Petrogenesis of opaque minerals in kimberlite : Elliott County, Kentucky

Author

Jeffrey J. Agee

Date of Award

8-1980

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Lawrence A Taylor

Committee Members

Harry Y McSween, John Higgins

Abstract

Two kimberlite pipes in Elliott County, Kentucky contain inclusions of altered crustal rocks, ultramafic xenoliths and a diverse megacryst suite in a groundmass of olivine, picroilmenite, chromite, titaniferous magnetite, pleonaste, perovskite, calcite, and various hydrous silicates. The megacryst assemblage consists of olivine, garnet, picroilmenite, phlogopite, clinopyroxene and orthopyroxene. Rare clinopyroxene-ilmenite and rutile-iImenite intergrowths also occur.

Ilmenite megacrysts (8.50 to 13.5% Mg0) have reversed-zoned rims (13.8 to 16.5% Mg0). These rims are surrounded by a perovskite and titaniferous magnetite reaction rim. Ilmenite megacryst cores show correlation of Mg0 with Fe₂0₃ ; groundmass ilmenites (12.6 to 17.0% Mg0) show a similar trend, but have a wider range of Fe₂0₃. Ilmenite in clinopyroxene-ilmenite intergrowths (13.0 to 13.5% Mg0) is at the high Mg0 end of the megacryst trend. The clinopyroxenes have equilibration temperatures of 1295-1335°C. (Di-En solvus). A rutile-ilmenite intergrowth is surrounded by ilmenite which is compositionally similar to the groundmass ilmenites (14.7% Mg0). Ilmenites at Elliott County plot as a portion of a parabolic trend on a Cr₂0₃ versus Mg0 diagram.

Spinels at Elliott County show the typical kimberlitic trend of Cr→Al-rich spinels, followed by late-stage formation of titaniferous magnetite. Cr and Al-rich spinels are believed to be related to the groundmass stage of crystallization, while the titaniferous magnetites are post magmatic.

Oxide minerals in the Elliott County kimberlite record three periods of formation. The early megacrysts and clinopyroxene-ilmenite intergrowths crystallized from a fractionating liquid at 150km. (T~ 1100-1400°C) as evidenced by Mg0 trends and association with silicate megacrysts. Subsequently, megacrysts reacted with a more Mg-enriched liquid, resulting in formation of Mg-rich rims. Groundmass minerals which crystallized from this magma are more magnesian than the earlier more fractionated megacrysts. The final event is the reaction of ilmenite to form perovskite titaniferous magnetite.

Recommended Citation

Agee, Jeffrey J., "Petrogenesis of opaque minerals in kimberlite : Elliott County, Kentucky. " Master's Thesis, University of Tennessee, 1980.
https://trace.tennessee.edu/utk_gradthes/15349