III. Geodynamic Cycle of Volatile Elements


Thin section microphotograph under plane polarised light. Characteristic lattice network of low-pressure serpentinisation is visible. Here, the ‘oceanisation’ process (i.e. hydrothermal metamorphism in ocean floor) represented by the networked lizardite, which is even cross cut by antigorite formed during the initial stage of the subduction at higher pressure conditions.

Serpentinite’s role in geochemical recycling during subduction. Alpine serpentines of various metamorphic facies potentially record element transfer and fluid circulation in a subduction zone, as deep as 100-150 km. During oceanic metamorphism, it incorporates fluid-mobile elements (B, Li, As, Sb, Rb, Ba, Cs, Sr, U et Pb) following fluid movements. Then, the results from a study of Lanzo massif demonstrates that the serpentinites have evolved in a closed system, because they do not show significant compositional variations of fluid-mobile trace elements (Debret et al., 2013). It should be noted that the Lanzo massif represents an oceanic lithosphere metamorphosed to eclogite facies, which also followed various stages of serpentinization and de-serpentinization during subduction.

It was also shown that the stored halogen (F and Cl) as well as S in serpentinites are lost during the lizardite -> antigorite transformation, even if it is not a major dehydration events (Debret et al. 2014b). In addition, these natural serpentinites show that the antigorite dehydration generates very high oxygen fugacity conditions, at hematite-magnetite buffer (Debret et al. 2015). This has implications on the redox potential of the fluid released to the mantle wedge, which ultimately generates the arc magmas.




Stability of hydrous phases in subduction zone. The electric conductivity measurements have been implemented in the multi anvil press. The first results on the dehydration of the lawsonite allow to explain, for the first time, the extremely high electric conductivity observed in some slabs without having to invoke the presence of brine (Manthilake et al., 2015).


 Figure : Electric conductivity profile as a function of the depth, for the subduction zone of NE Japan and North and Central Chili. The pink coloured zone corresponds to the electric conductivity of the fluid released from lawsonite dehydration. The inset shows the stability conditions of lawsonite (Okamoto and Maruyama, 1999) and black, green, and red lines show the geothermal gradient of Tohoku, North and Central Chili respectively (Syracuse et al. 2010). The dehydration of Lawsonite is indicated by the yellow star. (After Manthilake et al., 2015).








Storage of water in the mantle. While the source of MORBs contains around 100 ppm of water by weight, the minerals of the mantle can contain more and more water as the depth increases. We thus showed that water storage capacity of the mantle can reach ~800 ppm of H2O by weight around 350 km of depth. The value is in agreement with the content measured in the source of OIB (Férot et Bolfan-Casanova, 2012).

Figure: Storage capacity of water in the terrestrial mantle. The comparison between the experimental data of solubility of water in NAMs (red curve) and seismology suggests that the Low Velocity Zone observed around 350 km of depth indicates dehydration melting of an upwelling mantle containing around 850 ppm H2O by weight.




Behaviour of halogen elements in subduction zone. The goal is to understand how halogen elements are distributed during dehydration and melting of the mantle wedge. We have demonstrated the weak F mobility compared to Cl during the dehydration of the slab. Various partition coefficients (fluid/mineral and mineral/melt) have been determined (Dalou et al., 2012, 2014 ; Wu and Koga, 2013). In conclusion, the small partition coefficients of F between fluid and amphibole implies that slab melting is required to transport significant quantities of Cl and F as implied in the study of Le Voyer et al. (2010). The parameters controlling incorporation of Cl in minerals have been established.


Figure:  Variation of partition coefficients of Cl between cpx and melt as a function of mineral composition. Jadeite content of the orthopyroxene and Ca-Tschermack content in clinopyroxene. According to Dalou et al. (2012).




Carbon in the Mantle.  With regard to the melting relations of silicate-carbon, the thorough study of the literature suggests the existence of a thermal divide in the system eclogite-CO2 which would explain the divergences between the groups of experimentalists (Hammouda & Keshav, 2015). In addition, our experiments using synchrotron show that a disturbed géotherm (hot) can lead to the calcic carbonatite eruption, with limited degassing (Hammouda et al., 2014). Concerning the carbon cycle, the geochemical modelling of recycling by the radiogenic isotopes (Sr, Nd, Pb, This) makes it possible to conclude that, in the case of oceanic carbonatites (Cap Verde, the Canaries), carbon is of recycled origin and not primordial (Doucelance et al., 2010 ; 2014).



Figure: Effect of the composition and speciation of the fluid on the solidus of peridotite, according to Hammouda and Keshav (2015).

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