NEW INSIGHT INTO VOLATILE SOURCES AND MAGMATIC PROCESSES OF ALKALINE ROCKS IN THE NAVAJO VOLCANIC FIELD FROM CHLORINE ISOTOPES AND CHLORINE-FLUORINE TRENDS

The dominant bulk-rock geochemical signatures and trends of rocks in the Navajo volcanic field (NVF) are well established, but the volatile histories of these rocks are not well understood. New insight into the types and trends of volatiles in rocks of the NVF reveal that magmatic gases are F rich and have variable but generally depleted Cl concentrations. NVF volatiles were derived from a source with distinctly positive δ³⁷Cl-isotope signatures.

δ³⁷Cl values of phlogopite separates range from -0.46 ‰ to +2.38 ‰, which are consistent with an enriched-mantle source containing a recycled-crustal component. Alternatively, fractionation of ³⁷Cl and ³⁵Cl during degassing could have generated the positive values. Complex degassing histories and dynamic changes in magma chemistry are indicated by the multiple generations of phlogopite, and complex zoning trends of Cl and F in phlogopite crystals.

Electron microprobe analyses of phlogopite crystals in samples of all rock types in the NVF indicate complex trends in the partitioning of Cl and F with decreasing MgO and increasing FeO. Even within phlogopite crystals in a single sample the Cl-F trends vary widely. These trends indicate that these gas-rich magmatic systems involved dynamic movement of volatiles during crystallization, possibly with multiple periods of degassing.

The Cl-F data for NVF rocks indicate that magmatic-gas phases generated during the eruption of related magmas were F rich with the least-evolved rocks containing the highest concentration of F. The Cl-F trends in crystal phases of NVF rocks, and oxygen and carbon isotope data of carbonate rich veins, signify that melts associated with these rocks contained a significant, highly mobile, magmatic-volatile component. We propose that magmatic driven eruptions were dominant in these systems, rather than the acquisition of volatile components from reservoirs external to the magmas.