Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma

The join forsterite-anorthite-diopside with 0.3% MgCr2O4 at 1 atm and its petrologic significance. A preliminary report.

“…of the temperature maximum moves from the SiO2-rich side to the SiO2 poor side of the Fo-An-Di plane. The addition of MgCr2O4 to the join Fo-An-Di makes diopsidess, chromian and SiO2-poor (Table 3), and therefore similar phenomena as demonstrated by Presnall et al (1978) is expected. Yoder and Tilley (1962) have proposed the basalt tetrahedron consisting of nor mative Qz-O1-Di-Ne as a fundamental model of basaltic rocks and have divided this tetrahedron into two parts by the plane olivine-plagioclase-diopside; one represents tholeiites and the other alkalic basalts.…”

“…4D). Presnall et al (1978) have demonstrated that the position of the thermal divide on the fo+an+diss+liquid line is dependent on the diopside composi tion, and as the diopside becomes aluminous and SiO2-poor at higher pressure, the position Fig. 3.…”

“…This suggests that the join Fo-An-Di-0.3MgCr2O4 is not thermal divide in the system Fo-CaA12SiO6-Di-SiO2 (simplified basalt tetrahedron, Presnall et al, 1978). According to Onuma (1982), the "pierc ing points" fo+an+diss+liquid and fo+ an+spss+liquid are present in the join FoAn-Di-0.2MgCr2O4 (Fig.…”

“…The oxidation states of Cr are problem in experimental petrology as discussed by Schreiber (1978) and Ikeda and Yagi (1978) by Presnall et al (1978) . This suggests that the join Fo-An-Di-0.3MgCr2O4 is not thermal divide in the system Fo-CaA12SiO6-Di-SiO2 (simplified basalt tetrahedron, Presnall et al, 1978).…”

Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy

“…Experiments were conducted along the quartz-fayalite-magnetite buffer, so that iron is mainly present as FeO in the silicate melt. Comparing these data with phase equilibria in the CaO-MgO-Al 2 O 3 -SiO 2 (CMAS) simplified system (Andersen, 1915;Osborn and Tait, 1952;Kushiro, 1972;Presnall et al, 1978Presnall et al, , 1979Longhi and Boudreau, 1980;Sen and Presnall, 1984;Liu and Presnall, 1990) allows us to constrain the phase relations for the entire plausible range of Mg-number of lavas that might be present on the surface of Mercury. At the high Mg-numbers represented in the Mercury and CMAS compositions, the low-Ca pyroxene stability fields along the olivine liquidus surface are protoenstatite, followed by orthoenstatite and then pigeonite (Longhi and Pan, 1988).…”