Basltos Mit

12.109 Petrology Basalts, basalt series, and basalt classification Basalt in the generic sense = plagioclase + clinopyroxene +/- (olivine, nepheline, hypersthene, quartz, melilite, magnetite). Thebasalt tetrahedron is a useful tool for classifying basalts. The tetrahedron is constructed using the normative components olivine, quartz, clinopyroxene, and nepheline.

There are six volumes in thetetrahedron that define the different groups: Name I Quartz tholeiite II Tholeiite III Olivine tholeiite IV Transitional basalt or olivine basalt V Alkaline oliv basalt VI High Al basalt OversaturatedSaturated Undersaturated Critically undersaturated High Al from plag Normative components Quartz and hypersthene Hyp Olivine and hyp No normative hyp, only oliv Oliv and nepheline Plagioclase

Notethat there is a low pressure thermal divide (oliv-plag-cpx) that separates alkali olivine basalts from tholeiites. Basalt series – We can think of three end member series, which represent the majortypes of basalt magmas and their fractionation products. Alkali olivine basalt series

AOB > hawaiite > mugearite > trchyte > phonolite (pantellerite) Rock series characteristic of oceanic islandsand continental rift environments Tholeiite basalt series Tholeiite > andesite > dacite > rhyolite Rock series characteristic of ocean floor and continental arc or island arc settings Also, highalumina basalt can be the parent of this series

Are basalts produced by melting a single source? Or melting source regions of variable composition? Melting experiments on simple systems and on naturalbasalts showed that the range of observed basalts could be derived by melting a single source region – IF melts were produced at different pressures Alkali olivine basalts come from deep Olivinetholeiites come from melting at shallower depths Evidence – shrinking of olivine primary phase volume with increasing pressures. Melts at the olivine + high-Ca pyroxene (cpx) + low Ca pyroxene (opx) become...