Abstract In the case study described here the quantitative modal mineralogy of a number of core samples was determined with the objective of using these modes to calibrate geochemical logs. Modal estimates were obtained for the core samples by quantitative X-ray diffraction, infrared spectroscopy, point counting of thin sections, and indirectly by calculation from a complete chemical analysis of the samples. In the case of calculated modes, three different algorithms were applied. A by-product of this particularly complete dataset is the possibility of evaluating the most accurate method of modal analysis, and although no certain conclusion is reached on this point the analysis of these data does demonstrate the difficulty of obtaining accurate modal estimates. The core samples, taken at regular intervals through a sand, sandy-shale sequence, capped by a carbonate unit, have a mineralogy which, although dominated by quartz, includes feldspars, carbonates, and clays (illite, kaolinite) together with minor phases. There was generally good agreement between methods in the estimation of quartz, total carbonate, albite, kaolinite, total clay and pyrite. The results for illite and K-feldspar were poor, a reflection of their relatively low concentrations (<10%), and problems of compositional co-linearity in the calculated modes.
Abstract DSDP/ODP Hole 504B, located in the Eastern Equatorial Pacific in 5.9 Ma old crust, penetrates both basaltic pillows and dykes down to a depth of 1562.1 m below the sea floor. Core recovery is poor, averaging 230ut falling to a 12% average in the deepest sections. Geochemical logs give a continuous measure of elemental (Si, Ca, Fe, S, Ti, Gd, H, Cl, K, Th, U) abundances. Correlation between laboratory (XRF) geochemical data and log data is difficult because of the lack of accurate depth information for the cores. Pattern recognition techniques are of little help for locating sample depths in the uniform basaltic lithologies. Non-hierarchical clustering techniques prove the existence of relationships between core descriptions and gross geochemical variations; major lithostratigraphic zones are readily identified from the log-derived data.
[1] The data and analysis presented in this paper provide an assessment of lava morphologies and the geochemistry of lavas from the Oman ophiolite. In order to provide detailed constraints on the construction of the upper oceanic crust, a continuous volcanic transect (300 m-thick) was sampled at high-frequency in the Semail ophiolite along Wadi Shaffan. The Wadi Shaffan section is composed mainly of pillow lavas interbedded with massive flows and occasional hyaloclastites. The sampling performed along Wadi Shaffan implies temporal variations in the activity of the ridge. The section is characterized by chemical compositions consistent with those of V1-Geotimes volcanism. The Wadi Shaffan transect was built through two main petrological and geochemical sequences of volcanic activity. Trace element ratios (e.g. Zr/Nb and La/Yb) allow us to distinguish two main sequences with two different parental magmas. Differences in the degree of partial melting are required to explain these trace element ratio variations. Beyond these differences in parent melt composition, variations in trace element abundances (TiO2, Zr, REE) involve differentiation processes prior to emplacement. In the lower sequence, less differentiated lavas are in the upper part of the cycle. Magma mixing is proposed to explain this reversed geochemical evolution through time. In the upper sequence, geochemical analysis suggests a different magma chamber process. This sequence consists of multiple events of magma emplacement. Variations in trace element abundance suggest four magmatic cycles. Each magmatic cycle is characterized by primitive lavas evolving to more differentiated lavas with time. The upper sequence lavas appear to be in equilibrium with clinopyroxene and lower sills from the MTZ (Mantle-Crust Transition Zone) and with lower gabbros. We propose a model in which the upper sequence lavas were directly derived from the MTZ and lower gabbro sills and then transported to the surface without interaction with higher crustal levels. G Geochemistry Geophysics Geosystems
The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non‐uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.
Skiff Bank is a bathymetric and gravimetric high located ~350 km southwest of the Kerguelen Islands in the northern portion of the Kerguelen Igneous Province (KIP). Ocean Drilling Program Site 1139 was drilled at a water depth of 1415 m on Skiff Bank’s western edge. Hole 1139A penetrated to a depth of 695 meters below seafloor (mbsf) with sediment recovered from the upper 462 m and igneous basement recovered from the lower 233 m. A total of 19 basement units were identified, including variably welded trachytic to rhyolitic volcanic and volcaniclastic rocks (Units 1–5) and 14 lava flows with intermediate to mafic compositions (Units 6–19) (Shipboard Scientific Party, 2000). Core observations indicate that the basement units recovered from Hole 1139A display unique alteration patterns, in terms of both the intensity of alteration and secondary mineralogy, when compared to other drill holes from the Kerguelen Plateau. Basement Units 6–17 contain highly altered breccias, commonly corresponding to flow tops, and more massive and somewhat less altered flow interiors. The breccias are highly oxidized and cemented by calcite and clay minerals. Calcite veins and stringers are common throughout, and the groundmass has a bleached appearance due to the replacement of primary igneous minerals by secondary calcite. The lowermost basement Units 18 and 19 are highly to completely altered by intense oxidation to hematite or by nearly complete bleaching to produce a final mineral assemblage that 1Saccocia, P.J., Teagle, D.A.H., Telford, R.H., Eastley, N., and Brewer, T.S., 2004. Data report: Downhole major and trace element chemistry of volcanic rocks from Skiff Bank, Kerguelen Plateau (ODP Site 1139). In Frey, F.A., Coffin, M.F., Wallace, P.J., and Quilty, P.G. (Eds.), Proc. ODP, Sci. Results, 183, 1–12 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Department of Earth Sciences and Geography, Bridgewater State College, Bridgewater MA 02325, USA. psaccocia@bridgew.edu 3Southampton Oceanography Centre, School of Ocean and Earth Science, University of Southampton, Southampton SO14-3ZH, UK. 4Department of Geology, University of Leicester, Leicester LE1 7RH, UK.
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