Sphene (titanite): phase relations and role as a geochronometer
Sphene (titanite): phase relations and role as a geochronometer
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DOI：
10.1016/s0009-2541(00)00240-0
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发表时间：
2001-02
期刊：
影响因子：
3.9
通讯作者：
B. Frost;K. Chamberlain;John C. Schumacher
中科院分区：
文献类型：
--
作者：
B. Frost;K. Chamberlain;John C. Schumacher
有用的U-Pb同位素数据可以从黑煤（或钛铁矿，CaTiSiO 5）中获得，因为：（1）它是一种分布广泛的副矿物，（2）它可以在其结构中掺入铀，（3）它具有高的闭合温度。在火成岩中，相对氧化的岩石中，如中间成分的变辉岩中，异戊烯含量最高。这些岩石具有高的Ca/Al比，其中，相对于钛铁矿+石英或钛铁矿+钙长石，硅铝石是稳定的。在变质岩中，镁铁质和钙硅酸盐岩石中的辉石在最高温度下是稳定的。它主要存在于绿片岩、蓝片岩和角闪岩相中，尽管在钙质岩石中其稳定性可延伸至麻粒岩相。近年来的研究表明，斜长角闪岩的封闭温度位于角闪岩相的上限。由于辉石在变质过程中容易反应，U-Pb辉石年龄可能产生变质结晶的年龄，而不是通过简单的扩散重置。因此，变质辉石可能产生复杂的U-Pb系统，包含整个岩石变质历史的信息。火成岩和正片麻岩中的Sphene具有10 ~ 100 ppm的初始U含量和10 ~ 1000的初始U/Pb比值，这些比值可能产生高精度的U-Pb年龄。大理岩、钙硅酸盐和变碎屑岩中的Sphene与火成岩中的Sphene的成分范围相似，但变基性岩中的Sphene的初始U含量可能小于1 ppm，初始U与普通Pb的比值可能小于1，因此不适合用于地质年代学。这些低铀楔最常见于弱变质变基性岩中。从具有中等初始U/普通Pb比值的异戊烯中提取年龄信息的策略包括从共存的低U相中估计异戊烯的普通Pb同位素组成，使用U-Pb和Pb-Pb等时线图，以及逐步浸出方法来改善206 Pb/204 Pb散布。通过将辉石成分与变质或热液反应相关联，辉石的年龄测定可用于直接测定变质、变形和热液蚀变的年代。
Useful U–Pb isotopic data may be obtained from sphene (or titanite, CaTiSiO5) because: (1) it is a widespread accessory mineral, (2) it can incorporate uranium in its structure, and (3) it has a high closure temperature. In igneous rocks, sphene is most abundant in relatively oxidized rocks, such as metaluminous rocks of intermediate composition. These rocks have the high Ca/Al ratios wherein sphene is stabilized relative to ilmenite+quartz or ilmenite+anorthite. In metamorphic rocks, sphene is stable to the highest temperatures in mafic and calc-silicate rocks. It is found mostly in greenschist, blueschist, and amphibolite facies, although in calcic rocks its stability may extend into granulite facies. Recent studies show that the closure temperature for sphene lies at the upper limit of amphibolite facies. Because sphene reacts readily during metamorphism, U–Pb sphene ages are likely to yield the age of metamorphic crystallization, rather than resetting by simple diffusion. For this reason, metamorphic sphene may yield complex U–Pb systematics that contain information on the whole metamorphic history of the rock. Sphene from igneous rocks and orthogneisses has initial U contents ranging from 10 to over 100 ppm and ratios of initial U to common Pb ranging from 10 to 1000, ratios that may potentially yield high precision U–Pb ages. Sphene in marbles, calc-silicates, and metagraywackes has a similar range in composition to that from igneous rocks, but sphene from metabasites may have initial U contents of less than 1 ppm and ratios of initial U to common Pb lower than 1, making them unsuitable for geochronology. These low-U sphenes are most commonly found in weakly metamorphosed metabasites. Strategies to extract age information from sphene with moderate initial U/common Pb ratios include estimation of common Pb isotopic composition of sphene from coexisting low-U phases, use of U–Pb and Pb–Pb isochron plots, and step-wise leaching methods to improve206Pb/204Pb spread. By correlating sphene compositions to metamorphic or hydrothermal reactions, age determinations on sphene can be used to directly date metamorphism, deformation, and hydrothermal alteration.