各種岩系間のチタノマグネタイト中の微量成分(Al2O3, MgO)量とTiO2量の違いについての考察

抄録

Groundmass titanomagnetites are chemically distinguished from, phenocrystic ones: those in a groundmass contain less amounts of Al2O3 and MgO than phenocrystic ones. And groundmass titanomagnetites are more Ti-rich than phenocrystic ones except for a few instances from alkaline rocks. The comparison of chemical analyses of titanomagnetites from various rock series reveals that titanomagnetites are chemically distinguished based on Al2O3+MgO and TiO2 contents among different rock series. The Al2O3+MgO contents in the titanomagnetites, both from phenocrysts and groundmass are highest in those from kimberlite, intermediate from alkaline rock and low from non-alkaline rock. The ulvospinel contents in the titanomagnetites from calc-alkaline rocks and island arc tholeiites are low and fall into a relatively narrow range. Otherwise oceanic and continental tholeiites have Ti-rich titanomagnetites usually containing much more 16% of TiO2. These features suggest that the composition of titanomagnetite is dependent upon the chemistry of magmas as well as their crystallization conditions. The Al2O3-and MgO-poor titanomagnetite may be crystallized near the surface from SiO2-saturated magma with a character poor in Al2O3 and MgO contents. The calc-alkaline magma is inferred to be originally poor in TiO2 than the oceanic and continental tholeiite magmas.