Editing Kimberlite (section)

== Petrology ==

Both the location and origin of kimberlitic magmas are subjects of contention. Their extreme enrichment and geochemistry have led to a large amount of speculation about their origin, with models placing their source within the sub-continental lithospheric mantle (SCLM) or even as deep as the transition zone. The mechanism of enrichment has also been the topic of interest with models including partial melting, assimilation of subducted sediment or derivation from a primary magma source.

Historically, kimberlites have been classified into two distinct varieties, termed "basaltic" and "micaceous" based primarily on petrographic observations.<ref>Wagner, P. A., 1914: The diamond fields of South Africa; Transvaal Leader, Johannesburg.</ref> This was later revised by C. B. Smith, who renamed these divisions "group I" and "group II" based on the isotopic affinities of these rocks using the [[neodymium|Nd]], [[strontium|Sr]], and Pb systems.<ref>Smith, C. B., 1983: Lead, strontium, and neodymium isotopic evidence for sources of African Cretaceous kimberlite, Nature, 304, pp. 51–54.</ref>  Roger Mitchell later proposed that these group I and II kimberlites display such distinct differences, that they may not be as closely related as once thought. He showed that group II kimberlites show closer affinities to [[lamproite]]s than they do to group I kimberlites. Hence, he reclassified group II kimberlites as orangeites to prevent confusion.<ref>{{cite book|last1=Mitchell|first1=Roger Howard|title=Kimberlites, Orangeites, and Related Rocks|date=1995|publisher=Springer US|location=Boston, MA|isbn=978-1461519935}}</ref>

=== Group I kimberlites ===

Group-I kimberlites are of CO<sub>2</sub>-rich [[ultramafic]] potassic igneous rocks dominated by primary [[Forsterite|forsteritic olivine]] and carbonate minerals, with a trace-mineral assemblage of magnesian [[ilmenite]], chromium [[pyrope]], [[almandine]]-pyrope, chromium [[diopside]] (in some cases subcalcic), [[phlogopite]], [[enstatite]] and of Ti-poor [[chromite]]. Group I kimberlites exhibit a distinctive inequigranular texture caused by macrocrystic ({{convert|0.5|-|10|mm|in|disp=or|abbr=on}}) to megacrystic ({{convert|10|-|200|mm|in|disp=or|abbr=on}}) phenocrysts of olivine, pyrope, chromian diopside, magnesian ilmenite, and phlogopite, in a fine- to medium-grained groundmass.<ref>{{Cite journal |last1=Sarkar |first1=Soumendu |last2=Giuliani |first2=Andrea |last3=Dalton |first3=Hayden |last4=Phillips |first4=David |last5=Ghosh |first5=Sujoy |last6=Misev |first6=Sarah |last7=Maas |first7=Roland |date=2023-06-20 |title=Derivation of Lamproites and Kimberlites from a Common Evolving Source in the Convective Mantle: the Case for Southern African 'Transitional Kimberlites' |url=http://dx.doi.org/10.1093/petrology/egad043 |journal=Journal of Petrology |volume=64 |issue=7 |doi=10.1093/petrology/egad043 |issn=0022-3530|hdl=20.500.11850/623387 |hdl-access=free }}</ref>

The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, is dominated by carbonate and significant amounts of forsteritic olivine, with lesser amounts of pyrope garnet, Cr-[[diopside]], magnesian ilmenite, and [[spinel]].

=== Olivine lamproites ===

Olivine lamproites were previously called group II kimberlite or orangeite in response to the mistaken belief that they only occurred in South Africa. Their occurrence and petrology, however, are identical globally and should not be erroneously referred to as kimberlite.<ref>{{cite journal |last1=Francis |first1=Don |last2=Patterson|first2=Michael|title=Kimberlites and aillikites as probes of the continental lithospheric mantle|journal=Lithos|date=April 2009|volume=109|issue=1–2|pages=72–80 |doi= 10.1016/j.lithos.2008.05.007|bibcode=2009Litho.109...72F }}</ref> Olivine lamproites are [[ultrapotassic igneous rocks|ultrapotassic]], [[peralkaline rock]]s rich in volatiles (dominantly H<sub>2</sub>O). The distinctive characteristic of olivine lamproites is [[phlogopite]] macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Al-poor phlogopite requiring Fe to enter the tetrahedral site). Resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine are common but not essential constituents.

Characteristic primary phases in the groundmass include zoned pyroxenes (cores of diopside rimmed by Ti-aegirine), spinel-group minerals (magnesian [[chromite]] to titaniferous [[magnetite]]), Sr- and [[Rare-earth element|REE]]-rich [[perovskite]], Sr-rich [[apatite]], REE-rich phosphates ([[monazite]], daqingshanite), potassian barian [[hollandite]] group minerals, Nb-bearing [[rutile]] and Mn-bearing [[ilmenite]].

=== Kimberlitic indicator minerals ===

Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with chemical compositions that indicate they formed under high pressure and temperature within the mantle. These minerals, such as chromium diopside (a [[pyroxene]]), chromium spinels, magnesian ilmenite, and pyrope garnets rich in chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.