Chemistry of serpentinized peridotites from ODP Hole 109-670A

In this paper we describe textural relationships in hydrated upper mantle peridotites emplaced at a nonconstructive ridge segment. Development of serpentinites and partially serpentinized peridotites takes place in four main stages: (1) pervasive serpentinization forming mainly lizardite, (2) a tensional stage forming chrysotile + talc + chlorite, (3) a deformational stage forming antigorite + tremolite, and (4) a late local tensional stage forming another generation of chrysotile veinlets. Mineral chemistry of serpentine pseudomorphs reflects in part primary mineral compositions. Olivine pseudomorphs are typically nickeliferous and depleted in aluminum and chromium. Orthopyroxene pseudomorphs have lower nickel contents and relatively high iron, aluminum, and chromium contents. Clinopyroxene pseudomorphs have very low nickel contents and relatively high aluminum and chromium contents. These chemical patterns in the serpentinites can be used to help discriminate between harzburgitic and lherzolitic protoliths. Oxygen isotopes and mineral parageneses suggest serpentine is derived from circulation of hydrothermal (200 infinity C) fluids through the peridotite body. Crystallization of tremolite, talc, and chlorite may have occurred at temperatures up to 525 infinity C if C02/H20 ratios were less than 0.25. Open fissures developing in aging upper mantle provide paths for important seawater circulation through a thin basaltic carapace down to shallow mantle rocks.

本文阐述了侵位于非增生脊段的含水上地幔橄榄岩（hydrated upper mantle peridotites）的结构组构关系。蛇纹岩（serpentinites）与部分蛇纹石化橄榄岩的形成可划分为四个主要阶段：（1）普遍蛇纹石化作用，主要形成利蛇纹石（lizardite）；（2）张性阶段，形成纤蛇纹石（chrysotile）、滑石（talc）与绿泥石（chlorite）；（3）变形阶段，形成叶蛇纹石（antigorite）与透闪石（tremolite）；（4）晚期局部张性阶段，形成新一代纤蛇纹石细脉。
蛇纹石假象矿物的矿物化学特征在一定程度上反映了原岩的矿物组成：橄榄石假象矿物通常含镍且贫铝、铬；斜方辉石（orthopyroxene）假象矿物镍含量较低，铁、铝、铬含量相对较高；单斜辉石（clinopyroxene）假象矿物镍含量极低，铝、铬含量相对较高。
蛇纹岩中的上述化学特征可用于区分方辉橄榄岩（harzburgitic）与二辉橄榄岩（lherzolitic）原岩。氧同位素与矿物共生组合表明，蛇纹石由200℃的热液流体（hydrothermal fluids）循环穿过橄榄岩体形成。若CO₂/H₂O比值小于0.25，透闪石、滑石与绿泥石的结晶温度可达525℃。老化上地幔中发育的开放裂隙为海水大规模循环提供了通道，使海水穿过薄玄武岩壳（basaltic carapace）抵达浅部地幔岩石。