Primitive Mantle Composition Research Articles

Geochemistry of mafic-ultramafic rocks of the 3.33 Ga Kromberg type-section, Barberton greenstone belt, South Africa: Implications for early Earth geodynamic processes

In conjunction with major, trace and rare earth element data, this study presents the first whole-rock LuHf isotope analyses of mafic-ultramafic rocks of the c. 3.3 Ga Kromberg type-section in the Barberton greenstone belt of South Africa. Three compositionally distinct volcanic rock types are identified namely Group 1 metabasalts, Group 2 metabasalts and komatiitic metabasalts. Group 1 metabasalts are FeMg tholeiitic with compositions that plot close to the Primitive Mantle composition, have isochron intercept εHf(t) value of 0.01 ± 0.4 and Eoarchean Hf model ages. Group 2 metabasalts have very high Al2O3/TiO2 ratios (c. 50), small negative Nb anomalies, LREE-depleted slightly concave up REE patterns, and negative εHf(t) values. The komatiitic basalts variably record small negative Nb anomalies in a primitive mantle normalized plot. Group 1 metabasalts and cumulate peridotites (wehrlites) yield a LuHf isochron age of 3.340 ± 0.190 Ga (95% confidence level). The highly variable major and trace element geochemistry of the rocks in the sequence implies highly dynamic and variable mantle melting environments. The Group 2 metabasalts have geochemical characteristics similar to that of boninites and are interpreted to have formed from a shallow, refractory mantle source. Group 2 metabasalts are argued to have formed in a proto-plate tectonic back-arc basin involving crustal contamination, whereas Group 1 metabasalts likely formed from within a deep mantle plume.

Effects of phosphorus on partial melting of the Martian Mantle and compositions of the Martian Crust

Phosphorus is estimated to be ten times more enriched in the martian mantle compared to the terrestrial mantle. Yet, its effects on primary melt composition and melting phase relations in martian systems is unknown. We performed piston-cylinder experiments at a constant upper mantle pressure of 2 GPa and temperatures of 1210–1450 °C using a model martian primitive mantle composition with P2O5 content of 0 and 0.5 wt.%. All experiments produced an assemblage of olivine + orthopyroxene + melt ± pigeonite ± apatite ± spinel. Our experimental results, in combination with a previous study at similar P-T conditions and major element bulk composition but containing 0.2 wt.% bulk P2O5, show that the addition of phosphorus dramatically increases the abundance of more polymerized residual mineral such as orthopyroxene while decreasing the proportion of less polymerized residual phases such as olivine, especially at low extent of melting (∼11 wt.%). Such effects lead to lower SiO2 concentrations in the near-solidus melt by up to 10 wt.% for mantle P2O5 of 0.2 and 0.5 wt.%. Increasing bulk P2O5 to 0.5 wt.% also leads to slightly elevated CaO/Al2O3 ratio and increased FeO* concentration in mantle-derived melts with the latter likely due to formation of Fe-O-P complexes in the liquid. Our study suggests that elevated phosphorus in the martian mantle has important consequences regarding the composition and mineralogy of the crust, partly made with primary melts, and of the upper mantle. Because of elevated P, variably melt-depleted upper mantle of Mars is likely to be richer in orthopyroxene compared to the terrestrial mantle and the elevated P content is partly responsible for several geochemical attributes of martian basalts compared to those on Earth. Extrapolating our experimental results to a range of pressures, we suggest a depletion of P in the mantle through time, which likely contributed to major elemental compositional differences between ancient Gusev and Gale crater basalts and more recent martian meteorites.

Olivine as an indicator of polygenic assemblage of inclusions from Late Cenozoic volcanic rocks in the Tunka Valley, Baikal Rift Zone

Research subject. Olivine of deep-seated inclusions from Late Cenozoic volcanic rocks of the Tunka valley.Materials and methods. Electron probe microanalysis of minerals with determination of calcium in olivine by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A complex of methods of quantitative chemical analysis for determination of major oxides in rocks.Results. Deep-seated nodules from Late Cenozoic volcanic rocks in the Tunka Valley characterize the root part of the Slyudyanka metamorphic subterrane, formed during collisional accretion of the Khamar-Daban terrane to the Siberian paleocontinent in the Early Paleozoic and reactivated in the Late Cenozoic. From bulk compositions of deep-seated nodules, groups of xenoliths (restites, rocks closed to the primitive mantle composition, and metasomatites) and cognate rocks (magmatic and magmatic-metasomatic genesis) were distinguished. The polygenetic character of this assemblage is designated by variations of major and trace-element abundances in olivine. In olivine of xenoliths, forsterite (Fo) ranges from 86 to 91% under abundances of NiO – 0.2–0.5, MnO – 0.1–0.2, and CaO < 0.16 wt %. In olivine of magmatic rocks and those of mixed (magmatic-metasomatic) genesis, Fo decreases to 64% with a decrease in NiO (Ni/Mg ratio) and an increase in MnO and CaO. Magmatic olivine shows 0.16–0.21 wt % MnO and about 0.1 wt % CaO, in contrast to olivine of magmatic-metasomatic genesis, enriched in these oxides to 0.5 wt % and 0.45 wt %, respectively, with an increase in the Mn/Fe and Ca/Fe ratios. Olivine trends of the shallow polygenetic assemblage of inclusions are emphasized when compared with those of the deeper ones from volcanic rocks of the Vitim and Oka plateaus that represent a transition from the garnet to the spinel-pyroxene facies in mantle peridotites.Conclusion. Olivine compositions are indicative of the shallow origin of peridotite and pyroxenite nodules from the crust and crust–mantle transition in the root part of the Slyudyanka collision zone reactivated in the Late Cenozoic.

Constraints on the age and geodynamic setting of the iron formations and anhydrite Fe-(Ba) deposits in the Bulunkuole Group of the Taxkorgan area, NW China

• Bulunkuole Group (BG) and associated mineral deposits is both early Cambrian in age. • Volcanic-sedimentary rocks in the BG reveal a back-arc basin depositional setting. • Iron formations deposited in newborn back-arc basin from redox-stratified water column. • Anhydrite Fe-(Ba) deposits formed in restricted sulfate-rich water during back-arc basin collapse. Here we present detailed field observations, zircon U-Pb-Hf isotopes, whole-rock geochemical, and Sm-Nd isotopic data for the volcanic-sedimentary rocks within the Bulunkuole Group (BG) in the Taxkorgan area, northwestern China in order to reveal significant information on the formation age and tectonic setting of the iron formations (IFs) and anhydrite Fe-(Ba) deposits hosted by this group. Exposed rocks in the BG are broadly grouped into three subgroups: the lowermost Subgroup 1 is a bimodal volcanic suit (basalts and dacites) interbedded with IF layers; the overlying Subgroup 2 is a clastic sedimentary sequence hosting abundant anhydrite Fe-(Ba) mineralization; and the uppermost Subgroup 3 is dominated by carbonate rocks. Zircon U-Pb ages provide constraints on the formation age of the BG. Specifically, the bimodal volcanism occurred at ~ 540–521 Ma and the sedimentary rocks deposited after 540 Ma. Basalts from the BG bimodal suit are characterized by slightly enriched LREE patterns and pronounced to insignificant negative Nb anomalies when normalized to primitive mantle compositions. These features are comparable with those of modern N-MORB and back-arc basin basalts. Combined with highly heterogeneous Nd isotopes ( ε Nd (t) = 0.2–4.4), these suggest that their magmatic precursors were derived from partial melting of a depleted mantle modified by slab-derived fluids, and interacted with minor crustal components during ascending. By comparison, the BG dacites display gently sloped to flat LREE patterns with a FII-type felsic affinity and a range of negative to positive ε Nd (t) values (-5.8~+0.7). They are inferred to be generated by partial melting of the old continental crust at relatively shallow levels with variable addition of juvenile material. The BG clastic sediments were dominated by greywacke and arkose. Their diagnostic geochemical features, detrital zircon age spectra, and in situ zircon Hf isotopic data indicate that they were mainly derived from weathering of the early Cambrian felsic rocks. Together, these characteristics above, integrated with previous studies, suggest that the BG and related IFs and anhydrite Fe-(Ba) deposits were formed on an active continental margin (possibly back-arc basin) during the southward subduction of the Proto-Tethys Ocean beneath the Taxkorgan-Tianshuihai Terrane.

Constraining Ancient Magmatic Evolution on Mars Using Crystal Chemistry of Detrital Igneous Minerals in the Sedimentary Bradbury Group, Gale Crater, Mars

AbstractUnderstanding magmatic processes is critical to understanding Mars as a system, but Curiosity's investigation of dominantly sedimentary rocks has made it difficult to constrain igneous processes. Igneous classification of float rocks is challenging because of the following: (1) the possibility that they have been affected by sedimentary processes or weathering, and (2) grain size heterogeneity in the observed rock textures makes the small‐scale compositions measured by rover instruments unreliable for bulk classification. We avoid these ambiguities by using detrital igneous mineral chemistry to constrain models of magmatic processes in the source region for the fluvio‐deltaic Bradbury group. Mineral chemistry is obtained from X‐ray diffraction of three collected samples and a new stoichiometric and visual filtering of ~5,000 laser induced breakdown spectroscopy (LIBS) spots to identify compositions of individual igneous minerals. Observed mineral chemistries are compared to those produced by MELTS thermodynamic modeling to constrain possible magmatic conditions. Fractionation of two starting primary melts derived from different extent of adiabatic decompression melting of a primitive mantle composition could result in the crystallization of all minerals observed. Crystal fractionation of a subalkaline and an alkaline magma is required to form the observed minerals. These results are consistent with the collection of alkaline and subalkaline rocks from Gale as well as clasts from the Martian meteorite Northwest Africa 7034 and paired stones. This new method for constraining magmatic processes will be of significant interest for the Mars 2020 mission, which will also investigate an ancient volcaniclastic‐sedimentary environment and will include a LIBS instrument.

Petrogenesis of the mafic-ultramafic Canindé layered intrusion, Sergipano Belt, Brazil: Constraints on the metallogenesis of the associated Fe–Ti oxide ores

The Canindé layered intrusion is hosted by a Neoproterozoic volcano-sedimentary sequence and contains Fe–Ti mineralization in the Canindé Domain of the Brazilian Sergipano Belt. The petrochemistry of this layered complex and the metallogenesis of the associated Fe–Tioxide mineralization are still enigmatic. The intrusion comprises three main outcropping rock units from north to south: Unit I contains melanotroctolite and olivine gabbro/gabbronorite, Unit II consists of gabbro and (leuco)troctolite, and the Fe–Timineralized Unit III contains leucogabbro with stringers or lenses of pegmatitic gabbro, apatite-bearing gabbro, hornblende-bearing gabbro, Fe-diorite, oxide gabbro, gabbro with Fe–Ticumulate layers, and magnetitite. The mineralized Unit III contains low-Cr magnetite and ilmenite ± hercynite occurring disseminated and as massive mineralization in layers and lenses within the leucogabbro, close to the contact aureoles with the surrounding calc-silicate and amphibolite rocks. LA-ICP-MS U-Pb zircon dating of a gabbro sample from Unit II yields an age of 703.5 ± 1.6 Ma. The age-corrected ƐNd (t = 703 Ma) in the Canindé intrusion range from + 1.3 to + 4.3. Together with the depleted 87Sr/86Srt ratios of 0.7032–0.7040, these data strongly indicate a relatively depleted mantle source, with limited assimilation of crustal material. The low Sr-values may indicate a contribution from the subcontinental lithospheric mantle (SCLM) and the observed isotopic heterogeneity can be accounted for by replenishment by distinct magma pulses and/or minor but varying crustal assimilation. Except for the magnetitites, all Canindé cumulates are enriched in LILE (Rb, K, Sr, Ba, and Pb). Their Nb–Ta and Zr–Hf depletions relative to the primitive mantle composition (Nb/Nb* 0.4–0.6; Zr/Zr* of 0.6–0.8) reflect their relative incompatibility in the early cumulates, and are at least partly compensated by Nb- and Zr-enriched magnetitites. Based on the observed petrochemical characteristics of the cumulates and comparison with similar layered intrusives, the inferred parental magma is similar to a hydrous tholeiitic basalt (Mg# 50–58) with intermediate Ti-content (1–2 wt% TiO2). Our petrogeochemical data further indicate that the Canindé rocks are predominantly related through cumulus-forming processes, with some assimilation in the border zones. Apatite saturation preceded magnetitite formation and was concentrated in apatite gabbro cumulates with ≤ 6 wt% MgO. The resulting phosphor-loss would have lowered Fe-solubility in the remaining liquid, and may thus have triggered formation of massive magnetitite Fe–Ti ore bodies at the top of the intrusion. Finally, no economic Ni–Cu resources were identified in the intermediate portions and drill core data are still needed to assess the potential for Cr–PGE resources in the unexposed lower portions.

Age and compositional evolution of Sahand volcano in the context of post-collisional magmatism in northwestern Iran: Evidence for time-transgressive magmatism away from the collisional suture

Sahand volcano along with the two neighbouring volcanic systems of Saray and Sabalan are three Late Miocene-Quaternary volcanoes that formed in the wake of the collision between the Arabian and Eurasian continental plates along the Neo-Tethyan suture zone in northwestern Iran. Sahand volcano is an isolated, extensive (3707 m elevation; >3000 km2 area) stratovolcanic complex, which is located ~165 km north of the Neo-Tethyan suture zone and was constructed over ~45 km thick continental crust. Andesitic to dacitic pyroclastic deposits, ignimbrites, and lava flows form the now strongly eroded Paleo- and Meso-Sahand edifice. Paleo-Sahand rocks are mostly restricted to the central part the edifice, where they underlie Meso-Sahand deposits. By contrast, Neo-Sahand volcanic rocks intruded older volcanic successions within and outside a largely eroded caldera margin, which likely formed during the Meso-Sahand stage. Neo-Sahand units comprise several basaltic andesitic to rhyolitic domes in the center of the complex as well as small parasitic cones along with subvolcanic dikes. High spatial resolution and sensitivity U-Pb geochronology using secondary ionization mass spectrometry (SIMS) define the formation of Paleo-, Meso-, and Neo-Sahand to ca. 8 to 3 Ma, ca. 1 Ma, and ca. 600 to <173 ka, respectively, with dating mostly focusing on Neo-Sahand. Antecrystic zircon crystals in Sahand volcanic deposits also indicate that post-collisional magmatism initiated locally at about the same time or slightly earlier than at Saray volcano to the west of Sahand. The initiation of volcanic activity subsequently migrated to the northeast over a distance of ~300 km away from the Neo-Tethyan suture, where activity started at Sabalan volcano at ca. 4.5 to 2.9 Ma. This time corresponds to a magmatic lull of Sahand before the Meso-Sahand eruptive activity resumed at ca. 1 Ma. Subsequently, Neo-Sahand and Neo-Sabalan erupted synchronously throughout the middle and possibly into the late Pleistocene. Paleo- and Neo-Sahand volcanic rocks are characterized by enrichments of large-ion lithophile (LILE) and light rare earth elements (LREE) relative to high-field strength and heavy rare earth elements (HREE), respectively, and prominent negative Ti, Nb, and Ta anomalies relative to primitive mantle compositions. The least evolved magmas at Sahand have formed by 1–5% partial melting of garnet- and spinel-peridotite and were subsequently overprinted through assimilation – fractional crystallization (AFC) processes acting in the continental crust. Although Sahand and Sabalan volcanic rocks share many geochemical characteristics indicative of a continental subduction setting, Sahand is less potassic and less enriched in incompatible trace elements compared to Sabalan. Another difference is that Sahand primitive magmas were formed by smaller degrees of partial melting in the mantle than is the case for Sabalan. Both effects likely result from variable distances from the suture zone and crustal thickening away from the suture towards the northeast. Overall, the apparent age progression and geochemical variation of the post-collisional volcanism, lack of asthenospheric mantle components, and the subduction-affinity of Sahand and Sabalan volcanic rocks supports a scenario where mantle melting was triggered by slab roll back and break-off shortly after continental collision, followed by migration of the slab edge towards the northeast at plate-like velocities.

A Gondwana pre-break up related magmatism in Bafoussam area, west Cameroon: source characteristics and 40Ar/39Ar geochronology

The Gondwana pre-break up related formations in Bafoussam area are transitional calc-alkaline doleritic dykes exhibiting high Alumina and low Ti-Mg contents. Their REE compositions are similar to those of E-MORB (Nd/Nb≈1, Zr/Nb≤20). A high partial melting of about 20% of Garnet peridotites source having primitive mantle composition, is inferred to the studied rocks. They are slightly evolved 50&lt;Mg#&lt;54, and its clinopyroxenes are augite of high temperature (600°C-1100°C). The studied dykes were emplaced in a within-plate tectonic setting and yield a 40Ar-39Ar plateau ages of 229±7Ma. These ages are slightly different from those of dolerites from the Oban-Obudu massif (Nigeria), from the basaltic dyke of the Cameroon Volcanic Line and from the Karoo-Ferrar mafic magmatism, all interpreted as magmatism that preceded the break-up of Gondwana. The emplacement age of 229±7Ma of dolerites from Bafoussam area relates them to the very early stage of Gondwana pre-break up magmatism in central Africa.

Petrogenesis and tectonic setting of the Tuyeh-Darvar Granitoid (Northern Iran): Constraints from zircon U-Pb geochronology and Sr-Nd isotope geochemistry

Tuyeh-Darvar granitoid, which outcrop ca 45 km Sw of Damghan city, in the Eastern Alborz zone, comprise mainly the pluton emplaced into the Barut Formation of Lower Cambrian ages. Zircon U-Pb ages show Carboniferous ages (325 ± 3 Ma) for the formation of this granitoid. The granitoid is mostly metaluminous, ferroan and alkalic monzonite to monzodiorite. "These rocks have high values of FeOT/MgO and Ga/Al, high concentrations of K2O+Na2O, low abundances of MgO and transitional elements. Plots normalized to chondrite and primitive mantle compositions show strong enrichments of LREE relative to HREE and of LILE relative to HFSE, accompannied by negative anomalies of Nb and Sr."They contain Fe-rich hydrous mafic minerals and magnetite. These features are typical of A-type granites. Sr-Nd isotopic geochemistry, with initial ɛNd values from −1.1 to −1.5 and initial 87Sr/86Sr ratios between 0.70562 and 0.70678, are consistent with magmatic differentiation from mafic melts produced from an enriched mantle source. However, other models such as melting of mafic crust or mixing of components from depleted mantle and continental crust cannot be discarded. On the basis of the U-Pb zircon age (325 ± 3 Ma) and the known magmatic tectonic regime in Iran during the Paleozoic, it is suggested that the pluton, formed in a rift environment related to extensional structures of the Alborz block in Early Carboniferous time.

Garnet‐controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

AbstractDeep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower–upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660 km depth. We use subsequent forward modelling of mineral assemblages, seismic velocities, and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature‐dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite‐rich assemblages at undisturbed thermal conditions. Neglecting this ultralow‐velocity zone at upwelling sites can cause a miscalculation of the lower–upper mantle boundary on the order of 20 km.

Age of Plutonic Rocks from the Vema Fracture Zone (Central Atlantic) and Nature of Their Mantle Sources

This study presents the results of petrographic, geochemical, and isotope geochronological analyses of rock samples from the Southern flank of the Vema transform fault (Atlantic), which were dredged on cruises 19-th and 22-nd of the R/V Akademik Nikolai Strakhov. The sample suite includes both fresh and metamorphosed gabbros, dolerites, serpentinites, metapyroxenites. Zircons separated from three gabbro samples recovered at three different stations were used for in situ U–Pb dating by LA-ICP-MS. The ages reveal a strong linear relationship with a distance from the axis of the Mid-Atlantic ridge, which allowed us to estimate the rate of spreading in this segment of the Mid-Atlantic Ridge. It can be concluded that the estimated spreading rate of 16.2 ± 0.8 mm/yr was constant over the past 15 Myr. The mutual consistency of all U-Pb zircon and 39Ar–40Ar amphibole ages (Cipriani et al., 2009) obtained from the sampled transect suggests the temporal continuity of magmatic events that led to the formation of the original gabbroic rocks and their transformation during subsequent metamorphism. Rb—Sr isotope data show that hydrothermal activity took place in the presence of seawater between 14.7 and 9 Ma in the spreading axis region. Variations in the Nd isotopic composition in the time sequence of magmatic events indicate a high degree of chemical and isotopic heterogeneity of the ascending mantle material which became later entrained in the melting region beneath a spreading zone. Melting of the sources with primitive mantle composition (eNd ~ + 8 to +9) as well as enriched sources took place in the time interval between ~ 17 and 14.7 Ma and at about 8 Ma. The enriched source material is most likely represented by ancient mafic substratum.

Petrology of Jurassic and Cretaceous basaltic formations from the Parnaíba Basin, NE Brazil: correlations and associations with large igneous provinces

Abstract The basaltic Mosquito and Sardinha formations in the Parnaíba Basin are related to the opening of the Atlantic Ocean at the Triassic–Jurassic boundary and in the early Cretaceous, respectively. The Mosquito Formation consists of tholeiitic flows with both high-Ti (&gt;1.5 wt%) and low-Ti (&lt;1.5 wt%) compositions and the petrogenetic characteristics of enriched mantle reservoirs. The Mosquito Formation basalts have an initial 87 Sr/ 86 Sr isotopic composition of 0.70296–0.70841 and a low Nd isotopic composition (0.512245–0.512677) associated with an enrichment in large ion lithophile and high field strength elements relative to primitive mantle compositions. The Sardinha Formation is composed of high-Ti and low-Ti tholeiitic dykes with subordinate alkali basalts. The Sardinha Formation rocks have trace element and isotopic features associated with enriched mantle end-members. The initial isotopic compositions range from 0.702859 to 0.706703 and 0.512184 to 0.512671 for 87 Sr/ 86 Sr and 143 Nd/ 144 Nd, respectively. The concentrations of large ion lithophile and high field strength elements are elevated relative to primitive mantle values. Although the Mosquito and Sardinha formations share some similarities, they can be differentiated by their unique petrographic characteristics and trace element concentrations. These differences allow the discrimination of the basaltic magmatism in the Parnaíba Basin and their association with large igneous provinces, such as the Central Atlantic Magmatic Province or the Paraná–Etendeka Magmatic Province. Supplementary material: Microprobe analyses for clinopyroxene (Table A1), plagioclase (Table A2) and olivine (Table A3) for the Mosquito and Sardinha formations, along with mixing calculation parameters for the geochemical model (Table A4) are available at: https://doi.org/10.6084/m9.figshare.c.3985437

Zinc isotope composition of the Earth and its behaviour during planetary accretion

The terrestrial planets are depleted in volatile elements with respect to chondritic meteorites, their possible building blocks. However, the timing, extent and origin of volatile depletion is debated. Zinc is a moderately volatile element (MVE), whose stable isotopic composition can distinguish when and where this depletion took place. Here, we report data for 40 ultramafic rocks comprising pristine upper mantle peridotites from the Balmuccia orogenic lherzolite massif and Archean komatiites that together define the Zn isotope composition of the Earth's primitive mantle. Peridotites and komatiites are shown to have indistinguishable Zn isotopic compositions of δ66Zn=+0.16±0.06‰ (2SD), (with δ66Zn the per mille deviation of 66Zn/64Zn from the JMC-Lyon standard), implying a constant Zn isotope composition for the silicate Earth since 3.5Ga. After accounting for Zn sequestration during core formation, the Earth falls on the volatile-depleted end of a carbonaceous chondrite array in δ66Zn-Zn/Mg space, implying Earth avoided modification of its MVE budgets during late accretion (e.g. during a giant impact), in contrast to the Moon. The Moon deviates from the chondritic array in a manner consistent with evaporative loss of Zn, where its δ66Zn co-varies with Mn/Na, implying post-nebular volatile loss is more pronounced on smaller bodies. Should the giant impact deliver the Earth's volatile complement of Pb and Ag, it cannot account for the budget of lithophile MVEs (e.g. Zn, Rb, Mn), whose abundances reflect those of Earth's nebular building blocks. The Earth initially accreted from material that experienced chemical- and mass-dependent isotopic fractionation akin to carbonaceous chondrites, though volatile depletion was more pronounced on Earth.

Origin of dyke swarms in Wadi El Redi-Wadi Lahami area, southern Eastern Desert of Egypt

Dykes predominate within the Neoproterozoic rocks, especially granites, of Wadi El Redi-Wadi Lahami area in the southern Eastern Desert of Egypt. The dyke swarms form three major suites: from the oldest to the youngest, they are basaltic andesite—Suite 1 (E-W and ENE-WSW), rhyolite—Suite 2 (NE-SW), and andesite—Suite 3 (NNE-SSW, NNW-SSE, and NW-SE). Despite the wide ranges of the dyke compositions, the feldspar and amphibole are usually the essential forming minerals. The plagioclase arrays between Ab0.9An99.10 in the basaltic andesite and Ab98.80An0.70 in the rhyolite, while sanidine ranges from Or44.60Ab49.70 to Or98.40Ab1.60. Amphibole in Suite 1 and 3 (Al2O3, TiO2, Na2O, and K2O are the lowest and those of SiO2 and CaO are the highest) samples are usually magnesio-hornblende, whereas it is edenite and tschermakite in Suite 2 dykes. Despite all parent magmas have calc-alkaline affinity, some elements such as Ni show an erratic behavior against the progressing differentiation from one magma chamber and implying for an assimilation of the country rocks. The high contents of amphibole, the depletion in Ti, and the enrichment in large-ion lithophile elements (such as K, Rb, Ba, Sr, and Ba) compared to the primitive mantle composition are consistent with parent hydrous melts generated due to extension above the subduction zone. The estimated compositions of liquids in equilibrium with amphiboles and the pressures at which they crystallized (4.61–7.8 kbar for the Suite 2 and 1.5–2 kbar for the Suites 1 and 3) are greatly varied. These are indications for a difference in the source regions of the parent magmas of the studied dykes. It is supposed that the Suite 1 and 2 dykes are a conjugate set emplaced due to the NW-SE crustal extension in the Arabian-Nubian shield, whereas the Suite 3 dykes generated due to the rifting along the Red Sea.

Volcanological and petrogenetic characteristics of komatiites of the 3.3 Ga Saw Mill Complex, Weltevreden Formation, Barberton Greenstone Belt, South Africa

Abstract The Saw Mill Complex (SMC) is a 1 275 m thick layered komatiitic sequence in the 3.3 Ga Weltevreden Formation, uppermost stratigraphic unit of the Onverwacht Group in the northern part of the Barberton Greenstone Belt, South Africa. A series of ultramafic complexes in the Weltevreden Formation have previously been interpreted as layered ultramafic intrusions, consisting of thick, layered ultramafic units of peridotite, pyroxenite, dunite, and gabbro. However, recent work on the Pioneer complex of the Weltevreden Formation has demonstrated an extrusive origin of komatiitic flows and tuffs. The Weltevreden Formation has not been studied in the detail of other Onverwacht Group units, but it is likely composed of a number of individual tectonically juxtaposed terrains brought in by thrust faulting during deposition of Fig Tree Group sediments and felsic volcanics in a magmatic arc setting post-dating the Onverwacht mafic and ultramafic units, which are currently regarded as products of plume-based eruptive centers. This study represents an ongoing effort to elucidate the structure, stratigraphy, and petrogenesis of the Weltevreden Formation. A new 1:1000 scale map and stratigraphic section of the SMC reinterpret the igneous complex as extrusive, with lithologic units interpreted as layered komatiitic flows and interbedded tuffs. Flows accumulated olivine, forming distinct layers from bottom to top of: olivine adcumulate (dunitic lithologies),poikilitic orthopyroxene with abundant olivine inclusions (pyroxenitic lithologies), andolivine orthocumulate (peridotitic lithologies), and sometimes with a cap ofolivine-depleted pyroxene-rich orthocumulates (komatiitic basalts). The chemical variation of flows is almost entirely controlled by olivine accumulation, with dunitic komatiites representing up to 70% accumulated olivine and 30% trapped liquid, and peridotitic komatiites representing only 25% accumulated olivine, while spinifex-textured komatiites are representative of quenched liquid. Minor (up to 1%) chromite accumulation partly affects bulk chemistry, while orthopyroxene accumulation may have an effect on bulk chemistry of SMC pyroxentitic komatiites. Layered flows are up to 120 m thick and some display spinifex-textured chilled upper margins. The original liquid was near primitive mantle composition that likely represents a two-stage melting process. The SMC was rapidly emplaced and was not near a major sedimentary depositional system, as no sediment other than tuff was deposited. Flows preserved in the SMC dominantly formed as open flow pathways, allowing for large quantities of olivine to accumulate in the lower portions of flows. Some flows formed as closed systems, possibly as a result of deforming tuffs below the flows to form lava lakes. Large cm-sized lapilli, including those with aerodynamic shapes, suggest subaerial explosions and possibly near-vent environments (though littoral rootless vents can develop 100’s of km’s from actual volcanic vents). Pillow structures have not been observed in the SMC. Geochemically, the SMC is very similar to Weltevreden Formation komatiites recently studied in the Pioneer complex. They have Al2O3/TiO2 near 30, Gd/Lu normalized values between 0.8 and 1.2, maximum liquid compositions of approximately 33% MgO, and olivines up to Fo94. Oxygen fugacity of the mantle source is most likely to have change in log(fO2) of 0.00 relative to nickel-nickel-oxide buffer. SMC komatiites likely erupted at temperatures in excess of 1615°C based on MELTS modeling, and represent some of the hottest volcanism experienced on the early Earth.

186Os–187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Os-rich alloys

Abyssal peridotites are oceanic mantle fragments that were recently processed through ridges and represent residues of both modern and ancient melting. To constrain the nature and timing of melt depletion processes, and the composition of the mantle, we report high-precision Os isotope data for abyssal peridotites from three ocean basins, as well as for Os-rich alloys, primarily from Mesozoic ophiolites. These data are complemented by whole-rock highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), trace- and major-element abundances for the abyssal peridotites, which are from the Southwest Indian (SWIR), Central Indian (CIR), Mid-Atlantic (MAR) and Gakkel Ridges. The results reveal a limited role for melt refertilization or secondary alteration processes in modifying abyssal peridotite HSE compositions. The abyssal peridotites examined have experienced variable melt depletion (2% to >16%), which occurred >0.5Ga ago for some samples. Abyssal peridotites typically exhibit low Pd/Ir and, combined with high-degrees of estimated total melt extraction, imply that they were relatively refractory residues prior to incorporation into their present ridge setting. Recent partial melting processes and mid-ocean ridge basalt (MORB) generation therefore played a limited role in the chemical evolution of their precursor mantle domains. The results confirm that many abyssal peridotites are not simple residues of recent MORB source melting, having a more complex and long-lived depletion history.Peridotites from the Gakkel Ridge, SWIR, CIR and MAR indicate that the depleted MORB mantle has 186Os/188Os of 0.1198356±21 (2SD). The Phanerozoic Os-rich alloys yield an average 186Os/188Os within uncertainty of abyssal peridotites (0.1198361±20). Melt depletion trends defined between Os isotopes and melt extraction indices (e.g., Al2O3) allow an estimate of the primitive mantle (PM) composition, using only abyssal peridotites. This yields 187Os/188Os (0.1292±25), and 186Os/188Os of 0.1198388±29, both of which are within uncertainty of previous primitive mantle estimates. The 186Os/188Os composition of the PM is less radiogenic than for some plume-related lavas, with the latter requiring sources with high long-term time-integrated Pt/Os. Estimates of primitive mantle HSE concentrations using abyssal peridotites define chondritic Pd/Ir, which differs from previous supra-chondritic estimates for Pd/Ir based on peridotites from a range of tectonic settings. By contrast, estimates of PM yield supra-chondritic Ru/Ir. The cause of enhanced Ru in the mantle remains enigmatic, but may reflect variable partitioning behavior of Ru at high pressure and temperature.

A primitive mantle source for the Neoarchean mafic rocks from the Tanzania Craton

Mafic rocks comprising tholeiitic pillow basalt, dolerite and minor gabbro form the basal stratigraphic unit in the ca. 2.8 to 2.6 Ga Geita Greenstone Belt situated in the NW Tanzania Craton. They outcrop mainly along the southern margin of the belt, and are at least 50 million years older than the supracrustal assemblages against which they have been juxtaposed. Geochemical analyses indicate that parts of the assemblage approach high Mg-tholeiite (more than 8 wt.% MgO). This suite of samples has a restricted compositional range suggesting derivation from a chemically homogenous reservoir. Trace element modeling suggests that the mafic rocks were derived by partial melting within the spinel peridotite field from a source rock with a primitive mantle composition. That is, trace elements maintain primitive mantle ratios (Zr/Hf = 32–35, Ti/Zr = 107–147), producing flat REE and HFSE profiles [(La/Yb)pm = 0.9–1.3], with abundances of 3–10 times primitive mantle and with minor negative anomalies of Nb [(Nb/La)pm = 0.6–0.8] and Th [(Th/La)pm = 0.6–0.9]. Initial isotope compositions (ɛNd) range from 1.6 to 2.9 at 2.8 Ga and plot below the depleted mantle line suggesting derivation from a more enriched source compared to present day MORB mantle. The trace element composition and Nd isotopic ratios are similar to the mafic rocks outcropping ∼50 km south. The mafic rocks outcropping in the Geita area were erupted through oceanic crust over a short time period, between ∼2830 and ∼2820 Ma; are compositionally homogenous, contain little to no associated terrigenous sediments, and their trace element composition and short emplacement time resemble oceanic plateau basalts. They have been interpreted to be derived from a plume head with a primitive mantle composition.

Se–Te fractionation by sulfide–silicate melt partitioning: Implications for the composition of mantle-derived magmas and their melting residues

Partitioning of Se and Te has been measured between coexisting sulfide liquid, monosulfide solid solution (MSS) and silicate melt at 0.9–1.5 GPa, 1200–1300 °C, fO2 controlled near the fayalite–magnetite–quartz buffer (FMQ-1.2 to -1.6) and 3–22 wt% FeO in the silicate melt. Both elements are highly compatible in the sulfide phase relative to silicate liquid (Dsulfide phase/silicate liquid > 600), with the identity of the sulfide dictating the sense of Se–Te fractionation. Whereas the measured DTe/DSe is ∼5–9 for sulfide liquid/silicate liquid partitioning, MSS/silicate melt partitioning fractionates Te from Se in the opposite sense, with DTe/DSe of ∼0.5–0.8. At fixed fO2, DSulLiq/SilLiq values for both Se and Te decrease ∼8-fold over the range in silicate melt FeO content investigated. The relative values of DSulLiq/SilLiq for Cu to Se increase with increasing FeO in the silicate melt, such that DCu exceeds DSe only for melts with >11 wt% FeO. Hence the standard belief that DCu>DSe as indicative of sulfide removal should be carefully assessed in the context of the FeO content of the magmas involved.Assuming a chondritic mantle Se/Te, predicted MSS and sulfide liquid compositions are generally in accord with natural mantle sulfides, in terms of their designation as MSS or sulfide liquid, based on independent criteria. However, additional variability is likely due to Te redistribution in accessory platinum group minerals (PGM), or that some sulfides are metasomatic. Calculations show that the Se/Te ratio of silicate melt derived from a sulfide liquid-saturated mantle is significantly higher, and more variable, than for silicate melt in equilibrium with residual MSS; modest sulfide liquid removal at low pressure, however, likely obscures the Se/Te fractionation imposed by the source sulfide phase. Models indicate that the composition of MORB is consistent with melts produced from sulfide-bearing sources with chondritic Se/Te, and source sulfur contents higher than estimates for depleted mantle. The calculated composition of sulfide-saturated melting residues show relatively little deviation in Se/Te if MSS is residual, but a sharp drop in this ratio for sulfide liquid control. Although the data are scattered, a portion of the peridotite array near the primitive mantle composition is consistent with model trends, and suggests control by residual MSS.

N-isotope composition of the primitive mantle compared to diamonds

The nitrogen isotopic composition of the Earth's primitive mantle is controversial. Chromium-isotopic ratios of various terrestrial minerals and rocks, and chondritic meteorites are consistent with the silicate Earth being a mixture of enstatite and carbonaceous chondrites. From their relative proportions and N-isotope compositions we estimate that the bulk primitive mantle δ15N is −7±3‰. The negative value, as also evidenced by mantle-derived oceanic basalts and diamonds, is an intrinsic long-term feature of Earth's mantle. Some enstatite chondrite-like δ15N values down to −24‰ measured in very rare diamonds could be interpreted as a heterogeneous mantle. δ15N values in oceanic island basalts derived from the deep mantle have three components: deep mantle of ~−9‰ consistent with estimate, recycled sediments of about 15‰, and atmospheric N incorporated from groundwater and/or subducted atmospheric N (Mohapatra and Murty, 2000a; Chem. Geol. 164, 305–320). Some enriched δ15N values in MORB and OIB result from degassing fractionation. Shift of the upper mantle from an initial −7‰ to −5‰ by the Neoarchean can be explained by a combination of sediment recycling through subduction and upper mantle magma degassing processes.

Geochemical and Sr–Nd isotope variations within Cretaceous continental flood-basalt suites of the Canadian High Arctic, with a focus on the Hassel Formation basalts of northeast Ellesmere Island

Early- to mid-Cretaceous flood-basalt suites of the northeast Canadian High Arctic assigned to a High Arctic Large Igneous Province (HALIP) were studied for their whole-rock geochemistry and Sr–Nd isotopes. Data from basalt flows within the upper Albian to lower Cenomanian Hassel Formation of northeast Ellesmere Island are compared with former published data and new inductively coupled plasma mass spectrometry data of the stratigraphic equivalent Strand Fiord basalts and the older, late Hauterivian to Aptian Isachsen basalts from Axel Heiberg Island. The transitional to mildly alkaline aphyric Hassel basalts, with ocean island basalt (OIB)-like geochemical signatures in parts, have an Ar–Ar whole-rock age of on average 96.4 ± 1.6 Ma. They represent two geochemically different flow units without a fractional crystallization relationship: the high-phosphorous (HP) and low-phosphorous (LP) basalts. The Hassel HP basalts differ from the LP basalts by additionally higher Ba, K, Rb, Th and LREE contents, a pronounced positive Eu anomaly (Eu/Eu* = 1.74–1.76), as well as lower Ta, Nb, Zr and Hf concentrations. The Nd and Sr isotope ratios of the Hassel HP basalts [e Nd(t) of −1.3 to −1.4, 87Sr/86Sr(t) of 0.70706–0.70707] and the LP basalts [e Nd(t) of 4.5–4.9, 87Sr/86Sr(t) of 0.7038–0.7040] indicate an origin from different mantle sources. The geochemically similar tholeiitic Isachsen (ca. 130–113 Ma) and Strand Fiord basalts (ca. 105–95 Ma) are also incompatible element enriched relative to the primitive mantle, however, with negative Sr–P anomalies as well as partially negative K, Ta and Nb anomalies. In terms of incompatible element ratios (Zr/Nb, Nb/Th), several mantle components are involved in the formation of the flood-basalt suites: a component with primitive mantle composition, an OIB-like component (probably subducted and recycled oceanic crust) and an enriched lithospheric component. The latter component, probably metasomatized subcontinental lithospheric mantle, has contributed mainly to the Isachsen, Strand Fiord and Hassel HP basalts. In the Sr–Nd isotope system, the enriched lithospheric component is characterized by hypothetical values of e Nd(t) of approximately −4, and 87Sr/86Sr(t) of approximately 0.7083. The HALIP-related flood-basalt suites of the Canadian High Arctic are interpreted to be part of a continental rift zone that was active during the opening of the Amerasia Basin of the Arctic Ocean and the development of the oceanic Alpha-Mendeleev Ridge into a Large Igneous Province.