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USGS2011

Syn-extensional plutonism and peak metamorphism in the albion-raft river-grouse creek metamorphic core complex

The Cassia plutonic complex (CPC) is a group of variably deformed, Oligocene granitic plutons exposed in the lower plate of the Albion-Raft River- Grouse Creek (ARG) metamorphic core complex of Idaho and Utah. The plutons range from granodiorite to garnet-bearing, leucogranite, and during intrusion, sillimanite- grade peak metamorphism and ductile attenuation occurred in the country rocks and normal-sense, amphibolite-grade deformation took place along the Middle Mountain shear zone. U-Pb zircon geochronology from three variably deformed plutons exposed in the lower plate of the ARG metamorphic core complex revealed that each zircon is comprised of inherited cores (dominantly late Archean) and Oligocene igneous overgrowths. Within each pluton, a spread of concordant ages from the Oligocene zircon overgrowths is interpreted as zircon recycling within a long-lived magmatic system. The plutons of the CPC have very low negative whole rock ??Nd values of -26 to -35, and initial Sr values of 0.714 to 0.718, consistent with an ancient, crustal source. Oxygen isotope ratios of the Oligocene zircon overgrowths from the CPC have an average ??18O value of 5.40 ?? 0.63 permil (2SD, n = 65) with a slight trend towards higher ??18O values through time. The ??18O values of the inherited cores of the zircons are more variable at 5.93 ?? 1.51 permil (2SD, n = 29). Therefore, we interpret the plutons of the CPC as derived, at least in part, from melting Archean crust based on the isotope geochemistry. In situ partial melting of the exposed Archean basement that was intruded by the Oligocene plutons of the CPC is excluded as the source for the CPC based on field relationships, age and geochemistry. Correlations between Ti and Hf concentrations in zircons from the CPC suggest that the magmatic system may have become hotter (higher Ti concentration in zircon) and less evolved (lower Hf in zircon concentration) through time. Therefore, the CPC represents prolonged or episodic magmatism system (32-25 Ma), and the intrusions were each accompanied by sillimanite-grade deformation and extension. The Oligocene magmatism and peak metamorphism preserved in the ARG metamorphic core complex are likely related to regional trends in mantle-derived magmatism that led to protracted heating, melting and mobilization of the deeper crust.

American Journal of Science↗

USGS2010

In situ sulfur isotope analysis of sulfide minerals by SIMS: Precision and accuracy, with application to thermometry of ~3.5Ga Pilbara cherts

Secondary ion mass spectrometry (SIMS) measurement of sulfur isotope ratios is a potentially powerful technique for in situ studies in many areas of Earth and planetary science. Tests were performed to evaluate the accuracy and precision of sulfur isotope analysis by SIMS in a set of seven well-characterized, isotopically homogeneous natural sulfide standards. The spot-to-spot and grain-to-grain precision for δ34S is ± 0.3‰ for chalcopyrite and pyrrhotite, and ± 0.2‰ for pyrite (2SD) using a 1.6 nA primary beam that was focused to 10 µm diameter with a Gaussian-beam density distribution. Likewise, multiple δ34S measurements within single grains of sphalerite are within ± 0.3‰. However, between individual sphalerite grains, δ34S varies by up to 3.4‰ and the grain-to-grain precision is poor (± 1.7‰, n = 20). Measured values of δ34S correspond with analysis pit microstructures, ranging from smooth surfaces for grains with high δ34S values, to pronounced ripples and terraces in analysis pits from grains featuring low δ34S values. Electron backscatter diffraction (EBSD) shows that individual sphalerite grains are single crystals, whereas crystal orientation varies from grain-to-grain. The 3.4‰ variation in measured δ34S between individual grains of sphalerite is attributed to changes in instrumental bias caused by different crystal orientations with respect to the incident primary Cs+ beam. High δ34S values in sphalerite correlate to when the Cs+ beam is parallel to the set of directions < uuw>, from [111] to [110], which are preferred directions for channeling and focusing in diamond-centered cubic crystals. Crystal orientation effects on instrumental bias were further detected in galena. However, as a result of the perfect cleavage along {100} crushed chips of galena are typically cube-shaped and likely to be preferentially oriented, thus crystal orientation effects on instrumental bias may be obscured. Test were made to improve the analytical precision of δ34S in sphalerite, and the best results were achieved by either reducing the depth of the analysis pits using a Köhler illuminated primary beam, or by lowering the total impact energy from 20 keV to 13 keV. The resulting grain-to-grain precision in δ34S improves from ± 1.7‰ to better than 0.6‰ (2SD) in both procedures. With careful use of appropriate analytical conditions, the accuracy of SIMS analysis for δ34S approaches ± 0.3‰ (2SD) for chalcopyrite, pyrite and pyrrhotite and ± 0.6‰ for sphalerite. Measurements of δ34S in sub-20 µm grains of pyrite and sphalerite in ∼ 3.5 Ga cherts from the Pilbara craton, Western Australia show that this analytical technique is suitable for in situ sulfur isotope thermometry with ± 50 °C accuracy in appropriate samples, however, sulfides are not isotopically equilibrated in analyzed samples.

Chemical Geology↗