Kimberlit: Perbedaan revisi

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Kimberlit telah menarik perhatian yang lebih besar daripada volume relatifnya. Ini terutama disebabkan oleh sifatnya sebagai pembawa intan dan ksenolit [[batu delima]] [[peridotit]] dari [[mantel bumi|mantel]] ke permukaan Bumi. Turunannya yang mungkin dari kedalaman adalah lebih besar daripada jenis [[batuan beku]] lainnya, dan komposisi [[magma]] ekstrem yang ia cerminkan dari rendahnya kadar [[silika]] dan tingginya [[unsur kelumit]] yang tak-kompatibel, membuat pemahaman akan [[petrogenesis]] kimberlit menjadi penting. Dalam hal ini, kajian kimberlit berpotensi menyediakan informasi tentang komposisi mantel dalam dan tentang proses pelelehan yang terjadi di atau di dekat antarmuka antara [[litosfer]] benua [[kraton]]ik dan mantel [[astenosfer]] di dekatnya yang berkonveksi.
== Morfologi dan vulkanologi ==
Kimberlit hadir sebagai [[intrusi (geologi)|intrusi]] vertikal yang menyerupai wortel, yang disebut 'pipa'. Bentuk wortel klasik ini terbentuk karena proses intrusif kompleks dari magma kimberlitik yang mewariskan proporsi CO<sub>2</sub> dan H<sub>2</sub>O yang sama besarnya dalam sistem ini, yang menghasilkan tahapan pendidihan yang meledak-ledak yang menyebabkan kobaran api vertikal besar-besaran (Bergman, 1987). Penggolongan kimberlit didasarkan pada pengenalan fasies batu yang berbeda-beda. Fasies yang berbeda-beda ini berhubungan erat dengan gaya aktivitas magmatik khusus, yaitu kawah, diatrema, dan batuan [[hipabisal]] (Clement dan Skinner 1985; dan Clement, 1982).
The [[morphology (materials science)|morphology]] of kimberlite pipes, and the classical carrot shape, is the result of explosive [[Volcanic pipe|diatreme]] [[volcanism]] from very deep [[Earth's mantle|mantle]]-derived sources. These volcanic explosions produce vertical columns of rock that rise from deep magma reservoirs. The morphology of kimberlite pipes is varied but generally includes a sheeted dyke complex of tabular, vertically dipping feeder dykes in the root of the pipe which extends down to the mantle. Within {{convert|1.5|-|2|km|mi|abbr=on}} of the surface, the highly pressured magma explodes upwards and expands to form a conical to cylindrical [[diatreme]], which erupts to the surface. The surface expression is rarely preserved, but is usually similar to a [[maar|maar volcano]]. The diameter of a kimberlite pipe at the surface is typically a few hundred meters to a kilometer (up to 0.6 mile).
Two [[Jurassic]] kimberlite [[Dike (geology)|dikes]] exist in [[Pennsylvania]]. One, the Gates-Adah Dike, outcrops on the [[Monongahela River]] on the border of [[Fayette County, Pennsylvania|Fayette]] and [[Greene County, Pennsylvania|Greene]] Counties. The other, the Dixonville-Tanoma Dike in central [[Indiana County, Pennsylvania|Indiana]] County, does not outcrop at the surface and was discovered by miners.<ref name="multiple">Berg, T.M., Edmunds, W.E., Geyer, A.R. and others, compilers (1980). Geologic Map of Pennsylvania: Pennsylvania Geologic Survey, Map 1, scale 1:250,000.</ref>
== Petrologi ==
Both the location and origin of kimberlitic magmas are areas of contention. Their extreme enrichment and geochemistry has 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 subdivided into two distinct varieties termed 'basaltic' and 'micaceous' based primarily on petrographic observations (Wagner, 1914). This was later revised by Smith (1983) who renamed these divisions Group I and Group II based on the isotopic affinities of these rocks using the Nd, Sr and Pb systems. Mitchell (1995) 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 actually show closer affinities to lamproites than they do to Group I kimberlites. Hence, he reclassified Group II kimberlites as orangeites to prevent confusion.
=== Kimberlit Golongan I ===
Group-I kimberlites are of CO<sub>2</sub>-rich [[ultramafic]] potassic igneous rocks dominated by a primary mineral assemblage of [[forsteritic]] [[olivine]], 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=comma|abbr=on}}) to megacrystic ({{convert|10|-|200|mm|in|disp=comma|abbr=on}}) phenocrysts of olivine, [[pyrope]], chromian diopside, magnesian ilmenite and phlogopite, in a fine to medium grained groundmass.
The groundmass mineralogy, which more closely resembles a true composition of the igneous rock, contains forsteritic [[olivine]], [[pyrope]] garnet, Cr-[[diopside]], magnesian [[ilmenite]] and [[spinel]].
=== Kimberlit Golongan II ===
Group-II kimberlites (or '''orangeites''') are [[ultrapotassic igneous rocks|ultrapotassic]], [[peralkaline igneous rocks|peralkaline]] rocks rich in volatiles (dominantly H<sub>2</sub>O). The distinctive characteristic of orangeites is [[phlogopite]] macrocrysts and microphenocrysts, together with groundmass micas that vary in composition from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). 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 REE-rich [[perovskite]]; Sr-rich [[apatite]]; REE-rich phosphates ([[monazite]], daqingshanite); potassian barian [[hollandite]] group minerals; Nb-bearing [[rutile]] and Mn-bearing [[ilmenite]].
=== Mineral indikator kadar kimberlit ===
Kimberlites are peculiar igneous rocks because they contain a variety of mineral species with peculiar chemical compositions. These minerals such as potassic [[richterite]], chromian diopside (a [[pyroxene]]), chromium spinels, magnesian ilmenite, and garnets rich in [[pyrope]] plus chromium, are generally absent from most other igneous rocks, making them particularly useful as indicators for kimberlites.
These indicator minerals are generally sought in stream sediments in modern [[alluvium|alluvial material]]. Their presence may indicate the presence of a kimberlite within the erosional watershed which produced the alluvium.
== Geokimia ==
The geochemistry of Kimberlites is defined by the following parameters:
* [[Ultramafic]]; MgO >12% and generally >15%
* [[Ultrapotassic]]; Molar K<sub>2</sub>O/Al<sub>2</sub>O<sub>3</sub> >3
* Near-primitive Ni (>400 ppm), Cr (>1000 ppm), Co (>150 ppm)
* [[Rare earth element|REE]]-enrichment<ref>Nixon, P.H., 1995. The morphology and nature of primary diamondiferous occurrences. Journal of Geochemical Exoloration, 53: 41-71.</ref>
* Moderate to high LILE enrichment; ΣLILE = >1,000 ppm
::LILE = large ion lithophile elements<ref>[ Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland]</ref>
* High H<sub>2</sub>O and CO<sub>2</sub>
== Keekonomian ==
Kimberlites are the most important source of primary [[diamonds]]. Many kimberlite pipes also produce rich [[alluvial]] or [[eluvial]] diamond [[placer deposit]]s. Only about 1 in 200 kimberlite pipes contain gem-quality diamonds.
The deposits occurring at [[Kimberley, South Africa|Kimberley]], [[South Africa]] were the first recognized and the source of the name. The Kimberley [[diamond]]s were originally found in [[weathering|weathered]] kimberlite which was colored yellow by [[limonite]], and so was called ''[[yellow ground]]''. Deeper workings encountered less altered rock, [[serpentinite|serpentinized]] kimberlite, which miners call ''[[blue ground]]''.
See also [[Udachnaya pipe]].
The blue and yellow ground were both prolific producers of diamonds. After the yellow ground had been exhausted, miners in the late 19th century accidentally cut into the blue ground and found gem quality diamonds in quantity. The economic importance of the time is that with flood of diamonds being found, the miners were undercutting each other's price of the diamonds and eventually decreased the diamonds' value down to cost in a short time.<ref>"South Africa: A New History of the Development of the Diamond Fields" (1902): New York Times Archives [PDF file], ''New York Times''.</ref>
== Catatan ==