Geochemistry and petrogenesis of Proterozoic diabase in the southern Death Valley region of California

Diabase sills and dikes of Proterozoic age intrude crystalline basement and the overlying Crystal Spring Formation in the southern Death Valley region of California. Despite pervasive deuteric alteration, analyses of relict plagioclase (An_66-45), titaniferous augite, and ilmenite permit the calculation of initial crystallization temperatures of 1,165±25° C for plagioclase and 1,110±25° C for augite with an oxygen fugacity of 10^–11 atm. The early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase.Geochemical arguments show that deuteric alteration has had little effect on the whole rock chemistry which has a mildly alkaline, transitional character. Mathematical models support the interpretation that chemical variations are principally the result of flow differentiation. Also, about 15 to 25% olivine or augite may have fractionated from the melt prior to intrusion. The mildly LREE enriched composition (normalized La/Yb=4.27 to 6.13) is best modelled by a 5% melt fraction from a moderately LREE-enriched source. Melt derived from a model garnet peridotite with accessory amphibole is compatible with the observed major and trace element chemistry. The diabase is compositionally akin to other middle Proterozoic basaltic rocks in North America which formed in extensional settings, and it is interpreted as a manifestation of widespread extensional tectonism.     

The chemistry of sublimates collected directly from lava fountains at Kilauea Volcano,Hawaii

During 1970, it was occasionally feasible to collect sublimate from directly above the lava fountain in the crater of Mauna Ulu on the east rift zone of Kilauea Volcano, when the level of the lava pool had dropped within the crater. Collecting equipment was suspended down the steep wall to a position above the fountain. Collections were made on quartz wool held within open-ended quartz tubes and, for silica detection, on stainless steel wool in a stainless steel tube. The main components in the sublimate were, in order of decreasing concentration for the best sample, Na, Ca, Al, Fe, Mg, K, B, Si, Ti, Zn, H⁺, NH₄⁺, Cu, Ni in the form of sulfates, chlorides and fluorides.In order to investigate the forms in which the sublimate ions occur under different conditions of temperature and oxidation, the equilibrium compositions of the compounds most likely to be present were calculated. This was done for those important components for which thermodynamic data are available, using a computer program to calculate the minimum free energy for the mixture. The results indicate that, for primary conditions of high temperature and low oxygen partial pressure, the halides were the most likely form of the metallic compounds. Particulate sulfates appear under increasing oxidizing conditions caused by the access of air. These conclusions were reinforced by collections made from holes drilled through the thin crust of a lava lake formed during the same eruption.     

The calculation of the statistical counting error for 222Rn scintillation counting

The calculation of the statistical counting error for alpha-scintillation counting of²²²Rn is complicated by the fact that two of the short-lived daughter products of²²²Rn are alpha emitters. All observed daughter-product counts attributable to parent decays that have already been detected must be excluded in the calculation of this error. An equation is derived which takes this into consideration.     

Verification of route-based road weather forecasts

Route-based road weather forecasting is increasingly becoming the standard methodology for winter maintenance decision making (i.e. whether or not to salt the road network) by the highway industry in the UK. Route-based forecasting requires, for the first time, the accuracy of forecasts around routes and away from sensor sites to be verified. This is essential so that end users have confidence in the models’ ability to accurately predict road surface temperature at every point around their road network. A new methodology for verifying route-based forecasts is proposed that uses clustering techniques to create clusters of forecast points with similar geographical and infrastructure characteristics. This facilitates the analysis of forecast statistics at the cluster level, which is found to improve statistical assessment of model performance since verification can be achieved at a much higher resolution than the current methodology allows. Furthermore, verification of the full spatial extent of a route-based forecast can be achieved with fewer forecast points since the majority of thermal variations around the road network are well represented by the clustering solutions. A new sampling strategy is proposed that potentially enables verification at the full spatial and temporal resolution.     

Geochemistry and chronology of the Bunburra Rockhole ungrouped achondrite

Bunburra Rockhole is a unique basaltic achondrite that has many mineralogical and petrographic characteristics in common with the noncumulate eucrites, but differs in its oxygen isotope composition. Here, we report a study of the mineralogy, petrology, geochemistry, and chronology of Bunburra Rockhole to better understand the petrogenesis of this meteorite and compare it to the eucrites. The geochemistry of bulk samples and of pyroxene, plagioclase, and Ca‐phosphate in Bunburra Rockhole is similar to that of typical noncumulate eucrites. Chronological data for Bunburra Rockhole indicate early formation, followed by slow cooling and perhaps multiple subsequent heating events, which is also similar to some noncumulate eucrites. The ²⁶Al‐²⁶Mg extinct radionuclide chronometer was reset in Bunburra Rockhole after the complete decay of ²⁶Al, but a slight excess in the radiogenic ²⁶Mg in a bulk sample allows the determination of a model ²⁶Al‐²⁶Mg age that suggests formation of the parent melt for this meteorite from its source magma within the first ~3 Ma of the beginning of the solar system. The ²⁰⁷Pb‐²⁰⁶Pb absolute chronometer is also disturbed in Bunburra Rockhole minerals, but a whole‐rock isochron provides a re‐equilibration age of ~4.1 Ga, most likely caused by impact heating. The mineralogy, geochemistry, and chronology of Bunburra Rockhole demonstrate the similarities of this achondrite to the eucrites, and suggest that it formed from a parent melt with a composition similar to that for noncumulate eucrites and subsequently experienced a thermal history and evolution comparable to that of eucritic basalts. This implies the formation of multiple differentiated parent bodies in the early solar system that had nearly identical bulk elemental compositions and petrogenetic histories, but different oxygen isotope compositions inherited from the solar nebula.     

Denitrification in anoxic sediments supported by biological nitrate transport

Biologically available nitrogen (fixed N) is removed from the oceans by metabolic conversion of inorganic N forms (nitrate and ammonium) to N₂ gas. Much of this removal occurs in marine sediments, where reaction rates are thought to be limited by diffusion. We measured the concentration and isotopic composition of major dissolved nitrogen species in anoxic sediments off the coast of California. At depths below the diffusive penetration of nitrate, we found evidence of a large nitrate pool transported into the sediments by motile microorganisms. A ∼20‰ enrichment in ¹⁵N and ¹⁸O of this biologically transported nitrate over bottom water values and elevated [N₂] and δ¹⁵N-N₂ at depth indicate that this nitrate is consumed by enzymatic redox reactions with the production of N₂ as the end product. Elevated N₂O concentrations in pore waters below the nitrate diffusion depth confirm that these reactions include the denitrification pathway. A data-constrained model shows that at least 31% of the total N₂ production in anoxic sediments is linked to nitrate bio-transport. Under suboxic/anoxic regimes, this nitrate bio-transport augments diffusive transport, thus increasing benthic fixed nitrogen losses and the reducing burial efficiency of sedimentary organic matter.     

Spinelloid phases in the nickel gallosilicate system

The phase diagram of the NiO-Ga₂O₃- SiO₂ system has been investigated at high temperatures (up to 1550° C) and 1 atm. pressure. The only ternary phases observed in this system are the spinelloid phases I, II, and V, forming on the NiGa₂O₄-Ni₂SiO₄ pseudo-binary join at temperatures above 1400° C. Our results show that phase V is stable at higher temperature than phase I, while phase II may be metastable. The stability of phases I and V has been confirmed by the successful growth of single crystals from a silica-rich flux. Phases I, II, and V have been characterized by powder X-ray diffraction and transmission electron microscopy. The dimensions of their orthorhombic unit-cells are: (I) a= 5.7741(5), b=11.712(1), c=8.2387(9) Å; (II) a= 5.765(1), b= 17.619(3), c= 8.238(2) Å; (V) a= 5.7914(4), b=8.7809(7), c=8.2346(6) Å. High resolution electron microscopy has also revealed the formation of microscopic intergrowths similar to those previously observed for the spinelloid phases of the NiO-Al₂O₃-SiO₂ system.