Application of a textural geospeedometer to the late-stage magmatic history of MIL 03346

Abstract

Dynamic crystallization experiments performed on Fe-rich, Al-poor basalt are employed as a textural calibration set to quantify the late-stage igneous history of nakhlite Miller Range (MIL) 03346. The ratio of crystal-melt surface area to volume typifying morphologically distinct populations of Ca-pyroxene has been shown to vary as a strong function of cooling rate (Hammer 2006). Furthermore, a texture of phenocrysts surrounded by finer-grained groundmass crystals arises by sequential nucleation events during constant-rate cooling, but multiple populations nucleate only if the cooling rate is less than or equal to 72 degrees C h^(-1). Textural analysis of meteorite MIL 03346 reveals at least two distinct populations. The Ca-pyroxene phenocryst and microphenocryst three dimensional (3D) aspect ratios are 112 +/- 8.3 and 1530 +/- 160 mm^(-1), respectively. By comparison with the calibration set, the range of cooling rates consistent with 3D aspect ratios of both populations in MIL 03346 is ~20 degrees C h^(-1). An additional experiment was performed approximating a conductive heat transfer profile in order to interpret and apply results of constant-rate cooling experiments to the natural cooling of magma. Results suggest that the textures of constant-rate experiments parallel the initial period of rapid cooling in natural magma. Initial cooling rates of ~20 degrees C h^(-1) in the lava hosting MIL 03346 occur in conductively solidifying lava at depths of ~0.4 m, constraining the minimum total thickness to greater than or equal to 0.8 m. Crystal accumulation beginning in a subsurface reservoir and continuing after lava emplacement as an inflated pahoehoe sheet satisfies all textural constraints on the late-stage igneous history of MIL 03346.