INSIGHT INTO TRENDS OF FLUORINE AND CHLORINE IN MAFIC VOLCANIC ROCKS OF THE NAVAJO VOLCANIC FIELD: POTENTIAL IMPLICATIONS FOR MAGMA-VOLATILE SOURCES

The types and roles of volatile species involved in the production and evolution of mantle-derived rocks of the Navajo volcanic field (NVF) are not well understood or constrained. Insight into the concentrations and trends of different volatiles is critical to understand magma sources and conditions during the genesis and emplacement of melts.

Bulk-rock F data varies from 0.10 to 0.50 weight percent and in general increases with higher MgO and decreases with increasing FeO and SiO₂. Preliminary bulk-rock Cl data range from < 0.01 to 0.05 weight percent with no significant trends. Katungite samples with 33 to 36 weight % SiO₂ are the most enriched in F and have the highest total F+Cl concentrations of rocks in the field. Melanephelinite and minette samples also have high F:Cl ratios but have lower F+Cl concentrations overall.

Electron microprobe analyses of phlogopite and apatite were done to understand trends of F and Cl during crystallization. Most samples from different suites have multiple generations of phlogopite. Early-formed crystals tend to form phenocrysts and megacrysts that have poikilitic cores or are slightly altered and ragged. Later phases form smaller and better developed phenocrysts or groundmass crystals. Zoning in phlogopite is complex. Many phlogopite crystals have MgO-rich cores and rims enriched in FeO and TiO₂, but reverse and oscillatory zoning are also evident and dominant in some populations. A slight to significant increase in F (up to 2 weight percent) is apparent with increasing MgO whereas Cl has a slight increase, decrease or constant trend with increasing MgO. Apatite in all samples are F rich with concentrations 10 to 20 times Cl. Apatite crystals in different rocks have highly variable trends in Cl versus F with a slight increase or decrease in Cl with increasing F. The most noticeable trends are slight to dramatic increases in Cl with increasing SO₂ for many apatite populations in given samples.

Our preliminary data indicate that magmas that gave rise to the NVF rocks had a wide range of magmatic F and Cl signatures, but the data for the katungite samples reveal differences that hint at a distinct magma-volatile source. Individual rock samples from different suites have complex trends in Cl and F that tend to indicate multiple generations of crystallization under variable magmatic conditions.