Chemistry of minerals of ODP Hole 103-637A peridotites

The Galicia margin lies northwest of the Iberian Peninsula and is a passive ocean margin with thin sedimentary cover. Altered peridotite was recovered from ODP Site 637, on the north-trending ridge at the western edge of the margin, near the oceanic/continental crust boundary.
The altered ultramafics were originally clinopyroxene-rich upper mantle harzburgites and are now extensively serpentinized (>85%) and cut by very late-stage carbonate veins. Despite pervasive late, low-temperature alteration, evidence of early, high-temperature alteration remains. Alteration is apparent as (1) amphibole rims on clinopyroxene (>800°C), (2) hornblende + tremolite (450° to 800°C), (3) breakdown of hornblende to form tremolite + chlorite (<450°C), (4) zoned Cr-spinels, (5) hydration of orthopyroxene and olivine to serpentine, (6) serpentine veins, (7) replacement of pyroxene and olivine by calcite, and (8) calcite veins and vugs.
Both the relict igneous and the high-temperature alteration minerals (amphiboles) show evidence of brittle deformation. Subsequent low-temperature alteration veins and minerals are deformed only in faulted and brecciated zones. This textural evidence suggests that the low-temperature alteration occurred after emplacement of the ultramafics at the surface. Serpentine fills tension fractures in orthopyroxene, and both serpentine and calcite fill tension cracks in olivine.
The high-temperature alterations in these samples are similar to those found in oceanic fracture zone and ophiolite ultramafics. This widespread occurrence of high-temperature alteration suggests that hot fluids were pervasive in these ultramafic blocks.
Localization of high-temperature alteration close to large carbonate veins suggests channelization of the late, low-temperature fluids. Earlier hydrations (e.g., high-temperature alterations and serpentinization) were pervasive.

加利西亚边缘位于伊比利亚半岛西北部，属于沉积盖层较薄的被动大陆边缘。蚀变橄榄岩采自该边缘西缘北倾海脊上的大洋钻探计划（Ocean Drilling Program, ODP）637站位，站位紧邻洋陆壳边界。

这些蚀变超镁铁质岩原属富含单斜辉石的上地幔方辉橄榄岩，现今已发生广泛蛇纹石化（蛇纹石化程度＞85%），并被极晚期碳酸盐脉切穿。尽管普遍经历了晚期低温蚀变，但仍保留有早期高温蚀变的证据。蚀变现象具体表现为：(1) 单斜辉石表面形成角闪石边（形成温度＞800℃）；(2) 出现角闪石+透闪石组合（形成温度450~800℃）；(3) 角闪石分解形成透闪石+绿泥石（形成温度＜450℃）；(4) 出现环带结构铬尖晶石；(5) 斜方辉石与橄榄石发生水合作用转变为蛇纹石；(6) 形成蛇纹石脉；(7) 方解石交代辉石与橄榄石；(8) 形成方解石脉与晶洞。

残留火成矿物与高温蚀变矿物（角闪石）均显示出脆性变形的证据。后续形成的低温蚀变脉体与矿物仅在断裂及角砾岩化带中发生变形。这一结构证据表明，低温蚀变发生于超镁铁质岩侵位至地表之后。蛇纹石充填于斜方辉石的张裂隙中，而蛇纹石与方解石均充填于橄榄石的张裂缝内。

本批样品中的高温蚀变特征与大洋断裂带及蛇绿岩套超镁铁质岩中所见的蚀变特征相似。高温蚀变的广泛分布表明，热流体曾广泛渗透于这些超镁铁质岩块中。

高温蚀变集中分布于大型碳酸盐脉附近，这表明晚期低温流体存在流体通道化现象。早期水合作用（如高温蚀变与蛇纹石化）曾广泛发生。