CHEMICAL SIGNATURES OF PEROVSKITE IN KATUNGITE DIKES OF THE NAVAJO VOLCANIC FIELD: IMPLICATIONS FOR MAGMA SOURCES AND CONDITIONS

A series of west-trending ultramafic and ultrapotassic katungite dikes exposed in the west-central part of the Navajo volcanic field (NVF) contain high concentrations (up to 5% volume) of perovskite along with associated olivine, phlogopite, mellilite, and apatite. Perovksite in these rocks forms rounded to pyramid-shaped crystals, “flower” aggregates, and “necklaces” around phenocrysts of olivine.

Perovskite in NVF katungites have an ideal molecular formula CaTiO₃ (96-98 mol. %). LREE in the perovskite crystals are enriched 10,000 to 30,000 times more than chondrite with variable Nb₂O₃ (0.185-0.59 wt %), La₂O₃ (0.126-0.726 wt %), Ce₂O₃ (0.01-0.955 wt %), Pr₂O₃ (0.014-0.282 wt %), Nd₂O₃ (0.106-0.807 wt %), and SrO (0.15-0.44 wt %). Overall, the perovskite crystals are more ceroan, and also exhibit elevated concentrations of U (40 to 100 ppm) and Th (300-2000 ppm). Prominent chemical-zonation patterns in the perovskite indicate a complex history of crystallization during magma transport and emplacement in which LREE tend to decrease in concentration from core to rim whereas CaO and SrO show a general increase.

Application of the oxygen barometer of Bellis & Canil (2007) to data from perovskite in NVF katungites reveals a wide range of oxygen fugacities relative to the Ni-NiO buffer. Calculated ΔNNO values vary from -4 to 0, typically increase with increasing Fe₂O₃, and tend to become more negative from the cores to rims of individual perovskite crystals. This suggests that the katungite magmas were in a moderately to slightly reduced state during emplacement, and were buffered by a C-H-O volatile phase that increased with crystallization and degassing. Most elements in the perovskite crystals increase in concentration with higher Fe₂O₃ and more positive ΔNNO which hints that the distribution of elements in the perovksite was closely allied to the oxidation state of the magma. We interpret the ΔNNO data as further evidence that the magmas for the katungite dikes were generated by melting of a different mantle source than NVF minettes. A variety of geochemical trends indicate that the minettes were generated by partial melting of metasomatized lithospheric mantle whereas the katungite magmas formed by partial metling of a deeper, CO₂-rich mantle source with signatures similar to oceanic island basalts.