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.
