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Mineralogy and Geochemistry |
Borehole Research Group, Lamont-Doherty Geological Observatory, Columbia University, Palisades, New York 10964, USA
Elemental analyses of rock compositions have been measured in situ with a geochemical logging tool (GLT) in drillholes through igneous and metamorphic terrains. Comparisons of dry weight percent oxide analyses from laboratory measurements on core samples and the log-derived results demonstrate that accuracies are fair in Ocean Drilling Program Hole 504B basalts, good in Cajon Pass, California granites, and very good in Moodus, Connecticut metamorphic rocks and Palisades, New York diabases. Improved accuracies of GLT measurements are attainable through the use of a boron sleeve to isolate the borehole fluid from wellbore effects, and in smaller diameter, land-based boreholes. Precisions within each well are very good in all cases. That is, only a linear calibration shift was required to bring core and laboratory analyses into general agreement in all cases. Variations in mineralogy, derived from inversion models of the log-derived elemental abundances at each well, produce a lithostratigraphic interpretation that is useful for determining the structure of the oceanic crust (Hole 504B), the fractionation sequence of basaltic intrusions in a new rift basin (Palisades Sill), the deformation history near the San Andreas Fault (Cajon Pass), and the variations in degree of metamorphism across a shallow decollement surface caused by collision tectonics (Moodus).
* On sabatical leave from Schlumberger Offshore Services, New Orleans, La., USA.
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  D. Goldberg and K. Burgdorff Natural fracturing and petrophysical properties of the Palisades dolerite sill Geological Society, London, Special Publications, 2005; 240: 25 - 36. [Abstract] [PDF]
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