different methodologies do not take account the petrographic relationship between carbonates and serpentine (Fuchs et al. 1973;Zolensky et al. 1997;Brearley 2006;Rubin et al. 2007;Vacher et al. 2018;Lee et al. 2013;2014), which can constrain the evolution of the fluid over time. In addition, different types of carbonates are present in CM chondrites (Vacher et al. 2017;Lee et al. 2013;2014) but no specific attention has been paid on their respective oxygen isotopic compositions. The objectives of this paper are thus to determine the oxygen isotopic composition of petrographically characterized carbonates in order to quantify the thermal evolution of hydrated asteroids. To do so, we surveyed a suite of different CM chondrites characterized by varying degrees of alteration: CM2.6/2.7 Maribo (van Kooten et al. 2018), CM2.5 Murchison (Rubin et al. 2007), CM2.4/2.7 Jbilet Winselwan (King et al. 2018) and CM2.0 Mukundpura (Rudraswami et al. 2018). Based on oxygen isotopic compositions of carbonates, we propose a new isotopic alteration model that reconciles petrographic observations and formation temperatures of CM carbonates.

Material and methods

SEM imaging

Calcite grains were located in carbon coated (i) polished sections of Murchison, Mukundpura and Jbilet Winselwan (samples provided by the Muséum National d'Histoire Naturelle in Paris) and (ii) thin section of Maribo (section provided by the Natural History Museum of Denmark in Copenhagen) using a Scanning Electron Microscope (SEM) JEOL JSM-6510 equipped with an energy dispersive X-ray detector (Bruker-AXS XFlash, silicon drift detector) at Centre de Recherches Pétrographiques et Géochimiques (CRPG, Nancy, France). Back-Scattered Electron (BSE) imaging were performed with a 3 nA electron beam current operated at 15 kV.

SIMS oxygen isotope analyses of calcite

Calcite oxygen isotopic compositions were analysed using a CAMECA IMS 1270 E7 at CRPG laboratory. A Cs+ primary ion beam (~15x10 µm spot area) with a current of ~5 nA was used in order to collect 16 O -, 17 O -and 18 O -secondary ions in multi-collection mode using three Faraday cups (L'2, FC2 and H1). Charge compensation was applied using a normalincidence electron gun. Mass Resolving Power (MRP = M/ΔM) was adjusted to ~7000 to resolve interference from 16 OH -on the 17 O -peak and achieve maximum flatness on the top of the 16 O -and 18 O -peaks (entrance and exit slits of FC2 were adjusted to ~70 µm and ~170 µm, respectively). 16 O -and 18 O -secondary ions were collected on L'2 and H1, respectively (slit 1, MRP ≈ 2500). Pre-sputter on a large area (~2020 µm) was applied before each measurement during 60s in order to remove carbon coating at the surface of the calcite grains. Acquisition time was set to ~5s and measurements were repeated over 30 cycles to achieve counting statistics ~0.2‰ (1σ) for δ 18 O and ~0.3‰ for δ 17 O. Isotope ratios ( 17 O/ 16 O and 18 O/ 16 O) are presented in per mil (‰) relative to Standard Mean Ocean Water (SMOW):

(1) where x represents 17 O or 18 O and SMOW the ratios of the SMOW standard. We measured two in-house terrestrial standards to define the Terrestrial Fractionation Line (TFL): (i) quartz (SiO 2 ) from Brazil (δ 18 O = 9.6‰, 16 O = 2  10 9 cps and 18 O = 4.2  10 6 cps) and (ii) calcite (CaCO 3 ) from Mexico (δ 18 O = 23.6‰, 16 O = 2  10 9 cps and 18 O = 4.2  10 6 cps; 0.03 wt% of MgO). Instrumental Mass Fractionation (IMF) for calcite matrix was determined from our inhouse Mexican calcite standard at the beginning and end of each analytical session. IMF values for each sample analysis were then calculated by accounting for the linear deviation over the time of the IMF values. Typical measurement errors (2σ), accounting for errors on each measurement and the external reproducibility of the standard, were estimated to be ~0.5‰ for δ 18 O, ~0.6‰ for δ 17 O, and ~0.7‰ for Δ 17 O (i.e., Δ 17 O = δ 17 O -0.52 × δ 18 O), where Δ 17 O represents the departure from the TFL.

Results

In the four CM chondrites surveyed in this work, calcite grains surrounded by Fe-Srich serpentine/tochilinite (hereafter T1 calcite; Fig. 1a) are ubiquitous whereas serpentinefree polycrystalline calcite grains containing Fe-Ni sulfide inclusions (hereafter T2 calcite; S2)

Discussion

Together, T1 and T2 calcite grains define a mass-independent trend with δ 17 O = (0.61 ± 0.03)  δ 18 O -(3.3 ± 1.1) (Fig. 1c-d) that is similar, within errors, to those commonly reported in different CM chondrites (Vacher et al. 2018). As a first approximation, this trend suggests that the O isotopic compositions of CM carbonates is essentially controlled by variable degrees of isotopic exchanges between 16 O-rich anhydrous silicates and a 17,18 S2) with Δ 17 O = -2.6 ± 1.0, which artificially produce the aforementioned mass-independent trend when taken together with T1 calcites (Fig. 1d). This demonstrates that petrographic However, these values correspond to the initial water composition and do not represent the oxygen isotopic compositions of fluids from which carbonates precipitated, which had Earthlike compositions with Δ 17 O ≈ 0‰ (Fig. 1c; Vacher et al. 2016;Verdier-Paoletti et al. 2017).

Quantitative estimates of the O isotopic compositions of the parental fluids of carbonates have been obtained by CO 2 clumped-isotope thermometry (Δ 47 ; Guo & Eiler 2007), which corresponds to anomalous enrichments of mass 47 (i.e., 13 C 18 O 16 O) in CO 2 derived from H 3 PO 4 digestion of carbonates (Ghosh et al. 2006). Based on measurements performed on the CM chondrites Murchison and Murray, dominated by T1 calcites, Guo & Eiler (2007) estimated that the O isotopic compositions of their alteration fluids ranged from δ 18 O = 2‰ and Δ 17 O = -0.6‰ (hereafter W min ; Fig. 2a) to δ 18 O = 6.8‰ and Δ 17 O = -0.5‰ (hereafter W max ; Fig. 2a). Because these fluids experienced oxygen isotopic exchanges with the anhydrous CM chondrite protolith (δ 18 O = -3.8‰, δ 17 O = -6.5‰; Fig. S2), they define two trends that delimit the possible O isotopic compositions of CM alteration fluids (blue shaded area in Fig. 2a). Considering these extreme trends, the precipitation temperatures of each T1 calcite grain can be calculated according to the isotopic fractionation factor  (Watkins et al. 2013), which corresponds to the distance between the minimum and maximum trends and the O isotopic compositions of carbonates in the three oxygen isotope diagram. This estimation leads to respective minimum (T min ) and maximum (T max ) precipitation temperatures of -9 ± 11 °C and 5 ± 14 °C for Maribo, 19 ± 22 °C and 50 ± 34 °C for Murchison, 15 ± 21 °C and 33 ± 29 °C for Jbilet Winselwan and 12 ± 17 °C and 34 ± 22 °C for Mukundpura (1σ; Table 1 and Fig. 3a). On average, this gives minimum and maximum precipitation temperatures for all T1 calcites of 7 ± 22 °C and 29 ± 32 °C, respectively (1σ). S1) imply that T1 calcites precipitated along a temperature gradient from alteration fluids characterized by a fixed Δ 17 O value. On the other hand, T2 calcites (this study) and serpentine (Clayton & Mayeda 1984) formed from a more 16 O-rich fluid that resulted from protracted isotopic exchange with the 16 O-rich anhydrous protolith and thus evolved toward negative Δ 17 O values (Fig. 2b). Following the same methodology as for T1 calcites, we calculated the formation temperatures of T2 calcites to be significantly higher, with respective minimum and maximum temperatures being 109 ± 11 °C and 158 ± 22 °C for Jbilet Winselwan and 110 ± 11 °C and 139 ± 55 °C for Mukundpura (1σ; Table 1 and Fig. 3a); respective average values are 109 ± 29 °C and 141°C ± 52°C (1σ). The same calculation for bulk serpentine compositions (n = 5; Clayton & Mayeda 1999) gives minimum and maximum formation temperatures of 28 ± 8 to 56 ± 13 °C and 73 ± 7 to 98 ± 11 °C, respectively, depending of the fractionation factor considered (Früh-Green et al. 1996;Zheng 1993;1σ; Table 1 and Fig. 3a).

Our petrographic and isotopic approaches reveal that T1 calcites precipitated at lower temperatures than serpentines and T2 calcites (Fig. 3a). These results support petrographic observations suggesting that T2 calcites correspond to a later stage of alteration, as highlighted by (i) the absence of serpentine rims and (ii) the existence of a fine-grained rim (FGR) suggesting that they replaced chondrule silicates (Fig. 3b; Lee et al. 2014;Lindgren et al. 2017). This implies that fluid circulation in CM parent bodies, and thus the formation of CM secondary phases, occurred during a prograde thermal evolution (Figs. 2b and 3b;Vacher et al. 2019), with T1 calcites forming first at T = -10 to +50 °C, followed by the precipitation of (Fe,S)-rich serpentine (mostly cronstedtite; Pignatelli et al. 2016;2017) and tochilinite at T = 30-100 °C and finally T2 calcites at T = 110-160 °C (Fig. 3a, Table 1).

According to X-ray diffraction studies (Howard et al. 2011;2009), serpentine represent the most abundant mineral in CM chondrites (75 vol% in average). Their formations (posterior to that of T1 calcites) would thus affect the isotopic evolution of the alteration fluids, leading to a slight shift toward 16 O-enriched values. As this isotopic shift is not taken into account in our model, this implies that the precipitation temperatures estimated for T2 calcites (Fig. 3a, Table 1) should be considered as maximum values. However, as the fractionation factor  is significantly lower for serpentine-water than for calcite-water (i.e., 1000ln⍺serpentine-water = 6.3 vs 1000ln⍺calcite-water = 17.1 at 100°C; O'Neil et al. 1969;Fruh-Green et al., 1996), this approximation does not affect our main conclusions that T1 calcites precipitated at lower temperature than T2 calcites.

As the uncertainties on the O isotopic compositions of primordial water accreted by CM chondrites could also affect our conclusions, we tested our results with different initial O isotopic compositions (Fig. 4). According to the currently favoured self-shielding model, primordial water is hypothesized to have had a large 17,18 O enrichment (i.e., δ 17 O = δ 18 O ≈ 180‰; Sakamoto et al. 2007) plotting on a line of slope 1 in a three oxygen isotope diagram. However, mass balance calculations performed on the O isotopic compositions of CM chondrites at bulk and mineral scales suggest more modest enrichments in the heavy oxygen isotopes with δ 17 O = 35 ± 9‰ and δ 18 O = 55 ± 13‰ (Fujiya 2018). Hence, we tested our results by using primordial CM water O isotopic compositions corresponding to (i) a composition intermediate between W min and W max (W int ) and (ii) the values proposed by Fujiya (2018) (W F , Fig. 4a). Isotopic exchange between these fluid compositions and the anhydrous CM protolith (Marrocchi et al. 2018) thus defines two other lines on the δ 17,18 O diagram (b and d in Fig. 4a, with lines a and c corresponding to the trends defined by W min and W max , respectively) from which the precipitation temperatures of T1 and T2 calcites can be calculated according to the fractionation factor  (Watkins et al. 2013). The results for experienced more heating than B-type asteroid Bennu (Hamilton et al. 2019;Kitazato et al., 2019). Although late shock heating could have induced such features, their different thermal history could also be the result of distinct accretion ages. The returned samples from Osiris-REx and Hayabusa2 in the near future will likely contain hydrated minerals (Kitazato et al. 2019;Lauretta et al. 2019) whose in-situ isotopic analyses would provide information on the thermal alteration processes and formation histories of asteroids Ryugu and Bennu.

Conclusions

In this letter, we report the results of in-situ oxygen isotope analyses performed on alteration phases (calcium carbonates) from a suite of different hydrated meteorites (CM chondrites) to quantitively estimate the thermal evolution of hydrated asteroids. Based on our isotopic results, we propose a new isotopic model that reconciles formation temperatures and petrographic observations of secondary minerals whose isotopic compositions recorded a gradual increase of the temperature (up to 250°C) during a prograde evolution of the temperature, regardless the oxygen isotopic composition of the initial water. These results are fundamental because they imply that hydrated asteroids accreted relatively late in the protoplanetary disk, as their earlier accretion would have led to higher alteration temperatures due to higher concentrations of radioactive 26 Al. Although more precise radioactive dating and numerical modelling are required, our study provides a key method to quantitatively estimate the respective thermal histories of the asteroids Bennu (Lauretta et al. 2019) and Ryugu (Sugita et al. 2019;Watanabe et al. 2019) upon the return of samples to Earth.

The Muséum National d'Histoire Naturelle (Paris) and The Natural History Museum of Denmark (Copenhagen) are also thanked for providing samples of Mukundpura and sections of Maribo chondrites. We thank Pr. Frederic Rasio for his editorial handling and the anonymous reviewer for his comments that contributed to improve the quality of the manuscript. This research was funded by l'Agence Nationale de la Recherche through grant ANR-587 14-CE33-0002-01 SAPINS (PI Yves Marrocchi). This is CRPG contribution #2714. Fig. S3: Time (Myr) required for the complete reequilibration of early precipitated T1 calcite as a function of grain size (radius) and temperature, calculated using oxygen self-diffusion parameters for calcite (Farver 1994). The grey shaded region at 10 Myr corresponds to the duration estimated for fluid circulations in asteroidal parent bodies (Fujiya et al. 2012;Doyle et al. 2015;Petitat et al. 2011).

Fig

  Fig. 1b) have only been observed in two sections (Jbilet Winselwan and Mukundpura). The O isotopic compositions of T1 calcite grains measured herein vary widely, with δ 18 O values ranging from 23.1 to 44.3‰, δ 17 O from 11.1 to 23.4‰ and Δ 17 O from -2.8 to +1.8‰ (Figs 1c & S1, TableS1), whereas T2 calcite grains have homogeneous compositions with δ 18 O values ranging from 12.6 to 18.4‰, δ 17 O from 4.2 to 8.1‰ and Δ 17 O from -4 to -0.8‰(Figs 1c &

Fig. 1 :

 1 Fig. 1: Backscattered electron images of (a) a T1 calcite grain (Cal) surrounded by a Fe-S-rich Serpentine/Tochilinite rim (Srp/Tch) in the matrix of Murchison and (b) a T2 calcite grain that is free of serpentine/tochilinite rim, but is instead surrounded by a rim of Fine-Grained Matrix (FGR; white dashed line) in the matrix of Mukundpura. White circles represent the locations of SIMS analytical spots. (c) δ 17 Oδ 18 O plot for T1 (circles) and T2 (diamonds) calcites from the CM chondrites Maribo (black), Murchison (green), Jbilet Winselwan (orange) and Mukundpura (red) (2σ errors). Schematic δ 17 O-δ 18 O diagrams represent the linear correlations (black solid lines) obtained considering (d) T1 and T2 calcites, defining a mass-independent trend with a slope of 0.61 ± 0.03 and (e) only T1 calcites, defining a mass-dependent trend with a slope of 0.53 ± 0.06. TFL = Terrestrial Fractionation Line (δ 17 O = 0.52  δ 18 O); PCM = Primary Chondrule Minerals line (δ 17 O = 0.987  δ 18 O -2.7).

  O-rich fluid (Fig. 1d; Verdier-Paoletti et al. 2017; Marrocchi et al. 2018). However, the O isotopic compositions of T1 calcites alone define a distinct trend with δ 17 O = (0.53 ± 0.06)  δ 18 O -(1.2 ± 2.2) that is indistinguishable, within errors, from the TFL (i.e., Δ 17 O = -0.4 ± 1.0; Fig. 1e). Conversely, T2 calcites exhibit clustered δ 17 O -δ 18 O values (Figs 1c & S1, Table

  observations of carbonates are essential for understanding the O isotopic evolution of asteroidal fluids and the constraints they bear. Based on mass-balance calculations, tentative attempts at estimating the O isotopic composition of primordial water accreted by CM chondrites have led to widely ranging and contrasted results due to assumptions on the O isotopic composition of the anhydrous protolith and asteroidal thermal evolution (δ 18 O fluid = 16-55‰, δ 17 O fluid = 9-35‰ and Δ 17 O fluid = 0.9-6.6‰; Verdier-Paoletti et al. 2017; Clayton & Mayeda 1999; Fujiya 2018).

Fig. 3 :

 3 Fig. 3: (a) Minimum (T min ) and maximum (T max ) precipitation temperatures of T1 calcites (circles), serpentines (brown triangles) and T2 calcites (diamonds) calculated as a function of their formation sequence/time (errors are 1σ) using previously reported fractionation factors for calcite (O'Neil et al., 1969) and serpentine (Früh-Green et al. 1996; Zheng 1993). Colours are as in Fig. 1. (b) Schematic representation of the formation sequence of (1) T1 calcite, (2) serpentine around T1 calcite and (3) T2 calcite, as deduced from petrographic observations and the mean ∆ 17 O values of these three phases (see Fig. 2c).

  The large range of δ 17 O-δ 18 O values and constant Δ 17 O values observed in T1 calcites (Fig.1c-e, Table

Fig. 4 :

 4 Fig. 4: (a) δ 17 O-δ 18 O plot showing the four trends (see text) used to calculate the influence of the initial oxygen isotopic composition of primordial water on the thermal path interpreted for water-rich asteroids. (b, c) δ 17 O-δ 18 O plots showing the results of the model for Mukundpura and Jbilet Winselwan, respectively.In both cases, the average formation temperatures of T2 calcites are systematically higher than those estimated for T1 calcites (red dashed lines), even when accounting for variability in the δ 17 O and δ 18 O values of calcites in Jbilet Winselwan and Mukundpura (grey triangles).

Fig. S2 :

 S2 Fig. S2: δ 17 O-δ 18 O plot showing the O isotopic composition of the anhydrous CM protolith (i.e., the theoretical anhydrous bulk CM composition: δ 18 O = -3.8‰ and δ 17 O = -6.5‰; grey star) determined from the linear correlation of the CM bulk composition (green circle(Clayton & Mayeda 1999; Hewins et al. 2014)). The anhydrous CM protolith corresponds to the intercept between the bulk CM trend (δ 17 O = 0.69 × δ 18 O -3.8; n = 36) and the PCM line (δ 17 O = 0.987 × δ 18 O -2.7; (Ushikubo et al. 2012)). CCAM: Carbonaceous chondrite anhydrous minerals line.

Table 1 :

 1 Mean O isotopic compositions of secondary phases and their calculated minimum (T min ) and maximum (T max ) formation temperatures(O'Neil et al., 1969). Uncertainties are 1σ.

	Meteorite	Petrologic	Secondary	Sample	Mean δ 18 O	Mean ∆ 17 O	Mean Tmin	Mean Tmax
		subtype	phase	number	(‰)	(‰)	(°C)	(°C)
	Maribo	CM2.6/2.7 a T1 calcite	25	38.3 (± 3.1) -1.2 (± 0.5)	-9 (± 11)	5 (± 14)
	Murchison	CM2.5 b	T1 calcite	24	33.7 (± 4.5) 0.4 (± 0.8)	19 (± 22)	50 (± 34)
	Jbilet Winselwan	CM2.4/2.7 c T1 calcite T2 calcite	10 2	31.4 (± 4.1) -1.6 (± 1.2) 17.5 (± 0.6) -1 (± 0.3)	15 (± 21) 109 (± 11)	33 (± 29) 158 (± 22)
	Mukundpura	CM2.0 d	T1 calcite T2 calcite	8 13	33.5 (± 3.8) -0.7 (± 0.6) 15.4 (± 2) -2.6 (± 1)	12 (± 17) 110 (± 32)	34 (± 22) 139 (± 55)
	Bulk CM	-	Serpentine e	5	11.7 (± 0.6) -2.2 (± 0.3)	28 (± 8) f 73 (± 7) g	56 (± 13) f 98 (± 11)

g References: a (van Kooten et al. 2018); b (Rubin et al. 2007); c (King et al. 2018); d (Rudraswami et al. 2018); e (Clayton & Mayeda 1999); f (Früh-Green et al. 1996); g

(Zheng 1993)

Table S1 :

 S1 Oxygen isotopic compositions of T1 calcite grains in the CM chondrites Maribo, Murchison, Shaded rows denote multiple analyses of the same grain.

	424	CC-3	32.2	0.7	17.1	1.5	0.4	1.6
	425	CC-2 CC-7	32.3 32.5	Jbilet Winselwan and Mukundpura. 0.6 14.3 0.7 0.5 14.1 0.7	-2.4 -2.8	0.8 0.7
		# CC-9 CC7 CC-8 CC21-1 CC-1 CC21-2 CC-10 CC22 Mean CC20 StDev CC27 CC15 CC30 CC13 CC10 CC14 CC11 CC1 CC4-2 CC12 CC4-1 CC8-1 CC5 CC11 CC17 CC7-1 CC9 Mean CC33 StDev	δ 18 O 33.4 32.7 33.7 33.3 34.1 33.9 36.1 33.7 31.4 34.1 4.1 34.9 27.4 35.8 30.8 37.3 30.9 37.7 31.8 38 35.9 41.6 36.2 38.3 36.5 38.3 38.2 39.3 33.5 39.5 3.8	2σ 0.5 0.3 0.5 0.3 0.5 0.3 0.5 0.3 0.4 0.2 0.7 0.3 0.6 0.6 0.6 0.2 0.5 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.6 0.3 0.3	δ 17 O 15.4 15.5 15.1 16.5 16.4 16.3 19.1 17.1 14.8 16.9 2.5 16.5 13.3 17.8 15.4 18.1 15.1 18.4 17.1 18.6 18.5 19.9 17.8 18.9 18.2 17.8 18.5 20 16.7 19.1 1.9	2σ 0.7 0.5 0.8 0.8 0.7 0.7 0.8 0.5 0.6 0.5 0.4 0.5 0.4 0.6 0.4 0.5 0.4 0.5 0.4 0.5 0.4 0.6 0.4 0.6 0.4 0.5 0.5	∆ 17 O -1.9 -1.5 -2.4 -0.8 -1.3 -1.3 0.3 -0.4 -1.6 -0.9 1.2 -1.7 -0.9 -0.8 -0.6 -1.3 -1 -1.3 0.6 -1.1 -0.2 -1.7 -1 -1.1 -0.8 -2.2 -1.4 -0.4 -0.7 -1.4 0.6	0.7 2σ 0.8 0.8 0.8 1 0.8 0.9 0.8 0.9 0.6 0.7 0.6 0.7 0.6 1 0.5 0.7 0.6 0.8 0.6 0.7 0.6 0.8 0.6 0.8 0.8 0.5
		CC32	39.5	0.3	19.6	0.7	-0.9	0.9
		CC13	40	0.5	20.5	0.6	-0.3	1
		CC3	40.2	0.3	20	0.6	-0.9	0.7
		CC28	40.3	0.2	19.5	0.6	-1.4	0.8
		CC29	40.5	0.3	19.6	0.6	-1.5	0.6
		CC12	40.6	0.3	19.7	0.5	-1.4	0.8
		CC15	40.9	0.4	20.4	0.5	-0.9	0.6
		CC2	41.3	0.3	19.7	0.5	-1.7	0.8
		CC18	42	0.3	20.5	0.5	-1.4	0.8
		CC1	44.3	0.3	21.5	0.6	-1.5	0.8
		Mean	38.3		18.7		-1.2
		StDev	3.1		1.6		0.5
		CC-7-2	25.3	0.2	14.5	0.4	1.3	0.3
		CC-7-3	25.6	0.2	14.6	0.4	1.3	0.4
		A-CC-5-2	29.1	0.5	15.8	0.4	0.7	0.6
		A-CC-5-3	36.6	0.2	20	0.3	1	0.3
		A-CC-8-2	29.2	0.6	15.9	0.6	0.7	0.7
		CC-1-2	29.2	0.3	16.2	0.4	1	0.4
		A-CC-11-1	30.1	0.5	14.5	0.6	-1.1	0.7
		CC-6	30.5	0.3	16.7	0.3	0.8	0.4
		A-CC-3-2	31.3	0.9	16.5	0.8	0.3	1.1
		A-CC-3-1	34.4	0.7	18.6	0.7	0.7	0.9
		A-CC-10	31.5	0.9	15.5	0.6	-0.9	1.0
		A-CC-13	32.6	0.3	17.6	0.3	0.7	0.4
		A-CC-7	33	0.5	17.1	0.6	0	0.7
		CC-18	33	0.2	18	0.3	0.9	0.3
		A-CC-2-2	34.4	0.6	17.6	0.6	-0.3	0.8
		CC-3	34.6	0.3	17.8	0.4	-0.2	0.4
		CC-2	36.8	0.2	19	0.3	-0.1	0.3
		CC-5	37.3	0.6	19.4	0.6	0	0.8
		CC-10	37.5	0.2	20.7	0.3	1.2	0.3
		CC-12	38.2	0.3	21	0.3	1.1	0.3
		A-CC-1	38.6	0.6	20.9	0.6	0.8	0.8
		CC-11	38.8	0.3	20.2	0.3	0	0.4
		CC-9	40.3	0.3	20.1	0.3	-0.8	0.4
		CC-8	41.7	0.4	23.4	0.3	1.8	0.4
		Mean	33.7		18		0.4
		StDev	4.5		2.4		0.8
		CC-6	23.1	0.7	11.1	0.8	-0.9	1.0
		CC-5	25.2	0.4	11.3	0.7	-1.9	0.7
		CC-11	31.5	0.5	13.5	0.8	-2.8	0.9

Table S2 :

 S2 Oxygen isotopic compositions of T2 calcite grains in the CM chondrites Jbilet Winselwan and Mukundpura. Shaded rows denote multiple analyses of the same grain.

	#	δ 18 O	2σ	δ 17 O	2σ	∆ 17 O	2σ
	CC15	17.1	0.4	8.1	0.9	-0.8	0.9
	CC14	17.9	0.5	8	0.8	-1.3	0.0
	Mean	17.5		8.1		-1
	StDev	0.6		0		0.3
	CC3-1	12.4	0.6	5.6	0.3	-0.9	0.5
	CC3-4	12.5	0.6	4.3	0.4	-2.2	0.6
	CC3-3	13	0.6	4.6	0.4	-2.2	0.6
	CC3-2	15	0.5	6.5	0.4	-1.2	0.6
	CC17-1	14.7	0.6	4.9	0.4	-2.7	0.6
	CC17-2	17.7	0.6	6.1	0.4	-3.1	0.6
	CC5-1	14.8	0.7	5.6	0.4	-2	0.6
	CC5-5	15	0.8	4.2	0.5	-3.6	0.7
	CC5-3	15.8	0.7	4.6	0.4	-3.6	0.7
	CC5-2	16.6	0.5	4.7	0.4	-3.9	0.5
	CC5-4	16.7	0.6	4.7	0.5	-4	0.6
	CC6-1	17.2	0.6	6.5	0.4	-2.5	0.6
	CC9	18.6	0.8	7.3	0.4	-2.4	0.7
	Mean	15.4		5.4		-2.6
	StDev	2.0		1.0		1.0

Jbilet Winselwan and Mukundpura (the only meteorites containing T2 calcites in this study) systematically show that T2 calcites precipitated at higher temperatures than T1 calcites (Fig.4b-c), regardless of the oxygen isotopic composition used for primordial water. Depending on the isotopic trend considered (a, b, c or d in Fig.4a), the average precipitation temperatures of T1 calcites range from 10 to 100 °C, whereas T2 calcites formed between 110 and 245 °C (Fig.4b-c). We note that the absolute temperatures at which secondary phases formed is directly affected by the oxygen isotopic composition of primordial chondritic water, whose precise determination is thus fundamental to better quantifying the thermal evolution of hydrated asteroids.

with the isotopic measurements. Laurette Piani is thanked for helpful scientific discussions.

 S1 andS2and Clayton & Mayeda 1999) indicates the following formation sequence: (1) T1 calcites, (2) serpentines and (3) T2 calcites.

Another possible source of uncertainty in estimating precipitation temperatures arises if T1 calcites experienced post-precipitation isotopic reequilibration, especially if CM chondrites experienced peak temperatures as high as 250 °C. However, this appears unlikely as T1 calcites systematically show mass-dependent oxygen isotopic variations (Supplementary Fig. S3), whereas isotopic exchange between initial water and anhydrous silicates during reequilibration would have induced Δ 17 O variations (Fig. 2a). In addition, according to the values of oxygen self-diffusion in calcite (Anderson 2007;Farver 1994), 1-10 4 Gyr are required to isotopically reequilibrate calcite grains of 5µm in size at temperatures ≤200 °C (Fig. S3). Such results thus strengthen our conclusion that hydrated asteroids experienced a prograde thermal evolution with T1 calcites precipitating first, followed by serpentine and then T2 calcites.

By taking into account the petrographic type of carbonates, our hydrothermal temperature estimates for CM chondrites are higher than previously proposed (Clayton & Mayeda 1984;Benedix et al. 2003;Guo & Eiler 2007) but remain low (<250 °C) compared to the peak of thermal metamorphism experienced by other groups of carbonaceous chondrites, such as CO or CV chondrites (up to 500-600 °C; Bonal et al. 2007;Busemann et al. 2007;Cody et al. 2008;Ganino & Libourel 2017). This implies the relatively late accretion of water-rich asteroids in the protoplanetary disk, as water-poor asteroids that accreted earlier experienced significantly higher temperatures due to the radioactive decay of 26 Al. Based on ε 54 Cr anomalies, it has been proposed than CM chondrites accreted ∼3.7-5.0 Myr after the formation of CV calcium-aluminium-rich inclusions (CAIs; Fujiya et al. 2012;Doyle et al. 2015;Sugiura & Fujiya 2014), after a five-fold decrease in the abundance of 26 Al, whereas CO and CV chondrites accreted ∼2.1-2.4 and ∼2.4-2.6 Myr after CV CAIs, respectively (Doyle et al. 2015). Interestingly, recent spectral data provided by the Osiris-REx and Hayabusa2 asteroid sample return missions suggest that C-type asteroid Ryugu has


Yifei Ma

Yongrui Bao

Wenjie Zhang

Xixiang Ying

Didier Stien

Four lignans from Portulaca oleracea L. and its antioxidant activities

Keywords: Portulaca oleracea L., lignans, antioxidant activity

A new lignan, named oleralignan (1) and three known compounds (+)-syringaresinol (2), (+)-lirioresinol A (3) and monomethyl 3,30,4,40-tetrahydroxy-δ-truxinate (4) were isolated from the P. oleracea for the first time. The compound (1) were identified by 1D, 2D NMR spectroscopic methods and high resolution electrospray ionization time-of-flight mass spectrometry. In addition, it was found that the four lignans presented the scavenging activities in 1,1-diphenyl-2-picryl-hydrazyl (DPPH) radical quenching assay more than that of hydroxyl anisole (BHA).

Introduction

Portulaca oleracea L. belonging to the family of Portulacaceae, was widely used as a potherb in the Mediterranean, Central European and Asian countries, which was listed in the World Health Organization as one of the most used medicinal plants and has been given the term 'Global Panacea' [START_REF] Wu | New CuCl 2 -induced glucoside esters and other constituents from Portucala oleracea[END_REF]. As a traditional Chinese medicine, it is well-known for the function of removing heat, counteracting toxicity, cooling blood, hemostasia and antidysentery (The Pharmacopoeia Commission of PRC 2015). At the same time, it also exhibits a wide range of pharmacological effects, such as antiinflammatory [START_REF] Meng | The anti-inflammation and pharmacokinetics of a novel alkaloid from Portulaca oleracea L[END_REF], antibacterial [START_REF] Chan | Combating against methicillin-resistant Staphylococcus aureus -two fatty acids from Purslane (Portulaca oleracea L.) exhibit synergistic effects with erythromycin[END_REF], antioxidant [START_REF] Dkhil | Antioxidant effect of purslane (Portulaca oleracea) and its mechanism of action[END_REF], wound-healing activities [START_REF] Rashed | Simple evaluation of the wound healing activity of a crude extract of Portulaca oleracea L. (growing in Jordan) in Mus musculus JVI-1[END_REF], antitumor [START_REF] Shen | Purification and characterization of an antitumor polysaccharide from Portulaca oleracea L[END_REF]) attributing to its components, i.e., alkaloids (Li et al. 2017;[START_REF] Xu | A novel alkaloid from Portulaca oleracea L[END_REF]Zhao et al. 2018;[START_REF] Jiang | An isoindole alkaloid from Portulaca oleracea L[END_REF][START_REF] Xiu | A new alkaloid from Portulaca oleracea L. and its antiacetylcholinesterase activity[END_REF], flavonoids [START_REF] Yang | New flavonoids from Portulaca oleracea L. and their activities[END_REF], phenolic acids [START_REF] Erkan | Antioxidant activity and phenolic compounds of fractions from Portulaca oleracea L[END_REF], terpenoids [START_REF] Acedo | Chemical composition of purslane (Portulaca oleracea L.) and anti-inflammatory activity of its lipid components[END_REF], coumarins [START_REF] Awad | Lipid content and antimicrobial activity of phenolic constituents of cultivated Portulaca oleracea L[END_REF]), a-linolenic acid, b-carotenend [START_REF] Liu | Fatty acids and beta-carotene in australian purslane (Portulaca oleracea) varieties[END_REF] and trace elements [START_REF] Petropoulos | Chemical composition and yield of six genotypes of common purslane (Portulaca oleracea L.): an alternative source of omega-3 fatty acids[END_REF] etc. Recently, the lignans was found from the Portulaca oleracea L. for the first time in our laboratory including a new lignan, together with 3 known lignans isolated from the Portulaca oleracea L. for the first time (Figure 1). Lignans and norlignans constitute abundant classes of phenylpropanoids, and these compounds have received widespread interest, mainly because they have various clinically important biological activities [START_REF] Suzuki | Biosynthesis of lignans and norlignans[END_REF], such as antitumour, antioxidant, antimitotic, antiviral activity and etc. [START_REF] Raghavendra | An efficient synthesis of new lignan scaffolds as antimicrobial inhibition agents[END_REF]. Therefore, its antioxidant activities against DPPH free radical-scavenging was investigated, and butylated hydroxyanisole (BHA) is being as the antioxidant that is similar to lignans with the phenolic hydroxyl groups as a free radical absorbents [START_REF] Rahmatzadeh | Antioxidative effect of BHA in soya bean lecithin-based extender containing glycerol or dmso on freezing capacity of goat semen[END_REF]. δ 7.15 (H-6), 7.28 (H-7) and7.58 (H-8) anddefines a 1,2,3 By comparison of the 1 H and 13 C NMR spectral data with literature, compounds 2-4 were identified as the (+)-syringaresinol (2) [START_REF] Monthong | þ)-syringaresinol lignan from new species Magnolia thailandica[END_REF]), (+)-lirioresinol A (3) [START_REF] Liu | Chemical constituents from qianliang tea[END_REF]), monomethyl 3,3',4,4'-tetrahydroxy-δ-truxinate (4) [START_REF] Liang | Acylated flavonol glycosides and δ-truxinate derivative from the aerial parts of Lysimachia clethroides[END_REF], respectively.

Antioxidant activity

The DPPH radical scavenging method [START_REF] Xiong | Antioxidative effects of phenylethanoids from cistanche deserticola[END_REF] was used to determine the antioxidant activities of the compounds, which was improved to evaluate the four lignans isolated from P. oleracea. The results indicated that the four lignans presented higher antioxidant activity than that of BHA (Table S2 in supplementary material) because of its more phenolic hydroxyl group around the lignans framework playing an important role in scavenging DPPH radical [START_REF] Miyake | Antioxidative catechol lignans converted from sesamin and sesaminol triglucoside by culturing with aspergillus[END_REF]. Figure S17 showed that the antioxidant activities of the four lignans were dose-dependent.

Experimental

General experimental procedures

UV spectrum was obtained from a HITACHI U-3010 spectrophotometer (Hitachi Ltd, Tokyo, Japan), the IR spectrum and data obtained from IR200 Reagent Plant (Tianjin, China, purity ≥99.9%). All other reagents were of analytical grade purchased from Jinfeng Chemical Factory (Tianjin, China), and the water was WAHAHA purified water (Shenyang, China).

Isolation and identification

A sample (150 kg) of the air-dried aerial parts P. oleracea was treated twice with 10 volumes of water for 2 hours each time to afford a crude extract ( 21kg). The extract was concentrated and subjected to a 100-200 mesh silica-gel column chromatography (61 x 55 cm, approximately, 150 kg) with a 2 volumes of ethyl acetate 3 times as the isocratic elution, affording the fraction evaporated (180 g). Then, the fraction eluted with water, 30%, 50%, 70% and pure ethanol was chromatographed respectively on a 80-100 mesh polyamide resin column (120 x 8 cm, approximately, 2.5 kg) to obtain five fractions (4 L each). The fraction of 50% extracting was condensed under the reflux to obtain the extract of 58 g, which was subjected to a 200-300 mesh silica-gel column (120 x 8 cm, approximately, 2.5 kg) for further purification with ethyl acetate, ethyl acetate and methanol (5:1, 2:1, 1:2, 1:4, v/v) as the gradient eluant, obtaining five fractions (Frs. 1-5, 500 mL each). The fractions (38 g), which turns cyan when exposed to ferric chloride reagent, were applied to an ODS column chromatography (20-40 lm, 150 g, Ultimate XB-C18, u3 x 70 cm) and eluted with methanol (40%, 50%, 60%, 70%, respectively). At medium pressure, five fractions (A1-A5, 100 mL each) were obtained. A3 (0.46 g) was changed to cyan when exposed to ferric chloride reagent, and then purified on a Sephadex LH-20 column (100 g, u2 x 150 cm) using methanol solvent to give fractions B1-B6, and then B3 was prepared by ultra-high performance liquid chromatography (UHPLC), using Acetonitrile-0.1% formic acid as the mobile phase, with flow rate of 1.0 mL/min, and obtained compound 1 (4 mg, purity of >99% with UHPLC, tR 9.606 min, Acetonitrile-0.1% formic acid, 40:60, v/v), compound 2 (5 mg, purity of >98% with UHPLC, tR 5.94 min, Acetonitrile-water, 30:70, v/v), compound 3 (7 mg, purity of >98% with UHPLC, tR 6.07 min, Acetonitrile-0.1% formic acid, 32:68, v/v), compound 4 (4 mg, purity of >97% with UHPLC, tR 28.23 min, MeOH-0.1% formic acid, 10:90, v/v).

Oleralignan (1): yellow-green powder; UV (CH 3 OH) kmax 291, 236 nm; IR (KBr)

vmax 3393,2925,2850,1600,1515,1478,1256,1203,1159,1125,1021,859,790, 762 cm -1 . 1 H and 13 C NMR data (Table S1 in

Conclusion

In this study, the lignans was found in the Portulaca oleracea L. for the first time including a new lignans named oleralignan (1), together with three known compounds 2, 3 and 4 isolated from the plant for the first time. The antioxidant activity assay indicated that the compound 1, 2, 3 and 4 showed more stronger activities of scavenging DPPH radical than that of BHA, with IC 50 value of 20.64, 17.34, 16.46 and 8.00 µM, respectively.

Figure 1 .

 1 Figure 1. Structure of compounds 1-4.

  -trisubstituted phenyl ring. H-6 and H-8 both correlate with a carbon at d 126.1 which must be attributed to C-4a. Quaternary carbons C-8a (δ 129.8) and C-5 (δ 138.2) can be assigned based on their HMBC correlation with H-7. Correlation H-8/C-1, correlations of H-1 with C-2, C-3, C-4a and C-8, and correlations of H-4 with C-2, C-3, C-5, C-8a define another tetrasubstituted fused aromatic ring. A methoxy group can be placed in C-3 based on MeO/C-3 correlation and the weak COSY correlation of the methyl group with H-4. The molecular formula and the carbon chemical shift indicate that C-2 bears a hydroxyl group. Another 3-spin system was and COSY experiment. It includes protons at δ 7.03 (H-2'), 6.91 (H-5') and 6.90 (H-6') and defines a 1,3,4-tribstituted aromatic ring. A methoxy group was placed in C-3' based on correlation H-5'/C-3' and on the correlation of the methyl protons with C-3'. A hydroxyl substituent should therefore be in C-4'. Eventually, this additional phenyl ring was connected to C-5 based on the correlations H-6/C-1' and H-2'/C-54-hydroxy-3-methoxyphenyl)-3-methoxynaphthalen-2-ol and was given the trivial name oleralignan (1).

  supplementary material); HR-ESI-TOF-MS m/z 297. 1093 [M+H] + (calcd. for C 18 H 17 O 4 , 297.1121).3.4. Assay of scavenging DPPH radicalsScavenging of DPPH free radical activity was the basis method of the antioxidative activities of determining the compounds[START_REF] Sharma | DPPH antioxidant assay revisited[END_REF]. Each compound has been diluted with methanol to five different concentrations ranging from 2.5 to 40 µM, and BHA as the positive control was also diluted to the same concentrations. Thereafter, the different concentrations of sample and BHA (1 mL) was separately mixed with 80 µM DPPH (1 mL) which dissolved in methanol. The methanol was used to zero the spectrophotometer. The mixtures were allowed to stand for 20 min, and measured at 517 nm at room temperature. The DPPH solution (1 mL) added to methanol (1 mL) were used as the blank group. The sample solution (1 mL) and methanol (1 mL) were used as the control group. Each sample was examined five times in parallel. Radical scavenging activity was expressed as the clearance percentage and calculated using the following formula[START_REF] Mensor | Screening of Brazilian plant extracts for antioxidant activity by the use of dpph free radical method[END_REF]:DPPH radical scavenging effect (%) = (1 -(A sample -A control )/=A blank ) × 100%where, A control is the absorbance of the control group, A sample is the absorbance of the test compound, and A blank is the absorbance of the blank group. The antioxidant capacities were evaluated by the values of IC 50 , and values are expressed as the means ± SD for n = 5.

  spectrophotometer (Thermo Electron Corporation, Waltham, MA). The NMR spectra in DMSO-d 6 or MeOD was recorded by using an AVANCE 500 MHz

	instrument (Bruker Corporation, Switzerland). Relative molecular mass was
	recorded using a 6520 quadrupole-time-offlight mass spectrometer (Agilent,
	Palo Alto, CA). Purity was detected on a Nexera X2 UHPLC LC-30A system
	(Shimadzu, Kyoto, Japan), using a Kromasil C18 column (150 mm x 4.6 mm, 5
	mm, Dalian Johnsson Separation Science and Technology Corporation). In the
	separation process, column chromatography (CC) included silicagel (100-220
	and 200-300 mesh, Qingdao Marine Chemical Co., Qingdao, China), poly-
	amide resin (80-100 mesh, Taizhou Luqiao Sijia Biochemical Plastic Factory,
	Zhejiang, China) and ODS (20-40 lm, GE Healthcare, Marlborough, MA). The
	TLC was prepared with silica gel GF254 (Qingdao Marine Chemical Co.,
	Qingdao, China).
	3.2. Plant materials and chemicals
	The whole herbs of P. oleracea were collected in Shijiazhuang (Hebei, China) in
	June 2017, and identified by Prof. Xixiang Ying. The voucher specimen (No.
	20171001) was displayed at Liaonig University of Traditional Chinese
	Medicine. 1,1-Diphenyl-2-picrylhydrazyl radical (DPPH) (purity ≥99%) was

purchased from Sigma Co. (USA). BHA (purity ≥98%) was purchased from Shanghai Xiangrui Biological Technology Co., Ltd. (Shanghai, China). Methanol, acetonitrile and formic acid are all of HPLC grade provided by Damao Chemical

Funding

This work was supported by the National Natural Science Foundation of China (Grant 81573546).

Disclosure statement

The authors have no conflicts of interest to disclose.