上地幔流体的性质和作用——从女山幔源二辉橄榄岩捕虏体获得的证据

THE NATURE AND ROLE OF FLUIDS IN THE UPPER MANTLE EVIDENCE IN MANTLE-DERIVD IHERZOLITE XENOLITHS FROM NUSHAN,CHINA

玄武岩浆从地幔深处挟带上来的超镁铁质捕虏体含有地幔流体的直接证据,它们对于阐明地幔深部地质作用(部分熔融作用、地幔交代作用)的机制十分重要。女山幔源二辉橄榄岩的研究结果表明,初始的地幔流体除富含CO_2外,尚含有H_2O和少量CO、CH_4、SO_2、C1、F。地幔中的初始流体(挥发组分)在源部是溶解于地幔橄揽岩的高压固体矿物相中,它们在地幔上隆减压时出溶,形成细小的最早期流体包裹体。这些自由流体相在一定部应聚集,使得地幔固相线(和液相线)下降,引起上地幔发生减压部分熔融。初始的部分熔融主要发生于矿物边界,以单斜辉石最易遭受,形成矿物边缘的海绵状部分熔融带。当地幔剪切流动时,初始的部分熔融体珠滴联合成大的珠滴,并开始渗透,在变形橄榄岩中相连成脉状网络,晚期含C0_2岩浆包裹体及填隙玻璃即为其代表。由于剪裂作用,熔体(和流体)压裂开上覆橄榄岩,产生岩墙状通道,并在上地幔的某些低压区聚集成岩浆库。流体和熔体均为地幔交代作用的介质。在流体参与下,上地幔低度部分熔融产生的熔体将高度富集不相容元素。这些熔体和流体与亏损地幔间的相互作用,可以带来不相容元素重大的局部富集。当流体(和熔体)中H_2O浓度低时,仅发生隐交代作用;当H_2O浓度较高时,部分熔融程度也较高,相应有含水相(如角闪石等)成核,出现矿物交代作用。

Ultramafic xenoliths, entrained by basaltic magmas from mantle depth, contain direct evidence of mantle fluids which are of crucial importance in interpreting the mechanism of geological processes (partial melting, mantle metasomatism) in mantle depth. The results of study on the mantle-derived lherzolites from indicate that besides CO_2, there are H_2O and small amounts of CO, CH_4, SO_2, Cl and F. The initial fluids (volatiles) at mantle depth may be dissolved in high-pressure solid mineral phases of mantle peridotite, and they may be exsolved under the decompressive conditions of mantle up welling and produce the minute earliest fluid inclusions. These free fluid phases accumulate in some regions of mantle, and they result in lowering of mantle solidus (and liquidus) and may initiate decompressively partial melting in the upper mantle. The initial partial melting occurs mainly at mineral grain boundaries, the clinopyroxene is more easily undergone partial melting, and forms the spongy partial melting zones in the rims of mineral crystals. During shear flow of the mantle the initial partial melt drops might coalesce and form larger drops that begin to permeate and form a network of connected melt veins in the deforming peridotite, which is represented by the younger CO_2-magmatic inclusions and interstitial glasses. Due to shear fracturing, the melts (and fluids) pressure and fracture the overlying peridotite, creating a dykelike conduit, and flow towards a low pressure region in the mantle where a magma reservoir might form gradually. Fluids and melts are all the metasomatic agents for mantle metasomatism. Melts formed by small degrees of fluid-assisted partial melting of up-per mantlt will be highly rich in incompatible elements. Interaction between these melts, fluids and depleted mantle can bring about substantial local enrichments. When the concentration of H_2O in fluids (and melts) is lower, there occurs only the cryptic metasomatism; When the conentration of H_2O is higher, the degrees of partial melting are higher and hydrous metasomatic phases (e.g, amphibole)nucleate, and the modal metasomatism occurs.