The trace elemental and isotopic signatures in apatite can be modified during hydrothermal alteration. This study investigates the suitability of apatite as an indicator of the source, chemistry, and evolution of magma and hydrothermal fluids. In situ textural, elemental, and O-Sr-Nd isotope analyses were performed on apatite in thin sections, from fresh and propylitically altered pre- and synmineralized dioritic porphyries from the Black Mountain porphyry Cu deposit in the Philippines.All studied apatite crystals have similar subhedral to euhedral shapes and are homogeneous in the grayscale in backscattered electron images. In cathodoluminescence images, the apatite in fresh and altered rocks displays yellow to yellow-green and green to brown luminescence, respectively. Apatite in fresh rocks has a higher Cl and Mn content, and lower Fe, Mg, Sr, Pb, and calculated X_OH-apatite, compared to apatite in altered rocks. The content of F, rare earth elements (REEs), Y, U, Th, and Zr, and the Sr-Nd isotope signatures of apatite from fresh and altered rocks are similar in all apatite grains (⁸⁷Sr/⁸⁶Sr = 0.7034–0.7042 vs. 0.7032–0.7043, εNd(t) = 5.3–8.0 vs. 5.1–8.4). The X-ray maps and elemental and oxygen isotope signatures across individual apatite crystals are typically homogeneous in apatite from both fresh and altered rocks. The distinct luminescence colors, coupled with distinct mobile element compositions (Cl, OH, Mn, Mg, Fe, Sr, Pb), indicate modification of primary magmatic apatite during interaction with hydrothermal fluids. The similarities in Sr isotope ratios (⁸⁷Sr/⁸⁶Sr = 0.7032–0.7043) but slight differences in O isotope signatures (δ¹⁸O = 6.0 ± 0.3‰ vs. 6.6 ± 0.3‰) in apatite from fresh and altered rocks are consistent with the magma and hydrothermal fluids having the same source and suggest significant phase separation in the hydrothermal fluids given that ¹⁸O preferentially fractionates into the residual liquid relative to ¹⁶O during phase separation. The similarity of immobile element (REE, Y, U, Th, and Zr) contents in both populations of apatite, consistency of textures and Nd isotope compositions, and absence of obvious dissolution-reprecipitation features all suggest that altered apatite retains some magmatic characteristics. The apatite in fresh rocks has oxygen isotope compositions similar to that of zircons from the same sample (δ¹⁸O = 5.9 ± 0.3‰), indicating little to no oxygen isotope fractionation between zircon and apatite and that apatite can be a good proxy for the oxygen isotope composition of the magma. Based on the Cl contents of the magmatic and replacement apatite, and assuming their equilibrium with high-temperature magma fluid and replacement hydrothermal fluid, respectively, the calculated Cl content of the early magmatic fluid and the later replacement fluid can be estimated to be 6.4 to 15.1 wt % and ~0.25 ± 0.03 wt %, respectively. This indicates a depletion of Cl from the early high-temperature fluid to the replacement fluid, consistent with phase separation.This study demonstrates that cathodoluminescence, elemental compositions (such as Cl, Mn, Mg, Fe, Sr, Pb) and Sr-O isotope signatures in apatite can be modified during hydrothermal alteration, whereas other components (REE, Y, U, Th, and Zr) and the Nd isotope composition are preserved. These features can be used to constrain the origin, chemistry, and evolution of the primary magma and ore-forming hydrothermal fluids.

中文翻译：

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菲律宾黑山斑岩矿床的磷灰石结构、成分和 O-Sr-Nd 同位素特征记录岩浆和热液流体特征

磷灰石中的微量元素和同位素特征可以在热液蚀变过程中改变。这项研究调查了磷灰石作为岩浆和热液流体的来源、化学和演化指标的适用性。对来自菲律宾黑山斑岩铜矿床的新鲜和青石化蚀变的预矿化闪长斑岩和同化闪长斑岩的薄切片磷灰石进行了原位结构、元素和 O-Sr-Nd 同位素分析。所有研究的磷灰石晶体都具有相似的从亚面体到自面体形状，并且在背散射电子图像的灰度中是均匀的。在阴极发光图像中，新鲜岩石和蚀变岩石中的磷灰石分别显示黄色至黄绿色和绿色至棕色发光。新鲜岩石中的磷灰石具有较高的Cl和Mn含量，并具有较低的Fe，Mg，Sr，_{OH-磷灰石，}与蚀变岩石中_的磷灰石相比。F、稀土元素 (REE)、Y、U、Th 和 Zr 的含量以及来自新鲜和蚀变岩石的磷灰石的 Sr-Nd 同位素特征在所有磷灰石颗粒中均相似 ( ⁸⁷ Sr/ ⁸⁶ Sr = 0.7034– 0.7042 与 0.7032–0.7043，εNd(t) = 5.3–8.0 与 5.1–8.4）。单个磷灰石晶体的 X 射线图以及元素和氧同位素特征在来自新鲜岩石和蚀变岩石的磷灰石中通常是均质的。不同的发光颜色，加上不同的移动元素组成（Cl、OH、Mn、Mg、Fe、Sr、Pb），表明在与热液相互作用期间原生岩浆磷灰石发生了变化。Sr 同位素比的相似性 ( ⁸⁷ Sr/ ⁸⁶Sr = 0.7032–0.7043）但来自新鲜和蚀变岩石的磷灰石中 O 同位素特征的细微差异（δ ¹⁸ O = 6.0 ± 0.3‰ vs. 6.6 ± 0.3‰）与具有相同来源的岩浆和热液流体一致，并表明考虑到¹⁸ O 相对于¹⁶ O 优先分馏到残余液体中，热液流体中的显着相分离O 相分离过程中。两种磷灰石群中固定元素（REE、Y、U、Th和Zr）含量的相似性、质地和Nd同位素组成的一致性以及不存在明显的溶解-再沉淀特征都表明蚀变的磷灰石保留了一些岩浆特征。新鲜岩石中的磷灰石的氧同位素组成与同一样品中的锆石（δ ¹⁸O = 5.9 ± 0.3‰)，表明锆石和磷灰石之间几乎没有氧同位素分馏，磷灰石可以很好地代表岩浆的氧同位素组成。根据岩浆和置换磷灰石的Cl含量，假设它们分别与高温岩浆液和置换热液平衡，计算出的早期岩浆液和后期置换液的Cl含量为6.4～分别为 15.1 重量 % 和 ~0.25 ± 0.03 重量 %。这表明 Cl 从早期高温流体到置换液的消耗，与相分离一致。该研究表明，阴极发光、元素组成（如 Cl、Mn、Mg、Fe、Sr、Pb）和 Sr-O磷灰石中的同位素特征可以在热液蚀变过程中改变，而其他成分（REE、Y、U、Th 和 Zr）和 Nd 同位素组成则保留下来。这些特征可用于限制原生岩浆和成矿热液流体的起源、化学和演化。