Mathematics – Logic
Scientific paper
Jan 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51..143r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 1, pp.143-154
Mathematics
Logic
3
Scientific paper
Basaltic lavas form the isolated Quaternary volcanic centers of Diamond Craters and occur at Cow Lakes in S.E. Oregon. The exposed rocks are olivine and plagioclase phyric alkali olivine basalts. The holocrystalline groundmasses contain plagioclase, titanaugite, olivine, Fe-Ti oxides and minor apatite, and occur in the high-alumina (greater than 17%) petrographic province of the northwestern Great Basin. The whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas. The solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions. Two of the lavas from Diamond Craters have whole rock and phenocryst compositions consistent with their being natural melts. Rock compositions that reflect the effects of crystal separation (open systems) are recognized by significant differences in predicted versus observed phenocryst modes. The P-T conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints. The initial melts are inferred to have equilibrated at 0.8-1.0 GPa and 1210-1240°C. The majority of the lavas from Diamond Craters have compositions that reflect olivine and plagioclase crystal separation. These lavas can be related to two possible parent rock compositions by small amounts (2-8% by weight) of olivine and plagioclase separation.
Nicholls Jim
Russell Kasey J.
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