Abstract
The chemical and isotopic compositions of volcanic arc lavas often show evidence for involvement of a sedimentary component during magma genesis. Determining where this sedimentary component is added to arc magmas is of vital importance for constraining the extent to which sediments and volatiles are recycled at subduction zones. Lavas from Martinique in the Lesser Antilles arc have wide ranging isotopic compositions extending to highly radiogenic values (e.g. 87/Sr/86Sr up to ∼0.710) that could, in principle, be explained by sediment addition to the mantle source or by crustal assimilation in the upper plate. We use Sr isotopic compositions of plagioclase from Martinique plutonic xenoliths to provide evidence supporting the crustal assimilation hypothesis. Plagioclase from plutonic xenoliths formed in the mid-crust (∼12 km) show a restricted range of unradiogenic Sr isotope ratios (87Sr/86Sr = 0.7041–0.7042) whereas plagioclase from upper crustal plutonic xenoliths (∼6 km) show greater intra-sample variation and more radiogenic Sr isotopic compositions up to 87Sr/86Sr = 0.7047. This trend is also observed in plutonic xenolith whole rock 87Sr/86Sr. Combined, these results indicate that the range of Sr isotope compositions becomes larger and more radiogenic in Martinique magmas as a result of sediment assimilation at shallow crustal levels. This is supported by Assimilation-Fractional Crystallization modeling, which shows that assimilation of chemically and isotopically heterogenous crustal sediments can produce the isotopic variation in Martinique plutonic xenoliths and lavas. Our results highlight the importance of constraining crustal contributions from the upper plate before using arc lava geochemistry to quantify sediment and volatile recycling at subduction zones and assessing potential heterogeneity of arc mantle sources.
Highlights
The chemical and isotopic compositions of arc magmas are generally considered to reflect contributions from the mantle wedge combined with fluids and/or melts derived from subducting oceanic crust and sediments (e.g. Kelemen et al, 2003 and references therein)
The new Sr isotopic data from plutonic xenoliths is used to test whether the crustal/sedimentary component in Martinique lavas is introduced from the subducting slab or upper plate crust
The Sr isotopic composition of plagioclase from plutonic xenoliths has been used to evaluate the origin of the wide isotopic range and highly radiogenic compositions of Martinique lavas
Summary
The chemical and isotopic compositions of arc magmas are generally considered to reflect contributions from the mantle wedge combined with fluids and/or melts derived from subducting oceanic crust and sediments (e.g. Kelemen et al, 2003 and references therein). Many arc magmas have trace element and isotopic compositions displaced from MORB toward crustal compositions Davidson et al, 2005), which may be explained by input from the subducting slab, or assimilation of arc crust during magma ascent and storage. Geochemical constraints upon the relative importance of slab vs upper plate contributions to arc magmas have clear implications for understanding sediment recycling at subduction zones. Modification of trace element concentrations in arc lavas by assimilation of crustal material can cause misleading estimates of the amount of sediment recycled into the mantle. Assimilation of crustal sediments modifies the volatile budget of arc magmas (e.g. Chadwick et al, 2007; Deegan et al, 2010), which in turn affects eruption explosivity, it is vital to constrain the relative contributions of the overlying vs. subducting plate to quantify recycling in arc systems
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