A hypabyssal kimberlite dike in southwestern Pennsylvania (USA), emplaced through Proterozoic basement and Phanerozic cover, contains a xenocryst and xenolith assemblage typical of material sampled within the subcontinental lithosphere, including xenocrysts of Cr-rich pyrope, magnesiochromite, Cr-rich diopside, and peridotite xenoliths. Temperatures and depths of equilibration of the clinopyroxene (840 °C and 130 km to 1350 °C and 170 km) indicate some sampling in the field of diamond stability. Diamonds have not been reported, however, and the chemistry of the garnet (lherzolite, Cr-poor megacryst, and Group II eclogite) and spinel (<56.0 wt.% Cr2O3) are consistent with diamond absence and the off-craton tectonic setting of the kimberlite. An unusual feature of this suite is that, unlike most mantle xenolith/xenocryst spinel, some of those from Masontown have an unusually high silica content (to 0.59 wt.% SiO2). The significance of the high silica content is unclear, but may be related to an ultrahigh-pressure precursor chromite polymorph with a calcium ferrite structure, which can accommodate Si in solid solution.
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Research Article|
July 01, 2015
Mantle Xenocrysts from the Masontown, Pennsylvania Kimberlite: an Ordinary Mantle with Si-enriched Spinel
Daniel J. Schulze
;
Daniel J. Schulze
§
Department of Earth Sciences and Department of Chemical and Physical Sciences, University of Toronto, Mississauga, Ontario, Canada L5L 1C6
§Corresponding author e-mail address: daniel.schulze@utoronto.ca
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B. Carter Hearn, Jr.
B. Carter Hearn, Jr.
U.S. Geological Survey, Reston, Virginia, USA 22092 and Smithsonian Institution, Washington, D.C., USA
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The Canadian Mineralogist (2015) 53 (4): 767–773.
Article history
received:
02 May 2015
accepted:
19 May 2015
first online:
16 Feb 2018
Citation
Daniel J. Schulze, B. Carter Hearn; Mantle Xenocrysts from the Masontown, Pennsylvania Kimberlite: an Ordinary Mantle with Si-enriched Spinel. The Canadian Mineralogist 2015;; 53 (4): 767–773. doi: https://doi.org/10.3749/canmin.1500056
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Index Terms/Descriptors
- chain silicates
- chromite
- clinopyroxene
- diamond
- dikes
- diopside
- eclogite
- Fayette County Pennsylvania
- garnet group
- Grenvillian Orogeny
- igneous rocks
- inclusions
- intrusions
- kimberlite
- lherzolite
- mantle
- metamorphic rocks
- native elements
- nesosilicates
- orthosilicates
- oxides
- Pennsylvania
- peridotites
- plutonic rocks
- Precambrian
- pressure
- pyrope
- pyroxene group
- silicates
- silicon
- solid solution
- spinel
- ultrahigh pressure
- ultramafics
- United States
- upper mantle
- xenocrysts
- xenoliths
- magnesiochromite
- Masontown Pennsylvania
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