AN EXPERIMENTAL AND ANALYTICAL INVESTIGATION OF CLINOPYROXENE SECTOR ZONING IN TWO HAWAIIAN MAGMAS

Clinopyroxene (cpx) crystals in two alkalic Hawaiian magmas exhibit contrasting types of sector zoning. Compositionally-distinct subdomains within intrusive ijolite from Kaua'i island have well-defined sector boundaries that are identifiable in polarized light microscopy (PLM) and backscattered electron (BSE) imaging, and quantifiable using electron probe microanalysis (EPMA). Ankaramite lava from Haleakala on Maui provide sector zoning not easily recognizable in PLM, BSE imaging, or spot EPMA (Welsch et al., 2016). These cpx crystals contain textural subdomains, but their external morphologies are similar to those in the ijolite, allowing for a direct comparison of sector enrichment with principal sections indexed in SHAPE software (Dowty 1980, 1987). Maui cpx display (Al + Ti + Na)_{−111} = (Si + Mg + Cr + Ca)_{hk0}, whereas the ijolite cpx exhibit (Al + Ti + Na + Cr)_{110} = (Si + Mg + Ca)_{{-434},{-111},{100}}.

The sector trends are interpreted within a framework from Kouchi et al. (1983), which relates enrichment trends to the undercooling (-ΔT = T_{crystallization} - T_cpx-in) where growth occurs. The basal {-111} sectors of the ankaramite cpx are richest in Al₂O₃, associated with relatively high undercooling (-ΔT = 45 °C); ijolite Al₂O₃ enrichment on {110} is most consistent with lower undercooling (-ΔT < 25°). Experiments have been initiated to explore the importance of melt composition, oxygen fugacity (fO₂), and undercooling on sector zoning systematics, which informs us on the implications of magmatic crystallization environments that are preserved in the natural samples. An element enrichment trend similar to the natural ankaramite was produced in 1-atmosphere gas-mixing experiments utilizing fO₂ at 2 log units below the fayalite-magnetite-quartz buffer, one superheating dwell (+TΔ = 25° for 7h), and a cooling trajectory (-ΔT = 24°C & 1°C/h cooling rate over 24h). Because cpx show poor faceting in the experimental charge, orientation indexation using interfacial angles was not possible in the synthetic crystal. However, EPMA indicate that Si and Mg are concentrated into one domain, and Ti, Al, Ca, Na, and Cr are concentrated into another.