On multi-phase mineral inclusions associated with microdiamond formation in mantle-derived peridotite lens at Bardane on Fjortoft, west Norway
On multi-phase mineral inclusions associated with microdiamond formation in mantle-derived peridotite lens at Bardane on Fjortoft, west Norway
复制标题
DOI：
10.1127/0935-1221/2005/0017-0031
复制
发表时间：
2005-01-01
影响因子：
2.1
通讯作者：
Van Roermund, HLM
中科院分区：
文献类型：
--
作者：
Carswell, DA;Van Roermund, HLM
本文报道了在挪威西部Fjortoft的Bardane的地幔衍生橄榄岩体中暴露的石榴石韦氏体荚体样品中与微钻石形成相关的显微结构和矿物包裹体的研究。据解释，该荚果最初由正辉石+斜辉石+石榴石+ Cr尖晶石（Cr*约为75）+小橄榄石的巨晶组合组成，可能在中元古代（1651 +/- 47 Ma）地幔底流/羽流中结晶，温度约为1410℃，GPa为3.2 GPa。观察到的微金刚石显然不是这种早期（M-1）共生的一部分，而是在变形引起的流体渗透事件之后形成的。金刚石和其他碳相矿物包裹体很少单独出现，而是与其他大多数微尺度矿物包裹体形成复合聚集体，这些矿物包裹体包括云母、千硅石、Cr尖晶石（Cr*接近55）、菱镁矿、Ba-Mg碳酸盐岩、Fe-Ni硫化物、cl -磷灰石、金红石、锆石和独居石。因此，交代流体显然将其他元素（如K、Ba、Ti、Fe、S、P、Cl、Zr、Cl、Th和Nb）引入了橄榄岩体，除了碳。后向散射电子扫描电镜（SEM）图像表明，引入的矿物包裹体在空间上与M-2代正辉石、斜辉石、石榴石和Cr尖晶石（Cr*接近55）密切相关，它们既存在于变形和高度应变的M-1巨晶中，也存在于物理破碎的M-1巨晶之间，形成颗粒的冕状边界网络。迄今为止，人们发现微金刚石很少作为多相矿物包裹体的组成部分保存下来，这些包裹体被双重包裹在M-2 Cr尖晶石中（Cr*接近55），然后又被包裹在M-2代石榴石中。大多数碳相包裹体现在由无序的石墨组成，人们认为最有可能取代了先前存在的金刚石。根据巴尔丹内溶出M-2矿物组合的Sm-Nd误差年龄（518 +/- 78 Ma），认为M-2矿物组合形成于古生代加里东造山运动时期，与引入的含微金刚石矿物包裹体套件密切相关。这个确定的年龄很可能是M-1组合的表观年龄和M-2变形引发的溶蚀和流体渗透时间的混合年龄。因此，微金刚石的形成被解释为可能发生在加里东造山旋回的加拿大阶段，当时岩石圈地幔碎片被引入波罗的海边缘大陆地壳板块，经历了短暂的深部俯冲和随之而来的超高压榴辉岩相变质作用。将碳和其他元素引入橄榄岩体的超临界流体很可能来自于大陆地壳片麻岩封闭板块内的递进脱水和脱碳反应。微金刚石的形成被认为发生在巴丹橄榄岩中M-2矿物组合形成的P-T条件约为875 +/- 25℃，4.1 +/- 0.2 GPa。
This paper reports a study of the microstructures and mineral inclusions associated with microdiamond-formation in samples of a garnet websterite pod exposed within a mantle-derived peridotite body at Bardane on Fjortoft, western Norway. This pod is interpreted to have originally comprised a megacrystic assemblage of orthopyroxene + clinopyroxene + garnet + Cr-spinel (Cr* approximate to 75) + minor olivine that probably crystallised within a mid-Proterozoic (1651 +/- 47 Ma) mantle diapir/plume at conditions of around 1410 degrees C and 3.2 GPa. The observed microdiamonds are clearly not part of this early (M-1) paragenesis but formed later, following a deformation-induced fluid infiltration event.The diamond and other carbon-phase mineral inclusions rarely occur in isolation, instead in composite aggregates with other, mostly micro-scale, mineral inclusions that include phlogopite, kalsilite, Cr-spinel (Cr* approximate to 55), magnesite, Ba-Mg carbonate, Fe-Ni sulphide, Cl-apatite, rutile, zircon and monazite. Thus a metasomatic fluid has clearly introduced other elements (such as K, Ba, Ti, Fe, S, P, Cl, Zr, Cl, Th, and Nb) into this peridotite body, in addition to carbon.Back-scattered electron SEM images demonstrate that the introduced suite of mineral inclusions is spatially intimately associated with M-2 generation grains of orthopyroxene, clinopyroxene, garnet and Cr-spinel (Cr* approximate to 55) that occur both as internally-ex solved phases within deformed and highly-strained M-1 megacrysts and as a coronitic boundary network of grains precipitated between physically-fragmented M-1 megacrysts.To date the microdiamonds have been found to be only rarely preserved as a component of multi-phase mineral inclusions that are doubly armoured in M-2 Cr-spinel (Cr* approximate to 55) in turn enclosed within M-2 generation garnet. Most carbon phase inclusions are now composed of disordered graphite thought most likely to have replaced pre-existing diamond.The M-2 mineral assemblage, with which the introduced microdiamond-bearing mineral inclusion suite is intimately associated, is considered to have formed during the Palaeozoic Caledonian orogeny on the basis of a Sm-Nd errorchron age of 518 +/- 78 Ma for the internally exsolved M-2 mineral assemblage at Bardane. This determined age is most likely a mixed age between the apparent age of the M-1 assemblage and the time of M-2 deformation-triggered exsolution and fluid infiltration. Thus microdiamond formation is interpreted to have probably occurred during the Scandian phase of the Caledonian orogenic cycle when this fragment of lithospheric mantle was introduced into a slab of Baltica margin continental crust that experienced short-lived deep-level subduction and attendant ultra-high pressure eclogite-facies metamorphism. The supercritical fluid responsible for the introduction of carbon and other elements into this peridotite body most likely originated from prograde dehydration and decarbonation reactions within the enclosing slab of continental crust gneisses. The microdiamond formation is considered to have occurred at the P-T conditions of ca. 875 +/- 25 degrees C and 4.1 +/- 0.2 GPa estimated for the formation of the M-2 mineral assemblage within the Bardane peridotite body.