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 (permil SMOW) values of
25.5 a 0.8 (lo) and delta(13)C (permil PDB) values of -0.4 a 0.6 (lo).
Samples in the tremolite zone (750-1900 m) have delta(18)O values
ranging from 19.4 to 27.7 permil with a median value of 25.2 permil; and
delta(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 (delta(18)O) and -9.1 permil
(delta(13)C). Despite this shift, most forsterite zone samples retain
sedimentary isotopic compositions with median delta(18)O values of 25.0
permil and delta(13)C values of - 1.2 permil. 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 now 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 Xco(2), of 0.3. Although not in
agreement with estimates based on phase petrology (Xo < 0.05), such
discrepancy in the carbon mass balance is not limited to 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.