Other
Scientific paper
Aug 1983
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983e%26psl..64..231c&link_type=abstract
Earth and Planetary Science Letters, Volume 64, Issue 2, p. 231-243.
Other
4
Scientific paper
Oxygen isotopic compositions of 27 whole rock and mineral samples from the three intrusive phases of the Stony Mountain stock, San Juan Mountains, Colorado, have been measured and are combined with published analyses to obtain a history of the meteoric water circulation that cooled the intrusions. The order of intrusion was: the Outer Diorite, the Gabbro and the Inner Diorite. The new whole rock data give δ18O (SMOW) + 1.9 to + 3.7‰ for the Gabbro, and + 0.4 to + 4.9‰ for the Inner Diorite. Five mineral analyses are reported for the Inner Diorite: three feldspars (ranging from + 0.2 to + 4.3), one quartz (+8.2), and one pyroxene (+5.3).
All three intrusions are significantly depleted in 18O relative to normal igneous rocks. This depletion occurred by subsolidus exchange of oxygen isotopes, as shown by the correlation between the extent of depletion and the grain size, and by the non-equilibrium 18O concentrations in coexisting minerals. The patterns of 18O depletion, which are complex, suggest that separate hydrothermal circulation systems were set up by the cooling of each intrusion in turn.
The minimum water to rock ratio required to effect the observed depletion in the Inner Diorite is approximately 0.18, although the ratio for the total flux was probably much greater. The mineral data show marked 18O depletion in the feldspars and little, if any, depletion in the quartz. Application of oxygen diffusion data for feldspar and quartz leads to the conclusion that the effective temperature for oxygen isotope exchange was below 600°C and above 400°C. Furthermore, these data place constraints on convective cooling times for this intrusion. Minimum times for cooling of the 500-m-diameter Inner Diorite through this temperature range are on the order of 14 to 900 years. For comparison, a maximum time limit of 4000 years would be needed if cooling occurred only by conduction.
Crowley Julia C.
Giletti Bruno J.
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