Distribution of halogens between fluid and apatite during fluid-mediated replacement processes
Distribution of halogens between fluid and apatite during fluid-mediated replacement processes
复制标题
DOI：
10.1016/j.gca.2015.08.023
复制
发表时间：
2015-12-01
影响因子：
5
通讯作者：
Putnis, Andrew
中科院分区：
文献类型：
--
作者：
Kusebauch, Christof;John, Timm;Putnis, Andrew
磷灰石(Ca-5(PO4)（3)(OH， F, Cl)）是岩浆岩和变质岩中卤素的主要寄主之一，它与热液流体中的卤素（F, Cl, Br， I）和OH结合形成端元F-磷灰石、Cl-磷灰石和OH-磷灰石的三元固溶体，在流-岩相互作用中起着独特的作用。本文通过实验研究了Cl-apatite在地壳条件下（400-700℃，0.2 GPa）与不同水溶液（不同浓度的KOH， NaCl, NaF，并掺杂NaBr， NaI）相互作用形成新磷灰石的过程。利用实验结果计算了卤素在磷灰石与流体之间的分配系数。由于溶蚀-再沉淀耦合作用，形成新的磷灰石作为cl -磷灰石的假晶替代。此外，一些实验还产生了新的磷灰石，也作为外延过度生长。新磷灰石的组成主要受其析出的流体相的复杂特征支配，并取决于流体的组成、温度和液矿比。替换磷灰石呈现出成分分带性，这是由于局部平衡共存的流体与新形成的磷灰石组成演化所致。磷灰石/流体中F的分配系数取决于流体中F的浓度，从高浓度时的75 （460 μ g/g F）增加到低浓度时的300 (46 μ g/g F)，表明F在磷灰石中的相容性很高。在高盐溶液中没有观察到磷灰石中Cl-浓度与流体Cl-浓度的相关性，新磷灰石的组成更受热液中OH-浓度的支配。较大卤素Br和I的分配系数较低，Br和I的分配系数分别在0.7 * 10(-3)~ 152 * 10（-3）和0.3 * 10(-3)~ 17 * 10（-3）之间。Br的D值似乎比i高一个数量级。这些数据允许基于晶格应变模型对其他卤素的D值进行估计，该模型显示序列的D- f接近120，DOH接近100，DCl接近2.3，D-Br接近0.045，D- i接近0.0025。这项实验研究的结果有助于更好地理解演化流体的流体-岩石相互作用，因为它使热液衍生磷灰石的成分能够用作地壳条件下卤素的流体探针。进一步说明矿物置换是生成不同组成磷灰石的关键反应之一。(C) 2015 Elsevier Ltd.版权所有。
Apatite (Ca-5(PO4)(3)(OH, F, Cl)) is one of the main host of halogens in magmatic and metamorphic rocks and plays a unique role during fluid-rock interaction as it incorporates halogens (i.e. F, Cl, Br, I) and OH from hydrothermal fluids to form a ternary solid solution of the endmembers F-apatite, Cl-apatite and OH-apatite. Here, we present an experimental study to investigate the processes during interaction of Cl-apatite with different aqueous solutions (KOH, NaCl, NaF of different concentration also doped with NaBr, NaI) at crustal conditions (400-700 degrees C and 0.2 GPa) leading to the formation of new apatite. We use the experimental results to calculate partition coefficients of halogens between apatite and fluid. Due to a coupled dissolution-reprecipitation mechanism new apatite is always formed as a pseudomorphic replacement of Cl-apatite. Additionally, some experiments produce new apatite also as an epitaxial overgrowth. The composition of new apatite is mainly governed by complex characteristics of the fluid phase from which it is precipitating and depends on composition of the fluid, temperature and fluid to mineral ratio. Furthermore, replaced apatite shows a compositional zonation, which is attributed to a compositional evolution of the coexisting fluid in local equilibrium with the newly formed apatite. Apatite/fluid partition coefficients for F depend on the concentration of F in the fluid and increase from 75 at high concentrations (460 mu g/g F) to 300 at low concentrations (46 mu g/g F) indicating a high compatibility of F in apatite. A correlation of Cl-concentration in apatite with Cl- concentration of fluid is not observed for experiments with highly saline solutions, composition of new apatite is rather governed by OH- concentration of the hydrothermal fluid. Low partition coefficients were measured for the larger halogens Br and I and vary between 0.7 * 10(-3) - 152 * 10(-3) for Br and 0.3 * 10(-3) - 17 * 10(-3) for I, respectively. Br seems to have D values of about one order of magnitude higher than I. These data allow an estimation of the D values for the other halogens based on a lattice strain model which displays a sequence with D-F of similar to 120, DOH of similar to 100, DCl of similar to 2.3 D-Br similar to 0.045, and D-I similar to 0.0025. Results from this experimental study help to better understand fluid-rock interaction of an evolving fluid, as it enables the composition of hydrothermally derived apatite to be used as a fluid probe for halogens at crustal conditions. It further shows the importance of mineral replacement as one of the key reactions to generate apatite of different composition. (C) 2015 Elsevier Ltd. All rights reserved.