Subduction Zone Metamorphic Pathway in Deep-Earth Carbon Cycling

研究地表碳被输送到地球深处的过程是理解现代挥发物循环和长期地球脱气和大气演化的关键。虽然俯冲作用被认为是碳返回地幔的主要方式，但人们对这一过程的效率以及这种深俯冲碳的化学形式（可能是碳酸盐和还原性碳，后者代表有机质）知之甚少。在整个地质时期，火山爆发导致的深潜碳返回大气的变化，可能导致大气中二氧化碳含量的显著变化，从而导致全球表面温度的变化。人为（人类）向大气中增加的二氧化碳，主要是由于化石燃料的燃烧，叠加在这些较长时间框架上，部分与俯冲带输入和火山输出有关的大气二氧化碳浓度的“自然”变化。本研究将通过对高压和超高压变质岩的现场研究来研究碳的俯冲路径，为最近的碳循环理论和实验研究提供有价值的现场“地面真相”。这项研究将是更全面地描述关键岩性（沉积岩、基性岩和超基性岩）中弧前到弧下变质碳通量的第一步，这些岩性被认为将碳传递到火山锋下甚至更深处。工作将主要集中在法国和意大利阿尔卑斯山的地方，允许检查蛇绿岩和相关海底沉积物在广泛的高压(HP)/超高压（UHP）条件下变质。对于每个岩性和地点，野外观察、岩石学研究和稳定同位素分析将重点放在确定任何经历过的脱碳，并将这些从岩石研究中得到的观察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.