Groups Of Xenoliths Research Articles

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

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

Sub-continental lithospheric mantle deformation in the Yerer-Tullu Wellel Volcanotectonic Lineament: A study of peridotite xenoliths

Volumetrically, the lithospheric mantle comprises the bulk of the continental lithosphere, yet the mechanisms by which the lithospheric mantle is deformed during rifting are unresolved. Stretching and thermo-mechanical erosion are often cited mechanisms for facilitating lithospheric deformation during continental rift development; however, the infiltration of melt into the lithosphere during rift development also results in chemical alteration of the lithospheric mantle. The purpose of this study is to test the potential mechanisms by which the continental lithospheric mantle is chemically altered during rift development. Here we present a study of mantle xenoliths derived from the lithospheric mantle in Ethiopia, which has been deformed during rifting. The data collected suggest that the lithospheric mantle beneath this region exhibits evidence of focused magma-lithosphere interaction, resulting in four distinct types of peridotite xenoliths: a) deformed xenoliths representing the ancestral lithospheric mantle; b) granular xenoliths representing overprinted lithospheric mantle; c) replacement dunite xenoliths that are evidence of pervasive melt-lithosphere interaction; and d) cumulate xenoliths representing remnants of a metasomatic agent. The deformed xenoliths have a high Mg# (>89) and exhibit little, if any, interaction with melt. The remaining xenolith groups exhibit lower Mg# (<89), suggestive of magma-lithosphere interaction. The high Ni content in olivine and depleted incompatible elements concentrations in orthopyroxene of the granular xenoliths are inconsistent with simple metasomatic enrichment. Furthermore, the presence of dunite containing olivine with low Ca and Sc, and spinel with a Cr# of ~60 is suggestive of a replacement dunite. These data support a melt channeling model over a pervasive chromatographic metasomatism event. Given that the xenoliths examined in this this study are derived from a lithospheric shear zone, we conclude that melt channeling is likely facilitated by the focusing of melts along topography at the lithosphere-asthenosphere boundary, and shear-induced porosity.

Constraints on plateau architecture and assembly from deep crustal xenoliths, northern Altiplano (SE Peru)

Newly discovered xenoliths within Pliocene and Quaternary intermediate volcanic rocks from southern Peru permit examination of lithospheric processes by which thick crust (60–70 km) and high average elevations (3–4 km) resulted within the Altiplano, the second most extensive orogenic plateau on Earth. The most common petrographic groups of xenoliths studied here are igneous or meta-igneous rocks with radiogenic isotopic ratios consistent with recent derivation from asthenospheric mantle ( 87 Sr/ 86 Sr = 0.704–0.709, 143 Nd/ 144 Nd = 0.5126–0.5129). A second group, consisting of felsic granulite xenoliths exhibiting more radiogenic compositions ( 87 Sr/ 86 Sr = 0.711–0.782, 143 Nd/ 144 Nd = 0.5121–0.5126), is interpreted as supracrustal rocks that underwent metamorphism at ~9 kbar (~30–35 km paleodepth, assuming a mean crustal density of 2.8 g/cm 3 ) and ~750 °C. These rocks are correlated with nonmetamorphosed rocks of the Mitu Group and assigned a Mesozoic (Upper Triassic or younger) age based on detrital zircon U-Pb ages. A felsic granulite Sm-Nd garnet whole-rock isochron of 42 ± 2 Ma demonstrates that garnet growth took place in Eocene time. Monazite grains associated with quenched anatectic melt networks in the same rocks yield ion microprobe U-Pb ages ranging from 3.2 ± 0.2 to 4.4 ± 0.3 Ma (2σ). These disparate geochronologic data sets are reconciled by a model wherein Mesozoic cover rocks were transferred to >30 km depth beneath the plateau in the Eocene and progressively heated until at least Pliocene time. Isothermal decompression and partial melting ensued as these rocks were entrained as xenoliths in volcanic host magmas and transported toward the surface. Mafic granulites and peridotites from the same xenolith suite comprise the basement of the metasedimentary sequence, exhibiting isotopic characteristics of Central Andean crust. Calculated equilibrium pressures for these basement rocks are >11 kbar, suggesting that the basement-cover interface lies beneath the northernmost Altiplano at ~30–40 km below the surface. Together, these results indicate that crustal thickening under the northernmost Altiplano started earlier than major latest Oligocene and Miocene uplift episodes affecting the region and was coeval with a flat slab–related regional episode of deformation. Total shortening must have been at least 20% more than previous estimates in order to satisfy the basement to cover depth constraints provided by the xenolith data. Sedimentary rocks at >30 km paleodepth require that Andean basement thrusts decapitated earlier Triassic normal faults, trapping Paleozoic and Mesozoic rocks below the main decollement. Magma loading from intense Cenozoic plutonism within the plateau probably played an additional role in transporting Mesozoic cover rocks to >30 km and thickening the crust beneath the northern Altiplano.

The petrology, geochronology and significance of Granite Harbour Intrusive Complex xenoliths and outcrop sampled in western McMurdo Sound, Southern Victoria Land, Antarctica

Granite Harbour Intrusive Complex xenoliths in McMurdo Volcanic Group rocks and in situ outcrops have been studied from Mount Morning, western McMurdo Sound, Antarctica. Calc-alkalic samples have whole rock signatures and normative compositions similar to the Dry Valleys 1b suite, and zircon grains in one specimen yield a 545.2 ± 4.4 Ma crystallisation age. This supports subduction-related magmatism initiating in Southern Victoria Land by 545 Ma. A second group of xenoliths is alkalic, with titanite grains in one xenolith from this group dated at 538 ± 8 Ma. Whole rock chemistry, normative compositions and geochronology of the alkalic group are comparable to the Koettlitz Glacier Alkaline Suite (KGAS). The position of a proposed lower crustal discontinuity that may form a significant basement suture in the McMurdo Sound region is newly constrained to the east of Mount Morning, perhaps along the trace of the Discovery Glacier. The boundary between East and West Antarctica may also pass along the trace of the Discovery Glacier if, as previously hypothesised, its location is controlled by the basement suture. A significant basement suture may also have provided the necessary egress for the (regionally) early and sustained magmatic activity observed at Mount Morning over the last 24 million years.

Evidence from mantle xenoliths for lithosphere removal beneath the central Rio Grande Rift

Seismic tomography beneath the Central Rio Grande Rift (RGR) at ∼34°N shows a low P and S wave velocity zone in the mantle that extends up the base of the Moho. This low-velocity region has been interpreted by (Gao et al., 2004) to be the result of convective removal of a portion of the once >100km thick Proterozoic lithosphere. The amount of extension in the central RGR is thought to be low (∼25%) and thus cannot account for the amount of lithosphere thinning suggested by seismic tomography. We measured whole rock and mineral major element, trace element, and isotopic compositions of spinel-peridotite xenoliths erupted along the central axis of the rift (Elephant Butte) and the eastern margin of the Colorado Plateau (Cerro Chato) to determine their depth of origin and mantle provenance and to test the delamination hypothesis. If lithosphere removal has not occurred and the low P and S wave velocities are instead the result of hydration or melt infiltration in the lithosphere, then xenoliths erupted on the rift axis should have geochemical compositions similar to Proterozoic sub-continental lithospheric mantle (SCLM). At Cerro Chato, on the margin of the Colorado Plateau, xenoliths were derived from ∼60km depth and have geochemical signatures similar to Proterozoic sub-continental lithospheric mantle (e.g. refractory major element compositions, LREE-enrichment, enriched Sr and Nd isotopes, unradiogenic Os isotopes). At Elephant Butte, along the central rift axis, two distinct groups of xenoliths are present. The majority of xenoliths from Elephant Butte are LREE-depleted and have fertile major element compositions. Additionally, these xenoliths have isotopic signatures similar to the range for DMM (e.g. 87Sr/86Sr ranging from 0.7018 to 0.7023, εNd ranging from 7 to 21, and 187Os/188Os ranging from 0.122 to 0.130). We interpret this group of xenoliths to be derived from asthenospheric mantle. A less-abundant group of xenoliths at Elephant Butte are LREE enriched, have refractory major element compositions, enriched Sr, Nd, and Pb isotopes, and unradiogenic Os isotopes. These are characteristic of Proterozoic SCLM. Both groups of xenoliths from Elephant Butte are derived from ∼45km depth. We interpret the suite of xenoliths at Elephant Butte to have sampled what was recently the base of the Proterozoic SCLM. We conclude that a portion of the mantle lithosphere has been removed which allowed modern convecting mantle (DMM) to be emplaced at the base of the pre-existing SCLM.

Cryptic metasomatism in clino- and orthopyroxene in the upper mantle beneath the Pannonian region

Abstract Clino- and orthopyroxenes in anhydrous spinel peridotite xenoliths from Pliocene alkali basalts of the western Pannonian Basin have been analysed for trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Clinopyroxenes show highly variable mantle normalized REE (rare earth elements) patterns but basically can be classified into three major groups: LREE-depleted, LREE-enriched and U-shaped patterns. As the REE patterns of clinopyroxenes usually reflect the REE patterns of the host peridotite, the three major REE patterns define three geochemically different groups of xenoliths. LREE-depleted xenoliths generally have undeformed protogranular textures, while the more deformed xenoliths with porphyroclastic and equigranular textures have LREE-enriched trace element patterns. The U-shaped pattern is very distinctive and is generally associated with poikilitic textures. The HREE content of the clinopyroxenes suggest that most of the xenoliths experienced less than15% partial melting, with the lowest degree occurring in the LREE-depleted xenoliths, and the highest degree in LREE-enriched xenoliths. Cryptic metasomatism frequently accompanies deformation. Metasomatic enrichment of incompatible trace elements can be observed not only in clinopyroxenes but also in coexisting orthopyroxenes. The metasomatic agents were probably alkaline mafic melts of asthenospheric origin and some may relate to upper Cretaceous alkali lamprophyre magmatism. Geochemical signatures of subduction-related melts or fluids have not been found in the anhydrous LREE-enriched xenoliths, although poikilitic xenoliths with U-shaped normalized REE patterns may indicate the influence of subduction-related melts.

Geochemical processes in peridotite xenoliths from the Premier diamond mine, South Africa: Evidence for the depletion and refertilisation of subcratonic lithosphere

This paper serves to document evidence for lithospheric depletion and enrichment processes through the analysis of mineral and bulk rock compositional trends in garnet-bearing peridotite xenoliths from the Premier kimberlite. The xenolith suite at Premier shows petrographic and compositional trends comparable to kimberlite-derived peridotite xenoliths world-wide, although hot, deformed xenoliths are unusually abundant. Observed differences between xenolith groups e.g. between coarse and deformed xenoliths follow trends which, by now, have been well documented for xenolith suites world-wide. However, there are also important similarities in mineral and bulk chemistry between lherzolite and harzburgite xenoliths from Premier. For instance, xenoliths containing garnets with 4 to ~ 10 wt.% Cr 2O 3 are recognised amongst both varieties, and all of these have bulk Al 2O 3 < 2 wt.%, with many < 1 wt.% Al 2O 3. Furthermore, examples with Y in garnet < 10 ppm and/or Zr < 30 ppm are recognised amongst all coarse xenolith varieties analysed. This is also a feature of the deformed harzburgites containing subcalcic garnet, and it is evident that xenolith deformation is not necessarily always accompanied by melt-related metasomatism. Evidence for high levels of depletion as measured by low bulk Al 2O 3 is not only seen in subcalcic and calcic harzburgite xenolith varieties, but is also a feature of a number of coarse and deformed lherzolites. This suggests that all xenolith varieties probably derive from the same (or similar), highly depleted protolith(s) but with varying levels of metasomatic overprint associated with a refertilisation process.

Eclogite and pyroxenite xenoliths from off-craton kimberlites near the Kaapvaal Craton, South Africa

Mantle xenoliths brought to the surface by kimberlite magmas along the south-western margin of the Kaapvaal craton in South Africa can be subdivided into eclogites sensu stricto, kyanite eclogites and orthopyroxene eclogites, all containing omphacite, and garnet clinopyroxenites and garnet websterites characterised by diopside. Texturally, chemically (major elements) and thermally, we observe an evolution from garnet websterites (T EG = 742–781 °C) towards garnet clinopyroxenites (T EG = 715–830 °C) and to eclogites (T EG = 707–1056 °C, mean value of 913 °C). Pressures calculated for orthopyroxene-bearing samples suggest upper mantle conditions of equilibration ( P = 16–33 kb for the garnet websterites, 18 kb for a garnet clinopyroxenite and 23 kb for an opx-bearing eclogite). The overall geochemical similarity between the two groups of xenoliths (omphacite-bearing and diopside-bearing) as well as the similar trace element patterns of clinopyroxenes and garnet suggest a common origin for these rocks. Recently acquired oxygen isotope data on garnet (δ 18O gnt = 5.25–6.78 ‰ for eclogites, δ 18O gnt = 5.24–7.03 ‰ for garnet clinopyroxenites) yield values ranging from typical mantle values to other interpreted as resulting from low-temperature alteration or precursors sea-floor basalts and associated rocks. These rocks could then represent former magmatic oceanic rocks that crystallised from a same parental magma as plagioclase free diopside-bearing and plagioclase-bearing crustal rocks. During subduction, these oceanic rock protoliths equilibrated at mantle depth, with the plagioclase-bearing rocks converting to omphacite and garnet-bearing lithologies (eclogites sensu largo), whereas the plagioclase-free diopside-bearing rocks converted to diopside and garnet-bearing lithologies (garnet websterites and garnet clinopyroxenites).

Petrological and seismic studies of the lithosphere in the earthquake swarm region Vogtland/NW Bohemia, central Europe

New petrological and geochemical data of upper mantle and lower crustal xenoliths from a Quaternary tephra deposit in Mýtina, Czech Republic, are discussed in the frame of previous geophysical results (receiver functions, reflection seismology) of the western Eger/Ohře Rift area. The Vogtland/NW Bohemia region is well known for intraplate earthquake swarms, which are usually associated with volcanic activity. As previously reported, 3He/ 4He data of CO 2 emissions in mofettes and mineral-water springs point at ongoing magmatic processes in this area. Using teleseismic P receiver functions, an approximately 40-km-wide Moho updoming (from 31 to 27 km) and indications for a seismic discontinuity at 50 to 60 km depth were observed beneath the active CO 2-degassing field. The studied xenolith suite probes a lithospheric profile within the structural and gas geochemical anomaly field of the western Eger Rift. With regard to texture, composition, p– T estimates and origin, five xenolith groups can be discriminated. Upper crustal xenoliths (quartzites, phyllites, mica schists) resemble crystalline country rocks at surface. One noritic xenolith (6 kbar, 800 °C) could represent a sample of the lower crust. Clinopyroxenites and hornblendites probably represent cumulates of the nephelinitic magma or fragments of magmatic veins. Porous wehrlites and one hornblende peridotite xenolith reflect a metasomatied upper mantle. Megacrysts of Ti-rich amphibole, olivine, clinopyroxene, and phlogopite could be fragments of pegmatitic veins or high-pressure phenocrysts. Most of the ultramafic nodules (xenoliths and megacrysts) formed at pressures between 6 and 11 kbar (22 to 38 km depth), at temperatures well above regional geotherms of the Bohemian Massif calculated from surface heat flow studies. Orthopyroxene-bearing spinel-lherzolite xenoliths were not observed. Our petrographical, geochemical, and thermobarometric results indicate a lithospheric mantle strongly altered by magmatic processes. This metasomatism can cause slower than typical uppermost-mantle seismic velocities in a greater area and might help to explain observed seismic anomalies.

Evidence from Antarctic mantle peridotite xenoliths for changes in mineralogy, geochemistry and geothermal gradients beneath a developing rift

Garnet and spinel peridotite xenoliths associated with the Phanerozoic Lambert–Amery Rift in eastern Antarctica contain evidence for several stages in the development of the mantle beneath the rift. Despite the fact that equilibria were only partly attained, a combination of petrography, whole-rock geochemistry, mineral chemistry and thermobarometry can be used to decipher four stages prior to entrainment of the xenoliths in the host magma during the initial stages of the breakup of Antarctica, India and Madagascar. The first chronological stage is represented by harzburgitic protoliths represented by rare occurrences of low-Ca olivines and orthopyroxenes in spinel lherzolites: these yield the lowest temperatures of 830–850 °C, and are also characterized by distinct trace element contents; lower Ti, Cr, V and Zn in olivine and orthopyroxene, and additionally lower Cu, Ni, Ga and Li in orthopyroxene. Some garnets are subcalcic, indicating that the spinel–garnet lherzolites also formed from harzburgitic protoliths. The second stage is the formation of garnet due to a pressure increase probably related to collision at 1.1 Ga. The third stage is marked by the growth of clinopyroxene, demonstrably in cpx-poor spinel lherzolites but probably in all xenolith groups: equilibrium of clinopyroxene with olivine and orthopyroxene was not attained in all samples, so that the non-judicious use of thermobarometers can produce bewildering results. The fourth stage is an enrichment episode that affected all spinel–garnet peridotites and about half of the spinel peridotites. During this stage, reaction rims were produced on the clinopyroxenes that formed during stage 3, the modal content of olivine and Mg/(Mg + Fe) in the rocks was reduced, CaO, Al 2O 3 and trace elements were enriched, and garnets were almost completely transformed to kelyphites. A later stage is documented by interstitial glasses and films around spinels related to infiltration of melt from the host magma. These post-date, and are more enriched in alkalies than, partially melted rims on clinopyroxenes, demonstrating that all the three earlier episodes were pre-entrainment events. Pressures indicated by the spinel + garnet lherzolites are restricted to 20–24 kbar at 1040–1180 °C. Early harzburgitic assemblages are interpreted to represent an earlier, cooler geotherm, whereas the kelyphite assemblages indicate temperatures 180–200 °C hotter than the main xenolith geotherm. This event also caused recrystallization of the clinopyroxene rims and is attributed to heating during rifting, but not due to the host magma itself. The preservation of evidence for three progressively hotter geotherms can be related to the upward movement of isotherms during the development of the sub-rift mantle.

Serpentinization and dehydration in the upper mantle beneath Fuerteventura (eastern Canary Islands): Evidence from mantle xenoliths

Mantle xenoliths from Fuerteventura (spinel-bearing harzburgites, dunites, and rare lherzolites and wehrlites) in the easternmost part of the Canary Islands chain, are highly deformed and show a wide range in textures from protogranular and porphyroclastic with olivine and orthopyroxene porphyroclasts, to rocks with “fibrous textures” (no orthopyroxene porphyroclasts, large, porous clusters of fibrous orthopyroxene, olivine porphyroclasts criss-crossed by stringy trails of fibrous orthopyroxene and fluid inclusions). The whole group of xenoliths covers a wider compositional range (e.g. Fo 86.6–92.7, 0.18–0.44 wt.% NiO, and up to 0.25 wt.% CaO in olivine; < 0.01–2.5 wt.% TiO 2 and 0.7–5.5 wt.% Al 2O 3 in clinopyroxenes in harzburgites and lherzolites, and relatively Ti–Fe 3+-rich spinels in some rocks). Rocks with orthopyroxene porphyroclasts cover a restricted compositional range typical of depleted oceanic peridotites (e.g. Fo 90.7–91.4, 0.36–0.41 wt.% NiO, and < 0.10 wt.% CaO in cores of olivine porphyroclasts; ≤ 0.12 wt.% TiO 2, 1.0–1.8 wt.% Al 2O 3 in clinopyroxene; low Ti and Fe 3+, and high Cr in spinel). These rocks are interpreted as pieces of the highly refractory oceanic lithosphere that accreted to the oldest oceanic crust in this part of the Atlantic Ocean, later modified by different processes. This mantle was highly depleted due to melt extraction, and had a composition similar to that of the most refractory peridotites collected along the Mid-Atlantic Ridge. Also the oceanic mantle beneath the other Canary Islands appears to have had similar composition before the onset of the Canary Islands magmatism. The “fibrous textures” in many xenoliths from Fuerteventura are interpreted as the results of partial serpentinization, later followed by heating and dehydration during the formation of Fuerteventura. The Fuerteventura magmatism has also caused metasomatism and reactions in the lithospheric mantle beneath Fuerteventura, leading to decreased Si and increased Fe+Ca+Na±Ti.

Single-crystal X-ray diffraction of spinels from the San Carlos Volcanic Field, Arizona: Spinel as a geothermometer

Fourteen spinels from two types of mantle xenoliths from the San Carlos Volcanic Field in Arizona were characterized using single-crystal X-ray diffraction and electron microprobe analysis. The dominant feature seen in the chemistry of spinels from the Group I xenoliths is the extensive substitution of Cr for Al (Cr0.20Al1.76 to Cr0.83Al1.10) correlated with Mg for Fe2+ (Mg0.69Fe2+ 0.31 to Mg0.80Fe2+0.19). Although Group II spinels display consistently low Cr values, they also show a well-correlated substitution of Mg for Fe2+ (Mg0.63Fe2+ 0.37 to Mg0.69Fe2+0.31). Unit-cell parameters for spinels from the Group I xenoliths range from 8.1259 to 8.2167 Å, while those from the Group II xenoliths range from 8.1247 to 8.1569 Å. The cell parameters are linearly correlated with Fe2+ and Cr contents. Cation distributions were determined from experimental bond lengths and refined site occupancies using the algorithm of Lavina et al. (2002). The San Carlos spinels display variable degrees of order, with inversion parameters ranging from 0.10 to 0.16 for Group I and from 0.17 to 0.22 for Group II. Closure temperatures were computed with the Princivalle equation, giving averages of 808(37) °C for spinels from Group I xenoliths and 822(62) °C for samples from Group II xenoliths. We show that these results are reasonable, and thus extend the use of the Princivalle equation, or at least its functional form, to samples with significant Cr and Fe2+ contents. This study demonstrates that, in spite of the extensive chemical variability of the San Carlos spinels, and given that the origins of the two groups of xenoliths are different, the oxygen coordinates remain fixed, suggesting that the oxygen coordinate is a function of thermal history.

Geochemistry of pyroxenitic and hornblenditic xenoliths in alkaline lamprophyres from the Spanish Central System

The alkaline lamprophyres and diabases of the Spanish Central System carry a heterogeneous suite of xenoliths which includes scarce pyroxenitic and hornblenditic types that can be divided in two groups: (a) pyroxenite xenoliths, including spinel clinopyroxenites and spinel websterites with granoblastic textures, and (b) hornblende-bearing clinopyroxenites and hornblendites (here after called hornblenditic xenoliths) characterised by the presence of Ti-rich kaersutitic amphibole and magmatic textures. Both groups of xenoliths can be assigned to the Al-augite series of Wilshire and Shervais (1975) [Wilshire, H.G., Shervais, J.W., 1975. Al-augite and Cr-diopside ultramafic xenoliths in basaltic rocks from western United States. Phys. Chem. Earth 9, 257–272] with Al-rich and Cr-poor mafic phases. Clinopyroxenes show a very similar trace element composition in all of the ultramafic xenoliths, characterised by convex-upward chondrite-normalised REE patterns and low contents of incompatible elements such as Rb, Ba, Th and Nb. Kaersutite in the amphibole-bearing xenoliths shows a similar convex-upward REE pattern as clinopyroxene. Whole-rock and mineral geochemistry support an origin as cumulates from alkaline to subalkaline melts for most of the pyroxenites and hornblendites that have been studied. The Sr–Nd isotope ratios of pyroxenite xenoliths display two extreme compositional poles: one clinopyroxenite plots in the OIB field towards depleted values ( 87Sr/ 86Sr = 0.7028 and εNd = 6.2), whereas the other pyroxenites plot in enriched lithospheric fields (0.705 to 0.706 and −2.8 to −3.4, respectively), which implies that different magmas have been involved in their genesis. The hornblenditic xenolith suite has a very homogeneous isotopic composition, close to the isotopically depleted values of high εNd and low 87Sr/ 86Sr ratios of one of the pyroxenite xenoliths. Some of these ultramafic xenoliths fall within the isotopic compositional range of their host alkaline dykes, which also define a bipolar compositional field, suggesting that most of them are cogenetic with the lamprophyres. P–T estimates yield temperatures in the range of 970–1080 °C and pressures mainly from 0.9 to 1.2 GPa for pyroxenites, whilst hornblenditic xenoliths give lower (and probably underestimated) pressures (0.7–0.9 GPa). This pressure range is in agreement with pyroxenites being formed by an underplating event at the upper mantle–lower crust boundary, whereas pressure estimates for hornblenditic xenoliths suggest equilibration within the lower crust.

The late Cretaceous lithospheric mantle beneath the Central Andes: Evidence from phase equilibria and composition of mantle xenoliths

Temperature estimates and chemical composition of mantle xenoliths from the Cretaceous rift system of NW Argentina (26°S) constrain the rift evolution and chemical and physical properties of the lithospheric mantle at the eastern edge of the Cenozoic Andean plateau. The xenolith suite comprises mainly spinel lherzolite and subordinate pyroxenite and carbonatized lherzolite. The spinel lherzolite xenoliths equilibrated at high- T (most samples >1000 °C) and P below garnet-in. The Sm–Nd systematics of compositionally unzoned clino- and orthopyroxene indicate a Cretaceous minimum age for the high- T regime, i.e., the asthenosphere/lithosphere thermal boundary was at ca. 70 km depth in the Cretaceous rift. Major elements and Cr, Ni, Co and V contents of the xenoliths range between values of primitive and depleted mantle. Calculated densities based on the bulk composition of the xenoliths are <3280 kg/m 3 for the estimated P– T conditions and indicate a buoyant, stable upper mantle lithosphere. The well-equilibrated metamorphic fabric and mineral paragenesis with the general lack of high- T hydrous phases did not preserve traces of metasomatism in the mantle xenoliths. Late Mesozoic metasomatism, however, is obvious in the gradual enrichment of Sr, U, Th and light to medium REE and changes in the radiogenic isotope composition of an originally depleted mantle. These changes are independent of the degree of depletion evidenced by major element composition. 143Nd/ 144Nd i ratios of clinopyroxene from the main group of xenoliths decrease with increasing Nd content from >0.5130 (depleted samples) to ca. 0.5127 (enriched samples). 87Sr/ 86Sr i ratios (0.7127–0.7131, depleted samples; 0.7130–0.7134, enriched samples) show no variation with variable Sr contents. Pb i isotope ratios of the enriched samples are rather radiogenic ( 206Pb/ 204Pb i 18.8–20.6, 207Pb/ 204Pb i 15.6–15.7, 208Pb/ 204Pb i 38.6–47) compared with the Pb isotope signature of the depleted samples. The large scatter and high values of 208Pb/ 204Pb i ratios of many xenoliths indicates at least two Pb sources that are characterized by similar U/Pb but by different Th/Pb ratios. The dominant mantle type in the investigated system is depleted mantle according to its Sr and Nd isotopic composition with relatively radiogenic Pb isotope ratios. This mantle is different from the Pacific MORB source and old subcontinental mantle from the adjacent Brazilian Shield. Its composition probably reflects material influx into the mantle wedge during various episodes of subduction that commenced in early Paleozoic or even earlier. Old subcontinental mantle was already replaced in the Paleozoic, but some inheritance from old mantle lithosphere is represented by rare xenoliths with isotope signatures indicating a Proterozoic origin.

Composition of the lower crust of the Arabian Plate: a xenolith perspective

Petrological and geochemical data for a suite of mafic granulite xenoliths in Cenozoic alkali basalts from Saudi Arabia, Jordan and Syria provide a unique opportunity to explore the composition and nature of the lower crust beneath the Arabian Plate. Two mineralogically and chemically distinct groups of xenoliths occur. Group I is composed of two pyroxenes and plagioclase approximately in equal amounts. Group II is plagioclase rich and has variable proportions of orthopyroxene and clinopyroxene. The xenolith mineral assemblages and geothermobarometry of coexisting minerals suggest that these xenoliths represent basaltic cumulates that crystallized under high-pressure conditions in the lower crust. The xenoliths possibly form a part of a lower crustal gabbroic intrusive complex that underlies the Arabian Plate and may represent mafic roots of an arc complex of Pan-African age beneath Arabia. The xenolith data are compatible with available geophysical models on crust thickness and layering. The crust is between 20 and 40 km thick, and its lower part consists of mafic meta-igneous granulites. The chemical averages of xenoliths from different parts of the Arabian Plate are more mafic than the estimated present-day average of model lower crust.

Sulfur and selenium systematics of the subcontinental lithospheric mantle: Inferences from the Massif Central xenolith suite (France)

Selenium has been analyzed in addition to S in 58 spinel peridotite xenoliths collected in Cenozoic alkali basalts from the Massif Central (France). The S concentration range now available for this suite, calculated from 123 samples, is the largest ever reported for alkali basalt-hosted xenoliths (<3–592 ppm). Likewise, the Se concentrations range between 0.2 and 67 ppb. No partial melting signature can be identified from the S and Se systematic. Half of the analyzed xenoliths have lost S during supergene weathering. By contrast, neither surficial alteration, nor loss of chalcophile elements during eruption can explain the regional-scale variations of S and Se concentrations. A first group of lherzolite xenoliths sampled in Southern Massif Central, from volcanic centers older and spatially unrelated to the Massif Central plume that triggered the Cenozoic volcanism, contains between 20 and 250 ppm S (with occasional S concentrations up to 592 ppm) and 12–67 ppb Se. It is clear that the highest S values, originally interpreted as representing S abundances in the primitive mantle, were in fact enriched by metasomatism. Highly variable S and Se contents (<5–360 ppm; 9–52 ppb) have also been observed in peridotite xenoliths collected in the Northern Massif Central, from volcanic centers mostly older than the plume. Like Group I xenoliths, these Group II xenoliths were strongly metasomatized by volatile-rich carbonated/silicated melts which precipitated Cu-rich sulfides. A third group of xenoliths from Plio-Quaternary basalts spatially related to the Massif Central Plume are uniformly poor in S (10–60 ppm) and Se (9–29 ppb). In this Group III, poikiloblastic textured xenoliths have lost most of their S and Se budget by peridotite-melt interactions at high melt/rock ratios. Taken as a whole, the Massif Central xenolith suite provides further evidence for strong heterogeneities in the S and Se budget of the sub-continental lithospheric mantle. However, the few LREE-depleted fertile lherzolites that escaped strong metasomatic alterations suggest a S- and Se-depleted primitive mantle reservoir compared to currently accepted primitive mantle estimates.

Composition and Cretaceous thermal structure of the upper mantle beneath the Damara Mobile Belt: evidence from nephelinite-hosted peridotite xenoliths, Swakopmund, Namibia

Abstract: Mantle xenoliths have been discovered in a Cretaceous nephelinite plug in the Damara Belt of NW Namibia, providing a new window into the thermal and compositional structure of the lithospheric mantle that separates the Congo and Kaapvaal cratons of southern Africa. The xenoliths range in size from 1.5 to 5 cm, and comprise lherzolite and harzburgite (± spinel), with predominantly granuloblastic textures. Two groups of spinel peridotites are identified on the basis of mineral chemistry. The first group has refractory compositions with Mg-number progressively increasing from olivine (91.3–92.0), through orthopyroxene (91.8–92.5) to clinopyroxene (93.7–94.3), low Al 2 O 3 (2.4–2.9 wt%), Na 2 O (0.25–0.63 wt%) and TiO 2 (&lt;0.05 wt%) contents in clinopyroxene, and Cr-rich spinels (Cr/(Cr+Al)=0.34–0.47). The second group has less refractory mineral compositions with Mg-number of 90–91 for all three silicate phases, clinopyroxene with elevated Al 2 O 3 ( c . 6.5 wt%), Na 2 O ( c . 1.7 wt%) and TiO 2 ( c . 0.45 wt%) contents, and Al-rich spinels (Cr/(Cr+Al)=0.11). In terms of mineral chemistry and textures, both groups of xenoliths can be classified as Type I peridotites and, unlike spinel lherzolites from the Okenyenya complex 100 km to the NE, they show no signs of modal metasomatism. Calculated oxygen fugacities indicate equilibration of the xenoliths at −0.29 to 1.0 log units above fayalite–magnetite–quartz (FMQ), which for the fertile peridotites places them among the most oxidized samples recorded from continental settings. Temperatures of equilibration range from 871 to 1236 °C for T BKN and from 844 to 1128 °C for T Ca-in-opx . No significant difference in temperature was found between the two groups of lherzolites. Pressures calculated from the Ca content of olivine in equilibrium with clinopyroxene are in the range 1.4–2.0 GPa, with the fertile lherzolites originating at deeper levels than the refractory group. A local geotherm substantially hotter ( c . 90 mW m −2 ) than that beneath the contemporaneous Gibeon kimberlites ( c . 45 mW m −2 ) is indicated and probably relates to tectonothermal events associated with local and regional Cretaceous magmatism. The inferred mantle stratigraphy suggests the juxtapositioning of relatively fertile Phanerozoic-type mantle beneath refractory peridotite similar to that of Proterozoic lithosphere sampled by the Gibeon kimberlites 300 km to the SE. This may have occurred during formation of the Damara Belt and be related genetically to the linear belt of mafic volcanic rocks emplaced during late Proterozoic time and preserved today as the Matchless amphibolite.

AMPHIBOLE-PHLOGOPITE MANTLE XENOLITHS FROM THE KERGUELEN ARCHIPELAGO: EVIDENCE OF A MAJOR METASOMATIC EVENT BY ALKALINE SILICATE AND CARBONATE-RICH MELTS

Alkali basaltic rocks from the Kerguelen Islands have entrained numerous phlogopite- and/or amphibole-bearing ultramafic to mafic xenoliths. The xenoliths can be subdivided into mantle wall-rocks (harzburgites, dunites, Type I xenoliths) and high pressure magmatic segregates (clinopyroxenites, hornblendites, glimmerites; type II xenoliths) that generally form composite xenoliths with mantle wall-rocks. A phlogopite megacryst-bearing lava was also incorporated in this study. Chemical compositions of amphiboles and phlogopites from both xenolith groups are characterized by high titanium contents and are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites, alkali basalts and in peridotites or pyroxenites from orogenic lherzolite massifs. Phlogopite shows homogeneous trace element compositions and important incompatible trace element fractionation processes characterized by strong enrichment in Rb, Ba, Nb, Ta, Pb, Sr, Ti and slightly in Zr, Hf. Amphibole displays compositional variation between samples but they are commonly high in LIL element and show systematic negative Zr, Hf, Pb and Sr anomalies. Hornblendite dykelets from composite xenolith display major and trace element compositions closely similar with Kerguelen lamprophyric lavas. Clinopyroxenes from type I, type II and composite hydrous Kerguelen xenoliths display tight similarities in their trace element contents (Fig. 1). These later display an enrichment in LREE (La/Yb)N=10 with large negative anomalies in Rb, Ba, Pb, Sr, Ti, Zr and Y. They are also similar to poikilitic harzburgite (anhydrous) clinopyroxene. The calculated liquids in equilibrium with clinopyroxene may be compared with ultramafic and alkaline lamprophyric melts. One sample of phlogopite-bearing dunite displays clinopyroxene with very large anomalies in Rb, Ba, Nb, Ta, Zr, Hf, Pb, Sr and Ti which is characteristic of an interaction with a carbonatitic melt. The dD value of -86 to -70‰ vs SMOW for mica and amphibole of both types of Kerguelen xenoliths and of the phlogopite megacrysts are very homogeneous and within the common mantle range. These data are in agreement with neon isotopic data that indicate homogeneous and relatively primitive compositions of the volatile sources. The phlogopite- and/or amphibole-bearing ultramafic to mafic xenoliths from Kerguelen Islands may be all related to the circulation of highly alkaline mafic silicate melts into the upper mantle. Interstitial amphibole and phlogopite of harzburgites and dunites formed by a diffuse percolation of such melts within the upper mantle (porous flow). Evidence from composite xenoliths indicates channellized circulation of similar melts propagated by hydraulic fracture resulting in a network of dykes in the Kerguelen upper mantle. The high pressure magmatic xenoliths are evidenced of the cristallisation of highly alkaline magmas into the upper mantle and the lower crust. The mica-bearing lavas (“lamprophyres”) are the surface expression of this highly alkaline magmatic activity. The ubiquity of the volatile-rich high alkaline magmatic activity and its isotopic homogeneity is inferred to be the latest and largest metasomatic event related to the late intraplate activity of the Kerguelen mantle plume.

Isotopic and trace element compositions of upper mantle and lower crustal xenoliths, Cima volcanic field, California: Implications for evolution of the subcontinental lithospheric mantle

Ultramafic and mafic xenoliths from the Cima volcanic field, southern California, provide evidence of episodic modification of the upper mantle and underplating of the crust beneath a portion of the southern Basin and Range province. The upper mantle xenoliths include spinel peridótite and anhydrous and hydrous pyroxenite, some cut by igneous‐textured pyroxenite‐gabbro veins and dikes and some by veins of amphibole ± plagioclase. Igneous‐textured pyroxenites and gabbros like the dike rocks also occur abundantly as isolated xenoliths inferred to represent underplated crust. Mineral and whole rock trace element compositions among and within the different groups of xenoliths are highly variable, reflecting multiple processes that include magma‐mantle wall rock reactions, episodic intrusion and infiltration of basaltic melts of varied sources into the mantle wall rock, and fractionation. Nd, Sr, and Pb isotopic compositions mostly of clinopyroxene and plagioclase mineral separates show distinct differences between mantle xenoliths (εNd = −5.7 to +3.4; 87Sr/86Sr = 0.7051 – 0.7073; 206Pb/204Pb = 19.045 – 19.195) and the igneous‐textured xenoliths (εNd = +7.7 to +11.7; 87Sr/86Sr = 0.7027 – 0.7036 with one carbonate‐affected outlier at 0.7054; and 206Pb/204Pb = 18.751 – 19.068), so that they cannot be related. The igneous‐textured pyroxenites and gabbros are similar in their isotopic compositions to the host basaltic rocks, which have εNd of +5.1 to +9.3; 87Sr/86Sr of 0.7028 – 0.7050, and 206Pb/204Pb of 18.685 – 21.050. The igneous‐textured pyroxenites and gabbros are therefore inferred to be related to the host rocks as earlier cogenetic intrusions in the mantle and in the lower crust. Two samples of peridotite, one modally metasomatized by amphibole and the other by plagioclase, have isotopic compositions intermediate between the igneous‐textured xenoliths and the mantle rock, suggesting mixing, but also derivation of the metasomatizing magmas from two separate and distinct sources. Sm‐Nd two‐mineral “isochrons” yield apparent ages for petrographically identical rocks believed to be coeval ranging from ∼0 to 113±26 Ma, indicating the unreliability of dating these rocks with this method. Amphibole and plagioclase megacrysts are isotopically like the host basalts and probably originate by mechanical breakup of veins comagmatic with the host basaltic rocks. Unlike other Basin and Range localities, Cima Cr‐diopside group isotopic compositions do not overlap with those of the host basalts.

Origin of the continental crust in the Colorado Plateau: Geochemical evidence from mafic xenoliths from the Navajo Volcanic Field, southwestern USA

Mafic xenoliths from the Navajo Volcanic Field (NVF) in the Colorado Plateau are represented by four lithologies, in order of decreasing abundance: garnet granulite, amphibolite, gabbro, and pyroxene granulite, Eight geochemical groups are defined based chiefly on REE (rare earth element) and HFSE (high field strength element) distributions. Mg number exhibits a broad negative correlation with SiO 2, and most xenoliths plot in the hy-di-ol-plg volume of the CIPW normative basalt tetrahedron. REE distributions include groups with LREE enrichment (Gps I, IV, VII), LREE depletion (Gp II), HREE depletion (Gps III, V), flat REE patterns (Gp VI), and u-shaped patterns (Gp VIII). With the exception of Groups VI and VII, all of the NVF mafic xenoliths show negative Ta Nb anomalies on primitive-mantle-normalized diagrams; also, all groups show negative Th anomalies, and all but Gp IV show positive Sr anomalies. Thorium and Sr anomalies appear to reflect introduction of Rb, Ba, Sr, and K during xenolith alteration. Nickel changes rapidly in concentration with only a small change in Zr, suggesting the rocks are dominantly mafic cumulates. Geochemical results also indicate that the mafic xenoliths are not genetically related to the host minettes. Associated spinel lherzolite xenoliths are depleted in incompatible elements, and cannot serve as sources for any of the mafic xenoliths, but they may represent restite left during extraction of the xenolith protolith liquids. Garnet lherzolite xenoliths, which are enriched in incompatible elements, could have served as sources for the LREE-enriched mafic xenolith groups. If there is no sampling bias in the NVF mafic xenoliths, mafic xenolith Groups IV, V, VI, and VII dominate in the lower crust of this region. A component of enriched lower crust, as represented by xenolith Group IV and by xenoliths from the San Francisco Volcanic Field, occurs only northwest of a boundary extending from near Flagstaff, Arizona, into northwestern New Mexico and may be coincident with a buried Early Proterozoic crustal boundary. Published geochemical and Nd, Sr, and Pb isotopic data from Proterozoic surface rocks and from NVF mafic xenoliths indicate that the crust and mantle lithosphere beneath the Colorado Plateau is dominantly Early proterozoic (2.0–1.75 Ga) in age. With the possible exception of gabbros from Moses Rock, there is no evidence among the mafic crustal xenoliths for later magmatic underplating. Incompatible element distributions are consistent with an origin for the crust by Early Proterozic arc accretion, and they do not reflect a contribution from submarine plateaus. Also, a negative Eu anomaly must exist in both the upper and lower crust in this region.