Sub-mm Scale Research Articles

A noble gas profile across a Hawaiian mantle xenolith: Coexisting accidental and cognate noble gases derived from the lithospheric and asthenospheric mantle beneath Oahu

A noble gas profile across a garnet pyroxenite xenolith from Salt Lake Crater, Oahu, Hawaii, provides information about the scale and origin of noble gas heterogeneities within such rocks. Variations in both absolute and relative noble gas concentrations are large and comparable to those observed between individual Salt Lake Crater pyroxenite xenoliths. 3He/4He varies from 7.7 to 9.4 times the atmospheric value (Ra) and correlates inversely with 40Ar/36Ar, which ranges between 4100 and 9700. Neon krypton and xenon isotopes are uniform and indistinguishable from air, with the exception of excess 129Xe. Overall, the observed noble gas compositions reflect a derivation from depleted MORB-type mantle sources.The spatial distribution of noble gas signatures within the xenolith and the observed correlation between helium and argon isotopes suggest the presence of two different noble gas components which are trapped in different phases and are unevenly distributed within the xenolith. 40Ar/36Ar and 1/36Ar correlate inversely, indicating that atmospheric contamination is insignificant. Hence, the observed isotopic variations reflect mixing of two mantle-derived noble gas components. Correlations between HeAr isotopes and CO2/H2O in different pyroxenites from Salt Lake Crater, including our sample, reveal that the first component is characterized by highly radiogenic helium and argon isotopes and related to abundant secondary CO2-rich fluid inclusions. Given the high diffusivity of He at mantle temperatures (Hart, 1984), the observed helium isotope heterogeneities on a sub-mm scale require that the fluids were introduced concurrently with eruption. This interpretation is supported by the low entrapment depths of fluid inclusions in Salt Lake Crater pyroxenites (<30 km; Murck et al., 1978). This implies that the fluids are genetically related to the host magma itself and reflect its composition. The second noble gas component is interpreted as being magmatic, i.e., cognate to the basaltic magma from which the pyroxenite precipitated within the mantle. It is proposed that this component resides inside the mineral lattices and was trapped during magmatic crystallization.Our results indicate that the depleted lithospheric mantle source of the post-erosional host magma (Honolulu Volcanic Series) is characterized by 40Ar/36Ar ≥ 10000, R/Ra ≤ 7.5, and excesses in129Xe. Noble gas signatures of the asthenospheric mantle source parental to the pyroxenite-producing magma are less radiogenic with respect to these isotopes. Combining noble gas and strontium, neodymium, and lead isotope evidence, we propose that the asthenosphere beneath Oahu was originally similar to depleted MORB-type mantle, but became slightly modified by noble gases and other incompatible trace elements derived from the Hawaiian mantle plume prior to partial melting.This study provides evidence that noble gas isotopes and solid radiogenic isotopes are coupled in the mantle sources of basalts (e.g., Allègre et al., 1983), but decoupled in xenoliths (e.g., Vance et al., 1989).

The Microsphere Plate: a new type of electron multiplier

Microsphere Plates (MSPs), a new type of electron multiplier, consisting of sintered disks of glass beads, have recently become available from El-Mul Technologies Ltd. The principles of MSP operation are similar to those of microchannel plates (MCPs). We present a survey of the gain, resistance, dark noise, count rate capability, charge abstraction lifetime and image characteristics of a number of standard microsphere plates (of thickness 0.7 and 1.4 mm), operated both singly and as two-stage multipliers. Modal gains lay in the range 2.4–43 pC with pulse height relative FWHM values as low as 52%. MSP dark noise is relatively low (⋍0.5 counts cm 2s −1) for well-chosen lower level discriminator settings. The image response is globally uniform, having local variations in image intensity on sub-mm scale, corresponding to the structure of MSP. Spatial resolution of an MSP detector may be better than 250 μm FWHM. No dependence of the gain on the angle of radiation incidence was observed for an angular range of 90°±25° (relative to the MSP surface), while the 2540 Å UV relative detection efficiency varied by 10–15% over the same range. Lifetest measurements revealed that after removal of 8 × 10 −4C cm −2, the detector modal gain fell from 8 pC to half of its initial value. This fall in gain was compensated by raising the bias voltage.

Microelectrode studies of organic carbon degradation and calcite dissolution at a California Continental rise site

Fine scale porewater profiles of resistivity, O 2, pH, and pCO 2 were measured in situ with microelectrodes at a site on the California continental rise. They are reported here with more traditional measurements of shipboard porewater NO 3 −, alkalinity and TCO 2, and sediment organic C and CaCO 3 profiles. A numerical model encompassing diffusion, advection, and multiple reaction terms is applied to these data to characterize organic carbon degradation and CaCO 3 dissolution processes near the sediment-water interface. A benthic O 2 flux of 58 μmol cm −2 yr −1 over the top 1 mm sediment is estimated based on one measured sub-mm depth scale O 2 profile. The numerical model indicates that this high O 2 consumption rate is related to a very labile fraction of organic C which has a degradation rate constant of approximately 4.5 × 10 −8 s −1 (1.4 yr −1 ), and which is not mixed downward by biological mixing. Seasonality in the benthic O 2 consumption rate of this site may be attributed to variations in the input rate of this highly labile organic C. Degradation rate constants of the bulk organic C by oxygen reduction and by nitrate reduction are estimated to be about the same (1 × 10 −9 s −1 ). The organic C rain rate responsible for the bulk organic C is estimated to be 53 μmol cm −2 yr −1 . This estimate exceeds the average rain rate determined by sediment trap collections at this site. Saturation state profiles, calculated from the in situ pH and pCO 2 data and from modelling the in situ O 2 and pH data, both indicate that the porewater is undersaturated with respect to calcite to a depth of at least 8 cm. The dissolution rate constant of calcite in this CaCO 3-poor station is determined to be 5–10% day −1 (assuming the reaction order is 4.5 ), which is close to the minimum estimates for environments richer in calcite. A uniform calcite dissolution rate constant, therefore, may be adequate for representing carbonate dissolution in global-scale models of ocean chemistry.