Fluids in equilibrium with peridotite minerals: Implications for mantle metasomatism
Fluids in equilibrium with peridotite minerals: Implications for mantle metasomatism
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DOI：
10.1016/0016-7037(86)90347-9
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发表时间：
1986-05
影响因子：
5
通讯作者：
Mark E. Schneider;D. Eggler
中科院分区：
文献类型：
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作者：
Mark E. Schneider;D. Eggler
在15-20 kbar压力和600-1100° C下，测定了氧化物在H2O和H2O-CO2流体中与橄榄岩和几种单矿物平衡时的溶解度。来自H2O流体的溶质富含标准石英和长石，而不仅仅是碱，并且不富集Mg、Fe或Ti。与金云母橄榄岩平衡的流体比与角闪石橄榄岩平衡的流体含有更多的溶质。金云母橄榄岩和角闪石橄榄岩在水溶液中的典型反应分别为：1.05金云母+1.33斜方辉石+8.62水= 9.62水+1.38橄榄石0。59角闪石+1.64斜方辉石+0.05尖晶石+32.33H2O = 33.33流体+1.85橄榄石+0.43透辉石对角闪石橄榄岩和硬玉橄榄岩的实验表明，H2O-CO2流体中的溶质与H2O溶质不同，其摩尔Na Al> 1。以此类推，与金云母橄榄岩相平衡的H2O-CO2溶质比H2O溶质的过铝性更低。这些关系表明流体中存在碱-CO 3络合作用。H2O-CO2流体的溶质含量远低于H2O流体，即使在低CO2-H2O比下也是如此。在深度超过约70公里的亚固体大陆岩石圈中，与橄榄岩围岩平衡的流体受到相平衡的约束，富含H2 O，并与金云母和碳酸盐共存。向上运动的溶质欠饱和含水流体可能会沥滤岩石圈的这一部分;在主要元素中，K的相对损耗最大。流体必须保持饱和沉淀反应与围岩，然而，因此大的流体/岩石比地幔交代在深度低于70公里。熔体是更有效的交代剂，在这个意义上，只需要小比例的结晶熔体/岩石。向上流经岩石圈金云母-碳酸盐岩区的含水流体将在浅于约70 km的深度通过橄榄岩围岩的角闪岩化-碳酸盐化作用沉淀。该过程将添加少量的主要元素，特别是K，相对于所产生的角闪石（约为1 K2 O至3700角闪石）。然而，随着时间的推移，该过程可能会产生一个交代带，代表碱性岩浆作用的可能来源。
Solubilities of oxides in H 2 O and H 2 O-CO 2 fluids in equilibrium with peridotites and with several single minerals have been determined at 15–20 kbar pressure and 600–1100° C. Solutes from H 2 O fluids are rich in normative quartz and feldspars, rather than in alkalies alone, and are not enriched in Mg, Fe, or Ti. Fluids in equilibrium with phlogopite peridotite contain much more solute than fluids in equilibrium with amphibole peridotite. Typical reactions for solution of phlogopite peridotite and amphibole peridotite in H 2 O fluid are, respectively, in weight proportions: 1.05 phlogopite+ 1.33 orthopyroxene+ 8.62 H 2 O= 9.62 fluid+ 1.38 olivine0. 59 amphibole+ 1.64 orthopyroxene+ 0.05 spinel+ 32.33 H 2 O= 33.33 fluid+ 1.85 olivine+ 0.43 diopside Experiments on amphibole peridotite and on Jadeite peridotite indicate that solutes in H 2 O-CO 2 fluids have molar Na Al> 1, unlike H 2 O solutes. By analogy, H 2 O-CO 2 solute in equilibrium with phlogopite peridotite is less peraluminous than H 2 O solutes. These relations suggest alkali-CO 3 complexing in fluids. Solute contents of H 2 O-CO 2 fluids are far less than of H 2 O fluids, however, even for low CO 2 H 2 O ratios. Fluids that equilibrate with peridotitic wallrocks in subsolidus continental lithosphere, at depths in excess of about 70 km, are constrained by phase equilibria to be H 2 O-rich and to coexist with phlogopite and carbonate. Upward-moving solute-undersaturated hydrous fluids may leach that portion of the lithosphere; the greatest relative depletion, among major elements, will be in K. Fluids must remain saturated during precipitation reaction with wallrocks, however, and hence large fluid/rock ratios are required for mantle metasomatism at depths below 70 km. Melts are more efficient metasomatic agents, in the sense that only small ratios of crystallizing melt/rock are required. Hydrous fluids that flow upward through the phlogopite-carbonate region of the lithosphere will precipitate, at depths shallower than about 70 km, by amphibolitization-carbonatization of peridotite wallrocks. That process will add small amounts of major elements, notably K, relative to amphibole produced (on the order of 1 K 2 O to 3700 amphibole). Nevertheless, integrated over time, the process may produce a metasomatized zone that represents a possible source for alkaline magmatism.