Petrology of high-MgO bronzite andesite resembling boninite at DSDP Hole 60-458

A high-MgO andesite which is texturally similar to boninite and a variolitic basalt collected from Site 458, about 100 km west of the Mariana Trench, have been studied through microprobe analyses and melting experiments at high water pressures. The boninite-type andesite is very similar in composition and texture to a boninite from Bonin Islands, except that the former is more calcic than the latter. The variolitic basalt contains magnesian pigeonite (Ca12Mg74Fe14) in cores of augite microphenocrysts. This pigeonite crystallized at temperatures above 1200°C. In the melting experiments of the boninite-type rock, clinopyroxene crystallizes as a liquidus phase at pressures at least above 8 kbar. No olivine crystallizes near the liquidus temperatures, indicating that the magma of this rock cannot be in equilibrium with the upper mantle periodotite (lherzolite) at depths at least greater than 25 km. The boninite-type rock is probably a product of fractional crystallization of a more primitive magma (e.g., olivine-bearing boninite magma) by separation of olivine and orthopyroxene. The magma of the variolitic basalt also cannot be in equilibrium with the upper mantle peridotite, and may be a product of fractional crystallization of a more primitive basaltic magma.

Barium/Calcium and Cadmium/Calcium ratios of sediment core RC13-229

A new benthic foraminiferal Ba/Ca and Cd/Ca data set from core RC13-229 in the deep Cape Basin indicates only small variations in bottom water nutrient concentrations in Circumpolar Deep Water (CPDW) over the last 450 kyr. Variability in the Ba record is characterized by somewhat higher values during glacial periods, consistent with a reduction in the flux of Ba-depleted North Atlantic Deep Water to the Southern Ocean during glacial periods. The small changes in the Ba and Cd records contrast with the large and systematic increase in CPDW nutrients during glacial periods implied by the benthic delta13C record. This discrepancy, essentially an extension of the well-known Southern Ocean Cd-delta13C conflict, is evaluated by transforming RC13-229 paleochemical data into carbonate parameters using the modern oceanic relationships between delta13C, Cd, and SumCO2 and between Ba and alkalinity. Calculations using Cd/Ca to estimate past variations in CPDW SumCO2 and Ba/Ca to estimate past variations in CPDW alkalinity yield carbonate ion concentrations that exceed calcite saturation throughout the record length, with generally higher carbonate ion values associated with glacial intervals (opposite in sense to the RC13-229 %CaCO3 record). Substituting delta13C to estimate SumCO2 leads to extreme calcite undersaturation at this site during glacial periods, clearly inconsistent with the preservation of calcite throughout the length of RC13-229. Accepting the carbon isotope record as a direct measure of past variations in CPDW SumCO2 concentrations requires that both the Cd and Ba evidence for limited nutrient and alkalinity changes be disregarded.