Thirteen peridotite xenoliths from Bultfontein, S Africa, are characterized by recrystallization zones and both grain-boundary and coarser veins with abundant mica, amphibole, and diopside, and minor Ba-Sr-Ca-Zr-Cr-titanate (crichtonite structure; enriched in light rare earths), Cr-spinel, and Mg-ilmenite Although a genetic relationship to the MARID-suite (mica-amphibole-rutile-ilmenite-diopside) is suggested by the overall mineralogy and high concentrations of trace elements, some details must be considered if the peridotites are to represent veined and metasomatized wall-rock to a magma body which formed MARID "pegmatites" The micas and amphiboles of the two suites do not overlap in composition, and two peridotites show at least two veining events Perhaps the peridotites were invaded by several Ti-rich liquids, one of which evolved to yield MARID pegmatites One distinct peridotite has linear veinlets of mica, Zr-Nb-rutile, and serpentine, which cut the rock fabric The micas are richer in Ti than t

Mantle metasomatism in 14 veined peridotites from Bultfontein mine, South Africa

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diAusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with diAerent Cr contents formed at diAerent depths ranging from 50 km (where T » 600 ∞ Ca ndP a 15 kbar) to 95 km (where T » 800 ∞C and P a 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM21O38, where A a Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2)x(OH)x, where M a Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb2O5), Cr (up to6 wt% Cr2O3), and OH (up to0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclu- sions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate in- clusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The com- posite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in meta- somatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc ba- salts, providing a possible mechanism for HFSE deple- tion in these rocks.

/pdf/mineral-inclusions-in-pyrope-crystals-from-garnet-ridge-1r2jvh0veb.pdf

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

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

Experimental evidence for the role of accessory phases in magma genesis

Two new opaque minerals are described from metasomatized peridotites and heavy media concentrates from four kimberlites in the Republic of South Africa. Lindsleyite and mathiasite are the Ba and K members of the AM2lO3s series, characterized by predominant A-specific, large cations (Sr = crichtonite; Na : landauite; Pb = senaite; ca = loveringite, and U + REE : davidite), and by M-formula small cations which are dominantly Ti, cr, Fe, Mg, Zr, and Nb. Approximately TOVI of the A-site is fitled by either Ba or K in the new minerals, and selective solid solution is present with Ba-Sr and K-Ca exchange. Typical electron microbeam analyses based on 38 oxygens, give the following formulae: lindsleyite: A = Ban.62Srs a1Ca6.sePb6.e1Ke sTNaa.s6REEs q3 M : Ti1 1.62rs.67A16. 1 1 Cr3.seFe2.s7Mg 1 aqNbo.os mathiasite: A = Ifu.ooCao 22516.;5Baa roNao.osREEo.os M : Ti 1 3. 16Zrs 62Ct237F a2.zaMg r.o:Cao.zzNbo oq REE in both minerals share the unusual characteristic of davidite and loveringite in possessing LREE- and HREE-enrichment, with depletion of the middle lanthanides. The space group for both minerals is R3 or R3 and the strongest reflections are 2.13, 1.80, 1.59, and 1.444 (all100) for lindsleyite, and 2.25(90),2.14 (100), and 1.44 A (tOO) for mathiasite. Hexagonal unit cell parameters for lindsleyite are a : 1.037 nm, c = 2.052 nm, V : l.9ll nm3, and for mathiasite are a = 1.035 nm, c : 2.058 nm, V = 1.909 nm3. Calculated densities for the Ba and K members are 4.63 and 4.60 gm./cm3, respectively. Both minerals are black and have a metallic luster and conchoidal fracture. The minerals are pale tan in reflected light, exhibit weak reflection pleochroism and weak-moderate reflection anisotropy. Spectral reflectances for lindsleyite and mathiasite are very similar and are comparable to ilmenite and rutile. R1 and R2 in air and oil, respectively, for lindsleyite are: 470 nm = 17.9-183%,5.9-6.1%; 546 nm : 17.0-17.3%;5.3-5.5%;589 nm : 16-7-17.Uvo,5.1-5.3%;650 nm : 16.5-16.9vo,5.0-5.2%. Similar data for mathiasite are:

/pdf/lindsleyite-ba-and-mathiasite-k-two-new-chromium-titanates-3rl7rnwc1b.pdf

I. H. Campbell

Papers

The crystal structure of loveringite, a new member of the crichtonite group

Additional data on loveringite (Ca,REE)(Ti,Fe,Cr) 21 O 38 and mohsite discredited