方解石-橄榄石-流体三相体系氧同位素交换的实验研究  

Experimental studies of oxygen isotope exchange in the system calcite-forsterite-fluid

作　　者：郑永飞[1] M.Satir P.Metz Z.D.Sharp 

机构地区：[1]中国科学技术大学地球和空间科学系,合肥230026 [2]Institute for Mineralogy,Petrology and Geochemistry,University of Tuebingen [3]Institute for Mineralogy and Petrography,University of Lausanne

出　　处：《地球化学》1999年第5期411-420,共10页Geochimica

基　　金：科学技术部攀登计划预选项目(编号:95-预-39);国家自然科学基金(编号:49453003);德国科学基金会(DFG)基金

摘　　要：在６８０ ℃和５００ ＭＰａ 以及岩石学相平衡条件下， 首次对方解石与镁橄榄石之间在大量超临界ＣＯ２Ｈ２ Ｏ 流体存在的条件下进行了氧同位素交换实验。结果发现，在实验刚刚开始的一段时间内氧同位素交换很快，主要是以溶解／ 重结晶的机制进行的；而在以后的时间里氧同位素交换要慢得多， 变成以扩散机制为主。进一步， 反应产物方解石相对于镁橄榄石亏损１８ Ｏ，指示方解石的溶解／重结晶速率大于镁橄榄石。流体相的存在导致具不同溶解／重结晶速率的矿物与流体之间的氧同位素交换速率不同， 结果使矿物矿物或矿物流体体系的氧同位素分馏在实验过程中不是单向地朝着平衡值方向移动，而是简单地超越平衡值，甚至出现“倒转”现象。只是当体系达到化学平衡后，溶解／重结晶作用显著减慢或停止，扩散作用才成为驱动体系朝向同位素平衡的主要机制。这时，方解石中氧的扩散速率等于或略大于镁橄榄石。因此，在应用实验测定的矿物氧扩散速率计算水岩反应的时间尺度时， 必须首先排除溶解／ 重结晶和矿物蚀变引起的同位素交换。Experiments of oxygen isotope exchange between calcite and forsterite were carried out at 680 ℃ and 500 MPa in the presence of a large amount of supercritical CO2 H2O fluid The experiments were conducted in standard cold seal pressure vessels within the T P XCO stability field of the calcite forsterite assemblage The results show that the oxygen isotope exchange reactions between calcite and forsterite and between calcite and the fluid proceed in two stages: the first through a very rapid surface controlled mechanism of “Ostwald ripening” recrystallization, the second through a slow lattice controlled mechanism of diffusion Furthermore, for the given fluid composition, calcite shows a greater rate of dissolution recrystallization than forsterite, suggesting that calcite prefers to exchange oxygen isotopes with fluid relative to forsterite As a result, oxygen isotope fractionations between calcite and forsterite and between the minerals and the fluid can simply pass the equilibrium fractionations and even become “reversed”rather than undirectional shift toward the equilibrium values Once diffusion becomes a primary mechanism for further isotope exchange in the three phase system, the rate of oxygen diffusion in calcite is equal to or slightly smaller than that in forsterite The present results imply that isotopic exchange due to dissolution/recrystallization and mineral alteration has to be precluded when calculating the time scale of water rock interactions by using oxygen diffusivity in minerals

关 键 词：实验研究 化学动力学 氧同位素交换 方解石 橄榄石 流体 

分 类 号：P578.61[天文地球—矿物学] P579[天文地球—地质学]