Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction
Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction
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DOI：
10.1111/jmg.12130
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发表时间：
2015-06-01
影响因子：
3.4
通讯作者：
Vrabec, M.
中科院分区：
文献类型：
--
作者：
Janak, M.;Froitzheim, N.;Vrabec, M.
本文报道了在东阿尔卑斯山奥斯特拉尔山超高压变质地体中，首次在斯洛文尼亚Pohorje山脉的变质地壳岩石中发现了金刚石和碳硅石。显微观察和拉曼光谱表明，金刚石以包裹体的形式存在于石榴石中，呈不均匀分布。在光学显微镜下，含金刚石包裹体呈立方体至圆形，粉红色、黄色至棕褐色。所研究的金刚石的拉曼光谱显示了一个尖锐的SP(3)键合碳的一级峰，大多数情况下集中在1332到1330 cm(-1)之间，半峰宽在3到5 cm(-1)之间。几个光谱显示了无序石墨化(sp(2)键)碳的典型拉曼光谱。详细的观察表明，金刚石要么以单矿物、单晶包裹体的形式存在，要么与多相包裹体中的碳化硅(碳化硅)、二氧化碳和CH4共生。这种罕见的钻石记录表明，金刚石和碳硅石是在还原条件下从超临界流体中结晶出来的。热力学模拟表明，含金刚石片麻岩获得了3.5 Gpa和800-850℃的温压条件，与榴辉岩和石榴石橄榄岩相似。我们认为，金刚石是在晚白垩世(约95-92 Ma)大陆俯冲过程中，碳质沉积物在地幔深度超过100公里时发生超高压变质作用而形成的。钻石的发现证实了波霍耶山的超高压变质作用，波霍耶山是奥斯特罗拉山单元俯冲最深的部分。
We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh-pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond-bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp(3)-bonded carbon, in most cases centred between 1332 and 1330cm(-1), with a full width at half maximum between 3 and 5cm(-1). Several spectra show Raman bands typical for disordered graphitic (sp(2)-bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single-crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions(.) This rare record of diamond occurring with moissanite as fluid-inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond-bearing gneisses attained P-T conditions of 3.5GPa and 800-850 degrees C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100km during continental subduction in the Late Cretaceous (c. 95-92Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.