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Conditions and systems for on-line combustion of effluents from capillary gas chromatographic columns and for removal of water vapor from product streams were tested. Organic carbon in gas chromatographic peaks 15 s wide and containing up to 30 nanomoles of carbon was quantitatively converted to CO2 by tubular combustion reactors, 200 x 0.5 mm, packed with CuO or NiO. No auxiliary source of O2 was required because oxygen was supplied by metal oxides. Spontaneous degradation of CuO limited the life of CuO reactors at T > 850 degrees C. Since NiO does not spontaneously degrade, its use might be favored, but Ni-bound carbon phases form and lead to inaccurate isotopic results at T < 1050 degrees C if gas-phase O2 is not added. For all compounds tested except CH4, equivalent isotopic results are provided by CuO at 850 degrees C, NiO + O2 (gas-phase mole fraction, 10(-3)) at 1050 degrees C and NiO at 1150 degrees C. The combustion interface did not contribute additional analytical uncertainty, thus observed standard deviations of 13C/12C ratios were within a factor of 2 of shot-noise limits. For combustion and isotopic analyses of CH4, in which quantitative combustion required T approximately 950 degrees C, NiO-based systems are preferred, and precision is approximately 2 times lower than that observed for other analytes. Water must be removed from the gas stream transmitted to the mass spectrometer or else protonation of CO2 will lead to inaccuracy in isotopic analyses. Although thresholds for this effect vary between mass spectrometers, differential permeation of H2O through Nafion tubing was effective in both cases tested, but the required length of the Nafion membrane was 4 times greater for the more sensitive mass spectrometer.

Detailed structural relationships at the margins of ophiolites can provide important information about the mode of emplacement of ophiolitic terranes. In this study we use geologic mapping and gravity data to determine the structural and tectonic relations between a Middle to Late Jurassic volcanic arc complex (the Smartville Ophiolite block) and a pre‐Late Jurassic melange terrane in the northern Sierra Nevada foothills. The volcanic arc complex overlies the melange along a complex fault zone. Although this fault zone is steeply dipping wherever it is observed, map patterns suggest that the Smartville Upper Volcanic unit was originally emplaced along a subhorizontal fault that has been subsequently deformed. Two‐dimensional interactive computer analysis of gravity data from over 200 stations shows that the Smartville Upper Volcanic unit is a relatively shallow feature with a maximum thickness ranging from 0.5 to 3 km and that it was thrust as a sheet over the previously deformed melange. Both units were subsequently folded, giving rise to the steeply dipping faults observed at the surface. Recognition of low‐angle thrusting along the northern margin of the Smartville Ophiolite block has important implications for the mode of emplacement of the block. The most plausible mechanism for the thrusting of this particular oceanic island arc sequence onto the continental margin involves a subduction zone dipping westward beneath the Smartville Ophiolite block, similar to the present‐day situation in the Molucca Sea collision zone.

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