CuFeNiS mineral assemblages in upper-mantle peridotites from the Table Mountain and Blow-Me-Down Mountain ophiolite massifs (Bay of Islands area, Newfoundland): Their relationships with fluids and silicate melts
CuFeNiS mineral assemblages in upper-mantle peridotites from the Table Mountain and Blow-Me-Down Mountain ophiolite massifs (Bay of Islands area, Newfoundland): Their relationships with fluids and silicate melts
复制标题
桌山和 Blow-Me-Down 山蛇绿岩地块（纽芬兰群岛湾地区）上地幔橄榄岩中的 CuFeNiS 矿物组合：它们与流体和硅酸盐熔体的关系
DOI：
10.1016/0024-4937(87)90024-7
复制
发表时间：
1987
期刊：
影响因子：
3.5
通讯作者：
J. Lorand
中科院分区：
文献类型：
--
作者：
J. Lorand
桌山和 Blow-Me-Down 山蛇绿岩中的塑性变形超镁铁质岩石包括轻度贫化的二辉橄榄岩的基底单元、强烈贫化的方辉橄榄岩的中间层序和上部的纯粒岩区，也称为被玄武岩熔体或岩浆流体强烈渗滤的过渡区。对包括所有上述岩石类型的 35 个样品的 100 个抛光薄片进行了 CuFeNiS 矿物组合的研究。大多数含有微量的CuFeNi硫化物、原生金属和局部砷化镍。不透明组合的成分特征及其在岩石中的结构位置表明，目前的CuFeNiS矿物是通过低至100°C的广泛的固相线下再平衡而衍生自上地幔硫化物成分。由于封闭系统中的亚固相线再平衡，原始成分（主要是镍黄铁矿、少量磁黄铁矿和黄铜矿）以硫化物包裹体的形式保留在过渡带的铬铁矿中。相反，硅酸盐和尖晶石填隙的硫化物组合与还原性蛇纹石化流体广泛反应，产生缺硫硫化物，例如黑硫铜矿和马基纳矿以及天然金属（天然铜和细铜矿）。氧化还原条件的微观变化和蛇纹石化过程中硅酸盐中铁的去除可能解释了粒间组合的特殊“逐粒”平衡状态。尽管存在低温蚀变，但从基底二辉橄榄岩到中间方辉橄榄岩，硫化物成分逐渐减少，而硫化物含量在过渡带逐渐增加（按体积计可达0.2%）。第一种模式与地幔熔融过程中硫化物成分的低熔点性质一致。微观结构标准，例如橄榄石新生细胞中不存在硫化物包裹体，表明硫化物成分晚于过渡区的塑性变形。因此，硫化物富集模式归因于硫饱和的玄武岩岩浆渗透到残余的纯花岗岩中。
Plastically deformed ultramafic rocks in the Table Mountain and Blow-Me-Down Mountain ophiolites comprise a basal unit of slightly depleted Iherzolites, an intermediate sequence of strongly depleted harzburgites and an upper zone of dunites, also referred to as a transition zone intensively percolated by basaltic melts or magmatic fluids. Thirty-five samples including all of the above rock types have been investigated on 100 polished thin sections for CuFeNiS mineral assemblages. Most of them contain traces of CuFeNi sulfides, native metals and locally Ni arsenide. Compositional features of opaque assemblages as well as their textural sites in the rocks indicate that the present CuFeNiS minerals derive from an upper-mantle sulfide component through extensive subsolidus re-equilibration down to 100°C. The primitive component (predominant pentlandite, minor pyrrhotite and chalcopyrite) is preserved as sulfide inclusions in chromites of the transition zone, due to a subsolidus re-equilibration in a closed system. On the contrary, sulfide assemblages interstitial to silicates and spinel have extensively reacted with reducing serpentinizing fluids to produce sulfur-deficient sulfides such as heazlewoodite and mackinawite and native metals (native copper and awaruite). Microscale variations of redox conditions and the removal of Fe from the silicate during serpentinization may account for the peculiar “grain-by-grain” equilibrium state of intergranular assemblages. In spite of low-temperature alteration, a gradual depletion in sulfide component has been recognized from the basal lherzolites to the intermediate harzburgites while the sulfide content gradually increases in the transition zone (up to 0.2% by volume). The first pattern is consistent with the low-melting nature of the sulfide component in mantle melting processes. Microstructural criteria such as the absence of sulfide inclusions in olivine neoblasts demonstrate that the sulfide component postdates plastic deformation of the transition zone. The sulfide-enrichment pattern is thus ascribed to the percolation of a sulfur-saturated basaltic magma into residual dunites.