Papers by Alexander Rocholl
EGU General Assembly Conference Abstracts, Apr 1, 2018
Supplementary File 2. Representative microprobe analyses of olivine in the Aladag peridotites and... more Supplementary File 2. Representative microprobe analyses of olivine in the Aladag peridotites and chromitites.
Supplementary File 5. Representative microprobe analyses of chromite in the Aladag peridotites an... more Supplementary File 5. Representative microprobe analyses of chromite in the Aladag peridotites and chromitites.
Supplementary File 1. Sampling and analytical methods.
Geostandards and Geoanalytical Research, 2022
Goldschmidt2021 abstracts, 2021
Journal of Volcanology and Geothermal Research, 2018
North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an ... more North Su is the central volcanic edifice of the SuSu Knolls hydrothermal district, located in an extensional transform zone of the eastern Manus back-arc basin. The active submarine volcano hosts a vigorous hydrothermal system with black and white smokers, diffuse vents, and accumulation of native sulfur. Among its dacitic to andesitic lavas are remarkable hybrid rocks, containing highly magnesian olivine (Fo 81-94) phenocrysts with thin reaction rims in a dacitic matrix. We carried out a detailed petrological and geochemical study of these rocks in order to understand their formation, to constrain temperature and depth of magma storage, and to identify links between magmatic processes and hydrothermal activity. Phenocryst compositions suggest that the hybrid lavas formed by mixing between Mg-rich andesite to basaltic andesite with boninite affinities and dacitic to rhyodacitic magma. The resulting melts show strong enrichment of large ion lithophile elements, moderate enrichment of light rare earth elements and depletion of Nb, Ta and Ti, characterizing them as back-arc basin magmas. Compositional zonations at the outermost rims of olivine phenocrysts suggest a period of no more than a few days between the inferred mixing event and eruption. The hybrid magma had a relatively high temperature of 1000 C, as is indicated by mineral-melt thermometers. Mineral-host melt barometers yield a wide range of pressures with a mean of-21 206 MPa, reflecting low equilibration pressure and limited barometer performance. In addition, many clinopyroxene phenocrysts lack equilibrium between their strongly zoned rims and the host melt. The H 2 O (0.8-4.8 wt%) and CO 2 (11-360 ppm) contents of melt inclusions in clinopyroxene and plagioclase phenocrysts provide better constraints on the pressure range for pre-eruptive magma storage, yielding 40-150 MPa equivalent to 1-5 km beneath North Su's summit. The sulfur-H 2 O relationship in melt inclusions reflect a degassing trend that is consistent with formation of aqueous-sulfurous magmatic fluid, inferred to produce highly acidic vent fluids at North Su (Seewald et al., 2015). Our results suggest that a dynamic magmatic system, with magma mixing occuring shortly before ascent and eruption, causes rapid changes in composition and quantity of exsolved fluids. Because of the short distance to the seafloor, such fluctuations are rapidly transferred to the hydrothermal systems. This may be the main cause for the observed temporal, spatial and compositional variations in hydrothermal activity at North Su, and possibly at other vent fields in the Southeastern Ridges of the Manus Basin.
Ore Geology Reviews, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the ad... more This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Geochim Cosmochim Acta, 2004
We observed nanocrystalline diamonds in magmatic rocks from Hawaii (Salt Lake Crater). They occur... more We observed nanocrystalline diamonds in magmatic rocks from Hawaii (Salt Lake Crater). They occur in mantle xenoliths (Ga-pyroxenites) in melt inclusions in ortho- and clinopyroxene. The xenoliths are incorporated in the host lava and have been transported from the Earth's interior to the surface by volcanic eruptions. Consequently, such xenoliths allow an insight into the structure, the chemical composition and the P-T conditions of the Earth's mantle. Salt Lake Crater pyroxenites are interpreted as high-pressure basaltic cumulates trapped and adiabatically cooled within the Hawaiian lithosphere at 1000o - 1150oC and 1.6 - 2.5 GPa (50-80 km). The melt inclusions were investigated by using TEM and AEM. Specimen preparation was performed by focused ion beam technique (FIB) at the GeoForschungsZentrum Potsdam (GFZ). Promising melt inclusions in pyroxene have been selected from thin sections. FIB technique uses oil-free vacuum to avoid contamination of the foil. The resulting...
Journal of African Earth Sciences, 2005
... siderite or clay ironstone. 2.5. Effia Nkwanta Beds (210 m). This formation is characterized ... more ... siderite or clay ironstone. 2.5. Effia Nkwanta Beds (210 m). This formation is characterized by bright colors and is divided into three units: lower, middle, and upper beds on the basis of lithostratigraphy. Lower Effia Nkwanta Beds ...
Geostandards and Geoanalytical Research, 1997
Geostandards and Geoanalytical Research, 1998
Despite the growing application of various microanalytical in situ techniques in geochemistry (e.... more Despite the growing application of various microanalytical in situ techniques in geochemistry (e.g. laser ablation ICP-MS and secondary ion mass spectrometry), there is an obvious shortage of appropriate reference materials. The most important requirements for a microbeam "standard" include a good knowledge of its major and trace element composition and chemical homogeneity, with the latter required to be better than or comparable to the analytical precision. Basalt glass BCR-2G is a new geochemical reference material designed and prepared for microa n a l y t i c a l a p p l i c a t i o n s b y t h e U n i t e d S t a t e s Geological Survey (Reference Materials Project USGS). It has been prepared by fusion of BCR-2 powder, a basalt sampled at the same locality as the well-e s t a b l i s h e d U S G S re f e re n c e m a t e r i a l B C R-1 (Columbia River Basin, NW Oregon). First analyses by laser plasma ionisation mass spectrometry (LIMS; Seufert and Jochum 1997) and by ICP-AES, WD-XRF, electron microprobe and laser ablation ICP-MS (USGS 1996) indicate that the major, minor and trace element compositions of BCR-2 and BCR-2G are identical within error to that of BCR-1. Moreover, these investigations suggest that samples from different parts of the solidified BCR-2G melt "cake" are fairly uniform in chemical composition (USGS 1996). The present study aims to complement the presently limited and preliminary information on the chemical composition and homogeneity of BCR-2G. New data on the concentration and variation of ten major and
Geochimica et Cosmochimica Acta, 1996
... and asthenospheric mantle beneath Oahu A. ROCHOLL, * E. HEUSSER, T. KIRSTEN, J. OEHM, and H. ... more ... and asthenospheric mantle beneath Oahu A. ROCHOLL, * E. HEUSSER, T. KIRSTEN, J. OEHM, and H. RICHTER MaxPlanckInstitut fOr ... Microscopic investigation suggests that alteration, as indicated by dusty grain boundaries and cleavages, is linked to mantle metasomatism ...
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Papers by Alexander Rocholl