Sr isotopes in arcs revisited: tracking slab dehydration using δ88/86Sr and 87Sr/86Sr systematics of arc lavas

Abstract

Dehydration of the subducting slab is a crucial process in the generation of hydrous convergent margin magmas, yet the exact processes of how and where the slab dehydrates and how these fluids are transported to the mantle wedge remain obscure. Strontium is a "fluid-mobile" element and as such well suited to investigate the source of slab-derived fluids. We employ mass-dependent Sr isotope systematics (δ 88/86 Sr; the deviation in 88 Sr/ 86 Sr of a sample relative to NIST SRM 987) of primitive arc lavas, in tandem with conventional radiogenic 87 Sr/ 86 Sr measurements, as a novel tracer of slab dehydration. To characterise the δ 88/86 Sr composition of subduction zone inputs, we present new δ 88/86 Sr data for subducting sediments, altered oceanic crust and MORB. Calcareous sediments are isotopically lighter and carbonate-free sediments are isotopically heavier than mid-ocean ridge basalts (MORB). Samples of the altered oceanic crust display elevated 87 Sr/ 86 Sr but only the most intensely altered sample has significantly higher δ 88/86 Sr than pristine MORB. Mafic arc lavas from the Aegean and Mariana arc invariably have a mass-dependent Sr isotope composition that is indistinguishable from MORB and lower 87 Sr/ 86 Sr than upper altered oceanic crust. This δ 88/86 Sr-87 Sr/ 86 Sr signature of the arc lavas, in combination with their high but variable Sr/Nd, can only be explained if it is provided by a fluid that acquired its Sr isotope signature in the deeper, less altered part of the subducted oceanic crust. We propose a model where the breakdown of serpentinite in the slab mantle releases a pulse of fluid at sub-arc depths. These fluids travel through and equilibrate with the overlying oceanic crust and induce wet partial melting of the upper altered crust and sediments. This hydrous melt is then delivered to the mantle source of arc magmas as a single metasomatic component. From mass balance it follows that the slab-derived fluid contributes >70 % of the Sr budget of both Mariana and Aegean arc lavas. Whereas this fluid-dominated character is unsurprising for the sediment-poor Mariana arc, the Aegean arc sees the subduction of 3-6 km of calcareous sediments that were found to exert very little control on the Sr budget of the arc magmas and are overwhelmed by the fluid contribution.

Molybdenum isotopes unmask slab dehydration and melting beneath the Mariana arc

“…The hydrous melt component is from subducted sediments, given its high Th/Nb and less radiogenic Nd isotopes 2 . However, the conclusion that AMOC (average 87 Sr/ 86 Sr = ~0.7045 5 ) and marine sediment (average 87 Sr/ 86 Sr = ~0.710 6 ) act as respective sources for the aqueous fluid and hydrous melt is inconsistent with Mariana arc lavas having unradiogenic 87 Sr/ 86 Sr (0.7031–0.7041), and fresh mid-ocean ridge basalt (MORB)-like Pb isotopes 7 – 10 , and mantle-like oxygen isotopic compositions 11 . Recent geochemical and geophysical observations suggest that the dehydration of serpentinites may play a more critical role in explaining these observed elemental and isotopic paradoxes.…”

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

“…Elliott et al 1997;Class et al 2000;Turner and Foden 2001;Zamboni et al 2016). On the other hand, a recent study of Klaver et al (2020) using δ 88/86 Sr as a tracer of Sr concluded that the slab-derived fluid accounts for > 70% of the Sr budget of the Mariana and Aegean arc lavas. Considering that the Aegean arc subducts 3-6 km of Sr-rich calcareous sediments, this observation casts severe doubts on the perceived importance of the sediment melts for arc magma generation.…”

Halogen (F, Cl) Concentrations and Sr‐Nd‐Pb‐B Isotopes of the Basaltic Andesites From the Southern Okinawa Trough: Implications for the Recycling of Subducted Serpentinites

“…Nevertheless, based on the thermal structure of the Ryukyu subduction zone, the sediments are expected to be melted beneath the southern Okinawa Trough (Figure 9). Recent studies have shown that Sr/Nd fractionation is negligible during sediment melting (Klaver et al., 2020; Martindale et al., 2013; Skora et al., 2015). Thus, the addition of sediment melts is indistinguishable with the addition of bulk sediments (Klaver et al., 2020).…”

“…The mechanism, however, remains uncertain and requires further investigation. One possible explanation is that serpentinite fluids dehydrated from the slab mantle react with the overlying oceanic crust and, thus, inherit some compositional characteristics from the crustal materials (S. Chen, Hin, et al., 2019; Klaver et al., 2020).…”

“…Recent studies have shown that Sr/Nd fractionation is negligible during sediment melting (Klaver et al, 2020;Martindale et al, 2013;Skora et al, 2015). Thus, the addition of sediment melts is indistinguishable with the addition of bulk sediments (Klaver et al, 2020).…”