Triple oxygen isotopes of meteoric hydrothermal systems - implications for palaeoaltimetry

We use triple oxygen isotopes of altered granitic rocks to determine the isotope composition of meteoric waters in a fossil hydrothermal system, the low δ¹⁸O Eocene Idaho Batholith, originally studied by Criss and Taylor (1983). In doing so we: 1) test whether meteoric water values estimated from previous δ¹⁸O and δD analyses on quartz, feldspar and biotite are robust and 2) determine the palaeoelevation of the Eocene highlands that are presently constrained primarily by the δ¹⁸O and δD of paired muscovite and quartz from core complexes and altered granites. Our calculated δ¹⁸O values of meteoric water are higher than estimates that use combined feldspar δ¹⁸O and biotite δD measurements in these hydrothermally altered granites and δD values from muscovite from nearby core complexes (Mulch et al., 2004). Both methods are consistent with a high elevation (∼3.1 to 4.7 km) Eocene highland in the northwestern U.S. Cordillera.