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Zircon melt inclusions in mafic and felsic rocks of the Bushveld Complex – constraints for zircon crystallization temperatures and partition coefficients
Geochimica et Cosmochimica Acta ( IF 4.5 ) Pub Date : 2020-11-01 , DOI: 10.1016/j.gca.2020.08.027
D. Gudelius , A. Zeh , R.R. Almeev , A.H. Wilson , L.A. Fischer , A.K. Schmitt

Abstract Melt inclusions in zircon represent time capsules, which provide deep insights into igneous rock formation including timing, physicochemical conditions, and the compositional evolution of cogenetic magmas with respect to major, trace, and volatile elements. However, their full potential as petrogenetic indicators, in particular their usability and consistency as geothermometers, is poorly investigated. Therefore, we present new mineralogical and chemical data for recrystallized and homogenized melt inclusions and host zircon from different mafic and felsic rocks of the Bushveld Complex (BC), South Africa. Samples include rutile-bearing cumulate rocks of the Marginal and Critical zones, as well as rutile-free, magnetite-ilmenite-titanite-bearing diorites and granites of the Upper Zone, Rashoop Granophyre and Lebowa Granite Suite. All melt inclusions, irrespective of rock type, have rhyolitic compositions with SiO2 contents ranging from 65 to 78 wt.%, and H2O from 1.6 to 4.0 wt.%, whereas trace element contents differ systematically between rock types. In rutile-bearing mafic rocks, melt inclusions commonly show higher Ti contents (>800 ppm), higher Th/U ratios (up to 38), and lower REE contents (ƩREE 150 ppm). Liquidus temperatures of melt inclusions obtained from normative Qz-Ab-Or and rhyolite-MELTS modelling indicate melt entrapment mostly at 930–850 °C (at 200 MPa), tailing down in some samples to 700 °C. For rutile-bearing rocks, these temperatures overlap with those obtained by TiO2-in-melt and Ti-in-zircon geothermometry. For all rutile-free mafic and felsic rocks, reduced TiO2 activities of aTiO2 ∼ 0.3 are required for Ti-in-zircon geothermometry, and TiO2rutile ∼ 30 wt.% for TiO2 saturation geothermometry, to match temperatures from other geothermometers. Furthermore, partition coefficients obtained from melt inclusion - host zircon pairs are within error identical for mafic and felsic rocks and also reveal no systematic dependency on melt inclusion size, composition and entrapment temperature. The results of this study demonstrate that melt inclusions in zircon are a powerful tool for geothermometry and to constrain magma compositions, H2O contents, and TiO2 activities, which are critical for the understanding of magmatic processes shaping Earth’s crust.

中文翻译:

Bushveld Complex 镁铁质和长英质岩石中的锆石熔体包裹体——锆石结晶温度和分配系数的限制

摘要 锆石中的熔体包裹体代表了时间胶囊,它提供了对火成岩形成的深入见解,包括时间、物理化学条件以及共生岩浆在主要、微量和挥发性元素方面的成分演化。然而,它们作为岩石成因指标的全部潜力,特别是它们作为地温计的可用性和一致性,却没有得到很好的研究。因此,我们提供了来自南非布什维尔德复合体 (BC) 不同基性和长英质岩石的再结晶和均质熔体包裹体和主锆石的新矿物学和化学数据。样品包括边缘带和临界带的含金红石堆积岩,以及上带无金红石、含磁铁矿-钛铁矿-钛铁矿的闪长岩和花岗岩、Rashoop Granophyre 和 Lebowa Granite Suite。无论岩石类型如何,所有熔体包裹体都具有流纹岩成分,SiO2 含量范围为 65 至 78 重量%,H2O 含量为 1.6 至 4.0 重量%,而微量元素含量因岩石类型而异。在含金红石的基性岩石中,熔体包裹体通常表现出较高的 Ti 含量(>800 ppm)、较高的 Th/U 比(高达 38)和较低的 REE 含量(ƩREE 150 ppm)。从规范的 Qz-Ab-Or 和流纹岩-MELTS 模型获得的熔体包裹体的液相线温度表明熔体滞留主要在 930–850 °C(200 MPa),在一些样品中拖尾到 700 °C。对于含金红石的岩石,这些温度与通过 TiO2-in-melt 和 Ti-in-zircon 地温测量法获得的温度重叠。对于所有不含金红石的基性和长英质岩石,锆石中的 Ti 测温需要将 TiO2 活性降低至 0.3,和 TiO2 金红石 ~ 30 wt.% 用于 TiO2 饱和地热测量,以匹配来自其他地热计的温度。此外,从熔体包裹体 - 主锆石对获得的分配系数对于基性岩和长英质岩石在误差范围内相同,并且也没有显示对熔体包裹体尺寸、成分和截留温度的系统依赖性。这项研究的结果表明,锆石中的熔体包裹体是一种强大的地温测量工具,可以限制岩浆成分、H2O 含量和 TiO2 活性,这对于了解塑造地壳的岩浆过程至关重要。从熔体包裹体获得的分配系数 - 主锆石对对于基性岩和长英质岩石在误差范围内是相同的,并且也没有显示对熔体包裹体尺寸、成分和截留温度的系统依赖性。这项研究的结果表明,锆石中的熔体包裹体是一种强大的地温测量工具,可以限制岩浆成分、H2O 含量和 TiO2 活性,这对于了解塑造地壳的岩浆过程至关重要。从熔体包裹体获得的分配系数 - 主锆石对对于基性岩和长英质岩石在误差范围内是相同的,并且也没有显示对熔体包裹体尺寸、成分和截留温度的系统依赖性。这项研究的结果表明,锆石中的熔体包裹体是一种强大的地温测量工具,可以限制岩浆成分、H2O 含量和 TiO2 活性,这对于了解塑造地壳的岩浆过程至关重要。
更新日期:2020-11-01
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