气候变化领域集成服务门户(CCPortal): Cognate versus xenocrystic olivines in kimberlites

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DOI	10.1016/j.earscirev.2021.103771
	Cognate versus xenocrystic olivines in kimberlites – A review
	Moore A.; Costin G.; Proyer A.
发表日期	2021
ISSN	0012-8267
卷号	221
英文摘要	Models for a xenocryst origin for kimberlite olivines emphasise the similarity between their core compositions and those in mantle peridotites. While this permits a xenocryst origin, it does not provide proof, as magmas generated in equilibrium with mantle olivines could, in principle, crystallize initial olivines matching those in the source region. Further, in several kimberlites, there is a striking disparity between the compositional range of olivine cores and that in associated mantle peridotite xenoliths from the same locality. Olivine-liquid Mg-Fe exchange coefficients and Ni partition coefficients permit equilibrium between Mg-rich mantle olivines (Mg # ~ 94–93) and magmas matching kimberlite bulk rock compositions. Glass inclusions in olivine megacrysts from the Monastery kimberlite, with compositions which overlap the range of archetypal Group I kimberlites, were interpreted to represent original liquids trapped at pressures of 4.5–6 GPa. These glass inclusions provide direct petrographic support for primitive melts matching kimberlite bulk chemistry in the lower SCLM. A majority of kimberlitic olivines show normal (decreasing Mg #) core to rim zonation. Cores of normal-zoned kimberlitic olivines are typically homogeneous, but collectively define a field with a range in Mg # and invariant or slightly decreasing Ni towards more Fe-rich compositions. The most Mg-rich cores of normal-zoned olivines typically have Mg # in the range 94–93, but there are marked differences in the Fe-rich extreme of the normal-zoned population between different kimberlite clusters. Olivine rims typically define a field characterized by steeply decreasing Ni, coupled with invariant or slightly increasing or decreasing Mg #, which invariably overlaps the Fe-extreme of core compositions of the relatively Mg-rich, normal-zoned olivines. Consequently, while there is a sharp inflection in chemical gradient between the respective fields of cores and rims, they nevertheless define a continuous compositional field. Trace element modelling demonstrates that these zonation patterns can be explained in terms of a Raleigh crystallization model. Most, if not all kimberlites are characterized by a subordinate group of olivine macrocrysts with cores that are Fe-rich relative to the field for rims, and thus show reverse zonation, which are interpreted to be linked to the Cr-poor megacryst suite. Rare Mg-rich olivines (relative to rims), have high-pressure inclusions of garnet, clinopyroxene and orthopyroxene. When present, such inclusions often show disequilibrium features such as internal chemical zonation. This points to a very short mantle residence time prior to entrainment by the host kimberlite, indicating a link to the Cr-rich megacryst suite rather than mantle peridotites. In addition to a variable, but generally subordinate proportion of olivines derived from Cr-poor and Cr-rich megacrysts, xenocrysts derived from disaggregated mantle peridotites will undoubtedly be present. While their proportions are difficult to quantify, the collective evidence points to a cognate origin for a majority of kimberlitic olivines. A kimberlite magma ascent model is proposed which provides a framework for understanding both olivine compositional variation and apparently enigmatic internal and external olivine morphology. © 2021 Elsevier B.V.
英文关键词	Cognate phenocryst; Kimberlite; Megacrysts; Olivine; Trace element modelling; Xenocrysts
语种	英语
scopus关键词	Trachinotus falcatus
来源期刊	EARTH-SCIENCE REVIEWS
文献类型	期刊论文
条目标识符	http://gcip.llas.ac.cn/handle/2XKMVOVA/209196
作者单位	Department of Geology, Rhodes University, Artillery Road, Makhanda (Grahamstown)6139, South Africa; Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, Houston, TX 77005, United States; Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana
推荐引用方式 GB/T 7714	Moore A.,Costin G.,Proyer A.. Cognate versus xenocrystic olivines in kimberlites – A review[J],2021,221.
APA	Moore A.,Costin G.,&Proyer A..(2021).Cognate versus xenocrystic olivines in kimberlites – A review.EARTH-SCIENCE REVIEWS,221.
MLA	Moore A.,et al."Cognate versus xenocrystic olivines in kimberlites – A review".EARTH-SCIENCE REVIEWS 221(2021).