Sulfur Isotopic Evidence on the Age of Recycled Surface Material in the Tristan-Gough Plume Source

我们今天所知道的地球经过了超过45亿年的进化，经历了增生、灾难性的撞击、海洋地壳和大陆的形成以及板块构造的建立。了解这种进化是非常困难的，因为这种进化的记忆存在于岩石中，但几乎所有的岩石都在我们动态的地球上被循环利用。一个例外是海洋岛屿（如夏威夷，加拉帕戈斯群岛），它们是由核心-地幔边界的上升流（地幔柱）形成的，代表了很久以前地球表面条件的物质。但那是多久以前了？这一点通常还不为人所知。虽然可以测定火山岩的年龄，但不能测定火山岩来源的年龄。最近开发的一种解决这个问题的工具着眼于硫。大约24.5亿年前，当氧气急剧增加时，大气发生了变化，这就是所谓的大氧化事件。在24.5亿年之前和之后，与大气接触的硫的组成是非常不同的。因此，它可以确定硫是在24.5亿年前还是之后在地球表面存在的。先前的研究发现，24.5亿年前地球表面的硫到达了3000公里深的地核-地幔边界，并在岩浆中被带到地表。这是一个很好的例子，我们的星球的表面如何塑造它的内部，一直到核心。在这个项目中，硫将首次用于研究一个主要的羽流系统，该系统在大约130年前形成了覆盖巴西大部分地区的洪水玄武岩，随后是火山线，标志着板块在静止的地幔羽流上的通过。这个项目将把这个主要羽流系统来源的年龄限制在24.5亿年以下或更早。它还将帮助我们了解年龄超过24.5亿年前的重要物质是如何形成地幔柱的。地幔柱的来源可以如此古老，这让研究人员感到惊讶。看着火山喷发，并知道今天的熔岩是什么形式的，在没有更高的生命形式存在的时候，已经在地球表面，一直旅行到地核-地幔边界，并在地球历史的一半以上重新出现，这是令人兴奋的。南太平洋两个较小的地幔柱（皮特凯恩岛和南库克岛的曼盖亚岛）的玄武岩中含有的硫的历史超过2.45亿年，已经证明了地表硫通过俯冲和地幔柱的最终上升而持续存在。然而，到目前为止，尚不清楚这种古老的表面物质是羽流源的常见成分还是稀有成分。在这个项目中，首次测量了与主要的长寿命羽流系统有关的岩石中的硫同位素，南大西洋的特里斯坦-高夫羽流系统，具有最初的洪水玄武岩省Parana，以及与非洲大低横波速度省相关的核心-地幔边界的根。本项目通过对特里斯坦岛和高夫岛火山岩中硫化物包裹体的详细分析，提出了以下主要假设：特里斯坦-高夫羽流中的再循环成分是太古宙时代的还是更年轻的？先前的研究表明，高夫特征至少从100 Ma以来一直存在，因此太古宙时代将表明，一个主要羽源的相当大的体积包含旧的再循环表面物质。2. 太古宙硫的缺失或存在与Sr-Nd-Hf-Pb同位素组成中海洋玄武岩的全球端元有何关系？皮特凯恩和特里斯坦-高夫都有丰富的地幔1端元组成。硫同位素是否支持由相似年龄的类似地球化学分馏事件形成，还是不同的演化路径导致相似的成分？3. 硫同位素对大气、深海热液甚至微生物沉积物成分的贡献意味着什么？该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Earth as we know it today evolved over more than 4.5 billion years through accretion, catastrophic impacts, formation of oceanic crust and continents and establishing plate tectonics. Learning about this evolution is extremely difficult because the memory of this evolution is in rocks but almost all of them have been recycled in our dynamic Earth. An exception are ocean islands (e.g. Hawaii, Galapagos) that are formed by upwellings (mantle plumes) from the core-mantle boundary and represent material that witnessed surface conditions of the Earth a long time ago. But how long ago? This is generally not yet known. Although the age of a volcanic rock can be measured, the age of the source of a volcanic rock cannot. A recently developed tool to address this question looks at sulfur. The atmosphere changed about 2.45 billion years ago when oxygen increased dramatically, the so-called Great Oxygenation Event. The composition of sulfur in contact with the atmosphere before and after this 2.45 billion year mark are very different. Thus it can be determined if sulfur was on the surface of Earth before or after 2.45 billion years ago. Previous studies have found that sulfur from the surface of Earth more than 2.45 billion years ago traveled to the core-mantle boundary some 3000 km deep and back up where it was carried in magmas to the surface. This is an excellent example how the surface of our planet shapes its interior all the way down to the core. In this project sulfur will be used for the first time to investigate one of the major plume systems that have formed flood basalts covering most of Brazil about 130 Ma ago followed by lines of volcanoes marking the passage of the plates over a stationary mantle plume. This project will constrain the age of the source of this major plume system to younger or older than 2.45 billion years. It will also help our understanding how volumetrically significant material older than 2.45 billion year-old material is in the formation of mantle plumes. It has surprised researchers that mantle plume sources can be so old. It is exciting to look at a volcanic eruption and know that what forms lava today had been at the surface of Earth when no higher life forms yet existed, traveled all the way to the core-mantle boundary, and came back up over more than half of Earth's history. The persistence of recycled surface sulfur through subduction and eventual rise in a mantle plume has been demonstrated for two smaller South Pacific plumes (Pitcairn and Mangaia island of Cook-Austral) with basalts including sulfur older than about 2.45 Ga. However, to this point it is not clear whether such old surface material is a common or rare component in plume sources. In this project for the first time sulfur isotopes are measured in rocks related to a major long-lived plume system, the Tristan-Gough plume system in the South Atlantic, with an initial flood basalt province, the Parana, and with a root at the core-mantle boundary related to the African large low shear wave velocity province. This project addresses the following major hypothesis through a detailed analyses of sulfide inclusions in volcanic rocks from Tristan and Gough island: 1. Is the recycled component in the Tristan-Gough plume of Archean age or younger? Previous work showed that the Gough signature persists since at least 100 Ma, thus an Archean age would indicate that a significant volume of a major plume source contains old recycled surface material. 2. How does the absence or presence of Archean sulfur relate to the global endmembers of oceanic basalts in Sr-Nd-Hf-Pb isotopic compositions? Pitcairn and Tristan-Gough both have enriched mantle 1 endmember compositions. Do sulfur isotopes support formation by similar geochemical fractionation events of similar age, or, do different evolution paths lead to similar compositions? 3. What do sulfur isotopes imply regarding contributions from atmospheric, deep ocean hydrothermal or even microbial sediment components?This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.