FLUID CHANGES ALONG A TRANSECT OF QUARTZITE FROM STRONG TO WEAK: WHERE DID THE WATER GO?

Detailed petrography, cathodoluminescence (CL), electron backscatter diffraction (EBSD), and oxygen isotopes from Secondary Ion Mass Spectrometry (SIMS) spot analyses on Harkless quartzite collected along a 4 km traverse reveal changes to the intragranular fluids involved in mechanical weakening of quartz. The Harkless quartzite, in the White Mountains of California, has been affected by regional metamorphism in the greenschist facies and can be mapped into the contact aureoles of plutons where it is intensely recrystallized and deformed at temperatures high enough for partial melting within ~200 m of the pluton contact. Quartzites 4km from the EJB pluton exhibit sedimentary grain shapes and CL images reveal cements (clastic cores and cement rims with similar crystal structure). SIMS oxygen isotope ratios (¹⁸O/¹⁶O) from these grains reveal that cores and rims have been isotopically homogenized to ~16‰ (d¹⁸O VSMOW) and we assume this occurred during greenschist facies regional metamorphism prior to pluton emplacement and is consistent with background values collected elsewhere in the range. At 2.5 km from the pluton, d¹⁸O has been shifted to ~14‰ and fluctuates between 13 & 14‰ into the contact. Intense fracturing and healing, seen with CL, seems to play a role in introducing water to grain interiors early in the transect. Some grains have lower d¹⁸O values adjacent to boundaries or within healed fractures but most of the of the samples exhibit no observable trends with little fluctuation. Because the SIMS automatically also collects mass 17 (¹⁶OH), we are also able to obtain the amount of H in these quartzites using a standard also measured with FTIR. H amounts are highly variable but one trend is between cores and edges of recrystallized grains; cores varying from 10 to 40 ug/g H₂O and edges can be up to several hundred ug/g H₂O. It is clear that the shifted isotopes in quartzites closer to the pluton record exchange with water but H data record very low amounts of water in these quartzites, especially in the core regions of grains. The higher amounts of H in rims suggests that H is more recently mobile. Our analysis suggests that only small amounts of water are necessary for weakening of naturally deformed quartz, similar to other recent work, or that the water has been removed during deformation and recrystallization or during cooling/annealing of the pluton-aureole system.