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Oxygen isotope studies of Jurassic fossil hydrothermal systems, Mojave Desert, southeastern California

Solomon, G. Cleve and Taylor, Hugh P., Jr. (1991) Oxygen isotope studies of Jurassic fossil hydrothermal systems, Mojave Desert, southeastern California. In: Stable isotope geochemistry : a tribute to Samuel Epstein. Special publication (Geochemical Society). No.3. Geochemical Society , San Antonio, TX, pp. 449-462. ISBN 9780941809023.

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Whole-rock oxygen isotope analyses of 66 Jurassic plutonic and subvolcanic granodiorites and monzogranites from the Rodman-Ord Mountains (ROM) area in the Mojave Desert range from δ¹⁸O = -3.2 to + 9.4. These data define an elongate WNW-ESE zone of ¹⁸O depletion, where intrusive igneous rocks with original δ¹⁸O values of +7.5 to +9 have been partially altered and veined by epidote, chlorite, and sericite, and depleted in ¹⁸O by two to ten per mil over an area of more than 1000 km². These effects were produced by exchange with heated low-¹⁸O meteoric ground waters in association with a series of volcanic centers. These centers are identified by ovoid-shaped areas of extreme ¹⁸O depletion where δ¹⁸O < +2. Earlier Triassic plutons (4 samples) and later Cretaceous plutons (10 samples) from the ROM area do not display any analogous ¹⁸O depletions, indicating that the identifiable hydrothermal events are confined to the Jurassic igneous episode, which clearly had to be epizonal in nature. Reconnaissance ¹⁸O/¹⁶O analyses of another 20 early Mesozoic granodiorites and monzogranites elsewhere in Southern California demonstrate that the ROM-type low-¹⁸O effects extend well to the southeast along the major Jurassic rift zone and associated calderas proposed by BUSBY-SPERA (1988). This graben depression extends from Yerington, Nevada (where the associated Jurassic hydrothermal waters were higher in ¹⁸O and probably marine in origin) southeastward across a Jurassic coastline and through the subaerial ROM area (where the hydrothermal fluids were continental meteoric in origin); the low-¹⁸O rift-zone continues across southeastern California and then into southern Arizona where only scattered oxygen isotope data are available. In the ROM area the low-¹⁸O patterns are truncated by the Cretaceous plutons and they are also offset by displacements along NW-trending late Cenozoic strike-slip faults. The δ¹⁸O systematics can be used to map offsets on these faults, confirming that they are relatively small. For example, the right-lateral offset on the Camp Rock Fault is constrained by the ¹⁸O/¹⁶O patterns to be only 5 ± 2 km.

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Additional Information:We wish to take this opportunity to salute Samuel Epstein as he continues to build upon and expand his life-long contributions to the field of stable isotope geochemistry. He has taught us both a great deal over the years. We also wish to thank Keith A. Howard, Leon T. Silver, Robert E. Powell, John H. Dilles, and Robert E. Criss for helpful discussions and for their aid in obtaining some of the samples studied in this work. Financial support for this research was provided by the National Science Foundation, Grants No. EAR-83-13106, EAR-88-16413, and EAR-90-19190.
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Caltech Division of Geological and Planetary Sciences5062
Series Name:Special publication (Geochemical Society)
Issue or Number:3
Record Number:CaltechAUTHORS:20221115-161130327
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:117868
Deposited By: Tony Diaz
Deposited On:15 Nov 2022 22:47
Last Modified:17 Nov 2022 17:04

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