Isotopic Compositions of Plagioclase From Plutonic Xenoliths Reveal Crustal Assimilation Below Martinique, Lesser Antilles Arc

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.

A Crustal Control on the Fe Isotope Systematics of Volcanic Arcs Revealed in Plutonic Xenoliths From the Lesser Antilles

“…Whole-rock isotope data from Martinique plutonic xenoliths was originally published in Brown et al (2021) 3). These ranges do not overlap each other, with Martinique lavas and plutonic xenoliths having more radiogenic Sr and Pb, and less radiogenic Nd than those at Statia.…”

“…The petrology, mineral chemistry and whole rock major and trace elements of a large sample set of sample set of Martinique and Statia plutonic xenoliths were originally described by Cooper et al (2016) and Cooper et al (2019) respectively. Whole-rock isotope data from Martinique plutonic xenoliths were originally published in Brown et al (2021) but are compared to the record from Statia here. To provide key context for stable Fe isotope analysis, we chose a representative subset of the plutonic xenolith samples described in these studies, covering a range of mineral assemblages and whole-rock geochemistry (Supplementary Data).…”

“…The radiogenic isotope compositions of Sr, Nd, and Pb were measured in the Arthur Holmes isotope Geology Laboratory at the University of Durham (United Kingdom), see Brown et al (2021) for full analytical techniques. Briefly, whole rock powders were digested in concentrated HF + HNO 3 acids, before being processed on columns containing Eichrom Sr Spec resin.…”

“…There is a long-standing debate as to whether geochemical variations in the LAA and other volcanic arcs are attributed to heterogeneity in the sub arc mantle source via addition of slab derived sediments (White and Dupré, 1986;Carpentier et al, 2008;Labanieh et al, 2010;Labanieh et al, 2012;Hu et al, 2021) versus subsequent crustal modification by assimilation processes (Davidson, 1986;Davidson, 1987;Van Soest et al, 2002;Davidson and Wilson 2011;Bezard et al, 2014;Bezard et al, 2015;Brown et al, 2021). Geochemical evidence for sediment and crustal assimilation during storage in LAA magmatic plumbing systems is provided by an excess in incompatible elements in lavas compared with fractional crystallisation models and by covariations between Sr and Pb isotopes and indices of differentiation (Roobol and Smith, 2004;Davidson and Wilson, 2011).…”

“…Plutonic xenoliths are direct portions of this crystal mush and therefore represent a volumetrically greater proportion of the crust than the melt dominant magmas which reach the surface in eruptions. Thus, the formation of crystal mush in the deeper portions of volcanic plumbing systems likely plays a key role in governing the geochemical and isotopic composition of arc magmas (Cooper et al, 2016;Cooper et al, 2019;Brown et al, 2021). In this study, we present the first δ 56 Fe record from plutonic xenoliths, in order to test if the Fe isotope systematics of sub-arc mantle inputs can be inferred from arc lavas, or are modified through subsequent differentiation processes in the sub-arc crust during storage and ascent.…”