<p style="text-align: justify">Melt inclusions and magmatic processes in subduction zones<Br />

Melt inclusions are small volumes of melt trapped during crystal growth. When hosted by high-Fo olivines, melt inclusions could represent primary melt unaffected by superficial processes such as degassing, magma mixing or crustal contamination.
Melt inclusion composition in one sample could display large variations, as large as the variation recorded for the corresponding island or even arc. These variations could record slab influence or magmatic evolution, but could also represent isotopic diffusion.
In the case of St. Vincent basaltic scoriae (Lesser Antilles arc), the chemical and isotopic variations in melt inclusions trapped in high Fo olivines have been suggested to represent primary magmas and to record the influence of at least 3 different slab fluids (e.g., Bouvier et al., 2010). Indeed, the combination of light elements and stable isotopes analysis has the potential to reveal the influence of different slab fluids on the mantle wedge. On the other hand, Li isotopes, a stable isotopic system, displays a large range (-10 to +7‰) which cannot be explain by simple slab fluid contributions (Figure 1). Because of the fast Li diffusion rate, the Li isotopic variations could reflect isotopic fractionation by Li diffusion in or out of the MI, or the influence of a slab fluid with specific δ7Li signature. Results of δ7Li, δ18O and P profiles in olivines combined to δ18O and δ7Li in their hosted melt inclusions do not suggest Li diffusion from melt inclusions to the host. Correlation with melt inclusions water content and δ7Li in olivine rather suggests Li isotopes fractionation during non-equilibrium fluid extraction from the slab and/or during fluids migration within the mantle before melt inclusions entrapment
 

Figure 1: Li isotopes compared to Li contents in melt inclusions. Light Li isotopic compositions, associated with low Li contents, could not be explained by “simple” slab fluid influence, based on Rayleigh equations.

 

For more information:

Bouvier A.-S., Deloule, N., Métrich, M. (2010) “Magma genesis of St. Vincent and Grenada (Lesser Antilles) primary magmas: new insights from trace elements in olivine-hosted melt inclusions”, Journal of Petrology 51, 1597-1615.

 

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