A RE-OS INVESTIGATION OF MONOGENETIC BASALT VOLCANISM IN SE OREGON

The Jordan Valley Volcanic Field (JVVF) of southeastern Oregon provides an opportunity to examine the roles of source and process in the evolution of continental, monogenetic basalt volcanism. The JVVF is situated at the northern edge of the Owyhee Plateau portion of the Oregon Plateau province and lies within a transition between the ancient cratonic and Mesozoic and younger accreted terranes at ca. 117 degrees longitude. Two chemically and chronologically distinct JVVF eruptive sequences are identified: young (250 - 1.5 ka) mildly alkaline and older (5.5 - 0.5 Ma) tholeiitic basalts. The Sr, Nd, and Pb isotope characteristics of these basalts have been interpreted to represent primarily local mantle source heterogeneities within the context of regional and temporal mantle isotope heterogeneity. Here we report the initial results of a rhenium (Re) - osmium (Os) investigation of representative samples from the range of elemental and Sr-Nd-Pb isotope diversity previously documented in the JVVF.

Os isotope and concentration data has been obtained for 6 samples (3 alkaline, 3 tholeiitic), with Os concentrations and 187Os/188Os ranging from 2.7 to 58.8 ppt and 0.1683 to 0.2449, respectively. These values fall within the ranges reported for regional primitive tholeiitic magmas. The Os isotope ratios are above those obtained for bulk oceanic and continental mantle. The data set displays positive correlations between Os concentrations and indices of basalt differentiation (MgO, Ni) but the Os isotope ratios are not coupled to these indices. Rather, the Os isotopic compositions co-vary with other isotope and trace element ratio parameters that previously have been argued to result from primarily melting of and mixing between heterogeneous mantle domains. An aspect of this heterogeneity may derive from mafic, cpx-rich lithologies in the mantle lithosphere, an assertion supported by positive correlations between JVVF area Sc and Os concentrations. Moreover at least the very radiogenic Os isotopic compositions must reflect some overprinting from crustal interactions, and Re-Os data on additional whole rock, matrix, and olivine separates from JVVF basalts and spatially associated rhyolites (crustal melt proxies?) will be used to quantify the relative roles of mantle versus crustal reservoirs and processes.