ISOTOPIC EVIDENCE FOR THE ORIGIN AND EVOLUTION OF CO2-RICH VOLATILES FROM OLIGOCENE TO MIOCENE MANTLE MAGMAS, SOUTHWESTERN COLORADO AND NORTHWESTERN NEW MEXICO

Oligocene to Miocene mafic to ultramafic rocks exposed in the Navajo volcanic field and dikes on the northern San Juan basin (NVSJ) contain calcite in vugs, veins, and breccias. O, C and Sr isotope signatures of bulk carbonate samples in these rocks were used to test hypotheses on the history of volatiles related to this pulse of mantle magmatism.

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/⁸⁶Sr_i data form NVSJ carbonate samples are similar to ⁸⁷Sr/⁸⁶Sr_i for related rocks hinting at the same melt source. Generally, carbonates have higher ⁸⁷Sr/⁸⁶Sr_iratios 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/⁸⁶Sr_irock and F over the +6 to +24 range of δ¹⁸O_calcite 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.