PLAGIOCLASE FLOTATION CUMULATE IN FERROBASALTS OF THE WESTERN SNAKE RIVER PLAIN: IMPLICATIONS FOR EVOLUTION OF PLANETARY MAGMA OCEANS

Pleistocene basalt flows of the western Snake River Plain are characterized by high FeO* (?13.5%) at moderate MgO (5.6-8.6%), resulting in low mg#s that reflect extensive fractional crystallization of primitive parent magmas. Many of these lavas contain plagioclase flotation cumulates that formed within covered lava channels; these flotation cumulates provide an analogue for anorthosite formation in the lunar magma ocean.

Lava flows with flotation cumulates comprise three zones: a central, plagioclase porphyry with intersertal to intergranular textures in the groundmass, an upper diktytaxitic zone comprising plagioclase laths with large voids and minor intergranular mafics and glass, and a lower aphyric zone of ferrobasalt, with 16-17% FeO*. Mass balance calculations show that the diktytaxic zone contains 30-50% porosity, represented by the ferrobasalt base, if the central plagioclase porphyry is assumed to represent the bulk composition. Detailed outcrop maps show that successive lava flows flowed beneath previously emplaced flows, inflating and plastically deforming the aphyric ferrobasalt zone in the overlying flow. Plagioclase flotation cumulates beneath this ferrobasalts ceiling display a horizontal contact between the diktytaxitic and plagioclase porphyry zones; we interpret this horizon to represent the contact between interstitial melt (below) and volcanic gasses (above), and suggest that interstitial melt was displaced by the rising gasses.

Plagioclase phenocrysts are An65 in both the porphyry and diktytaxitic zones, suggesting formation the plagioclase framework by simultaneous crystallization throughout the flow, followed by sinking of the dense, interstitial ferrobasaltic liquid to the bottom of the flow. This interpretation is supported by olivine phenocrysts (Fo68) trapped in the plagioclase framework, and by partially replaced intergrowths containing Fo80-89 olivine. The Fo-rich olivine relicts imply a parent magma that was significantly more magnesian than observed. We suggest that planetary anorthosites represent plagioclase saturated melts that crystallize en mass to form buoyant rafts of plagioclase plus trapped mafics; interstitial ferrobasalts liquids are forced out by rising magmatic gasses, and sink back into the underlying magma.