FLUORINE VARIATIONS IN IGNEOUS PHENOCRYSTS FROM CONTINENTAL MAGMAS

Igneous minerals contain trace to minor quantities of fluorine that can be used to track the geochemical behavior of F in magmatic systems. These minerals include both F-bearing hydrous silicates (amphiboles, micas) and nominally F- free anhydrous minerals such as pyroxenes and olivine. Secondary Ion Mass Spectrometry (SIMS) was used to quantify F concentrations in such minerals from four continental magmatic rock suites of diverse origin in order to examine the geochemical behavior of F during magmatic evolution and to establish inter-mineral partitioning. Samples included clinopyroxene and phlogopite phenocrysts in minette dikes from Agathla Peak, The Thumb, Wetherill Mesa, and Johnson Canyon diatremes from the Navajo Volcanic Field (NVF); olivine phenocrysts in meimechite of the 250 Ma Siberian LIP; olivine phenocrysts in picrite from the Karoo LIP; and kaersutite and augite megacrysts from the Dish Hill alkali basalt, CA.

In the NVF minettes, F contents of clinopyroxene (38-140 ppm) and phlogopite (3000 – 9000 ppm) phenocrysts are correlated positively with Mg# and negatively with TiO₂. Similar correlations are recorded in clinopyroxene (48 – 94 pm) and kaersutite (1100 – 1900 ppm) megacrysts that precipitated at mantle depths from alkali basalt magmas at Dish Hill. It remains uncertain if these trends of decreasing F content with melt evolution record decreasing F in coexisting melts due to hydrous mineral precipitation, vapor saturation, or the effects of changing mineral-melt partition coefficients for F, or a combination of these factors. In contrast to the above, olivine phenocrysts (13 – 47 ppm) display a negative correlation of F with Mg# and a positive correlation with TiO₂ suggesting an increase in F concentration during crystallization. Olivine phenocrysts from meimechite (12-67 ppm) show a positive correlation with both Mg# and TiO₂. However, the meimechites olivines are reversely zoned to Mg-rich rims and thus also suggest an increase in F concentration during crystallization. These results demonstrate the capacity of nominally anhydrous minerals to record the volatile evolution of continental magmas, but they also indicate the need to evaluate the effects of composition-dependent mineral-melt partitioning in clinopyroxene.