Kimberlite Complex Research Articles

Comparative geochemistry of West African kimberlites: Evidence for a micaceous kimberlite endmember of sublithospheric origin

A suite of largely unaltered, aphanitic, mica-bearing hypabyssal kimberlites from the Koidu kimberlite complex of the West African Craton have been investigated to determine their geochemical affinity relative to Group I (nonmicaceous) and Group II (micaceous) kimberlites of southern Africa. Comparison is made with altered kimberlites from Liberia, other West African and global kimberlites. Based on major element oxides, the Koidu kimberlites, though mica-bearing, show closest compositional similarity with the Group IA kimberlites of southern Africa. Based on major and trace elements, the Koidu kimberlites show an unusual geochemical signature. This signature is similar to that of the distinctive, micaceous Aries kimberlite of northwest Australia, and includes high Nb U (most samples > 46), Ce Sr(>0.4) , Ta Hf(>2) , and Nb Zr(>1) ratios and low P 2O 5/Ce ∗10 4(<27), Ba Rb(<32) , and U Th(<0.2) ratios compared with Group I kimberlites. Koidu kimberlites can be readily discriminated from Group II kimberlites by their higher Ti K(>0.4) and Mb La(>1) ratios and lower Ba Nb(<10) and Pb Ce(<0.06) ratios. The compositions of Liberian kimberlites are leached of mobile incompatible elements, but least affected samples show affinity to Group I. Guinea kimberlites appear to be of two types: one having affinity with Group IA and the other, micaceous variety, having affinity with the Aries kimberlite. Kimberlites with an Aries geochemical signature appear to exist on some other cratons, e.g., the Kundelungu kimberlites (Zaire) and two mica-bearing Group I kimberlites (S. Africa). The Koidu kimberlites exhibit compositionally-dependent isotopic heterogeneity though initial ϵ Nd and ϵ Sr values are broadly asthenospheric (i.e., near bulk earth) similar to Group I and Aries. A compositional spectrum appears to exist between nonmicaceous Group I kimberlites through mica-bearing Koidu kimberlites to extreme endmembers of the Aries type. This spectrum can be modelled as partial melts of heterogeneous peridotite sources which incorporate a potassic, high-Nb source component. The component may represent a fluoro-K-richterite-bearing peridotite residue derived by melt extraction from subduction-zone metasomatized peridotites. Such materials may have been trapped, together with former oceanic lithosphere, in the Transition Zone of the mantle. In response to lower mantle upwelling, diapiric uprise accompanying reduced volatile degassing of the kimberlite source may occur. Because of differences in oxidation potential across the 400 km discontinuity, reactions in the ascending diapir will lead to redox melting and ultimately segregation of the kimberlite melt at the base of the thermal boundary layer (P ~ 13 GPa) separating the subcratonic lithosphere from the convective mantle.

Oxide Mineralogy and Magnetic Properties of the Koidu Kimberlite Complex, Sierra Leone, West Africa

SUMMARY A good correlation between opaque mineralogy and rock magnetic properties has been established in a study of a small number of oriented samples from four kimberlite dykes and one pipe in the Koidu complex of Sierra Leone. The pipe sample is mineralogically distinctive, in that it alone contains deuteric titanomaghaemite after titanomagnetite, and haematite after chlorite. All of the kimberlites contain xenocrystic, upper-mantle derived ilmenite that has undergone sub-solidus reduction to form Mg-Ti magnetite. Three of the dykes contain serpentinized olivine with microscopic to sub-microscopic magnetite and identifiable metal Ni-Fe alloys, and finely textured Mg titanomagnetite and magnetite as atolls surrounding complexly zoned Cr-Mg- A1 spinels. Consistent with these observations, Curie and blocking temperatures range from 520 to 590°C and high alternating field (AF) coercivities imply single domain or pseudo-single domain particles as the bulk carrier of natural remanent magnetization (NRM). In the pipe sample, however, the remanence persisting above 600°C is comparatively strong. Preliminary remanent vectors, isolated by either thermal or AF demagnetization, from four of the five samples are moderately consistent, and yield a mean (n = 4) of D = 323, Z = -17, k = 18, and ag5 = 22.

Petrochemistry of eclogites from the Koidu Kimberlite Complex, Sierra Leone

Petrography, mineral and bulk chemistry of upper mantle-derived eclogites (garnet and clinopyroxene) from the Koidu Kimberlite Complex, Sierra Leone, are presented in the first comprehensive study of these xenoliths from West Africa. Although peridotite-suite xenoliths are generally more common in kimberlites, the upper mantle sample preserved in Pipe Number 1 at Koidu is exclusively eclogitic, making this the fifth locality in which eclogite is the sole polymineralic xenolith in kimberlite. Over 2000 xenoliths were collected, of which 47 are described in detail that include diamond, graphite, kyanite, corundum, quartz after coesite, and amphibole eclogites. Grossular-pyrope-almandine garnets are chromium-poor (<0.72 wt% Cr2O3) and fall into two distinct groups based on magnesium content. High-MgO garnets have an average composition of Pyr67Alm22Gross11, low-MgO garnets are grossular- and almandine-rich with an average composition of Gross34Pyr33Alm33. Clinopyroxenes are omphacitic with a range in jadeite contents from 7.7 to 70.1 mol%. Three eclogites contain zoned and mantled garnets with almandine-rich cores and pyrope-rich rims, and zoned clinopyroxenes with diopside-rich cores and jadeite-rich rims, and are among a very rare group of eclogites reported on a world-wide basis. The bulk compositions of eclogites have ranges comparable to that of basalts. High-MgO eclogites (16–20 wt% MgO) have close chemical affinities to picrites, whereas low-MgO eclogites (6–13 wt% MgO) are similar to alkali basalts. High-MgO eclogites contain high-MgO garnets and jadeiterich clinopyroxenes. Low-MgO eclogites contain low-MgO garnets, diopside and omphacite, and the group of primary accessory phases (diamond, graphite, quartz after coesite, kyanite, and corundum); grospydites are peraluminous. Estimated temperatures and pressures of equilibration of diamond-bearing eclogites, using the diamond-graphite stability curve and the Ellis and Green (1979) geothermometer, are 1031°–1363° C at 45–50 kb.K D values of Fe-Mg in garnet and clinopyroxene range from 2.3 to 12.2. Diamonds in eclogites are green, yellow, and clear, and range from cube to octahedral morphologies; the entire spectrum in color and morphology is present in a single metasomatized eclogite with zoned garnet and clinopyroxene. Ages estimated from Sm-Nd mineral isochrons range from 92–247 Ma. ɛNd values range from +4.05 to 5.23. Values of specific gravity range from 3.06–3.60 g/cc, with calculated seismic Vp of 7.4–8.7 km/s. Petrographie, mineral, and bulk chemical data demonstrate an overall close similarity between the Koidu xenolith suite and upper mantle eclogites from other districts in Africa, Siberia and the United States. At least two origins are implied byP-T, bulk chemistry and mineral compositions: low-MgO eclogites, with diamond and other accessory minerals, are considered to have formed from melts trapped and metamorphically equilibrated in the lithosphere; high-MgO eclogites are picritic and are the products of large degrees of partial melting, with equilibration in the asthenosphere. Fluid or diluted melt metasomatism is pervasive and contributed here and elsewhere to the LIL and refractory silicate incompatible element signature in kimberlites and lamproites, and to secondary diamond growth.

Statistical Classification of Garnets from Kimberlite and Associated Xenoliths: Addendum

Previous articleNext article No AccessStatistical Classification of Garnets from Kimberlite and Associated Xenoliths: AddendumJ. B. Dawson and W. E. StephensJ. B. Dawson Search for more articles by this author and W. E. Stephens Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 84, Number 4Jul., 1976 Article DOIhttps://doi.org/10.1086/628216 Views: 4Total views on this site Citations: 18Citations are reported from Crossref Copyright 1976 University of ChicagoPDF download Crossref reports the following articles citing this article:Agnes T. Fung, Stephen E. Haggerty Petrography and mineral compositions of eclogites from the Koidu Kimberlite Complex, Sierra Leone, Journal of Geophysical Research: Solid Earth 100, no.B10B10 (Sep 2012): 20451–20473.https://doi.org/10.1029/95JB01573Doris V. Hills, Stephen E. Haggerty Petrochemistry of eclogites from the Koidu Kimberlite Complex, Sierra Leone, Contributions to Mineralogy and Petrology 103, no.44 (Jan 1989): 397–422.https://doi.org/10.1007/BF01041749J.B. Dawson, J.V. Smith Relationships between eclogites and certain megacrysts from the Jagersfontein kimberlite, South Africa, Lithos 19, no.3-43-4 (Oct 1986): 325–330.https://doi.org/10.1016/0024-4937(86)90031-9M. Ito Kimberlites and their ultramafic xenoliths from western Kenya, TMPM Tschermaks Mineralogische und Petrographische Mitteilungen 35, no.33 (Jan 1986): 193–216.https://doi.org/10.1007/BF01082085R. L. Hervig, J. V. Smith, J. B. Dawson Lherzolite xenoliths in kimberlites and basalts: petrogenetic and crystallochemical significance of some minor and trace elements in olivine, pyroxenes, garnet and spinel, Transactions of the Royal Society of Edinburgh: Earth Sciences 77, no.33 (Nov 2011): 181–201.https://doi.org/10.1017/S026359330001083XColin H. Donaldson Kinetics of pyrope megacryst reactions in ascending basaltic magma – relevance to high-pressure magmatic crystallization at Elie Ness, East Fife, Geological Magazine 121, no.66 (May 2009): 615–620.https://doi.org/10.1017/S0016756800030752 References, (Jan 1984): 435–466.https://doi.org/10.1016/B978-0-444-42273-6.50037-7 References, (Jan 1984): 361–393.https://doi.org/10.1016/B978-0-444-42274-3.50036-6 Suzanne Mahlburg Kay , William T. Snedden , Brayton P. Foster , and Robert W. Kay Upper Mantle and Crustal Fragments in the Ithaca Kimberlites, The Journal of Geology 91, no.33 (Sep 2015): 277–290.https://doi.org/10.1086/628771SUZANNE Y. WASS, J. D. HOLLIS Crustal growth in south-eastern Australia?evidence from lower crustal eclogitic and granulitic xenoliths, Journal of Metamorphic Geology 1, no.11 (Mar 1983): 25–45.https://doi.org/10.1111/j.1525-1314.1983.tb00263.xD.B. Clarke, G.G. Pe-Piper Multiply exsolved clinopyroxene megacrysts from the Frank Smith Mine, Cape Province, South Africa, Lithos 16, no.11 (Jan 1983): 75–84.https://doi.org/10.1016/0024-4937(83)90036-1R.L. Hervig, J.V. Smith, I.M. Steele, J.B. Dawson Fertile and barren AlCr-spinel harzburgites from the upper mantle: Ion and electron probe analyses of trace elements in olivine and orthopyroxene: Relation to lherzolites, Earth and Planetary Science Letters 50, no.11 (Oct 1980): 41–58.https://doi.org/10.1016/0012-821X(80)90118-1N. P. Lock, J. B. Dawson Garnet-olivine reaction in the upper mantle: evidence from peridotite xenoliths in the Letseng-la-Terae kimberlites, Lesotho, Transactions of the Royal Society of Edinburgh: Earth Sciences 71, no.11 (Nov 2011): 47–53.https://doi.org/10.1017/S0263593300013481J. B. DAWSON, J. V. SMITH, J. S. DELANEY Multiple spinel–garnet peridotite transitions in upper mantle: evidence from a harzburgite xenolith, Nature 273, no.56655665 (Jun 1978): 741–743.https://doi.org/10.1038/273741a0F.C. Bishop, J.V. Smith, J.B. Dawson Na, K, P and Ti in garnet, pyroxene and olivine from peridotite and eclogite xenoliths from African kimberlites, Lithos 11, no.22 (Apr 1978): 155–173.https://doi.org/10.1016/0024-4937(78)90006-3J. B. Dawson, J. S. Delaney, J. V. SmitH Aspects of the mineralogy of aln�itic breccia, Malaita, Solomon Islands; comparison with continental kimberlites, Contributions to Mineralogy and Petrology 67, no.22 (Jan 1978): 189–193.https://doi.org/10.1007/BF01046575 W. E. Stephens , and J. B. Dawson Statistical Comparison between Pyroxenes from Kimberlites and Their Associated Xenoliths, The Journal of Geology 85, no.44 (Sep 2015): 433–449.https://doi.org/10.1086/628317