Small-degree Melts Research Articles

Petrography and petrogenesis of some Indian basaltic achondrites derived from the HED parent body: Insights from electron microprobe analyses

Three Indian achondrites, viz., Bholghati howardite, Lohawat howardite and Pipliya Kalan eucrite and two other achondrites, viz., Bereba eucrite and Johnstown diogenite are studied for their petrography and mineral chemistry. All these achondrites are derived from the HED parent body. Both Bholghati and Lohawat howardites are polymict breccias and contain pieces of eucrites and diaogenites (lithic clasts), pyroxene and minor olivine as mineral clasts, and small proportion of ilmenite and pure iron metal. Eucrite clasts are noncumulate basaltic in nature, whereas diogenite clasts are mostly composed of orthopyroxene with minor clinopyroxene and anorthite. Both howardite samples contain orthopyroxene, pigeonite and augite. Notable characteristics observed in Lohawat howardite include crystallization of orthoenstatite first at a high-temperature followed by ferrosilite, pigeonite olivine and augite from a basaltic melt. Piplia Kalan eucrite is noncumulate, unbrecciated and basaltic in nature and display ophitic/sub-ophitic or hypidiomorphic textures. It contains ~60% pyroxenes (clinoenstatite and pigeonite) and ~40% plagioclase feldspars (bytownite to anorthite). The observed mineralogy in the Piplia Kalan eucrite suggests its crystallization from a high-temperature basaltic melt crystallized at low pressure. Two other achondrite samples, viz., Bereba eucrite and Johnstown diogenite are also studied. The Bereba eucrite shows cumulate nature which is probably formed by small-degree melts of ilmenite-bearing gabbro, whereas the Johnstown diogenite crystallized from a slow cooling of a Ca-poor basaltic melt derived from cumulates formed from the magma ocean, similar to the origin of the noncumulate eucrites.

Polybaric melting of a single mantle source during the Neogene Siverek phase of the Karacadağ Volcanic Complex, SE Turkey

Siverek plateau basalts represent the Neogene activity of the Karacadağ Volcanic Complex in southeast Turkey and can be divided into two groups based on incompatible element concentrations. Group 1 is largely basaltic, containing some alkali basalts, while Group 2 consists of alkali basalts, trachybasalts and tephrites. The lavas display a range in major element concentrations that are consistent with restricted amounts of differentiation in the crust. Melts from both groups have experienced variable, small amounts of interaction with crustal rocks, which is responsible for most of the isotopic heterogeneity and caused significant Ba-enrichment. Neither fractional crystallisation nor crustal contamination can account for the differences in trace element enrichment observed between the two groups. Group 1 is derived mainly from the spinel lherzolite field by >1% partial melting. Group 2 lavas were derived from very similar mantle but by smaller degrees of melting and contain a larger relative contribution from garnet-lherzolite. The Siverek plateau lavas are indistinguishable from contemporaneous magmatism in the Karasu Valley of southern Turkey and in northernmost Syria. Together, these plateau basalt fields represent mantle upwelling and melting beneath the thinned and/or weakened Arabian Plate as it migrated northwards during the Neogene.

U-series isotope systematics of mafic magmas from central Oregon: Implications for fluid involvement and melting processes in the Cascade arc

The Cascade arc is the warm-slab subduction zone global end member, where a broad variety of primitive magmas with highly variable slab fluid signatures have erupted in close spatial and temporal proximity. A number of petrogenetic models have been proposed to explain the occurrence of such diverse magmas, but the source(s) of these magmas and the timing of fluid addition to the sub-arc mantle remain controversial. We present uranium-series isotope data ( 238U– 230Th– 226Ra) for eighteen mafic lavas from the Three Sisters region of the central Oregon Cascades, and for a further six lavas from the rear-arc Newberry Volcano. The majority of these samples have geochemical characteristics (e.g. Nb/Zr, Ba/Zr, 87Sr/ 86Sr) consistent with previously described calc-alkaline basalts from this region of the arc, and indicative of limited fluid involvement at some stage in their genesis. Trace element and long-lived radiogenic isotope modeling suggests that this fluid was derived from dehydration of subducting sediments, and was added to an enriched, garnet-bearing mantle wedge source. The trace element systematics of the lavas are consistent with small degree (< 10%) melts of this fluid-modified source. All samples display ( 230Th/ 238U) and ( 226Ra/ 230Th) ≥ 1, similar to values measured in fresh MORB and other parts of the arc. Results of a dynamic melting model support the interpretation that these lavas are small degree melts of an asthenospheric source, and do not allow for a lithospheric mantle source. However, the U-series data do not permit us to determine whether fluid addition was the trigger for melting, or whether the lavas were generated from a secular equilibrium source that had experienced fluid addition > 350 ka prior to melting. Regardless, modern fluid input is limited and melting is dominantly occurring in response to upwelling and decompression.

Geochemical and Sr–Nd isotopic constraints on the genesis of the Cenozoic Linzizong volcanic successions, southern Tibet

The Linzizong volcanic successions that crop out in the Lhasa terrane, southern Tibet have been conventionally regarded as the products of northward subduction of the Neotethyan oceanic slab beneath South Asia. This study reports geochemical data of 100+ volcanic rocks from the Lhasa terrane to better constrain the temporal–spatial distribution and petrogenesis of the Linzizong volcanism. The Linzizong volcanic successions consist dominantly of calc-alkaline rocks that erupted from ca. 69 to 43Ma and show typical arc-lava geochemical features marked with LILE enrichment and HFSE depletion in the spidergram. Their Sr and Nd isotope ratios [εNd(T)=+3.3 to −2.4; ISr=0.7048–0.7072] are generally similar to those of the associated Gangdese I-type granitoids. The Linzizong volcanism is characterized by a flare-up period (ca. 50Ma) that shows significant geochemical variations, manifested by the coexistence of five types of volcanic rocks: (1) the main suite of calc-alkaline rocks [SiO2=45–80wt.%; La=12–45ppm; εNd(T)=+3.8 to −4.9; ISr=0.7037–0.7105] that could be interpreted by partial melting of the mantle wedge followed by assimilation and fractional crystallization (AFC) processes with >10% crustal contamination and/or magma mixing; (2) the low-K, low-REE suite (SiO2=48–61wt.%; K2O=0.5–1.1wt.%; La=7–10ppm) that has the highest Nd isotope ratios [εNd(T)=+5.9 to 3.5], suggesting a juvenile mantle origin possibly related to decompression melting of the asthenosphere; (3) the shoshonitic suite [SiO2=53–71wt.%; K2O=3.8–6.7wt.%; εNd(T)=−2.8 to −6.1] from small-degree melting of the metasomatized lithospheric mantle; (4) the high-REE suite [SiO2=61–75wt.%; La=36–87ppm; εNd(T)=−2.6 to −3.2] originating from remelting of a newly underplated basaltic lower crust; and (5) the evolved suite of rhyolitic flows and ignimbrites (SiO2>65wt.%) that have the lowest εNd(T) values of −14 to −18, representing remelting products of the basement or continental crust of the Lhasa terrane. Such geochemical heterogeneities are attributed to breakoff of the subducted Neotethyan slab under southern Tibet that occurred in the early stage of the India–Asia collision.

Young volcanism in the Borborema Province, NE Brazil, shows no evidence for a trace of the Fernando de Noronha plume on the continent

We present 40Ar/39Ar ages for four volcanic bodies from a group of volumetrically minor alkaline plugs, necks, and dikes in northeastern Brazil, previously ascribed to passage over the purported Fernando de Noronha plume. The rocks are relatively primitive (9.5–14.7wt.% MgO), typically nepheline-normative basanites with ocean island basalt (OIB)-like trace-element compositions similar to alkalic basalts from the Fernando de Noronha archipelago. Highly fractionated REE coupled with relative depletions of K and Rb indicates that the silica undersaturated magmas were generated by small degrees of melting in the presence of residual garnet and a hydrous metasomatic phase. Three of the four units (Caracarazinho, Cabugizinho da Arara and Serra Preta de Bodó) were heretofore undated. The fourth body (Cabelo de Negro) was included to facilitate comparison with published K–Ar dates. 40Ar/39Ar age determinations by the laser incremental-heating method on duplicate grains of groundmass reveal the youngest continental volcanism in Brazil, with emplacement ages between 8.9±0.5 and 7.1±0.3Ma. Our age for Cabelo de Negro (7.9±0.3Ma) is roughly 20Ma younger than the published K–Ar date for this plug. The reproducibility of our duplicate analyses and the consistency of the plateau, ideogram and isochron ages for this sample attest to the reliability of the new 40Ar/39Ar results. Our geochronological results show that volcanic activity on the continent did not shut down prior to the onset of volcanism on the island of Fernando de Noronha. Both areas were active contemporaneously for at least 5Ma. We argue that the extended duration, small volume and lack of a clear age progression suggest that this example of alkaline intraplate volcanism is more likely the surface manifestation of the upwelling flow seen in an edge-driven convection mode, rather than tracking passage over a deep-seated mantle plume. This hypothesis is supported by xenolith thermobarometery, heat-flow data and seismic tomography, which collectively provide evidence for a mild thermal anomaly (upwelling) in the upper mantle under the eastern-most equatorial Brazil that extends into the Atlantic and appears to travel with the plate. Small-scale convection is also consistent with the seismic evidence for a downwelling to about 600km depth in the mantle beneath the cratonic margin of eastern Brazil. Given the inherently sluggish upwelling associated with such small-scale convection, it appears that the entrainment of metasomatically enriched lithospheric mantle is likely vital to melt generation. If so, the spatial distribution of edge-driven magmatism may provide a tool for mapping fertility of the subcontinental lithospheric mantle.

Trace element partitioning between mica- and amphibole-bearing garnet lherzolite and hydrous basanitic melt: 2. Tasmanian Cainozoic basalts and the origins of intraplate basaltic magmas

Oligocene volcanics from Oatlands in Tasmania, Australia, include olivine tholeiites, alkali olivine basalts, nepheline basanites and olivine nephelinites. They have compositional characteristics that are typical of intraplate basalts worldwide. They are generally enriched in incom- patible elements relative to the primitive mantle and are strongly enriched in Nb, Ta and light rare earths, but not heavy rare earths. At the same time, they have Sr and Nd isotope compositions that are similar to those in some incompatible-element-depleted mid-ocean ridge basalts (E-type MORB). Experimentally obtained mineral/melt partition coefficients for an Oatlands basanite allow the relative concentrations of incompatible elements in the volcanics to be produced by small degrees of melting (B1%) of a source similar to the E-type MORB source of Workman and Hart (2005). However, the absolute concentrations that can be achieved in this way are much less than present in the most incompatible-element-enriched basanites and nephe- linites at Oatlands. This contradiction can be explained by open-system melting under the influence of a conductive geotherm. This would have involved upwardly migrating near-solidus melts from the asthenosphere cooling along a sub-adiabatic geotherm. Cooling of the melts would have caused them to re-crystallize and accumulate in the overlying mantle, thereby enriching both the new host rocks and any residual melts in incompatible elements. This would also have increased the buoyancy of the host rocks leading to upwelling and further (decompression) melting of incom- patible-element-enriched peridotite. We were able to use our partition coefficients to quantitatively model the develop- ment of incompatible-element enrichments in the Oatlands magmas by these processes. Our explanation is consistent with the characteristically scattered but widespread distri- butions and long time scales of intraplate volcanism in a broad variety of tectonic settings. This is because the con- ditions required to initiate volcanism (i.e. those of near-sol- idus melting of the asthenosphere) are relatively easy to produce and can therefore be caused by both near-surface tectonics and deeper mantle processes. Furthermore, the super-enrichments of incompatible elements in some intra- plate volcanics can be attributed to the influence of normal geothermal gradients on melting processes. Without the very strong fractionation imposed by this combination of factors, the Oatlands volcanics would more closely resemble mid- ocean-ridge basalts.

Late Cretaceous alkaline sills of the south Tethyan suture zone, Pakistan: Initial melts of the Réunion hotspot?

During the closure of the Tethyan Ocean, Triassic to early Jurassic marine sediments containing alkali dolerite and ultramafic lamprophyre sills were accreted to the margin of Greater India. The 69.7 ± 0.2 Ma weighted mean 40Ar- 39Ar plateau age of the sills, their location, and their incompatible element and Nd–Pb–Sr isotopic composition, which is similar to that of recent Réunion shield lavas, are consistent with their derivation from the Réunion hotspot. Mantle melt modelling suggests that the alkali dolerites are the result of < 3% melting of a moderately incompatible element-depleted, garnet-bearing source. However, some of the ultramafic lamprophyres have lower heavy rare-earth element, Nb and Zr contents that cannot be explained by melting of the same mantle source as the alkali dolerites. This unusual feature can be explained by small-degree melting of a metasomatised mantle source containing small amounts (< 0.02 vol.%) of residual zircon. Isotopic signatures of the ultramafic lamprophyres are similar to those of the alkali dolerites, implying that metasomatism was not significantly older than the melting event that formed the sills. The metasomatism may have been due to small-degree melting and re-solidification at the leading edge of the starting head of the Réunion plume. This alkaline magmatism may represent the arrival of the Réunion plume at the base of the Indian lithosphere several million years before the onset of the main phase of Deccan flood basalt volcanism.

Chemical variations and regional diversity observed in MORB

An assemblage of MORB analyses ( n = 792 samples), including a suite of new, high-precision LA-ICP-MS measurements ( n = 79), has been critically compiled in order to provide a window into the chemical composition of these mantle-derived materials and their respective source region(s), commonly referred to as the depleted MORB mantle (DMM). This comprehensive MORB data set, which includes both “normal-type” (N-MORB, defined by (La/Sm) N < 1.00) and “enriched-type” samples (E-MORB, (La/Sm) N ≥ 1.00), defines a global MORB composition that is more enriched in incompatible elements than previous models. A statistical evaluation of the true constancy of “canonical” trace element ratios using this data set reveals that during MORB genesis Ti/Eu, Y/Ho and Ce/Pb remain constant at the 95% confidence-level; thus, the ratios recorded in MORB (Ti/Eu = 7060 ± 1270, 2σ; Y/Ho = 28.4 ± 3.6, 2σ; Ce/Pb = 22.2 ± 9.7, 2σ) may reflect the composition of the DMM, presuming the degree of source heterogeneity, component mixing and conditions of melting/crystallization of the DMM are adequately recorded by global MORB. Conversely, Ba/Th, Nb/U, Zr/Hf, Nb/Ta, Sr/Nd, and Th/U are shown to fractionate as a function of MORB genesis, and thus these ratios do not faithfully record the composition of the DMM. Compared to samples from the Pacific and Indian Oceans, MORB derived from Atlantic ridge segments are characterized by statistically significant (≥ 95% confidence-level) enrichments in both highly incompatible elements (e.g., light REE, TITAN group elements, Sr, Ba, Pb, Th, and U) as well as less incompatible elements (e.g., heavy REE), indicating: i) a prominent recycled source component; ii) variable proportions of pyroxenite in the Atlantic source region; and/or, most likely iii) smaller degrees of melting and/or greater extents of fractional crystallization due to slower ridge spreading rates. Conversely, Pacific MORB has the most depleted regional signatures with regard to highly incompatible elements (e.g., Ba, Pb, Th, and U), likely due to faster ridge spreading rates. Indian Ocean MORB exhibit limited variation in incompatible element enrichments/depletions but are generally the most depleted in more compatible elements (e.g., Ti, Cr, Sc, and heavy REE), potentially due to distinct source characteristics or deep source melting in the garnet field. Atlantic, Pacific and Indian MORB can also be distinguished by trace element ratios, particularly Ce/Pb and Th/U, which is distinct at the > 99% confidence-level. Global MORB, and by inference the DMM, are characterized by enrichments in Y/Ho and depletions in Th/U relative to the chondritic ratios, and are complementary to the continental crust. However, the median of global MORB and the bulk continental crust both have sub-chondritic Ti/Eu and Nb/Ta ratios, suggesting an under-represented Ti- and Nb-rich reservoir in the Earth, potentially refractory, rutile-bearing eclogite at depth in the mantle.

Timing, relations and cause of plutonic and volcanic activity of the Siluro-Devonian post-collision magmatic episode in the Grampian Terrane, Scotland

Abstract: Calc-alkaline magmatism in the Grampian Terrane started at c . 430 Ma, after subduction of the edge of continental Avalonia beneath Laurentia, and it then persisted for at least 22 Ma. Isotope dilution thermal ionization mass spectrometry U–Pb zircon dating yields 425.0 ± 0.7 Ma for the Lorn Lava Pile, 422.5 ± 0.5 Ma for Rannoch Moor Pluton, 419.6 ± 5.4 Ma for a fault-intrusion at Glencoe volcano, 417.9 ± 0.9 Ma for Clach Leathad Pluton in Glencoe, and, in the Etive Pluton, 414.9 ± 0.7 Ma for the Cruachan Intrusion and 408.0 ± 0.5 Ma for the Inner Starav Intrusion. The Etive Dyke Swarm was mostly emplaced during 418–414 Ma, forming part of the plumbing of a large volcano (≥2000 km 3 ) that became intruded by the Etive Pluton and was subsequently removed by erosion. During the magmatism large volumes (thousands of km 3 ) of high Ba–Sr andesite and dacite were erupted repeatedly, but were mostly removed by contemporaneous uplift and erosion. This volcanic counterpart to the ’Newer Granite' plutons has not previously been fully recognized. The intermediate magmas forming both plutons and volcanoes originated mainly by partial melting of heterogeneous mafic to intermediate lowermost crust that had high Ba–Sr derived from previous melting of large ion lithophile element (LILE)-enriched mantle, possibly at c . 1.8 Ga. This crustal recycling was induced by heat and volatiles from underplated small-degree melts of LILE- and light REE-enriched lithospheric mantle (appinite–lamprophyre magmas). The post-collision magmatism and uplift resulted from breakoff of subducted oceanic lithosphere and consequent rise of asthenosphere. Supplementary material: Geochronological methods, tabulated data and additional figures are available at http://www.geolsoc.org.uk/SUP18343 .

The geochemistry of Middle Jurassic dykes associated with the Straumsvola–Tvora alkaline plutons, Dronning Maud Land, Antarctica and their association with the Karoo large igneous province

AbstractJurassic dykes of western Dronning Maud Land (Antarctica) form a minor component of the Karoo large igneous province. An extensive local dyke swarm intrudes Neoproterozoic gneisses and Jurassic syenite plutons on the margins of the Jutulstraumen palaeo rift in the Svedrupfjella region. The dykes were intruded in three distinct episodes (~204, ~176 and ~170 Ma). The 204 Ma dykes are overwhelminglylow-Ti, olivine tholeiites including some primitive (picritic) compositions (MgO &gt;12 wt.%; Fe2O3 &gt;12 wt.%; Cr &gt;1000 ppm; Ni &gt;600 ppm). This 204 Ma event precedes the main Karoo volcanic event by~25 Ma, so anycorrelations to the wider province are difficult to make. However, it mayrecord the earliest phase of rift activity along the Jutulstraumen. The 176 Ma dyke event is more intimately associated with the two syenite plutons. The dykes are alkaline (basanite/ tephrite) and were small-degree melts from an enriched, locallyderived source and underwent at least some degree of interaction with a syenitic contaminant. This ~176 Ma dyke event is widespread elsewhere in the Karoo (southern Africa and Dronning Maud Land). Later-stage (170 Ma) felsic (phonolite–comendite) dykes intrude the 176 Ma basanite–tephrite suite and represent the last phase of magmatic activityin the region.

Geochemistry of Triassic flood basalts from the Yukon (Canada) segment of the accreted Wrangellia oceanic plateau

A large part of the accreted Middle to Late Triassic Wrangellia oceanic plateau is exposed as a linear belt (< 30 km × 300 km) in southwest Yukon. The first major- and trace-element, and isotopic compositions of the Nikolai Formation in Yukon are presented here, along with compositions for underlying Paleozoic arc rocks. The Nikolai Formation in Yukon is predominantly massive tholeiitic subaerial flows (~ 1000 m) with no intervening sediments and a thin zone of pillow breccia along the base (< 100 m). The Nikolai basalts unconformably overlie Late Paleozoic volcanic arc and marine sedimentary sequences and are overlain by Late Triassic limestone, which grades upwards into pelagic sediments. The Nikolai Formation is comprised of two distinct lava types: low-titanium basalts form most of the lower stratigraphy and high-titanium basalts form the upper parts of the volcanic stratigraphy. All of the low-titanium basalts (0.5–1.0 wt.% TiO 2; 5.6–11.3 wt.% MgO) have prominent negative HFSE anomalies, whereas the high-titanium basalts (1.4–2.3 wt.% TiO 2; 5.8–8.7 wt.% MgO) do not have HFSE anomalies and are more LREE-enriched. The low-titanium basalts are characterized by mostly higher initial ε Hf (+ 11.1 to + 15.8) and lower initial ε Nd (+ 2.3 to + 6.8) than the high-titanium basalts (initial ε Hf = + 10.4 to + 12.0; initial ε Nd = + 6.6 to + 9.0), and their Pb isotopic compositions overlap. Incongruent dynamic melting modeling of trace element compositions indicate the low-titanium basalts could have been derived from small degrees of melting (< 5%) of Paleozoic sub-arc lithospheric mantle that was HFSE-depleted and evolved with high 176Hf/ 177Hf. The high-titanium basalts formed from melting of Pacific plume-type mantle, similar to the source of the Caribbean Plateau. Plume-derived melts dominated the upper stratigraphy of the oceanic plateau as a result of increased decompression melting of the underlying mantle plume in response to thinning of the lithosphere.

Primary carbonatite melt from deeply subducted oceanic crust

Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here we provide experimental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.

Simple models for dynamic melting in an upwelling heterogeneous mantle column: Analytical solutions

Several lines of evidence suggest that the melt generation and segregation regions of the mantle are heterogeneous, consisting of chemically and lithologically distinct domains of variable size and dimension. Partial melting of such heterogeneous mantle source regions gives rise to a diverse range of basaltic magmas. In order to better assess the role of source heterogeneity during mantle melting, we have undertaken a theoretical study of trace element distribution and fractionation during concurrent melting and melt migration in an upwelling, chemically heterogeneous, two-porosity double lithology melting column. Analytical solutions for the abundance of a trace element in the matrix and channel were obtained under the assumptions that the porosity, melt and solid velocities, and solid–melt partition coefficients are constant and uniform. For simplicity, we neglected diffusion and dispersion in the melt. Chemical source heterogeneities of arbitrary size and shape were integrated into the simple melting models by allowing trace element abundance in the source region to vary as a function of time and space. Concurrent melting and melt migration in an upwelling heterogeneous mantle may be approximated as a quasi-steady state problem in which time-dependent concentration patterns produced by melting of heterogeneous source regions are superimposed on a reference steady-state concentration distribution established by melting of the ambient or background mantle. Chromatographic fractionation is especially important for the matrix melt and solid when chemical heterogeneities are involved during melting and melt migration in the mantle, giving rise to significant phase-shift between two incompatible trace elements in the matrix melt and scattered correlations among incompatible trace elements in residual peridotites. Mixing is the chief mass transfer process in the dunite channel where the chromatographic effect is negligible for most of the incompatible trace elements. The lack of chromatographic fractionation among incompatible trace elements and isotopic ratios in MORB suggests either most MORB are channel melts or mixing in magma conduit and chamber is very efficient such that the phase-shift is averaged out during magma transport and storage processes. Advection brings melt produced by smaller-degree of melting in the deeper part of the melting column to the overlying melting region, increasing the incompatible trace element abundance in the matrix and the channel. This advection-induced self-enrichment is especially important when heterogeneous sources are involved and may account for some of the enriched incompatible trace element patterns observed in residual peridotite that were previously interpreted to be a result of mantle metasomatism. Systematic studies of high-resolution spatially correlated mantle samples may help to constrain the melting history and the size and nature of chemical heterogeneities in the mantle.

TiO 2 enrichment in ocean island basalts

An extensive dataset of ocean island basalt (OIB) compositions highlight that their titanium (Ti) contents are generally too high to be derived from melting of a peridotitic mantle. Even vanishingly small degree melts of a primitive mantle source have insufficient Ti to reproduce the compositions of many OIB. A Ti-enriched material is thus a necessary additive to many OIB sources. Of the enriched components commonly invoked to explain the range in OIB radiogenic isotopic signatures, only the addition of small amounts (∼ 1–10%) of recycled mafic crust is compatible with all geochemical constraints. Since such mafic crust starts melting deeper than its host peridotite, small degree melts formed beneath thick lithosphere will preferentially sample this enriched heterogeneity. This is compatible with the observation that the most Ti rich OIB also have the most enriched and variable signatures in terms of radiogenic isotope ratios. However, recycled mafic crust itself is insufficiently extreme isotopically to be able to account for the full range of isotopic ratios in OIB, therefore further, associated components (e.g. sediments, metasomatic veins) seem necessary in some cases.

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotope compositions for clinopyroxenes from anhydrous spinel peridotite and garnet ± spinel pyroxenite xenoliths of Pan-African lithospheric mantle from Jordan, including the first high-precision double-spike Pb isotope measurements of mantle clinopyroxene. Clinopyroxenes from the peridotites are variably Th–U–LILE–LREE enriched and display prominent negative Nb, Zr and Ti anomalies. MREE–HREE abundances can generally be modelled as partial melting residues of spinel lherzolite with primitive-mantle-like composition after extraction of 5–10% melt, whereas the enrichments in Th–U–LILE–LREE require a Pan-African or later metasomatic event. The large range of Nd, Sr, Pb and Hf isotope ratios in both peridotites and pyroxenites (e.g. εNd ∼ 1·4–17·5; 206Pb/204Pb ∼ 17·2–20·4; εHf ∼ 0·6–164·6) encompasses compositions more radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopes cover almost the entire range of oceanic basalt values. εHf values are some of the highest ever recorded in mantle samples and are decoupled from εNd in the same samples. Marked correlations between Sr–Nd–Pb isotopes, LILE–LREE enrichments and HFSE depletion suggest that the metasomatizing agent was a carbonatitic-rich melt and isotopic data suggest that metasomatism may have been related to Pan-African subduction. The metasomatic melt permeated depleted upper mantle (<16 kbar) during Pan-African subduction at ∼ 600–900 Ma, and the variably metasomatized material was then incorporated into the Arabian lithospheric mantle. There is no evidence for recent metasomatism (<30 Ma) related to the Afar plume like that postulated to have affected southern Arabian lithospheric mantle. Hf isotopes in the mantle clinopyroxenes are unaffected by metasomatism, and even some strongly overprinted lithologies record ancient (>1·2 Ga) pre-metasomatic Lu–Hf signatures of the depleted upper mantle that was the protolith of the Arabian lithospheric mantle. The ‘resistance’ of the Lu–Hf isotopic system to later metasomatic events resulted in the development of extremely heterogeneous Hf isotopic signatures over time that are decoupled from other isotopic systems. No mantle sample in this study exactly matches the chemical and isotopic signature of the source of Jordanian intraplate basalts. However, the xenolith compositions are broadly similar to those of the source of Arabian intraplate basalts, suggesting that the numerous Cenozoic intraplate volcanic fields throughout Arabia may be the product of melting upper mantle wedge material fertilized during Pan-African subduction and incorporated into the Arabian lithospheric mantle. We propose a model whereby the proto-Arabian lithospheric mantle underwent a major melting event in early Proterozoic–late Archean times (at the earliest at ∼1·2 Ga). Island-arc volcanism and major crust formation occurred during the Pan-African orogeny, which liberated fluids and possibly small-degree melts that migrated through the mantle creating zones of enrichment for certain elements depending upon their compatibility. Immobile elements, such as Nb, were concentrated near the base of the mantle wedge providing the source of the Nb-rich Jordanian volcanic rocks. More mobile elements, such as LILE and LREE, were transported up through the mantle and fertilized the shallow mantle source of the Jordanian xenoliths. Following subduction, the mantle wedge became fossilized and preserved distinct enriched and depleted zones. Lithospheric rifting in the Miocene triggered partial melting of spinel-facies mantle in the lower lithosphere, which mixed with deeper asthenospheric garnet-facies melts as rifting evolved. These melts entrained segments of variably carbonatite-metasomatized shallow lithospheric mantle en route to the surface.

New insights into the genesis of Indian kimberlites from the Dharwar Craton via in situ Sr isotope analysis of groundmass perovskite

In situ Sr isotopic analyses of kimberlitic perovskite are more representative of primary magmatic compositions than conventional bulk-rock analyses of the same samples, because the latter are variably compromised by contamination and alteration processes. Bulk-rock Sr isotopic data obtained for 18 intrusions from the adjacent Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India, exhibit a high degree of scatter (∼0.701-0.709) and have indistinguishable initial isotope ratios. In contrast, laser ablation perovskite results display strikingly uniform and distinct initial 87Sr/86Sr compositions for each field of 0.70312-0.70333 and 0.70234-0.70251, respectively. The increased resolution provided by these new data permits the evaluation of key aspects of kimberlite genesis. It is argued that lithospheric and crustal contamination had a negligible impact on the perovskite Sr isotope compositions, and that these values are representative of the primary melt component in each field. The results cast doubt on models of kimberlite formation that invoke either the small degree melting of metasomatized subcontinental lithospheric mantle, or derivation from unusually enriched asthenospheric mantle. The data are more compatible with a source region for the Dharwar kimberlites that was similar to a common mantle component such as prevalent mantle (PREMA) or focal zone (FOZO). © 2007 The Geological Society of America.

Geochemistry of Cenozoic basalts and mantle xenoliths in Northeast China

Volcanic activity in Northeast China and adjacent regions is widespread, with eruption ages ranging from Late Cretaceous (80 Ma) to about 300 years ago. Volcanic rock types are principally basanite, alkali olivine basalt, and tholeiite, with minor evolved trachyte and rhyolite. Ultramafic xenoliths, mainly spinel lherzolite and harzburgite, are common in alkali olivine basalt and basanite. We present new major and trace element data from 11 volcanic fields, and helium isotope data from ultramafic xenoliths of three of those fields. 3He/ 4He ratio ranges from 5 to 7 times the atmospheric ratio, indicating that previously reported high 3He/ 4He ratios are likely due to the contamination by cosmogenic 3He. There is currently no evidence for a high- 3He/ 4He mantle plume component beneath NE China. Sr–Nd–Pb isotopic data from the literature are reinterpreted to result from mixing between a FOZO end-member and a LoMu end-member. The LoMu end-member appears to reside within continental lithosphere. Major and trace elements do not indicate significant contributions from a subducted slab. Differences in depth and the extent of partial melting in the asthenosphere, both locally within individual fields and regionally between the different volcanic fields, explain most of the correlations between major and trace elements observed in this study, with deeper melting usually associated with a smaller degree of melting. Highly potassic basalts are associated with the LoMu end-member and are interpreted to be the products of lithosphere melting.

Flat rare earth element patterns as an indicator of cumulate processes in the Lesser Qinling carbonatites, China

The Lesser Qinling carbonatite dykes are mainly composed of calcites. They are characterized by unusually high heavy rare earth element concentrations (HREE; e.g. Yb > 30 ppm) and flat to weakly light rare earth element (LREE) enriched chondrite-normalized patterns (La/Yb n = 1.0–5.5), which is in marked contrast with all other published carbonatite data. The trace element contents of calcite crystals were measured in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Some crystals show reduced LREE from core to rim, whereas their HREE compositions are relatively constant. The total REE contents and chondrite-normalized REE patterns from the cores of carbonate crystals are similar to those of the whole rock. The carbon and oxygen isotopic compositions of calcites fall within the range of primary, mantle-derived carbonatites. The initial Sr isotopic compositions (0.70480–0.70557) of calcites are consistent with an EM1 source or mixing between HIMU and EM1 mantle sources. However these sources cannot produce carbonatite parental magmas with a flat or slightly LREE enrichment pattern by low degrees of partial melting. Analyses of carbonates from other carbonatites show that carbonates have nearly flat REE pattern if they crystallize from a LREE enriched carbonatite melt. This implies that when carbonates crystallize from a carbonatite melt the calcite/melt partition coefficients ( D) for HREE are much greater than the D for the LREE. The nearly flat REE patterns of the Lesser Qinling carbonatites can be explained if they are carbonate cumulates that contain little trapped carbonatite melt. Strong enrichment of HREE in the carbonatites may require their derivation by small degrees of melting from a garnet-poor source.

Oxygen–osmium isotope systematics of West Maui lavas: A record of shallow-level magmatic processes

New δ^(18)O and ^(187)Os/^(188)Os data from late shield-building stage lavas from West Maui volcano, Hawai'i, indicate that a range of distinct shallow-level processes can affect compositions of plume-generated ocean island magmas. The relative ages of the samples studied were determined either on the basis of field relationships, or from collection depth in a water well. The stratigraphically lower lavas have [Os] = 61–399 ppt, initial ^(187)Os/^(188)Os = 0.1311–0.1324 and δ^(18)O_(olivine) = 4.53–4.88‰. The stratigraphically higher lavas have [Os] = 20–273 ppt, initial ^(187)Os/^(188)Os = 0.1323–0.1390 (with the exception of one low [Os] sample with ^(187)Os/^(188)Os = 0.1517), and δ^(18)O_(olivine) = 4.73–5.21‰. These compositions are generally similar to those of Kea-type Hawaiian lavas from Mauna Kea and Kilauea volcanoes, but in detail, there are some significant differences between the West Maui lavas and other Kea-type lavas. The stratigraphically lower lavas have ^(187)Os/^(188)Os similar to those previously observed for Mauna Kea and Kilauea lavas, and define a nearly flat ^(87)Sr/^(86)Sr vs. ^(187)Os/^(188)Os trend that is consistent with assimilation by these magmas of small degree melts of Pacific oceanic crustal gabbros. The stratigraphically higher West Maui lavas extend to ^(187)Os/^(188)Os higher than observed at Mauna Kea or Kilauea. This probably results from interaction of these magmas with the hydrothermally altered volcanic edifice during the waning phases of the shield building stage. This process is likely to affect the isotope systematics of Os and O, but will have little effect on those of Hf, Nd, Pb or Sr.

Storage capacity of H 2O in nominally anhydrous minerals in the upper mantle

The H 2O storage capacity of nominally anhydrous minerals or rocks is the concentration of water that can be sequestered in the mineral(s) without stabilizing a hydrous fluid or melt. The storage capacity of the upper mantle is considerably greater than generally appreciated, as recent studies show that H 2O uptake in olivine is ∼3 times that originally inferred by Kohlstedt et al. [D.L. Kohlstedt, H. Keppler, D.C. Rubie, Solubility of water in the α, β and γ phases of (Mg,Fe) 2SiO 4, Contrib. Mineral. Petrol. 123 1996 345–357.] and, at least at low pressure, pyroxene stores considerably more H 2O than olivine. Consequently, H 2O has smaller influence on small degree melting than inferred previously. Combining data on the storage capacity of olivine with constraints on partition coefficients between olivine, pyroxene, and garnet, we estimate that the storage capacity of the upper mantle just above the 410 km discontinuity is > 0.4 wt.%. Owing to the increasing mode of garnet at the expense of pyroxene, there is likely to be a local maximum in storage capacity between 350 and 400 km, and a local minimum just above the onset of wadsleyite stability. Although published data suggest that the storage capacity of wadsleyite diminishes with increasing temperature, the storage capacity of the transition zone likely is considerable because Fe-bearing wadsleyite has a larger storage capacity than Mg 2SiO 4. Peridotite upwelling from the transition zone will undergo partial melting above the 410 km discontinuity only if it has more H 2O than the local storage capacity (i.e., > 0.4 wt.%), and the dehydrated residue cannot be drier than this unless it melts further under conditions where the storage capacity is less. Because residues of partial melting at 410 km have much more H 2O than the 50–200 ppm H 2O in the average upper mantle, they cannot be principal sources for the upper mantle. If hydrous melting occurs at 410 km, further upwelling of the residual peridotite will result in continued melting throughout the upper mantle, unless the storage capacity increases with decreasing depth. The partition coefficient of H 2O between wadsleyite and olivine is ∼5, which is less extreme than previously assumed. Consequently, the effect of H 2O on the depth and thickness of the 410 discontinuity may not be pronounced and typical (10 km) discontinuity thickness can be reconciled with up to ∼400 ppm H 2O.