Bilel Nefzi
email: bilel.nefzi@loria.fr

Dawood Khan
email: dawood.khan@inria.fr

Ye-Qiong Song
email: ye-qiong.song@loria.fr

TBoPS: a Tree based distributed Beacon only Period Scheduling mechanism for IEEE 802.15.4

IEEE 802.15.4 standard specifies a beacon-enabled mode which provides a synchronization environment using beacon transmissions. However, this mode is designed for single hop networks and its use in multi-hop networks is not straightforward. The main challenges of using beacon-enabled mode in multi-hop networks are how to efficiently schedule beacon transmissions to avoid direct and indirect beacon collisions and how to make a schedule tolerant to the clock drifts due to the low cost components. In this paper, we present TBoPS, a novel technique for scheduling beacons in the cluster tree topology. TBoPS uses a dedicated period called beacon only period (BOP) to schedule beacons at the beginning of IEEE 802.15.4 superframe. The advantages of TBoPS is that every beacon-enabled node selects a beacon schedule distributively during association. We analysed the robustness of TBoPS to clock drifts. We also show through simulations that all nodes in the network are synchronized and follow the same superframe structure.

I. INTRODUCTION

Cyber-physical systems (CPS) may monitor and interact with the physical world thanks to the recent advances on wireless sensor and actuator networks (WSAN). When interacting with the real world, real-time properties must be ensured. Providing common time awareness across these WSAN is critical in CPS since the intelligent entity is led to make decisions based on coherent temporal information. Let us take the example of a simple multi-hop WSAN-based CPS for intrusion detection and tracking1 . The detection of an intruder, possibly by multiple sensors, will be reported to the sink, which will trigger the execution of the intruder position estimation (e.g. based on kalman estimation), and then sending of a command to guide mobile robots (mobile sensors) to cover the estimated possible target (intruder) position, which is often an ellipse area whose size depends on the round trip time (RTT) between the intruder detection and the command reception by the mobile sensors. It is obvious that long RTT results in larger area to cover, so needs more mobile sensors to move and to cover the estimated region, and thus consumes more energy. Moreover, the coordinated moving of the mobile sensors needs a common sense of time among them. Otherwise it is impposible to correlate data and make good decision. IEEE 802. 15.4 [1] standard is widely used in Wireless Sensor Netwoks (WSNs) because it meets many of the requirements of these networks. The MAC layer provides two access modes; the beacon-enabled mode and the non beacon-enabled mode. The beacon-enabled mode provides a synchronization environment using beacon transmissions. Therefore, the MAC sublayer uses a superframe structure, defined as the time between two successive beacon transmissions. The superframe can be divided into an active period, during which nodes can transmit data, and a sleep period, during which nodes go to sleep in order to preserve energy. This mode works perfectly in the case of star networks. However, for large scale multi-hop networks, many beacon-enabled nodes (FFD) should transmit beacons in order to synchronize the network.

The main challenge is how to schedule beacon transmissions of these nodes to avoid collisions. In addition, only a subset of beacon enabled nodes should transmit beacon to synchronize the entire network. The existing approaches suppose that these nodes are already known and well distributed in the network. This hardens the deployment process. Our approach efficiently builds the cluster tree to optimize the synchronization process. An FFD becomes a beacon broadcaster only if it has children. If the association rules minimize the number of cluster-heads (i.e. FFDs which have children), the beacon scheduling process is eased.

In this paper, we present a Tree based distributed Beacon only Period Scheduling mechanism (TBoPS) for IEEE 802.15.4 multi-hop wireless sensor networks. TBoPS adopts a beacon only approach in which a period of time, called BOP, is reserved for beacon transmissions at the beginning of each superframe. This BOP is followed by the working period (WP) which is used for data transmission. TBoPS procedure is activated for a particular associated FFD if another node tries to associate to it. The advantage of the approach in dense networks is that it minimizes the number of beacon broadcasters while maintaining the superframe structure for all nodes in the network.

The rest of paper is organized as follows. In the next section, we briefly present the beacon-enabled mode IEEE 802.15.4. Then we describe the related work in section III. TBoPS details are given in section IV. Section V gives the clock-drift analysis which derives the maximum superframe duration. Section VI In beacon-enabled mode, beacon frames are periodically sent by a central device, referred to as PAN coordinator, to identify its PAN and synchronize nodes that are associated with it. The PAN coordinator defines a superframe structure characterized by a Beacon Interval (BI) specifying the time between two consecutive beacons, and a Superframe Duration (SD) corresponding to the active period, defined as:

BI = aBaseSuperf rameDuration * 2 BO (1) SD = aBaseSuperf rameDuration * 2 SO (2) ∀0 ≤ SO ≤ BO ≤ 14 (3)
BO and SO are called Beacon Order and Superframe Order, respectively. The Beacon Interval may optionally include an inactive period (for SO < BO), in which all nodes may enter into a sleep mode, thus saving energy.

By default, nodes compete for medium access using slotted CSMA/CA during the Contention Access Period (CAP). The IEEE 802.15.4 protocol also provides a Contention-Free Period (CFP) within the superframe, in which a node may request the PAN coordinator to allocate Guaranteed Time Slots (GTS). In this paper, we consider the physical layer operating in the 2.4 GHz frequency band and with a 250 kbps data rate.

It can be easily observed in (1) that low duty-cycles can be configured by setting small values of the superframe order (SO) as compared to beacon order (BO), resulting in greater sleep (inactive) periods. The advantage of this synchronization with periodic beacon frame transmissions from the coordinator is that all nodes wake up and enter sleep mode at the same time. However, with multiple coordinators sending beacon frames, each with its own beacon interval, sending beacon is a challenging problem due to frame collisions. Frame collisions may be direct, which occur when two or more neighbour coordinators transmit their beacons at the same time. They me be indirect, which occur when two or more two-hop neighbour coordinators transmit their beacons at the same time. In the latter case, common neighbour nodes between these coordinators are affected.

III. RELATED WORK

Two approaches are used for beacon scheduling in IEEE 802.15.4; the Time Division (TD) approach and the Beacon Only Period (BOP) approach.

In the TD approach, the beacons of a coordinator are always scheduled in the sleep period of its neighbor coordinators and two-hop coordinators (the ones which send beacons). This is approach is the one suggested by IEEE 802.15.4 standard because it does not modify the structure of the superframe. This solution has two major drawbacks. The first one is that it imposes the use of a very low duty cycle since the sleep period have to be long enough to include the active periods of all coordinators. The second drawback is that coordinators are not able to communicate directly with each other. A consequence is that broadcasting is not possible any more. Koubaa et al. [START_REF] Koubaa | A time division beacon scheduling mechanism for ieee 802.15.4/zigbee cluster-tree wireless sensor networks[END_REF] proposed a centralized beacon scheduling mechanism which uses the TD approach. MeshMAC [START_REF] Panneer Muthukumaran | Meshmac: Enabling mesh networking over ieee 802.15.4 through distributed beacon scheduling[END_REF] implements a distributed mechanism for the TD approach. In MeshMAC the node needs first to be associated as a end device. Then it finds a suitable schedule by collecting neighbours and neighbours' neighbours beacon schedule.

In the BOP approach, an portion of the time, called BOP, is reserved for beacon scheduling transmission at the beginning of the superframe. During this period, a coordinator picks a Contention-Free Time Slot and transmits its beacon. We selected the BOP approach for two reasons. First, it ensures that the active periods of all nodes in the network are synchronized thus easing message broadcasting and ensuring fast message delivery (if generated during the active period). Second, it allows coordinators to uses different SD values. This is an interesting feature for data gathering scenarios. The coordinators located at higher depths collect fewer packets. Thus the active portion of the superframe have to be smaller than those of coordinators located at lower depths (near the PAN coordinator). One of the drawbacks of this solution is that it alters the standard. Additionally, the use of the CFP, as defined by the standard, is not possible any more and the coordinators have to use a higher layer protocol to establish these kind of CFPs . Lu et al. [START_REF] Lu | A New Beacon Scheduling Mechanism for Mesh Wireless Personal Area Networks based on IEEE 802.15.4[END_REF] proposed a beacon scheduling mechanism which uses the BOP approach.

In all of these approaches, the coordinators are already known before network start. This increases the deployment complexity, especially in large networks, since every beacon broadcaster has to be deployed in a precise location to ensure global network synchronization.

IV. TBOPS IN DETAILS

A. General description

We suppose that the PAN coordinator node is already defined. Once the PAN coordinator node starts, it starts immediately using the superframe structure by periodically broadcasting the beacons. The PAN coordinator defines the superframe length (BI), the maximum active period length (SD) and the maximum number of slots d M AX in the BOP. These values are embedded in the beacon payload. The BOP is used to transmit and receive beacons and the active period is used for regular message transmission and reception. The BOP is divided into d M AX frame slots, numbered from 1 to d M AX . The PAN coordinator always uses the first slot to transmit beacons. Other coordinators choose a frame slot (from 1 to d M AX ) for beacon schedules whenever they become beacon broadcasters. End devices always join the network as a leaf node.

The next sections detail the association and TBoPS procedures

B. Association

Figure 2 shows the finite state machine of the association procedure. It is executed by all nodes in the network. The root node is considered associated directly. Any other node starts the procedure by listening to the medium. It keeps listening until it receives either a beacon or a Hello message. In the first case, it synchronizes itself and follows the superframe structure. Moreover, it can determine when the WP begins. If it receives a hello message, it means that the network is already in the WP. In both cases, the node knows now when it should transmit frames. In the next state, the node sends a HelloRequest message and then starts the collection of HelloResponse messages for a certain period. HelloResponse procedure is illustrated in Algorithm 2. The node determines the RSSI of the received HelloResponse message and records all the information in the neighbour table. At the expiration of the collection period, the node selects the best father and sends an association request to it. If the list is empty, the node goes to the listen to medium state. If the association is granted by the selected father, the node is now associated. If the father is already a beacon broadcaster, it can start tracking its father's beacon. If the node is the first child of the father, it must wait until the father finds a beacon schedule. During this time, the node cannot accept association requests. If the father fails to find a beacon schedule, the node must dissociate from it and selects another parent. If the association is not granted or no response is received, the node shall select the next best father in the list and repeat the procedure.

A node A selects a father according to the following rules.

• {N eigh} is the list of neighbour coordinators.

• A select a subset {EligibleN eigh} from {N eigh} which has RSSI > T hresh where T hresh is a constant defined to ensure a good quality link. • A select from {EligibleN eigh} the father which has the highest number of children. In case of multiple choices, it chooses the father with the minimum depth. The association rules minimize the number of cluster-heads in the network.

C. TBoPS procedure

Algorithm 1 describes the TBoPS procedure. It is executed by a coordinator which becomes a father. The PAN coordinator is considered associated directly and starts transmitting beacons upon start. The procedure starts by sending a HelloRequest to message and then starts the collection of HelloResponse messages for a certain period of time. Although the node may already have many of the information gathered previously, this step ensures that it has the most up to date information. The coordinator obtains neighbour coordinators, neighbours' neighbour coordinators and fathers' neighbour end devices beacon schedules. After the expiration of the collection period, the node executes SelectaBeaconSlot() procedure to select a slot (illustrated in Algorithm 3). If a beacon slot is found, the node advertises it as being temporarily reserved. Then it waits for a period of time. If during this time, a neighbour node informs it, through a ConflictAdvertisement,that this schedule is already taken, the coordinator repeat this step. If the advertisement time expires, the beacon slot is definitely assigned to this coordinator. When beacon slot selection fails, the coordinator must inform the new son so that it can deassociate and find a new father.

V. CLOCK-DRIFT ANALYSIS

Let t i ∈ R + be the release time of a beacon and t j such that t j > t i be the next release time of a beacon by a cluster head. Let ∆ be the beacon inter-arrival period for some cluster head, therefore, ∆ = t j -t i . In synchronized cluster-tree, beacons are the references for the system wide scheduling. Therefore, a stable beacon is at very base of system performance. The clock-drift in beacon generation may cause a synchronization errors if son nodes miss multiple beacons from their father. The clock drift of WSN nodes is independent from each other, the value of which is dependent of the quality of the clock oscillator used on the WSN board. Therefore, we need to provide a bound such that if multiple beacons are missed leaf nodes remain synchronized despite clock-drift for cluster heads (CH) in the scheduling path. Therefore, let Π be set of clocks representing beacon generation periods in a scheduling path such that Π = {π 1 , π 2 , . . . , π n } be a set of clocks representing a scheduling path having n nodes with π 1 being the clock of the root node. The periodic generation clock π i for cluster head C i is given as π i (t) = (1 ± δ i ) * t + φ i . Where t is the time, φ ∈ R + is the phase-offset, and δ ∈ R is the clock-drift.

If π(t) is the clock time at t and π(t + ∆t) is the clock time at some time t + ∆t then the drift can be given as:

δ(t) = π(t + ∆) -π(t) ∆ -1 (4)
The value of ±δ is a clock drift rate often expressed as a ratio in parts per million (ppm) or microsecond/second, this value can be found from a data sheet of a WSN board. Therefore, the change in the clock after a time interval ∆t can be given as:

Φ(∆t) = |∆t * δ| (5)
The clocks, therefore, can be represented as π i (t) = (1 ± δ i )t (i.e., with zero offset).

Example 1: The crystal oscillators commonly found in WSN nodes usually have a drift rating of the order of value between 10 4 to 10 6 that is, two similar but uncalibrated oscillators will drift apart between 1 and 100 microseconds every second.

For the correct beacon synchronization condition when a father cluster head (FCH) is sending its beacon, all its child nodes heads (CN) must be in receiving mode. However, at the moment when a FCH starts sending its beacon in its allocated slot the CN may not be in their corresponding receive slot due to phase errors from clock drift, as illustrated in figure 3. In figure 3, if node 0 sends beacon during its the entire allocated slot, then node 1 may not be able to receive this beacon successfully, due to un-aligned slots. Taking the clock of node 0 as a reference, the clock of node 1 may drift in either direction as shown by case 1 and case 2 (i.e., backward or forward). In such a situation, the nodes may have a different view of the current slot number within the superframe. In order to cope with this problem we use guard-bands (GB), such that we have a bound for drift with respect to GB in an clustertree. This will ensure that a beacon are not transmitted during the entire sending slot but only in the middle, as illustrated in figure 4. Therefore, the FCH will not transmit at the beginning or the end of its slot (for duration defined by GB), in order to accommodate for the clock drift of receiving CN. If SL is the slot length in seconds and BT T is the beacon transmission time in seconds and GB is the guard-band in seconds, then the maximum phase error due to the clock drift between FCH and CN at any time t should be bound as:

max{Φ} ≤ SL -BT T -GB (6)
The cluster head C i transmits beacons every T (C i ) and its CN should be able to receive this beacon in its appropriate beacon slot. During this beacon period of T (C i ) for C i the phase error of CN can be found using equation [START_REF] Panneer Muthukumaran | Meshmac: Enabling mesh networking over ieee 802.15.4 through distributed beacon scheduling[END_REF] as:

Φ(T (C i )) = T (C i ) * |δ| (7)
Where δ is the clock drift of the CN. Thus, from equations (6 & 7) we have the bound for the waiting time of C i such that its CN are synchronized as:

T (C i ) ≤ SL -BT T -GB |δ| (8)
This ensures that the slots remain synchronized between FCH and CN. Therefore, the superframe duration shall be less than this period.

VI. CLUSTER-TREE FORMATION PERFORMANCE

The simulation is setup in Matlab and consists of a scenario represented by a square area with each side equal to 100 meters. The WSN nodes are assumed to be uniformly distributed in this area. We assume unit disc model for the node communication ranges, where each node's communication range is assumed to be uniformly distributed within a range of 25 to 30 meters (to simulate the different battery power levels of the nodes). A more realistic model like the one presented by Zuniga et et al. [START_REF] Zuniga | Analyzing the transitional region in low power wireless links[END_REF] could be also used. We leave this as a future work. The simulations are carried out for various node densities in the square area. The simulations have the flexibility to fix the root node (by us) or choose one randomly and thereof begin the cluster tree formation based on the rules discussed in earlier sections. The objective of the simulation is to study the effectiveness of the proposed clustertree formation algorithm in terms of cluster-tree depth and the number of cluster-heads.

The simulation results are depicted in Figure 5. Figure 5 shows the results which have been averaged over six different simulations for each node density. We see that there is very little variation in the cluster-head count and the cluster-tree depth over different node densities. This result implies that the number of nodes in the clusters is increasing while the depth and cluster-head count remains some what constant, that is the density of nodes in the clusters is increasing. Which was our objective, to being with, when we formed the rules for tree formation in section IV-B.

VII. CONCLUSION

In this paper we proposed a new beacon scheduling approach based on collision free beacon frame of each coordinator (cluster head) in the network organized in the cluster tree topology. An algorithm is provided to construct an optimized cluster-tree. We used a dedicated period (beacon only period) to schedule beacon at the beginning of IEEE 802.15.4 superframe. We derived the maximum superframe length given clock drift and guard interval. We simulated effects of association rules in cluster head selections. Our algorithm keeps the number of cluster heads almost constant although the number of nodes increases in the network. As a future work, we will implement this mechanism and measure its performance in a real world scenario.

VIII. ACKNOWLEDGEMENT

Fig. 1 :

 1 Fig. 1: Superframe

Fig. 2 :

 2 Fig. 2: Finite State Machine of the association mechanism.

  message repeat Collect Hello responses and beacon slot advertisements until (Collection time expires) if No collected Hello responses then Go to Step 1 end Step 2 if SelectaBeaconSlot()!=-1 then Advertise selected slot as being temporarily reserved repeat if ConflictAdvertisement received then Go to Step 2 end Advertise selected slot as being definitively reserved until (Advertisement time expired) else Inform the new son that beacon selection failed end End of TBoPS procedure Algorithm 1: TBoPS procedure HelloResponse() / * This is in addition to information described in the association procedure of the standard * / if I am a coordinator then Send -my beacon schedule -Neighbour beacon schedules -SD and BI -My address, depth and Number of children -The RSSI of the received HelloRequest else -Send my father's schedule end end Algorithm 2: HelloResponse procedure

3 :Fig. 3 :

 33 Fig. 3: Cases of clock drift for the receiving nodes.

Fig. 4 :

 4 Fig. 4: Bound with drift and guard-bands.

Fig. 5 :

 5 Fig. 5: Simulation results of cluster tree formation

The intrusion detection and tracking is the main scenario that we are dealing with within the Quasimodo project: http://quasimodo.loria.fr/

This work was partially supported by Quasimodo project under No. ANR 2010 INTB 0206 01 in France and No. NSFC 61061130563 in China


Giada Iacono-Marziano

Fabrice Gaillard

Bruno Scaillet

Alexander G Polozov

Virginie Marecal

Michel Pirre

Nicholas T Arndt

Extremely reducing conditions reached during basaltic intrusion in organic matter-bearing sediments

Keywords: magma-organic matter interaction, graphite native iron saturation, gas emissions, carbon monoxide, Siberian Traps, Disko Island

Redox conditions in magma are widely interpreted as internally buffered and closely related to that of their mantle source regions. We use thermodynamic calculations to show that hightemperature interaction between magma and organic matter can lead to a dramatic reduction of the magma redox state, and significant departure from that of the original source. Field studies provide direct evidence of the process that we describe, with reported occurrences of graphite and native iron in igneous mafic rocks, implying very reducing conditions that are almost unknown in average terrestrial magmas. We calculate that the addition of 0.6 wt% 2 organic matter (in the form of CH or CH 2 ) to a standard basalt triggers graphite and native iron crystallisation at depths of few hundred meters.

Interaction with organic matter also profoundly affects the abundance and the redox state of the gases in equilibrium with the magma, which are CO-dominated with H 2 as the second most abundant species on a molar basis, H 2 O and CO 2 being minor constituents. The assimilation of only 0.1 wt% organic matter by a basalt causes a decrease in its oxygen fugacity of 2-orders of magnitude. The assimilation of 0.6 wt% organic matter at depths < 500 m implies minimum CO content in the magma of 1 wt%, other gas components being less than 0.1 wt%. In the light of our calculations, we suggest that the production of native ironbearing lava flows and associated intrusions was most likely accompanied by degassing of CO-rich gases, whose fluxes depended on the magma production rates.

Introduction

Volcanic gases may have critically influenced the evolution of planetary atmospheres and biogeochemical cycles [START_REF] Holland | Volcanic gases, black smokers, and the Great Oxidation Event[END_REF][START_REF] Kump | Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago[END_REF][START_REF] Gaillard | Atmospheric oxygenation caused by a change in volcanic degassing pressure[END_REF]. The classic view is that the nature and the abundance of gas emanations reflect redox state and volatile abundances of the magma source region in the mantle [START_REF] Jambon | Earth degassing and large-scale geochemical cycling of volatile elements[END_REF][START_REF] Carmichael | The redox states of basic and silicic magmas-a reflection of their source region[END_REF]. Volatiles are released to the atmosphere during volcanic degassing and re-introduced into the mantle by subduction (e.g. [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF][START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF]. Mantle redox state is considered to have undergone little evolution over much of Earth history [START_REF] Canil | Vanadium in peridotites, mantle redox and tectonic environments: Archean to present[END_REF][START_REF] Li | The constancy of upper mantle fO(2) through time inferred from V/Sc ratios in basalt[END_REF][START_REF] Trail | The oxidation state of Hadean magmas and implications for early Earth's atmosphere[END_REF], resulting in H 2 O-CO 2 -(SO 2 ) dominated volcanic gases for most magmas, regardless of their tectonic setting and age [START_REF] Li | The constancy of upper mantle fO(2) through time inferred from V/Sc ratios in basalt[END_REF][START_REF] Gaillard | Atmospheric oxygenation caused by a change in volcanic degassing pressure[END_REF][START_REF] Trail | The oxidation state of Hadean magmas and implications for early Earth's atmosphere[END_REF]. This picture is corroborated by geochemical surveys of the composition of volcanic gases emitted by active volcanoes (e.g. [START_REF] Symonds | Volcanic gas studies: methods, results, and applications[END_REF][START_REF] Aiuppa | Forecasting Etna eruptions by real-time observation of volcanic gas composition[END_REF][START_REF] Aiuppa | Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits[END_REF][START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF][START_REF] Metrich | Magma and volatile supply to post-collapse volcanism and block resurgence in Siwi Caldera (Tanna Island, Vanuatu Arc)[END_REF], showing that volcanic species are mainly composed of H 2 O, CO 2 and SO 2 . Recent works show, however, that magma-host rock interaction contributes additional gases that can enhance or even overwhelm the mantlederived contribution [START_REF] Svensen | Release of methane from a volcanic basin as a mechanism for initial Eocene global warming[END_REF][START_REF] Svensen | Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming[END_REF][START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF][START_REF] Iacono-Marziano | Limestone assimilation and the origin of CO 2 emissions from the Alban Hills (Central Italy): constraints from experimental petrology[END_REF], 2009; [START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF][START_REF] Troll | Crustal CO2 liberation during the 2006 eruption and earthquake events at Merapi volcano, Indonesia[END_REF], and locally impacts the magma redox state [START_REF] Ganino | Interaction of magma with sedimentary wall rock and magnetite ore genesis in the Panzhihua mafic intrusion, SW China[END_REF]. During continental flood magmatism, contact metamorphism of country rocks is proposed to have controlled the nature of gas emissions and their environmental consequences [START_REF] Svensen | Release of methane from a volcanic basin as a mechanism for initial Eocene global warming[END_REF][START_REF] Svensen | Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming[END_REF][START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF][START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF]. The high biological impact ascribed to the emplacement of the Siberian Traps and the Emeishan igneous province, for instance, has been attributed to the occurrence of carbonates, evaporites and organic matter-rich rocks in the sedimentary sequences traversed by ascending magmas [START_REF] Svensen | Release of methane from a volcanic basin as a mechanism for initial Eocene global warming[END_REF][START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF][START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF]. Previous studies focused on low temperature (<650°C) contact metamorphism, which generates carbon and sulphur gases (CO 2 , CH 4 , SO 2 ) and halocarbons [START_REF] Svensen | Release of methane from a volcanic basin as a mechanism for initial Eocene global warming[END_REF][START_REF] Svensen | Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming[END_REF][START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF][START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF][START_REF] Retallack | Methane release from igneous intrusion of coal during Late Permian extinction events[END_REF]. The high temperature contribution of magmatic assimilation has been generally neglected, being difficult to quantify [START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF]. Here we study the high temperature (1200°C) assimilation of carbonaceous rocks into basaltic magmas, by simulating the incorporation of organic matter in the form of CH or CH 2 , and investigating its effect on the redox state of the magma, its mineralogical assemblage and its volatile content. Crystalmelt-gas equilibria are first explored via thermodynamic calculations, taking into account both magmatic and sedimentary contributions to gas production. The conditions of basaltorganic matter interaction in relevant geological settings will be then presented.

Numerous studies have investigated the composition and the thermodynamics of volcanic gases, paying particular attention to the redox processes governing their composition (e.g. [START_REF] Giggenbach | redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF][START_REF] Giggenbach | Relative importance of thermodynamic and kinetic processes in governing the chemical and isotopic composition of carbon gases in high-heatflow sedimentary basins[END_REF][START_REF] Gerlach | Oxygen buffering of Kilauea volcanic gases and the oxygen fugacity of Kilauea basalt[END_REF][START_REF] Moretti | On the oxidation state and volatile behavior in multicomponent gas-melt equilibria[END_REF]. [START_REF] Giggenbach | redox processes governing the chemistry of fumarolic gas discharges from White Island, New Zeland[END_REF] dealt with C-O-N-H-S gases, taking into account the role of the rock matrix in buffering their redox conditions. [START_REF] Gerlach | Oxygen buffering of Kilauea volcanic gases and the oxygen fugacity of Kilauea basalt[END_REF] investigated C-O-H-S-Cl-F species and modelled their exchanges with the melt in terms of transfer of oxygen from the gas to the lava. Moreover, [START_REF] Giggenbach | Relative importance of thermodynamic and kinetic processes in governing the chemical and isotopic composition of carbon gases in high-heatflow sedimentary basins[END_REF] explored the role of organic sedimentary material in the production of natural carbon gasses. Compared to these studies, the main originality of our work is that we combine: (i) high temperature magmatic conditions, (ii) gas-melt equilibria, i.e. not only oxygen exchange between gas and melt is taken into account, but also the solubility of some gas species in the melt, (iii) open system conditions, i.e. both magmatic and non-magmatic volatiles are considered, (iiii) saturation of graphite and native iron in the melt, as we deal with reduced condition.

Methods: thermodynamic calculations

We modelled the composition of volcanic gases resulting from interaction with organic matter and magma degassing. To reach this aim we used gas-melt thermodynamic calculations that take into account S-H-O-C gaseous species at temperatures and pressures in equilibrium with basaltic liquids [START_REF] Gaillard | The sulphur content of volcanic gases on Mars[END_REF][START_REF] Gaillard | Atmospheric oxygenation caused by a change in volcanic degassing pressure[END_REF]. We considered that the melt, prior to interaction with organic compounds, is basaltic in composition with 8 wt% FeO.

At the initial conditions (P = 200 MPa, T = 1200°C) the basaltic melt is saturated in CO 2 , H 2 O and S: 0.08, 0.50 and 0.10 wt% respectively [START_REF] Mavrogenes | The relative effect of pressure, temperature and oxygen fugacity on the solubility of sulphide in mafic magmas[END_REF]Iacono-Marziano et al., in press). We investigated the roles of the initial oxidation state and gas content of the magma prior to interaction and tested the effect of coal (CH) or hydrocarbon (CH 2 ) incorporation. The initial oxygen fugacity (fO 2 ) of the magma varied between the Fayalite-Magnetite-Quartz (FMQ) and the Nickel-Nickel Oxide (NNO) buffers, as generally observed in basaltic magmas [START_REF] Li | The constancy of upper mantle fO(2) through time inferred from V/Sc ratios in basalt[END_REF], yielding Fe 2 O 3 /FeO mole ratios between 0.07 and 0.11. The initial (i.e. at 200 MPa) gas content of the magma was between 0.3 and 0.7 %, yielding total volatile contents (dissolved in the melt + bubbles) of 0.50 wt% H 2 O, 0.10 wt% S and 0.24-0.64 wt% CO 2 , which are typical of undegassed basalts [START_REF] Dixon | Carbonatite and silicate melt metasomatism of the mantle surrounding the Hawaiian plume: Evidence from volatiles, trace elements, and radiogenic isotopes in rejuvenated-stage lavas from Niihau, Hawaii[END_REF].

We tested initial dissolved water content between 0.2 and 1 wt% and found it to have a negligible influence on our calculations. Starting conditions are listed in Table 1. Calculations show also that varying the total FeO content of the basalt between 8 and 13 wt% changes by less than 5% the concentration of the species with mole fractions higher than 0.1.

The gas composition was calculated following [START_REF] Gaillard | The sulphur content of volcanic gases on Mars[END_REF] and [START_REF] Gaillard | Atmospheric oxygenation caused by a change in volcanic degassing pressure[END_REF]. This methodological approach is long known as the "Equilibrium constants and mass balance method" [START_REF] Holloway | Igneous fluids[END_REF] to calculate equilibria in the C-O-H system, briefly recalled below. The mole fractions of the different gas species were calculated from the thermodynamic constants of the following reactions at P and T (after [START_REF] Shi | Thermodynamic modelling of the C-H-O-S fluid system[END_REF].

C [graphite] + O 2 = CO 2 [fluid]
(1)

CO [fluid] + ½O 2 = CO 2

[fluid]

(2)

H 2 [fluid] + ½O 2 = H 2 O [fluid] (3)

CH 4 [fluid] + O 2 = CO 2 [fluid] + 2H 2 O [fluid] (4) ½ S 2 [fluid] + O 2 = SO 2 [fluid]
(5)

H 2 S [fluid] + ½O 2 = ½ S 2 [fluid] + H 2 O [fluid] (6)
Melt-gas equilibria were also considered:

H 2 O [fluid] + O 2-[melt] = 2 OH -[melt] (7) CO 2 [fluid] + O 2-[melt] = CO 3 2-[melt] (8) S 2 [fluid] + 2 O 2-[melt] = O 2 + 2 S 2-[melt] (9) 2 FeO [melt] + ½ O 2-[gas] = Fe 2 O 3 [melt] (10)
Equilibria (1) to ( 6) and (10) were simultaneously solved at every given pressure and temperature, under the requirement that the amounts of H, S, O and C remain constant in the system (gas + melt + graphite). Seven redox couples were considered for each calculation, involving sulphur (with 3 different redox states), carbon (with 4 different redox states) and hydrogen (with 2 different redox states), together with iron (2 redox states). Each equilibrium constant for equilibria from (1) to ( 6) is compiled in [START_REF] Symonds | Calculation of multicomponent chemical equilibria in gassolid-liquid systems: Calculation methods, thermochemical data and applications to studies of high-temperature volcanic gases with examples from Mount St. Helens[END_REF], while solubility laws for H 2 O, CO 2 and S (equilibria 7-9) were taken from Iacono-Marziano et al.

(in press) and [START_REF] Mavrogenes | The relative effect of pressure, temperature and oxygen fugacity on the solubility of sulphide in mafic magmas[END_REF]. The ferric-ferrous equilibrium (10) was computed after [START_REF] Kress | The compressibility of silicate liquids containing Fe2O3 and the effect of composition, temperature, oxygen fugacity and pressure on their redox states[END_REF]. The concentrations of reduced C species (e.g.

CO, CH 4 ) in the basaltic melt was neglected in our calculations, as they are known to have very low solubility in a basaltic melt at ~200-300 MPa (Pan et al., 1992;[START_REF] Morizet | C-O-H fluid solubility in haplobasalt under reducing conditions: An experimental study[END_REF].

In conclusion, our approach combines existing experimental studies performed on specific volatiles into a multi-component model, under the assumption of gas-melt equilibrium.

Organic matter was incrementally added up to 0.8 wt%. Two types of organic compounds were taken into account, CH and CH 2 , which are representative of mature organic matter that is most likely to interact with basaltic magmas (e.g. coal or petroleum). To simplify our calculations, we ignore the possible presence of oxygen in the organic matter, as the oxygen content generally decreases with the maturity of the organic matter [START_REF] Hetenyi | Oxygen index as an indicator of early organic maturity[END_REF].

Degassing is simulated by decreasing pressure, keeping the masses of the different components (H, S, C, O) constant. Gas compositions are calculated at variable final pressures and shown below for 10 and 0.1 MPa. Figure 1 in the Supplementary Material schematically summarizes the strategy of the numerical calculations.

Results

Fluid phase composition

Assimilation of organic matter has dramatic effects both on the amount and the redox state of the gases in equilibrium with the basalt (Fig. 1). When no organic matter is assimilated the fluid phase consists mainly of H 2 O and CO 2 (respectively 0.52-0.66 and 0.15-0.33 on a molar basis, with 0.05-0.14 SO 2 , and H 2 , CO, H 2 S and S 2 <0.05) as typically observed in volcanic gas emissions from basalts worldwide [START_REF] Symonds | Volcanic gas studies: methods, results, and applications[END_REF][START_REF] Edmonds | Vapor segregation and loss in basaltic melts[END_REF].

Figure 1 (see also Figure 2 in the Supplementary Material) shows the influence of the initial conditions (initial gas content of the magma and initial fO 2 ) on gas phase compositions at 0.1

MPa resulting from the assimilation of CH or CH 2 . For CH addition, we show how gas composition varies for initial fO 2 of NNO or FMQ, and for an initial gas proportion of 0.3 or 0.7% of the magma. More oxidizing conditions (initial fO 2 of NNO) slightly favours the survival of oxidized species (e.g. H 2 O, CO 2 ) and S species in general (H 2 S, S 2 , SO 2 ) for a given amount of organic matter added to the system. Higher initial gas contents (0.7 wt%) also slightly promote oxidized species relative to reduced ones. The most extreme cases are therefore represented by: (i) fO 2 = NNO and gas proportion = 0.7% (Fig. 1a), and (ii) fO 2 = FMQ and gas proportion = 0.3% (Fig. 1b). CH 2 addition slightly enhances the production of H 2 respect to that of CO. Similarly to CH addition, the two most extreme cases are shown in Fig. 1c andd. In all cases, with the assimilation of only 0.25-0.35 wt% CH or CH 2 , the gas composition changes from CO 2 -H 2 O dominated to strongly reduced and CO-H 2 dominated. In general, the effects of the initial fO 2 and gas fraction are minor, as are differences arising from whether CH or CH 2 is added: in all cases the degassing trends are very similar and the amount of the dominant species (i.e. those with mole fractions >0.1) changes by less than 30%, for a given amount of organic matter assimilated (Fig. SM2).

Graphite and native iron saturation

An important consequence of the addition of organic matter is the crystallization of graphite from the magma (equilibrium 1). Graphite saturation depends only slightly on factors such as the type of organic matter assimilated (CH or CH 2 ), the initial redox conditions, and the initial gas content of the magma but strongly on pressure and therefore on depth (Fig. 2). At pressures higher than 60 MPa, graphite crystallizes from the magma when low amounts of organic matter are assimilated (0.1-0.3 wt%). Coal and oil ingestion at 1200°C and graphite crystallization can be simplified as:

(x) CH + (y) O magma → (y) CO + (1/2 x) H 2 + (x-y) C (11) (x) CH 2 + (y) O magma → (y) CO + (x) H 2 + (x-y) C (12)
where x and y are stochiometric coefficients that are essentially affected by pressure. For a given amount of organic matter assimilated (x), high pressure favours graphite crystallization (i.e. low y), while low pressure enhances CO and H 2 production, by subtracting oxygen to the magma (i.e. high y). At shallow depths (i.e. low pressure), increasing CH or CH 2 addition therefore causes a dramatic decrease in magma fO 2 , whereas at deeper conditions the decrease in fO 2 is less pronounced. Reactions ( 11) and ( 12) conveniently summarize the overall process of assimilation of organic matter by a basaltic melt, however our calculations also take into account other C-H-O and S species (i.e. CO 2 , H 2 O, CH 4 , S 2 , SO 2 , H 2 S; see reactions 1 to 6), which have concentrations lower than 1 wt%, and are therefore neglected here for simplifying.

Figure 3 shows the evolution of the redox state of a magma with an initial gas content of 0.3 wt % at 200 MPa as a function of the assimilated organic matter. The fO 2 of the magma rapidly decreases with the addition of organic matter from the FMQ buffer (initially imposed conditions). For only 0.1 wt% CH assimilated, fO 2 decreases by more than 2 log-units. For more than 0.5 wt% CH assimilated, fO 2 decreases by ~6 log units (FMQ-6). When the fO 2 of the magma reaches FMQ-6, native iron crystallizes [START_REF] Gaillard | The sulphur content of volcanic gases on Mars[END_REF] and buffers magma redox state, until all FeO is exhausted. The amount of organic matter needed to reach graphite and native iron saturation due to the variable initial conditions (i.e. type of organic matter assimilated, initial gas fraction and initial fO 2 of the magma), varies by less than 30% at a given pressure (Fig. 2). At graphite saturation, native iron crystallization occurs with decreasing pressure by graphite volatilization, as expressed by the reaction:

C graphite + FeO magma → CO gas + Fe native (13) Reaction ( 13) implies that native iron formation consumes graphite and produces CO, in addition to that supplied by reactions ( 11) and ( 12).

We calculate that the critical pressure for native iron saturation to occur is 10 MPa. At higher pressures, the redox state of the magma is not reduced enough for native iron to be stable.

This critical pressure mainly depends on magma temperature and to a lesser extent on its total iron content. A hotter and Fe richer magma would crystallize native iron at slightly higher depths (e.g. a magma with 13 wt% total FeO at 1200°C is saturated in native iron at 12 MPa).

Reactions (11-13) can help illustrating what happens to a basaltic magmas ascending through sedimentary sequences containing organic matter-rich layers. When a magma is contaminated deep in a basin with oil or shale, graphite crystallizes for very low amounts of organic matter assimilated (Fig. 2). This graphite is then consumed during ascent to degas carbon monoxide, and to produce, at depths of few hundred meters (varying as a function of the density of the sedimentary host rocks), native iron if significant amounts (more than 0.5 wt%) of organic matter are assimilated (Fig. 2). If the ascending magma also flows through coal-bearing sediments at shallow depths, coal assimilation is even more likely to furnish the amount of organic matter necessary to reach iron saturation. When the magma becomes saturated in native iron, further addition of organic matter does not increase CO degassing, unless pressure is decreased.

A probable side effect of native iron separation that is not taken into account by our model is sulphide precipitation. Native iron separation decreases the total iron content of the melt and therefore also its FeO content, consequently decreasing the maximum amount of sulphur that it can dissolve [START_REF] Gaillard | The sulphur content of volcanic gases on Mars[END_REF]: sulphides are therefore likely to precipitate in response to native iron precipitation. However, this process would not produce large amounts of sulphides, given the relatively low S content of the magma, and would probably happen after the precipitation of relatively large amounts of native iron.

Effect of temperature on gas composition

The effect of temperature on the composition of the gas phase was calculated between 1200 and 300°C. The gas phase produced at 1200°C at graphite and iron saturation (0.6 wt% CH added) was then re-equilibrated at lower temperatures: Figure 4 shows its composition at two different final pressures (0.1 and 10 MPa, Fig. 4a andb, respectively). While at magmatic temperatures (>1000 °C) the gas phase is CO-H 2 -dominated, it is mainly constituted of H 2 O, CH 4 and CO 2 when the temperature decreases below 500°C. Such low temperature gases are close in composition to those resulting from thermal metamorphism of country rocks [START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF].

Field evidence of magma-organic matter interactions

The presence of igneous graphite in mafic rocks has been interpreted as an evidence of magma-organic matter interaction in different geological contexts, e.g. at the Karoo Basin, South Africa [START_REF] Henning | Hydrothermal Cu-Ni-Au-Ag mineralization in a granodiorite sill north of Cradock, Republic of South Africa[END_REF], or at Borrowdale in Cumbria, United Kingdom [START_REF] Ortega | The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism[END_REF]. However, the occurrence of native iron in basaltic extrusive or intrusive rocks is probably the clearest evidence of extremely reducing conditions reached during magmatic assimilation of carbonaceous rocks (crystallization of native iron implying oxygen fugacities lower than FMQ-6); three unambiguous cases are presented below.

Disko Island, Greenland

The first occurrences of native iron in basaltic magmas were found at the island of Disko in central West Greenland [START_REF] Nordenskjöld | Account of an expedition to Greenland in the year 1870, Part I-V[END_REF]. Since then, disseminated native iron has been described in several basalts at Disko, associated with wustite, carbides and sulphides [START_REF] Steenstrup | On the Existence of Nickel-iron with Widmannstatten's Figures in the Basalt of North Greenland[END_REF]Pedersen, 1979aPedersen, ,b 1985;;[START_REF] Ulff-Møller | Solidification history of the Kitdlidt lens: immiscible metal and sulphide liquids from a basaltic dyke on Disko, central West Greenland[END_REF]. Numerous graphite rich xenoliths have been also found throughout the iron bearing basalts and interpreted as thermally metamorphosed fragments from the underlying bituminous shales on the basis of their mineral assemblages and sedimentary structures [START_REF] Melson | Plagioklase-Spinel-Graphite Xenoliths in Metallic Iron-Bearing Basalts, Disko-Island, Greenland[END_REF][START_REF] Pedersen | Non-stoichiometric magnesian spinels in shale xenoliths from a native iron-bearing andesite at Asuk, Disko, Central West Greenland[END_REF]Pedersen, , 1979b)).

Disko lavas belong to the Paleocene volcanic succession related to the North Atlantic mantle plume [START_REF] Pedersen | The Ilugissoq graphite andesite volcano, Nuussuaq, central West Greenland[END_REF]. They were emplaced in the Nuussuaq Basin (part of the West Greenland Basin), whose Cretaceous and Tertiary sedimentary sequences comprise abundant carbonaceous sediments including coal seams up to 2 m thick [START_REF] Medenbach | Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak, Greenland, and Bühl, Federal Republic of Germany[END_REF], which severely contaminated picritic magmas (see [START_REF] Pedersen | The Ilugissoq graphite andesite volcano, Nuussuaq, central West Greenland[END_REF] for a review). The crustally contaminated rocks occur as dykes, necks, lava flows and tuffs and contain variable amounts of graphite and native iron. At the classic locality of Asuk, a ~100 m thick sequence of sediment-contaminated lavas, mostly iron-bearing, occurs in which baked, party glassy, shale xenoliths are frequent, some still preserving sedimentary bedding (Pedersen, 1979a). [START_REF] Pedersen | The Ilugissoq graphite andesite volcano, Nuussuaq, central West Greenland[END_REF] proposed that the abundance of graphite and native iron in the Disko and Nuussuaq rocks depends on the pressure at which they equilibrated: using thermodynamic estimations of the equilibrium between graphite and a C-H-O gas phase at high temperature and pressure [START_REF] French | Experimental control of oxygen fugacities by graphite-gas equilibriums[END_REF][START_REF] French | Some geological implications of equilibrium between graphite and a C-H-O gas phase at high temperatures and pressures[END_REF], they predicted an equilibration pressure of 0.1 MPa for Disko lavas and 50-100 MPa for the native iron free graphite andesite tuffs from Nuussuaq. This is in a very good agreement with what our calculations predict.

Bühl, Germany

Native iron-bearing xenoliths have also been observed in Miocene alkali basalts at Bühl near Kassel, in West Germany [START_REF] Medenbach | Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak, Greenland, and Bühl, Federal Republic of Germany[END_REF]. The xenoliths have been found in a basalt pipe that intruded Tertiary coal seams and have been interpreted as the products of a reduction under near surface conditions (<10 MPa) of manganese-bearing iron minerals contained in Tertiary sediments and coal seams cut by the erupting basalts [START_REF] Medenbach | Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak, Greenland, and Bühl, Federal Republic of Germany[END_REF]. Carbon from coal has been considered as the reducing agent [START_REF] Medenbach | Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak, Greenland, and Bühl, Federal Republic of Germany[END_REF].

Siberian Platform, Russia

The highest concentrations of native iron in magmatic rocks have been observed in the North and North-west of the Siberian Platform, in several trap intrusions hosted by the Middle Carboniferous-Permian coal-bearing terrigenous deposits [START_REF] Ryabov | Native iron (-platinum) ores from the Siberian Platform trap intrusions[END_REF]. West Siberian Basin [START_REF] Saunders | A mantle plume origin for the Siberian traps: uplift and extension in the West Siberian Basin, Russia[END_REF].

Several lines of evidence suggest that the temperature of ore formation was 750-1200 °C and that at least some of the native iron formed before the crystallization of silicates, e.g. native iron inclusions occur in plagioclase, olivine and pyroxene [START_REF] Ryabov | Native iron (-platinum) ores from the Siberian Platform trap intrusions[END_REF].

Although sulphides are commonly associated with the native iron, the total sulphur content of the mineralization is < 0.2 wt%. Carbon isotopic compositions of graphite and carbides are similar to that of the organic matter [START_REF] Ryabov | Native iron (-platinum) ores from the Siberian Platform trap intrusions[END_REF]. More generally, the occurrence of varying amounts of finely dispersed carbonaceous matter and its carbon isotopic composition suggest that nearly all ore-bearing intrusions in the northwest of the Siberian Platform have been contaminated by carbonaceous matter to different extents [START_REF] Ryabov | Bitumen, graphite and carbonaceous matter in the rocks and ores of northwestern Siberian Platform: occurrence and isotope composition of carbon[END_REF][START_REF] Ryabov | Native iron (-platinum) ores from the Siberian Platform trap intrusions[END_REF]. Carbonaceous matter occurs in the form of amygdales in dolerite of explosive breccias, and also as small rounded inclusions in plagioclase [START_REF] Ryabov | Bitumen, graphite and carbonaceous matter in the rocks and ores of northwestern Siberian Platform: occurrence and isotope composition of carbon[END_REF][START_REF] Ryabov | Native iron (-platinum) ores from the Siberian Platform trap intrusions[END_REF]. Abundant bitumen-filled amygdules (schungite) in volcanic units of the Noril'sk region and massive reduction of sulphate to sulphide during the formation of the major Ni sulphide deposits of the Norilsk-Talnakh region [START_REF] Aiuppa | Mantle-derived magmas and magmatic Ni-Cu-(PGE) deposits[END_REF] provide additional evidence of interaction with hydrocarbons.

Gas emissions produced by assimilation of organic matter

Native iron-bearing intrusions and lava flows described in the previous section are hosted in, or associated with, coaliferous strata and bituminous shales, suggesting that graphite and native iron saturation was triggered by assimilation of organic matter (mainly coal). Our calculations restrict native iron crystallization to pressures lower than 10 MPa (Fig. 2), i.e.

350-500 m, depending on the density of the overlying sedimentary rocks, however the ingestion of organic matter could have happened also at deeper levels (e.g. where petroleum accumulations occur). The occurrence of native iron constrains the minimum amount of organic matter assimilated (either CH or CH 2 ) to be 0.5 wt % (Fig. 2). Moreover, the coexistence of graphite and native iron also constrains the composition of the fluid phase. At 0.1 MPa, the fluid phase in equilibrium with graphite and native iron is CO-dominated, regardless the initial conditions (Fig. 1, Fig. SM1, and Table 2). Figure 5a presents the same results as Figure 1d, with the difference that the composition of the fluid phase is expressed in wt% of the magma. We calculated that graphite and native iron-saturated lavas like those observed at Disko and Bühl degassed 1.3 wt% CO, the other gas components representing less than 0.1 wt% (Table 2). These estimations take into account only the contribution of reactions ( 11) and ( 12), without considering the additional production of CO associated to native iron formation (reaction 13) and are hence minima. We also recall that an oxygen-free organic matter was considered in our calculations; the presence of oxygen in the assimilated organic matter would result in a higher degree of assimilation and therefore in a higher CO production. Fluid bubbles hosted in the groundmass glass of native iron-bearing Disko basalts were investigated by heating and cooling microscopic techniques, revealing strongly reduced volatile molecules most likely formed during the magmatic stage [START_REF] Solovova | Reduced magmatic fluids in basalt from the island of Diskp, central West Greenland[END_REF]. The estimated fluid composition at ~1180°C and pressures lower than 10 MPa is CO-H 2 dominated [START_REF] Solovova | Reduced magmatic fluids in basalt from the island of Diskp, central West Greenland[END_REF].

As native iron was observed not only in basaltic flows, but also in subvolcanic intrusions that were probably emplaced a few hundred meters deep (e.g. in North Siberia, see previous section), we also calculated the composition of the fluid phase in equilibrium with graphite and native iron at 10 MPa (i.e. the maximum depth for native iron crystallization calculated at 1200°C). At 10 MPa, saturation in graphite and native iron is reached for ~0.5 wt% CH or CH 2 addition. Figure 5b and Table 2 show that, although the coexisting fluid phase is still largely CO-dominated, calculated CO contents of the magma are slightly lower (~1.0 wt%), and those of the other components slightly higher (but always <0.1 wt%) than at 0.1 MPa.

Again, these estimations consider only reactions ( 11) and ( 12) and not (13). In the absence of robust constraints on the degree of metallisation of iron in the native iron-bearing basalts we cannot correctly estimate the amount of CO produced by reaction (13), which is nevertheless potentially significant. A maximum estimate can be done by considering that all FeO initially present in the basalt (8 wt%) has been transformed into native iron: this would have produced additional 3.1 wt% CO, which then totalizes ~4.2 wt% of the magma (Table 2). Even if this additional contribution is not considered, both Figure 5 and Table 2 clearly show how the ingestion of only 0.6 wt% of organic matter by a basalt strongly enhances magma degassing:

the total volatile content of the magma increases from ~0.9 wt% in the absence of assimilation to ~1.5 wt% at 0.1 MPa (Fig. 5a, Table 2), and from ~0.5 wt% to ~1.2 wt% at 10 MPa (Fig. 5b, Table 2).

If the temperature of magmatic gases decreases during ascent after separation from the magma, their composition would change from CO toward H 2 O-CH 4 -CO 2 -dominated (Fig. 4b).

For instance, at 300°C the calculated gas composition is ~ 46 wt% H 2 O, 28 wt% CH 4 and 26 wt% CO 2 .

The enhanced magmatic degassing of C species (including CO at high temperatures and CH 4 -CO 2 at low temperature) is likely to have environmental implications when the associated magma production rate is high. The emplacement of Disko and Nuussuaq lavas that belong to the North Atlantic Large igneous Province [START_REF] Storey | 40Ar/39Ar geochronology of the West Greenland Tertiary volcanic province[END_REF] could have caused environmental damages, due to the large volumes of magma that were contaminated by organic matter-rich sediments [START_REF] Pedersen | The Ilugissoq graphite andesite volcano, Nuussuaq, central West Greenland[END_REF]. Lethal hazard for animal life has been invoked at a local scale due to intense CO and CO 2 emanations [START_REF] Pedersen | The Ilugissoq graphite andesite volcano, Nuussuaq, central West Greenland[END_REF]. A global impact of these gas emissions can also be speculated: magmatic activity in Western Greenland precedes the Paleocene-Eocene hyperthermal (rapid and transient global warming events) and several authors have suggested that the two events can be related (e.g. [START_REF] Kennett | Abrupt deep-sea warming, palaeoceanographic changes and benthic extinctions at the end of the Palaeocene[END_REF][START_REF] Wignall | Large igneous provinces and mass extinctions[END_REF].

Siberian magmatism/volcanismis another case in point: it has often been proposed to have caused the end-Permian crisis, and the strong perturbation of the Earth's carbon cycle, globally marked by a negative carbon isotope excursion (e.g. [START_REF] Wignall | Large igneous provinces and mass extinctions[END_REF]. In particular, the interaction between magmas and carbonaceous sediments has been proposed as the major cause of the environmental impact of the Siberian Trap emplacement (e.g. [START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF]. In a companion paper (Iacono-Marziano et al., submitted to EPSL) we quantify the gas production potential of a single intrusive event in coal-bearing sediments at the Siberian Traps and investigate the dispersion of these gases in the atmosphere. We also discuss the potential impact of these gas emissions on the environment, distinguishing short term implications at a local scale from long term ones at the global scale.

The process described and quantified here could concern not only LIP sized events, but also much smaller intrusions/eruptions. Virtually any active volcanic area whose sedimentary substrata comprise layers of organic-rich material can produce important amounts of CO, with potentially dangerous consequences for nearby human beings.

Conclusions

The long term volatile cycle is generally described as the exchange between two major reservoirs, the mantle and the atmosphere, where volcanism has the role of conveying volatiles from the mantle to the surface (e.g. [START_REF] Hilton | Noble gases and volatile recycling at subduction zones[END_REF][START_REF] Wallace | Volatiles in subduction zone magmas: concentrations and fluxes based on melt inclusion and volcanic gas data[END_REF]. Recent studies have however revealed that magma-host rock interaction may contribute additional gases, in particular when host rocks are composed of carbonates, evaporites or organic matter [START_REF] Svensen | Release of methane from a volcanic basin as a mechanism for initial Eocene global warming[END_REF][START_REF] Svensen | Hydrothermal venting of greenhouse gases triggering Early Jurassic global warming[END_REF][START_REF] Svensen | Siberian gas venting and the end-Permian environmental crisis[END_REF][START_REF] Iacono-Marziano | Limestone assimilation and the origin of CO 2 emissions from the Alban Hills (Central Italy): constraints from experimental petrology[END_REF], 2009;[START_REF] Ganino | Climate changes caused by degassing of sediments during the emplacement of large igneous province[END_REF]. In this paper, we show that another crucial effect of magma-host rock interactions is that they can impact severely on the redox state of the magma and thus that of volcanic gases.

Very minor amounts of organic matter (<0.5 wt%) assimilated by the magma decrease its redox state by several log-units. The assimilation of only 0.6wt% of organic matter almost doubles its total volatile content and produces unusual CO-dominated gases. We therefore conclude that while the long-term deep volatile cycle controls the background level of volcanic degassing, occasional interaction between magma and sedimentary host rocks may yield peak values of abnormal gas compositions, representing a potentially major perturbation of the volatile cycles. Gas abundances (in wt% of the magma) produced by high temperature assimilation of organic matter. Given ranges account for the variations in initial conditions (initial fO 2 and gas fraction of the magma), type of organic matter assimilated, i.e. CH or CH 2 and amount of organic matter necessary to reach graphite-native iron saturation (see Section 3.1 for explanation). b) Gas species produced by the degassing of a basaltic magma (initial H 2 O, CO 2 and S contents of 0.50, 0.08 and 0.10 wt%, respectively) without assimilation. * carbon monoxide produced by reaction 13 in the text.

Figure Captions

Fig. 1 Gas compositions at 0.1 MPa expressed in mole fraction as a function of the amount of organic matter assimilated. The effects of different initial conditions of oxygen fugacity (NNO or FMQ), gas proportion in the magma (0.3 or 0.7 %) and type of organic matter assimilated (CH or CH 2 ) are shown. Four different configurations listed in Table 1 are shown: #7 (a), #1 (b), #13 (c), #10 (d). Black and red lines indicate the amount of organic matter necessary to reach graphite and native iron saturation, respectively.

Fig.2

Graphite and iron saturation as a function of pressure (and roughly of depth) for CH and CH 2 addition. Calculations for CH assimilation were performed using initial fO 2 of FMQ and NNO and initial gas fraction of 0.3 and 0.7%. Only the two extreme cases (fO 2 = FMQ, gas fraction = 0.3% and fO 2 = NNO, gas fraction = 0.7%) are shown for CH 2 . Variations between the different cases are always within 30%. The presence of graphite alone in a magmatic rock does not clearly indicate the amount of organic matter assimilated by the basaltic magma, because of the strong dependence on the depth of the intrusion emplacement. On the contrary, the coexistence of graphite and native iron precisely constraints the minimum amount of CH or CH 2 (0.5-0.8 wt%) assimilated at depths lower than 350-500 m (depending on the density of the sedimentary host rocks).

Fig. 3

Variation of the oxygen fugacity of the magma (expressed as logarithmic units relative to the FMQ buffer) as a function of the amount of coal assimilated. Calculations were performed at 1200°C and 10 MPa using pre-assimilation fO 2 of FMQ and volatile contents of 0.50 wt% of H 2 O, 0.10 wt% of S and 0.24 wt% CO 2 (corresponding to a gas fraction of 0.3 % at 200 MPa). Red and black lines indicate native iron and graphite saturation, respectively. Gas contents of the magma (expressed in wt%) at 0.1 MPa (a) and 10 MPa (b) as a function of the amount of organic matter assimilated. Calculations were performed using initial fO 2 of FMQ and initial gas fraction of 0.3 %. The lines indicate the amount of organic matter necessary to reach graphite and iron saturation and the composition of the coexisting gas phase.

  The occurrence and composition of native iron in the different intrusions (Maimecha, Khininda, Khungtukun intrusions, Dzhaltul igneous complex and Ozernaya Mountain intrusion) are very similar. Two types of native iron have been recognized: fine disseminations (0.2-0.5 mm) in dyke-like bodies and nodules (up to 200 kg in weight) concentrated in the upper zone of the intrusions (or of the first intrusive phase in the Dzhaltul complex). Dyke-like bodies are 0.3-3.9 m-thick and contain fragments of sedimentary rocks. The 2-40 m thick nodule-bearing horizons are located within a 40-110 m-thick olivine gabbro zone where they are associated with graphite, carbides and finely dispersed carbonaceous matter. The quantity and size of nodule increase upwards in the ore-bearing horizons. The current depths of native iron mineralizations are between 40 and 600 m. Subsidence of several hundred meters throughout the Mesozoic and Cenozoic has been, however, estimated for the

Fig. 4

 4 Fig. 4 Calculated gas compositions as a function of temperature for ~0.6 wt% CH assimilated (required for both graphite and iron saturation). Two different final pressures are shown: 0.1 MPa (a) and 10 MPa (b).

Fig. 5

 5 Fig. 5

  Fig. 1

  Fig.3

  Fig.5

Table 1 . Numerically tested configurations of organic matter assimilations by basalts. N° Redox conditions a Gas content b Volatile contents c H 2 O (wt%) S (wt%) CO 2 (wt%)

 1

	Organic
	matter d

calculations have been conducted at 1200°C, with pressure varying from 200 to 0.1 MPa. a Initial redox conditions of the basaltic magma at 200 MPa and 1200°C b Initial gas content of the basaltic magma at 200 MPa and 1200°C c Initial volatile contents (dissolved in the melt + in the form of bubbles) of the basaltic magma at 200 MPa and 1200°C d Type of organic matter assimilated Table 2. Fluid phase composition in equilibrium with graphite and native iron.

	Emitted	10 MPa	0.1 MPa	10 MPa	0.1 MPa
	species	(wt%) a	(wt%) a	without assimil.	without assimil.
				(wt%) b	(wt%) b
	CO	1.00-1.09	1.46-1.50	0.02	0.03
	CO*	3.11	2.22
	CO 2	0.08	0.00	0.35	0.31
	H 2	0.04	0.04-0.07	0.00	0.00
	H 2 O	0.03-0.04	0.00	0.07	0.40
	H 2 S	0.02-0.03	0.00	0.02	0.03
	CH 4	0.02	0.00	0.00	0.00
	S 2	0.00	0.00	0.02	0.04
	SO 2	0.00	0.00	0.03	0.14
	Tot	1.19-4.41	1.50-3.79	0.51	0.95
	a)

Acknowledgements G.I.M. was founded by the European Community's Seventh Framework Programme (grant agreement n° PIEF-GA-2008-220926). F.G. was supported by the ERC grant number 279097.

This work benefited from conversations with Jean-Robert Disnar and Jérémy Jacob. We thank B. Marty for the careful handling of the manuscript, H. Svensen and an anonymous reviewer for their constructive comments that greatly improved the paper.