Gt. Roselle et al., Stable isotope evidence of heterogeneous fluid infiltration at the UbehebePeak Contact Aureole, Death Valley National Park, California, AM J SCI, 299(2), 1999, pp. 93-138
Stable isotope ratios of carbon and oxygen are used to define quantitativel
y the effects of magmatic fluid infiltration in marbles contact metamorphos
ed by the 173 Ma Ubehebe Peak quartz monzonite, Death Valley National Park;
California. In previous studies of fluid infiltration, quantitative interp
retation 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 o
f 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 obta
ined for an additional 31 samples. Unmetamorphosed samples (1900-3000 m fro
m the intrusion) have delta(18)O (permil SMOW) values of 25.5 a 0.8 (lo) an
d delta(13)C (permil PDB) values of -0.4 a 0.6 (lo). Samples in the tremoli
te zone (750-1900 m) have delta(18)O values ranging from 19.4 to 27.7 permi
l 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 s
amples retain sedimentary isotopic compositions with median delta(18)O valu
es 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 b
e the result of magmatic infiltration. The effects of this infiltration wer
e quantified by identifying samples with isotopic alteration that can only
be attributed to infiltration. The results show that magmatic fluid infiltr
ation was limited in extent and very heterogeneous. There is no evidence fo
r infiltration of isotopically reactive fluids beyond 850 m from the intrus
ive 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 geom
etry of the hydrothermal now system was mainly vertical and away from the p
luton. Infiltration was restricted to large, nearly vertical, "tube-like" z
ones of increased permeability. These higher permeability zones likely refl
ect an initial heterogeneity of the host rocks and show no significant evid
ence for reaction enhanced permeability
Given the heterogeneity of the system and a lack of knowledge about many ba
sic parameters controlling fluid infiltration, it is shown that the best me
thod of calculating the amount and composition of the infiltrating fluid ma
y be a mass balance approach (fluid/rock ratio). The application of mass ba
lance models is discussed and shown to be valid only under limited conditio
ns. Since infiltration at Ubehebe Peak was largely vertical, the observed i
sotope 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 d
ata to be evaluated. The InSide model uses the ratio of the areal amounts o
f infiltrative alteration to calculate a fluid composition. Fluid amounts c
annot be obtained from this model. Results for the Ubehebe Peak data show t
hat the infiltrating fluid had an average Xco(2), of 0.3. Although not in a
greement with estimates based on phase petrology (Xo < 0.05), such discrepa
ncy in the carbon mass balance is not limited to aureole and is a common pr
oblem in many other aureoles.
The statistically representative Ubehebe Peak data set provides the most ac
curate picture of aureole-scale fluid infiltration presently available. Alt
hough in many ways this study quantifies the heterogeneous nature of contac
t metamorphic fluid infiltration, it also highlights some serious problems
in predicting the amount and composition of infiltrating fluids. Data gaine
d from studies such as this, however, will lead to an increased understandi
ng of fluid infiltration and contribute to the development of more accurate
models.