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，大洋板块的俯冲将海洋地壳和沉积物运送到地幔中。然而，人们对地幔中俯冲包裹体——海洋地壳和沉积物——的命运知之甚少。一个长期存在的假设认为，俯冲的物质在地幔中残留了很长一段时间，但未知，然后在浮力上涌的地幔区域循环回到地球表面，并在热点地区融化。如果这一假设是正确的，那么在热点地区喷发的洋岛玄武岩（OIB）应该显示出与地质过去俯冲带注入地幔的地壳原岩相关的地球化学特征。然而，对热点熔岩中古俯冲物质的地球化学特征的明确探测一直存在困难。该提案将使用质量独立分馏硫（MIF-S）同位素特征的测量——使用两种互补的技术——在关键的热点熔岩中追踪地壳从表面经过地幔再返回的循环。MIF-S是最近在HIMU（高238U/204Pb）的典型地区Mangaia的2000万年火山岩中的橄榄石斑晶和橄榄石硫化物中发现的。在~2.45 Ga之前，广泛存在的陆地MIF-S同位素特征完全是通过大气光化学反应产生的，因此在年轻热点熔岩中发现的MIF-S同位素特征似乎为地幔源OIB中俯冲太古代表面物质的保存提供了“时间戳”和“特征”。该提案是一项为期1年的试点研究，旨在对OIB中EM1（皮特凯恩熔岩）和EM2（萨摩亚）的其他两个地幔端段的熔岩中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宙再循环成分。我们还将针对Loihi和萨摩亚火山玻璃中的大型硫化物球进行离子探针和标准硫萃取及同位素比质谱（IRMS）技术的分析，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，以确定足以进行硫同位素分析的原生地幔硫化物，如果发现，将用离子探针测量硫化物，如果足够大，将用硫提取和IRMS进行测量。分析活动将使我们能够解决海洋热点是否普遍显示太古宙原岩再循环的证据。这一提议对回收假设进行了前所未有的检验。

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.