Cr-rich Clinopyroxene Research Articles

Cr-poor and Cr-rich clinopyroxene and garnet megacrysts from southern African Group 1 and Group 2 kimberlites: Clues to megacryst origins and their relationship to kimberlites

Controversies surround the origin of kimberlite megacrysts, including whether and how they are genetically related to their host kimberlites, the relationship between the Cr-poor and Cr-rich suites and the dominant processes responsible for elemental and isotopic variations of megacrysts from a given kimberlite. We present new in-situ major and trace element and Sr isotopic results for clinoyroxene and garnet megacrysts from four southern African kimberlites: Colossus and Orapa (Group 1 kimberlites on the Zimbabwe craton), and Kalkput and Bellsbank (Group 2 kimberlites on the western Kaapvaal craton), that include both Cr-poor and Cr-rich megacryst varieties. Cr-poor megacrysts are present at Colossus, Orapa and Kalkput and the data exhibit tight, well-defined trends on major element diagrams as well as incompatible and rare earth element abundances similar to those previously reported for Cr-poor megacrysts. Cr-rich megacrysts, which are also present at Orapa and are the only variety present at Bellsbank, generally have higher Mg# values, lack well-defined major element trends and show stronger incompatible element enrichments as well as more radiogenic 87Sr/86Sri ratios than Cr-poor megacrysts from the same kimberlite group. Thermobarometry indicates that the Cr-poor megacrysts equilibrated at temperatures of ≈1200 to 1450 °C and pressures of 4.5 to 7.5 GPa. Cr-rich megacrysts, in contrast, extend to temperatures and pressures as low as 700 °C and 3 GPa, respectively. This indicates that, in the studied suites, Cr-poor megacrysts equilibrated at high temperatures in the lower lithosphere (>135 km), whereas Cr-rich megacrysts typically equilibrated at lower temperatures and pressures. Within the Cr-poor megacrysts from Group 1 and Group 2 kimberlites, there is a clear correspondence between kimberlite group, diagnostic incompatible element ratios (e.g., Nb/La) and Sr isotope ratios that parallel the differences noted between whole-rock Group 1 and Group 2 kimberlites. In the case of Cr-poor megacrysts, similar calculated melt compositions in equilibrium with garnet and clinopyroxene from the same kimberlite were obtained using recent high-pressure mineral‑carbonated melt partition coefficients. This suggests formation in conditions close to trace element equilibrium, and is consistent with crystallization from primitive melts with kimberlite-like trace element compositions. In the case of Cr-rich megacrysts, differences in the compositions of melts in equilibrium with clinopyroxene and garnet tend to be larger, and melts in equilibrium with Cr-rich clinopyroxene tend to show significantly greater incompatible element enrichments than those of estimated near-primary kimberlite melts. This could be due to the different behaviour of clinopyroxene and garnet during metasomatic melt-rock interaction, but the apparent disequilibrium between clinopyroxene and garnet could also be due to some of the Cr-rich megacrysts actually being peridotitic xenocrysts. We propose a model for the origin of southern African megacrysts in which carbonated protokimberlite melts formed stockwork-like bodies of variable size in the deep lithosphere (>130 km), which fed networks of melt-filled veins extending into the surrounding and overlying mantle. Crystallization of larger melt bodies resulted in megacryst assemblages dominated by Cr-poor megacrysts, and the incompatible element and isotopic characteristics of these dominantly reflect those of the protokimberlite melt. In contrast, crystallization of smaller melt bodies and their vein networks resulted in megacryst assemblages dominated by Cr-rich megacrysts, which formed as a result of extensive assimilation and metasomatic melt-rock interaction between protokimberlite and peridotite wallrock at low melt/rock ratios, particularly in the middle to shallow lithosphere where pre-existing potassic metasomatic heterogeneities are prevalent. The Cr-rich nature and enrichments in incompatible elements and radiogenic Sr in the Cr-rich megacrysts reflect extensive interaction of their parental magmas with this metasomatized peridotite.

Petrology and Nd–Sr isotopic composition of alkaline lamprophyres from the Early to Late Cretaceous Mundwara Alkaline Complex, NW India: evidence of crystal fractionation, accumulation and corrosion in a complex magma chamber plumbing system

Abstract The Early to Late Cretaceous Mundwara alkaline complex (comprising the Musala, Mer and Toa plugs) displays a broad spectrum of alkaline rocks closely associated in space and time with the Deccan large igneous province in NW India. Petrology and Nd–Sr isotopic data on the two youngest and altogether compositionally different lamprophyre dykes of the Mundwara alkaline complex are presented in this paper to understand their petrogenesis and also to constrain the magmatic processes responsible for generation of the rock spectrum in the complex (pyroxenite, picrite ankaramite, carbonatite, shonkinite, olivine gabbro, feldspathoidal and foid-free syenite). The two lamprophyre dykes occurring in the Mer and the Musala hills are referred to as basaltic camptonite I and camptonite II, respectively. The basaltic camptonite I is highly porphyritic and contains olivine, clinopyroxene and magnetite macrocrysts embedded within the groundmass of microphenocyrsts composed of clinopyroxene, phlogopite, magnetite and feldspar. Camptonite II, however, with a more or less similar texture, contains amphibole, biotite, magnetite and clinopyroxene within the microphenocrystic groundmass of amphibole, biotite, apatite and feldspar. Pyroxenes are chemically zoned and display corrosion of the cores, revealing that they are antecrysts developed during the early stages of magma evolution and later on inherited by more evolved magmas. The mineral chemistry and trace element composition of the lamprophyres reveal that fractional crystallization was a dominant process. Early segregation of olivine + Cr-rich clinopyroxene + Cr-spinel from a primary hydrous alkali basalt within a magmatic plumbing system is inferred, which led to the generation of basaltic camptonitic magma (M1) forming the Mer hill lamprophyre. Subsequently, progressive fractionation of pyroxene and Fe–Ti oxides from the basaltic camptonitic (M1) magma generated camptonitic (M2) magma forming the Musala hill lamprophyre. Both lamprophyre dykes on the Sr–Nd isotopic array reflect plume-type asthenospheric derivation, which largely corresponds to the Réunion plume and other alkaline rocks of the Deccan large igneous province. Our study brings out a complex sequence of processes such as crystal fractionation, accumulation and corrosion in the magmatic plumbing system involved in the generation of the Mundwara alkaline complex.

Elucidating the processes affecting highly primitive lavas of the Borgarhraun flow (northern Iceland) using trace elements in olivine

Olivine is typically the first phase to crystallise from basaltic melts and its chemistry can therefore inform on the earliest stages of magmatic evolution, not recorded by later crystallising phases. Despite the potential of olivine for understanding primitive differentiation, limited analytical capabilities have previously restricted the range of elements that can be routinely measured. Consequently, important processes controlling early magma evolution may have been overlooked or misidentified. This study reports a wide range of minor and trace elements in forsteritic (up to Fo92.2) olivine macrocrysts from the primitive Borgarhraun lava flow in northern Iceland.We define two distinct populations of olivine based on their forsterite (Fo) content and then apply minor and trace element data to discern mixing and crystallisation of subtly different high-MgO parental melts. High-Fo (90.9–92.2 mol%) olivines show approximately linear trends between Cr and other incompatible trace elements (Li, Na, Ca, Ti, Al and Y), implying mixing and concurrent crystallisation of two highly primitive melts. Low-Fo (87.4–90.0 mol%) olivines show trends that indicate mixing and crystallisation of multiple, genetically distinct and less primitive melts. The outermost 50 μm of the olivine microcrysts record diffusive re-equilibration of the olivine macrocrysts to a single, significantly more evolved carrier liquid over an ascent timescale of 70–250 days. Compared to the rest of Iceland, the Borgarhraun olivine macrocrysts are distinguished by their Cr contents, which extend from 97 to 1150 ppm. The uniquely steep trend in Fo vs. Cr can be explained by early crystallisation of Cr-spinel and Cr-rich clinopyroxene, stabilised by high pressures of differentiation (>0.8 GPa). Chromium-forsterite systematics may therefore be a powerful tool for qualitatively assessing relative pressures of crystallisation for different magmatic systems. Collectively, our new dataset clearly demonstrates the importance of measuring trace elements in olivine for identifying the formative stages and conditions of basaltic magmatic systems.

Unique sodic–calcic skarn hosted by ultramafic rocks and albitite at the Ulsan skarn deposit, Gyeongsang Basin, South Korea

The Ulsan Fe–W skarn deposit is located within the Cretaceous Gyeongsang Basin in the southeastern part of the Korean Peninsula. The skarn and mineralization formed along contacts between the Paleogene Gadaeri granite, limestone, partially serpentinized ultramafic rocks (dunite and harzburgite), and hornfelsed volcanic and siliciclastic rocks. These various country rock types produced a system with multiple types of skarn. Typically, skarn within limestone occurs as clinopyroxene–magnetite skarn, clinopyroxene–garnet skarn, and garnet skarn, but the unique host rocks (ultramafic rocks + albitite) at Ulsan also produced unusual skarn types such as a sodic–calcic skarn system. Na–Ca skarn formed by chemical interactions between albitized granites and adjacent ultramafic rocks. This Na–Ca skarn contains unusual Na-rich clinopyroxene (acmite), Na-rich amphiboles, Cr-rich clinopyroxene, Cr-rich amphiboles, Sr-bearing plagioclase, and Ba-bearing K-feldspar. Clinopyroxene and amphibole have Na2O concentrations up to 6.5 wt%, and these minerals contain up to 12.1 and 2.5 wt% Cr2O3, respectively. Na was derived from albitite, and the formation of Cr-bearing acmite (kosmochlor) and Na-amphibole (richterite) appears to have been related to the release of Cr into the fluid phase by the breakdown of chromite in ultramafic rocks during prograde skarn formation. K-feldspar K–Ar dating yields an age of 50.3 ± 1.7 Ma for the prograde garnet skarn. The related Gadaeri granite exhibits a high-K calc-alkaline signature and a weakly metaluminous to peraluminous I-type affinity with a total alkalinity (Na2O + K2O) of 8.5–8.8 wt%. The Gadaeri granite shows an enriched REE pattern compared with albitite, as well as negative Eu anomalies, which together with the K/Rb values (200–500) are suggestive of a highly evolved magma from a continental-margin setting.

On the Chemical Composition and Possible Origin of Na–Cr-Rich Clinopyroxene in Silicocarbonatites from Samalpatti, Tamil Nadu, South India

Mineralogical and chemical data are presented for a suite of Na–Cr-rich clinopyroxenes associated with chromite, winchite (sodium-calcium amphibole), titanite and calcite in Mg-Cr-rich silicocarbonatites from the Samalpatti carbonatite complex, Tamil Nadu, South India. The Mg-Cr-rich silicocarbonatites occur as 10–30 cm large enclaves in pyroxenites. The chemical composition of the pyroxenes differs among individual enclaves, with variable proportions of diopside, kosmochlor and jadeite-aegirine end-members. These compositions fill a previously unoccupied space in the kosmochlor-diopside-jadeite+aegirine ternary plot, indicating a distinct origin of kosmochlor-rich pyroxene compared with previous findings from diverse settings. The Na–Cr-rich clinopyroxene has low ΣREE = 9.2 ppm, with slight enrichment in LREE (LaN = 7), coupled with low HREE (YbN = 0.6), and flat HREE, paralleled by a significant fractionation of Nb/Ta (2408) and Th/U (26.5). Sodic metasomatism (fenitization) associated with either carbonatite emplacement at shallow levels or during carbonatite ascent through the upper mantle most likely was the major process operating in the area. We suggest two scenarios of the formation of Na–Cr-rich pyroxene: (1) from mantle-derived chromian mineral phases (spinel and/or garnet) through fenitization, with subsequent corrosion by growing winchite due to volatile influx; (2) via metasomatic reaction of Cr-rich garnet in mantle peridotite due to reaction with Na-rich carbonatite melt. Collectively, the appearance of kosmochlor may play an important role in deconvolving metasomatic processes, and fenitization in particular. If combined with petrologic experiments, it could improve our understanding of the origin and subsequent history of chemical signatures of carbonate-rich materials in the mantle.

Cr-rich clinopyroxene megacrysts from the Grib kimberlite, Arkhangelsk province, Russia: Relation to clinopyroxene–phlogopite xenoliths and evidence for mantle metasomatism by kimberlite melts

To provide new insights into the origin of megacrysts and metasomatism of the subcontinental lithospheric mantle (SCLM), we present a detailed petrographic and geochemical investigation of clinopyroxene-phlogopite xenoliths and clinopyroxene megacrysts from the Grib kimberlite (Arkhangelsk diamond province, Russia). Clinopyroxene megacrysts and clinopyroxene from clinopyroxene-phlogopite xenoliths have similar petrography, major and trace element compositions, and are therefore classified as Cr-rich megacrysts. Geothermobarometry suggests that Cr-rich clinopyroxenes originate from within the SCLM (3.6–4.7GPa and 764–922°C). Phlogopite from clinopyroxene-phlogopite xenoliths have low-Ti and -Cr compositions that overlaps with phlogopite megacrysts from the Grib kimberlite. The clinopyroxene-phlogopite rocks within the SCLM are the main source for Cr-rich clinopyroxene and low-Ti phlogopite megacrysts in the Grib kimberlite matrix.Trace element compositions of studied Cr-rich clinopyroxenes have similar geochemical features to clinopyroxenes megacrysts occurrences worldwide and overlap with clinopyroxenes from phlogopite-garnet peridotite xenoliths from the Grib kimberlite. The strong depletion in Ti, Nb, Ta and to a lesser extent in Zr and Hf in clinopyroxene reflects equilibrium with Ti-oxides, such as ilmenite. The clinopyroxene-phlogopite xenoliths could be the final product of metasomatism of garnet peridotites within the SCLM beneath the Grib kimberlite. The calculated equilibrium of clinopyroxene melt compositions suggests that the metasomatic agents were derived from silicate-bearing kimberlite melts.The presence of veinlets infilled with kimberlitic mineral assemblages in clinopyroxene grains suggests that the clinopyroxene-phlogopite rocks experienced intense interactions with kimberlite melt after their formation, but before their entrainment into the host kimberlite magma. This interaction resulted in the formation of high-Ti and -Cr phlogopite and high-Ti clinopyroxene rims, zones and grains with spongy textures.Finally, we propose the sequence of metasomatic events that occurred in the SCLM and the subsequent formation of the Grib kimberlite.

Mineral Chemistry of Wehrlite Xenoliths Hosted in Basalts from the SW of Hosséré Dammougalré (Adamawa Plateau, Cameroon): Thermobarometric Implications

Wehrlite samples (size: ~4 cm) hosted in basaltic lavas from the SW of Hosséré Dammougalré are located in the western Adamawa Plateau. Porphyritic and allotriomorphic texture characterize respectively host Basalt and wehrlite xenoliths. The phenocrysts of olivine (Fo68−74), and Ti-magnetite are scattered in host basalt. Wehrlite xenoliths (~4 cm size) contain Cr-rich clinopyroxene (diopise-augite), olivine (Fo76−88) and chromiferous spinel. Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/spinel vary between 944°C and 1102°C. The wehrlite olivine crystals with low Fo (vi and Aliv contents. The studied spinel-bearing wehrlite xenoliths represent probably the residual portions of the upper mantle, which are an important source of information about lithospheric composition and thermal evolution beneath the Adamawa Plateau.

Metasomatized lithosphere–asthenosphere interaction during slab roll-back: Evidence from Late Carboniferous gabbros in the Luotuogou area, Central Tianshan

Late Carboniferous igneous rocks are widespread in the western Tianshan, but the tectonic settings for these rocks remain controversial. We report a plagioclase 40Ar/39Ar age, and geochemical, Sr–Nd isotope and LA–ICPMS clinopyroxene trace element data for gabbros in the Luotuogou region. The tholeiitic Luotuogou gabbros give a Late Carboniferous (312±1Ma) 40Ar/39Ar age and are characterized by high and variable εNd(t) values ranging from +3.7 to +7.8. They have geochemical features of both intra-plate and island arc magmatic rocks, i.e., relatively high TiO2 (0.6–2.2wt.%), Nb (4.2–24ppm) and Zr (51.4–283ppm) contents combined with variable and slightly high Nb/La ratios (0.24–1.8, mostly>0.7), and negative to positive Nb anomalies. The gabbros contain zoned clinopyroxenes, with Mg- and Cr-rich cores. Their parental magmas, as calculated using trace element data from Cr-rich (>3000ppm) clinopyroxene cores and clinopyroxene/basaltic liquid partition coefficients, show enrichments in incompatible elements, and prominent negative to slightly positive Nb anomalies, indicative of the influence of subduction-related compositions in their mantle source. These features indicate that the Luotuogou gabbros were most likely formed by interactions between asthenospheric and metasomatized lithospheric mantle. They were most plausibly formed by mixing between the asthenospheric mantle-derived and metasomatized lithosphere mantle-derived melts. Mixing was the result of asthenosphere upwelling triggered by roll-back of the subducted Paleo-Junggar Oceanic Plate rather than mantle plume-related rifting or post-collisional break-off during the Late Carboniferous.

Clinopyroxene from basaltic rocks of the Erzgebirge - Krušné hory Mts. - implications for modelling the magmatic plumbing system

*Corresponding author The Erzgebirge-Krusne hory area is part of the NW flank of the Eger-Ohře Rift. Within this rift-related area, basaltic rocks with low ( 3.5 wt. %) of TiO 2 occur. The distribution of these basaltic rock types shows strong spatial dependency. Ti-rich basalts are concentrated in the western Erzgebirge-Krusne hory Mts. with a sharp change at longitude 13.3 °E. Clinopyroxene and olivine are the most abundant phenocrysts. The zoning and chemistry of the clinopyroxene act as a recorder of the different stages of the magmatic plumbing systems. Seven different types of clinopyroxene phenocrysts are defined with four distinct chemical types: Cr-rich clinopyroxene, green-core py roxene, Mg-rich clinopyroxene and finely zoned Ti-rich clinopyroxene with sector zoning. Chemical and petrographic analysis of individual zones within clinopyroxenes allows the qualitative description of distinct steps of the multistage, regionally diverse processes yielding to the formation of both Ti-rich basalts in the W as well as Ti-poor basalts in the E. Two different types of asthenospheric melts are suggested, with increased activities of H 2 O and potassium in the western Erzgebirge Mts. During ascent they incorporated xenoliths and xenocrysts of both the mantle (peridotite, olivine and Cr-rich clinopyroxene) and the crustal (granitic and gneissic rocks, quartz and feldspar) origin. Furthermore, green cores of clinopyroxene were introduced into the magmas. Fractionation processes took place within lithospheric magma chambers, having been accompanied by reaction of xenocrysts and xenoliths with the melt. The residence time in the magma chambers associated with the eastern and western parts of the Erzgebirge appears to have been totally different: short in the eastern and long in the western parts. Within the latter, no mantle-derived xenoliths, xenocrysts, or magmatic olivines are found. It is inferred that these xenoliths and crystals settled to the bottom of the magma chambers forming cumulates and therefore did not come to the surface. In contrast, the basaltic rocks of the East still contain mantle-derived xenoliths and xenocrysts as well as magmatic olivine. The size and amount of clinopyroxene phenocrysts in the West significantly exceed that of the East. Moreover, the crystallization of giant clino pyroxenes was restricted to the western region. Mixing of the magmas with Ti-rich melts, presumably derived from an eclogitic source, took place in crustal magma chambers. In any case, the proportion of Ti-rich melts in the West significantly exceeded that of the East.

Cooperation of upper and lower boundary-layer fractionations in a sheet-like intrusion: Composition and microstructure of the Aosawa dolerite sill in Yamagata Prefecture, northeastern Japan

Homogeneous fractionation, involving crystal nucleation, growth, and separation from a melt-dominant magma, and boundary-layer fractionation, involving the separation of a fractionated interstitial melt from a crystal-dominant boundary layer and its mixing with the main magma body, have been advocated as two major differentiation mechanisms in a crustal magma chamber. In this study, we focus on one of the dolerite sills in the Aosawa area, Yamagata Prefecture, Japan, to elucidate the roles of the two mechanisms in the differentiation of a sheet-like magma body. The intrusion is concordantly intruded into black mudstone and is ∼ 100 m thick and more than 5 km in lateral extension. The chilled margins contain olivine (5.3 vol%) and plagioclase (1.9 vol%) as phenocrysts. There is an absence of clinopyroxene in the chilled margins, and there are systematic sill-scale variations in the whole-rock major- and trace-element contents that require the addition or removal of clinopyroxene. These findings suggest sill-scale differentiation, involving the transportation of crystals that nucleated and grew in the sill, and/or the transfer of the residual melt. The downward increases in mode and size of clinopyroxene and in the size of the most dominant plagioclase, all with maxima near the bottom, and the occurrence of pigeonite rimming augite only near the bottom, suggest a slower cooling rate in the lower boundary layer compared to the upper one. Clinopyroxene crystals with Cr-rich cores that show textures suggesting rapid growth, such as remarkable sector zoning, melt inclusions, and small euhedral plagioclase, are more abundant at 5-15 m above the lower contact. These observations lead us to believe that clinopyroxene nucleated and grew near the upper boundary layer, where large supersaturation occurred, and then settled to concentrate in the lower boundary layer, where slower cooling provided suitable conditions for pigeonite crystallization. The upward increase in the incompatible elements and the decrease in compatible elements from the zone containing a high concentration of Cr-rich clinopyroxene suggest that a fractionated melt, which was formed through enhanced overgrowth on settled clinopyroxene, was transported upwards. The sill-scale magmatic differentiation in the Aosawa dolerite intrusion took place through crystal settling from the roof boundary layer, followed by the upward transportation of the fractionated melt from the bottom boundary layer. The lower-boundary-layer fractionation was probably very effective because the settling of the crystals thickened the bottom boundary layer to facilitate an effective melt-crystal separation.

On the onset and evolution of the Ross-orogeny magmatism in North Victoria Land — Antarctica

An extensive geochemical (major-, trace-element and oxygen isotope) and geochronological (U/Pb geochronology on zircon) characterisation of Ol-bearing ultramafic–mafic cumulates and gabbro-diorites recently discovered in the southwestern region of the Wilson Terrane (North Victoria Land — Antarctica) was undertaken in order to constrain source characteristics, magma evolution and emplacement history in this sector of the Cambro–Ordovician Ross Orogeny. Textural and chemical data indicate disequilibria among mineral phases of these Ol-bearing cumulate rocks. Corroded Cr-rich clinopyroxene with high Mg# (0.82) and Ca-rich plagioclase likely represent xenocrysts from an old magmatic system, and are not in equilibrium with the host cumulate assemblage (Ol + Opx + Amp + Bt + Pl). The calculated liquid in equilibrium with the xenocrysts (“melt 1”) exhibits exceptionally high La/Yb ratios and Th–U concentrations, suggesting a strong sediment influx in the mantle source, possibly from the subducted slab. Because of the extensive fractional crystallisation (Fo 70; Mg# = 76) and crustal contamination (e.g., δ 18O Opx = 7.13–7.47‰) the trace element composition of the liquid in equilibrium with the cumulate assemblage (“melt 2”) does not reveal the nature of the mantle source not the differentiation processes. The gabbroic parental liquid for the main cumulate assemblage was not produced by assimilation and fractional crystallisation (AFC) of the equilibrium “melt 1” calculated from the xenocrystic paragenesis. Rather, it likely represents a new pulse of magma originated from a different mantle source, which then evolved through AFC into gabbro-diorites. Zircon U–Pb dating of one gabbro-diorite yields a crystallisation age of 489 ± 4 Ma. Zircon in the Ol-bearing cumulates yielded two ages populations at 521 ± 2 Ma and 502 ± 3 Ma, which we interpret to represent the actual age difference between the two magmatic systems. This age difference suggests that poorly differentiated melts with adakite-type signature intruded the crust prior to the generation of the large volumes of gabbroic and dioritic magmas. These early pulses represent up to now the oldest proof of subduction related mantle melts in North Victoria Land and predate the diffuse igneous activity dominated by intermediate- to felsic products.

Geochemistry of the Uintjiesberg kimberlite, South Africa: petrogenesis of an off-craton, group I, kimberlite

The Uintjiesberg kimberlite diatreme occurs within the Proterozoic Namaqua–Natal Belt, South Africa, approximately 60 km to the southwest of the Kaapvaal craton boundary. It is a group I, calcite kimberlite that has an emplacement age of ∼100 Ma. Major and trace element data, in combination with petrography, are used to evaluate its petrogenesis and the nature of its source region. Macrocryst phases are predominantly olivine with lesser phlogopite, with very rare garnet and Cr-rich clinopyroxene. Geochemical variation amongst the macrocrystic samples (Mg# 0.85–0.87, SiO 2=27.0–29.3%, MgO=26.1–30.5%, CaO=10.9–13.5%) is shown to result from ∼10% to 40% entrainment and partial assimilation of peridotite xenoliths, whereas that shown by the aphanitic samples (Mg# 0.80–0.83, SiO 2=19.1–23.0%, MgO=17.9–23.9%, CaO=16.5–23.7%) is consistent with ∼7–25% crystal fractionation of olivine and minor phlogopite. Changing trajectories on chemical variation diagrams allow postulation of a primary magma composition with ∼25% SiO 2, ∼26% MgO, ∼2.3% Al 2O 3, ∼5%H 2O, ∼8.6% CO 2 and Mg#=0.85. Forward melting models, assuming 0.5% melting, indicate derivation of the primary Uintjiesberg kimberlite magma from a source enriched in light rare earth elements (LREE) by ∼10× chondrite and heavy REE (HREE) by 0.8–2× chondrite, the latter being dependent on the proportion of residual garnet. Significant negative Rb, K, Sr, Hf and Ti anomalies present in the inferred primary magma composition are superimposed on otherwise generally smooth primitive mantle-normalized trace element patterns, and are inferred to be a characteristic of the primary magma composition. The further requirement for a source with chondritic or lower HREE abundances, residual olivine with high Fo content (∼Fo 94) suggests derivation from a mantle previously depleted in mafic melt but subsequently enriched in highly incompatible elements prior to kimberlite genesis. These requirements are interpreted in the context of melting of continental lithospheric mantle previously enriched by metasomatic fluids derived from a sublithospheric (plume?) source.

Feldspar from carbonate-rich silicate metasomatism in the shallow oceanic mantle under Kerguelen Islands (South Indian Ocean)

Some highly depleted harzburgitic mantle xenoliths from two different localities in the Kerguelen Islands display a secondary mineral assemblage unusual for both oceanic and continental settings. Petrographic and chemical evidence indicate reaction of primary orthopyroxene and spinel with an infiltrating melt to form Na–Cr-rich clinopyroxene+olivine+Cr-rich spinel±apatite±rutile±carbonate and a Si–Al–Na-rich phase, which is either a glass and/or a Na-rich plagioclase. Major-element compositions of secondary minerals strongly indicate that their formation resulted from the infiltration of a sodium-rich carbonate–silicate melt with a low water content through the host depleted harzburgite. The trace-element signatures of metasomatic clinopyroxene and glass indicate that the metasomatic agent was enriched in Sr, P, Cl, Th, U, and LREE and low in K, Rb and HFSE (Nb, Ta, Zr, Hf, Ti). The infiltration of this metasomatic agent led to heterogeneous crystallisation and wall-rock reactions, which are recorded by the mode and the major-element variations of minerals. The trace-element composition of clinopyroxene suggests that the carbonate to silicate melt proportion in the metasomatising agent was locally variable, resulting in places in trace-element signatures typical of alkali silicate melt metasomatism. The migration of sodic, carbonate-rich silicate melt is inferred to result in feldspar-bearing metasomatic assemblages formed by reaction of the melt with the host peridotite if carbonate is not stable at shallow depth in the oceanic upper mantle. The metasomatic imprint was acquired shortly before eruption and formed areas of felsic composition, heterogeneous at the mm scale. This study demonstrates the close relationship between migrating carbonate and silicate melts in the oceanic mantle beneath Kerguelen Islands.

Petrogenesis of late Hercynian calc-alkaline dykes of mid-eastern Sardinia: petrographical and geochemical data constraining hybridization process

During the final stages of the emplacement of the Hercynian batholith, a calc-alkaline dyke swarm intersected the crystalline basement of Sardinia. In mid-eastern Sardinia, these dykes range in composition from basaltic andesite to dacites-rhyolites with oligophyric and porphyritic textures, respectively. Plagioclase is the dominant phase among the phenocrysts, with subordinate brown and green amphibole; biotite, K-feldspar and quartz are abundant in more differentiated compositions. The biotite Rb-Sr age of this magmatism ranges between 291 and 271 Ma. Compositional data can be modelled using two separate trends: 1) AFC-type processes with fractionation of An-rich plagioclase, orthopyroxene and Cr-rich clinopyroxene are responsible for compositional variations amongst the most basic rocks; 2) andesitic to rhyolitic magmas evolve starting from basic magmas which are aligned along the first trend due to hybridization process. The complex and long-lived mafic magma and the crust interaction indicate a MASH-like scenario characterized by different magma chambers, as proposed for the late-Hercynian magmatism in other areas.

Ultramafic inclusions from San Carlos, Arizona: Petrologic and geochemical data bearing on their petrogenesis

Ultramafic inclusions from San Carlos, Arizona, are classified into two groups. Group I inclusions are dominated by magnesian (Mg/Mg + ΣFe= 0.86 – 0.91), olivine-rich peridotites containing Cr-rich clinopyroxene and spinel. The less abundant Group I pyroxenites (containing Mg- and Cr-rich pyroxenes) occur as discrete inclusions and as portions of composite inclusions where they have a sharp, planar interface with lherzolite. Group II inclusions are dominated by clinopyroxene-rich peridotites containing Al- and Ti-rich augite and commonly abundant, Al-rich spinel. Compared to Group I inclusions, they are more Fe-rich (Mg/Mg + ΣFe= 0.62 – 0.78) and more hetereogeneous in composition and modal proportions. Similar groups occur at many ultramafic inclusion localities. Our petrographic and geochemical results lead to the following conclusions. Olivine-rich Group I inclusions are not genetically related to the host basanite, and they are formed from two components. Component A is a partial melting residue; it comprises the major portion of these inclusions and determines the modal mineralogy and major and compatible trace element composition. Component B results from a small degree (<5%) of garnet peridotite melting (probably, within the low-velocity zone). This highly LIL-element-enriched melt has migrated upwards into the overlying component A where it crystallized primarily as clinopyroxene and amphibole, and thus, introduced LIL elements into the residual component A. Subsequent cooling and subsolidus recrystallization have removed textural evidence of this mixing. This model has also been proposed for olivine-rich Group I inclusions from Victoria, Australia. At Victoria and San Carlos some relatively clinopyroxene-rich Group I lherzolites are not contaminated by component B, and they represent the best estimates of upper mantle composition prior to melting. Group I orthopyroxenites may be fragments of tectonic layers formed in lherzolite, but they could also be early cumulates (now metamorphosed) from the melt in equilibrium with component A. Group I clinopyroxenites have geochemical features of clinopyroxene in equilibrium with a magma. Thus, they could also represent early cumulates (now metamorphosed) from a magma unrelated to the host basanite. Alternatively, their geochemical characteristics could result from more complex models such as residues from partial remelting of pyroxenite dikes and veins or intradike segregation processes such as filter pressing. All Group II inclusions studied appear to be cumulates derived from a SiO 2-undersaturated magma, possibly an early magma in the same volcanic episode which culminated with eruption of the host basanite. The poikilitic texture of amphibole-rich (kaersutite) inclusions is consistent with a cumulate origin. The bulk compositions of Group II inclusions are not equivalent to typical basaltic compositions.

Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States

Ultramafic xenoliths in basalts from the western United States are divided into Al-augite and Cr-diopside groups. The Al-augite group is characterized by Al, Ti-rich augites, comparatively Fe-rich olivine and orthopyroxene, and Al-rich spinel, the Cr-diopside group by Cr-rich clinopyroxene and spinel and by Mg-rich olivine and pyroxenes. Both groups have a wide range of subtypes, but the Al-augite group is dominated by augite-rich varieties, and the Cr-diopside group by olivine-rich lherzolites.