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Monticellite in group-I kimberlites: Implications for evolution of parental melts and post-emplacement CO2 degassing

Citation

Abersteiner, AB and Kamenetsky, VS and Pearson, DG and Kamenetsky, MB and Goemann, K and Ehrig, K and Rodemann, T, Monticellite in group-I kimberlites: Implications for evolution of parental melts and post-emplacement CO2 degassing, Chemical Geology: An International Journal ISSN 0009-2541 (In Press) [Refereed Article]

DOI: doi:10.1016/j.chemgeo.2016.08.029

Abstract

Monticellite is a magmatic and/or deuteric mineral that is often present, but widely varying in concentrations in Group-I (or archetypal) kimberlites. To provide new constraints on the petrogenesis of monticellite and its potential significance to kimberlite melt evolution, we examine the petrography and geochemistry of the minimally altered hypabyssal monticellite-rich Leslie (Canada) and Pipe 1 (Finland) kimberlites. In these kimberlites, monticellite (Mtc) is abundant (2545 vol%) and can be classified into two distinct morphological types: discrete and intergrown groundmass grains (Mtc-I), and replacement of olivine (Mtc-II). Primary multiphase melt inclusions in monticellite, perovskite and Mg-magnetite contain assemblages dominated by alkali (Na, K, Ba, Sr)-enriched Ca-Mg-carbonates, chlorides, phosphates, spinel, silicates (e.g. olivine, phlogopite) and sulphides. These melt inclusions probably represent snapshots of a variably differentiated kimberlite melt that evolved in-situ towards carbonatitic and silica-poor compositions. Although unconstrained in their concentration, the presence of alkali-carbonates and chlorides in melt inclusions suggests they are a more significant component of the kimberlite melt than commonly recorded by whole-rock analyses. We present petrographic and textural evidence showing that pseudomorphic Mtc-II resulted from an in-situ reaction between olivine and the carbonate component of the kimberlite melt in the decarbonation reaction: This reaction is supported by the preservation of abundant primary inclusions of periclase and to a lesser extent Fe-Mg-oxides in monticellite, perovskite and Mg-magnetite. Based on the preservation of primary periclase inclusions, we infer that periclase also existed in the groundmass, but was subsequently altered to brucite. We suggest that CO2 degassing in the latter stages of kimberlite emplacement into the crust is largely driven by the observed reaction between olivine and the carbonate melt. For this reaction to proceed, CO2 should be removed (i.e. degassed), which will cause further reaction and additional degassing in response to this chemical system change (Le Chatelier's principle). Our study demonstrates that these proposed decarbonation reactions may be a commonly overlooked process in the crystallisation of monticellite and exsolution of CO2, which may in turn contribute to the explosive eruption and brecciation processes that occur during kimberlite magma emplacement and pipe formation.

Item Details

Item Type:Refereed Article
Keywords:Kimberlite Monticellite Periclase Melt inclusions Alkali-carbonate CO2 degassing
Research Division:Earth Sciences
Research Group:Geology
Research Field:Igneous and Metamorphic Petrology
Objective Division:Mineral Resources (excl. Energy Resources)
Objective Group:Other Mineral Resources (excl. Energy Resources)
Objective Field:Mineral Resources (excl. Energy Resources) not elsewhere classified
Author:Abersteiner, AB (Mr Adam Abersteiner)
Author:Kamenetsky, VS (Professor Vadim Kamenetsky)
Author:Kamenetsky, MB (Dr Maya Kamenetsky)
Author:Goemann, K (Dr Karsten Goemann)
Author:Rodemann, T (Dr Thomas Rodemann)
ID Code:119062
Year Published:In Press
Deposited By:Central Science Laboratory
Deposited On:2017-07-25
Last Modified:2017-07-25
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