Peridotite Component Research Articles

Mapping the Structure and Metasomatic Enrichment of the Lithospheric Mantle Beneath the Kimberley Craton, Western Australia

AbstractThe lithology, geochemistry, and architecture of the continental lithospheric mantle (CLM) underlying the Kimberley Craton of north‐western Australia has been constrained using pressure‐temperature estimates and mineral compositions for >5,000 newly analyzed and published garnet and chrome (Cr) diopside mantle xenocrysts from 25 kimberlites and lamproites of Mesoproterozoic to Miocene age. Single‐grain Cr diopside paleogeotherms define lithospheric thicknesses of 200–250 km and fall along conductive geotherms corresponding to a surface heat flow of 37–40 mW/m2. Similar geotherms derived from Miocene and Mesoproterozoic intrusions indicate that the lithospheric architecture and thermal state of the CLM has remained stable since at least 1,000 Ma. The chemistry of xenocrysts defines a layered lithosphere with lithological and geochemical domains in the shallow (<100 km) and deep (>150 km) CLM, separated by a diopside‐depleted and seismically slow mid‐lithosphere discontinuity (100–150 km). The shallow CLM is comprised of Cr diopsides derived from depleted garnet‐poor and spinel‐bearing lherzolite that has been weakly metasomatized. This layer may represent an early (Meso to Neoarchean?) nucleus of the craton. The deep CLM is comprised of high Cr2O3garnet lherzolite with lesser harzburgite, and eclogite. The peridotite components are inferred to have formed as residues of polybaric partial mantle melting in the Archean, whereas eclogite likely represents former oceanic crust accreted during Paleoproterozoic subduction. This deep CLM was metasomatized by H2O‐rich melts derived from subducted sediments and high‐temperature FeO‐TiO2melts from the asthenosphere.

Relicts of Neo-Tethyan mantle wedge in the Indo-Burma Range, India: Record of carbonate metasomatism and Neo-Tethyan mantle evolution

Several different geochemical signatures, i.e., mid-oceanic ridge (MOR) and supra subduction zone (SSZ), are frequently reported from ophiolite belts. Such bi-modal geochemical signatures are generally interpreted in terms of formation in two contrasting tectonic settings: divergent and convergent settings with associated petrogenetic processes. Whilst MOR-like and SSZ-like geochemical signatures are well understood in general terms, their combined occurrence in the peridotite component of ophiolite belts is not fully understood. Here, we describe the geochemically comparable Nagaland and Manipur ophiolites which are part of a same belt located in the Indo-Burma Range, India, and represent part of the eastern Tethys regime. In this study we explore the mechanisms which are responsible for this dual geochemical signature in a contiguous ophiolite belt formed during the closure of eastern Neo-Tethys. The existing and new whole-rock Nd isotopic signatures in the serpentinized peridotites from the Manipur ophiolite reveal that the dual geochemical signatures observed in the peridotites are due to patchy metasomatism of the mantle wedge. Thus, the entire mantle section peridotite in the Nagaland and Manipur ophiolites represents a relic of that Neo-Tethyan mantle wedge and this is also supported by the occurrence of high Cs/Th and low U/Th in the serpentinised peridotites. Further, variation of LaN/YbN, SmN/HfN, Ti/Eu, Zr/Hf, Ca/Al and Mg# observed in the secondary and primary clinopyroxenes of the studied peridotites can be explained by an influx of carbonate-rich fluid derived from subducted pelagic limestone. Elemental zoning and associated modeling of clinopyroxenes also clarify that the mantle metasomatism and different degrees of partial melting in the mantle wedge were responsible for the heterogeneity of the Neo-Tethyan mantle preserved in the Nagaland and Manipur ophiolites of the Indo-Burma Range.

Iron isotope constraints on the lithological heterogeneity of the upper mantle in the South China Sea

The South China Sea is located at the junction of the Eurasian, Indian, Australian, and Pacific plates, and thus its evolution is complex and the nature of its sub-ridge mantle remains unclear. Here we present new iron (Fe) isotopic data for mid-ocean ridge and intraplate basalts from the South China Sea region, in order to examine the origins of compositional heterogeneity in the sub-ridge mantle beneath the South China Sea. The investigated mid-ocean ridge basalts are from International Ocean Drilling Program Sites U1431 and U1433 in the eastern and southwestern sub-basins of the South China Sea, respectively. The primary melts of the Site U1431 and U1433 basalts have mean δ56Feprim = +0.06‰ ± 0.04‰ (2SD; n = 10) and +0.02‰ ± 0.02‰ (2SD; n = 3), respectively. The primary melts of the intraplate rocks have highly variable δ56Feprim (+0.04‰ to +0.22‰). Our new and previously published data show that the δ56Feprim values and Fe/Mn ratios of the Site U1431 and U1433 basalts are positively correlated (r2 = 0.42). This result indicates that the heterogeneous Fe isotopic compositions of the studied basalts reflect distinct mantle source lithologies (i.e. pyroxenite and peridotite) that have also been identified from the chemistry of olivine phenocrysts. Co-variations between δ56Feprim values and Mg–Sr–Nd isotopes for the South China Sea basalts suggest that the end-member components in the sub-ridge mantle include an enriched recycling-related pyroxenitic component with high δ56Fe and low δ26Mg, and a ubiquitous depleted subduction-modified peridotitic component with low δ56Fe and high δ26Mg.

Olivine chemistry reveals compositional source heterogeneities within a tilted mantle plume beneath Iceland

High-Fo olivine (Fo = Mg/(Mg+Fe) mol%) is an ideal proxy for establishing the compositions of primary melts and their mantle sources. This has been exploited in establishing lithological variations in the mantle source regions of oceanic basalts, including in Iceland. However, previous studies on Icelandic olivine lack spatial and temporal coverage. We present high-precision in-situ major, minor and trace element analyses of Fo-rich olivine from a suite of 53 primitive basalts erupted in the neovolcanic rift and flank zones of Iceland, as well as in older regions of Quaternary and Tertiary crust. Most of these samples have previously been analysed for 3He/4He, which ranges from 6.7 to 47.8 RA, the largest span reported for any oceanic island. By combining trace elemental variability with 3He/4He, we assess the extent of lithological variability in the Icelandic mantle plume. Trace-element ratios that are likely to preserve information about mantle source regions (e.g., Mn/Fe, Ni/(Mg/Fe), Ga/Sc, Zn/Fe and Mn/Zn) suggest a peridotitic mantle source in all rift-related volcanic regions, as well as in the off-rift flank zones of Öræfajökull and Snæfellsnes. However, a signal of a more pyroxenitic mantle lithology is clearly visible in olivine from the South Iceland Volcanic Zone, which represents the southward propagation of the Eastern Rift Zone, while olivine from Tertiary lavas suggests a mixed peridotite-pyroxenite source composition. We are able to identify four components present in the Icelandic mantle: a lithologically heterogeneous plume component with 3He/4He >MORB; a depleted MORB-like peridotite; an isotopically enriched MORB-like peridotite; and a peridotitic component with 3He/4He <MORB, sampled in the off-rift flank zone at Öræfajökull. The spatial distribution of these four components can be explained by a northward tilted mantle plume. This has previously been proposed by geophysical and geochemical studies of both Hawaii and Iceland suggesting that the plume geometry could be a contributing factor in controlling some of the spatial variation in source lithology beneath ocean islands.

Petrogenesis and mantle source characteristics of volcanic rocks on Jeju Island, South Korea

Volcanic rocks on Jeju Island, South Korea, are dominantly alkaline suites (basalt to trachyte) with small amounts of intercalated tholeiitic (TH) basaltic rocks. The alkaline suites can be subdivided into high-Al alkaline (HAA) and low-Al alkaline (LAA) sub-suites. We determined the major- and trace-element abundances, and Sr–Nd–Pb–Mg isotopic compositions for 60 samples from four boreholes and nine outcrops on Jeju Island to better understand their petrogenesis and mantle source characteristics. The samples yield whole-rock40Ar/39Ar ages of ~968–24 ka. The HAA suite formed slightly earlier than the other suites. On a primitive-mantle-normalized trace-element diagram, Jeju basaltic rocks exhibit typical oceanic island basalt (OIB)-like large-ion lithophile element (LILE) enrichments without significant Nb or Ta depletions. No meaningful correlation was observed between SiO2 and Sr–Nd–Pb–Mg isotope compositions. However, some samples, especially the TH and HAA suites, exhibit positive Eu and Sr anomalies, coupled with elevated 87Sr/86Sr, Δ7/4Pb, and Pb/Ce ratios, indicating an important role of ancient anorthosite assimilation during its evolution. Furthermore, they show negative K and Rb anomalies. The basaltic rocks have fractionated Nb/Ta and Zr/Hf ratios with negative Ti anomalies. They have elevated Dy/Yb ratios within the range of melts derived from garnet lherzolite, but have Yb contents much higher than those of garnet lherzolite melts, instead plotting close to those of eclogite-derived melts. They also have high Fe/Mn ratios (>~60) and FeO/CaO–3MgO/SiO2 values (>0.45), similar to melts from pyroxenite/eclogite sources. The Sr–Nd–Pb isotopic compositions display a good linear array between depleted mid-ocean-ridge basalt (MORB) mantle and enriched mantle type 2 (EM2). There are no discernible 143Nd/144Nd, Δ8/4Pb, and δ26Mg isotopic differences among the three suites, implying their derivation from a compositionally homogeneous source by variable degrees of melting. The Mg isotopic compositions yield δ26Mg values of −0.53‰ to −0.20‰, extending to a much lower value than normal mantle (δ26Mg = −0.25‰ ± 0.04‰). These observations suggest the presence of recycled rutile-bearing eclogite, carbonate, and sedimentary components in the mantle source of the Jeju volcanic rocks together with a peridotite component. No significant spatiotemporal changes in the mantle sources and melting conditions were observed.

Primary magmas and mantle sources of Emeishan basalts constrained from major element, trace element and Pb isotope compositions of olivine-hosted melt inclusions

Olivine-hosted melt inclusions within lava retain important information regarding the lava’s primary magma compositions and mantle sources. Thus, they can be used to infer the nature of the mantle sources of large igneous provinces, which is still not well known and of the subject of debate. We have analysed the chemical compositions and Pb isotopic ratios of olivine-hosted melt inclusions in the Dali picrites, Emeishan Large Igneous Province (LIP), SW China. These are the first in-situ Pb isotope data measured for melt inclusions found in the Emeishan picrites and allow new constraints to be placed on the source lithology of the Emeishan LIP. The melt inclusions show chemical compositional variations, spanning low-, intermediate- and high-Ti compositions, while their host whole rocks are restricted to the intermediate-Ti compositions. Together with the relatively constant Pb isotope ratios of the melt inclusions, the compositional variations suggest that the low-, intermediate- and high-Ti melts were derived from compositionally similar sources. The geochemical characteristics of melt inclusions, their host olivines, and whole-rocks from the Emeishan LIP indicate that Ca, Al, Mn, Yb, and Lu behave compatibly, and Ti, Rb, Sr, Zr, and Nb behave incompatibly during partial melting, requiring a pyroxenite source for the Emeishin LIP. The wide range of Ti contents in the melt inclusions and whole-rocks of the Emeishan basalts reflects different degrees of partial melting in the pyroxenite source at different depths in the melting column. The Pb isotope compositions of the melt inclusions and the OIB-like trace element compositions of the Emeishan basalts imply that mixing of a recycled ancient oceanic crust (EM1-like) component with a peridotite component from the lower mantle (FOZO-like component) could have underwent solid-state reaction, producing a secondary pyroxenite source that was subsequently partially melted to form the basalts. This new model of pyroxenite melting could explain the geochemical variations among the low-, intermediate- and high-Ti basalts for the Emeishan LIP and challenges the prevailing belief that the source of the Emeishan basalts is peridotite.

Metasomatized Mantle Xenoliths as a Record of the Lithospheric Mantle Evolution of the Northern Edge of the Ahaggar Swell, In Teria (Algeria)

Mantle-derived xenoliths hosted by melilitite lavas from In Teria (Ahaggar, SE Algeria) include garnet and spinel peridotites, pyroxenite and phlogopite megacrysts. The spinel and garnet peridotites record an early deformation event, which formed porphyroclastic microstructures and olivine crystal preferred orientations, followed by static infiltration of hydrous alkaline melts. This metasomatic stage (stage 1) is characterized by the crystallization of phlogopite in the garnet and spinel peridotites, amphibole in the spinel peridotites and clinopyroxene in the garnet peridotite, which record chemical equilibration with an alkaline silicate melt. These early events were largely overprinted by carbonatitic metasomatism (stage 2), which is observed only in the spinel peridotites. Spinel peridotite major and trace element compositions, as well as the compositions of newly formed minerals, are characteristic of interaction with carbonate melt, associated with strong enrichment in incompatible trace elements in clinopyroxene. This second stage was followed by crystallization of pyroxenites (stage 3) in vein conduits, probably segregated from alkaline melts. We propose a scenario in which the different metasomatic imprints record successive stages of interaction between lithospheric mantle and sublithospheric melts throughout the Cenozoic. In Sr–Nd isotope space, the host melilitites and several xenoliths are clustered and plot close to the HIMU mantle end-member. However, some peridotite xenoliths are shifted towards more radiogenic 87Sr/86Sr values. In 207Pb/204Pb–206Pb/204Pb and 208Pb/204Pb–06Pb/204Pb space the In Teria samples define a relatively large domain characterized by high 206Pb/204Pb and 208Pb/204Pb, consistent with a contribution of an HIMU component, considered to represent a sublithospheric signature. The highest 87Sr/86Sr values are comparable with those ascribed to the EM1 mantle end-member, representing the signature of the lower continental lithosphere, and are probably inherited from the pre-metasomatic lithospheric mantle beneath In Teria. Numerical modelling of porous percolation of melt of sublithospheric origin through an EM1-like lithospheric mantle protolith reproduces the In Teria peridotite compositions, using moderately sub-chondritic Sr/Nd values for the peridotite (e.g. In Teria garnet peridotite) and moderately super-chondritic Sr/Nd values in the melt (approximately ocean island basalt values). A few spinel peridotites require a component characterized by a 143Nd/144Nd signature higher than both the EM1 end-member and the local Ahaggar basalts; the 208Pb/204Pb compositions of several samples point to a component with a depleted mid-ocean ridge basalt (MORB) mantle (DMM) signature. Thus the lithospheric mantle beneath In Teria probably did not have a uniform EM1 signature before the onset of metasomatism; it included a DMM peridotite component as well as some peridotites with elevated 143Nd/144Nd values recording long-term LREE depletion.

Interface partition coefficients of trace elements in carbonate-silicate parental media for diamonds and paragenetic inclusions (experiments at 7.0-8.5 GPa)

Interface partition coefficients KD of a representative set of trace elements (TE) in the partly molten diamond-forming peridotite-eclogite- carbonatite system are experimentally determined at 7.0-8.5GPa. The experimental data evidence that trace-element partition does not depend on the melt composition, with heavy rare-earth elements (HREE) concentrating mainly in garnet. Model TE partition coefficients for the natural diamond-producing carbonatite melts of mantle chambers are calculated based on TE concentrations in minerals of peridotite and eclogite parageneses of diamond inclusions, on the one hand, and on the experimental KD coefficients, on the other. The results show that the TE of the parental media are mostly the mantle peridotite components, with the parental media being depleted in medium (Ba, La, Ce, Pr, Nd, Sm, Eu, Gd) and heavy (Tb, Dy, Ho, Er, Yb, Lu, Hf) rare-earth elements relative to the primitive peridotite. The elevated contents of Sr, Nb, and Ce in the completely miscible carbonate-silicate melts might be due to the participation of “metasomatic agents” in the formation of chambers of diamond-forming carbonatite magmas.

Nature and Evolution of Primitive Vesuvius Magmas: an Experimental Study

Two mafic eruptive products from Vesuvius, a tephrite and a trachybasalt, have been crystallized in the laboratory to constrain the nature of primitive Vesuvius magmas and their crustal evolution. Experiments were performed at high temperatures (from 1000 to ≥1200°C) and both at 0·1 MPa and at high pressures (from 50 to 200 MPa) under H2O-bearing fluid-absent and H2O- and CO2-bearing fluid-present conditions. Experiments started from glass except for a few that started from glass plus San Carlos olivine crystals to force olivine saturation. Melt H2O concentrations reached a maximum of 6·0 wt % and experimental fO2 ranged from NNO – 0·1 to NNO + 3·4 (where NNO is nickel–nickel oxide buffer). Clinopyroxene (Mg# up to 93) is the liquidus phase for the two investigated samples; it is followed by leucite for H2O in melt <3 wt %, and by phlogopite (Mg# up to 81) for H2O in melt >3 wt %. Olivine (Fo85) crystallized spontaneously in only one experimental charge. Plagioclase was not found. Upon progressive crystallization of clinopyroxene, glass K2O and Al2O3 contents strongly increase whereas MgO, CaO and CaO/Al2O3 decrease; the residual melts follow the evolution of Vesuvius whole-rocks from trachybasalt to tephrite, phonotephrite and to tephriphonolite. Concentrations of H2O and CO2 in near-liquidus 200 MPa glasses and primitive melt inclusions from the literature overlap. The earliest evolutionary stage, corresponding to the crystallization of Fo-rich olivine, was reconstructed by the olivine-added experiments. They show that the primitive Vesuvius melts are trachybasalts (K2O ∼ 4·5–5·5 wt %, MgO = 8–9 wt %, Mg# = 75–80, CaO/Al2O3 = 0·9–0·95) that crystallize Fo-rich olivine (90–91) as the liquidus phase between 1150 and 1200°C and from 300 to <200 MPa. Primitive Vesuvius melts are volatile-rich (1·5–4·5 wt % H2O and 600–4500 ppm CO2 in primitive melt inclusions) and oxidized (from NNO + 0·4 to NNO + 1·2). Assimilation of carbonate wall-rocks by ascending primitive magmas can account for the disappearance of olivine from crystallization sequences and explains the lack of rocks representative of olivine-crystallizing magmas. A correlation between carbonate assimilation and the type of feeding system is proposed: carbonate assimilation is promoted for primitive magma batches of small volumes. In contrast, for longer-lived, large-volume, less frequently recharged, hence more evolved, cooler reservoirs, magma–carbonate interaction is limited. Primitive magmas from Vesuvius and other Campanian volcanoes have similar redox states. However, the Cr# of Vesuvius spinels is distinctive and therefore the peridotitic component in the mantle source of Vesuvius differs from that of the other Campanian magmas.

IMMOBILE TRACE ELEMENT SYSTEMATICS OF OCEANIC ISLAND BASALTS: THE ROLE OF OCEANIC LITHOSPHERE IN CREATING THE GEOCHEMICAL DIVERSITY

The Earth’s mantle is known to be heterogeneous at different scales, which has been generally linked to the presence of diverse mantle reservoirs, some of which are believed to have remained isolated for long periods of time. When oceanic island basalts (OIBs) are subdivided into five distinct end-member groups on the basis of Sr-Nd-Pb isotope systematics, which include DM, EM1, EM2, HIMU and C, trace element systematics do not appear to be effective discriminators as isotopes, though an end-member signature may become dominant relative to others within a specific ratio range. Melting of a lithologically heterogeneous source or melting of distinct sources, which is followed by melt mixing, appears to be an important mechanism in creating variable geochemi- cal signatures in OIB genesis. Ratio-based trace element modeling suggests that sole involvement of eclogitic components cannot explain the entire elemental variation observed in OIBs; a peridotitic component must have been involved in the genesis of all types of end-member signatures. Combined trace element and isotope systematics are consistent with the involvement of metasomatized oceanic lithosphere (crust + lithospheric mantle) with/without sediments. This mechanism, which involves slab components uprising within the plumes, may also have been the main reason causing the geochemical diversity in the Tethyan mantle.

Deciphering the Trace Element Characteristics in Kilbourne Hole Peridotite Xenoliths: Melt–Rock Interaction and Metasomatism beneath the Rio Grande Rift, SW USA

Significant differences between bulk-rock lithophile trace element budgets and the sum of the contributions from the constituent minerals are common, if not ubiquitous, in peridotite xenoliths. Notwithstanding the possible contributions from fluid inclusions and grain-boundary glass and micro-phases, it is often difficult to reconcile the bulk-rock incompatible element budgets with those of the silicate phases. In the absence of modal metasomatism this discrepancy is often attributed to the ‘catch-all’yet often vague process of cryptic metasomatism.This study presents comprehensive petrological descriptions, major and trace element and Sr^Nd isotope data for variably metasomatized bulk-rock peridotites, host basalt, and constituent peridotite mineral phases from spinel lherzolite and harzburgite xenoliths from the Kilbourne Hole volcanic maar, New Mexico, USA. Similar measurements were also made on hand-picked interstitial glass from one of the most highly metasomatized samples in an attempt to unravel the sources, effects, and relative timings of multiple metasomatic events. Reaction textures around clinopyroxene grains are evident in the most metasomatized samples. These are accompanied by films of high-SiO2 interstitial glass, which is not in equilibrium with the primary silicate minerals. Trails of glassy melt inclusions terminate in these films against which the margins of the primary minerals appear partially resorbed. Furthermore, different styles of high field strength element fractionation [e.g. (Nb/Ta)N vs (Zr/Hf)N] are evident in the bulk-rocks and the clinopyroxenes that they host. In all of the Kilbourne Hole peridotites analysed, hand-picked, optically clean clinopyroxenes preserve a more unradiogenic Sr isotope signature than the corresponding bulk-rock. Hand-picked interstitial glass from KH03-16 reveals the most radiogenic Sr/Sr of any component recovered from these xenoliths (Sr/Sr1⁄4 0·708043 0·00009; [Sr]1⁄4 81ppm). Similarly, the Nd/Nd of the glass is amongst the most evolved of the peridotite components (Nd/Nd1⁄4 0·512893 0·000012; [Nd]1⁄4 10 ppm). However, the host basalt (Sr/Sr1⁄4 0·703953 0·00012; Nd/Nd1⁄4 0·512873 0·000013), similar in composition to that of the nearby, contemporaneous, Potrillo Volcanic Field basalts, contains nearly an order of magnitude more Sr and more than three times more Nd ([Sr]1⁄4 655 ppm; [Nd]1⁄4 34 ppm) than the interstitial glass. Despite the high Sr and Nd contents of the host basalt the evidence for basalt infiltration is scant, although the effects of melt^rock interaction, both in antiquity and more recently, are preserved in several xenoliths. Mixing between clinopyroxene and the host basalt cannot account for the full range of bulk-rock Sr^Nd isotope ratios; nearly half of the xenoliths require an additional component that could involve varying amounts of interstitial glass. In detail the behaviour of Sr and Nd isotopes has been decoupled,

Diamond-forming efficiency of chloride-silicate-carbonate melts

According to the carbonatite model of the diamond formation [Litvin, 2007] based on the large volume of mineralogical and physicochemical experimental information, carbon-silicate-carbonate (carbonatitic) melts with widely variable compositions are the growth medium for most mantle diamonds and inclusions in them. In addition to the major completely miscible carbonate and silicate components (minerals of peridotite and eclogite assemblages), such melts contain minor soluble components (oxides, phosphates, chlorides, C-O-H-N fluids, and others), as well as minor completely immiscible and insoluble solid and melt phases (sulfides, metals). The diamond-forming efficiency of silicate-carbonate melts clearly corresponds to the important criterion of syngenesis of diamond with its silicate and carbonate inclusions. High-pressure experimental associations not only comprise the whole set of minerals typical for inclusions in diamonds of peridotitic (olivine, garnet, clino-, and orthopyroxenes) and eclogitic (garnet and clinopyroxene) type, but demonstrate the characteristic features of minerals of diamond paragenesis. These comprise significant admixtures of Na in garnets and K in clinopyroxenes, which are the reliable indicators of crystallization of these minerals from alkaline silicate-carbonate melts. Experimental investigations of multicomponent peridotite-carbonatite and eclogite-carbonatite systems at a standardized pressure of 8.5 GPa within the narrow pressure range of 1760–1820°C demonstrated that the diamond-forming efficiency of their carbon-bearing melts had concentration limitations [Bobrov and Litvin, 2009]. The compositions effective for the diamond nucleation comprise only significantly carbonatite parts of the systems and are limited by the concentration barriers of diamond nucleation (CBDN) in the cases of K-Na-Ca-Mg-Fe-carbonatite, Ca-Mg-, and Kcarbonate compositions (at the concentrations of 30, 25, and 30 wt % of peridotite components and 35, 30, and 45 wt % of eclogite component, respectively). This means that the inhibitory influence of peridotite and eclogite components dissolved in carbon-bearing carbonatite melts on diamond nucleation is observed only at their relatively low concentrations. We established the diamond-forming efficiency of chloride [Litvin, 2003] and chloride

Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley)

We present a comprehensive set of data on compositions of melt inclusions and earliest crystallized mineral phases from mafic lavas of Roccamonfina and Ernici, situated in a central sector of the string of Pliocene–Quaternary potassic volcanic centres along the Tyrrhenian border of peninsular Italy. Studied samples of mafic lavas (4.4–7 wt.% MgO) cover a wide spectrum of potassium levels, and represent magmas considered to be parental to the ultrapotassic leucite-bearing high-K series (HKS, 4–8 wt.% K 2O) and to shoshonitic (1.5–5%) and subalkaline (< 1.5%) series, here collectively referred to as medium-low potassic series (M-LKS). Highly variable compositions of melt inclusions in olivine hosts (Fo = 89–91.5) from single lava samples indicate that all parental magmas are composed of diverse collections of primary melts, consistent with extraction from heterogeneous vein-type mantle lithologies. Major and trace-element systematics provide evidence that primitive HKS and M-LKS magmas originated from separate domains and not through changes in the proportion of vein and wall-rock peridotite during progressive melting of a common source. We infer that parental HKS magmas are largely derived from the vein portion of a heterogeneous phlogopite (± amphibole ± apatite) wehrlitic mantle. Contrasting major element, volatile and trace-element signatures of melt inclusions from medium-K lavas point to an amphibole-bearing wehrlitic source for shoshonitic magmas, probably with a subordinate role of phlogopite. Finally, a population of silica-undersaturated potassium-poor melt inclusions with extreme CaO/Al 2O 3 ratios (> 1.2) and fluid-depleted signatures suggests that subalkaline magmas originate either from the same source following the exhaustion of amphibole, or from a separate wehrlitic–pyroxenitic (± apatite ± carbonate?) assemblage. Our melt inclusion data are consistent with a mixed metasomatic imprint by siliceous potassium-rich and carbonate-rich (carbonatitic) potassium-poor melts. Siliceous melt components dominated in the HKS sources, but a minor group of potassium-poor melt inclusions in HKS samples, with compositions not represented by erupted products, carries trace-element signatures pointing to metasomatism by carbonatite-like melts. The melt inclusions show a general inverse relationship between fO 2 and potassium enrichment, suggesting that primitive potassium-poor magmas are the most oxidized (NNO + 0.5), whereas primitive HKS magmas have the lowest oxidation state (NNO − 1.6). Mineral chemistry of (near-)liquidus assemblages varies systematically according to the signature of the primary melts from which they crystallized. Calcium contents of Mg-rich olivines and clinopyroxenes are unrelated to the calcium content of the melts but are higher in HKS than in M-LKS samples. Forsterite-rich olivines contain Cr-spinel inclusions with Cr/(Cr + Al) ratios that tend to be higher in M-LKS (0.58–0.80) than in HKS (0.48–0.67) samples. We infer that this parameter, particularly at the more elevated values, is predominantly controlled by the metasomatic veins, and does not necessarily reflect the (‘refractory’) nature of the pre-metasomatic peridotite component. We surmise that primitive HKS and M-LKS melts of Roccamonfina–Ernici are derived by lower degrees of partial melting of vein lithologies with a stronger metasomatic imprint of the carbonatitic component and more subordinate involvement of pristine wall-rock peridotite than their equivalents of the northern Roman Province. The distinctive HKS and M-LKS sources point to a layered vein-permeated mantle column. Melting possibly occurred in response to a heat pulse induced by exposure to hot asthenospheric mantle, facilitated by post-collisional slab tearing and shallow break-off.

Source components of the Gran Canaria (Canary Islands) shield stage magmas: evidence from olivine composition and Sr–Nd–Pb isotopes

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (87Sr/86Sr = 0.70315–0.70331, 143Nd/144Nd = 0.51288–0.51292, 206Pb/204Pb = 19.55–19.93, 207Pb/204Pb = 15.60–15.63, 208Pb/204Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with 87Sr/86Sr = 0.70337, 143Nd/144Nd = 0.51291, 206Pb/204Pb = 19.36, 207Pb/204Pb = 15.61 and 208Pb/204Pb = 39.07 (EM-type end member), and pyroxenite with 87Sr/86Sr = 0.70309, 143Nd/144Nd = 0.51289, 206Pb/204Pb = 20.03, 207Pb/204Pb = 15.62 and 208Pb/204Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.

MINERALOGY AND PETROLOGY OF ASSOCIATED Mg-RICH ULTRAPOTASSIC, SHOSHONITIC, AND CALC-ALKALINE ROCKS: THE MIDDLE LATIN VALLEY MONOGENETIC VOLCANOS, ROMAN MAGMATIC PROVINCE, SOUTHERN ITALY

In the Middle Latin Valley, in the Roman Magmatic Province, southern Italy, volcanism developed during the Late Pleistocene, with eruption of high-Mg magmas characterized by a low content of phenocrysts. Small volumes of magma, transported to the surface rapidly, favored by an extensional post-collision regime, led to small monogenetic centers aligned along two main tectonic trends. The absence of a large magmatic reservoir allowed the occurrence of a fairly large amount of mafic, strongly primitive volcanic rocks with a fairly large enrichment in potassium. They range in composition from plagioclase-free ultrapotassic melilite-bearing (kamafugites) to ultrapotassic leucitites and plagioclase leucitites, to trachybasalts (shoshonites), and calc-alkaline basaltic rocks (subalkaline). The composition of minerals from the four different groups of rock is reported. The Fe–Mg distribution between olivine crystals and whole rocks indicates equilibrium crystallization. The clinopyroxene shows the typical trend of alkaline potassic and ultrapotassic rocks, ranging in composition from diopside to hedenbergite. Melilite-bearing ultrapotassic rocks are found beside feldspar-bearing ultrapotassic and potassic primitive rocks. Magnesiochromite is the main spinel; it has been found enclosed in olivine cores. Olivine–spinel pairs indicate that the magmas from which they crystallized were in equilibrium with a strongly depleted mantle source. An origin from a common mantle source, in terms of a peridotitic component, is inferred for the different varieties of ultrapotassic to subalkaline primitive rocks. This source was characterized by different degrees of metasomatic enrichment arranged in a network of veins, possibly within the lithospheric upper mantle.

Controls on magmatism in an island arc environment: study of lavas and sub-arc xenoliths from the Tabar–Lihir–Tanga–Feni island chain, Papua New Guinea

The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area.

Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187 Os/ 188 Os; however, the postorogenic lavas are characterized by radiogenic 187 Os/ 188 Os ratios (0.139‐0.559). Simple modeling shows that only very large degrees of melt depletion (>25%‐30%) can lower source Re/Os ratios sufficiently to permit timeintegrated development of subchondritic 187 Os/ 188 Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25%. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere.

Melt Generation Beneath Ocean Ridges: Re-Os Isotopic Evidence from the Polar Ural Ophiolite

We present Re and Os isotope data for a set of well characterized rocks from the crustal and mantle sections of the Voykar Massif, Polar Urals Ophiolite Complex and investigate the nature and possible role of mantle heterogeneities in melt generation and formation of the oceanic lithosphere under spreading centers. The origin of the mid-ocean ridge basalt (MORB), especially the mechanisms of melt formation and extraction, are issues that are still poorly understood. In particular, recent indications that osmium isotopes may be systematically more radiogenic in MORB than in oceanic peridotites (Snow and Reisberg, 1995; Schiano et al, 1997) have rejuvenated an old debate about chemical and isotopic equilibrium between melt and residue and about the role of local heterogeneities in controlling the composition of the melt. Moreover, neodymium isotope studies of crustal and mantle lithologies exposed in ophiolite complexes have indicated that the igneous rocks of the oceanic crust are not always in equilibrium with the bulk of the underlying peridotites. Finally, there is circumstantial evidence from Lu-Hf systematics and from Th-U partitioning data that MORB melting may occur in the presence of garnet. All of this has revived speculation that MORB melts may be produced from mixed sources consisting of a peridotite component and a garnet pyroxenite component, derived from subducted oceanic crust (All+gre and Turcotte, 1986; Hirschmann and Stolper, 1996). If MORB really are pooled melts derived from separate and compositionally different source components, then the various crust-mantle lithologies exposed in ophiolites may provide more detailed information on probable source materials and their interaction.

Phase relationships in the system KAlSiO4-Mg2SiO4-SiO2-H2O as a model for hybridization between hydrous siliceous melts and peridotite

The system KAlSiO4-Mg2SiO4-SiO2-H2O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi3O8-SiO2-H2O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H2O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.