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

我们今天所知的地球经过45亿多年的演化，经历了吸积、灾难性撞击、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是在岩石中，但几乎所有的岩石都在我们动态的地球中被回收。一个例外是海洋岛屿（例如夏威夷、加拉帕戈斯），它们是由地核-地幔边界的隆起（地幔柱）形成的，代表了很久以前地球表面条件的物质。但是多久以前？这一点一般还不清楚。虽然火山岩的年龄可以测量，但火山岩的来源的年龄却无法测量。最近开发的解决这个问题的工具着眼于硫。大约在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。在这24.5亿年之前和之后，与大气接触的硫的成分是非常不同的。因此，可以确定硫在24.5亿年前或之后是否存在于地球表面。先前的研究发现，24.5亿年前来自地球表面的硫迁移到了3000公里深的地核-地幔边界，并在岩浆中返回到地表。这是一个很好的例子，我们星球的表面如何塑造它的内部一直到核心。在该项目中，硫将首次用于调查约130 Ma前形成覆盖巴西大部分地区的洪水玄武岩的主要羽流系统之一，随后是标志着板块通过固定地幔羽流的火山线。该项目将把这一主要羽流系统来源的年龄限制在24.5亿年以下。这也将有助于我们理解超过24.5亿年的物质在地幔柱形成中的体积重要性。令研究人员惊讶的是，地幔柱的来源可以如此古老。令人兴奋的是，看看火山爆发，并知道今天的熔岩形式在地球表面，当时还没有更高的生命形式存在，一直旅行到地核-地幔边界，并在地球历史的一半以上回来。通过俯冲和最终上升的地幔柱的表面硫循环的持久性已被证明为两个较小的南太平洋羽流（皮特凯恩岛和库克-南极的Mangaia岛）与玄武岩，包括硫的年龄超过约2.45 Ga。然而，到目前为止，尚不清楚这种古老的表面物质是羽流源中常见的还是罕见的成分。在该项目中，首次测量了与一个主要的长寿命羽流系统有关的岩石中的硫同位素，该系统是南大西洋的特里斯坦-戈夫羽流系统，具有最初的洪水玄武岩省，巴拉那，以及与非洲大的低剪切波速度省有关的核幔边界处的根。本项目通过对特里斯坦-戈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：1。特里斯坦-高夫地幔柱中的回收成分是太古代还是更年轻？先前的工作表明，高夫签名至少持续了100 Ma，因此太古代的年龄将表明，一个主要的羽流源的显着体积包含旧的回收表面材料。2.太古代硫的存在或缺失与大洋玄武岩端元Sr-Nd-Hf-Pb同位素组成有何关系？皮特凯恩和特里斯坦-高夫都有丰富的地幔1端元成分。硫同位素是否支持由相似年龄的相似地球化学分馏事件形成，或者，不同的演化路径导致相似的组成？3.关于大气、深海热液甚至微生物沉积物成分的贡献，硫同位素意味着什么？该奖项反映了NSF的法定使命，并被认为是值得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的支持。

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.