Abstract
The thickness of the crystal mush on magma chamber floors can be constrained using the offset between the step-change in the median value of dihedral angles formed at the junctions between two grains of plagioclase and a grain of another phase (typically clinopyroxene, but also orthopyroxene and olivine) and the first appearance or disappearance of the liquidus phase associated with the step-change in median dihedral angle. We determined the mush thickness in the Rustenburg Layered Suite of the Bushveld Complex at clinopyroxene-in (in Lower Main Zone) and magnetite-in (in Upper Zone). We also examined an intermittent appearance of cumulus apatite in Upper Zone, using both the appearance and disappearance of cumulus apatite. In all cases, the mush thickness does not exceed 4 m. These values are consistent with field observations of a mechanically rigid mush at the bases of both magnetitite and chromitite layers overlying anorthosite. Mush thickness of the order of a few metres suggests that neither gravitationally-driven compaction nor compositional convection within the mush layer is likely to have been important processes during solidification: adcumulates in the Bushveld are most likely to have formed at the top of the mush during primary crystallisation. Similarly, it is unlikely either that migration of reactive liquids occurs through large stretches of stratigraphy, or that layering is formed by mechanisms other than primary accumulation.
Highlights
The thickness of the crystal mush on the floor of magma chambers, defined as the vertical interval separating an essentially rigid assemblage of cumulus and intercumulus grains within which any remaining liquid is trapped and immobile, and the interface with the overlying bulk magma, is of interest as it is the physical properties of this mush, Communicated by Timothy L
The appearance of unambiguously cumulus clinopyroxene occurs at 1377.15 m depth in the core, with unambiguously intercumulus clinopyroxene below 1378.3 m: the offset between the two markers denoting the top and bottom of the mush at the moment the bulk magma saturated in clinopyroxene is no more than ~ 2 m
Detecting stratigraphic intervals characterised by a thick mush requires many more data points than the few presented here: while this is possible in that part of UZ characterised by the cyclic appearance and disappearance of cumulus apatite (Tegner et al 2006), constraints on mush thickness in regions of stratigraphy far from horizons marking the arrival or disappearance of liquidus phases can only be obtained using detailed field observations
Summary
The thickness of the crystal mush on the floor of magma chambers, defined as the vertical interval separating an essentially rigid assemblage of cumulus and intercumulus grains within which any remaining liquid is trapped and immobile, and the interface with the overlying bulk magma, is of interest as it is the physical properties of this mush, Communicated by Timothy L. It was argued that the thickness of the crystal mush can be determined at the specific stratigraphic position recording the saturation of the bulk magma in a new liquidus phase (or when the bulk magma lost a liquidus phase). This is done by measuring the stratigraphic distance between the arrival (or disappearance) of the liquidus phase in the primocryst assemblage and the step-change in dihedral angle associated with the changing fractional contribution of latent heat to the enthalpy budget (Holness et al 2017a). The base of the mush is marked by the step-change in dihedral angle
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