Spinel Lherzolite Research Articles

MELT DIFFUSE REFERTILIZATION AND FOCUSED MIGRATION IN LITHOSPHERIC MANTLE OF A FOSSILE RIFTED MARGIN: THE MT. NERO PERIDOTITE (EXTERNAL LIGURIDES, ITALY)

MELT DIFFUSE REFERTILIZATION AND FOCUSED MIGRATION IN LITHOSPHERIC MANTLE OF A FOSSILE RIFTED MARGIN: THE MT. NERO PERIDOTITE (EXTERNAL LIGURIDES, ITALY)

EXHUMATION HISTORY OF SUBCONTINENTAL GARNET PYROXENITE-BEARING MANTLE FROM THE EXTERNAL LIGURIDE OPHIOLITES (NORTHERN APENNINE, ITALY): IMPLICATIONS FOR RIFTING AND MANTLE EXHUMATION PROCESSES AT OCEAN-CONTINENT TRANSITION

The External Liguride Jurassic ophiolites derive from an ocean-continent transition similar to the West Iberian margin. The mantle sequence consists of spinel and spinel-plagioclase lherzolites recording a lithospheric thinning history culminated into mantle exhumation at the ocean-floor. They represent unroofed subcontinental litosphere of the former Adria-Europe system largely affected by thermochemical erosion, heating and refertilization by asthenospheric melts during the Ligurian Tethys formation (Rampone et al., 1995; Piccardo et al., 2004). Peridotite bodies which escaped melt contamination and intrusion preserve relics of a deep-seated litospheric origin, i.e. occurrence of graphitebearing garnet clinopyroxenite and websterite layers. The garnet pyroxenite-bearing mantle from the External Liguride ophiolites therefore represents a unique tectonic sampling of deep levels of subcontinental lithosphere exhumed in an oceanic setting. Garnet clinopyroxenites are mafic rocks with a primary assemblage of pyrope-rich garnet + sodic Al-augite (Na2O ~2.5 wt%, Al2O3 ~12.5 wt%), with minor amounts of orthopyroxene, graphite, Fe-Ni sulphides and rutile. Decompression yielded Na-rich plagioclase (An50-45) exsolutions in clinopyroxene porphyroclasts and extensive development of symplectites composed of secondary orthopyroxene + plagioclase (An85-72) + Al-spinel ± clinopyroxene ± ilmenite at the interface between garnet and primary clinopyroxene. Further decompression is recorded by the development of an olivine+ plagioclase-bearing assemblage, locally under synkinematic conditions, at the expenses of two-pyroxene + Alspinel. Mg-rich garnet has been also found in websterite layers, which are commonly characterised by the occurrence of symplectites made of orthopyroxene + Al-spinel ± clinopyroxene. The enclosing peridotites are Ti-amphibole-bearing lherzolites with a fertile geochemical signature and a widespread plagioclase-facies mylonitic foliation, which preserve in places a spinel tectonite fabric. Lu-Hf and Sm-Nd mineral isochrons (220 ± 13 Ma and 186 ± 1.8 Ma, respectively) have been obtained from a garnet clinopyroxenite layer and interpreted as cooling ages. Geothermobarometric estimates for the high-pressure equilibration have yielded T = 1150-1200 °C and P = 2.6-3.0 GPa. The early decompression was associated with significant cooling, corresponding to T = 900-1050°C, and development of the spinel tectonite fabric in the lherzolites. Further decompression associated with plagioclase-olivine growth in both peridotites and pyroxenites was nearly isothermal. The shallow evolution was characterized by a widespread polyphase brittle deformation under decreasing temperature conditions, coupled with hydration. In particular, the brittle evolution includes an early amphibolite-facies veining stage, followed by low-temperature hydrothermal alteration and serpentinisation associated with polyphase cataclasis. The exhumation was likely accomplished as a two-step process starting during Late Palaeozoic continental extension. The low-pressure portion of the exhumation path, possibly including also the plagioclase mylonitic shear zones, was related to the Mesozoic (Triassic to Jurassic) rifting that led to continental break-up. In Jurassic times, the studied mantle sequence was involved into an extensional detachment structure up to sea-floor exhumation, which was associated with development of serpentinisation-related hydrothermal activity. Integrated values of cooling and uplift rates for the Jurassic rifting evolution are ~35°C/Ma and 0.6-1.4 mm yr-1.

CHARACTERISATION OF THE LITHOSPHERE BENEATH THE NORTH-WESTERN ETHIOPIAN PLATEAU: SPINEL LHERZOLITES FROM INJIBARA (LAKE TANA PROVINCE)

We have investigated xenoliths of spinel lherzolites in a Quaternary basanitic flow near Injibara (Tana Lake Province) to characterise the mantle beneath the north-western Ethiopian Plateau. Previous studies of mantle rocks from the Main Rift (e.g. Mega, South Ethiopia; Bedini et al., 1997), have indicated that peridotites suffered modal (apatite) or cryptic (enrichment/depletion in LILE) metasomatism. The metasomatic agents were identified as silicate melts migrating through the lithospheric mantle. At Injibara, lherzolites [Ol (46-69), Opx (19-31), Cpx (9-22), and Spl (2-7) in vol.%] show porphyroclastic to equigranular textures. These rocks suffered a modal metasomatic event, testified by growth of Ti-pargasite, and enrichment in major (Mg,Na,Ti) and trace elements (LREE,Rb,Ba,Th,U,Pb). Equilibration temperatures are 907-1015 °C (median values 960-970°C), in agreement with previous data obtained from Injibara mantle xenoliths (954-985°C, Conticelli et al., 1999). Fluid inclusions (<1-40 _m) occur only in the most preserved rocks, along intragranular trails within porphyroclasts, and as rare primary inclusions in Opx neoblasts. At room temperature, monophase (L) and biphase (L+V) inclusions are observed with rare birefringent solids, recognised as magnesite by Raman spectroscopy. Microthermometric data reveal the presence of CO2 that melts (TmCO2) instantaneously at @ -56.6°C. Most of inclusions homogenise to the liquid (ThLCO2) between –39.2 and 31°C, and only rare homogenise to the vapour (ThVCO2; 28.3 - 30.9°C). For a temperature of 970°C, a minimum pressure of 0.95 GPa is obtained by the highest density (1.11 g/cm3) fluid isochore. In conclusion, our data indicate that the mantle under Injibara (North-Western Ethiopian Plateau) underwent a different evolution with respect to the mantle beneath the Rift. In mantle rocks beneath Injibara, growth of amphibole and the LILE enrichment are strictly associated to the migration of a CO2-rich fluid, whereas in rift peridotites, apatite growth and variable LILE contents are related to interaction with silicate melts.

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

MANTLE EXHUMATION PROCESSES IN THE OCEAN CONTINENT TRANSITION OF MAGMA-POOR RIFTED MARGINS

Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites, nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth's history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace element geochemistry, it has been suggested that these lavas represent low-degree melts that have undergone little crystal fractionationen route to the surface. Many of these lavas also carry high-pressure mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that these magmas cooled to variable extents during their passage. We note subtle, yet important, differences in terms of CaO, Al2O3, CaO/AlP2O3, and CaO/MgO. High-pressure experimental melting studies in CMAS-CO2 (3-8 GPa) and natural lherzolitic systems (3GPa) demonstrate that at an isobar increasing F leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet (>- 85 km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with anhydrous ones, also have higher CaO content in the coexisting melt at a given P and T. Comparison of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally determined high-pressure melts indicates that melting of anhydrous mantle lherzolite or garnet pyroxenite is not able to explain many of the major element systematics of the lavas. However, high-pressure partial melts of carbonated lherzolite have the right major element trends. Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced partial melting. On continents, lavas from South Africa and certain localities in Germany are the products of a very low degree of partial melting, and those from Texas and certain other localities in Germany are products of a slightly more advanced degree of partial melting of a carbonated lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas does not necessarily mean that the erupted lavas represent direct mantle melts.

Cenozoic Magmatism of the North-Eastern Eurasian Margin: The Role of Lithosphere Versus Asthenosphere

Sikhote-Alin and Sakhalin are located in the Russian Far East flank of the northernmost part of the Sea of Japan. Magmatism in this region preceded, was concurrent with, and continued after the extension and sea-floor spreading (25–18Ma) that formed the Sea of Japan. Among the Sikhote-Alin and Sakhalin volcanic suites, Eocene–Oligocene (55–24Ma) lavas are characterized by greater large ion lithophile element and rare earth element enrichments compared with Early–Mid-Miocene (23–15Ma) tholeiites, and also show a depletion in high field strength elements (HFSE). The geochemical characteristics of the Eocene–Oligocene and Early– Mid-Miocene basalts are consistent with migration of the locus of magma generation beneath the Sikhote-Alin and Sakhalin areas from subduction-modified lithospheric mantle into mid-ocean ridge basalt (MORB)-source asthenosphere as spreading in the Sea of Japan progressed. Mid-Miocene–Pliocene (14–5Ma) lavas, erupted following the opening of the Sea of Japan, include alkaline and subalkaline basalts with wide ranges in trace-element abundances, varying between two distinct end-members: (1) volumetrically minor alkaline basalts with Zr–Nb and Sr–Nb–Pb isotope compositions similar to asthenosphere-derived, intra-plate–hotspot basalts from eastern China; (2) more abundant, lithosphere-derived, low-alkali tholeiites depleted in HFSE. The similarity of isotopic signatures coupled with systematically different rare earth element (REE) abundances in the Mid-Miocene–Pliocene and Chinese basalts are best modeled by similar extents of melting of spinel lherzolite and garnet lherzolite, respectively. The Mid-Miocene– Pliocene alkali basalts were generated by small degrees of partial melting of hot asthenosphere beneath a thin lithospheric lid; the thin lithospheric mantle beneath the Sikhote-Alin and Sakhalin region resulted from heating and extension associated with the opening of the Sea of Japan.

Composition and evolution of lithosphere beneath the Carpathian–Pannonian Region: a review

Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland). The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere. The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al 2O 3 with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE). Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene. Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.

Distribution of high field strength and rare earth elements in mantle and lower crustal xenoliths from the Southwestern United States: The role of grain-boundary phases

Spinel lherzolite and harzburgite xenoliths from the Basin and Range and Colorado Plateau Provinces in the southwestern United States (SWUS) show a broad range in incompatible element distributions from depleted to enriched, both within and between sites. The most fertile xenoliths occur in the Basin and Range Province and the most refractory in the Colorado Plateau or Transition Zone. Mass balance calculations indicate that up to 80% of the high field strength (Nb, Ta, Zr, Hf, Th, Ti) and rare earth elements (50% for Yb) in these xenoliths occur in phases along grain boundaries (or in microfractures). In lower crustal and eclogite xenoliths, up to 90% of these elements occur in grain-boundary phases. Nb/Ta, Zr/Hf, La/Sm and Nb/Th ratios and Nb-Ta anomalies in both types of xenoliths are also controlled by grain-boundary phases. To interpret these ratios in whole-rock analyses, it is critical to understand the timing and origin of the grain-boundary components in the xenoliths. Most of the enriched mantle xenoliths appear to have been enriched by metasomatic fluids related to silicate magmas and not to carbonatite magmas. Because of a complex multi-event history that affected the composition of the xenoliths, there is no simple geochemical relationship between magmatic or metamorphic history of the lower crust and upper mantle in the SWUS as reflected by the xenolith populations.

Metasomatism and melting history of a Variscan lithospheric mantle domain: evidence from the Puy Beaunit xenoliths (French Massif Central)

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F≈25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F≈17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.

Ultramafic Xenoliths from the Bearpaw Mountains, Montana, USA: Evidence for Multiple Metasomatic Events in the Lithospheric Mantle beneath the Wyoming Craton

Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al 2 O 3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al 2 O 3 and TiO 2 ) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0·5121 (close to the host minette values) to 0·5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0·5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas. The cumulate group ranges from clinopyroxene-rich mica peridotites (including abundant mica wehrlites) to mica clinopyroxenites. Most contain >30% phlogopite. Their mineral compositions are similar to those of phenocrysts in the host minettes. Their whole-rock compositions are generally poorer in MgO but richer in incompatible trace elements than those of the tectonite peridotites. Whole-rock trace element patterns are enriched in large ion lithophile elements (LILE; Rb, Cs, U and Pb) and depleted in HFSE (Nb, Ta Zr and Hf) as in the host minettes, and their Sr–Nd isotopic compositions are also identical to those of the minettes. Their clinopyroxenes are LREE-enriched and formed in equilibrium with a LREE-enriched melt closely resembling the minettes. The cumulates therefore represent a much younger magmatic event, related to crystallization at mantle depths of minette magmas in Eocene times, that caused further metasomatic enrichment of the lithosphere.

Genesis and evolution of the lithospheric mantle beneath the Buffalo Head Terrane, Alberta (Canada)

Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between ∼135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m 2. The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth. A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions. Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic 187Os/ 188Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle. A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions. If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events.

Origin of the Gabbro-Peridotite Association from the Northern Apennine Ophiolites (Italy)

The Northern Apennine ophiolites are remnants of the Middle Jurassic---Early Cretaceous lithosphere from the Ligurian Tethys. New trace element and Nd---Sr isotope investigations were performed on: (1) the rare gabbros associated with the subcontinental mantle rocks from the External Liguride ophiolites; (2) the gabbro--peridotite association from the poorly known ophiolitic bodies from Cecina valley (Southern Tuscany). Clinopyroxenes from the External Liguride and Cecina valley gabbros have similar trace element compositions, which are consistent with formation from normal mid-ocean ridge basalt (N-MORB) magmas. Sm---Nd mineral isochron ages are 179 9Ma for an External Liguride gabbro and 170 13Ma and 173 5 4 8Ma for two different gabbroic bodies from the Cecina valley ophiolites. These ages are interpreted to date the igneous crystallization of the gabbros and are slightly older than the oldest pelagic sediments of the Ligurian Tethys. Initial eNd (þ8 5 toþ8 9) and Sr/Sr of clinopyroxene are consistent with the interpretation that the studied gabbros were derived from N-MORB magmas. The least serpentinized mantle rocks from the Cecina valley ophiolites are porphyroclastic spinel lherzolites displaying a residual geochemical signature. They are similar to the least depleted residual peridotites from modern oceans. Nd and Sr isotopic ratios for separated mantle clinopyroxene are respectively higher (e.g. eNd 1⁄4 þ11) and lower than those of clinopyroxene from associated gabbros at the time of the gabbro intrusion. The gabbro---peridotite associations from the Northern Apennine ophiolites record the progression of the rifting process that led to opening of the Ligurian Tethys.

Platinum group element geochemistry of komatiites from the Alexo and Pyke Hill areas, Ontario, Canada

Thirty-three whole-rock drill core samples and thirteen olivine, chromite, and sulfide separates from three differentiated komatiite lava flows at Alexo and Pyke Hill, Canada, were analyzed for PGEs using the Carius tube digestion ID-ICP-MS technique. The emplaced lavas are Al-undepleted komatiites with ∼27% MgO derived by ∼50% partial melting of LILE-depleted Archean mantle. Major and minor element variations during and after emplacement were controlled by 30 to 50% fractionation of olivine Fo93–94. The emplaced lavas are characterized by (Pd/Ir)N = 4.0 to 4.6, (Os/Ir)N = 1.07, and Os abundances of ∼2.3 ppb. Variations in PGE abundances within individual flows indicate that Os and Ir were compatible (bulk DOs,Ir = 2.4–7.1) and that Pt and Pd were incompatible (bulk DPt,Pd < 0.2) during lava differentiation, whereas bulk DRu was close to unity. Analyses of cumulus olivine separates indicate that PGEs were incompatible in olivine (DPGEsOl-Liq = 0.04–0.7). The bulk fractionation trends cannot be accounted for by fractionation of olivine alone, and require an unidentified Os-Ir-rich phase. The composition of the mantle source (Os = 3.9 ppb, Ir = 3.6 ppb, Ru = 5.4 ppb, Pt and Pd = 5.7 ppb) was constrained empirically for Ru, Pt, and Pd; the Os/Ir ratio was taken to be identical to that in the emplaced melt, and the Ru/Ir ratio was taken to be chondritic, so that the absolute IPGE abundances of the source were determined by Ru. This is the first estimate of the PGE composition of a mantle source derived from analyses of erupted lavas. The suprachondritic Pd/Ir and Os/Ir of the inferred Abitibi komatiite mantle source are similar to those in off-craton spinel lherzolites, orogenic massif lherzolites, and enstatite chondrites, and are considered to be an intrinsic mantle feature. Bulk partition coefficients for use in komatiite melting models derived from the source and emplaced melt compositions are: DOs,Ir = 2.3, DRu = 1.0, DPt,Pd = 0.07. Ruthenium abundances are good indicators of absolute IPGE abundances in the mantle sources of komatiite melts with 26 to 29% MgO, as Ru fractionates very little during both high degrees of partial melting and lava differentiation.

Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

Synopsis Pyropic garnet occurs within garnet pyroxenite xenoliths and as discrete crystals in Carboniferous–Permian alkali basaltic tuffs and hypabyssal intrusions. Eight localities are now known from the Midland Valley together with another in the NW Highlands. The xenoliths are inferred to have been derived from minor pyroxenitic intercalations in spinel lherzolite within the lithospheric mantle. Although pyrope crystals at Elie Ness are regarded as high-pressure phenocrysts cognate with the basanitic host, it is indeterminable whether discrete garnets at the other localities are phenocrysts or xenocrystal fragments derived from garnet pyroxenites. All, however, whether phenocrystic, xenocrystic or integral components of pyroxenitic xenoliths, are regarded as cognate in the sense of being products of high-pressure crystallization of late Palaeozoic basanite magmas. Kaersutite, phlogopite, hercynitic spinel, ilmenite and pseudomorphs after orthopyroxene occur as minor components in some of the garnet pyroxenite xenoliths. Mineral compositional data (electron- and ion-microprobe) are presented, with comparisons to xenolith-megacryst suites elsewhere. The garnets have strong similarities with each other and with megacryst populations from alkali basalts outside the U.K. It is concluded that the pyroxenites originally crystallized from basaltic magmas as high-pressure cumulates from depths of &gt;50 km and represent some of the deepest samples from the Scottish lithosphere.

Li abundances in inclusions in diamonds from the upper and lower mantle

Lithium abundances in inclusions in diamonds of peridotitic, websteritic, eclogitic (including garnets with majoritic component) and lower mantle paragenesis were determined with the ion microprobe. The partitioning of Li both between olivine and clinopyroxene and clinopyroxene and garnet appears to be pressure-dependent in peridotites. More importantly, garnets containing a majorite component show a dramatic increase in Li solubility so that even for mildly majoritic garnets D cpx/majorite is smaller than one. Lower mantle ferropericlase is a sink for lithium, containing several hundred times more Li than coexisting Mg-perovskite. Lithium decreases in olivine inclusions with increasing forsterite content, which we interpret as reflecting a depletion event in the protoliths of the inclusions. We also infer that lithium was not re-enriched in the silicate phases by a metasomatic agent during diamond formation unlike other trace elements like REE and HFSE. Depletion on partial melting and no re-enrichment also seems to hold for the eclogitic suite of inclusions where cpx's (and deduced bulk composition) have similar Li abundances as in the peridotitic suite and are significantly lower than in crustal eclogites. For the lower mantle bulk, lithium estimates give values four to five times higher than primitive mantle. This may relate either to local enrichment through fluid/melt metasomatism similarly as for spinel lherzolites or high Li abundances in lower mantle diamonds sources are inherited from crustal protoliths.

An experimental investigation of the diffusive infiltration of alkalis into partially molten peridotite: Implications for mantle melting processes

When a silica‐undersaturated melt is juxtaposed with partially molten peridotite, alkali elements rapidly diffuse into the peridotite in a process referred to as the diffusive infiltration of alkalis (DIA). Four types of piston cylinder experiments were performed providing constraints on the DIA process: (1) simple phase relation experiments, (2) melt‐melt diffusion couples, (3) experiments examining the reaction between powders of basanite and spinel lherzolite (direct mixture experiments), and (4) diffusive infiltration‐reaction couples between basanite and partially molten peridotite. Melt‐melt diffusion couples constrain effective binary diffusion coefficients (EBDC) for all major elements except Na to be in the 10−6 to 10−7 cm2/s range at 1450°C and 0.9 GPa. The Na concentration profile is not binary being coupled to gradients in SiO2. EBDCs for Cl, Li, Rb, Sr, Ba, and La are in the 10−6 cm2/s range while Nb and Zr are in the 10−7 cm2/s range. A time series of direct mixture experiments shows that basanite reacts with lherzolite to reduce the orthopyroxene mode to a constant value within 15 min at 1300°C and 0.9 GPa. Quench modified melts in the direct mixture experiments have 50–51 wt.%. SiO2, independent of the basanite/lherzolite ratio. In contrast, infiltration‐reaction experiments show that sodium diffuses from basanite into partially molten peridotite in 10–30 min, resulting in quench modified melt pools with up to 64 wt.% SiO2 within the peridotite. Modal analysis shows that addition of alkalis causes orthopyroxene to incongruently break down to olivine plus silica rich melt. In experiments using a basanite with enriched Cl concentrations, a distinct boundary zone appears between the basalt‐peridotite interface and the peak in SiO2 concentration. Melts within this boundary zone have elevated CaO and Cl concentrations relative to both the basanite and melts within the peridotite beyond the boundary zone. The repeated formation of this boundary zone may indicate an important aspect of the DIA process, possibly responsible for the formation of anorthitic plagioclase and CaO rich melts.

Trace element partitioning on the Tinaquillo Lherzolite solidus at 1.5 GPa

The partition coefficient ( D) for any trace element in the mantle is a function of pressure, temperature and phase composition, the latter of which is controlled by the relative fertility of the peridotite. Models of mantle melting therefore require accurate determination of partition coefficients along the peridotite solidus. We have experimentally determined partition coefficients for clinopyroxene, orthopyroxene and olivine in equilibrium with melt, close to the anhydrous solidus of refractory Tinaquillo Lherzolite at 1.5 GPa, 1315 °C. All trace elements, apart from Sc, are incompatible in clinopyroxene, and the maximum D REE is D Er at 0.76. All trace elements are incompatible in orthopyroxene; the most compatible REE is Lu ( D Lu=0.18). Partition coefficients for olivine are generally <0.005, and only Ti and Li are moderately compatible ( D Ti=0.10; D Li=0.29). Compared to a previous study of trace element partitioning at the solidus of a more fertile peridotite (mid-ocean ridge basalt (MORB)-pyrolite-90, MPY-90) at 1.5 GPa, it is shown that, at fixed pressure, partition coefficients, especially for clinopyroxene, generally increase with increasing source fertility. D U/ D Th for clinopyroxene is 0.86±0.02 and for orthopyroxene is 1.49±0.03, significantly lower than D U/ D Th ratios previously determined for MPY-90 at 1.5 GPa. When compared with pyroxene partition coefficients experimentally determined on the MPY-90 solidus at 3.0 GPa, the partition coefficients for the majority of trace elements increase as pressure decreases along the mantle solidus. Notable exceptions are Li and Na which become more compatible as pressure increases along the solidus. The combined effects of pressure, temperature and source fertility mean that partition coefficients increase with decreasing pressure during decompression mantle melting beneath mid-ocean ridges. By expressing the variation in D as a function of melt fraction ( F), the new data can be used to model decompression melting taking into account changes in partition coefficients as melting proceeds. We show that the trace element composition of residual (or “trapped”) melts is sensitive both to source mineralogy (i.e. garnet–lherzolite versus spinel–lherzolite) and to the depth range over which melting occurs. The variation in partition coefficients during mantle melting therefore provides a valuable tool in the interpretation of melt inclusions. The variable partition coefficients have much less impact on the composition of aggregated melts.

Chromian spinel during melting experiments of dry peridotite KLB-1) at 1.0–2.5 GPa

The stability and chemistry of chromian spinel were determined for dry mantle (spinel lherzolite) melting at pressures between 1.0 and 2.5 GPa and temperatures between 1250 and 1500 ∞C. To find the most suitable sample container, we tested three different possibilities at 1.5 GPa: a Pt capsule, a Re/Pt capsule, and a graphite/Pt capsule. The oxygen fugacity of the run products with Pt, Re/Pt, and graphite/Pt are FMQ + 4 to 5, ‐2 to 0, and < ‐10, respectively. We conclude that the Re/Pt capsule is most suitable for melting experiments of dry peridotite at high temperatures and the oxygen fugacity similar to that of the terrestrial upper mantle, which has been estimated to be near FMQ. Using Re/ Pt capsules, batch-melting experiments of a dry spinel lherzolite from Kilborne Hole (KLB-1) were performed. Our experimental results and natural mantle peridotites are significantly different in terms of the stability field and the Cr/(Cr + Al) atomic ratio (= Cr') of chromian spinel. Spinel, which is common in natural mantle peridotite regardless of their bulk chemistry, disappeared at a much lower degree of melting in our experiments. The upper limit of the Cr' of spinel just before its disappearance decreases rapidly from 0.45 to 0.13 with increasing pressure, whereas natural peridotites have a wide range of Cr' from 0.1 to 0.8. Our dry experiments, compared with previous hydrous experiments, indicate that the upper limit of Cr' increases with increasing H2O content. These findings suggest that the petrological variation of natural mantle peridotite cannot be formed by simple batch melting but may be the result of more complex melting.

Petrological feature of spinel lherzolite xenolith from Oki‐Dogo Island: An implication for variety of the upper mantle peridotite beneath southwestern Japan

Abstract Spinel lherzolite is a minor component of the deep‐seated xenolith suite in the Oki‐Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; &lt; 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo86) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U‐shaped chondrite‐normalized rare‐earth element (REE) patterns. The abundance of Fe‐rich ultramafic and mafic cumulate xenoliths in Oki‐Dogo alkali basalts suggests that the later formation of those Fe‐rich cumulates from alkaline magma was the cause of Fe‐ and light REE (LREE)‐enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South‐west Japan arc, which are non‐metasomatized in terms of major‐element chemistry (e.g. Fo &gt; 89), and are rarely associated with Fe‐rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE‐enrichment in mantle rocks has been more prominent and prevalent than Fe and other major‐element enrichment during the metasomatism.

Geochemical characteristics and petrogenesis of Mesozoic basalts from the North China Craton: A case study in Fuxin, Liaoning Province

Occurrence of Cretaceous basalts in Fuxin County, Liaoning Province provides us an opportunity to understand Mesozoic mantle processes beneath the northern margin of the North China Craton (NNCC). Fuxin Jianguo basalts occur as volcanic channel phases with well-developed columnar jointings and contain few spinel lherzolite and pyroxenite xenoliths. They are poor in silica and rich in alkalis, Ti and Al, belonging to alkaline basalts. In trace element compositions, Jianguo basalts are moderately enriched in LREE and LILF., but not depleted in HFSE. They have low Sr and high Nd and Pb isotopic ratios. These geochemical characteristics suggest that Jianguo basalts originated from the depleted asthenosphere, representing an undifferentiated and uncontaminated primitive magma. Presence of these basalts indicates that the lithosphere beneath the region had thickness less than 65 km at the time of basalt eruption and was mainly composed of fertile pargasite-bearing spinel lherzolite and plagioclase pyroxenite. The voluminous basaltic-andesitic magmatism during the early Jurassic—late Cretaceous time indicates that the commencement and accomplishment of lithosphere thinning in the NNCC was much earlier than that in the southern margin, since the mafic-intermediate volcanism only occurred at the Cretaceous time in the southern margin and the basalts with an asthenosphere isotopic signature at the Tertiary. This shows that highly spatial and temporal heterogeneity existed in the Mesozoic lithosphere evolution.