THE ROLE OF CUMULATE PLAGIOCLASE ENTRAINMENT IN THE STEENS BASALT, SE OREGON: INSIGHTS FROM THE MAGMA CHAMBER SIMULATOR

Transcrustal magma systems are likely characterized by heterogeneous distribution of crystal cumulates and melt. We use a mass and energy constrained thermodynamic tool, the Magma Chamber Simulator (MCS [1]), to assess if interaction between magmas and cumulates produced abundant plagioclase in the Steens Basalt and, by extension, Giant Plagioclase Basalts (GPB) worldwide. The Steens Basalt exposure at Steens Mountain (SE Oregon) records evolution from more mafic lower Steens, where recharge outpaced fractionation, to more evolved upper Steens, where fractionation + modest crustal assimilation outpaced recharge [2 and references therein]. Lower Steens plagioclase are typically in Sr isotopic equilibrium with associated groundmass (GM) and have lower concentrations of incompatible trace elements like La and Ba, compared to upper Steens plagioclase [3]. A hypothesis that Steens GPB formed via entrainment of a plagioclase-bearing cumulate [3] was iteratively tested in MCS by varying input parameters, with model output compared to whole-rock and in situ mineral geochemical data. For more mafic Lower Steens magmas, cumulate entrainment facilitated by magma recharge into a midcrustal (0.4 GPa) reservoir yielded substantial plagioclase resorption. Upon cooling, plagioclase recrystallized, consistent with crystal-GM isotopic equilibrium and lower Ba and La. Isothermal decompression followed, with model results reproducing Lower Steens whole-rock and mineral data. Heat from Lower Steens magmatism thermally primed the crust as the magma system shoaled [2]. Assimilation + recharge + cumulate entrainment into a shallower (~0.1 GPa), more evolved Upper Steens magma body yielded some plagioclase resorption, followed by new growth, consistent with heterogeneous in situ isotopes and higher La and Ba. In addition, whole rock chemistry was replicated. In conclusion, plagioclase cumulate entrainment is essential for the formation of Steens GPB, and likely other GPBs. MCS modelling indicates that Steens eruptions were fed by a shoaling mid-upper crustal magma storage zone [2] and quantifies the recharge, cumulate entrainment, crystallization, and assimilation history.

[1] Bohrson et al. (2014) Jour of Petrol. 55, 1685–1171. [1] Moore et al. (2018) Geosphere. 14, 2505-2532. [3] Toth (2018) MS thesis, Central Washington University