REGIONAL IMPLICATIONS OF LOW-δ18O SKARNS OF THE JURASSIC MOJAVE SEGMENT OF THE SIERRA NEVADA ARC

Oxygen isotope ratios of skarn garnet can be used to trace hydrothermal fluid sources; regional and small-scale variation in δ¹⁸O can potentially resolve magmatic flare-ups, fluid pulses, and relative depths of hydrothermal systems.

Laser fluorination (LF) δ¹⁸O data from grossular-andradite garnet in five skarn locations in the Mojave segment of the Jurassic Sierra Nevada arc vary by ~14.5‰ (−8.9 to +5.6‰ VSMOW). These values are interpreted to reflect mixing of meteoric and magmatic-derived hydrothermal fluids. Individual skarn bodies may have internal variations of as much as ~13‰ or as little as ~1‰. Secondary ion mass spectrometry (SIMS) analysis of two skarn garnets from the Lucerne Valley region exhibit strong oscillatory zoning, and reveal cores with δ¹⁸O values of −9.6‰, internal variations of −9.4‰ to −3.3‰, and rims of −2.2‰ and −2.9‰ (precision ±0.3; 2σ). In general, SIMS δ¹⁸O values are decoupled from grossular and andradite mole fractions. Binary mixing models suggest that meteoric water may account for as much as ~85% of the hydrothermal fluid budget.

Skarn stable isotopes are particularly useful to reconstruct aspects of the paleogeography of the southwestern Cordillera. Jurassic skarns of the eastern Sierra Nevada generally lack low-δ¹⁸O values, and may indicate formation at depths greater than that at which brittle deformation in hydrothermal systems would allow infiltration of surface water; δ¹⁸O values of the Jurassic eastern California skarns suggest mixtures of igneous fluids with a strong influence of sedimentary protolith compositions. In contrast, the abundance and magnitude of negative δ¹⁸O values from Jurassic skarn garnets in the Mojave require that skarn-forming systems were charged with meteoric water. This finding shows that the Mojave hydrothermal system originally documented by Solomon and Taylor (1991) and Battles and Barton (1995) was and not dominated by sea water, but instead had a surface-water source, and indicates that the arc was inboard of the Jurassic shoreline. On-going efforts to directly date skarn minerals are designed to bracket the longevity of the regional hydrothermal system in the Mojave. Lastly, skarn mineral stable isotope compositions may lend insight to the overall composition of meteoric water and paleogeography during the Mesozoic.