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标题：No evidence for Archean eclogite-facies metamorphism
本地全文：下载
作者：Yi Zou ; Xu Chu ; Jialin Wu 等
期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2022
卷号：119
期号：39
DOI：10.1073/pnas.2208090119
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：Ning et al. ( 1) report Archean eclogite-facies metamorphic conditions (792 °C to 890 °C, 19.8 kbar to 24.5 kbar) whereby they argue that deep subduction and thus modern-style plate tectonics have operated since at least the Late Archean. However, the conclusion is compromised by flawed geobarometric estimates and petrography that is inconsistent with eclogite-facies metamorphism. Firstly, their rock samples do not meet the requirements of the two conventional geobarometers ( 2, 3) the authors use to calculate the peak metamorphic pressures. The garnet clinopyroxene (Grt-Cpx) geobarometer ( 2), based on the model reaction of Grossular + Pyrope = Diopside + CaTs ( Fig. 1 A ), was calibrated at high temperatures (1,200 °C to 1,550 °C; Fig. 1 B ), and was designed for mantle eclogites ( 2). The tetrahedral aluminum (Al iv) contents of Cpx in Ning et al. ( 1), given the analytical uncertainties, are indistinguishable from zero (<0.05) and are far outside of the calibration range of the geobarometer, since metamorphic temperatures are <1,200 °C ( Fig. 1 B ). To illustrate the consequences, we apply the geobarometer to published metamorphic studies. This geobarometer overestimates pressure by at least 7 kbar to 11 kbar for low Al iv (<0.05) metamorphic Cpx identical to that in Ning et al.’s peak pressure calculation ( Fig. 1 C ). The other rare earth element (REE)-in-Grt-Cpx geobarometer ( 3) requires high garnet Mg # (Mg/(Fe 2+ + Mg) > 0.4 in molar ratio)—a threshold none of their samples meet. To illustrate the consequences, this geobarometer yields systematically overestimated pressures for published samples with garnet Mg # < 0.4 ( Fig. 1 D ). Fig. 1. ( A) Model reaction of 2/3 Grs + 1/3 Prp = Di + CaTs for the Grt-Cpx geobarometer ( 2) (after ref. 5). ( B) Comparison between experimental calibration range of the Grt-Cpx geobarometer ( 2) and Al iv contents of metamorphic Cpx in Ning et al. ( 1) (after ref. 2). ( C) Validity test of the Grt-Cpx geobarometer ( 2) applied to published metamorphic rocks ( 6– 8). ( D) Validity test of the REE-in-Grt-Cpx geobarometer ( 3) applied to samples with low Mg # (<0.4) garnet ( 7, 9). Furthermore, the petrographic features presented in Ning et al. ( 1) are inconsistent with eclogite-facies metamorphism. Neither garnet exsolution from clinopyroxene nor rutile exsolution from garnet emphasized by the authors is compelling evidence for eclogite-facies metamorphism. Garnet exsolution from clinopyroxene forms during pressure increase, or cooling from high temperatures, or a combination of both ( Fig. 1 A ). Rutile exsolution from garnet is a common phenomenon in granulite-facies rocks undergoing cooling ( 4). Thus, the two exsolution textures together indicate cooling from high temperatures. In addition, phase equilibria simulations for their garnet pyroxenite indicate >60 vol % garnet at >16 kbar ( Fig. 2), in sharp contrast to the 30 vol % to 40 vol % garnet with limited decomposition textures in their rocks. Besides, plagioclases in their rocks are Na rich (>7 wt % Na 2O), so there should be sodic clinopyroxene at eclogite-facies conditions. However, neither sodic clinopyroxene nor its decomposed product of clinopyroxene + plagioclase symplectite is observed. Fig. 2. ( A) Phase equilibria diagram in the model system NCKFMASHTO calculated using bulk-rock composition of garnet pyroxenite (18SY19C-2) in Ning et al. ( 1). The thermodynamic dataset, solution models, Fe 2O 3/(Fe 2O 3+FeO) = 0.1, and water content constraint follow those of ref. 10. ( B) Isopleths of garnet volume percentage and mineral assemblages in Ning et al. ( 1). In summary, due to the misuse of geobarometers, the peak pressures are overestimated by at least 7 kbar to 11 kbar, which moves their peak metamorphic conditions squarely out of the eclogite-facies field. Without robust evidence for eclogite-facies metamorphism, the peak metamorphic pressures would not exceed 12 kbar at T > 890 °C, which reconciles with the observed mineral assemblages ( Fig. 2 B ) that are considered “retrograde” in Ning et al. ( 1). From our respective, the recalculated results are unremarkable relative to what is already known about predominantly high-temperature Archean metamorphic conditions. Thus, the samples of Ning et al. ( 1) do not support Archean eclogite-facies metamorphism, or the operation of modern-style plate tectonics in the late Archean.