How Did We Get Here? Insights From Local Mantle-Derived Kimberlites and Pyroxenites

Brenna G. Cole,* Jayden N. Ware* and Graham D. M. Andrews, Department of Geology and Geography, West Virginia University, Morgantown, WV 26506

Field (Broad Category): Geography/Geology (Agricultural & Environmental Sciences) 

Student’s Major: Geology 

Kimberlites and pyroxenites are ultramafic (i.e. rich in iron and magnesium) igneous rocks formed from melting of the Earth’s mantle and are the primary sources of diamonds. Both of these rocks occur as small dikes throughout Appalachia from Quebec to North Carolina, despite the region being dominated by sedimentary rocks. The Masontown kimberlite (type-II) dike in Fayette County, PA, and the Sugar Grove mica pyroxenite dike in Pendleton County, WV, are about 170 km apart and are both Jurassic (~160 Ma) in age. Kimberlites are dominated by the mineral olivine, whereas pyroxenites are dominated by pyroxenes; both are rich in phlogopite mica and xenoliths (mantle fragments in the kimberlite and crustal fragments in the pyroxenite). Our goal is to determine whether their emplacement mechanisms are similar. Using the Kimberlite Factory model of emplacement established by Brett et al. (2015) and thin section petrography, we have established that the Masontown kimberlite exhibits textural features consistent with ascent as a gas-filled, turbulent fracture quickly inundated by kimberlite magma. The primary evidence of this is rounded xenoliths surrounded by a selvage of olivine-free kimberlite, set in a matrix of olivine-rich kimberlite. Petrography of the pyroxenite and the xenoliths within it will allow us to compare the textures with the kimberlite. Pyroxenite dikes are very rare globally, and little to nothing is known of how they are emplaced; comparing the Sugar Grove dike to a contemporaneous kimberlite will allow us to assess if the Kimberlite Factory model is appropriate for this composition too. 

Funding: 

Program/mechanism supporting research/creative efforts: WVU 497-level course