Onset and Progression of Serpentinization and Magnetite Formation in Olivine-rich Troctolite from IODP Hole U1309D
Onset and Progression of Serpentinization and Magnetite Formation in Olivine-rich Troctolite from IODP Hole U1309D
复制标题
DOI：
10.1093/petrology/egp004
复制
发表时间：
2009-03-01
影响因子：
3.9
通讯作者：
Sharma, Shiv K.
中科院分区：
文献类型：
--
作者：
Beard, James S.;Frost, B. Ronald;Sharma, Shiv K.
综合海洋钻探计划（IODP） U1309D孔227岩心富橄榄石橄榄岩的蛇纹石化范围为10 ~ 90。发现了两段蛇纹岩化。第一种，在弱蛇纹石化的样品中占主导地位，是由蛇纹石（反长辉石，mg值为92）和水镁石（富马镁石，mg值为65）的混合物近似等化学取代橄榄石（Fo(8485)）。1型脉的矿物组成反映了橄榄石与早期蛇纹岩形成的水辉石蛇纹石之间的稀土交换。早期蛇纹岩脉（1型）细（005毫米），不规则，利用橄榄石中已有的裂缝。反长花岗岩的存在表明早蛇纹石化发生在t300c。1型矿脉反映岩石主导的蛇纹石化，在其历史早期就被孤立，并在除最具蚀变的样品外的所有样品中作为遗迹存在。蛇纹岩化的主要阶段表现为2型（平均mg值为96）穿通的蜥蜴辉石磁铁矿脉。2型矿脉定义一个吻合的叶理，宽度可达几毫米，似乎利用了先前存在的有利定向的1型矿脉。2型脉反映开系蛇纹岩化。这些矿脉中的磁铁矿是由水镁石和蛇纹石中的铁氧化形成的，而在体系中加入二氧化硅则将水镁石中的镁成分转化为蛇纹石。磁铁矿在矿脉内部形成一条或多条明显的条带，从不与残余橄榄石直接接触。一种类似于在1型脉中发现的水镁蛇纹石混合物，但含有蜥蜴石而不是反长辉石，通常存在于2型脉的边缘（即它们与残余橄榄石反应接触的地方）。我们将2型矿脉解释为一个稳态系统，其中水镁石在橄榄石接触处不断形成，然后在矿脉内部发生反应。水镁石的不断形成和破坏使该体系的a（SiO2）含量极低。磁铁矿和橄榄石在2型矿脉（或其他任何地方）中从不接触，因为橄榄石-出反应产生铁水辉石而不是磁铁矿。2型矿脉中蛇纹石的脱硅是富铁蛇纹石相对于低硅活性磁铁矿的固有不稳定性的反映。因此，蛇纹石中与磁铁矿平衡的蛇纹石组成是蛇纹石磁铁矿的函数，而不是蛇纹石橄榄石平衡的函数。
Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from 10 to 90. Two episodes of serpentinization are recognized. The first, dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo(8485)) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of FeMg exchange between olivine and the brucite serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin ( 005 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T 300C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A bruciteserpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivinevein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low a(SiO2) on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentinemagnetite and not serpentineolivine equilbria.