Scientific Publications from the Virginia Museum of Natural History
by James S. Beard
published 19 November 2010
The Mole Hill diatreme consists of picrobasalt with abundant megacrysts (0.5 mm to 2cm in maximum dimension) of clinopyroxene, Mg-Al-Fe spinel, and (less abundant) olivine. Cognate minerals include microphenocryst and groundmass plagioclase, olivine, clinopyroxene and Fe-Ti-(Cr) spinel. Clinopyroxene is the most abundant megacryst phase. Overall, the clinopyroxene in the megacryst cores is a high-Al, low-Cr augite with Mg# 78-88. Sieve textured rims approach groundmass clinopyroxene compositions. Olivine occurs as megacrysts and also as small (0.1- 0.5 mm) crystals of indeterminate origin. These may be phenocrysts, xenocrysts, or both. All non-groundmass olivine is zoned, becoming Fe- and Ca-rich (and approaching the composition of groundmass olivine) rimward. The most primitive olivine has Fo~90 and NiO as high as 0.75wt.%. More typically, olivine is Fo78-88 with NiO <0.5 wt.%. The megacryst/xenocryst olivine cores have higher Mg# and lower CaO than groundmass olivine. Megacrystic spinels are notably low in Cr, with Cr# <1, and variable Mg# ranging from 52-74. This variation is appears to be continuous, despite the lack of zoning in individual spinel xenocrysts. Plagioclase occurs only as a microphenocryst phase, with uniform An75 cores and rims as sodic as An58. Cognate clinopyroxene (Mg#67-78) is enriched in Ca and Ti relative to the megacrysts. Groundmass olivine has low NiO and high (0.3-0.6 wt.%) CaO. Groundmass spinels have ulvospinel contents near 50%, initially rising with Mg# (in Cr-rich microphenocrysts) then dropping. Although the lack of context for the megacrysts precludes a definitive understanding of their origin, megacryst chemistry (especially the low-Cr spinels and the overall abundance of clinopyroxene) suggests a clinopyroxene-rich source in the upper (e.g. spinel zone) continental lithosphere. This source is likely similar to the Al-augite suite clinopyroxenites and wehrlites that occur as xenoliths and as intrusive veins in composite xenoliths from alkali basalt provinces worldwide. Cognate mineral (groundmass minerals and microphenocrysts) compositions are consistent with crystallization from a slightly evolved alkali basalt melt.