Collaborative Research: Using Sulfur Isotopes to Identify Subducted Archean Crust in Modern Oceanic Hotspot Lavas

现代板块构造的一个结果是，俯冲的海洋板块将海洋地壳和沉积物输送到地幔中。然而，人们对地幔中的俯冲包-洋壳和沉积物-的命运知之甚少。一个长期存在的假说认为，俯冲物质在地幔中残留了一段时间，但时间未知，然后在地幔上涌的区域再循环回到地球表面，并在热点下熔化。如果这一假设是正确的，在热点爆发的洋岛玄武岩（OIB）应该表现出与地壳原岩，在过去的地质俯冲带注入地幔的地球化学特征。然而，它一直难以明确地检测地球化学特征的古代俯冲物质在热点熔岩。这项建议将利用两种互补技术测量关键热点熔岩中的质量独立分馏硫同位素特征，以追踪地壳从地表到地幔再到地幔的循环，最近在Mangaia的2 000万年前熔岩中的橄榄石斑晶和橄榄石中的硫化物中发现了质量独立分馏硫，Mangaia是典型的HIMU（高238 U/204 Pb）地点。广泛的陆地MIF-S同位素特征仅通过大气光化学反应产生，直到~2.45 Ga，因此在年轻热点熔岩中发现MIF-S同位素特征似乎为保存俯冲太古代表面物质提供了“时间戳”和“签名”。这项提议是一项为期一年的试点研究，目的是对OIB中发现的另外两个地幔端元EM 1（皮特凯恩熔岩）和EM 2（萨摩亚）的熔岩中的橄榄石硫化物和斑晶进行S同位素研究，以确定这些熔岩是否也含有太古代再循环成分。 我们还将针对Loihi和萨摩亚火山玻璃中的大型硫化物球，通过离子探针和标准硫提取和同位素比质谱（IRMS）技术进行分析，以进一步建立两种分析技术之间的直接比较。 将对深海橄榄岩硫化物进行调查，以确定硫同位素分析所需的足够大小的原生地幔硫化物，如果发现，将通过离子探针和（如果足够大）硫提取和IRMS测量硫化物。分析活动将使我们能够解决是否海洋热点一般表现出证据的太古代原岩回收。这一提议为再循环假说提供了前所未有的检验。

A consequence of modern plate tectonics is that subducting ocean plates transport oceanic crust and sediment into the mantle. However, the fate of the subducted package--oceanic crust and sediment--in the mantle is poorly understood. A long-standing hypothesis maintains that subducted materials residue in the mantle for an extended, but unknown, period of time and are then recycled back to the Earth's surface in regions of buoyantly upwelling mantle and melted beneath hotspots. If this hypothesis is correct, ocean island basalts (OIB) erupted at hotspots should exhibit geochemical signatures associated with the crustal protoliths that were injected into the mantle at a subduction zone in the geologic past. However, it has been difficult to unequivocally detect geochemical signatures of ancient subducted materials in hotspot lavas. This proposal will use measurements of mass independently fractionated sulfur (MIF-S) isotope signatures--made using two complementary techniques--in key hotspot lavas to trace crustal cycling from the surface, through the mantle and back again.MIF-S were recently discovered in olivine phenocrysts and olivine-hosted sulfides in 20 million-year-old lavas from Mangaia, the archetypal HIMU (high 238U/204Pb) locality. Widespread terrestrial MIF-S isotope signatures were generated exclusively through atmospheric photochemical reactions until ~2.45 Ga, and the discovery of MIF-S isotope signatures in young hotspot lavas therefore appears to provide a "timestamp" and "signature" for preservation of subducted Archean surface materials in the mantle sourcing OIB. This proposal is a 1-year pilot study that aims to perform a S-isotope study on olivine-hosted sulfides and phenocrysts in lavas from the other two mantle endmembers found in OIB, EM1 (in Pitcairn lavas) and EM2 (Samoa), to determine whether these lavas also host an Archean recycled component. We will also target large sulfides globules hosted in volcanic glass from Loihi and Samoa for analysis by both ion probe and standard sulfur extraction and isotope ratio mass spectrometry (IRMS) techniques, to further establish a direct comparison between the two analytical techniques. A survey of abyssal peridotite sulfides will be undertaken to identify primary mantle sulfides of sufficient size for sulfur isotopic analyses, and if found, the sulfides will be measured by ion probe and (if large enough) by sulfur extraction and IRMS. The analytical campaign will allow us address whether oceanic hotspots in general exhibit evidence for recycling of Archean protoliths. This proposal offers an unprecedented test of the recycling hypothesis.