Os isotopic composition of western Aleutian adakites: Implications for the Re/Os of oceanic crust processed through hot subduction zones

Abstract

Constraining the behaviour of Re and Os during eclogite melting is required to understand the Re and Os budget and 187Os/188Os of recycled slabs produced at warm subduction zones. It is particularly relevant to early Earth history, a period during which slab melting could have prevailed over dehydration due to higher mantle temperatures. There are however currently few constraints on Re and Os mobility during slab melting. Accordingly, we measured Os, Re and 187Os/188Os in primitive submarine lavas (Mg# ˃ 0.6) from the western Aleutian Arc. These include strongly adakitic rocks shown to be derived from eclogite melting (high-Mg# andesite, dacites and rhyodacites), as well as non-adakitic rocks (high-Mg# andesites, basaltic andesites and basalts) with variable sediment and fluid-derived slab contributions for comparison.The 187Os/188Os of the adakitic and non-adakitic volcanic rocks vary significantly but largely overlap. In both groups, the most radiogenic values occur in samples with the lowest Os concentrations, thus implicating crustal assimilation as the main cause of Os isotope variations. Adakitic and non-adakitic rocks least affected by crustal assimilation have overlapping 187Os/188Os of 0.141–0.149. We show that the source of the adakites is very unlikely to comprise significant eclogite-derived Os, which suggests no or minimal mobilization of Os during eclogite melting. Eclogitic Os is inferred to be retained in sulphides or replacement phases formed upon sulphide breakdown for which Os has high affinity, such as a platinum-group minerals (PGMs). The small Os budget of the adakites is most likely derived from limited reaction with the mantle wedge during ascent. Degassing has reduced Re contents in most samples, but not for end-member adakites (SiO2 > 67% and Sr/Y > 200; n = 4) that were erupted at seafloor depths > 2500 m. These undegassed samples have elevated Re concentrations (0.8–1.5 ppb) that are positively correlated with Sr/Y and so are interpreted to be primary magmatic concentrations resulting from the mobilization of Re from the slab. Re could either be derived from the eclogites or from the serpentinite-derived fluids fluxing eclogites during melting. The former scenario would produce recycled residual crusts with lower Re/Os than in unmelted eclogites while the latter would result in Re/Os ranging from similar to higher than prior to melting. In both cases, the Re/Os and therefore the time-integrated 187Os/188Os of residual crust produced at warm subduction zones involving slab melting are likely to be different from that processed at cooler typical modern subduction zones. Therefore, if slab melting was an important process during the early Earth, the use of Re and Os partitioning in modern subduction zones to model the source of magmas comprising old recycled oceanic crust, such as the HIMU (high μ = 238U/204Pb) ocean island basalts (OIBs), might lead to erroneous interpretations.