The mineral chemistry of new titanates from the jagersfontein kimberlite, South Africa: Implications for metasomatism in the upper mantle Article

Haggerty, SE. (1983). The mineral chemistry of new titanates from the jagersfontein kimberlite, South Africa: Implications for metasomatism in the upper mantle . GEOCHIMICA ET COSMOCHIMICA ACTA, 47(11), 1833-1854. 10.1016/0016-7037(83)90201-6

cited authors

  • Haggerty, SE

authors

abstract

  • New members of the crichtonite mineral series are described in which K, Ba, Ca and REE are in significant concentrations (5 wt% oxides) filling the A formula position in AM21O38. These phases are chromium (16 wt% Cr2O3) titanates (58 wt% TiO2) enriched in ZrO2 (5 wt%) and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle. The mineral senes coexists with Mg-Cr-ilmenite, Nb-Cr-rutile, and Ca-Cr (NbZr) armalcolite that have equally unusual chemistries. Kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in LREE and HREE with chondrite normalized abundances of 103 to 105. Crichtonite, armalcolite, and Nb-Cr-rutile occupy a compositional range in TiO2 contents bridging the gap between ilmenite and rutile, two minerals having a widespread distribution in kimberlites and mantle-derived nodule suites. In common with other associations, and based on similarities in mineral chemistry, it is concluded that these minerals formed at P = 20-30 kb, 900-1100°C by reaction of peridotite with metasomatizing fluids. Kimberlitic crichtonite may be expressed as spinel + Cr-ferropseudobrookite, and armalcolite is equivalent to Cr-geikielite + rutile in the system (FeMg)-TiO2-Cr2O3. This system contains a number of Cr-Ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure αPbO2-type crystallographic shear structures. The new minerals have exotic chemistries and the high K-affinities broaden the scope for the origin of alkalic rocks, the generation of highly potassic magmas in the upper mantle, and suggest that alkali metasomatism may be pervasive. © 1983.

publication date

  • January 1, 1983

published in

Digital Object Identifier (DOI)

start page

  • 1833

end page

  • 1854

volume

  • 47

issue

  • 11