Melting Relationship of The Rock Creek Flow, Columbia River Basalts, Idaho

Abstract

The Rock Creek Flow, a Miocene tholeiitic basalt of the Columbia River Plateau Province, is a geographically well defined flow more than 400 feet thick and belonging to the "Basaltic flood" type. Having received little prior geologic attention, it is now the object of an eight-university study with the ultimate goal of comparing its petrology to that of the lunar maria basalts. This thesis is an experimental study of the melting relationships (the solidification temperatures) of samples collected in vertical sequence at one outcrop of the Rock Creek Flow. These natural basalts were investigated experimentally at a series of temperatures. The remelt solidus of the Rock Creek Flow is 1025°C ± 6°C. The remelt liquidus of the flow is 1210°C ± 10°C , and estimated eruption temperature is 1175° C ± 15°C. The melting behavior of the Rock Creek Flow Basalt indicates that this flow had nearly isothermal crystallization except for chilled zones. Either a single out­welling of lava or many converging flow units formed this single major (more than 400 feet thick) flow unit prior to the solidifica­tion of lava. The petrology of the Rock Creek Flow was studied by Drs. Hooper and Rosenberg (1971.) and they made chemical analyses of samples collected in vertical sequence at five different out­crops of the Rock Creek Flow including the outcrop sampled for the determination of melting relationship. Their chemical analyses show that the Rock Creek Flow has a nearly isochemical nature having very little vertical chemical variation. Very probably this isochemical nature of the flow is responsible for the nearly isothermal crystallization determined by experimental study. The results of the melting relationship study of the Rock Creek Flow do not support any major crystal settling for this 440-foot-thick flow.