Chris Clark, Richard J. M. Taylor, Malcolm P. Roberts, Simon L. Harley
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A monazite grain hosted within mesoperthite records a spread of ages from 2.43 to 2.20 Ga and Y contents ranging between 400 and 1,700 ppm. This grain exhibits core to rim zoning in both Y and age, with the cores enriched in Y relative to the rim and younger ages in the core relative to the rim. A monazite grain that sits on a grain boundary between mesoperthite and garnet records the largest spread in ages—from 2.42 to 2.05 Ga. The youngest ages in this grain are within a linear feature that reaches the core and is connected to the grain boundary between the garnet and mesoperthite; the oldest ages are observed where monazite is in contact with garnet. Yttrium in the grain is enriched in the core and depleted at the rim with the strongest depletions where monazite is adjacent to grain boundaries between the silicate minerals or in contact with garnet. The unarmoured monazite grains have lower intercept ages of 1.85 Ga, which overlaps with the bulk of ages determined from the rutile and is coincident with a previously reported zircon age obtained through depth profiling from the Napier Complex. The age and chemical relationships outlined above illustrate decoupling between the geochemical and geochronological systems in monazite. Individual grains are suggestive of a range of processes that modify these systems, including volume diffusion, flux-limited diffusion and fluid-enhanced recrystallization, all operating at the scale of a single thin section and primarily controlled by host mineral microstructural setting. These findings illustrate how the development of simple partitioning coefficients (cf. garnet/zircon) and geospeedometry based on experimentally determined diffusion coefficients on grain separates may not be achievable. 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引用次数: 0
摘要
利用激光分流分析法对来自纳皮尔岩群的超高温(UHT)元青石寄主白铁矿的独居石、金红石和锆石晶粒进行了原位年龄和痕量元素测定,结果表明,U-Pb和痕量元素系统学的行为变化很大,这与单个晶粒的微观结构环境直接相关。被石榴石和石英铠装的独居石晶粒保留了两个一致的年龄 2.48 Ga 和 2.43 Ga,与之前测定的纳皮尔岩群超高温变质峰值年龄一致。铠装晶粒中的钇未分区,石榴石包裹的独居石中的钇含量为约 700 ppm,石英包裹的独居石中的钇含量为 400-1,600 ppm。中透辉石中寄生的独居石晶粒记录的年龄从 2.43 Ga 到 2.20 Ga 不等,Y 含量在 400 ppm 到 1,700 ppm 之间。该晶粒在Y和年龄方面都呈现出从岩芯到岩缘的分带,岩芯的Y含量相对于岩缘更高,岩芯的年龄相对于岩缘更年轻。位于中透辉石和石榴石晶界上的一个独居石晶粒记录了最大的年龄分布--从 2.42 Ga 到 2.05 Ga。该晶粒中最年轻的年龄位于一个线性特征内,该线性特征直达晶核,并与石榴石和中透辉石之间的晶界相连;最古老的年龄出现在独居石与石榴石接触的地方。晶粒中的钇在核心富集,在边缘贫化,在独居石邻近硅酸盐矿物之间的晶界或与石榴石接触的地方贫化最严重。未铠装独居石晶粒的截距年龄较低,为 1.85 Ga,这与从金红石中测定的大部分年龄相重叠,并且与之前报告的通过纳皮尔复合体深度剖面获得的锆石年龄相吻合。上述年龄和化学关系说明了独居石的地球化学和地质年代系统之间的脱钩。单个晶粒表明了改变这些系统的一系列过程,包括体积扩散、通量限制扩散和流体强化再结晶,所有这些过程都是在单个薄片的尺度上进行的,并且主要受主矿物微结构环境的控制。这些发现说明,根据实验确定的晶粒分离扩散系数来制定简单的分区系数(如石榴石/锆石)和地质速度测定法可能是无法实现的。不过,这也凸显了将多种附属矿物的年龄和痕量元素浓度与微观结构信息结合起来,在试图建立一个经历了长期热、变形和流体流动事件的古老岩石系统所经历的构造热事件的完整历史时的实用性。
Modification of the isotopic and geochemical compositions of accessory minerals controlled by microstructural setting
In situ age and trace element determinations of monazite, rutile and zircon grains from an ultrahigh temperature (UHT) metapelite-hosted leucosome from the Napier Complex using laser split-stream analysis reveal highly variable behaviour in both the U–Pb and trace element systematics that can be directly linked to the microstructural setting of individual grains. Monazite grains armoured by garnet and quartz retain two concordant ages 2.48 and 2.43 Ga that are consistent with the previously determined ages for peak UHT metamorphism in the Napier Complex. Yttrium in the armoured grains is unzoned with contents of ~700 ppm for the garnet-hosted monazite and in the range 400–1,600 ppm for the monazite enclosed within quartz. A monazite grain hosted within mesoperthite records a spread of ages from 2.43 to 2.20 Ga and Y contents ranging between 400 and 1,700 ppm. This grain exhibits core to rim zoning in both Y and age, with the cores enriched in Y relative to the rim and younger ages in the core relative to the rim. A monazite grain that sits on a grain boundary between mesoperthite and garnet records the largest spread in ages—from 2.42 to 2.05 Ga. The youngest ages in this grain are within a linear feature that reaches the core and is connected to the grain boundary between the garnet and mesoperthite; the oldest ages are observed where monazite is in contact with garnet. Yttrium in the grain is enriched in the core and depleted at the rim with the strongest depletions where monazite is adjacent to grain boundaries between the silicate minerals or in contact with garnet. The unarmoured monazite grains have lower intercept ages of 1.85 Ga, which overlaps with the bulk of ages determined from the rutile and is coincident with a previously reported zircon age obtained through depth profiling from the Napier Complex. The age and chemical relationships outlined above illustrate decoupling between the geochemical and geochronological systems in monazite. Individual grains are suggestive of a range of processes that modify these systems, including volume diffusion, flux-limited diffusion and fluid-enhanced recrystallization, all operating at the scale of a single thin section and primarily controlled by host mineral microstructural setting. These findings illustrate how the development of simple partitioning coefficients (cf. garnet/zircon) and geospeedometry based on experimentally determined diffusion coefficients on grain separates may not be achievable. However, it highlights the utility of combining age and trace element concentrations from multiple accessory minerals with microstructural information when trying to build a complete history of tectonothermal events experienced by an ancient rock system that has undergone a prolonged history of thermal, deformational and fluid flow events.
期刊介绍:
The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.