ISOTOPIC EVIDENCE FOR THE ORIGIN AND EVOLUTION OF CO2-RICH VOLATILES FROM OLIGOCENE TO MIOCENE MANTLE MAGMAS, SOUTHWESTERN COLORADO AND NORTHWESTERN NEW MEXICO
O and C isotope data for NVSJ carbonates record a complex paragenetic history. δ13C values are mostly -8‰ to -4‰ with a mean value of -5.3 ± 2.0‰, similar to δ13C for primary mantle carbonate. A subset of δ18O values are +5‰ to +10‰ which are within the range of values for magmatic carbonate in carbonatite and kimberlite. A majority of δ18O values, however, range from +10‰ to +24‰ revealing that magmatic volatiles were overprinted by processes that enriched 18O at some stage of melt emplacement and crystallization.
A subset of 87Sr/86Sri data form NVSJ carbonate samples are similar to 87Sr/86Sri for related rocks hinting at the same melt source. Generally, carbonates have higher 87Sr/86Sriratios reflecting different melt-volatile sources or crustal contamination by Paleozoic limestone. Field and petrologic evidence do not support crustal contamination.
We propose that CO2-H2O-F volatiles in NVSJ magmas came from distinct melt-volatile sources, similar to the interpretation of Nowell (1993). Our assertion is that low δ18O volatiles exsolved from asthenospheric mafic melts and interacted with volatiles and melts from carbonate-bearing metasomatized lithospheric mantle. This is consistent with the subtle increase of 87Sr/86Srirock and F over the +6 to +24 range of δ18Ocalcite in minette samples. This hypothesis is supported by other studies that document +21 to +25 δ18O for carbonate in mantle xenoliths from Pliocene alkaline basalts in the region. Incipient to extensive alteration of olivine and phlogopite phenocrysts in NVSJ rocks reveals that deuteric isotopic exchange with H2O-CO2 magmatic fluids was a factor for some of the variation in δ18O in calcite. Melt contamination with limestone cannot be ruled out, but requires almost complete digestion of xenoliths in feeder dikes that had relatively low volumes of magma and cooled quickly. Overall, the combined data are not consistent with models that invoke groundwater as the main source of volatiles in NVSJ magmas.