CHEMICAL ALTERATIONS OF THE HOLBROOK CHONDRITE RESULTING FROM TERRESTRIAL WEATHERING     

Everett K. Gibson  Donald D. Bogard 《Meteoritics & planetary science》1978,13(3):277-289

The effects of chemical weathering on the Holbrook, Arizona chondrite were studied via major, minor, and trace element analysis along with noble gas measurements. Three samples of Holbrook were studied: a fresh specimen which fell in 1912, a specimen collected in 1931 and a specimen collected in 1968. The weathering of the meteorite produced changes in the Fe and Mg concentrations. The amount of metallic iron decreases with increasing degree of weathering. The MgO content decreases and the total iron content increases slightly with increasing weathering of the specimen, but these concentration differences are not necessarily due to weathering. Elemental concentrations for Ti, Ca, Al, P, Mn, Ni and Cr did not change significantly during weathering. Sodium may have been lost during weathering, whereas K showed a slight enrichment in abundance with terrestrial age. Trace elements C, Rb and Sr increased at least two-fold during weathering of the meteorite. Abundances of the radiogenic and cosmogenic noble gases decreased with increasing terrestrial age. Concentration ratios of certain cosmogenic noble gas nuclides which are commonly used as indicators of gas loss and shielding in chondrites show unpredictable behavior in the weathered chondrite.  相似文献   

Electrical conductivity in the Earth's mantle inferred from CHAMP satellite measurements—I. Data processing and 1-D inversion     

Jakub Velímský  Zden&#;k Martinec  Mark E. Everett 《Geophysical Journal International》2006,166(2):529-542

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A qualitative risk assessment for the impacts of climate change on nationally and internationally important geoheritage sites in Scotland     

Rachel M.L. Wignall  John E. Gordon  Vanessa Brazier  Colin C.J. MacFadyen  Nick S. Everett 《Proceedings of the Geologists' Association. Geologists' Association》2018,129(2):120-134

Climate change is a significant concern for nature conservation in the 21?st century. One of the goals of the 2014 Scottish Climate Change Adaptation Programme is to identify the consequences of climate change for protected areas and to put in place adaptation or mitigation measures. As a contribution to the process, this paper develops a methodology to identify the relative level of risk to nationally and internationally important geological and geomorphological sites in Scotland from the impacts of climate change. The methodology is based on existing understanding of the likely responses of different types of geosite to specific aspects of climate change, such as changes in rainfall, rising sea levels or increased storminess, and is applied to assess the likelihood of damaging impacts on groups of similar geoheritage features in sites with similar characteristics. The results indicate that 80 (8.8%) of the ～900 nationally and internationally important geoheritage sites in Scotland are at ‘high’ risk from climate change. These include active soft-sediment coastal and fluvial features, finite Quaternary sediment exposures and landforms in coastal and river locations, active periglacial features, sites with palaeoenvironmental records, finite or restricted rock exposures and fossils. Using this risk-based assessment, development of indicative geoheritage climate-change actions have been prioritised for these sites. Depending on the characteristics of the sites, management options may range from ‘do nothing’ to rescue excavations and posterity recording. Monitoring is an essential part of the management process to trigger evidence-based interventions.  相似文献   

An abiotic origin for hydrocarbons in the Allan Hills 84001 martian meteorite through cooling of magmatic and impact‐generated gases     

Mikhail Yu. ZOLOTOV  Everett L. SHOCK 《Meteoritics & planetary science》2000,35(3):629-638

Abstract— Thermodynamic calculations of metastable equilibria were used to evaluate the potential for abiotic synthesis of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in the martian meteorite Allan Hills (ALH) 84001. The calculations show that PAHs and normal alkanes could form metastably from CO, CO², and H² below approximately 250–300°C during rapid cooling of trapped magmatic or impact‐generated gases. Depending on temperature, bulk composition, and oxidation‐reduction conditions, PAHs and normal alkanes can form simultaneously or separately. Moreover, PAHs can form at lower H/C ratios, higher CO/CO² ratios, and higher temperatures than normal alkanes. Dry conditions with H/C ratios less than approximately 0.01–0.001 together with high CO/CO² ratios also favor the formation of unalkylated PAHs. The observed abundance of PAHs, their low alkylation, and a variable but high aromatic to aliphatic ratio in ALH 84001 all correspond to low H/C and high CO/CO² ratios in magmatic and impact gases and can be used to deduce spatial variations of these ratios. Some hydrocarbons could have been formed from trapped magmatic gases, especially if the cooling was fast enough to prevent reequilibration. We propose that subsequent impact heating(s) in ALH 84001 could have led to dissociation of ferrous carbonates to yield fine‐grain magnetite, formation of a CO‐rich local gas phase, reduction of water vapor to H², reequilibration of the trapped magmatic gases, aromatization of hydrocarbons formed previously, and overprinting of the synthesis from magmatic gases, if any. Rapid cooling and high‐temperature quenching of CO‐, H²‐rich impact gases could have led to magnetite‐catalyzed hydrocarbon synthesis  相似文献   

Subduction disruption,slab tears: ca. 1 Ma true collision of an ~30-km-thick oceanic plateau segment recorded by Yakutat slab nascent tear magmatism     

Matthew E. Brueseke  Jeffrey A. Benowitz  Alexander T. Bearden  Michael Everett Mann  Daniel P. Miggins 《地学学报》2023,35(1):49-57

Distinguishing the initiation of actual collision from flat-slab subduction of oceanic buoyant highs along convergent margins is elusive because both can lead to inboard deformation and disrupt magmatic arcs. Volcanoes with nascent tear magmatic signatures provide a means to document both the occurrence and timing of actual oceanic buoyant high collision. There is a ~40-year debate on when the true collision of the Yakutat plateau began in Alaska. Three newly identified ca. 1 Ma volcanoes with a north-to-south trench perpendicular orientation, nascent tear geochemical signatures, overlaying an imaged Yakutat slab tear, provide constraints on the timing of Yakutat collision and slab tearing. The ca. 1 Ma slab tear is coincident with Yakutat slab segmentation, northern continental Aleutian Arc rejuvenation, cessation of Wrangell Arc magmatism, increased collisional zone exhumation and eastern Yakutat trench abandonment. The documentation of nascent slab tear volcanoes may help resolve similar debates in other convergent margin settings.  相似文献