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

Stable isotope ratios of carbon and oxygen are used to define quantitatively the effects of magmatic fluid infiltration in marbles contact meta- morphosed 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 18 O (permil SMOW) values of 25.5 0.8 (1) and 13 C (permil PDB) values of 0.4 0.6 (1). Samples in the tremolite zone (750-1900 m) have 18 O values ranging from 19.4 to 27.7 permil with a median value of 25.2 permil; and 13 C ranges from 5.1 to 0.5 permil with a median value of 0.9 permil. Forsterite zone samples (0-750 m) have isotopic ratios shifted to values as low as 11.1 permil ( 18 O) and 9.1 permil ( 13 C). Despite this shift, most forsterite zone samples retain sedimentary isotopic compositions with median 18 O values of 25.0 permil and 13C values of 1.2 permil. 18O 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 18 O, and 20 percent are depleted in 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 composi- tion. Fluid amounts cannot be obtained from this model. Results for the Ubehebe