The origin of microdiamonds in ultrahigh pressure metamorphic (UHPM) terranes recrystallized at subduction-zone depths≥100 km is a significant geological question, because it relates to the ways in which crustal carbon is recycled in the Earth. The most widely accepted hypothesis on the origin of metamorphic diamonds is crystallization from mobile C-bearing C-O-H fluids or melts (cf., Dobrzhinetskaya, 2012), although solid-state transformation from graphite has also been proposed. Recently, microdiamonds have been described in UHP ocean floor metasediments in the Lago di Cignana Unit (LCU) in the western Alps (Frezzotti et al., 2011). This new diamond discovery has several characteristics that are well suited to study carbon transport and redox gradients during deep subduction (Frezzotti et al., 2014): (a) host metamorphic rocks are of oceanic rather than continental affinity, (b) diamond formation occurred at low T (about 600°C) and at P consistent with subarc depths (≥ 3.2 Gpa), and ? diamond precipitated from an oxidized, carbonate-bearing H_2O-rich fluid buffered by mineral equilibria.
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机译：在俯冲区深度≥100公里的超高压变质（UHPM）地区的Microdimonds的起源≥100公里是一个重要的地质问题，因为它涉及地壳在地球上再循环的方式。在变质钻石起源上最广泛接受的假设是从携带的含有C-O-H流体或熔体结晶（CF.，Dobrzhinetskaya，2012），但也提出了来自石墨的固态变换。最近，Microdiamonds已在西阿尔卑斯山（FrezzottiTi等，2011）中的Lago Di Cignana（LCU）中的UHP海底MetaseDiment中描述了Microdiamonds。这个新的钻石发现有几种特征，非常适合在深俯口期间研究碳运输和氧化还原梯度（Frezzottti等，2014）：（a）宿主变质岩石是海洋而不是欧洲亲和力，（b）发生钻石形成在低T（约600°C）和与子宫颈深度（≥3.2GPa）一致的P处，以及？金刚石从氧化碳酸化的含碳酸盐的H_2O的富含矿物质平衡缓冲的富含金刚石。
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