Stable isotope evidence of heterogeneous fluid infiltration at the Ubehebe Peak contact aureole, Death Valley National Park, California

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ID: 299635
1999
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Abstract
Stable isotope ratios of carbon and oxygen are used to define quantitatively the effects of magmatic fluid infiltration in marbles contact metamorphosed by the 173 Ma Ubehebe Peak quartz monzonite, Death Valley National Park, California. In previous studies of fluid infiltration, quantitative interpretation of aureole-wide isotopic data has been difficult due to small data sets. For this study, sampling strategies were developed to obtain a data set that was large and unbiased enough to be statistically representative of the carbonates north of the Ubehebe Peak intrusion. A total of 357 samples of marble were analyzed for bulk carbonate isotopic ratios. Separate analyses of coexisting calcite and dolomite were also obtained for an additional 31 samples. Unmetamorphosed samples (1900-3000 m from the intrusion) have delta 18 O (per mil SMOW) values of 25.5+ or -0.8 (1sigma ) and delta 13 C (per mil PDB) values of -0.4+ or -0.6 (1sigma ). Samples in the tremolite zone (750-1900 m) have delta 18 O values ranging from 19.4 to 27.7 per mil with a median value of 25.2 per mil; and delta 13 C ranges from -5.1 to 0.5 per mil with a median value of -0.9 per mil. Forsterite zone samples (0-750 m) have isotopic ratios shifted to values as low as 11.1 per mil (delta 18 O) and -9.1 per mil (delta 13 C). Despite this shift, most forsterite zone samples retain sedimentary isotopic compositions with median delta 18 O values of 25.0 per mil and delta 13 C values of -1.2 per mil. delta 18 O values for igneous minerals show no evidence for interaction with heated meteoric or metamorphic fluids. The shifts in isotopic compositions within the marbles are interpreted to be the result of magmatic infiltration. The effects of this infiltration were quantified by identifying samples with isotopic alteration that can only be attributed to infiltration. The results show that magmatic fluid infiltration was limited in extent and very heterogeneous. There is no evidence for infiltration of isotopically reactive fluids beyond 850 m from the intrusive contact, and within this 850 m zone only 28 percent of the samples have been infiltratively altered with respect to delta 18 O, and 20 percent are depleted in delta 13 C compositions. The isotopic data, when evaluated in conjunction with geostatistical and petrologic data, indicate that the geometry of the hydrothermal flow system was mainly vertical and away from the pluton. Infiltration was restricted to large, nearly vertical, "tube-like" zones of increased permeability. These higher permeability zones likely reflect an initial heterogeneity of the host rocks and show no significant evidence for reaction enhanced permeability. Given the heterogeneity of the system and a lack of knowledge about many basic parameters controlling fluid infiltration, it is shown that the best method of calculating the amount and composition of the infiltrating fluid may be a mass balance approach (fluid/rock ratio). The application of mass balance models is discussed and shown to be valid only under limited conditions. Since infiltration at Ubehebe Peak was largely vertical, the observed isotope alteration patterns represent an infiltration side and not a front. Because this precludes the use of traditional mass balance calculations, a new infiltration side (InSide) model is proposed that allows the isotopic data to be evaluated. The InSide model uses the ratio of the areal amounts of infiltrative alteration to calculate a fluid composition. Fluid amounts cannot be obtained from this model. Results for the Ubehebe Peak data show that the infiltrating fluid had an average X CO2 of 0.3. Although not in agreement with estimates based on phase petrology (X CO2 <0.05), such discrepancy in the carbon mass balance is not limited to the Ubehebe Peak aureole and is a common problem in many other aureoles. The statistically representative Ubehebe Peak data set provides the most accurate picture of aureole-scale fluid infiltration presently available. Although in many ways this study quantifies the heterogeneous nature of contact metamorphic fluid infiltration, it also highlights some serious problems in predicting the amount and composition of infiltrating fluids. Data gained from studies such as this, however, will lead to an increased understanding of fluid infiltration and contribute to the development of more accurate models.
Reference Key
openalex_W2268445567 Use this key to autocite in the manuscript while using SciMatic Manuscript Manager or Thesis Manager
Authors Gregory T. Roselle, Lukas P. Baumgartner, John W. Valley
Journal american journal of science
Year 1999
DOI
10.2475/ajs.299.2.93
URL
https://doi.org/10.2475/ajs.299.2.93
Keywords Keywords not found

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