THERMODYNAMIC AND GEOCHEMICALLY CONSTRAINED MODELING OF THE GENESIS OF LOW-δ18O RHYOLITES IN THE CENTRAL SNAKE RIVER PLAIN

The >15,000 km³ of low-δ¹⁸O rhyolites (<4‰) found in the Central Snake River Plain (CSRP) require a source component that has exchanged oxygen at high temperatures with surface derived meteoric fluids. Meteoric water has rarely been shown to penetrate further than a few kilometers into the crust, so these rhyolites likely require a voluminous supply of material that, at some time, resided in the shallow crust.

In order to assess the geochemical suitability of potential source rocks, we modeled major elements using MELTS software; fractionation of trace elements during melting and crystallization; and Sr, Nd, Pb and O isotopes with simple mixing models. Abundant, low-δ¹⁸O igneous rocks, of Eocene age, have been identified in the region; including a low-δ¹⁸O volcanic unit south of the CSRP found during this study. All source rock compositions used are from unmodified geochemical analyses of samples collected from regions around the margins of the CSRP. The best fit MELTS models for petrogenesis indicate that ~75% partial melting of these Eocene rocks reproduces most CSRP compositions, with <25% fractional crystallization of observed phases necessary to reconcile some trace elements (e.g. Sr and Ba). For most samples, radiogeneic isotope constraints can be satisfied with the addition of 10-20% mantle derived basalts to the Eocene partial melts.

In addressing the necessary heat flow, we used simple thermodynamic calculations to quantify the minimum heat needed to produce hydrothermal alteration of a suitable degree to provide the source material for the CSRP rhyolites (ave. δ¹⁸O of 2‰). Accounting for heating meteoric water to ~300° C, and multiplying that by calculated water to rock ratios of ~1.5, alteration requires ~2000 joules/gram. Calculations using MELTS suggest the minimum amount of heat necessary to produce the CSRP rhyolites by melting silicic crust is ~2200 j/g, which suggests that alteration to low-δ¹⁸O values requires a heat flux of a similar magnitude to that required to produce silicic melts.

Our modeling suggests that 1) locally outcropping Eocene plutonic and volcanic rocks are a geochemically suitable source material for the CSRP rhyolites, and 2) if hydrothermal alteration of the source resulted from the same thermal event that produced CSRP magmatism, the required heat flux may be prohibitively high.