Megacryst assemblages in kimberlite from northern Colorado and southern Wyoming: Petrology, geothermometry-barometry, and areal distribution

Eggler, David H.; McCallum, M. E.; Smith, Craig B.

doi:10.1029/sp016p0213

Cited by 41 publications

(27 citation statements)

References 19 publications

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“…9 Bivariate plots for major and minor elements of cpx in polymineralic inclusions in Cr-pyrope. Data are compared against Cr-diopside hosts and spongy reaction rims (around polymineralic inclusions) from this study, and against megacrysts worldwide (Eggler et al 1979;Hunter and Taylor 1984;de Bruin 2005;Pivin et al 2009) kimberlitic calcite, interpreted to be the product of magmatic crystallization (Malarkey et al 2010).…”

Section: Strontium Isotopesmentioning

confidence: 99%

“…Their Cr-rich compositions may suggest a genetic link to the Cr-rich megacryst suite, first documented by Eggler et al (1979). To obtain P-T estimates of equilibration for Cr-diopside megacrysts, the single-cpx thermobarometer of Nimis and Taylor (2000) was used in combination with the compositional filters suggested by Grütter (2009).…”

Section: Classification and Thermobarometrymentioning

confidence: 99%

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The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Bussweiler

Stone

Pearson

et al. 2016

Contrib Mineral Petrol

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inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr 2 O 3 and Al 2 O 3 . This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside → forsterite + calcite + CO 2 , yielding the observed inclusion mineralogy and producing associated (CO 2 -rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.

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Section: Strontium Isotopesmentioning

confidence: 99%

Section: Classification and Thermobarometrymentioning

confidence: 99%

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Bussweiler

Stone

Pearson

et al. 2016

Contrib Mineral Petrol

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“…Bronzite MsO2 resembles orthopyroxene from Tanzanian kimberlites (Nixon and Condliffe, 1989). Chromium-rich orthopyroxene has been found in the kimberlites from ColoradoWyoming (Eggler et al, 1979) and from Fayette County (Hunter and Taylor, 1984). The large size of the minerals indicates that these orthopyroxene grains did not originate from disaggregation of peridotite inclusions.…”

Section: The Megacrystsmentioning

confidence: 99%

“…The central question pertains to whether the megacrysts are phenocrysts or xenocrysts, and has been addressed by several authors. Eggler et al (1979) distinguished two megacryst populations: a Cr-rich and a Cr-poor one. This criterion cannot be applied to the Kundelungu megacrysts except for garnets, which consist of two distinct groups (separated at 2.5 wt.% Cr203).…”

Section: Petrologic Significance Of Megacrysts and Ultramafic Inclusionsmentioning

confidence: 99%

“…Kimberlites characteristically contain large isolated crystals and ultramafic inclusions, whose abundance and granulometry vary from pipe to pipe (Nixon and Boyd, 1973b;Boyd and Clement, 1977;Eggler et al, 1979;Dawson, 1980;Garrison and Taylor, 1980;Gurney et al, 1979;Mitchell, 1986;Hunter and Taylor, 1984). Following Clement et al (1984) we will use the term 'macrocryst' in a descriptive, non-genetic sense.…”

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confidence: 99%

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Megacrysts and ultramafic xenoliths from Kundelungu kimberlites (Shaba, Zaire)

et al. 1995

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Some twenty kimberlite pipes outcrop along the eastern and western borders of the Kundelungu plateau, Shaba Province, Zaire. They are arranged roughly along two north-south trending alignments. The pipes probably intruded the Bangweulu Block, which stabilized around 1800 Ma. The exceptionally fresh kimberlites contain mantle-derived nodules (peridotites and eclogites), as well as megacrysts which may reach up to several cm in diameter. The most important megacrysts are garnets, ilmenites, clinopyroxenes, orthopyroxenes and olivines. Micas and diamonds are rarely observed. The clinopyroxenes can be subdivided in two groups: (1) a Ca-rich, low-T type, similar to the Cr-rich diopsides found in 'depleted' (granular) peridotites; and (2) subcalcic clinopyroxene comparable to the megacrysts and to the clinopyroxenes of 'fertile' (sheared) peridotites. The orthopyroxenes are less frequent and are Ca-poor enstatites (0.07-0.42 wt.% CaO) and Ti-bronzites (CaO <1.3 wt.%). All the analysed garnets are Ca-rich (>4.5 wt.% CaO) and all fall in the lherzolite field defined by Sobolev et al., 1973. The low-Ca garnets which appear in many diamond-bearing kimberlites have never been observed in Zaire, neither in the diamond-poor Kundelungu pipes nor in the diamond-rich Mbuji-Mayi pipes. The ilmenites define a trend close to the 'magmatic Mgenrichment trend'. The olivine macrocrysts have Fo contents comparable to those of peridotites (Fo90-93). The ultramafic nodules comprise lherzolites, harzburgites, pyroxenites, wehrlites and dunites. The granular textures and P-T equilibrium conditions (770-1380~ and 28-61 kbar) deduced from their mineral compositions, show clearly that they were derived from a mantle zone on the continentalgeotherm (90--190 km depth). The eclogite nodules, which are less frequent, contain only two mineral phases (pyropealmandine-grossular and omphacite), and the texture and the mineral compositions are similar to those of Roberts Victor eclogites. Our findings support the conclusion of Nixon and Condliffe (1989) that low-T peridotites, eclogites and pyroxenites derived from 'depleted' lithosphere, while Cr-poor garnet, subcalcic diopside and bronzite megacrysts cristallized from fertile asthenosphere.

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Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

Pivin

Debaille

Mattielli

et al. 2013

Proceedings of 10th International Kimberlite Conference

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Megacryst assemblages in kimberlite from northern Colorado and southern Wyoming: Petrology, geothermometry-barometry, and areal distribution

Cited by 41 publications

References 19 publications

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Megacrysts and ultramafic xenoliths from Kundelungu kimberlites (Shaba, Zaire)

Nd–Hf Isotope Systematics of Megacrysts from the Mbuji-Mayi Kimberlites, D. R. Congo: Evidence for a Metasomatic Origin Related to Kimberlite Interaction with the Cratonic Lithospheric Mantle

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