Ultramafic Mantle Xenoliths Research Articles

Metasomatic Interaction of Ultramafic Mantle Xenoliths with their Felsic HP–UHT Granulite Host (Moldanubian Domain, Bohemian Massif in Lower Austria)

Abstract The St. Leonhard granulite massif in Lower Austria, dominantly formed by kyanite-bearing felsic granulite, encloses countless up to 5 cm sized mantle xenoliths of garnet clinopyroxenite and peridotite. The mineralogical, textural and chemical consequences of a mutual metasomatic interaction at the contact between these xenoliths and the host orthopyroxene-bearing felsic granulite are described. Movement of Mg, Al and, especially, Ca from the garnet clinopyroxenite to the granulite and migration of K and Na in the opposite direction, caused the breakdown of clinopyroxene and formation of orthopyroxene–plagioclase symplectite coronae at the expense of the garnet clinopyroxenite xenoliths. Around the peridotite xenoliths, monomineralic orthopyroxene coronae have developed due to the supply of Si from the host granulite. The P–T conditions of this interaction were established to 900°C to 1000°C and 1.0 to 1.2 GPa by thermodynamic modelling. The duration of coronae growth was constrained to 13 to 532 ka based on modelling of Fe–Mg interdiffusion underlying secondary compositional zoning of garnet from the garnet clinopyroxenite xenolith extending to the coronae. The most significant change in the host granulite was caused by the supply of Ca from the garnet clinopyroxenite xenolith, which led to the breakdown of the Al2SiO5 phase—probably kyanite—and stabilization of orthopyroxene. K-feldspar-poor haloes surrounding mantle xenoliths formed due to the depletion of K in the granulite adjacent to the garnet clinopyroxenite. The observed origin of felsic–intermediate orthopyroxene-bearing granulite by transformation of felsic kyanite-bearing granulite through the metasomatic interaction with mantle xenoliths implies that the deep crustal chemical exchange between mantle- and crust-derived lithologies may have an important consequences on composition, thermal structure and geodynamic evolution of orogenic lower crust especially in hot collisional orogens, such as the European Variscides.

Geochemical evidence for a lithospheric origin of the Comoros Archipelago (Indian Ocean) as revealed by ultramafic mantle xenoliths from La Grille volcano

Petrology and fluid inclusions (FI) geochemistry are increasingly used in tandem to constrain the compositional features and evolution of the lithospheric mantle. In this study, we combine petrography and mineral chemistry with analyses of noble gases (He, Ne and Ar) and CO2 in olivine, orthopyroxene- and clinopyroxene-hosted FI, as well as radiogenic isotope (Sr-Nd-Pb) systematics of ultramafic xenoliths collected at La Grille volcano in Grande Comore Island, aiming at better characterizing one of the most enigmatic and controversial portions of the western Indian Ocean lithospheric mantle. Xenoliths have been divided in three groups on the basis of their textural features: Group 1 (Opx-bearing), Group 2 (Opx-free) and Group 3 (Cumulate). Overall, petrographic observations and mineral phase compositions indicate that the sampled lithospheric portion experienced variable degrees of melting (from 5% to 35%), recorded by Group 1 most refractory harzburgites and lherzolites, as well as modal metasomatic processes as evidenced by the crystallization of cpx at the expense of opx in Group 1 fertile lherzolites and wehrlite and by Group 2 xenoliths. Crystallization of slightly oversaturated basic silicate melts seems also to have occurred, as shown by Group 3 xenoliths. A positive trend between temperature and ƒO2 is evident, with Group 2 and 3 xenoliths testifying for hotter and more oxidised conditions than Group 1. The variability of the 4He/40Ar* ratio (0.02–0.39) in Group 1, significantly below typical values of a fertile mantle (4He/40Ar* = 1–5), can be explained by the variable degrees of partial melting coupled to metasomatic enrichment that may account for modifying 4He/40Ar*, as also indicated by the mineral composition. He-Ar-CO2 relationships support the presence of a metasomatic CO2-rich process post-dating the melt extraction and the cumulate formation. The air-corrected 3He/4He isotopic ratios (6.30 to 7.36 Ra) are intermediate between the MORB mantle signature (8 ± 1Ra) and the SCLM (6.1 ± 0.9 Ra). The Ne and Ar isotopic signatures (20Ne/22Ne, 21/Ne/22Ne and 40Ar/36Ar) are consistent with mixing between an air-derived component and a MORB-like mantle, supporting the hypothesis for a lithospheric origin of the Comoros magmas, and arguing against any deep mantle plume-related contribution. This is also corroborated by combining Ne with He isotopes, showing that La Grille ultramafic xenoliths are far from the typical plume-type compositions. Sr-Nd-Pb isotope systematics in opx and cpx from La Grille additionally support a MORB-type signature for the lithospheric mantle beneath the area.

Clues from Garnet-Spinel-Bearing Peridotite and Pyroxenite Xenoliths about the Formation of the Subcratonic Lithospheric Mantle—a Case Study from the Orapa Kimberlite Cluster

Abstract A series of 18 ultramafic mantle xenoliths from the Orapa and Letlhakane mines containing both garnet and spinel were investigated petrographically and by means of classical and multi-equilibrium geothermobarometry in order to understand their P–T evolution and infer tectonic processes consistent with that evolution, assuming that samples from the shallow subcontinental lithospheric mantle (SCLM), may have remained cool enough to preserve memory of its formation. Samples broadly classify into peridotites, mainly harzburgites, and pyroxenites, mainly (olivine-) websterites with less than 15 vol% olivine. The websterites all show thin garnet coronas around xenomorphic spinel clusters and often coarse garnet and very fine spinel exsolving from pyroxenes. Classical geothermobarometry results vary consistently between the 40 mW/m2 geotherm at high pressures and the 50 mW/m2 geotherm at lower pressures but partly fail the reliability test indicating some degree of disequilibrium, particularly, as may be expected, toward lower P and T. Eleven samples were analyzed in detail for mineral modes, and pseudosections were calculated for the four compositionally most extreme samples. All four samples show systematic divergence of the mode and composition isopleths indicating overall chemical disequilibrium, but partial isopleth convergence in two distinct P–T regions that were interpreted as the main equilibration stage and a stage of late ‘freeze-in’ of a few composition isopleths at lower temperature and higher pressure. Specifically, the P–T changes between stage 1 and stage 2 were from 900°C and 13 kbar to 750°C and 23 kbar for a websterite and from 850°C and 24 kbar to 730°C and 33 kbar for a harzburgite. For an olivine-orthopyroxenite and -clinopyroxenite the change was from 910°C, 17 kbar and 770°C, 17 kbar for stage 1, respectively, to less well-constrained conditions for stage 2 of 650°C, 36 kbar and 630°C, 20 kbar, respectively. This interpretation of a protracted or stepwise freeze-in along a P–T path holds even after considering thermodynamic, analytical and, as far as possible, geological uncertainties. The result is consistent with a ‘subcretion’ or ‘buoyant subduction’ model for the formation of the SCLM, specifically with the down-drag of an already subcreted lithospheric platelet by a subsequently arriving platelet. In the framework of the crust–mantle evolution in the Orapa region this most likely occurred in the Neoarchean, between 2.8 and 2.5 Ga.

Ultramafic mantle xenoliths in the Late Cenozoic volcanic rocks of the Antarctic Peninsula and Jones Mountains, West Antarctica

Abstract Abundant mantle-derived ultramafic xenoliths occur in Cenozoic (7.7–1.5 Ma) mafic alkaline volcanic rocks along the former active margin of West Antarctica, that extends from the northern Antarctic Peninsula to Jones Mountains. The xenoliths are restricted to post-subduction volcanic rocks that were emplaced in fore-arc or back-arc positions relative to the Mesozoic–Cenozoic Antarctic Peninsula volcanic arc. The xenoliths are spinel-bearing, include harzburgites, lherzolites, wehrlites and pyroxenites, and provide the only direct evidence of the composition of the lithospheric mantle underlying most of the margin. The harzburgites may be residues of melt extraction from the upper mantle (in a mid-ocean ridge type setting), that accreted to form oceanic lithosphere, which was then subsequently tectonically emplaced along the active Gondwana margin. An exposed highly depleted dunite–serpentinite upper mantle complex on Gibbs Island, South Shetland Islands, supports this interpretation. In contrast, pyroxenites, wehrlites and lherzolites reflect percolation of mafic alkaline melts through the lithospheric mantle. Volatile and incompatible trace element compositions imply that these interacting melts were related to the post-subduction magmatism which hosts the xenoliths. The scattered distribution of such magmatism and the history of accretion suggest that the dominant composition of sub-Antarctic Peninsula lithospheric mantle is likely to be harzburgitic.

Adakites and adakitic melts: Compositions of rocks, quenched glasses, and inclusions in minerals

Data set of rocks and glasses whose compositions correspond to the term “adakite” (SiO2 > 56 wt %, Sr > 400 ppm, Sr/Y > 18) was compiled from two large geochemical data bases. It was revealed that the adakitic melts are characterized by extremely low abundance as compared to adakitic rocks. Only 50 adakitic compositions (~0.5%) were identified in the data base that includes the major and trace element compositions of over 9700 quenched and melt inclusion glasses. It was established that only 22 of selected analyses characterize melt inclusion glasses, while other analyses represent residual glass or “pocket melts” in ultramafic mantle xenoliths. The question of a genetic relationship between adakitic rocks, adakitic melts, and melting of subsiding plate remains open. Original data on the Shiveluch volcanic center (Kamchatka) were used to demonstrate the formation of adakitic signatures through mineral accumulation.

The Harrat Al-Birk basalts in southwest Saudi Arabia: characteristic alkali mafic magmatism related to Red Sea rifting

Harrat Al-Birk volcanics are products of the Red Sea rift in southwest Saudi Arabia that started in the Tertiary and reached its climax at ~5 Ma. This volcanic field is almost monotonous and is dominated by basalts that include mafic–ultramafic mantle xenoliths (gabbro, websterite, and garnet-clinopyroxenite). The present work presents the first detailed petrographic and geochemical notes about the basalts. They comprise vesicular basalt, porphyritic basalt, and flow-textured basalt, in addition to red and black scoria. Geochemically, the volcanic rock varieties of the Harrat Al-Birk are low- to medium-Ti, sodic-alkaline olivine basalts with an enriched oceanic island signature but extruded in a within-plate environment. There is evidence of formation by partial melting with a sort of crystal fractionation dominated by clinopyroxene and Fe–Ti oxides. The latter have abundant titanomagnetite and lesser ilmenite. There is a remarkable enrichment of light rare earth elements and depletion in Ba, Th and K, Ta, and Ti. The geochemical data in this work suggest Harrat Al-Birk basalts represent products of water-saturated melt that was silica undersaturated. This melt was brought to the surface through partial melting of asthenospheric upper mantle that produced enriched oceanic island basalts. Such partial melting is the result of subducted continental mantle lithosphere with considerable mantle metasomatism of subducted oceanic lithosphere that might contain hydrous phases in its peridotites. The fractional crystallization process was controlled by significant separation of clinopyroxene followed by amphiboles and Fe–Ti oxides, particularly ilmenite. Accordingly, the Harrat Al-Birk alkali basalts underwent crystal fractionation that is completely absent in the exotic mantle xenoliths (e.g. Nemeth et al. in The Pleistocene Jabal Akwa Al Yamaniah maar/tuff ring-scoria cone complex as an analogy for future phreatomagmatic to magmatic explosive eruption scenarios in the Jizan Region, SW Saudi Arabia 2014).

Petrological characteristics of mantle xenoliths from the Azrou-Timahdite quaternary basalts, middle atlas, Morocco: A mineral chemistry perspective

Quaternary alkali basalts of the Azrou-Timahdite area contain a wide variety of ultramafic mantle xenoliths (e.g. lherzolites, wehrlites, pyroxenites and amphibolites). A comprehensive mineral chemistry studies using electron probe micro-analysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) allows us to propose a petrogenetic history of these ultramafic rocks.The studied mantle xenoliths contain spinel, which attests for their derivation from depths from 30 to 70 km, i.e. the intermediate zone of continental lithospheric upper mantle. The olivine forsterite (Fo) content is higher in lherzolites than in wehrlite, Fo88–91 and Fo80-83, respectively. The orthopyroxene is enstatite with relatively high Mg number (Mg#) (0.88–0.91). The clinopyroxene displays Mg# (0.88–0.92) similar to or slightly higher than that of olivine and orthopyroxene, indicating equilibrium between these phases. Progressive depletion of this ratio in lherzolite clinopyroxene suggests the generation of peridotitic rocks through a series of partial melting processes at different degrees. LREE enrichment in clinopyroxene combined with decoupling of Ti and Na (Fusible major elements) indicates mantle metasomatic processes, which possibly resulted in recrystallization of clinopyroxene. The wehrlitic lithologies are possibly the end product of the metasomatism. The LaN/YbN and Ti/Eu ratios of spine-bearing lherzolites and non-spongy clinopyroxenes combined with the low Ti contents and the Zr and Nb negative anomalies argue for an alkali-silicate metasomatism.Given the above scenario, we conclude that the mineralogical variations in the upper mantle beneath this part of the Middle Atlas Mountains can be attributed to a combination of mantle processes including partial melting that occurs in response of a mantle upwelling, and an alkali-silicate metasomatism.

제주도 현무암에 산출되는 함수광물(금운모와 각섬석): 모달교대작용의 증거

제주도의 초염기성-염기성 암편을 포획하고 있는 현무암은 조직적으로 일반 반정과 뚜렷하게 구별되는 금운모와 커슈타이트 결정을 산출하고 있다. 금운모와 커슈타이트는 모암인 현무암에서는 비교적 조립질 (2-10 mm)로 산출되며, 금운모는 초염기성 맨틀포획암에서, 커슈타이트는 염기성 반려암질 포획암에서 세립의 입자(1 mm 이하)로 간극을 채우거나, 미세맥으로 혹은 코로나로 뚜렷한 이차조직으로 산출된다. 금운모는 <TEX>$TiO_2$</TEX>(4.1-6.9 wt%)와 F(2.8-4.6wt%)가 풍부하고, mg#[=100Mg/(Mg+<TEX>$Fe^t$</TEX>) in mols, where <TEX>$Fe^t$</TEX> is total iron]는 88-80으로 매우 높다. 커슈타이트는 높은 <TEX>$TiO_2$</TEX>(5.6-6.11 wt%)와 비교적 낮은 mg#(68-64)를 보인다. 금운모와 커슈타이트는 서로 관련된 것은 아니며, 금운모의 형성은 상부맨틀에서 먼저 일어난 사건이고 커슈타이트의 형성은 하부지각에서 나중에 일어난 사건으로 해석된다. 이들 결정들 사이의 mg#와 조직적 특성은 제주도 암석권에서 맨틀의 심부일수록 금운모 부화작용을 경험하였으며, 천부일수록 각섬석 부화작용이 있었음을 지시하고 있다. 현무암에 금운모와 각섬석이 산출된다는 것은 제주도 상부맨틀/하부지각에 K-, Ti-를 함유한 휘발성 성분이 풍부한 멜트/유체가 이동하고 있다는 것에 대한 직접적인 증거가 되며, 제주도 하부 상부맨틀/하부지각도 다양한 모달 교대작용을 경험하였음을 의미한다. Phlogopite and kaersutite, showing distinctively different textural characteristics compared to the common phenocrysts, are observed in alkali basalt from Jeju Island. They occur as large crystals (2-10 mm) in host basalts, whereas fine-grained phlogopite and kaersutite occur in ultramafic mantle xenoliths and mafic gabbroic xenoliths, respectively, as an interstitial and microvein phases, or in corona textures (<1 mm). This textural characteristics of fine-grained grains clearly indicates secondary in origin. Phlogopite contains high <TEX>$TiO_2$</TEX>(4.1-6.9 wt%) and F(2.8-4.6 wt%) and relatively high mg#[=100Mg/(Mg+<TEX>$Fe^t$</TEX>) in mols, where <TEX>$Fe^t$</TEX> is total iron](88-80), whereas kaersutite has high <TEX>$TiO_2$</TEX>(5.6-6.11 wt%) and much lower mg#s(68-64). Our textural observations and the geochemical character of these hydrous minerals suggest that they were unrelated to each other and mica formation happened early in the upper mantle before the mantle xenoliths had been trapped. In contrast, kaersutite formation has happened later, probably during the late stage of crystallization as intracrustal processes. The presence of phlogopite and kaersutitic amphibole is a direct evidence for K-, Ti-, F- and <TEX>$H_2O$</TEX>-bearing fluid/melt percolation in the lithosphere beneath Jeju Island, indicating that they are product of interaction between host rock/peridotite/fluid-melt. Thus, the upper mantle/lower crust beneath Jeju Island are metasomatized to various extents, characterized by a change in major metasomatic hydrous minerals from phlogopite to amphibole with decreasing depth.

Spectral properties of carbonatized ultramafic mantle xenoliths and their host olivine basalts, Jabal Al Maqtal basin, South Eastern Desert, Egypt, using ASD FieldSpec spectroradiometer

Understanding the spectral properties of the carbonatized ultramafic mantle xenoliths and their host olivine basalts exposed at Jabal Al Maqtal strike-slip basin, south Eastern Desert, Egypt using portable ASD FieldSpec is the main task of this article. Field data revealed the presence of ultramafic mantle xenoliths at different stratigraphic levels within Jurassic olivine basalts. Ultramafic mantle xenoliths are recorded within highly altered olivine basalt topped by hard, compact brown conglomerate (erosional surface) of 1.5 m thick. They have different sizes (5–25 cm) and shapes (rounded, sub-rounded and elliptical). The spectral data are collected using ASD FieldSpec in sunny, free-cloud day. Data measurements were resampled as reference-target scan. Nearly all the mineralogical constituents of the studied ultramafic mantle xenoliths were replaced by Mg-Free (<0.5 wt%) calcium carbonate (calcite). Spectrally the ultramafic mantle xenoliths can be categorized into two main groups based on their Fe and Mn content which produce absorption features at shorter wavelength regions. The spectral profile of the first group shows a pronounced absorption features around 0.50 μm, 0.65 μm and 0.90 μm wavelength regions. These features are attributed to the electronic processes within the unfilled d-shells of transition Mn 2+ and Fe 2+ metal cations (MnO and Fe 2O 3 content are 2.11 wt% and 12.6 wt% respectively as indicated by XRF analyses). The spectral profile of the second group shows a very week absorption features in shorter wavelength regions which is attributed to the low MnO (0.7 wt%) and Fe 2O 3 (2.17 wt%) content. Both ultramafic mantle xenoliths groups show pronounced absorption features in longer wavelength regions (1.8–2.55 μm). Inspection of the spectral profiles of the mantle xenoliths revealed the presence of six absorption features around 1.80 μm, 1.95 μm, 2.00 μm, 2.35 μm, 2.4 μm. and 2.5 μm wavelength regions which are attributed to the vibrational processes of the carbonate ion. Absorption feature around 1.40 μm wavelength region is probably due to microcrystalline silica alteration. The spectral profile of fresh olivine basalt shows low flat reflectance curve whereas the reflectance curve of altered basalt shows three little absorption features around 1.40 μm, 1.95 μm and 2.35 μm wavelength regions. These absorption features are mainly due to calcite alterations.

Miocene basaltic magmatism in the Goldfield-Superstition volcanic province, central Arizona: Geochemistry, mineralogy, and petrology

Basaltic lavas form part of the Miocene (∼20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes. Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49–51 wt. %) and with MgO 8.1–10.2 wt. %. They are associated with lavas that are andesitic (SiO2 ∼59–62 wt. %; MgO 8–2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are ∼0.705. Olivines, rarely fresh, have Fo86–89 cores, and clinopyroxenes have Mg#s 86–89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 ∼45.5–47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba ∼1100 versus 1800 ppm; La ∼75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1–11.8 wt. %), and their87Sr/86Sr ratios are ∼0.705–0.706. Their olivines are Fo86–89, and their clinopyroxenes have Mg#s 86–90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 ∼2.6 wt. %), which has the lowest MgO (∼5.6–7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba ∼400 ppm; La ∼25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies. Middle-section basalts erupted among silicic lava and pyroclastic flows from ∼19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite (∼9.5 wt. % MgO;87Sr/86Sr ∼0.705) and Black Mesa basanite (SiO2 ∼44 wt. %; MgO ∼8 wt. %; CaO 14.7 wt. %;87Sr/86Sr ∼0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed (∼60 and ∼3 ppm; lowest Zr/Nb, ∼4 versus all others >7). Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with ∼50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions. Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after ∼60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after ∼50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting. This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning ∼2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.

Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression

The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, eNd = +6.6 to +7.0, and eHf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, eNd = −8.1 to +2.5, and eHf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.

Geochemical, Petrographic and Magnetic Characteristics of Spinel Lherzolite Mantle Xenoliths from Jabal Remah Volcano, Jordan

Peridotite ultramafic mantle xenoliths are occurring abundantly in the Harrat Al-Shaam basaltic province. Jabal Remah volcano is located in the Jordanian parts of the plateau and contains considerable amounts of mantle xenoliths within its pyroclastic successions. Mineralogical investigations show that the xenoliths are mostly of the spinel lherzolite type, which are characterized by a protogranular texture. Modally, the xenoliths are composed of olivine (55-65%), orthopyroxene (12-16%), clinopyroxene (10-15%) and spinel (~ 5%). The xenoliths are classified as Type I and belong to the Cr-diopside group. They are characterized by a high content of MgO (42.1-43.2%), Cr (2465-2538 ppm) and Ni (2196-2301 ppm). Three selected mantle-xenolith samples were analyzed for their magnetic characteristics. They were found to behave similarly in their ferrimagnetic phase, as indicated by their narrow hysteresis curves and because they never reach saturation even at high applied field. They exhibit ferrimagnetic hystertesis curves similar to world known mantle xenoliths, which is due to the presence of Fe in the form of oxides.

Petrology, Geochemistry and Magnetization of Remah Volcano North Eastern Jordan

Peridotite ultramafic mantle xenoliths are occurring abundantly in the Harrat Al-Shaam basaltic province. Jabal Remah volcano is located in the Jordanian parts of the plateau and contains considerable amounts of mantle xenoliths within its pyroclastic successions. Mineralogical investigations show that the xenoliths are mostly of the spinel Iherzolite type, which are characterized by a protogranular texture. Modally, the xenoliths are composed of olivine (55-65%), orthopyroxene (12-16 %) clinopyroxene, (10-15%) and spinel (- 5%). The xenoliths are classified as type I and belong to the Cr-diopside group. They are characterized by its higher content of MgO (42.1-43.2%), Cr (2460-2540 ppm) and Ni (2196-2301 ppm). Three selected spinel-lherzolite xenolith samples were analysed for their magnetic characteristics. They were found to behave similarly in their ferromagnetic phase, as indicated by their narrow hysteresis curves and because they never reach saturation even at high-applied field. They exhibit ferrimagnetic hystertesis curves similar to world known mantle xenoliths, which is due to the presence of Fe in the form of oxides.

Re–Os and S systematics of spinel peridotite xenoliths from east central China: Evidence for contrasting effects of melt percolation

Abstract Os isotopic compositions and Re and Os concentrations were determined for Cenozoic basalt-borne ultramafic xenoliths from the Subei Basin of east central China. Re–Os analyses were coupled with whole rock major and trace element and S abundance determinations, and with characterization of rock textures, modal phase proportions and sulfide petrography. The two main sampling areas, Lianshan and Panshishan, separated by only 6 km, have similar textures and major and moderately incompatible lithophile trace element compositions. The Os isotopic ratios of these two areas plot on the same trends relating 187Os/188Os to indices of melt extraction such as whole rock Yb content. These Os isotopic systematics suggest that both areas were affected by an early Proterozoic (∼1.8 Ga) melt extraction event. Thus the two areas apparently shared the same long term lithospheric history. Nevertheless, the sulfide abundances and whole rock S, Os and Re concentrations are strikingly lower in Lianshan than in Panshishan, and the two localities have different incompatible lithophile trace element signatures. These differences resulted from contrasting melt percolation styles between the two areas. Lianshan was affected by extensive percolation of sulfur undersaturated melts that removed Re, Os and S, while Panshishan experienced interaction with low degree or highly evolved melts that added Re, Cu and S, but had no affect on Os abundances. The lack of correlation between 187Os/188Os and 187Re/188Os, compared with the good correlation between 187Os/188Os and Yb, indicates that the perturbation of the Re and Os concentrations was fairly recent, and perhaps related to Mesozoic or Cenozoic lithospheric thinning in eastern China. The Lianshan Os concentrations are typical of those of off-cratonic mantle xenoliths, while the Panshishan Os concentrations are closer to those of orogenic peridotites. This suggests that the low Os concentrations, and by extension, the low concentrations of all of the highly siderophile elements (HSE) typically observed in ultramafic mantle xenoliths, may result from recent melt percolation processes, probably directly or indirectly related to the magmatism that brought the xenoliths to the surface. Thus ultramafic xenoliths may not provide reliable estimates of the HSE contents of the upper mantle, and variations in HSE abundances between xenolith localities should not be used to define global scale processes. Os and other HSE abundances may prove to be sensitive indicators of melt percolation, and may provide information about the degree of sulfur saturation of the melts. Despite the loss of Os during recent percolation, the Os isotopic ratios of most samples are nearly unmodified by this process, confirming the utility of this system for dating ancient melt extraction events.

Noble Gases in Ultramafic Mantle Xenoliths of the Persani Mountains, Transylvanian Basin, Romania

Suppl. 1, 513. Sarda, P., Staudacher, T. and All~gre, C.J.(1988), Earth Planet. Sci. Lett., 91, 73-88.

P-T Phase Relations of Silicic, Alkaline, Aluminous Mantle-Xenolith Glasses Under Anhydrous and C-O-H Fluid-saturated Conditions

High-pressure liquidus experiments on three silicic, aluminous, INTRODUCTION alkaline melts, modelled on glasses found in many mantle xenoliths, The compositions of silicate melts potentially in equishow that part of this compositional range is saturated with librium with the Earth’s mantle have long been of interest harzburgite (or possibly lherzolite) under anhydrous conditions. to petrologists and geochemists. Much useful information Under C–O–H fluid-saturated conditions with XH2O=0·5, phloabout such melts has come from the study of melt gopite mica is present along with anhydrous phases similar to those inclusions in phenocrysts in volcanic rocks and in the found under dry conditions. Phlogopite is the sole liquidus phase minerals of ultramafic mantle xenoliths. In addition, when XH2O=1·0. At XH2O=0·5 and 3·0 GPa, garnet, kyanite many mantle xenoliths contain silicate glasses as a discrete and carbonate minerals appear as near-liquidus phases and the phase, rather than (or in addition to) inclusions (see shape of the liquidus surface is reminiscent of that of the carbonated Appendix A for references). In this paper, we will refer peridotite solidus. Saturation of these liquids with harzburgite, and to discrete-phase xenolith glasses simply as xenolith comparisons with calculated melt silica activities, suggests that these glasses and those found as inclusions in minerals as liquids would face no chemical or thermal obstacles to circulating inclusion glasses. Xenolith glasses have been reported amongst and coexisting with harzburgitic mantle. Also, there is from mantle xenoliths hosted by alkaline, mafic magmas textural evidence that these melts may be mobile. Accordingly, these both from intra-plate and from subduction-related teckinds of liquids could act as cryptic metasomatic agents. If mantle tonic settings. The glasses have a wide range of comat ~45–90 km depth is pre-enriched in low-melting-temperature positions and textural relations: they are found as components, and probably volatiles, via the ascent and percolation intergranular blebs, irregular patches, veins, and thin of alkaline, mafic liquids (along geotherms that cross inflections in films that appear to wet all grain faces. Most glasses the solidus of CO2-bearing peridotite), then subsequent low-degree account for, at most, only a few volume percent of their partial melts could yield the liquids that are ultimately trapped as host xenolith, and some much less than 1 vol. %. xenolith glasses. Xenolith glasses have a very wide range of major

Helium, neon and argon isotope systematics in Kerguelen ultramafic xenoliths: implications for mantle source signatures

New noble gas data are presented for ultramafic mantle xenoliths, occurring in 10–20 Ma old volcanics from the Kerguelen Archipelago. Fusion results indicate that neon is isotopically primitive, whereas helium shows considerable isotopic variations, tending towards more radiogenic values. These fusion results of mantle xenoliths are the first report of a difference between helium and neon isotope systematics, which in most other studies show correlated isotope systematics [1–4]. Helium isotopic compositions obtained by crushing the same xenoliths yield values which are identical to or slightly lower than the theoretical values predicted by the neon data. Comparison of helium with mantle neon and argon reveals that the samples have experienced considerable helium loss, making the helium isotope systematics of the xenoliths prone to secondary disturbances. Caution is clearly required when interpreting helium isotopic compositions of mantle-derived samples, particularly in cases where helium abundances are low (also cf. [5]); combined noble gas studies provide a means of evaluating the helium isotopic signature in such cases. Our neon fusion results, together with helium and neon obtained by crushing, preserve evidence for a primitive mantle component in these Kerguelen xenoliths. This primitive component in the Kerguelen xenoliths may have been metasomatically introduced from the Kerguelen plume into upper mantle peridotite by CO 2-rich, silica-rich melts, now represented by cogenetic melt and fluid inclusions in the xenoliths [6].

Ultradeep, ultramafic mantle xenoliths: transmission electron microscopy preliminary results

The defect microstructure of ultradeep garnets from the Jagersfontein kimberlite pipe (South Africa) was investigated by transmission electron microscopy. These crystals exhibit a high density of dislocations, most of which form well-organized and regularly spaced subgrain boundaries. Free dislocations are also detected; most of them have a Burgers vector 1 2 〈111〉 (shortest lattice repeat), but a few 〈100〉 Burgers vectors are also characterized, especially in junctions. Small needle-shaped pyroxene exsolutions are also detected. They exsolved from the Si-rich garnets during assent to the Earth's surface. Their nucleation and growth required bulk diffusion, and this process may have affected the original dislocation microstructure. It is believed, however, that the observed dislocation pattern still reflects the original deformation regime of garnets at very high pressure and temperature and this dislocation pattern strongly suggests that garnets were highly ductile. Finally, a number of precipitates of a dense amorphous phase of pure carbon are also detected. They might result from the amorphization of small diamonds exsolved from the garnet as the pressure was progressively released.

Petrology and Trace Element Geochemistry of the Honolulu Volcanics, Oahu: Implications for the Oceanic Mantle below Hawaii

The Honolulu Volcanics comprises small volume, late-stage (post-erosional) vents along rifts cutting the older massive Koolau tholeütic shield on Oahu, Hawaii. Most of these lavas and tuff of the Honolulu Volcanics have geochemical features expected of near-primary magmas derived from a peridotite source containing Fo87–89 olivine; e. g. 100 Mg/(Mg + Fe2+) >65, >250 p. p. m. Ni, and presence of ultramafic mantle xenoliths at 18 of the 37 vents. Consequently, the geochemistry of the alkali olivine basalt, basanite, nephelinite and nepheline melilitite lavas and tuff of the Honolulu Volcanics have been used to deduce the composition of their mantle source and the conditions under which they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmas were derived by partial melting of a garnet (<10 per cent) Iherzolite source, which we infer to have been carbon-bearing, from analogy with experimental results. This source was isotopically homogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980; Nd, Roden et al., 1981), and we infer that the source was compositionally uniform in all major-element oxides except TiO2, in compatible trace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatible trace elements (P, Th, La, Ce). However, the source appears to have been heterogeneous in TiO2, Zr, Hf, Nb, and Ta, elements that were not strongly incompatible during partial melting. Some nepheline melilitite samples may be derived from a source with distinct Sc and heavy-rare-earth-elements (REE) abundances, or which had a phase or phases controlling the distribution of these elements. The relatively limited abundance range for several elements, such as Ti, Zr, Nb, is partly a consequence of the low degrees of melting inferred for the series (2 per cent for nepheline melilitite, 11 per cent for alkali olivine basalt), which failed to exhaust the source in minor residual phases. We infer that these residual phases probably included phlogopite, amphibole, and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle composition and the mantle source of mid-oceanic-ridge basalt the garnet peridotite source of the Honolulu Volcanics was increasingly enriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, and Hf all <P <Nd <Sr ∼Ce <La <Nb ∼Ta. A multi-stage history for the source of the Honolulu Volcanics is required because this enrichment was superimposed on a mantle that had been previously depleted in incompatible elements, as indicated by the relatively low 87Sr/86Sr ratio, high 143Nd/144Nd ratio and low contents of K, Rb, Ba, and Th. The composition of the source of the Honolulu Volcanics differs from the source of the previously erupted tholeiitic shield. The modal mineralogy of the source of the Honolulu Volcanics is not represented in the upper-mantle xenoliths, e. g. the garnet pyroxenite and olivine-poor garnet Iherzolite included within the lavas and tuff of the unit.