Carson Kinney, Jillian Kendrick, Manuel Duguet, Chris Yakymchuk
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We integrate field observations, whole-rock compositions, thermodynamic equilibrium and accessory mineral modelling with heat production and latency time modelling to provide insights into the partitioning of heat-producing elements between residue and melt during anatexis of metabasites as well as the resulting effects on metamorphic timescales and the production of tonalite–trondhjemite–granodiorite (TTG) suites. We model six metabasite compositions ranging from relatively fertile greenschist facies metabasites to melt-depleted residual mafic (upper-)amphibolites to granulites. Heat-producing elements are modelled to be partitioned between melt and residue; the dominant minerals in the residue that host these elements are apatite, hornblende, K-feldspar, and epidote. At 800–850°C epidote is no longer stable, and the melt fraction is predicted to contain roughly half of the heat production capacity for the system. Apatite and melt are expected to be the dominant repositories for Th and U during anatexis; zircon is predicted to be completely consumed by 850°C, whereas apatite persists to higher temperatures and allanite is expected only in minor modal abundances at high-P, low-T conditions. The partitioning of heat-producing elements into relatively low-density melt decreases the heat production of the residual system during anatexis. Due to their high density and affinity for U and Th, epidote and apatite retain heat production capacity in the residue during metabasite melting. Thermal latency modelling of metamorphism suggests that enriched metabasite compositions require 38–46 My to increase the temperature from ~650 to 850°C (solidus temperature to peak metamorphic temperature of the Kapuskasing uplift), whereas estimates are considerably shorter for depleted compositions (7–25 My). Four of the six samples modelled require 60–70 My to reach 1000°C from the solidus. 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引用次数: 0
摘要
钾、钍和铀衰变产生的热量在地球大陆地壳的分化和稳定过程中起着根本性的作用。这对于形成地球大陆核心的阿新世陨石坑的构造尤为重要。卡普斯卡辛隆起是阿基坦时代地壳横截面的罕见露头,提供了地壳熔化、分化和成分分层的快照。我们将野外观测、全岩成分、热力学平衡和附属矿物建模与产热和潜伏时间建模结合起来,深入研究了在偏闪长岩的安山过程中,产热元素在残余物和熔体之间的分配,以及由此对变质时间尺度和碳酸盐岩-特长闪长岩-花岗闪长岩(TTG)岩套的生成所产生的影响。我们模拟了六种偏闪长岩成分,从相对肥沃的绿辉石面偏闪长岩到熔体贫化的残余黑云母(上)闪长岩再到花岗岩。产热元素被模拟为在熔体和残余物之间分配;残余物中容纳这些元素的主要矿物是磷灰石、角闪石、K长石和闪长岩。在摄氏 800-850 度时,表土不再稳定,预计熔体部分将包含该系统大约一半的产热能力。预计磷灰石和熔体将成为安山期 Th 和 U 的主要储存地;预计锆石在 850°C 时将被完全消耗,而磷灰石将持续到更高的温度,预计在高 P、低 T 条件下,阳起石仅以较小的模态丰度存在。产热元素被分割到密度相对较低的熔体中,降低了残余体系在安氏过程中的产热量。由于表土和磷灰石的高密度和对铀和钍的亲和性,它们在偏闪石熔融过程中保留了残余物的产热能力。变质作用的热潜伏期模拟表明,富集偏闪长岩成分需要38-46 My才能将温度从大约650°C提高到850°C(卡普斯卡辛隆起的固结温度到变质峰值温度),而贫化成分的估计值要短得多(7-25 My)。在模拟的六个样品中,有四个需要 60-70 My 才能从固结温度达到 1000°C。我们的产热元素分配模型和预测的熔体比例表明,富集玄武岩成分是TTG岩浆最合理的来源,我们的加热时间模型表明,与放射源产热相比,地幔是变质岩厌氧作用的同等或主要热源。
Redistribution of heat-producing elements during melting of Archean crust
Heat generated from the decay of K, Th, and U plays a fundamental role in the differentiation and stabilization of Earth's continental crust. This is particularly important in the construction of Archean cratons that form the nuclei of Earth's continents. The Kapuskasing uplift is a rare exposure of an Archean-age crustal cross-section that provides a snapshot of crustal melting, differentiation, and compositional stratification. We integrate field observations, whole-rock compositions, thermodynamic equilibrium and accessory mineral modelling with heat production and latency time modelling to provide insights into the partitioning of heat-producing elements between residue and melt during anatexis of metabasites as well as the resulting effects on metamorphic timescales and the production of tonalite–trondhjemite–granodiorite (TTG) suites. We model six metabasite compositions ranging from relatively fertile greenschist facies metabasites to melt-depleted residual mafic (upper-)amphibolites to granulites. Heat-producing elements are modelled to be partitioned between melt and residue; the dominant minerals in the residue that host these elements are apatite, hornblende, K-feldspar, and epidote. At 800–850°C epidote is no longer stable, and the melt fraction is predicted to contain roughly half of the heat production capacity for the system. Apatite and melt are expected to be the dominant repositories for Th and U during anatexis; zircon is predicted to be completely consumed by 850°C, whereas apatite persists to higher temperatures and allanite is expected only in minor modal abundances at high-P, low-T conditions. The partitioning of heat-producing elements into relatively low-density melt decreases the heat production of the residual system during anatexis. Due to their high density and affinity for U and Th, epidote and apatite retain heat production capacity in the residue during metabasite melting. Thermal latency modelling of metamorphism suggests that enriched metabasite compositions require 38–46 My to increase the temperature from ~650 to 850°C (solidus temperature to peak metamorphic temperature of the Kapuskasing uplift), whereas estimates are considerably shorter for depleted compositions (7–25 My). Four of the six samples modelled require 60–70 My to reach 1000°C from the solidus. Our modelling of heat-producing element partitioning and predicted proportions of melt suggest that enriched basaltic compositions are the most reasonable source of TTG magmas and our heating time modelling indicates the mantle as an equal to dominant source of heat for metabasite anatexis compared with radiogenic heat production.
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.