The Origin and Evolution of Silica-saturated Alkalic Suites: an Experimental Study

Abstract

Experimental simulation of incremental crystal fractionation of a hy-normative hawaiite indicates that the spectrum of compositions from mildly alkalic hawaiite to sodic rhyolite found in silica-saturated alkalic suites of the ocean islands and continental hotspots and rifts can be produced by fractionation at 9Á3 kbar with bulk water contents 4$0Á5 wt % (in the hawaiite) at f O 2 $1Á5 log units below the fayalite-magnetite-quartz buffer (FMQ). Along this path, mildly alkalic basalt becomes increasingly alkalic because of the domination of clinopyroxene in the early fractionating assemblage and suppression of plagioclase. Kaersutite dominates at intermediate temperatures and results in stronger silica enrichment as the melt evolves to rhyolite. The fractionation assemblages are strongly pressuresensitive. At mid-crustal pressures, melts become potassic rather than sodic. At shallow conditions, the abundance of early olivine produces strong silica enrichment and subalkalic total alkalis to silica ratios. Natural mineral assemblages from silica-saturated alkalic suites show a polybaric history with fractionation at $30 km depth followed by decompression of liquids residual to this fractionation and crystallization, but not extensive fractionation, of lower-pressure assemblages. Equilibrium crystallization paths suggest that partial melting of hawaiite could produce the intermediate members of these suites provided that sufficient water was available in the source region.

Origin of mafic and ultramafic cumulates from the Ditrău Alkaline Massif, Romania

“…Experimental studies of hydrous mafic magmas crystallization indicate that usually olivine and clinopyroxene are the liquidus phases and amphibole saturates after these mineral phases (e.g., Nekvasil et al, 2004;Krawczynski et al, 2012. ) in accordance with our textural observations of the poikilitic samples.…”

“…Both temperature and melt composition can potentially affect our empirical barometer increasing the uncertainty of the pressure estimation. Therefore we analysed the effect of temperature and melt composition using the extensive experimental dataset of Nekvasil et al (2004). Fig.…”

A Ditrói Alkáli Masszívum ultramafikus kumulátum kőzeteinek petrogenetikája

“…In this paper the authors investigated the amphibole composition within rocks with a well-defined chemical/mineralogical composition (under well-defined conditions: temperature, pressure and H2O-content). Nekvasil et al (2004) reported that the rock types and amphibole compositions are similar to the DAM rock types and amphibole compositions, so it is an appropriate reference for the estimation of the pressure and temperature of alkaline systems. The best results were given for the ultramafic cumulate rocks' amphiboles by the oxo-hygro-thermobarometer provided by Ridolfi and Renzulli (2012) which was calibrated for alkaline systems.…”

“…A termobarométerek eredményeit összehasonlítottuk egymással és kísérleti adatokkal is. A kísérleti adatoknál Nekvasil et al (2004) munkáját vettük alapul, amelyben ismert összetételű kőzetekben, meghatározott kristályosodási körülmények között vizsgálták a keletkezett amfibolok összetételét. A Nekvasil et al (2004) munkájában használt kőzettípusok és azok amfibol adatai nagy hasonlóságot mutatnak a DAM kőzeteivel és amfibol összetételeivel, így megfelelő referenciaforrás egy alkáli rendszer nyomás és hőmérséklet becslésének megállapítására.…”

“…We have compared our calculations' results with the values obtained by Nekvasil et al (2004). In this paper the authors investigated the amphibole composition within rocks with a well-defined chemical/mineralogical composition (under well-defined conditions: temperature, pressure and H2O-content).…”