Mathematics – Logic
Scientific paper
Jun 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011e%26psl.306..163b&link_type=abstract
Earth and Planetary Science Letters, Volume 306, Issue 3-4, p. 163-174.
Mathematics
Logic
Scientific paper
This paper deals with strongly 18O-depleted (down to - 27.3‰ VSMOW) 1.9Ga Paleoproterozoic mid-grade metamorphic rocks found in the Belomorian Belt of Karelia (E. Baltic Shield). The protolith of these rocks is attributed to have been altered by glacial meltwaters during the world's first 2.4-2.3 Ga Paleoproterozoic "Slushball" glaciation, when Karelia was located near equatorial latitudes. We describe in detail three and report seven new localities with unusually depleted 18O signatures that now span 220 km across the Belomorian Belt. Hydrogen isotope ratios measured in amphibole, biotite and staurolite also display remarkably low values of - 212 to - 235‰. Isotope mapping in the three best exposed localities has allowed us to identify the world's most 18O depleted rock, located at Khitostrov with a δ18O value - 27‰. In Khitostrov samples, zircons have normal δ18O detrital cores and low-δ18O metamorphic rims. Mapping demonstrates that zones of δ18O depletion occur in a concentric pattern 100-400 m in dimension, and each locality displays significant δ18O and δD heterogeneity on a meter to centimeter scale, characteristic of meteoric-hydrothermal systems worldwide. The zone of maximum δ18O depletions usually has the highest concentration of metamorphic corundum, rutile, and zircon and also display doubled concentrations of insoluble trace elements (Zr, Ti, Cr, HREE). These results are explained by elemental enrichment upon mass loss during hydrothermal dissolution in pH-neutral meteoric fluid. Remarkably low-δ18O and δD values suggest that alteration could have only happened by glacial meltwaters in a subglacial rift zone. Many localities with δ18O depletions occur inside metamorphozed 2.4 Ga gabbro-noritic intrusions, or near their contact with Belomorian gneisses, implying that the intrusions were driving meteoric hydrothermal systems during the known 2.4 Ga episode of Belomorian rifting. Given that the isotopically-depleted localities now spread over 200 km, the extent of the Karelian ice cap is estimated to be at least that large. Svecofennian 1.9 Ga metamorphism is seen to cause metamorphic recrystallization of hydrothermally-altered rocks into coarse-grained assemblages, and causing local metasomatism through devolatilization of the underlying hydrous low-δ18O protolith, further depleting δD via volatilization. This process led to gem-quality rubies and kyanites that preserve these remarkable δ18O values in the geologic record.
Bindeman Ilya N.
Serebryakov N. S.
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