Phlogopite-bearing Peridotite Research Articles

Magmatic and post-magmatic evolution of the Newania carbonatite complex, Rajasthan, north-western India

AbstractThis work describes the mineralogy of dolomite carbonatite occurring at the Newania carbonatite complex, Rajasthan, north-western India. The mineralogy records the textural and compositional features of magmatic and post-magmatic stages of carbonatite evolution. Ferroan dolomite is the principal constituent and displays variable degrees of deformation, ranging from brittle-to-ductile deformation regimes. Apatite exhibits textural and compositional evolutionary trends from early-to-late stages of carbonatite evolution. Two varieties of amphibole are reported for the first time from this complex, ferri-winchite and cummingtonite; the former is magmatic and the latter is metamorphic in origin. The columbite–tantalite-series minerals are columbite-(Fe), and their paragenesis evolves from composite grains with pyrochlore to individual crystals. Pyrochlore is magmatic with U–Ta–Ti-rich compositions and shows evolution from calciopyrochlore to kenopyrochlore, followed by alteration during late-stages of carbonatite evolution. Monazite and baryte constitute the post-magmatic mineral assemblage; the former is hydrothermal and crystallised after precursor apatite, whereas the latter is associated exclusively with columbite–pyrochlore composites. On the basis of the mineralogy of the carbonatite, it is concluded that the parent magma was generated by low-degree partial melting of magnesite–phlogopite-bearing peridotite.

Mineralogical and geochemical characteristics of the Sedemte Prestola potassic alkaline pluton

The Sedemte Prestola pluton is an intermediate to acid plutonic body of Variscan age (308.7 ± 9.1 Ma). Despite its small size, several petrographic varieties are present: syenites, quartzsyentes and granites. It is composed of potassium feldspar, quartz, sodic-calcic amphibole; biotite, when present, is often rimmed by amphibole or strongly altered; zircon, titanite, apatite and ilmenite are the accessory phases. The rocks display pronounced potassic character with up to 10.4 wt% K2O, peralkaline tendency, and significant trace element enrichment for REE, Zr, Th, U, and Ba. The rocks also show enriched isotopic characteristic with 87/86Sri between 0.7107 and 0.7111, and 143/144Ndi between 0.51193 and 0.51182. The geochemical features of the Sedemte Prestola pluton imply orogenic geodynamic setting of formation as a product of more primitive magma fractionation. Isotopic characteristics and high LILE, Th and U contents support the derivation from enriched mantle source – phlogopite-bearing peridotites in the spinel stability field.

Phlogopite Formation in the Orthopyroxene–Garnet System in the Presence of H2O–KCl Fluid in Application to the Processes of Mantle Metasomatism

The results of experimental studies are presented for reactions in the orthopyroxene–garnet–phlogopite system in the presence of H2O–KCl fluid at 3–5 GPa and 900–1000°C, which model the processes of phlogopite formation in garnet peridotites and pyroxenites during alkaline metasomatism of the upper mantle. The experiments demonstrated regular variations in the composition of garnet, pyroxenes, and phlogopite depending on the KCl content of the fluid. With increasing KCl content of the fluid, enstatite and garnet become unstable, the Al2O3 content of enstatite decreases, and the amount of grossular and knorringite components in garnet are maximum at a KCl content of ~10 mol %. Our results illustrate well the regular variations in the compositions of the coexisting minerals and their zoning in phlogopite-bearing peridotites of the lithospheric mantle.

A new perspective on Cenozoic calc-alkaline and shoshonitic volcanic rocks, eastern Saveh (central Iran)

ABSTRACT Late Eocene – Oligocene volcanic rocks in the eastern Saveh region of the Urumieh-Dokhtar magmatic arc (UDMA) are representative of Paleogene magmatic activity in Iran. They show a wide range of silica-undersaturated to silica-oversaturated compositions, from basalt-trachy basalt and tephri-phonolite to trachyte-latite. Whole rock chemical compositions define a continuous assemblage of mafic and felsic rocks in terms of SiO2 (46.3 to 71.1 wt.%), Al2O3 (12.5–19.2 wt.%), K2O (2.06–12.23 wt.%) and TiO2 (0.41–1.75 wt.%) contents, Mg number (6.8–51) and K2O/Na2O ratios (0.53–30.2). Abundances of rare earth elements in the tephri-phonolite group are much higher than in the trachyte-latite and basalt-trachy basalt groups. Elevated contents of large ion lithophile elements such as Ba (191–7311 ppm) and Pb (6.49–118 ppm) as well as depletions in high field strength elements such as Nb (2.43–40.6 ppm), Ta (0.15–2.24 ppm), Zr (40.8–258 ppm) and TiO2 (0.40–1.77 wt.%) characterize these rocks. Initial ratios of 87Sr/86Sr from 0.7046 to 0.7074 and ɛNd(t) from +0.9 to +3.3 show that these were derived from parental magma extracted from partial melting of metasomatized mantle, namely amphibole- and/or phlogopite-bearing peridotite. Fractionation of feldspar, clinopyroxene, amphibole and phlogopite, and minor contamination with upper crust was responsible for the large variation of the rock types observed. We explain contemporaneous late Eocene – Oligocene calc-alkaline and high-K magmatic activity in the UDMA as due to the partial melting of both hot metasomatized asthenospheric and subcontinental lithospheric mantle in an extensional tectonic regime. This extensional regime probably developed due to the rollback of the subducted Neo-Tethys oceanic plate.

Genesis and Evolution of Mantle Melts of the Devonian Mafic-Ultramafic Rocks from the Eastern Azov Region (Dnieper-Donets Rift, Ukraine): Evidence from Clinopyroxene Geochemistry

The paper is dedicated to the study of the Devonian magmatic association of the Eastern Azov region, which is a part of the Pripyat-Dnieper-Donets rift zone. The association includes gabbros, peridotites, pyroxenites, and lamprophyre dikes of the Pokrovo-Kireevsky massif (PKM) and picrites, picrobasalts, and basalts of the Anton-Taram Formation (ATF). Analysis of clinopyroxenes of different generations from the PKM micaceous gabbro and the ATF alkaline picrite provided insight into the mantle source of these rocks and the evolution of melts, which determined the close spatiotemporal association of kimberlites, basites, ultramafic rocks, including alkaline varieties. Clinopyroxenes from the micaceous gabbro are composed of Cpx1 (Mg # = 0.87–0.88) or Сpx2 (Mg # = 0.80–0.81) cores and Cpx3 external zones (Mg # = 0.70–0.76). Clinopyroxenes from alkaline picrite are composed of Сpx2 cores (Mg # = 0.80–0.84) and external Cpx3 zones (Mg # = 0.71–0.78). Clinopyroxenes in general have an upward concave multielement pattern, with enrichment in LREE, depletion in Ba, Nb, and HREE, Zr–Hf negative anomaly, and, additionally, negative Sr-anomaly in Cpx2 and Cpx3. The calculated equilibrium melts for Сpx2 from the micaceous gabbro are very close in composition to this gabbro, and those for Cpx2 from the alkaline picrite coincide in composition with this picrite, and in general are close to ATF picritic lavas. The high Mg# and Cr content in cores Cpx1 indicate that this mineral was derived from the earliest weakly differentiated magma close to the primary melt. The presence of a negative Zr–Hf anomaly in Cpx1 geochemical patterns at ZrPM 0.8) melts, which generated picrobasalts and lamprophyres. The geochemical similarity of the early Cpx1 cores with clinopyroxenes from ilmenite-bearing mantle metasomatites is consistent with the assumption that ultrahigh-Ti primary melts of the Eastern Azov lamprophyres were derived from carbonated ilmenite-bearing, and likely, phlogopite-bearing (PIC) peridotites.

Stability of phlogopite in ultrapotassic kimberlite-like systems at 5.5–7.5 GPa

Hydrous K-rich kimberlite-like systems are studied experimentally at 5.5–7.5 GPa and 1200–1450 °C in terms of phase relations and conditions for formation and stability of phlogopite. The starting samples are phlogopite–carbonatite–phlogopite sandwiches and harzburgite–carbonatite mixtures consisting of Ol + Grt + Cpx + L (±Opx), according to the previous experimental results obtained at the same P–T parameters but in water-free systems. Carbonatite is represented by a K- and Ca-rich composition that may form at the top of a slab. In the presence of carbonatitic melt, phlogopite can partly melt in a peritectic reaction at 5.5 GPa and 1200–1350 °C, as well as at 6.3–7.0 GPa and 1200 °C: 2Phl + CaCO3 (L)↔Cpx + Ol + Grt + K2CO3 (L) + 2H2O (L). Synthesis of phlogopite at 5.5 GPa and 1200–1350 °C, with an initial mixture of H2O-bearing harzburgite and carbonatite, demonstrates experimentally that equilibrium in this reaction can be shifted from right to left. Therefore, phlogopite can equilibrate with ultrapotassic carbonate–silicate melts in a ≥ 150 °C region between 1200 and 1350 °C at 5.5 GPa. On the other hand, it can exist but cannot nucleate spontaneously and crystallize in the presence of such melts in quite a large pressure range in experiments at 6.3–7.0 GPa and 1200 °C. Thus, phlogopite can result from metasomatism of peridotite at the base of continental lithospheric mantle (CLM) by ultrapotassic carbonatite agents at depths shallower than 180–195 km, which creates a mechanism of water retaining in CLM. Kimberlite formation can begin at 5.5 GPa and 1350 °C in a phlogopite-bearing peridotite source generating a hydrous carbonate–silicate melt with 10–15 wt% SiO2, Ca# from 45 to 60, and high K enrichment. Upon further heating to 1450 °C due to the effect of a mantle plume at the CLM base, phlogopite disappears and a kimberlite-like melt forms with SiO2 to 20 wt% and Ca# = 35–40.

Metasomatic processes in the lithospheric mantle beneath the V. Grib kimberlite pipe (Arkhangelsk diamondiferous province, Russia)

New data on metasomatic processes in the lithospheric mantle in the central part of the Arkhangelsk diamondiferous province (ADP) are presented. We studied the major- and trace-element compositions of minerals of 26 garnet peridotite xenoliths from the V. Grib kimberlite pipe; 17 xenoliths contained phlogopite. Detailed mineralogical, petrographic, and geochemical studies of peridotite minerals (garnet, clinopyroxene, and phlogopite) have revealed two types of modal metasomatic enrichment of the lithospheric-mantle rocks: high temperature (melt) and low-temperature (phlogopite). Both types of modal metasomatism significantly changed the chemical composition of the peridotites. Low-temperature modal metasomatism manifests itself as coarse tabular and shapeless phlogopite grains. Two textural varieties of phlogopite show significant differences in chemical composition, primarily in the contents of TiO2, Cr2O3, FeO, Ba, Rb, and Cs. The rock-forming minerals of phlogopite-bearing peridotites differ in chemical composition from phlogopite-free peridotites, mainly in higher FeO content. Most garnets and clinopyroxenes in peridotites are the products of high-temperature mantle metasomatism, as indicated by the high contents of incompatible elements and REE pattern in these minerals. Fractional-crystallization modeling gives an insight into the nature of melts (metasomatic agents). They are close in composition to picrites of the Izhmozero field, basalts of the Tur’ino field, and carbonatites of the Mela field of the ADP. The REE patterns of the peridotite minerals make it possible to determine the sequence of metasomatic enrichment of the lithospheric mantle beneath the V. Grib kimberlite pipe.

Phlogopite bearing peridotite from the 25S OCC

Phlogopite bearing peridotite from the 25S OCC

Metasomatism of garnet peridotite from Jiangzhuang, southern Sulu UHP belt: constraints on the interactions between crust and mantle rocks during subduction of continental lithosphere

The Jiangzhuang ultrahigh-pressure (UHP) metamorphic peridotite from south Sulu, eastern China occurs as a layer within gneiss with eclogite blocks, and consists of coarse-grained garnet porphyroblasts and a fine-grained matrix assemblage of garnet + forsterite + enstatite + diopside ± phlogopite ± Ti-clinohumite ± magnesite. Both types of garnet are characterized by high MgO content and depletion of light rare earth element (LREE) and enrichment of heavy rare earth element, but the matrix garnet has lower MgO, TiO2 and higher Cr2O3 and REE contents. Diopside displays LREE enrichment, and has low but variable large-ion lithophile element (LILE) contents. Phlogopite is a major carrier of LILE. Ti-clinohumite contains high Nb, Ta, Cr, Ni, V and Co contents. The P–T conditions of 4.5–6.0 GPa and 850–950 °C were estimated for matrix mineral assemblages. Most peridotites are depleted in Al2O3, CaO and TiO2, and enriched in SiO2, K2O, REE and LILE. In contrast to phlogopite-free peridotites, the phlogopite-bearing peridotites have higher K2O, Zr, REE and LILE contents. Zircon occurs only in the phlogopite-bearing peridotites, shows no zoning, with low REE contents and Th/U ratios, and yields tight U–Pb ages of 225–220 Ma, indicating the peridotites experienced consistent Triassic UHP metamorphism with subducted supercrustal rocks. These data demonstrate that the Jiangzhuang peridotites were derived from the depleted mantle wedge of the North China Craton, and experienced various degrees of metasomatism. The phlogopite-free peridotites may have been subjected to an early cryptic metasomatism at UHP conditions of the mantle wedge, whereas the phlogopite-bearing peridotites were subjected to a subsequent strong metasomatism, characterized by distinctly enrichment in LILE, LREE, Zr and K as well as the growth of zircon and volatile-bearing minerals at UHP subduction conditions. The related metasomatism may have resulted from the filtration of fluids sourced mainly from deeply subducted supracrustal rocks.

LREE-rich hibonite in ultrapotassic rocks in southern India

We describe a rare terrestrial occurrence of hibonite (CaAl 12TiO 19) from the ultrapotassic rocks which occur as veinlets in graphite-bearing spinellite in the Achankovil Shear Zone (ACSZ), southern India. The ultrapotassic rocks are characterized by an unusual mineral assemblage, kalsilite + leucite + corundum + spinel + perovskite + apatite + hibonite ± phlogopite, along with mantle-derived graphite and Fe-sulfides. Three types of hibonite were identified in terms of chemical zoning as observed in back scattered images: (i) unzoned hibonite, (ii) zoned hibonite with brighter core and dark rim, and (iii) zoned hibonite with brighter core, a less bright intermediate zone and a dark outer rim corresponding to the variations in the distribution of LREEs (light rare earth elements), Ca, Al and Ti in the hibonite structure. The core regions of hibonite are highly enriched in LREEs and Sr, and depleted in Ca, Ti and Al, compared to the rim areas. Chondrite-normalized REE distribution patterns for hibonite exhibit extreme LREE enrichment (La N/Yb N = 763 to 374,494) with a negative Eu anomaly. Apatite displays MREE-enriched patterns. Partitioning of LREEs in hibonite and apatite crystallized from the melts is chiefly determined by their preference for hibonite > apatite. We propose a model that considers the formation of the LREE-rich Ti-depleted hibonite from ultrapotassic silica-undersaturated melts. We envisage that the melts were generated by low degree partial melting of a phlogopite-bearing peridotite that was fertilized in LREEs. The ultrapotassic rock that occurs as veins along the ACSZ represents a melt influx associated with deep-seated extension after the final amalgamation of the Gondwana supercontinent.

B 1 basaltic lavas in Vestfold–Jeløya area, central Oslo rift: derivation from initial melts formed by progressive partial melting of an enriched mantle source

The earliest (B 1) lavas in Vestfold and Jeløya, in the central part of the Permo-Carboniferous Oslo rift have been divided into four main groups: high Ti (HT) basalts (TiO 2>4.2 wt.%), low Ti (LT) basaltic rocks (TiO 2<3.7 wt.%), phonotephrites and ignimbrites. They are all highly enriched in strongly incompatible elements and have high La/Yb ratios. Initial ε Nd and ε Sr ratios of whole-rock samples and clinopyroxene separates (assumed age=295 Ma) are +6.0 to −1.6 and −17 to +57, respectively. Their trace element and isotopic compositions imply that, during ascent, the magmas were subjected to varying degrees of fractional crystallization, mixing between evolved and mafic magmas, and crustal contamination in the upper crust. Crustal contamination was most likely caused by rocks similar to the metasediments now exposed in the Bamble sector, south of the Oslo rift. The preserved lava sequences represent interfingering of lavas from at least three eruption centers. After extrusion, the lavas suffered post-magmatic alteration that led to mobilization of Rb, U and K, and increased 87Sr/ 86Sr ratios. Interpretation of magmatic signatures, inferred to have been preserved through contamination and alteration, allow us to make the following conclusions concerning the mantle sources of the Vestfold and Jeløya B 1 basalts. The formation of the HT-basalts involves highly Ti–P-rich, garnet-bearing mantle source rocks, e.g. clinopyroxenites, wehrlites, websterites, and amphibolites. The HT and LT basalts may have originated by progressive partial melting of a veined, enriched, garnet–phlogopite-bearing peridotite at the base of the lithosphere. Progressive melting of this complex source initially produced magnesian lavas with high REE and HFSE contents, followed by less enriched, more voluminous lavas. The early basalts were derived from an isotopically depleted source similar to the PREMA source. Contribution of older lithospheric material to the later basalts produced more enriched Sr–Nd isotopic signatures. As some of the most primitive basalts found within the European Permo-Carboniferous rifting event, these lavas are key to understanding the nature of the sublithospheric mantle beneath Europe at this time.

Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited

Kimberlite magmas from the Kimberley area of South Africa have sampled two main types of phlogopite-rich mafic xenoliths which represent deep mantle segregations from highly alkaline melts. The first group corresponds to the MARID rocks characterised by the mineral association mica (phlogopite)–amphibole (K-richterite)–rutile–ilmenite–clinopyroxene and the second group consists of the PIC rocks characterised by the mineral association mica (phlogopite)–ilmenite–clinopyroxene–minor rutile. The two groups are clearly distinguished from one another by their mineral paragenesis, by the major element composition of their phlogopite and ilmenite, by the trace element content of their clinopyroxene and by their clinopyroxene and whole rock Sr and Nd isotope ratios. The combined major and trace element variations are interpreted to indicate a genetic relationship between the PIC rocks and group I kimberlite magma, and between the MARID rocks and group II kimberlite magma. The two types of parental melts percolated through, and metasomatised, the upper mantle beneath the Kimberley area as indicated by the trace element characteristics of the clinopyroxenes of the studied phlogopite-bearing peridotites.

Phlogopite from the Serre ultramafic rocks, Central Calabria, Southern Italy

A 6–7 meter thick phlogopite-bearing peridotite “body” interlayered within a 200 m thick sequence of layered metagabbro and pyroxene-bearing felsic granulite outcrops in the Serre Massif, southern sector of the Calabrian Peloritan Arc (southern Italy). Phlogopite occurs throughout the whole peridotitic body, but in different modal amounts and in various textural sites. It occurs as Type-I, isolated and oriented grains in textural equilibrium with the other phases, either in the peridotite or in included orthopyroxenite veins; Type-II, in a kind of melange, as millimetric smearings of flakes which separate anastomosing phlogopite-rich and orthopyroxene-rich peridotitic portions; Type-III, in centimetric phlogopitite layers alternating with phlogopite-rich layers. The phlogopite has mg values [mg = Mg/(Mg+Fe)] in the range 0.90 to 0.94. It also shows appreciable variations of TiO 2 , Cr 2 O 3 and BaO contents, i.e., 1.26–2.44, 0.21–1.35, 0.15–2.02 wt%, respectively. The primitive mantle-normalized REE pattern of Type-I phlogopite shows a steep negative slope from La to Eu, followed by a slight Gd positive anomaly and a moderately positive slope from Tb to Lu. The REE patterns of Types-II and -III show negative slopes from La to Lu, but the Type-II pattern displays a marked Eu negative anomaly, and Type-III phlogopite has lower M- and HREE contents. The trace element spider diagrams show a generally decreasing trend in elemental amounts, in accordance with decreasing incompatibility, with evident K, Pb, Mo, P, Sn and Ti positive anomalies. The occurrence of phlogopite in peridotite from the Serre Massif probably indicates that the rocks underwent a severe metasomatic event in upper mantle and/or lower crust-mantle boundary conditions.

Geochemistry of the Lower Crustal Mafic-Ultramafic Complex at Finero, Ivrea Zone, N. Italy

In the Ivrea-Verbano Zone, mantle-derived marie rocks have been emplaced in the lower continental crust in contact with mantle peridotite bodies and granulite-facies metasedimentary rocks. One of these bodies, the marie intrusion in the Sesia valley near Varallo and Balmuccia, previously investigated by Voshage et al. (1990) and Quick et al. (1992), probably intruded 270 to 290 Ma ago and assimilated large amounts of crustal material by triggering extensive crustal anatexis. We have studied the mafic-ultramafic complex near Finero in order to compare it with the intrusion near Varallo and Balmuccia. Previous work by Voshage et al. (1987) indicated that in contrast with Balmuceia, the phlogopite-bearing mantle peridotite at Finero contains crustal-type isotopic signatures. This mantle peridotite forms the core of an antiform and is overlain by the three subunits of the marie complex, the strongly layered inner gabbro, the cumulus amphibole peridotite and the massive external gabbro.

Phase Relations and Chemistry of Richteritic Amphiboles in the System K2O-Na2O-CaO-MgO-Al2O3-SiO2-H2O (KNCMASH)

Knowledge of alkali-amphibole and phlogopite stability plays a key-role in the understanding of water storage and recycling in the upper mantle. The few experimental data so far available at P > 5.0 GPa yield an upper pressure stability of < 9 GPa at 1200~ for pure phlogopite and 13-14 GPa at 1200~ for pure KNaCa-r ich te r i t e KNaCaMgsSisO22(OH)2 (Tronnes 1990). In phlogopite-bearing natural peridotites Tronnes et al. (1988) observed growth of KNaCa-richterite at the expense of phlogopite at about l0 GPa and 10001200~ Sudo & Tatsumi (1990) stabilized pure K 2 C a M g 5 S i s O 2 2 ( O H ) 2 at 13 GPa and 1000~ The present study has been undertaken to better understand phase relations of richterite in the presence of Na in P, T and composition space as this may reduce the upper stability limit of

Open-system O-isotope behaviour and trace element enrichment in the sub-Eifel mantle

Spinel peridotites from the West Eifel, western Germany exhibit a wide range of δ 18O SMOW values for olivine ( +5.1 to +6.3‰), smaller ranges for pyroxenes ( +5.7 to +6.3‰ for orthopyroxene; +5.6 to +6.4‰ for clinopyroxene) and variable 18O/ 16O fractionations between olivine and pyroxenes ( Δ clinopyroxene-olivine = +0.6 to −0.3‰ and Δ orthopyroxene-olivine = +0.7 to −0.1‰). These features are characteristic of a disequilibrium distribution of O-isotopes between coexising olivine and pyroxenes. Enrichment in 18O is observed primarily in olivine from amphibole and/or phlogopite-bearing peridotites and correlates with degree of LREE/HREE enrichment, indicating that the processes responsible for trace element enrichment (i.e. mantle metasomatism) and O-isotope disequilibrium are linked. The 18O- and incompatible trace element-enriched fluid(s) cannot be generated by intra-mantle fractionation processes as presently understood, and are attributed to recycling of altered oceanic lithosphere and/or pelagic sediments into the mantle via subduction. This is supported by δD SMOW values for associated Eifel amphibole and mica ( −50 to −45‰ respectively) which fall midway between δD values for upper mantle ( −80‰) and seawater (0‰). Coexisting amphibole and mica also have distinctly different δ 18O values ( +7.0 and +5.9‰, respectively) which suggests that they did not originate from the same fluids/melts. This implies that the sub-Eifel mantle has experienced at least two distinct metasomatic episodes.

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with 143Nd/ 144Nd ranging from ∼ 0.51302 to 0.51355 and 87Sr/ 86Sr from ∼ 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path ∼ 2 Ga ago as a consequence of partial melting. The combination of high 143Nd/ 144Nd with high 87Sr/ 86Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity ( 143Nd/ 144Nd∼ 0.51256–0.51273, 87Sr/ 86Sr∼ 0.7044–0.7032). An evaluation of Sm Hf and Yb Hf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.

40Ar/ 39Ar analyses of phlogopite nodules and phlogopite-bearing peridotites in South African kimberlites

40Ar/ 39Ar analyses have been made on phlogopite-bearing peridotite nodules from Bultfontein and phlogopite nodules from Du Toitspan, Kimberley area, South Africa. Neither definite plateau nor isochron age could be obtained due to the occurrence of an excess 40Ar in phlogopite. However, the extrusion age of a phlogopite nodule from Du Toitspan has been estimated to be about 86 m.y. from the combination of the youngest 40Ar/ 39Ar age in the intermediate temperature fraction with Rb/Sr age data reported for this area. Excess 40Ar correlates with K-derived 39Ar in some phlogopites suggesting that it is trapped in K- or K-similar sites and has been incorporated during phlogopite formation. The occurrence of large amounts of excess 40Ar in phlogopite suggests that it was not formed at a shallow depth.

Rare gases in a phlogopite nodule and a phlogopite-bearing peridotite in South African kimberlites

Rare gas isotopes in a phlogopite nodule and a phlogopite-bearing peridotite nodule in South African kimberlites were studied to examine the state of rare gases in the deep interior of the kimberlite region. Within the experimental error of 1 ∼ 2%, rare gas isotopic compositions are atmospheric except for radiogenic 4He and 40Ar. No excess 129Xe was observed. In phlogopite, Ne is more depleted, whereas the heavier rare gases are more enriched than the atmospheric rare gases relative to 36Ar. Together with other data these results suggest that the state of rare gases in the upper mantle of the South African kimberlite region might have been changed from the typical primitive mantle by a process such as mixing of crustal materials.

Petrological aspects and evolution of the leucite bearing Lavas from Bufumbira, south west Uganda

Lavas ocouring in the western Rift of Central Africa are characterized by the presence of leucite both as phenocrysts and in the groundmass. Samples studied from the Bufumbira Bay area of the Birunga volcanic field, in south-west Uganda, form two distinct chemical series diverging from basanite compositions. One differentiates through phonolitictephrites to a trachyte and the other differentiates through leueitites to a phonolite. The low-pressure fractionation controls of both series are discussed in terms of the coexisting phases and ultramafic inclusions observed in the lavas. Equilibrium fusion of a mantle phlogopite-bearing peridotite combined with subsequent fractionation is the favoured petro-genetic process to produce a potash-rich basanite liquid, which is considered a likely parent of the Bufumbira lavas.