MANTLE MATERIALS AND CONSTRAINTS ON THE DEEP ANCIENT CARBON CYCLE

近年来，科学家们一直致力于了解碳循环，从地球内部的二氧化碳脱气和大气二氧化碳的动力学，到碳质沉积物的埋藏。沉积在海底的碳酸盐和其他含碳沉积物通过俯冲作用进入地幔。现代俯冲可以通过各种地球物理、遥感和地球动力学方法进行监测，但6亿年前的俯冲和古老的碳循环是很难探索的。在150公里以下，碳质岩石中的碳转化为钻石，钻石可以固定和储存碳长达35亿年。在极少数情况下，碳可以以碳酸盐流体的形式从地幔中逸出，并再次加入浅层碳循环。尽管进行了数十年的研究，但人们对碳循环最深处的部分知之甚少，地幔中含钻石岩石、钻石和碳酸盐熔体的行为和形成也没有得到很好的约束。这阻碍了我们对地球表面的碳通量的理解，这是缓解全球变暖所必需的。对我们来说，更重要的是了解碳通量随时间的演变。只有当我们确定循环中自然的全球变化的规模时，才能分离和评估碳通量的人为成分。这项拟议的研究提供了关于碳循环中最古老、最深(150-2900公里)部分的数据。这项研究将基于对非常稀有和具有科学价值的样品进行详细的岩石学、地球化学和矿物学检查。获取这些用于研究的标本极具挑战性，因为它们只能从花费数百万美元钻探和加工的钻石勘探公司那里获得。多年来与工业合作伙伴的合作使我能够从全球不同的钻石矿山收集到许多套钻石、地幔包体和金伯利岩。我将研究最近开放的非洲和巴西矿山的钻石收藏，以及将钻石带到地表的火山岩(金伯利岩)，以及这些火山中夹带的深层含钻石岩石的固体碎片(榴辉岩包体)。该研究计划将解决以下突出问题：1)榴辉岩是如何分布在古代大陆以下的深部内陆的，这告诉了我们关于古代俯冲的什么；2)什么样的深层流体形成了钻石，并为150公里以下的碳提供了储藏；3)大自然制造最有价值、最透明、最大的宝石级钻石的“配方”；4)阻止火山二氧化碳从金伯利岩浆逃逸到大气中的独特火山过程。总体而言，拟议的研究将限制古代俯冲对地球早期(3-0.6Gy前)深层碳循环的贡献，并表征导致碳储存和/或碳从地幔逃逸的过程。

In recent years scientists have focused their efforts on understanding the carbon cycle, from the CO2 degassing of the Earth’s interior and the dynamics of atmospheric CO2, to the burial of carbonaceous sediments. Carbonates and other carbon-bearing sediments deposited on the ocean floor are transported into the mantle by subduction. Modern subduction can be monitored by various geophysical, remote sensing and geodynamical methods, but the subduction older than 600 million years and the ancient carbon cycle are very difficult to explore. Below 150 km, carbon from carbonaceous rocks converts to diamond, which can fix and store the carbon for as long as 3.5 billion years. Under rare conditions carbon can escape from the mantle in the form of carbonatitic fluids and join the shallow carbon cycle again. Despite decades of research these deepest parts of the carbon cycle are the least understood, and the behavior and formation of diamondiferous rocks, diamonds and carbonatitic melts in the mantle are poorly constrained. This impedes our understanding of the carbon fluxes to and from the surface of the Earth, which is required to mitigate global warming. Even more important to us is to know the evolution of carbon fluxes over time. The man-made component of carbon fluxes can be isolated and assessed only if we determine the scale of the natural global change in the cycle. The proposed research provides data on this most ancient and deepest (150-2900 km) part of the carbon cycle. The research will be based on detailed petrographic, geochemical and mineralogical examination of very rare and scientifically valuable samples. Acquisition of these specimens for research is extremely challenging, as they can only be solicited from diamond exploration companies that spend millions of dollars on drilling and processing. Years of collaborations with industrial partners allowed me to collect many suites of diamonds, mantle xenoliths and kimberlites from various diamond mines globally. I will study diamond collections from recently opened African and Brazilian mines, as well as volcanic rocks that carry diamonds to the surface (kimberlites) and solid fragments of deep-seated diamondiferous rocks entrained in these volcanoes (eclogite xenoliths). The research program will address the following outstanding problems: 1) How eclogites are distributed in the deep interior below ancient continents and what this tells us about the ancient subduction; 2) What type of deep-seated fluids make diamond and provide depositories for carbon below 150 km; 3) Nature’s “recipe” for making the most valuable, clear and large gem-quality diamonds; 4) Distinct volcanic processes that prevent an escape of volcanic CO2 from the kimberlite magma into the atmosphere. Overall, the proposed research will constrain the contribution of ancient subduction to the deep carbon cycle in the early Earth, 3 - 0.6 Gy ago, and characterize the processes that lead to carbon storage and/or carbon escape from the mantle.