MANTLE MATERIALS AND CONSTRAINTS ON THE DEEP ANCIENT CARBON CYCLE

点击翻译按钮获取中文摘要

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.