南非2.06 Ga Phalabowa火成岩杂岩的高δ18O地幔源?

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Joshua T. Munro, C. Harris
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Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O-exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from -77 to -48‰ with a mean of -64 ± 9‰ (1σ, n=19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid-rock interaction in the carbonatite-foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, -3.19 to -5.60‰) overlaps with the unaltered, primary igneous carbonatite field.\n Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n=8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). 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引用次数: 0

摘要

2060±2 Ma Phalabowa杂岩是由多个岩浆脉冲形成的管状超镁铁质到碳酸岩侵入体。该杂岩由斜辉石、超镁铁质伟晶岩、碳酸盐岩和磷钾岩(橄榄石-磷灰石-磁铁矿-方解石组合)组成的主管组成,周围有许多较小的正长岩塞。Phalabowa杂岩中分析的所有岩石类型和矿物的δ18O值范围为2.24至18.3‰。然而,对最丰富和最坚固的矿物透辉石的24次分析的δ18奥值均在6.2至7.7‰之间。巴德雷质岩、橄榄石、透辉石、磁铁矿、磷灰石和赤铁矿的δ18O值被认为是岩浆的。大多数矿物对的δ18O值存在差异,这与高温至闭合温度下的岩浆平衡一致。碱长石和金云母的δ18O值变化较大,两种矿物可能都经历了亚固体O交换。岩相新鲜Phalabowa杂岩金云母的δD值范围为-77至-48‰,平均值为-64±9‰(1σ,n=19)。金云母的δD值与俯冲相关的岩浆水一致。尽管有岩相学证据表明碳酸盐岩-磷钾岩中存在流体-岩石相互作用,但碳酸盐岩δ13C和δ18O范围(δ18O,8.13至12.00‰;δ13C,-3.19至-5.60‰)与未改变的原生火成碳酸岩场重叠。由硅酸盐和氧化物矿物估算的岩浆δ18O值大多高于正常地幔岩浆(辉石岩,~7.6‰;磷钾石岩,7.2‰)。由赤铁矿估算的正长岩岩浆的δ18O价值为7.8±0.9‰(1σ,n=8),与全岩正长岩δ18O数值(8.7±0.4‰,1σ,n=5)平衡。局部基岩的平均体积δ18O值为8.6‰,地幔岩浆的实际同化比例(δ18O,5.7‰)不可能在辉石岩或磷钾石岩中产生δ18O。相反,提出Phalabowa杂岩岩浆的高δ18O值反映了地幔源的高δ。高δ18O值也是Bushveld杂岩勒斯滕堡层状岩组(2060至2055 Ma)的一个特征,该岩组可能具有类似的高δ18奥地幔源。卫星正长岩较高的δ18O值与交代围岩部分熔融的成因一致。
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A high-δ18O mantle source for the 2.06 Ga Phalaborwa Igneous Complex, South Africa?
The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.2 and 7.7‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O value that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O-exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from -77 to -48‰ with a mean of -64 ± 9‰ (1σ, n=19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid-rock interaction in the carbonatite-foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, -3.19 to -5.60‰) overlaps with the unaltered, primary igneous carbonatite field. Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n=8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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