Reply to "possible non-equilibrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O'Neil-Carmichael18O/16O geothermometer"
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Contributions to Mineralogy and Petrology↗
Geology topics
I. S. E. Carmichael
Publications and source records attributed to I. S. E. Carmichael.
Genetic relations among basic lavas and ultramafic nodules: Evidence from oxygen isotope compositions
??18O values of unaltered basic lavas range from 4.9 to 8.3 but different types of basalts are usually restricted to narrow and distinct ranges of isotopic composition. The average ??18O values for Hawaiian tholeiites, mid-ocean ridge tholeiites, and alkali basalts are 5.4, 5.7, and 6.2 permil, respectively. Potassic lavas and andesites tend to be more 18O rich with ??18O values between 6.0 and 8.0 permil. The differences among the oxygen isotopic compositions of most of these lavas can be attributed to partial melting of isotopically distinct sources. The oxygen isotope compositions of the sources may be a function of prior melting events which produce 18O-depleted partial melts and 18O-enriched residues as a consequence of relatively large isotopic fractionations that exist at high temperatures. It is proposed that lavas with relatively low ??18O values are derived from primitive, 18O-depleted sources whereas 18O-rich basalts are produced from refractory sources that have already produced partial melts. High temperature fractionations among silicate liquids and coexisting minerals can be used in conjunction with the oxygen isotope compositions of ultramafic nodules to place constraints on the genetic relations between some nodules and different types of basic lavas. ?? 1982 Springer-Verlag.
Contributions to Mineralogy and Petrology↗
Oxygen isotope thermometry of basic lavas and mantle nodules
Measurements have been made of the oxygen isotope and chemical composition of glass and phenocrysts in lavas and coexisting minerals in mantle nodules. Temperatures of formation of these assemblages have been estimated from various chemical thermometers and range from 855?? to 1,300?? C. The permil fractionations between coexisting orthopyroxene and clinopyroxene in the lavas and nodules are all near zero. The fractionations between pyroxene and olivine vary from +1.2 to -1.4 and are a smooth function of temperature over the entire range. This function is given by T(?? C)=1151-173?? (px-d)-68??2(px-d) and has an uncertainty of ??60?? (2??). At temperatures above 1,150?? C, olivine in the nodules becomes more18O-rich than coexisting clinopyroxene, orthopyroxene, and plagioclase. In combination with the experimental work of Muehlenbachs and Kushiro (1974), the olivine-pyroxene fractionations indicate that olivine also becomes substantially more18O-rich than basaltic liquids above 1,200?? C. Geothermometers based on the oxygen isotope equilibration of basaltic liquid with olivine, pyroxene, and plagioclase are presented. ?? 1981 Springer-Verlag.
Contributions to Mineralogy and Petrology↗
Strontium isotopes in Quaternary basalts of southeastern California
Strontium isotopic variations ( 87 Sr/ 86 Sr of 0.7029 to 0.7052) in Quaternary basalts of southeastern California, USA, are not readily explained by assimilation of crustal material similar to that contained as xenoliths in the basalt. Isotopic differences between chrome diopside (0.7016) and olivine (0.7087) from a lherzolite xenolith suggest that the isotopic variations in the basalts may be inherited from the source regions or modified by the upper parts of the mantle that were traversed by the lavas.
Earth and Planetary Science Letters↗
Sr87/Sr86 ratios of Quaternary lavas of the Cascade Range, northern California
Quaternary lavas from Mount Lassen, Mount Shasta, and Medicine Lake, which range in composition from olivine basalt to rhyolite, have Sr 87 /Sr 86 values that fall between 0.7030 and 0.7043, corrected to 0.7080 for the Eimcr and Amend (MIT) SrCO 3 standard. These values are very close to average (corrected) strontium isotopic values of some other circum-Pacific volcanic suites and of lavas of oceanic islands. The low ratios suggest a mantle origin. The notably high strontium content (1400 ppm) in the andesites from Mount Shasta also indicates a mantle origin at depths where plagioclase is absent as a stable phase in the residual material.
California↗