ROCKS THAT PAUL WOULD HAVE LOVED: PETROLOGY OF HAUYNE PHONOLITES AND TRACHYTES FROM TAHITI

Unusually fresh nosean and haüyne phenocrysts have been found in samples of alkalic volcanics from Tahiti. All samples contain microphenocrysts of Ti-rich clinopyroxene, sphene, apatite and variably titanian magnetite, as well as large feldspathoid phenocrysts, some contain small laths of sodic plagioclase or potassic feldspar, and most have abundant interstitial groundmass glass. Nosean or haüyne phenocrysts, distinguished by Na/Ca ratios, are all sulfate-rich. Ca-Na-K variations are sympathetic with variations in bulk rock chemistry. Phenocrysts in all samples show consistent core-to-rim decreases in Ca and K that record shifts in the composition of the melt, indicating crystal fractionation processes in the pre-eruptive magma. Minor Cl enrichment at nosean or haüyne rims, and discontinuous, apparently epitaxial, annular rims of end-member sodalite surrounding these crystals, indicate exhaustion of magmatic sulfate during feldspathoid phenocryst crystallization and a cessation of hauyne or nosean crystallization. Sufficient Cl was present in the remaining ne-normative melt to allow the abrupt shift from hauyne to sodalite crystallization. Abundance of sulfate in these rocks (in nosean and haüyne crystals), coupled with absence of any trace of sulfide minerals or any S in groundmass glass, indicates strong oxidation of magmatic sulfur and total depletion of sulfate in the melt during formation of feldspathoid phenocrysts. Cl enrichment in nosean and haüyne rims and the late shift from crystallization of nosean or haüyne to sodalite indicates a significant, probably syn-eruption increase in Cl/S ratio in the magma, possibly contamination by seawater but more likely related to sulfate depletion through feldspathoid crystallization. The bulk sulfur content of these rocks is high (up to 1.5-2 wt% SO2), due to abundant sulfate feldspathoid phenocrysts, and probably is close to, or slightly exceeds, reasonable sulfur solubilities of such magmas, although data is sparse on S solubilities in phonolitic liquids. Model calculations of the potential effects of haüyne fractionation on residual melt compositions suggest that removal of 10 or 15 modal% haüyne may significantly decrease magmatic Ca/Na ratio while dramatically increasing silica content, and may help to explain the unusual and well-known phonolite-trachyte trend in the Tahitian volcanic suite.