SHALLOW, HOT, MANTLE MELTING IN THE HIGH LAVA PLAINS, OREGON

Primitive basaltic magmas have erupted from the Cascade arc east across the extent of the Oregon High Lava Plains (HLP) since the Miocene. Holocene volcanism extends from Newberry Volcano behind the Cascade arc eastward to the Diamond Craters and Jordan Valley volcanic fields. Primitive basalts erupted at these centers are characterized by high Mg# (>70), high Ni (> 150 ppm) and geochemical signatures associated with subduction zone magmas (HFSE depletion, LILE enrichment). High pressure – temperature experiments and petrologic studies of these lavas provide estimates of their mantle melting temperature, pressure and pre-eruptive H₂O content. Across the HLP, mantle melts are extracted from shallow depths near the Moho. To the east at Jordan Valley, magmas were in equilibrium with mantle peridotite at 1 GPa, a pressure equivalent to the depth to the Moho beneath the volcano based on recent seismic evidence. In the west at Newberry volcano, experiments on primitive calc-alkaline basalts also place the depth of melt extraction at the top of the mantle near the Moho. Mantle potential temperature is ~1300 ^oC and is not anomalously high. Compositional evidence preserved in phenocryst minerals in basalts provides evidence of the pre-eruptive H₂O contents. Plagioclase – olivine pairs that crystallized under near liquidus conditions in shallow magma chambers show that the Newberry basalts have high H₂O contents, as high as 4 wt. %, whereas Jordan Valley lavas have very low H₂O contents (< 0.3 wt. %). Both wet and dry magmas are present at Newberry, and a few, nearly anhydrous high alumina olivine tholeiite lavas have been erupted on the Newberry edifice along with the more common high-H₂O, subduction-related, calc-alkaline basalts. To the east, drier magmas are more abundant, but H₂O-rich magmas are still present. Shallow melt extraction, from depths in the mantle near the Moho, imply that melting in the HLP occurs as buoyant diapirism where the mantle and derivative melts re-equilibrate and separate near the top of the melting column.