Subduction Zone Metamorphic Pathway in Deep-Earth Carbon Cycling

研究地表碳进入地球深处的过程是理解现代挥发性物质循环以及地球长期脱气和大气演化的关键。尽管俯冲作用被认为是碳返回地幔的主要途径，但人们对这一过程的效率以及这种深度俯冲的碳的化学形式(假设为碳酸盐和还原碳，后者代表有机物)知之甚少。随着地质时间的推移，深度俯冲的碳通过火山喷发返回大气的变化可能会导致大气中二氧化碳含量的显著变化，从而改变全球表面温度。人为(人类)向大气中增加的二氧化碳，主要是由于燃烧化石燃料，叠加在这些较长时间段的大气二氧化碳浓度的“自然”变化上，部分与俯冲带输入和火山输出有关。这项研究将通过对高压和超高压变质岩的实地研究来研究碳的俯冲途径，为最近的碳循环理论和实验研究提供有价值的野外“地面真相”。这项研究将是更充分地表征关键岩性(沉积、镁铁质和超镁铁质岩石)中从弧前到弧下的变质碳通量的第一步，人们认为这些岩性将碳输送到火山前缘以下的深处或更深处。工作将主要集中在法国和意大利阿尔卑斯山的某些地方，以便研究在各种高压/超高压条件下变质的蛇绿岩和相关海底沉积物。对于每一种岩性和地点，现场观察、岩石学研究和稳定同位素分析将集中于识别任何经历过的脱碳作用，将这些从岩石研究中观察到的结果与脱挥发历史的理论评估联系起来。

Study of the processes by which surface carbon is transported into the deep Earth is key to understanding modern volatiles recycling and longer-term Earth degassing and atmosphere evolution. Although subduction is considered the primary means by which carbon is returned to the mantle, little is known regarding the efficiency of this process and the chemical forms of this deeply subducted carbon (presumably as carbonate and as reduced carbon, the latter representing organic matter). Through geologic time, changes in the return of deeply subducted carbon to the atmosphere, via volcanic eruptions, could have led to significant change in the carbon dioxide content of the atmosphere and thus change in global surface temperatures. Anthropogenic (human) carbon dioxide additions to the atmosphere, largely due to burning of fossil fuels, are superimposed on these longer-timeframe, "natural" variations in atmospheric carbon dioxide concentration related in part to subduction zone inputs and volcanic outputs.This research will examine the subduction pathway for carbon via field studies of high- and ultrahigh-pressure metamorphic rocks, providing valuable field-based "ground truthing" of recent theoretical and experimental studies of carbon cycling. This study will represent a first step in more fully characterizing the forearc-to-subarc metamorphic carbon flux in key lithologies (sedimentary, mafic, and ultramafic rocks) thought to convey carbon to depths beneath volcanic fronts and greater. Work will mostly be concentrated on localities in the French and Italian Alps allowing examination of ophiolitic rocks and related seafloor sediments metamorphosed over a wide range of high-pressure (HP)/ultrahigh-pressure (UHP) conditions. For each lithology and locality, field observations, petrologic study, and stable isotope analyses will focus on identifying any decarbonation that was experienced, relating these observations from study of the rocks with theoretical assessments of devolatilization history.