Andy Beard

Petrology and geochemistry of xenoliths from the Northern Baltic shield: evidence for partial melting and metasomatism in the lower crust beneath an Archaean terrane

Lower crustal xenoliths entrained in a Paleozoic ultramafic lamprophyre breccia pipe on Elovy island, Kola peninsula, Russia, represent some of the oldest lower crustal material yet investigated from Europe. The xenoliths vary from feldspar-poor, garnetrich rocks which resemble eclogites, to feldspar-rich garnet granulites. Quartz-rich felsic granulites, as well as pyroxenites and amphibole-rich rocks are also present. The mafic granulites/eclogites represent a suite of gabbros and norites that is related by olivine fractionation. The igneous protoliths may have formed in a manner analogous to lower crustal rocks from most other European xenolith localities, i.e. by basaltic underplating, but magmatic cumulates are not in evidence. The Kola lower crust was subjected to one or more metasomatic events which introduced up to 45% phlogopite and/or amphibole into both eclogites/granulites and pyroxenites. The resulting rocks have strong enrichments in Rb, Ba, and K, indicating that the lower crust is not uniformly depleted in LIL and heat-producing elements. Siliceous (65% SiO2) and mafic (< 50% SiO2) lithologies coexist in migmatitic xenoliths, which provide evidence for partial melting processes and restite formation in mafic metaigneous lower crust. The relationship, if any, between partial melting and metasomatism is unclear.

Geochemistry and mineralogy of kimberlites from the Arkhangelsk Region, NW Russia: evidence for transitional kimberlite magma types

Abstract The Arkhangelsk kimberlite province (AKP) is situated in the north of the Baltic Shield within the buried southeastern portion of the Kola–Kuloi craton. It forms part of the extensive Devonian magmatic event of the northern Baltic Shield and Kola Peninsula. Two main groups of kimberlites can be distinguished within the province: (1) kimberlites from the diamondiferous Zolotitsa field that have geochemical and isotopic affinities with Group 2 kimberlites and lamproites; (2) diamond-poor Ti–Fe-rich kimberlites from other Arkhangelsk fields that have geochemical and isotopic affinities with Group 1 kimberlites. However, the Zolotitsa and Ti–Fe-rich kimberlites have mineralogical characteristics that are not typical for their respective assigned kimberlite group classifications. Both groups of Arkhangelsk kimberlites are apparently transitional to Group 1 kimberlites, Group 2 kimberlites and lamproites as they are defined elsewhere in the world. An associated kimberlite from the Mela Sill Complex has strong affinities with carbonatites. The low Al2O3, high Ni and Cr contents, and high Mg# in both groups of kimberlites indicate strongly depleted lherzolitic–harzburgitic mantle sources. Trace element patterns show a variable enrichment of incompatible elements and strong LREE enrichment. However, kimberlites from the Zolotitsa field have overall lower trace element abundances and less steep REE patterns, suggesting a higher degree of partial melt and/or a less enriched source compared to that of the Ti–Fe-rich kimberlites. A calciocarbonatite of the Mela Sill Complex has trace element and REE patterns typical of other carbonatites closely associated with kimberlites. 87 Sr / 86 Sr i and 143 Nd / 144 Nd i isotope compositions of the Arkhangelsk kimberlites and carbonatite reveal that at least two mantle sources are required to explain the isotopic variation: (1) most of the Zolotitsa and Mela kimberlites and the Mela carbonatite are derived from an ancient enriched lithospheric source (EMI); (2) the Ti–Fe-rich kimberlites are derived from a plume-related asthenospheric mantle source with an isotopic composition close to Bulk Earth. Present-day Pb isotope compositions reveal that the Zolotitsa kimberlites have values close to Group 1 kimberlites. However, the Ti–Fe-rich kimberlites generally have slightly more radiogenic Pb isotope values.

Mantle domains in the lithosphere beneath the French Massif Central: trace element and isotopic evidence from mantle clinopyroxenes

Lenoir et al. [Earth Planet. Sci. Lett. 181 (2000) 359] have suggested the existence of a lithospheric domain boundary at approximately 45°30′N within the French Massif Central because of differences in bulk rock geochemistry between mantle peridotite xenoliths from localities north and south of this latitude. New laser ablation ICP–MS analyses of trace element contents in clinopyroxenes from xenoliths from the northern domain, combined with Sr and Nd isotopic data for the same clinopyroxenes, clearly indicate significant differences between the northern and southern domains. LA–ICP–MS results show that the northern domain peridotites have experienced significantly more depletion than ones from the southern domain, having undergone almost twice as much fractional melting. Although they do not contain garnet at the present day, the northern domain peridotites appear to have contained garnet when they underwent melting. They can have extremely high Sm/Nd ratios (>0.5), indicating unusually strong depletion in Nd relative to Sm. However, they are also more enriched in LREE and Sr, and show significant fractionation of Zr from Hf, and of U from Th (Zr/Hf and Th/U≪chondritic values). This is due to metasomatism, possibly related to carbonitite infiltration. The Sr–Nd isotopic compositions of northern domain xenoliths are also unusual, in that many have high 143Nd/144Nd or high 87Sr/86Sr values which do not occur in samples from the southern domain. In contrast, the southern domain mantle shows low (MORB source) 87Sr/86Sr values not found in the northern domain. This confirms that significant differences exist between the two domains and offers strong support to the concept of the existence of lithospheric blocks with separate histories beneath the Massif Central. A remarkable similarity exists between the mantle beneath the northern domain of the Massif Central and that beneath the Rhon volcanic area (Germany) 1000 km to the northeast.

Petrogenesis of Devonian lamprophyre and carbonatite minor intrusions, Kandalaksha Gulf (Kola Peninsula, Russia)

Abstract Minor magmatic intrusions (dykes and explosion pipes) of lamprophyric and carbonatitic compositions occur on several islands in the Gulf of Kandalaksha (White Sea, Kola Peninsula, Russia). The lamprophyre dykes yielded K-Ar ages of 368 ± 15 Ma and 360 ± 16 Ma, similar to the majority of alkaline rocks from the Kola Alkaline Province. Mineralogical data (presence of perovskite and sodalite, absence of amphibole phenocrysts) and geochemical data (low SiO2 and A12O3, high MgO) indicate an ultramafic lamprophyre affinity for the investigated silicate rocks. The lamprophyres contain a wide variety of xenoliths including hornblende- and biotite-rich cumulate ultramafic rocks. The carbonatite intrusions have ferrocarbonatite affinities and one of them contains xenoliths of coarse-grained Si-rich calciocarbonatites, together with abundant hornblendites and glimmerites which resemble those in the lamprophyres. The calciocarbonatite xenoliths themselves contain fragments of mica- and hornblende-rich rocks. 40Ar-39Ar ages on phlogopite and amphibole from calciocarbonatite and hornblende-rich cumulate xenoliths are between 386 ± 1.0 Ma and 395.6 ± 4.4 Ma, indicating an early Devonian age and suggesting a close relationship between the calciocarbonatite xenoliths and the ultramafic cumulate xenoliths. Thus, the ferrocarbonatite host magma may have disrupted an older calciocarbonatite-hornblendite-glimmerite intrusion at depth and incorporated xenoliths from it. The presence of hornblendite and glimmerite xenoliths with similar parageneses and identical ages in both the ultramafic lamprophyres and ferrocarbonatites suggests a close relationship between the ferrocarbonatite and lamprophyric magmas. REE patterns of the lamprophyric dykes and calciocarbonatite xenoliths show strong similarities, indicating a petrogenetic relationship. The ultramafic lamprophyres have REE patterns which are indistingishable from the contemporaneous kimberlites and melilitites from the nearby Terskii Bereg area, south Kola Peninsula. Age-corrected Sr and Nd isotope compositions demonstrate that the calciocarbonatite xenoliths have close affinities with other Devonian carbonatites from Kola and Karelia, whereas the lamprophyres are similar to the Kola kimberlites and melilitites. Differences between the Sr and Nd isotopic ratios of the silicate and carbonatite magmas throughout the Kola Alkaline Province are probably due to different mantle-source components. The Kola carbonatites mainly show a depleted mantle signature whereas the lamprophyres, melilitites and kimberlites were derived from a more enriched mantle. However, some degree of assimilation of lower continental crust and late-stage hydrothermal alteration of the silicate magmas may also have occurred.

Geochemical constraints on restite composition and unmixing in the Velay anatectic granite, French Massif Central

The main anatectic granite of the Velay complex is unique among major French Massif Central Hercynian granitoids in that rather than having an entirely lower crustal source, it formed by mixing between partial melts of the meta-igneous lower crust and ‘upper crustal’ country rock schists and orthogneisses. The geochemical variations in the Velay main anatectic granites cannot, however, be explained by mixing alone as their compositions range to lower SiO2, with higher Al2O3, Fe2O3 and TiO2 and lower Na2O and CaO, than either end member in mixing. The variations are interpreted as being due to the presence of up to 35% restite in minimum melts of country rock compositions. Primary restites form equilibrium assemblages represented by biotite, ilmenite and surmicaceous enclaves which consist of biotite ± apatite, zircon and almandine. The main anatectic granites more rarely contain schist and gneiss enclaves, quartz resisters and plagioclase restites. Secondary restites are mainly represented by cordierite, and possibly K-feldspar, which formed by recrystallisation of primary biotite-rich restites. The unique characteristics of the Velay main anatectic granites are likely to be due, in part, to its late formation close to the end of the Hercynian orogeny. The metasedimentary lower crust may have become too refractory to yield large volumes of melt following partial melting to form the other major Massif Central granitoids. The heat necessary for partial melting at higher crustal levels was transferred from the lower crust by the intrusion of I-type granites and low volume diorites from the mantle. Upper crustal anatexis was mainly controlled by muscovite breakdown reactions (< 830 to 850 °C) and the liberation of water due to the recrystallisation of biotite to cordierite. The temperatures necessary for biotite breakdown were only achieved locally and resulted in the formation of high-LREE granites.

Buffering of melt oxygen fugacity by sulphur redox reactions in calc-alkaline magmas

Calculated oxygen fugacities of magmas from Lascar Volcano, Northern Chile, and the 1991 eruption of Mount Pinatubo, Phillipines, are seen to increase with decreasing temperature, relative to the fayalite-magnetite-quartz buffer. Isopleths of constant H2S/SO2 at moderate pressures (2–4 kbar) lie parallel to the data. We conclude that magma mixing prior to eruption liberated large quantities of SO2 gas by oxidation of dissolved sulphide in the mafic end-member. Oxygen fugacity of the magmas was then buffered by reduction of this gas to H2S, leading to increasingly oxidized magmas. Such a process might be used to explain the highly oxidized nature of other sulphur-rich evolved calc-alkaline magmas, notably the El Chichon trachyandesite.

Empirical evidence for the fractionation of carbon isotopes between diamond and iron carbide from the Earth's mantle

We have studied two samples of mantle diamond containing iron carbide inclusions from Jagersfontein kimberlite, South Africa. Syngenetic crystal growth is inferred using morphological characteristics. These samples provide an opportunity to investigate the isotopic partitioning of 13C in a terrestrial natural high-pressure and high-temperature (HPHT) system. The difference for the δ13C values between the diamond and coexisting iron carbide averaged 7.2 ± 1.3‰. These data are consistent with available data from the literature showing iron carbide to be 13C-depleted relative to elemental carbon (i.e., diamond). We infer that the minerals formed by crystallization of diamond and iron carbide at HPHT in the mantle beneath the Kaapvaal Craton. It is unclear whether crystallization occurred in subcratonic or sublithospheric mantle; in addition, the source of the iron is also enigmatic. Nonetheless, textural coherence between diamond and iron carbide resulted in isotopic partitioning of 13C between these two phases. These data suggest that significant isotopic fractionation of 13C/12C (Δ13C up to >7‰) can occur at HPHT in the terrestrial diamond stability field. We note that under reducing conditions at or below the iron-iron wustite redox buffer in a cratonic or deep mantle environment in Earth, the cogenesis of carbide and diamond may produce reservoirs of 13C-depleted carbon that have conventionally been interpreted as crustal in origin. Finally, the large Δ13C for diamond-iron carbide shown here demonstrates Δ13C for silicate-metallic melts is a parameter that needs to be constrained to better determine the abundance of carbon within the Earths metallic core.

Petrology and geochemistry of a cumulate xenolith suite from Bute: evidence for late Palaeozoic crustal underplating beneath SW Scotland

Xenoliths from Hawks Nib (Bute, SW Scotland), entrained in alkali basalt of late Carboniferous–early Permian age, generally possess unrecrystallized igneous cumulate textures. Most are inferred to derive from lower crustal (20–30 km) depths, with a small minority from the underlying restitic mantle. Compositions range from ultramafic cumulates (dunite, wehrlite, websterite, clinopyroxenite) to gabbros and anorthosites. The suite is unique in being dominated by poikilitic wehrlites and olivine clinopyroxenites. Major element variation in the cumulates is controlled by modal mineral variations. The ultramafic cumulates all have high REE contents and light REE (LREE)-enriched patterns, with websterites being the least enriched. Gabbroic xenoliths have uniform patterns in which both LREE and heavy REE are depleted relative to the middle REE, with small positive Eu anomalies. Present-day Sr–Nd isotopic data for the suite are the most depleted among Scottish xenoliths. The xenoliths are inferred to be products of young replenishments of the lower crust by basaltic magmas during continental underplating. Orthopyroxene-bearing lithologies crystallized from tholeiitic magmas whereas alkali basalts formed the clinopyroxene-rich samples; both parental magma-types were derived from similar mantle sources.

A melilite-bearing high-temperature calcic skarn, Camasunary Bay, Isle of Skye

Synopsis The occurrence of high-temperature calcic skarns is relatively rare, with around 30 being reported worldwide. Here, we report on a newly discovered occurrence of a melilite-bearing high-temperature calcic skarn at Camasunary Bay, Isle of Skye. The contact metamorphism was caused by the intrusion of the Cuillin Hills gabbro and the later intrusion of the Coire Uaigneich granophyre. The calcic skarn consists of a high-temperature assemblage of wollastonite, åkermanite–gehlenite, vesuvianite, tilleyite, spurrite. andradite–grossular garnet, monticellite and perovskite. It also includes the very rare Zr-rich garnet kimzeyite and baghdadite (Ca3(Zr,Ti)Si2O9), indicating transfer of Zr, Ti and other incompatible element-rich fluids derived from the hydrous Coire Uaigneich granophyre. The calcic skarn contains numerous cherty nodules and stringers, which bear a remarkable resemblance to the Cambrian–Ordovician dolostone exposures observed in other parts of Skye. suggesting that the protolith was originally an impure dolomite. The nature of the contact between the calcic skarn and surrounding Torridonian sedimentary rocks could not be directly observed, but elsewhere on Skye this contact is the Kishorn Thrust plane. This has implications for the simplified tectonic map accompanying the British Geological Survey map of Broadford (Sheet 71W), which now requires modification.

The origin of melanophlogite, a clathrate mineral, in natrocarbonatite lava at Oldoinyo Lengai, Tanzania

Abstract We report new observations of a clathrate mineral, melanophlogite [46SiO2⋅6(N2,CO2)⋅2(CH4,N2)], as part of a tuffaceous layer within a sample of the 2006 natrocarbonatite lava, whose composition reflects the typical magma erupted passively at Oldoinyo Lengai throughout the last ∼50 yr. The mineral has been identified by chemical composition, micro-X‑ray diffraction, and transmitted light optical characteristics. This is the first reported occurrence of a clathrate in an igneous carbonatite, and we conjecture that this mineral may be recognized elsewhere in alteration products of natrocarbonatite ash and in particular, combeite-bearing carbonatite lithologies. Specifically, melanophlogite is a rare polymorph of SiO2 with guest molecules (e.g., CH4, CO2, SO2, N2, OH, Xe, and Kr) within a silicate framework. It occurs in an ash pellet-rich layer within the natrocarbonatite lava, as abundant groundmass crystals and as cores of individual ash pellets, with pseudocubic and pseudohexagonal habits, ranging from 50 to 100 μm in size, with numerous inclusions of nepheline laths aligned parallel to the crystal margins. It has high-C contents (up to 2.25 wt%) and CO2 is considered to be the guest molecule due to crystallization within an alkaline carbonatitic-CO2-rich environment.