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Oxygen Isotope Fractionation between Phenocrysts and Melt: Equilibrium Estimation for the Alkaline Lavas of Changbaishan Volcano (Northeast China)
Petrology ( IF 1.0 ) Pub Date : 2020-05-31 , DOI: 10.1134/s0869591120030030 E. O. Dubinina , O. A. Andreeva , A. S. Avdeenko , I. A. Andreeva , Ji Jianqing
Petrology ( IF 1.0 ) Pub Date : 2020-05-31 , DOI: 10.1134/s0869591120030030 E. O. Dubinina , O. A. Andreeva , A. S. Avdeenko , I. A. Andreeva , Ji Jianqing
The mechanisms of the occurrence of nonequilibrium oxygen isotope fractionation between olivine and plagioclase phenocrysts and the groundmass of rocks (from alkaline basalts to alkaline rhyolites) of Changbaishan volcano are considered. The basaltic trachyandesite–trachyte–comendite–pantellerite rock series shows a systematic increase of the δ18O values of feldspar (from 5.4 to 7.8‰) and the rock groundmass (from 5.6 to 7.1‰). In contrast, the δ18О values of olivine decrease from 5.35‰ typical of mantle peridotites to significantly lower values (3.88–4.14‰). This is accompanied by changes in olivine composition from the Mg-rich (Fo = 74–79) to almost pure fayalite (Fo = 1), thus indicating a positive correlation between the Mg–number and δ18О of olivine. It is shown that feldspar and the rock groundmass are close to the oxygen isotopic equilibrium, whereas olivine–rock groundmass and olivine–feldspar pairs did not approach the oxygen isotopic equilibrium. The main mechanisms leading to the oxygen isotopic disequilibrium in the mineral–melt system are considered. An approach to the estimation of oxygen isotope equilibrium between the phenocrysts and melts is proposed. It takes into account the degree of melt polymerization (NBO/T ratio). This approach makes it possible to explain the positive correlation between δ18О values and olivine Mg-numbers in the Changbaishan lavas, as well as the occurrence of low δ18O values (down to +3‰) in ferrous olivine from alkaline–salic rocks. The δ18О values observed in the olivine and the rock groundmass reflect changes in the degree of melt polymerization during crystal fractionation. The proposed model does not require an additional geological process, in particular, the contribution of a low–δ18О material in the formation of the Changbaishan differentiated rock series.
更新日期：2020-05-31
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