Garnet Lherzolite Research Articles

A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form on both the ocean floor and the continental crust, are difficult to understand because of the diversity, heterogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanic rocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis. Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite with the presence of a compositional gap between SiO2 58–64 wt.%. Similarities in geochemical characteristics, modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocks argue against significant crustal contamination. Major and trace element evidences indicate that the melting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stability field. Melting models suggest small degree (<3%) partial melting of mantle bearing (6–10%) garnet for Mt. Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL, Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope systematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enriched mantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of the CVL lavas. Low Mg number (Mg# = 100 × MgO/(MgO + FeO)) and Ni, Cr and Co contents of the CVL mafic lavas reveal their crystallization from fractionated melts. The absence of systematic variation in Nb/Ta and Zr/Hf ratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolution of magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle heterogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

Petrogenetic significance of ocellar camptonite dykes in the Ditrău Alkaline Massif, Romania

Abstract Camptonite dykes intrude the rift-related Mesozoic igneous body of the Ditrau Alkaline Massif, Eastern Carpathians, Romania. We present and discuss mineral chemical data, major and trace elements, and the Nd isotopic compositions of the dykes in order to define their nature and origin. The dykes are classified as the clinopyroxene-bearing (camptonite-I) and clinopyroxene-free (camptonite-II) varieties. Camptonite-I consists of aluminian–ferroan diopside phenocrysts accompanied by kaersutite, subordinate Ti-rich annite, albite to oligoclase and abundant calcite–albite ocelli. Camptonite-II comprises K-rich hastingsite to magnesiohastingsite, Ti-rich annite, albite to andesine, abundant accessory titanite and apatite, and silicate ocelli filled mainly with plagioclase (An4–34). Age-corrected 143Nd/144Nd ratios vary from 0.51258 to 0.51269. The high eNd values of + 4.0 to + 6.1 which are consistent with intra-plate composition, together with light rare earth element (LREE), large ion lithophile element (LILE) and high field strength element (HFSE) enrichment in the camptonites is ascribed to the formation of small melt batches of a metasomatised sub-lithospheric mantle source. The presence of an asthenospheric ‘high μ’ ocean island basalt (HIMU-OIB)-type mantle component in the source region has also been revealed. A 1–4% degree of partial melting of an enriched garnet lherzolite mantle source containing pargasitic amphibole followed by fractionation is inferred to have been involved in the generation of the camptonites. They are deduced to be parental melts to the Ditrau Alkaline Massif.

Petrogenesis and tectonic implications of the middle Silurian volcanic rocks in northern West Junggar, NW China

Zircon U–Pb geochronological and geochemical analyses are reported for a suite of the middle Silurian volcanic rocks from northern West Junggar (NW China), southern Central Asian Orogenic Belt (CAOB), with the aim to investigate the sources, petrogenesis, and tectonic implications. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb analysis from an andesite yielded a concordant weighted mean 206Pb/238U age of 429 ± 3 Ma, indicating the presence of middle Silurian volcanic rocks in northern West Junggar. The andesite is tholeiite series and characterized by minor variations in compositions (SiO2 = 55.68–59.17 wt.%, Al2O3 = 14.56–17.7 wt.%, TiO2 = 0.55–1.23 wt.%, Na2O + K2O = 3.46–7.16 wt.%, and P2O5 = 0.15–0.37 wt.%), with wider MgO content (2.18–6.48 wt.%) and Mg# (57.4–77.9). All andesitic rocks are enriched in large-ion lithophile elements (LILEs; e.g. Rb, Ba, K, and Th) and light rare earth elements (LREEs), but strongly depleted in some high field strength elements (HFSEs; e.g. Nb, Ta and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.8–1). These features suggest that the andesitic magmas were derived from 2–8% partial melting of a garnet lherzolite depleted mantle source with subducted sediments metasomatized by slab-derived fluids. Combining the current study with those data in existing literature, we conclude that the middle Silurian volcanic rocks formed in an intra-oceanic subduction setting during consumption of the Irtysh–Zaysan Ocean, and further confirm the eastern extension of the early Palaeozoic Boshchekul–Chingiz volcanic arc of East Kazakhstan in China.

Petrology of the Guenfalabo ring-complex: An example of a complete series along the Cameroon Volcanic Line (CVL), Cameroon

In the Guenfalabo ring-complex (GRC), two non-comagmatic rock suites have been identified as a result of two volcanic episodes: Suite 1 (68.8±1.7Ma by K/Ar on trachyte) of peralkaline trachytes and pantellerites cogenetic with alkaline syenites, granites and rhyolitic flows and tuffs; Suite 2 (62±2Ma by K/Ar on basalt), a bimodal and complete series of alkali olivine basalts and associated microgabbro dykes, diorites, syenites and granites, cross-cutting the former. Kaersutite in the trachytes of Suite 1 has mantle-derived signatures: TiO2>4%, MgO<15%, FeO>8%, Ti=0.63 c.p.f.u. and Al=2, characteristic of kaersutites of HP and HT origin: 13–23kbar, 1100–1220°C. The trachytes are probably products of FC of a basaltic parent that did not attain higher crustal levels. The Suite 1 rocks are enriched in Rb, K, Zr, Nb, LREE, alkalis, and (Ce/Yb)N=7–15 probably due to some effect of metasomatism during the magma ascension. Fe–Ti enrichment is corroborated by the presence of ferropseudobrookite–ilmenite–ulvospinel in the syenites and ilmenite in the pantellerite. The Rb/Ba>1 in the trachytes (2.44, 26.7), pantellerite (6.33), alkaline granites (0.63–1.8) and the 87Sr/86Sr in the alkaline granites (=0.74060) depict the role of AFC. The ankaramites of the Suite 2 rocks are olivine-phyric (25%), Fo85–88, have 50–52% clinopyroxene (salite), 5% plagioclase (An55–36) and 7% Fe–Ti oxides. Trace element modeling indicates an origin from a basaltic magma of about 25% PM of spinel lherzolite mixed with a magma from <1% PM of garnet lherzolite (3–4% garnet) in a proportion of 1:4. The cogenetic alkali basalts and the microgabbro–diorite–syenite–granite that constitute the Suite 2 rocks, with a Daly gap of 54%>SiO2<58%, result from this Early Cenozoic magmatic event. The basalts have: Zr=225–253, Nb=98–111, Y=33–56, typical of FOZO, a HIMU-type OIB related magmas (Sr/Sri=0.70202–0.7034; Nd/Nd=0.51282–0.512545; 206Pb/204Pb=19.13, 207Pb/204Pb=15.59 and 208Pb/204Pb=38.64. The (La/Yb)N ratios (9–20) indicate a high REE fractionation and garnet-lherzolite source. Elemental ratios as La/Nb=0.49–0.81, Nb/Y=1.00–2.04 suggest crustal contamination of minor importance. The (La/Sm)N ratios (2–3.5) and LREE enrichment corroborate the contribution of a subcontinental lithospheric mantle (SCLM). REE spectra and minor element spidergrams are concordant for each rock suite.

Oligocene crustal xenolith-bearing alkaline basalt from Jandaq area (Central Iran): implications for magma genesis and crustal nature

The Oligocene alkaline basalts of Toveireh area (southwest of Jandaq, Central Iran) exhibit northwest–southeast to west–east exposure in northwest of the central-east Iranian microcontinent (CEIM). These basalts are composed of olivine (Fo70–90), clinopyroxene (diopside, augite), plagioclase (labradorite), spinel, and titanomagnetite as primary minerals and serpentine and zeolite as secondary ones. They are enriched in alkalis, TiO2 and light rare earth elements (La/Yb = 9.64–12.68) and are characterized by enrichment in large ion lithophile elements (Cs, Rb, Ba) and high field strength elements (Nb, Ta). The geochemical features of the rocks suggest that the Toveireh alkaline basalts are derived from a moderate degree partial melting (10–20%) of a previously enriched garnet lherzolite of asthenospheric mantle. Subduction of the CEIM confining oceanic crust from the Triassic to Eocene is the reason of mantle enrichment. The studied basalts contain mafic-ultramafic and aluminous granulitic xenoliths. The rock-forming minerals of the mafic-ultramafic xenoliths are Cr-free/poor spinel, olivine, Al-rich pyroxene, and feldspar. The aluminous granulitic xenoliths consist of an assemblage of hercynitic spinel + plagioclase (andesine–labradorite) ± corundum ± sillimanite. They show interstitial texture, which is consistent with granulite facies. They are enriched in high field strength elements (Ti, Nb and Ta), light rare earth elements (La/Yb = 37–193) and exhibit a positive Eu anomaly. These granulitic xenoliths may be Al-saturated but Si-undersaturated feldspar bearing restitic materials of the lower crust. The Oligocene Toveireh basaltic magma passed and entrained these xenoliths from the lower crust to the surface.

Origin of Basalts in a Hot Subduction Setting: Petrological and Geochemical Insights from Mt. Baker, Northern Cascade Arc

The northern Cascade arc is an end-member ‘hot’ subduction zone where slab dehydration may be essentially complete prior to reaching sub-arc depths, presenting a potential problem for a flux melting origin for arc basalts. Nevertheless, mafic lavas from the Mt. Baker volcanic field, the most magmatically productive volcano in the northern Cascade arc, record subduction input and compositions typical of calc-alkaline magmas. Relative to normal mid-ocean ridge basalt, the most primitive lavas at Mt. Baker show elevated abundances of large ion lithophile elements (LILE), Pb and light rare earth elements (LREE). High field strength elements (HFSE) and heavy REE (HREE) are depleted relative to LILE. Reconstructed primary magmas define three groups: Group I calcalkaline basalts (Sulphur Creek, Lake Shannon, Hogback) have lower SiO2, (La/Yb)N and LILE/HFSE, and are olivineand plagioclase-phyric; Group II high-Mg basaltic andesites (Tarn Plateau, Cathedral Crag) also contain clinopyroxene phenocrysts and have higher H2O, SiO2, (La/Yb)N and LILE/HFSE; Group III low-K olivine tholeiite (Park Butte) has the lowest (La/Yb)N but highest LILE/HFSE.The redox states of all groups lie between QFM (quartz^fayalite^magnetite) and QFM þ1·4. Pb, Sr and Nd isotope compositions are similar among all the analysed samples and are consistent with a depleted mantle source modified by input from the subducting slab. Trace element^isotope modeling indicates a subduction component composed predominantly of metabasaltderived fluid with lesser amounts of sediment melt and metabasalt melt. Group I basalts record the smallest melt fractions ( 5^7%), lowest water contents (1·5^2·1wt %), and highest temperatures and pressures of mantle segregation (up to 13548C, 1·5 GPa) from lherzolitic ( spinel) residues. Group II basaltic andesites show the greatest extents of mantle metasomatism, the highest water contents (2·7^3·7 wt %) and partial melt fractions (10^12%), and segregated from harzburgite at 12708C, 1GPa, consistent with pooling of melts at the Moho. Group III records P^Tconditions similar to Group I ( 1·4 GPa, 13268C) but melt fractions (12%) and mantle residues (harzburgite) are more similar to Group II, and H2O contents ( 2·1wt %) are intermediate. Melting beneath Mt. Baker was initiated by dehydration melting of amphibole peridotite at 95 km and 10208C, within the stability field of garnet lherzolite. Initial melt fractions were small ( 1^2%) and near water-saturation. Phase equilibria and trace element modeling show no evidence for garnet-bearing mantle residues, indicating that progressive melting during ascent of diapirs through the hot core of the convecting mantle wedge reduced H2O contents and erased any residual garnet signature. Because fluid release from the slab is restricted to the forearc, mantle hydrated at shallow depths in the serpentine and/or chlorite stability fields must be down-dragged to the region of amphibole stability to initiate dehydration melting.

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO2-undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.

Sr‐Nd‐Hf‐Pb isotope and trace element evidence for the origin of alkalic basalts in the Garibaldi Belt, northern Cascade arc

In the Garibaldi Belt, the northern segment of the Cascade arc, basalts at Bridge River Cones, Salal Glacier, and Mt. Meager (BSM volcanic centers) are alkalic, atypical for an arc setting. Subduction signatures are negligible or absent from primitive alkalic basalts from Salal Glacier and Bridge River, while altered oceanic crust may have contributed a minimal amount of fluid at Mt. Meager. More evolved BSM basalts display trace element signatures considered typical of arc lavas, but this is a consequence of deep crustal assimilation rather than primary input from the subducted slab. Primary BSM basalts represent 3–8% melts that segregated from enriched garnet lherzolite at significantly higher temperatures and pressures (70–105 km) than calc‐alkaline Cascade arc basalts. The BSM mantle source is significantly more incompatible element‐enriched than the depleted mantle tapped by calc‐alkaline Cascade arc basalts. The BSM basalts are also isotopically distinct from calc‐alkaline Cascade arc basalts, more similar to MORB and intraplate basalts of the NE Pacific and NW North America. The relatively deep, hot, and geochemically distinct mantle source for BSM basalts is consistent with upwelling asthenosphere. The BSM volcanic centers are close to the projected trace of the Nootka fault, which forms the boundary between the subducting Juan de Fuca plate and the near‐stagnant Explorer plate. A gap or attenuated zone between the plates may promote upwelling of enriched asthenosphere that undergoes low‐degree decompression melting to generate alkalic basalts that are essentially free of slab input yet occur in an arc setting.

40Ar–39Ar age and geochemistry of subduction-related mafic dikes in northern Tibet, China: petrogenesis and tectonic implications

The early Permian Xiaomiao mafic dike swarm in the East Kunlun orogenic belt (EKOB) provides an excellent opportunity to study the petrogenesis of such swarms developed in supra-subduction zone environments, and to investigate the early plate tectonic history of the Palaeo-Tethyan Ocean. Hornblende 40Ar–39Ar dating results indicate that the mafic dikes formed in the early Permian (277.76 ± 2.72 Ma). The Xiaomiao mafic hypabyssals have the following compositional range: SiO2 = 46.55–55.75%, MgO = 2.80–7.38%, Mg# = 36–61, and (Na2O + K2O) = 2.87–4.95%. Chemically, they display calc-alkali affinities, ranging in composition from gabbro to gabbroic diorite. All analysed dikes are enriched in light rare earth elements and large-ion lithophile elements (e.g. Rb and Ba), but are depleted in heavy rare earth elements and high field strength elements (e.g. Nb, Ta, and Ti). Their ISr and ϵNd(t) values range from 0.707 to 0.715 and –2.60 to +2.91, respectively. They are geochemically similar to subduction-related basaltic rocks (e.g. island arc basalt), but differ from E-MORB and N-MORB. Petrographic and major element data reveal that fractional crystallizations of clinopyroxene, olivine, hornblende, and Fe–Ti oxides may have occurred during magma evolution, but that crustal contamination was minor. Based on geochemical and Sr–Nd isotopic bulk-rock compositions, we suggest that the mafic dikes were likely generated by 10–20% partial melting of a spinel + minor garnet lherzolite mantle source metasomatized by subducted, slab-derived fluids, and minor sediments. Based on our results, we propose that the early evolution of the Palaeo-Tethyan Ocean involved the spreading and initial subduction of the Carboniferous to early Permian ocean basin followed by late Permian subduction, which generated the magmatic arc.

The effects of K2O on the compositions of near-solidus melts of garnet peridotite at 3 GPa and the origin of basalts from enriched mantle

Enrichment in K2O in oceanic island basalts (OIB) is correlated with high SiO2, low CaO/Al2O3, and radiogenic isotopic signatures indicative of enriched mantle sources (EM1 and EM2). These are also chemical characteristics of the petit-spot lavas, which are highly enriched in K2O (3–4 wt%) compared to other primitive oceanic basalts. We present experimentally derived liquids with varying concentrations of K2O in equilibrium with a garnet lherzolite residue at 3 GPa to test the hypothesis that the major element characteristics of EM-type basalts are related to their enrichment in K2O. SiO2 is known to increase with K2O at pressures less than 3 GPa, but it was previously unknown if this effect was significant at the high pressures associated with partial melting at the base of the lithosphere. We find that at 3 GPa for each 1 wt% increase in the K2O content of a garnet lherzolite saturated melt, SiO2 increases by ~0.5 wt% and CaO decreases by ~0.5 wt%. MgO and $$K_{D}^{{{\text{Fe}} - {\text{Mg}}}}$$ each decrease slightly with K2O concentration, as do Na2O and Cr2O3. The effect of K2O alone is not strong enough to account for the SiO2 and CaO signatures associated with high-K2O OIB. The SiO2, CaO, and K2O concentrations of experimentally derived partial melts presented here resemble those of petit-spot lavas, but the Al2O3 concentrations from the experimental melts are greater. Partitioning of K2O between peridotite and melt suggests that petit spots, previously considered to sample ambient asthenosphere, require a source more enriched in K2O than the MORB source.

Garnet and spinel in fertile and depleted mantle: insights from thermodynamic modelling

We performed thermodynamic calculations based on model and natural peridotitic compositions at pressure and temperature conditions relevant to the Earth's upper mantle, using well-established free energy minimi- zation techniques. The model is consistent with the avail- able experimental data in Cr-bearing peridotitic systems and can therefore be used to predict phase relations and mineral compositions in a wide range of realistic mantle compositions. The generated phase diagrams for six dif- ferent bulk compositions, representative of fertile, depleted and ultra-depleted peridotitic mantle, shown that the gar- net ? spinel stability field is always broad at low temper- atures and progressively narrows with increasing temperatures. In lithospheric sections with hot geotherms (ca. 60 mW/m 2 ), garnet coexists with spinel across an interval of 10-15 km, at ca. 50-70 km depths. In colder, cratonic, lithospheric sections (e.g. along a 40 mW/m 2 geotherm), the width of the garnet-spinel transition strongly depends on bulk composition: In fertile mantle, spinel can coexist with garnet to about 120 km depth, while in an ultra-depleted harzburgitic mantle, it can be stable to over 180 km depth. The formation of chromian spinel inclusions in diamonds is restricted to pressures between 4.0 and 6.0 GPa. The modes of spinel decrease rapidly to less than 1 vol % when it coexists with garnet; hence, spinel grains can be easily overlooked during the petrographical character- ization of small mantle xenoliths. The very Cr-rich nature of many spinels from xenoliths and diamonds from cratonic settings may be simply a consequence of their low modes in high-pressure assemblages; thus, their composition does not necessarily imply an extremely refractory composition of the source rock. The model also shows that large Ca and Cr variations in lherzolitic garnets in equilibrium with spinel can be explained by variations of pressure and temperature along a continental geotherm and do not necessarily imply variations of bulk composition. The slope of the Cr# (i.e. Cr/(Cr ? Al)mol) isopleths in garnet in equilibrium with spinel changes significantly at high temperatures, posing serious limitations to the applicability of empirical geo- barometric methods calibrated on cratonic mantle xenoliths in hotter, off-craton, lithospheric mantle sections.

Indicator minerals of diamond in the lamproitic diatreme, Kostomuksha region, Karelia

The mineralogy of a new lamproitic diatreme 200–250 m in diameter and 3 ga in area is studied in detail. The chemical and 3-D mineralogical analysis identify the diatreme rocks as strongly altered olivine lamproites with a large volume (50–60%) of xenoliths of strongly altered spinel (garnet) lherzolites and harzburgites-dunites. Numerous grains-xenocrysts of indicator minerals of diamond have been extracted from the heavy concentrates (the weight of the initial product is 742 g and the size is 100–500 μm) as a result of hydroseparation: (1) subcalcium (CaOav. 2.6 wt %) high-Cr (Cr2O3 av. 5.3 wt %) pyrope (50 grains); (2) chrome diopside (7 and 8 mol % of kosmochlor and jadeite components, respectively, >40 grains); (3) high-Cr chromite (Cr2O3 > 62 wt %); and (4) picroilmenite (MgO 12–13.8 wt %) and Cr-rutile (Cr2O3 1.1 wt %). Xenocrysts prove the mantle endogene (the level of garnet lherzolites) source of the magmatic center of lamproites and forecast the diamond potential of the new diatreme in the Kostomuksha ore district.

Age, petrogenesis and tectonic setting of the Thessalon volcanic rocks, Huronian Supergroup, Canada

The ∼2.5Ga Huronian Supergroup volcanic rocks in northern Ontario represent one of the better preserved Paleoproterozoic continental flood basalt sequences on Earth. The youngest age of Huronian Supergroup volcanism is refined to 2452.5±6.2Ma based on new U–Pb zircon analyses of the Copper Cliff rhyolite. Recent mapping and geochemical analyses of Thessalon Formation volcanic rocks define a well-developed chemostratigraphy; a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt–andesite volcanic series. These two mafic volcanic series are separated by felsic volcanic rocks (rhyolite and dacite). The mafic lavas are low-Ti tholeiites, dominated by basalt and andesite, and are characterized by enriched light rare earth element profiles and high Th contents (up to 12ppm). Most basalt–andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and, in some cases, limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle. All magma batches display prominent negative Nb–Ta and Ti anomalies on primitive mantle normalized multi-element diagrams, interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism. At least three distinct mantle source compositions are required to explain the range in the observed lava compositions. The most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high Th/Yb (∼10) and low Zr/Nb (∼11). This reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward Neoarchean subduction along the southern Superior craton margin.

PT conditions and trace element variations of picroilmenites and pyropes from placers and kimberlites in the Arkhangelsk region, NW Russia

Compositions of picroilmenite and pyrope concentrates from Carboniferous sandstones in the Arkhangelsk kimberlite province were analyzed by EPMA and LAM ICP MS in Analytic Center of V.S. Sobolev’s Institute of Geology and Mineralogy, SD RAS, Novosibirsk. The results from single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) for garnet, spinel, ilmenite and clinopyroxene suggest heating of the base of the lithospheric mantle to 1400°C (45mw/m2) at 7.0–7.5GPa and to 900°C (35mw/m2) at 3.5–5.5GPa in an interval corresponding to a lens enriched in chromite and clinopyroxene. The pipes from the eastern fields reveal smoother mantle geotherms and lower temperature PT paths. Mantle columns beneath the kimberlites from northern (Verkhotinskoe field) and western pipes (Kepinskoe field) show heating from the lithosphere base to 5.0GPa and stepped PT paths shown by chromites probably due to interaction with magmas which caused local Ti-enrichment near 3.0 and 5.5GPa. The PT paths in the mantle columns beneath the alnöite pipes reveal higher temperature and relatively shallow PT conditions with two major clusters around 3.0 and 5.0GPa. Trace element patterns for garnets vary from S-type typical of harzburgites to those with a hump in MREE (middle REE) typical for pyroxenites. Lherzolitic garnets with sinusoidal decrease of LREE show distinctive HFSE enrichment. Trace element ratios (Sm/Er)n and (La/Yb)n of garnets correlate positively with pressures estimates by single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) but only poorly with Cr2O3 content. Enrichment in HFSE of all garnets is related to metasomatism that accompanied the picroilmenite-forming event.Ilmenites reveal two compositional trends. One corresponds to fractionation within conduits at the lower mantle (6.0–7.0GPa) without contamination. A second trend at <6.0GPa, formed due to assimilation fractional crystallization (AFC), is characterized by Fe and Cr increase with decreasing pressure. Similar trace element patterns of the various in HREE in ilmenites, possibly partly due to garnet assimilation from wall rock peridotites. The PT conditions and geochemistry for the minerals from the Carboniferous sediments are similar to those from the Lomonosovskoe deposit and Arkhangelskaya pipe (Lehtonen et al., 2009).

The provenance of sub-cratonic mantle beneath the Limpopo Mobile Belt (South Africa)

Petrological, whole rock major element and mineral chemical analysis of mantle xenoliths from the Venetia kimberlite pipes (533Ma) in South Africa reveals an apparently stratified cratonic mantle beneath the Central Zone of the Limpopo Mobile Belt (LMB) that separates the Kaapvaal and Zimbabwe Cratons. Combined pressure–temperature (P–T) data and petrographic observations indicate that the mantle consists of an upper layer of Low-T coarse-equant garnet+spinel lherzolite (~50 to ~130km depth). This layer is underlain by a region of mixed garnet harzburgites and garnet lherzolites that are variably deformed (~130 to ~235km depth). An equilibrated geotherm did not exist at the time of kimberlite eruption (533Ma) and a localised heating event involving the introduction of asthenospheric material to the High-T lithosphere below 130km is inferred.Low-T garnet–spinel lherzolites are highly melt depleted (40% on average). In contrast, the High-T lithosphere (mostly at diamond stable conditions) consists of a mixed zone of variably sheared and melt depleted (30% on average) garnet harzburgite and mildly melt depleted (20% on average) garnet lherzolite. The chemistry of the High-T xenoliths contrasts with that of minerals included in diamond originating from the same depth. Inclusions suggest diamond crystallisation in a more melt depleted lithosphere than represented by either Low- or High-T xenoliths. High-T xenoliths are proposed to represent formerly melt depleted lithosphere, refertilised by asthenosphere-derived melts during the diapiric rise of a proto-kimberlitic melt pocket. This process is coupled to the positive temperature perturbation observed in the High-T xenoliths and may represent a common process in the lower lithosphere related to localised but intense tectono-magmatic events immediately preceding kimberlite eruption.The presence of clinopyroxene, garnet and abundant orthopyroxene in the Low-T lherzolite implies a history of melt depletion followed by metasomatic addition of Si–Al–Ca, forming high-temperature orthopyroxene from which clinopyroxene and garnet exsolved. Si enrichment is a characteristic feature of the majority of the Kaapvaal Craton to the south of the LMB but not of the Zimbabwe Craton to the north, implying a Kaapvaal origin. The provenance of the High-T lithosphere beneath the LMB is less well constrained as it is intensely modified by kimberlitic magmatism and diamond inclusion chemistry does not show significant systematic variation across the cratons. The presence of rare, mildly silica enriched high-temperature harzburgites suggests that a Kaapvaal origin for the entire lithosphere beneath the LMB is most likely.

Generation of adakites in a cold subduction zone due to double subducting plates

Adakites have been found in various tectonic settings, since the first report for the distinct lavas as a product of slab melting in Adak Island by Kay (J Volcanol Geotherm Res 4:117–132, 1978). In this study, we present geochemical data for an ‘adakite’ and ‘adakitic rock’ suite in central Japan with a cold subduction environment due to the two overlapping subducting plates: the Pacific plate and the Philippine sea plate. Based on the major, trace and isotopic compositions of the rocks, elemental transport from initial slab inventory at the trench to the volcanic rocks as a final product is quantitatively analyzed, considering the thermal structure, slab dehydration, elemental mobility, slab-fluid migration and melting of fluid-added mantle. The analysis demonstrates a large compositional impact of slab-fluid in the arc magma generation in central Japan. The melting conditions have been also estimated inversely by optimizing the predicted magma composition to the observed composition of volcanic rock, with the two parameters: the degree of melting and the proportion of spinel and garnet lherzolites involved in melting. Consequently, a moderately low degree of near-solidus melting of dominantly garnet lherzolite with a high fluid flux from the two overlapping slabs beneath the region has been argued to be responsible for the compositional characteristics, including the adakitic signatures, of the studied rocks. These results imply that the geochemical approach may provide useful constraints on the P–T condition of melting in the mantle wedge and the thermal structure in subduction zones, being complementary to the geophysical approach.

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia, South America

AbstractGeochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO2; 4.3–5.9 wt.% Na2O+K2O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO2; 4.0–4.4 wt.% Na2O+K2O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower 87Sr/86Sr and higher 143Nd/144Nd and 206Pb/204Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.

Pressure‐ and stress‐induced fabric transition in olivine from peridotites in the Western Gneiss Region (Norway): implications for mantle seismic anisotropy

AbstractThe effects of pressure, water and differential stress on the crystallographic preferred orientation (CPO) of olivine were investigated by an integrated study on 13 peridotite samples from four areas in the Western Gneiss Region (WGR) of the Norwegian Caledonides. The highly depleted dunites are mantle residues after Archean melt extraction, while the garnet peridotites were formed by refertilization of the dunites via melt percolation in the Proterozoic. These orogenic peridotites were tectonically emplaced into Proterozoic gneisses and metasedimentary rocks of the WGR and were recrystallized under high‐pressure (HP) to ultrahigh‐pressure (UHP) conditions when the Baltic Shield was subducted beneath Laurentia during the 420–400 Ma Scandian orogeny. The CPOs of olivine, enstatite and diopside were measured using the electron backscatter diffraction technique. The weakly deformed garnet wehrlites and dunites from the HP central WGR (the Gurskøy, Almklovdalen and Tafjord areas) show the A‐type olivine fabric defined by the [100] axis subparallel to the stretching lineation and the (010) plane subparallel to the foliation, which may be a relict fabric formed at high temperature and low strain rate in the mantle wedge prior to subduction. The [001](010) fabric (B‐type) is observed in strongly sheared HP garnet peridotites and dunites from the Almklovdalen and Tafjord areas, suggesting the dominant activation of [001](010) slip system at high stress and high strain rate. Recrystallized fine olivine grains may develop a fabric different from porphyroclasts and record a gradual fabric transition. The olivine CPO in garnet lherzolites from Otrøy in the northernmost WGR is characterized by the concentration of [001] axes parallel to the stretching lineation and [100] axes normal to the foliation, indicating a prevailing [001](100) slip (C‐type fabric) at P–T conditions of &gt;6 GPa and 850–950 °C. Despite very different deformation microstructures, Fourier transform infrared analysis reveals very low water contents (&lt;13 ppm H2O by the Paterson calibration) in olivine for both coarse porphyroclasts and recrystallized small grains, which is consistent with the high‐degree partial melting of the peridotites during the Archean. Therefore, a combination of UHP and low temperature plays a more important role than water in promoting the C‐type olivine fabric in a dry, cold and deeply subducted continental slab. With increasing subduction depths, the C‐type olivine fabric will predominate over the A‐type fabric and make the maximum P‐wave velocity and shear wave splitting of peridotites normal to the foliation. However, the stress‐induced fabric transition in olivine from the A‐type to the B‐type may be localized in shear zones and not distinguishable in seismic anisotropy observations.

Petrogenetic evolution of basaltic lavas from Balhaf–Bir Ali Plio-Quaternary volcanic field, Arabian Sea, Republic of Yemen

The Plio-Quaternary Balhaf–Bir Ali volcanic field (BBAVF) constitutes one of the largest volcanic fields in SE Yemen, covering some 500 km2. It comprises cinder cones complexes associated with vesicular lava flows and scoria–spatter cones. In many places, ultramafic xenoliths are encountered within these volcanics. The explosive volcanism is mainly of alkaline character including alkali olivine basalt, hawaiite and mugearite together with subordinate tuffaceous trachytes. Major, trace and REEs data from the basaltic rocks of the BBAVF are interpreted in terms of a mantle-lithospheric origin in which crustal contribution during the initial stage of rift magmatism has occurred. Magma genesis may have resulted from plume-derived melt introducing into the base of the lithosphere. A mantle plume source is proposed for the Balhaf–Bir Ali basaltic lavas that are here interpreted as having been generated by partial melting of garnet lherzolite in the uppermost part of asthenosphere. The magmatic evolution of Balhaf–Bir Ali volcanic field may be accounted for by the recent models developed for plume structure.

Trace element composition of parental magmas from mafic–ultramafic cumulates determined by in situ mineral analyses: The Juquiá mafic–ultramafic alkaline–carbonatite massif, SE Brazil

Major and trace element analyses of relict cores of cumulus minerals (olivine and clinopyroxene) from primitive rocks of the Juquiá mafic–ultramafic alkaline–carbonatite complex (Early Cretaceous) in the Ponta Grossa Arch Alkaline Province, southeastern Brazil, were used to calculate the equilibrium melt compositions. Olivine relict cores are compositionally restricted to the Fo83–74 interval, and they exhibit significant concentrations of Mn (2220–3001 ppm), Ni (1188–2327 ppm), Ca (175–649 ppm), Co (169–216 ppm), Zn (115–215 ppm), Ti (55–305 ppm), Cr (4–320 ppm) and P (31–154 ppm). Clinopyroxene relict cores are essentially diopside, with minor amounts of hedenbergite and tschermakite components. The trace element contents found in clinopyroxene cores are Ni (196–339 ppm), V (99–318 ppm), Sr (110–260 ppm), Zr (12–163 ppm), Sc (50–78 ppm), P (16–118 ppm), Zn (16–48 ppm) and Co (25–43 ppm). The rare earth element (REE) concentrations are variable (28–240 ppm), with the lighter elements present in the greatest abundance, particularly Ce (up to 83 ppm).The melt compositions in equilibrium with olivine and clinopyroxene cores from different samples suggest that distinct basanite magma batches have played a role in the formation of the Juquiá cumulate rocks. These calculated liquids have compositions that are quite similar to those of the nearby lamprophyre dikes. The CaO/Al2O3 values (>0.8) from the lamprophyre dikes and the high La/Zr and low Ti/Eu values from the calculated liquids point to a previous carbonatite metasomatic event in the magmatic source that lead to the formation of wehrlite veins in a peridotitic mantle. The variation in the Hf/Y ratio implies differences in cpx/gt modal proportions in the source. Non-modal mantle batch-melting models indicate that a homogeneous source with little variation in the degree of partial melting cannot explain the trace element differences among the calculated liquids. The models suggest that the geochemical differences represent (1) mixing between metasomatic vein partial melt and garnet lherzolite partial melt and (2) clinopyroxene/garnet modal ratios of the residual mantle.