Geochemistry, Geophysics, Geosystems

Journal Information
ISSN / EISSN : 15252027 / 15252027
Current Publisher: American Geophysical Union (AGU) (10.1029)
Former Publisher: Wiley (10.1002)
Total articles ≅ 4,475
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F.O. Marques, A. Hildenbrand, H. Zeyen, C. Cunha, S.S. Victória
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008754

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Tobias P. Fischer, Alessandro Aiuppa
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008690

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Andrea M. Erhardt, Alexandra V. Turchyn, J.A.D. Dickson, Aleksey Y. Sadekov, Paul D. Taylor, Mark A. Wilson, Daniel P. Schrag
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008448

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G.W. Ter Maat, K. Fabian, N. S. Church, S. A. McEnroe
Geochemistry, Geophysics, Geosystems, Volume 21; doi:10.1029/2019gc008761

Abstract:Realistic geometries of magnetite grains from the Stardalur volcano, Iceland, were obtained by FIB‐SEM nanotomography. These magnetite grains are subdivided by oxidation‐exsolution lamellae of ilmenite. Magnetic properties of these grains were modeled without internal stress using the three‐dimensional micromagnetic code MERRILL. The influence of grain shape and size were isolated by modeling hysteresis loops of the same grains with and without exsolution microstructures. The resulting coercivities Hc are up to 1.5 times higher, and the Mrs/Ms ratios are twice as high for the grains with exsolution than for those without. Both modeled values are a factor of 10 smaller than the measured bulk data from the same sample. This difference between stress‐free models and measured hysteresis loops suggests that the internal stress due to the formation of the oxidation‐exsolution lamellae is the dominant mechanism of coercivity and remanence enhancement. By comparing the approach‐to‐saturation behavior of modeled and measured hysteresis loops the internal stress is quantified to about 100 MPa. The formation of lamellae has two effects on magnetic properties. 1) the apparent grain size is geometrically reduced. This effect increases Mrs and Hc by up to a factor of two. 2) the formation of lamellae produces internal stress fields which provide additional anisotropy energy that deflect the magnetic spins and apparently increase Mrs and Hc by up to a factor of ten. Accordingly, stress dominates the remanent magnetic properties in the Stardalur basalts and may be the decisive effect explaining its unusual remanent‐dominated ground magnetic anomaly of up to 27.000 nT.
Luan J. Heywood, Susan M. Debari, James B. Gill, Susanne M. Straub, Julie C. Schindlbeck‐Belo, Ricardo D. Escobar‐Burciaga, Jon Woodhead
Geochemistry, Geophysics, Geosystems, Volume 21; doi:10.1029/2019gc008353

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E. Bowles‐Martinez, A. Schultz
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008831

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M. Zelenski, V. S. Kamenetsky, Yu. Taran, A. M. Kovalskii
Geochemistry, Geophysics, Geosystems, Volume 21; doi:10.1029/2019gc008802

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D. Quandt, P. Micheuz, W. Kurz, S. M. Bernasconi, D. Hippler, K. Krenn, C. A. Hauzenberger
Geochemistry, Geophysics, Geosystems, Volume 21; doi:10.1029/2019gc008745

Abstract:International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites, and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (< 30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35‐33 Ma and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (< 30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of > 95 % seawater and < 5 % basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20‐30 % basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.
Xiaomiao Li, Tingjiang Peng, Zhenhua Ma, Meng Li, Peiye Li, Zhantao Feng, Benhong Guo, Hao Yu, Xiyan Ye, Jun Zhang, et al.
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008682

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Tao Gou, Dapeng Zhao, Zhouchuan Huang, Liangshu Wang
Geochemistry, Geophysics, Geosystems; doi:10.1029/2019gc008812

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