MICRO-SCALE SILICON ISOTOPE HETEROGENEITY OBSERVED IN HYDROTHERMAL QUARTZ PRECIPITATES FROM THE >3.7 GA ISUA GREENSTONE BELT, SW GREENLAND

We present silicon isotope compositions of quartz crystals measured by secondary ion mass spectrometry (SIMS) from the northeastern part of the >3.7 Ga Isua Greenstone Belt where strain is relatively low compared with other parts of the belt. Four adjacent lithologies thought to represent silica precipitation under hydrothermal conditions were studied, including: (1) amygdaloidal, basaltic, pillow breccia, (2) quartz cement that surrounds the pillow fragments, (3) recrystallized bedded chert, and (4) chert clasts from an intraformational conglomerate. Overall, chert, chert clasts, and quartz cement have overlapping δ³⁰Si values, while quartz amygdules possess a larger range of ³⁰Si-depleted values. Silicon isotope values of quartz crystals that comprise amygdules range from δ³⁰Si_amyg = -5.09 to +0.2 ± 0.3 ‰. Such values span nearly the entire known range for silicon isotopes (roughly -5 to +3 ‰). By contrast, encasing quartz cement, separated only by millimeters, has δ³⁰Si_NBS-28 values that range from δ³⁰Si_cem = +0.01 to +0.83 ± 0.3 ‰. Silicon isotope compositions do not systematically vary with amygdule or crystal size, morphology, or distance from the brecciated cement contact. Quartz crystals from the bedded chert, and chert clasts possess a much smaller range of values (δ³⁰Si_chert = -1.61 to +1.24 ± 0.3 ‰; δ³⁰Si_clast = -0.78 to -0.23 ± 0.3 ‰). Because of the silicon isotope heterogeneity in the pillow breccia sample, we measured the oxygen isotope composition of cement and amygdules to test if oxygen isotopes preserve similar-scale heterogeneity. Oxygen isotope compositions of quartz crystals in both amygdules and cement display relatively similar and uniform values, ranging from 16.81 to 18.02 ± 0.5 ‰ and 16.89 to 18.09 ± 0.5 ‰, respectively. Such isotopic compositions lie within the expected range for metamorphosed quartz precipitates in the region (δ¹⁸O_chert = 12.9 - 20.4 ‰). Consistent with other studies, we interpret that oxygen isotopes were homogenized during metamorphism. By contrast, the wide range of δ³⁰Si values preserved in rocks from this study suggests the silicon isotope heterogeneity is primary. We propose that kinetic isotope fractionation during quartz precipitation under disequilibrium conditions in a hydrothermal setting best explains the data.