Convective fractionation: A mechanism to provide cryptic zoning (macrosegregation), layering, crescumulates, banded tuffs and explosive volcanism in igneous processes

Abstract

A large array of igneous and volcanic features has characteristics that are recognized in other disciplines as conclusive and direct evidence of convection in stratified and/or solidifying melts; e.g., macroscopic segregation (cryptic variation, zoning in magma chambers), mineral layering (in mafic intrusives), crescumulates (fingering) in the vertical and horizontal, banding (in pyroclastics), ‘rollover’ with attendant flashing of volatiles (explosive volcanism), etc. Some quantitative and qualitative aspects of convection in solidifying and/or stratified melts (e.g., mineral layer widths such as are observed in the Skaergaards) are examined to show consistency with field evidence. Convective fractionation does not possess the physical implausibilities of gravitational segregation (crystal settling). Neither is the field evidence as ambiguous if interpreted in terms of convective fractionation (which can explain amongst other things heavier material overlying lighter). Convective fractionation may operate on larger scales in the interior of planets.