Oxygen Isotope Zoning in Skarn Garnets: Evidence for Spatial and Temporal Fluid Source Variability in the Sierra Nevada and Mojave

Abstract:

Skarns provide insight to the depth, longevity, and dominant fluid regime associated with Sierra Nevada plutonism and Mesozoic magmatism in the Mojave National Preserve, which represent different spatial and temporal exposures of the Mesozoic arc. Skarns from these regions may serve as proxies for intricacies in the fluid source, and have the potential to resolve magmatic flare-ups and relative depths of emplacement.

Both laser fluorination (LF) and secondary ion mass spectrometry (SIMS) δ¹⁸O analyses of garnet from multiple Mojave (Lucerne valley) skarns indicate a strong, early influence of meteoric fluid despite the presence of relatively deep plutonism. LF data from individual whole garnets and garnet chips broken during sample preparation reveal variation from +4.2‰ to -8.8‰ (n = 24), with an average of approximately -4.0‰. The large spread in these LF data suggest that (A) δ¹⁸O reflects an average of varying δ¹⁸O (fluid) compositions spanning multiple garnet growth oscillations; or (B) multiple generations of garnets exist within individual skarns, the growth of each coinciding with changes in the hydrothermal source and composition.

SIMS analysis of two individual Mojave skarn garnets with oscillatory zoning (seen in backscatter electron images) reveal crystal cores with δ¹⁸O values of -9.6‰, internal variations of -9.4‰ to -3.3‰, and crystal rims of -2.2‰ and -2.9‰ (precision ±0.3; 2σ). In general, δ¹⁸O values negatively correlate with andradite compositions, with high andradite zones having lower δ¹⁸O values ([AND + CaTi] compositions range from 100 to 73).

Similar analyses (both SIMS and LF) of garnets from Sierra Nevadan skarns (Tungsten Hills region) show variation in δ¹⁸O values with LF data ranging from 5.4‰ to 6.2‰ (n = 8), with an average of 5.7‰, and an additional 2.7‰ value obtained from a garnet interior. SIMS data show δ¹⁸O compositional variation from 4.0‰ to 5.9‰. Data across the two Tungsten Hills garnets analyzed via SIMS show the range of isotope compositions possible within a single crystal. All data from the Tungsten Hills skarns suggest meteoric water was less influential farther north in the arc, and that major skarn formation was likely the result of pluton-related magmatic fluids. Magmatic fluids would produce garnets with δ¹⁸O > +6‰.