40Ar39Ar and KAr dating of altered glassy volcanic rocks: the Dabi Volcanics,P.N.G.

KAr and ${}^{40}\text{Ar}{}^{39}\text{Ar}$ ages have been determined for altered submarine tholeiitic and boninite (high-Mg andesite) lavas from the Dabi Volcanics, Cape Vogel Peninsula, Papua New Guinea. ${}^{40}\text{Ar}{}^{39}\text{Ar}$ whole rock total fusion and plateau ages identify a Late Paleocene age for the tholeiitic lavas (58.9 ± 1.1 Ma) and also for the boninitic lavas (58.8 ± 0.8 Ma). Apparent KAr ages for the same samples range from 27.2 ± 0.7 to 63.9 ± 4.5 Ma, and young KAr ages for glassy boninites are probably due to variable radiogenic ⁴⁰Ar (⁴⁰Ar¹) loss. These new ages effectively reconcile previously ambiguous age data for the Dabi Volcanics and indicate contemporaneous tholeiitic and boninitic volcanism occurring in southeast PNG during the Late Paleocene.Smectites, developed as alteration products after glass in oceanic lavas commonly do not retain ³⁹Ar during or subsequent to irradiation, but in some cases may contain ⁴⁰Ar1. In the absence of other factors modifying K and Ar contents, samples which have not lost ⁴⁰Ar¹ from smectite and suffer ³⁹Ar loss only, are interpreted to have been altered immediately subsequent to the crystallization of the lava; whereas samples which have lost ⁴⁰Ar¹ as well as ³⁹Ar may be the result of either recent alteration, or of continuous ⁴⁰Ar¹ loss since the time of crystallization.     

Spatiotemporal analysis of residential flood exposure in the Atlanta,Georgia metropolitan area

Natural Hazards - This research examines changes in residential built-environment flood exposure within the current boundaries of the Atlanta, Georgia metropolitan statistical area, by estimating...     

Melt density and the average composition of basalt

Densities of residual liquids produced by low pressure fractionation of olivine-rich melts pass through a minimum when pyroxene and plagioclase joint the crystallization sequence. The observation that erupted basalt compositions cluster around the degree of fractionation from picritic liquids corresponding to the density minimum in the liquid line of descent may thus suggest that the earth's crust imposes a density filter on the liquids that pass through it, favoring the eruption of the light liquids at the density minimum over the eruption of denser more fractionated and less fractionated liquids.     

Kinetics of arsenopyrite oxidative dissolution by oxygen

We used a mixed flow reactor system to determine the rate and infer a mechanism for arsenopyrite (FeAsS) oxidation by dissolved oxygen (DO) at 25 °C and circumneutral pH. Results indicate that under circumneutral pH (6.3-6.7), the rate of arsenopyrite oxidation, 10^{−10.14±0.03} mol m⁻² s⁻¹, is essentially independent of DO over the geologically significant range of 0.3-17 mg L⁻¹. Arsenic and sulfur are released from arsenopyrite in an approximate 1:1 molar ratio, suggesting that oxidative dissolution by oxygen under circumneutral pH is congruent. Slower rates of iron release from the reactor indicate that some of the iron is lost from the effluent by oxidation to Fe(III) which subsequently hydrolyzes and precipitates. Using the electrochemical cell model for understanding sulfide oxidation, our results suggest that the rate-determining step in arsenopyrite oxidation is the reduction of water at the anodic site rather than the transfer of electrons from the cathodic site to oxygen as has been suggested for other sulfide minerals such as pyrite.     

Os-Os systematics of Hawaiian picrites revisited: New insights into Os isotopic variations in ocean island basalts

Eighteen picrites (MgO > 13 wt.%) and three related basalts from six Hawaiian volcanoes were analyzed for ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os. Variations in these ratios reflect long-term Re/Os and Pt/Os differences in the mantle source regions of these volcanoes. ¹⁸⁷Os/¹⁸⁸Os ratios vary from ∼0.129 to 0.136, consistent with the range defined by previous studies of Hawaiian picrites and basalts. Samples with lower ¹⁸⁷Os/¹⁸⁸Os are mainly from Kea trend volcanoes (Mauna Kea and Kilauea), and the more radiogenic samples are mainly from Loa trend volcanoes (Mauna Loa, Hualalai, Koolau and Loihi). As previously suggested, differences in ¹⁸⁷Os/¹⁸⁸Os between volcanic centers are most consistent with the presence of variable proportions of recycled materials and/or pyroxenitic components in the Hawaiian source.¹⁸⁶Os/¹⁸⁸Os ratios vary from 0.1198332 ± 26 to 0.1198480 ± 20, with some samples having ratios that are significantly higher than current estimates for the ambient upper mantle. Although the range of ¹⁸⁶Os/¹⁸⁸Os for the Hawaiian suite is consistent with that reported by previous studies, the new data reveal significant heterogeneities among picrites from individual volcanoes. The linear correlation between ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁶Os/¹⁸⁸Os reported by a previous study is no longer apparent with the larger dataset. The postulated recycled materials and pyroxenites responsible for the dominant variations in ¹⁸⁷Os/¹⁸⁸Os are likely not responsible for the variations in ¹⁸⁶Os/¹⁸⁸Os. Such materials are typically characterized by both insufficiently high Os concentrations and Pt/Os to account for the ¹⁸⁶Os/¹⁸⁸Os heterogeneities. The lack of correspondence between ¹⁸⁶Os/¹⁸⁸Os variations and the Kea and Loa trends supports this conclusion.The primary cause of ¹⁸⁶Os/¹⁸⁸Os variations are evaluated within the framework of two mixing scenarios: (1) metasomatic transport of Pt and/or ¹⁸⁶Os-rich Os into some portions of the Hawaiian source, and (2) interaction between an isotopically complex plume source with a common, Os- and ¹⁸⁶Os-enriched reservoir (COs). Both scenarios require large scale, selective transport of Pt, Re and/or Os. Current estimates of HSE concentrations in the mantle source of these rocks, however, provide little evidence for either process, so the dominant cause of the ¹⁸⁶Os/¹⁸⁸Os variations remains uncertain.