MANTLE MATERIALS AND CONSTRAINTS ON THE DEEP ANCIENT CARBON CYCLE

近年来，科学家们把精力集中在了解碳循环上，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6亿年以上的俯冲和古碳循环非常难以探测。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，碳可以以碳酸流体的形式从地幔中逸出，并再次加入浅层碳循环。尽管经过了几十年的研究，这些最深层的碳循环部分仍然是最不为人所知的，而且对地幔中含金刚石岩石、钻石和碳酸盐岩熔体的行为和形成也知之甚少。这阻碍了我们了解进出地球表面的碳通量，而这是减缓全球变暖所必需的。对我们来说更重要的是了解碳通量随时间的演变。只有当我们确定了循环中全球自然变化的规模时，才能分离和评估碳通量的人为成分。拟议的研究提供了这个最古老、最深（150-2900公里）的碳循环部分的数据。这项研究将基于对非常罕见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。获得这些样本用于研究是极具挑战性的，因为它们只能从花费数百万美元进行钻探和加工的钻石勘探公司获得。多年来与工业伙伴的合作使我能够从全球各个钻石矿收集许多套钻石、地幔捕虏体和金伯利岩。我将研究最近开采的非洲和巴西矿山的钻石收藏，以及携带钻石到地表的火山岩（金伯利岩）和这些火山携带的深部含金刚石岩石的固体碎片（榴辉岩包体）。研究项目将解决以下突出问题：1)榴辉岩在古大陆下深部的分布规律及其对古俯冲作用的启示；2)什么类型的深层流体形成钻石并为150公里以下的碳提供储存库；3)大自然的“配方”，制造出最珍贵、最清澈、最大型的宝石级钻石；4)不同的火山过程阻止了金伯利岩岩浆中的火山二氧化碳逸出到大气中。总的来说，这项研究将限制古俯冲对地球早期（3 - 0.6 Gy以前）深部碳循环的贡献，并描述导致碳储存和/或碳从地幔逸出的过程。

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.