Anorthite--Forsterite and Anorthite--Enstatite Reactions and their bearing on the Basalt--Eclogite Transformation

Abstract

The reactions betweeon anorthite and forsterite and between anorthite and enstatite have been studied at high pressures and high temperatures with solid-media and gas-media apparatus. For the 1: 1 (molecular ratio) composition of anorthite and forsterite the following three assemblages have been obtained with increasing pressure at a constant temperature: anorthite³forsterite, clinopyroxene³orthopyroxene³spinel³anorthite, and garnet (pyropegrossularite series). Two univariant curves between these three assemblages have been determined. The amounts of pyroxenes relative to spinel and anorthite in the second assemblage increase and the contents of the Tschermak's components in the pyroxenes increase with increasing pressure. The 1:2 (molecular ratio) composition of anorthite and forsterite was also studied, and the univariant curve for the reaction clinopyroxene³orthopyroxene³spinel⇆ garnet (pyropegrossularite series)³forsterite has been determined. For the 1: 2 (molecular ratio) composition of anorthite and enstatite the following assemblages have been obtained with increasing pressure at a constant temperature: anorthite³enstatite, anorthite³orthopyroxene³clinopyroxene³quartz, and garnet (pyrope-grossularite series)³quartz. The univariant curve between the last two assemblages has been determined. Both the pyroxenes in the second assemblage contain Tschermak's components, which increase in content with increasing pressure at the expense of anorthite. These experimental results suggest that basalts, either saturated or undersaturated with silica, transform into aluminous pyroxene-rich assemblages at lower pressures or higher temperatures than the stability field of eclogites, and that pyroxene-spinel symplectites found in some metamorphosed gabbro, troctolite, and norite may have formed in the stability field of the pyroxene-rich assemblages. The stability field of the pyroxene-rich assemblages is probably included within the pressure-temperature conditions of the granulite facies. It is also suggested that the garnet-free peridotites, such as the peridotite inclusions in basaltic rocks and the intrusive peridotites in the orogenic belts, are stable at lower pressures or higher temperatures than garnet peridotites, such as the inclusions in kimberlite and intrusive garnet peridotite in the orogenic belts.