A RECORD OF SEISMIC BOILING PRESERVED IN THE OXYGEN AND HYDROGEN ISOTOPES OF QUARTZ VEINS FORMED AT THE BRITTLE-PLASTIC TRANSITION

Stable isotope investigations have demonstrated boiling of fluids in fairly shallow ore deposit settings. This process may be sustained at crustal depths < a few kilometers, but at greater depths, sustained boiling is prohibited by large hydrostatic pressures. Consequently, if evidence of boiling of non-magmatic waters in faults is shown at these greater depths, it may be best explained by fault dilations at seismic rates. Examples related to aftershock activity along subsidiary strands and jogs between larger strike-slip faults have been linked to major gold deposits and related to boiling or flash vaporization of fault fluids.

Quartz veins in the Saddlebag Lake pendant, Sierra Nevada, California, associated with the Late Cretaceous, transpressive Sierra Crest shear zone preserve two distinct isotope trends in dD-d¹⁸O space. The trends largely overlap in quartz d¹⁸O values (~14 to -3 per mil) and are distinct in their fluid inclusion dD values. The first trend has dD values as low as -135 per mil and is interpreted as an oxygen-buffered mixing system between earlier devolatization of an adjacent pluton and later incursion of meteoric-hydrothermal water along faults. The second trend only has dD values > -112 per mil and is interpreted as quartz precipitated from the fluids that formed the first trend, but after fractionation due to boiling during seismic ruptures. Vein microstructures and textures indicate that boiling took place during quartz precipitation and before subsequent recrystallization.

A particular focus of this study is a large (up to 11.5 meter-wide) incrementally grown quartz vein located along a fault: the vein consists of four distinct quartz domains defined by color and isotopic composition, all crosscut by younger quartz veins. Isotope values from the four domains all fall on the first (mixing) trend, and the younger crosscutting veins all fall on the second (boiling) trend. We’ve also examined a quartz-sealed jog formed by dilation of a right step-over, which has the lowest d¹⁸O values observed in both trends. These observations suggest that the stable isotope values of quartz veins from > 2 km depth may in some cases record evidence of boiling, thus providing evidence of episodic ruptures at seismic strain rates. This new test could be utilized as a tool for both paleoseimological and ore investigations.