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Many of the Columbia River flood basalts that erupted in the states of Oregon, Washington, and Idaho during the Miocene are remarkably iron rich (12-17 wt% total FeO) and under volatile-free conditions would have been too dense to erupt through continental crust. Their eruption through >20 km of sialic crust sets limits oil their minimum, pre-eruptive total volatile ([H.sub.2]O + C[O.sub.2]) concentrations. These limits are obtained by quantitatively comparing the average density, of the overlying crustal column with the calculated effect of dissolved [H.sub.2]O or exsolved C[O.sub.2] on the density of various Columbia River tholeiitic basalts as a function of depth. Minimum total volatile ([H.sub.2]O + C[O.sub.2]) concentrations range to >4 wt% and are therefore considerably higher than those found in tholeiitic basalts at oceanic islands, like Hawaii. These conclusions are consistent with the strong enrichments in other incompatible elements (e.g., K, P, Ba) observed in the Columbia River flood basalts. Such high concentrations of [H.sub.2]O + C[O.sub.2] would have had a marked effect on the solidus temperature of the source region, which may explain the remarkably large individual flow volumes (~[10.sup.3] [km.sup.3]) that characterize this flood-basalt province. Keywords: Columbia River, flood basalts, volatiles, melt density.

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic (super 187)Os/(super 188)Os; however, the postorogenic lavas are characterized by radiogenic (super 187)Os/(super 188)Os ratios (0.139-0.559). Simple modeling shows that only very large degrees of melt depletion (>25%-30%) can lower source Re/Os ratios sufficiently to permit time-integrated development of subchondritic (super 187)Os/(super l88)Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by Keywords: Re-Os isotopes, subcontinental lithospheric mantle, postorogenic lavas.

A well-known second-order mass extinction took place during the Pliensbachian and Toarcian Stages of the Early Jurassic. First recognized as a minor Pliensbachian peak in the global extinction rate, it has alternatively been interpreted as a regional response to the early Toarcian oceanic anoxic event. Detailed studies established it as a global long-term event spanning five successive ammonoid zones. Here we present a revised time scale based on high-precision U-Pb ages resolved to the zone level, which suggests that elevated extinction rates were sustained for about 4 m.y. and peak extinction occurred at 183 Ma. Recent isotopic dating of flood basalts from the southern Gondwanan Karoo and Ferrar provinces documents a culmination in volcanic activity ca. 183 Ma. The onset of volcanism is recorded as an inflection and start of a rapid rise of the seawater [sup.87]Sr/[sup.86]Sr curve. The synchrony of voluminous flood basalt eruptions and biotic crises, as already noted for three of the major mass extinctions, permits a causal relationship, which in this case may be mediated by widespread oceanic anoxia. Keywords: extinction, Toarcian, flood basalt, oceanic anoxic event, time scale.

The theory that the northern Taiwan volcanic zone is an extension of the Ryukyu volcanic front may not be feasible stemming from geochemical data which reveal wide differences in Nb/Zr and Ba/Zr basaltic rock ratios. Moreover, the termination of magmatism of the northern Taiwan volcanic zone was simply coincidental to the opening of the Okinawa Trough. Data suggest that volcanic rocks older than 2 Ma can be found in both eastern and western sides of the zone. Hence, magmatic activity did not cease due to opening of the Trough.