A MEASURED CORRELATION BETWEEN FLUORINE CONCENTRATIONS IN GEOTHERMAL SPRINGS AND GREAT BASIN CRUSTAL STRAIN RATES: A NEW GEOCHEMICAL TRACER FOR MANTLE FLUID INPUT

A significant unknown is the influence of mantle-derived fluids in affecting lithosphere rheology and strain localization within zones of continental deformation. We find a strong link between fluorine concentrations within geothermal springs and magnitudes of transtensional strain within the Great Basin region of the western U.S. This trend in fluorine concentrations correlates well with the ³He/⁴He ratios¹ consistent with the inference that the more dominant transtensional regions along the western edge of the Great Basin enables the escape of mantle-derived fluids through the middle-lower crust and into the upper brittle crust. Fluorine concentrations and ³He/⁴He ratios ^1,2,3,4 within geothermal sites are also generally correlated with positive dynamic topography, with occurrence often associated with the edges of the dynamically supported regions. Moreover, the occurrence of geologic deposits of fluorite records a similar spatial distribution as fluorine concentrations and ³He/⁴He measurements, with a marked association above mantle tomography⁵ velocity lows (both P and S). Recent work in the Rio Grande rift region⁶ suggests that fault associated fluorite veins there were derived from mantle fluids. We infer that abundant fluorite deposits likewise record events of mantle-derived fluid input into the crust, possibly during pulses of focused deformation. A major effort of our future work will be to obtain U-Pb ages for targeted deposits of fluorite to test the timing of these mantle-derived fluids. The dating of fault-associated fluorite vein deposits has the potential to test hypotheses regarding the timing of mantle fluid input and pulses of lithospheric strain and its link with mantle dynamics.

¹ Kennedyand van Soest, 2007, Science; ²Crossey et al., 2009, GSA; ³Karlstrom et al., 2013, Geology; ⁴Kennedy et al., 1997; ⁵Schmandt and Humphries, 2010, EPSL; ⁶Partey et al., 2009, Econ. Geol.