INSIGHT INTO THE VOLATILE HISTORIES OF MAGMAS OF THE NAVAJO VOLCANIC FIELD USING OXYGEN AND CARBON ISOTOPES

Alkaline mafic to ultramafic volcanic rocks of the Navajo volcanic field (NVF) commonly contain calcite in pockets, veins, and breccias. While some geologists maintain that the calcite deposits in the NVF originated from externally-derived, non-magmatic fluids and volatiles (e.g., meteoric or groundwater) others argue that the calcite is primarily a product of magmatic degassing. Twenty carbonate samples from NVF rocks were analyzed for carbon and oxygen isotopes to constrain the sources of the volatiles, and assess the contributions of magmatic and external sources.

The δ¹⁸O and δ¹³C values for calcite from NVF rocks vary from +13.4 to +24.4‰ and -9.64 to -2.23‰, respectively. Calcite samples from katungites have δ¹⁸O values of +13.4 to +17.1‰ and δ¹³C values of -8.87 to -3.69‰. These samples have isotopic compositions comparable to those compositions accepted for primary-magmatic carbonate in kimberlites and mantle carbonatites. For the one katungite-calcite sample with δ¹⁸O outside this range, field evidence clearly indicated that the magma interacted with an external source of fluids, and the rocks were altered.

Carbonate from minette samples have δ¹³C of -9.64 to -2.23‰, similar to those for primary-magmatic carbonates in mantle carbonatites and kimberlites. The δ¹⁸O values of +16.3 to +23.9‰ for these samples, however, plot in the upper part of the primary kimberlite field or above the field. Data for carbonate from minette samples define a broad range with a linear fractionation pattern that may lend evidence for elevated H₂O/CO₂ ratios in magmatic or deuteric volatiles.

Our preliminary interpretation of the carbon and oxygen isotope data from NVF rocks is that the formation of carbonates in these rocks was mostly from the release of CO₂-F rich gases and volatiles during the emplacement and cooling of mafic and ultramafic magmas. The trend of increasing δ¹⁸O may indicate post-emplacement circulation of deuteric fluids, at temperatures below 500°C, that were influenced by changes in CO₂/H₂O ratios. It is also possible that some samples with higher δ¹⁸O were a product of late, variable isotopic alteration from weathering or some other process. No evidence in support of such a process, however, has been found.