Apatite as probe for the halogen composition of metamorphic fluids (Bamble Sector, SE Norway)
Apatite as probe for the halogen composition of metamorphic fluids (Bamble Sector, SE Norway)
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磷灰石作为变质流体卤素成分的探针（挪威东南部 Bamble Sector）
DOI：
10.1007/s00410-015-1188-6
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发表时间：
2015
影响因子：
3.5
通讯作者：
Ane K.
中科院分区：
文献类型：
--
作者：
Kusebauch;Christof;Whitehouse;Martin J;Engvik;Ane K.
在区域交代事件(挪威东南部班布尔地区)中形成的交代磷灰石的卤素组成揭示了热液流体的组成和演化信息。高盐度流体渗入辉长岩体，导致角闪岩化和角闪岩化，岩浆富氯磷灰石与热液作用形成富OH和/或富F的磷灰石。剪切带附近蚀变程度较大的样品中的磷灰石具有最高的F(高达15,000微克/克)和最低的溴(4-25微克/克)浓度，而蚀变最少的样品中的磷灰石具有很低的F(30-200微克/克)和高的溴(30-85微克/克)。此外，单个交代磷灰石颗粒在F中呈带状分布，沿边缘和裂隙高度集中，核心区低F。碘浓度在0.18-0.70微克/克的低值范围内保持不变。我们将所有观察到的置换磷灰石的成分特征解释为流体-岩石相互作用过程中流体不断演化的结果。由于其高度的相容性，渗入流体中的F很早就被合并到重结晶的磷灰石中(靠近剪切带和单个磷灰石颗粒的边缘)。相反，作为一种不相容的卤素，溴在流体中变得丰富，在演化程度最高的流体中含量最高。利用置换磷灰石和流体之间的实验分配数据，我们计算了演化流体的F浓度从60降至~1t；1µg/g，Br从~1200降至~5000µg/g；流体的I浓度恒定在370µg/g左右。尽管预计氯的行为与溴相似，但置换磷灰石在整个蚀变序列中具有恒定的氯浓度(~1wt.%)，这可能是流体中相当恒定的氯活度的结果。单个磷灰石颗粒的氯稳定同位素值是不均匀的，范围从−1.2到+3.7n‰。高的δ37Cl值一般与交代磷灰石的富OH带相关，而低δ37Cl值在交代磷灰石的富F带和可能为岩浆成因的氯磷灰石中被测量到。尽管磷灰石δ37Cl值遵循总体趋势，但单个δ37Cl值似乎反映了界面流体在反应前沿的高度局域化组成。我们的观察表明，磷灰石可以作为F、Br和I的流体探针来检测流体的成分演化，这可以通过实验得出的分配系数来量化。氯和氯稳定同位素在高盐度流体和磷灰石之间的分配是复杂的，可能受更多未知因素的控制，而不仅仅是氯浓度。
Halogen composition of replaced apatite formed during a regional metasomatic event (Bamble Sector, SE Norway) reveals information about the composition and evolution of the hydrothermal fluid. Infiltration and pervasive fluid flow of highly saline fluids into gabbroic bodies lead to scapolitization and amphibolitization, where magmatic Cl-rich apatite reacts with the hydrothermal fluid to form OH- and/or F-rich apatite. Apatite from highly altered samples adjacent to the shear zone has highest F (up to 15,000 µg/g) and lowest Br (4–25 µg/g) concentrations, whereas apatite from least altered samples has very low F (30–200 µg/g) and high Br (30–85 µg/g). In addition, individual replaced apatite grains show a zonation in F with high concentrations along rims and cracks and low F in core regions. Iodine concentrations remain rather constant as low values of 0.18–0.70 µg/g. We interpret all observed compositional features of replaced apatite to be the result of a continuous evolution of the fluid during fluid–rock interaction. Due to its high compatibility, F from the infiltrating fluid is incorporated early into recrystallized apatite (close to shear zone and rims of individual apatite grains). In contrast, Br as an incompatible halogen becomes enriched in the fluid and is highest in the most evolved fluid. Using experimental partition data between replaced apatite and fluid, we calculated F concentrations of the evolving fluid to decrease from 60 to <1 µg/g and Br to increase from ~1200 to ~5000 µg/g; I concentrations of the fluid are constant in the order of 370 µg/g. Although Cl is expected to show a similar behavior as Br, replaced apatite has constant Cl concentrations throughout the alteration sequence (~1 wt.%), which is likely the result of a rather constant Cl activity in the fluid. Chlorine stable isotope values of individual apatite grains are heterogeneous and range from −1.2 to +3.7 ‰. Highδ37Cl values are generally correlated with OH-rich zones of replaced apatite, whereas lowδ37Cl values are measured in F-rich zones of replaced apatite and in Cl-apatite of probably magmatic origin. Though apatiteδ37Cl values follow the general bulk trend, the individualδ37Cl signature seems to reflect the highly localized composition of interfacial fluid at the reaction front. Our observations suggest that apatite can be used as a fluid probe for F, Br, and I to detect a compositional evolution of the fluid, which can be quantified by using experimentally derived partition coefficients. Partitioning of Cl and Cl stable isotopes between highly saline fluids and apatite is complex and likely controlled by more unknown factors than just the Cl concentration.
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DOI：
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