Ratios Of Clinopyroxenes Research Articles

Enrichment Nature of Ultrapotassic Rocks in Southern Tibet Inherited from their Mantle Source

Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.

Petrogenesis, platinum-group element geochemistry and geodynamic evolution of the Cretaceous Chilas gabbros, Kohistan island arc, NE Pakistan

Kohistan, an intra-oceanic arc, formed due to the India-Asia collision-related tectonics. The arc is comprised of mafic-ultramafic and crustal lithologies. Chilas Complex, one of the major mafic-ultramafic body that lies at the base of the Kohistan arc, is dominantly composed of massive gabbronorites with local ultramafic-mafic intrusive bodies. The ultramafic rocks were considered as the products of fractional crystallization whereas gabbronorites were reported to have formed at the root zone of the magma chamber either from melts or due to fractional crystallization, based on major and trace element data. To understand the petrogenesis and origin of the Chilas gabbros, we conducted platinum group elements (PGEs) geochemistry combined with whole-rock major and trace element compositions. Geochemical data of the Chilas gabbros show tholeiite series, likely derived from a depleted lithospheric mantle source via peridotite melts. Presence of hornblende and relatively higher Al/Ti ratios of clinopyroxene in the Chilas gabbros suggest hydrous magma, ascribed to mantle wedge metasomatism by slab-derived fluids. Poor correlations between MgO vs. Ni, Cr, and V, Ni vs. Cr, Dy/Yb vs. SiO2, Sr/Y vs. Eu/Eu* and relatively low TiO2 indicate a minimal role of fractional crystallization. Low contents of LOI (0–3.24 wt%), Ce/Ce* (0.92–1.01), and positive relation of La, Ta, and Hf with Zr strongly support the insignificant impact of hydrothermal alteration. Enrichment of PGEs, such as Pt (0.63–13.75 ppb), Pd (0.43–13.87 ppb), and lower Pd/Ir ratios (0.76–40.04) suggest low degree partial melting of the parent magma. Higher Pd, Pd/Ir ratios compared to Pt and Pt/Pd, combined with enrichment of PPGE over IPGE, suggest the participation of slab-dehydrated fluids and mantle metasomatism however Cu/Ir vs. Ni/Pd, V-Pd/Ir and La-Pd/Ir ratios point towards olivine fractionation in the magma. Lower Cu/Pd ratios, relationships of Ru and Pt against Cr, and Os diminution corroborate S-undersaturation in parental magma. The PGEs contents and Pd/Pt ratios attest depleted mantle source for their production. In view of Cu/Zr and Cu/Pd ratios, it is substantiated that the sulphide segregation cannot be a promising reason for PGEs depletion. Therefore, we infer PGEs depletions in parental magma are due to PGEs-depleted mantle source, though sulfide retention in mantle source cannot be eliminated. The geochemical fingerprints in tectonic diagrams and negative anomalies of Nb and Zr in the studied samples provide credible facts that favour arc magmatism. Based on major and trace element data and PGEs values, we consider that the Chilas gabbros were formed from a lithospheric mantle that was metasomatized by fluids generated at the mantle wedge and derived from mafic melts in an arc setting. Geochemical data and their plotting on various tectonic discrimination diagrams indicate Chilas Complex as a remnant of the Neo-Tethyan oceanic arc system.

Lithospheric transformation of the northern North China Craton by changing subduction style of the Paleo-Asian oceanic plate: Constraints from peridotite and pyroxenite xenoliths in the Yangyuan basalts

Mechanisms of lithospheric transformation beneath the North China Craton (NCC) were widely discussed, but the northern NCC was less studied compare to the eastern part. In this research, In-situ major and trace element and Sr isotopic compositions of spinel peridotite and pyroxenite xenoliths carried by the Yangyuan Tertiary basalt were analyzed to constrain subduction style of the Paleo-Asian oceanic plate (PAOP) and its influence on the northern NCC. These xenoliths were classified into dunite, pyroxenite, and two groups of lherzolites according to their petrography and chemical features. The petrography and mineral chemistry suggest that the dunite, pyroxenite, and Group 1 lherzolites are residues of partial melting which experienced subsequent hydrous metasomatism. Correlations between 87Sr/86Sr with the Zr/La and Zr/Nb ratios in clinopyroxenes from these xenoliths suggest that the metasomatic agents should be fluids derived from the subducted PAOP. Mineral RbSr isochron of the pyroxenite show that the minimum age of the metasomatism event should be 295 ± 38 Ma. Major element compositions of minerals in the Group 2 lherzolites are more fertile compare to the dunite and Group 1 lherzolites. Clinopyroxenes in these xenoliths exhibit LREE-depleted REE patterns with relatively high HREE contents and 87Sr/86Sr ratios (0.70403–0.70482). Chemical zonations are preserved in several clinopyroxenes with incompatible elements depleted from core to rim, which should be the results of melt depletion after metasomatism. Patches of silica-rich glasses were present along the grain boundaries. These glasses have sediment-like Pb isotopic compositions and trace elements distribution patterns and thus are supposed to represent the melt from recycled sediments. Combined with geophysical and tectonic evidences, we propose that roll-back of the southward low-angle subducted PAOP in the late stage of subduction contributed to remelting of the PAOP subduction-modified mantle wedge beneath the northern NCC.

Formation age and metasomatism of the sub-continental lithospheric mantle beneath southeast China: Sr-Nd-Hf-Os isotopes of Mingxi mantle xenoliths

This study presents comprehensive geochemical data of spinel- and garnet-facies mantle peridotites entrained in the Cenozoic basalts from Mingxi, southeastern China. Whole rock major elements suggest that most Mingxi peridotites have been subjected to polybaric melting, i.e., <15% for the lherzolites and ∼15–25% for the harzburgites. Presence of amphibole/phlogopite in a few samples indicates the local occurrence of modal metasomatism, whereas mineral chemistry suggests that most Mingxi peridotites have experienced cryptic metasomatism by various agents, including carbonatitic melts, silicate melts and Fe-Ti-rich melts. Clinopyroxenes in the harzburgites have 87Sr/86Sr and 143Nd/144Nd ratios of 0.703121–0.704557 and 0.512793–0.513106 (εNd of +3∼+9), which are more enriched than those in the lherzolites (87Sr/86Sr of 0.702238–0.703898, 143Nd/144Nd of 0.512918–0.514763, and εNd of +5∼+42). The 176Hf/177Hf ratios of clinopyroxenes in harzburgites are comparable to the lherzolites, i.e., 0.282888–0.285485 (εHf of +4∼+96) and 0.283259–0.285791 (εHf of +15∼+106), respectively. Neither Sm-Nd nor Lu-Hf isotopes yielded meaningful isochron ages owing to the metasomatic overprinting. Clinopyroxenes in two samples are characterized by extremely radiogenic 176Hf/177Hf ratios in association with less extreme 143Nd/144Nd ratios, resulting in a strong Nd-Hf decoupling relative to the mantle array. Whole rock Os isotopes of the Mingxi peridotites range from 0.11685 to 0.13035, yielding Re depletion (TRD) ages no older than 1.79Ga, and do not show a correlation with Re/Os ratios. In contrast, they display a roughly positive correlation with bulk rock Al2O3 contents, which can be explained by mixing of ancient mantle with fertile convecting upper mantle. Therefore, we suggest that the SCLM beneath Mingxi is juvenile and recently accreted from the convective mantle. The Mingxi peridotites with old Os ages reflect the ancient mantle in the convecting upper mantle, and have no relationship with the overlying old crust with ages of the Late Paleoproterozoic. The newly formed SCLM beneath Mingxi has been metasomatized by fluids/melts released from the subducted paleo-Pacific Ocean, which was initiated since the Mid-Permian.

Phase equilibria modelling using major and trace element compositions of zoned garnet and clinopyroxene from southwestern Tianshan eclogites, China

We present major and trace element zoning in clinopyroxene and garnet in the low-temperature eclogites from southwestern Tianshan ultrahigh-pressure metamorphic belt. The clinopyroxene crystals in the matrix exhibit three distinct growth zones, including the aegirine-rich core, a transition mantle and the omphacite rim. The aegirine core has weak Ce anomaly, flat light rare earth element pattern, and slightly enriched heavy rare earth element contents. The omphacite rim has lower trace element contents, and is depleted in large-ion lithospheric elements, light and heavy rare earth elements. The garnet zonation is featured by the core-to-rim increase in pyrope and grossular contents, and distinct rare earth element patterns: the core has left-leaning rare earth element patterns and the rim is depleted in both light and heavy rare earth elements. Phase equilibria modelling based on the phase assemblage and mineral zoning patterns reveal that the aegirine eclogite was heated and buried along the ca. 8°C/km geotherm in the cold subduction zone, and reached the peak metamorphic condition of 23–25kbar, 540–560°C. The jadeite content in the clinopyroxene increases along the prograde-to-exhumation P-T path (Jd: from 0.25 to 0.45), with the transitional mantle corresponds to the peak conditions. The omphacite rim with the highest jadeite content (Jd∼45) formed during decompression. For a Fe3+-rich clinopyroxene, the maximum in Jd component does not necessarily represent the peak pressure, so the conventional thermobarometers should be used with caution for mineral assemblages with such complicated zonation patterns. The variation of oxygen fugacity correlates with the changes in mineral assemblage, P-T condition, and the bulk-rock Fe3+/FeT ratio. The oxygen fugacity is not always positively correlated with the Fe3+/(Fe3++Al) or Fe3+/FeT ratio in clinopyroxene. Rare earth element zoning garnet and clinopyroxene are modelled based on mass balance. The modelled growth zonation closely resembles the observed compositional profiles, suggesting that rare earth elements are passively distributed among phases near chemical equilibrium during the mineral growth.

Mantle metasomatism did not modify the initial H2O content in peridotite xenoliths from the Tianchang basalts of eastern China

Metasomatism induced by melts/fluids is ubiquitous in the lithospheric mantle and can potentially modify the initial water content of the mantle. However, the preservation of correlations between H2O content and partial melting indices (e.g., Yb content in clinopyroxene, Cr/(Cr+Al) in spinel) and the lack of correlations between H2O content and metasomatic indices (e.g., La/Yb in clinopyroxene) in peridotite xenoliths from several localities suggest that variations in the initial H2O content were controlled by partial melting processes rather than by subsequent metasomatic event(s) (Hao et al., 2014; Denis et al., 2015). However, the applied partition coefficients of H2O between peridotite and melt (Dperidotite/melt=0.1–0.3) in the partial melting models cast doubts on the reasonability of such explanations. Whether metasomatism always modifies the initial H2O content of the lithospheric mantle remained a topic of debate. In this paper, we measure major and trace element concentrations and H2O contents of minerals in the peridotite xenoliths hosted by the Tianchang Cenozoic basalts in eastern China by using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The H2O contents (weight in ppm) of clinopyroxene, orthopyroxene and olivine are 70–280ppm, 35–140ppm and below detection limit (<2ppm H2O), respectively. Although water diffusion during xenolith ascent cannot be excluded for olivine, pyroxenes largely retain the initial H2O content of the mantle source, as supported by (1) the correlation between H2O content and major element content of pyroxene, and (2) the equilibrium H2O partitioning between clinopyroxene and orthopyroxene. The calculated whole-rock H2O contents range from 14 to 93ppm (average 52±25ppm) assuming 0.1 for the H2O partition coefficient between olivine and clinopyroxene. Although no hydrous minerals are found, the enrichment in light rare earth elements and large ion lithophile elements of clinopyroxene indicates cryptic mantle metasomatism. However, variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) are not correlated, suggesting that mantle metasomatism did not modify the initial H2O contents after the melting event. Instead, the H2O content correlates with the melting indices demonstrating that partial melting is the primary factor controlling the variations in the initial H2O contents. Notably, variations in whole rock H2O contents of the Tianchang peridotites can be modeled as a simple melting process of a MORB mantle source using the reported partition coefficients of H2O (Dperidotite/melt=0.005–0.03), providing a robust example suggesting that metasomatism does not always change the initial H2O content in the lithospheric mantle.

Nature and evolution of lithospheric mantle beneath the southern Ethiopian rift zone: evidence from petrology and geochemistry of mantle xenoliths

Mantle xenoliths hosted in Quaternary basaltic lavas from the Dillo and Megado areas of the southern Ethiopian rift are investigated to understand the geochemical composition and associated processes occurring in the lithospheric mantle beneath the region. The xenoliths are comprised of predominantly spinel lherzolite with subordinate harzburgite and clinopyroxenite. Fo content of olivine and Cr# of spinel for peridotites from both localities positively correlate and suggest the occurrence of variable degrees of partial melting and melt extraction. The clinopyroxene from lherzolites is both LREE depleted (La/Sm(N) = 0.11–0.37 × Cl) and LREE enriched (La/Sm(N) = 1.88–15.72 × Cl) with flat HREEs (Dy/Lu(N) = 0.96–1.31 × Cl). All clinopyroxene from the harzburgites and clinopyroxenites exhibits LREE-enriched (La/Sm(N) = 2.92–27.63.1 × Cl and, 0.45 and 1.38 × Cl, respectively) patterns with slight fractionation of HREE. The 143Nd/144Nd and 176Hf/177Hf ratios of clinopyroxene from lherzolite range from 0.51291 to 0.51370 and 0.28289 to 0.28385, respectively. Most of the samples define ages of 900 and 500 Ma on Sm–Nd and Lu–Hf reference isochrons, within the age range of Pan-African crustal formation. The initial Nd and Hf isotopic ratios were calculated at 1, 1.5, 2 and 2.5 Ga plot away from the trends defined by MORB, DMM and E-DMM which were determined from southern Ethiopian peridotites, thus indicating that the Dillo and Megado xenoliths could have been produced by melt extraction from the asthenosphere during the Pan-African orogenic event. There is no significant difference in 87Sr/86Sr ratios between the depleted and enriched clinopyroxene. This suggests that the melts that caused the enrichment of the clinopyroxene are mainly derived from the depleted asthenospheric mantle from which the xenoliths are extracted. Largely, the mineralogical and isotopic compositions of the xenoliths show heterogeneity of the CLM that could have been produced from various degrees of melt extraction, followed by metasomatism.

Partial melting control of water contents in the Cenozoic lithospheric mantle of the Cathaysia block of South China

Major and trace element and the H2O contents of minerals in peridotite xenoliths hosted by the Cenozoic basalts of Jiande in the Cathaysia block were evaluated using electron microprobe, laser-ablation ICP-MS and Fourier transform infrared spectroscopy, respectively. The correlations among the major elements of the minerals define a melting trend, and modeling of the Y and Yb contents in the clinopyroxenes indicates that the degree of partial melting ranges from 1% to 15%. Most samples (22 out of 29) show depleted chondrite-normalized rare earth element patterns and a degree of partial melting <4%. The H2O contents (weight in ppm) of the clinopyroxenes, orthopyroxenes and olivines are 390–590ppm, 160–330ppm and ∼0ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivines, pyroxenes largely preserve the H2O content they have in the mantle prior to sampling by the host basalts, as inferred from (1) the homogenous H2O content within single pyroxene grains, and (2) the equilibrium H2O partitioning between the clinopyroxene and orthopyroxene. Based on the mineral modes and assuming a partition coefficient of 10 for H2O between the clinopyroxene and olivine, the calculated whole-rock H2O contents range from 90 to 220ppm, similar to that of the MORB source. When combined with previously reported data for peridotites hosted by Cenozoic basalts at other localities of the Cathaysia block, the correlation of H2O content with melting index (such as Yb content in cpx, Cr# in spinel) suggests that extent of partial melting is the main factor controlling H2O abundance in these rocks. Nevertheless, the variations in whole-rock H2O contents cannot be perfectly modeled as a simple modal melting process using available experimental partition coefficients of H2O between peridotite and melt. The lack of coherent variations between the H2O contents of the whole rocks and the metasomatic index (La/Yb ratio in clinopyroxene) indicates that mantle metasomatism did not modify the initial H2O contents after the melting event(s). Based on the similarities in major and trace elements, and of H2O contents in the Cathaysia block peridotites and those inferred for the MORB source, we propose that the Cenozoic lithospheric mantle of the Cathaysia block is accreted from the upwelled and cooled asthenospheric mantle. In addition, the H2O contents of the Cenozoic lithospheric mantle of the Cathaysia block are much higher than those of the North China Craton of similar fertility (100ppm vs. 20ppm for whole rock H2O contents), where lithospheric thinning occurred during the Mesozoic. This implies that either the Cathaysia block did not undergo similar lithospheric thinning or that the mechanism of the lithospheric thinning was different.

He and Ar isotope geochemistry of pyroxene megacrysts and mantle xenoliths in Cenozoic basalt from the Changle–Linqu area in western Shandong

We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong. The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton. The 3He/4He ratios of pyroxene megacrysts, websterite, and dunites are 7.0–7.7 Ra, 7.4 Ra, and 7.0–7.7 Ra, respectively. They are related to the origin of host basalts, and are slightly lower than that of mid-ocean ridge basalts (MORB). Their 40Ar/36Ar ratios are much lower than that of MORB and close to that of air. The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle, and their REE and trace elements indicate that they experienced melt metasomatism and partial melting. The 3He/4He ratios of their olivine are slightly lower than that of MORB, but 3He/4He ratios of their clinopyroxene are low (2.3–7.1 Ra) and display an inverse correlation with (La/Yb)N. The 40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio. Combining existing studies of petrology, Sr–Nd–Pb isotopes, O isotopes, trace elements, and Mg isotopes, we infer that the juvenile lithospheric mantle in the Changle–Linqu area was metasomatized by oceanic crust-derived melts, which transfer the supracrustal Ar isotope signatures to the mantle sources. The low 4He abundance and low 3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.

Shonkinites and minettes of the Ryabinovyi massif (Central Aldan): composition and crystallization conditions

Dikes of biotitic shonkinites and minettes of the complex Ryabinovyi alkaline massif (Central Aldan) have been studied. The dikes are localized in a neck of K-picrites in the northeast of the massif, which intrudes gold-bearing microcline–muscovite metasomatites (Muscovitovyi site). The obtained data on the chemical and trace-element compositions of the rocks and minerals and study of melt inclusions in clinopyroxenes indicate that the biotitic shonkinites and minettes crystallized from the same deep-seated high-pressure alkaline ultrabasic magma during its evolution. Apparently, at the early stage of crystallization of diopside in the biotitic shonkinites, homogeneous carbonate–silicate melt was separated into immiscible fractions of silicate, carbonate–salt, and carbonate melts. The temperature of melt immiscibility was >1120–1190 °C, i.e., higher than the homogenization temperature of silicate inclusions in the diopside. The contents of trace elements in the biotitic shonkinites and rock-forming clinopyroxenes were one or two orders of magnitude higher than the mantle values. The Eu/Eu* ratios of both the considered rocks and the clinopyroxenes were close to those of chondrites, which testifies to their crystallization from mantle magma. The HREE/LREE ratio indicates that the magma source was localized at the depths where garnet-spinel assemblages existed. The negative Nb and Ti anomalies in the trace-element spectra and the high (>5) La/Nb ratios in the rocks and clinopyroxenes point to the influence of crustal material on the parental magma. Crystallization of magma took place in reducing conditions, which is evidenced by the low (4–7) Ti/V ratios in clinopyroxenes and the presence of chloride–sulfate inclusions in them. Since gold in the Ryabinovyi massif is associated with late sulfate–chloride and sulfate–carbonate fluids, it might have been transported by alkaline chloride–sulfate and carbonate (carbonatite) melts, found as inclusions in clinopyroxenes of the biotitic shonkinites, at the early stages of Mesozoic magmatism.

Destruction of the North China Craton: Delamination or thermal/chemical erosion? Mineral chemistry and oxygen isotope insights from websterite xenoliths

The destruction (or reactivation) of the North China Craton (NCC) is one of the important issues related to the Phanerozoic evolution of eastern China, although the processes of destruction remain debated. Two main mechanisms – delamination and thermal/chemical erosion – have been proposed based on the geochemistry of Mesozoic–Cenozoic basalts and entrained deep–seated xenoliths. A key criterion in distinguishing between these mechanisms is the nature of the melt, derived from delaminated crust or the asthenosphere, that modified the lithospheric mantle. Here we investigate the mechanism of destruction of the NCC based on mineral compositions and oxygen isotopic data from olivines, and strontium isotopic data for clinopyroxenes within websterite xenoliths from the Early Cretaceous Feixian basalts in the eastern NCC. Olivines in websterite xenoliths have higher Mg# (86–86.4), Ni content (2187–2468ppm), and lower Ca (983–1134ppm), Ti (58.1–76.1ppm), and Mn (1478–1639ppm) contents than olivine phenocrysts (Mg#=71.0–77.3, Ni=233–1038ppm, Ca=1286–2857ppm, Ti=120–300ppm, and Mn=2092–4106ppm) from Late Cretaceous basalts. Additionally, olivines in websterite xenolith have δ18O values of 7.10±0.21‰ to 8.40±0.21‰, evidently higher than those of typical mantle-derived olivines. Similarly, orthopyroxenes (Opx) and clinopyroxenes (Cpx) in the websterite xenoliths have much higher Mg# (86.3–89.2 and 87.5–90.3, respectively), and Ni contents (1097–1491ppm and 581–809ppm, respectively) than orthopyroxene- and clinopyroxene-phenocrysts (Opx: Mg#=82.2–83.9, Ni=730–798ppm; Cpx: Mg#=74.2–84.6, Ni=117–277ppm) from Late Cretaceous basalts. The 87Sr/86Sr ratios of clinopyroxenes in the websterite xenoliths range from 0.70862 to 0.70979, and are much higher than those of clinopyroxene grains from peridotite xenoliths and basalts. These data indicate that olivines are the residue of ancient lithospheric mantle that was modified intensively by a melt derived from recycled continental crust, and that the silicic and calcic metasomatic melt might have been derived from the partial melting of the subducted Yangtze slab and delaminated lower crust of the NCC. The existence of recycled continental crust in the Mesozoic lithospheric mantle implies that delamination was an important mechanism of destruction of the NCC.

Lithospheric Mantle Modification during Cenozoic Rifting in Central Europe: Evidence from the Księginki Nephelinite (SW Poland) Xenolith Suite

The Miocene Księginki nephelinite in SW Poland is part of the Central European Volcanic Province and occurs at the NE termination of the Cenozoic Ohře (Eger) Rift in the NW part of the Bohemian Massif. It contains numerous pyroxenite and peridotite xenoliths and clinopyroxene megacrysts. Pyroxenites (mostly olivine clinopyroxenites with no primary orthopyroxene) commonly have cumulate textures. Wehrlite and websterite xenoliths, texturally and chemically related to the pyroxenite suite, contain volcanic-textured intercumulus material similar to that of the host nephelinite. The Mg/(Mg + Fe) ratios of clinopyroxenes from the pyroxenite suite are 0·81–0·93; these are low-calcium]Ca 0·72–0·82 atoms per formula unit (a.p.f.u.)] and contain 0·22–0·37 a.p.f.u. Al. Clinopyroxene rare earth element (REE) patterns are slightly enriched in middle REE (MREE) and light REE (LREE) relative to heavy REE (HREE) (SmN/YbN = 2·4–4·3, LaN/YbN = 1·7–3·0) and are deflected downwards from Pr/Nd to La. The whole-rock REE patterns of the pyroxenite xenoliths are identical and show slight enrichment in LREE relative to HREE (LaN/Ybn = 3–5). The wehrlite cumulates are more LREE-enriched (LaN/Ybn = 13) and their REE patterns are identical to those of the host nephelinite. The peridotite xenoliths exhibit a range of textures from protogranular to mylonitic. Typically the peridotites (mostly harzburgites) consist of domains of olivine I (Fo88·9–91·9) and orthopyroxene I plus interstitial, texturally later clinopyroxene I [both with Mg/(Mg + Fe) = 0·89–0·93] and rare spinel [Cr/(Cr + Al) = 0·35–0·62]. The REE patterns of clinopyroxene I are similar to those of the pyroxenite suite. Fine grains of olivine II, commonly with forsterite contents identical to those of olivine I, but enriched in Ca, occur at the contacts between olivine I grains and in interstices or pools with glass. Clinopyroxene I commonly has spongy rims of clinopyroxene II, which are richer in Mg and poorer in Na. The peridotite xenoliths contain sparse fine-grained micrometre- to millimetre-scale aggregates consisting of (1) spinel, olivine II, clinopyroxene III and altered glass ± titanian biotite or (2) plagioclase, olivine II and clinopyroxene III ± glass. Olivine I in xenoliths containing these aggregates is typically Fo85·6–89·4, and the Mg/(Mg + Fe) ratio of pyroxenes is similar to that of olivine. The lithospheric peridotitic mantle beneath Księginki was infiltrated by alkaline silicate melts similar to the nephelinite lava prior to and during the volcanism. Pyroxenite cumulates crystallized from the melt in places where the flow was channelized. Locally melt infiltration was pervasive, leading to ‘Fe-metasomatism’ of the peridotites, which lowered the Fo content of the olivine to c. 86 %. The metasomatism was cryptic, and only the clinopyroxene trace element content was modified in peridotites not affected by pervasive melt infiltration or not in contact with pyroxenites. The mantle section affected by the metasomatism was located at a depth of ~35–50 km and was thermally equilibrated (temperatures of 1060–1120°C are recorded in both the pyroxenite and peridotite suites). The fine-grained aggregates are interpreted as remnants of the interstitial melt frozen after entrainment of the xenoliths during the eruption of the host nephelenite; they show that some of the peridotites resided at pressures allowing plagioclase crystallization, whereas the others were brought to the surface directly from spinel peridotite facies depths. The Księginki xenoliths provide a complete snapshot of the various effects of alkaline melt migrating through the lithospheric mantle during the initial stages of continental rifting.

Nonchondritic 142Nd in suboceanic mantle peridotites

The discovery that several solid Earth reservoirs have a superchondritic 142Nd/144Nd ratio led to the hypothesis that either the bulk silicate Earth is not chondritic or that a subchondritic reservoir lies hidden somewhere within the Earth's interior. One important reservoir, i.e., mid‐ocean ridge peridotites representing the main component of the upper oceanic mantle and the source of mid‐ocean ridge basalt, has never been tested for 142Nd/144Nd. We determined the 142Nd/144Nd ratio in clinopyroxene separated from two peridotites and a pyroxenite from the SW Indian Ridge and one peridotite from the northern Mid‐Atlantic Ridge. All samples analyzed have superchondritic 142Nd/144Nd ratios in line with mantle‐derived material measured to date, except for some ancient cratonic rocks.

Late orogenic mafic magmatism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion

The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths (<12 km). The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine (Fo88–80) with relatively sodic plagioclase (An75–60) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc.

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.

Petrochemistry of serpentinized peridotite from the Iberia Abyssal Plain (ODP Leg 173): its character intermediate between suboceanic and sub-continental upper mantle

Abstract Highly serpentinized peridotites derived from the upper mantle beneath a non-volcanic continental margin were sampled from the southern Iberia Abyssal Plain (Ocean Drilling Program Leg 173), and the primary mantle minerals were analysed for major and trace elements to petrologically characterize the upper mantle. The Cr -number (= Cr/(Cr + Al) atomic ratio) of the peridotite spinels varies widely from 0.1 to 0.6. The Na 2 O content of clinopyroxene is rather constant, 0.5–0.8 wt %. The chondrite-normalized rare earth element (REE) patterns have light REE (LREE) depleted convex-upward shapes. The LREE/HREE (heavy REE) ratio in clinopyroxene is constant irrespective of the degree of melt extraction of the sample as measured by the Cr -number of spinel. The trend of the peridotites’ mineral chemistry is different from both the simple melt extraction and the general mantle metasomatic trends. The geochemical character of the Iberia Abyssal Plain peridotite is intermediate between those of abyssal peridotites and peridotite xenoliths from continental regions. These geochemical features, especially for the trace elements in clinopyroxene, are rather similar to those in peridotite xenoliths from arcs. This chemical trend is probably the result of ‘open-system melting’, which involves melting simultaneously with enrichment of trace elements by the influx agent.

Determination of Ophiolitic and Nonophiolitic Gabbroic Rocks in Central Anatolia Using the Cr/Si Ratio in Clinopyroxene

Determination of Ophiolitic and Nonophiolitic Gabbroic Rocks in Central Anatolia Using the Cr/Si Ratio in Clinopyroxene

Eclogitic inclusions in diamonds: Evidence of complex mantle processes over time

The first ion-probe trace element analyses of clinopyroxene-garnet pairs both included within diamonds and from the eclogite host xenoliths are reported; these diamondiferous eclogites are from the Udachnaya and Mir kimberlite pipes, Yakutia, Russia. The major and trace element analyses of these diamond-inclusion and host-rock pairs are compared in order to determine the relative ages of the diamonds, confirm or deny genetic relationships between the diamonds and the eclogites, evaluate models of eclogite petrogenesis, and model igneous processes in the mantle before, during, and after diamond formation. The most striking aspect of the chemical compositions of the diamond inclusions is the diversity of relationships with their eclogite hosts. No single distinct pattern of variation from diamond inclusion minerals to host minerals is found for all four samples. Garnet and clinopyroxene inclusions in the diamonds from two samples (U-65/3 and U-66/3) have lower Mg#s, lower Mg, and higher Fe contents, and lower LREE than those in the host eclogite. We interpret such variations as due to metasomatism of the host eclogite after diamond formation. One sample, U-41/3 shows enrichment in diamond-inclusion MREE enrichment relative to the eclogite host and may indicate a metasomatic event prior to, or during, diamond formation. Bulanova [2] found striking differences between inclusions taken from within different portions of the very same diamond. Clinopyroxene inclusions taken from the central (early) portions of Yakutian diamonds were lower in Mg# and Mg contents (by up to 25%) than those later inclusions at the rims of diamonds. These trends are parallel to those between diamond inclusions and host eclogites determined for four of the five samples from the present study and may merely represent changing magmatic and/or P-T conditions in the mantle. Garnet trace element compositions are similar in relative proportions, but variable in abundances, between diamond inclusions and host eclogites. This is probably due to the rapid diffusion of trace elements in garnet under mantle temperatures and consequent alteration of the garnet, and not due to juvenile diamonds ‘locking in’ source heterogeneities (c.f., [3]). Trace element compositions of clinopyroxenes included in diamonds are generally similar to those in the host eclogite. However, one host clinopyroxene does show enrichment in the LREE compared to that in the inclusion and may be attributed to mantle metasomatism, not related to kimberlite transport. In another eclogite, M-46, the host clinopyroxene is depleted in the LREE and Fe, and enriched in the HREE and Mg, relative to the inclusion and is consistent with partial melting of the eclogite subsequent to diamond formation. Sm Nd ratios in clinopyroxenes appear to be little affected by these processes for most samples, allowing SmNd isotopic studies to yield important information about ancient protoliths. Eclogitic mineral inclusions in Yakutian diamonds appear consanguineous with the diamonds, a contention supported by the observations of Bulanova [2]. Therefore, ReOs whole-rock and Sm Nd clinopyroxene age determinations of the Udachnaya eclogites also yield the time of diamond formation, approximately 2.9 Ga [32,33].

Ion microprobe analyses bearing on the composition of the upper mantle beneath the Basin and Range and Colorado Plateau Provinces

Ion microprobe analyses of clinopyroxene from predominantly spinel‐bearing peridotite xenoliths found in volcanic rocks of the Colorado Plateau (Buell Park, Green Knobs localities) and the Basin and Range (Black Canyon, Lunar Crater localities) provinces are highly variable in light and heavy rare earth element (LREE, HREE), Sr, Zr and Ti contents. Abundances of these elements vary by 2–3 orders of magnitude. The LREE are fractionated from the HREE such that relatively LREE‐enriched and LREE‐depleted inclusions occur at all localities. However, LREE‐depleted clinopyroxenes predominate at the Colorado Plateau localities, whereas LREE‐enriched clinopyroxenes or those with chondritic REE patterns predominate at the Basin and Range localities. In addition to fractionation of the LREE from the HREE, the high field strength elements Zr and Ti are generally depleted relative to REE of similar compatibility. Although some of this depletion reflects subsolidus partitioning between orthopyroxene and clinopyroxene, the extremely low Ti abundances (50–500 ppm) of some clinopyroxenes must reflect very low bulk rock contents of Ti. This depletion does not correlate with LREE/HREE ratios. The bulk of the peridotites from the Colorado Plateau are interpreted to be residues from partial melting; extremely low Ce/Sm]cn ratios (less than 0.1) are similar to these ratios in clinopyroxenes from abyssal peridotites and require that the melting process in some cases approached ideal fractional melting; the very low Sr content of these clinopyroxenes (less than 10 ppm) suggest that clinopyroxene was not a residual phase. The association of these peridotites with LREE‐depleted websterites which may be crystal segregates from a magma similar to modern mid‐ocean ridge basalts reinforces the link between modern oceanic lithosphere and the upper mantle beneath the Colorado Plateau. In contrast, the clinopyroxenes from the Basin and Range tend to have relatively high Ti contents (greater than 2000 ppm) and only moderate fractionation of the LREE from the HREE or to be LREE‐enriched and have low Ti contents (as low as 100 ppm) but high Sr contents (to 220 ppm). The former clinopyroxenes are derived from fertile peridotites which could yield typical basaltic magmas upon melting or may have equilibrated with such magmas in the upper mantle. The latter clinopyroxenes are from metasomatic peridotites which likely reacted with a carbonatitic melt These features may be a consequence of widespread Cenozoic magmatism of the Basin and Range Province.

Discrimination of ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts by the Al/Ti ratio in clinopyroxene

Research Article| April 01, 1990 Discrimination of ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts by the Al/Ti ratio in clinopyroxene Robert R. Loucks Robert R. Loucks 1Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar Geology (1990) 18 (4): 346–349. https://doi.org/10.1130/0091-7613(1990)018<0346:DOOFNU>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Robert R. Loucks; Discrimination of ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts by the Al/Ti ratio in clinopyroxene. Geology 1990;; 18 (4): 346–349. doi: https://doi.org/10.1130/0091-7613(1990)018<0346:DOOFNU>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In mafic igneous suites typical of anorogenic igneous provinces (mid-ocean ridges, back-arc basins, continental rifts, and "hotspots"), clinopyroxenes of tholeiitic and silica-undersaturated rocks incorporate Al in clinopyroxene chiefly via the charge-coupled substitution viMgivSi2 ⇔ viTiivAl2 as the CaTiAl2O6 molecule. This substitution is promoted by high titania activity relative to silica in the melt, and hence is particularly characteristic of silica-undersaturated mafic rocks. Mafic igneous suites of convergent plate margins (i.e., arc tholeiites and calc-alkalic gabbros and basalts) characteristically incorporate Al in clinopyroxene largely as the CaFe3+AlSiO6 molecule. The viMgivSi ⇔ viFe3+ivAl substitution is promoted by relatively high fugacities of H2O and O2, a characteristic feature of arc-axis magmas. Recognition of cumulates that are complementary to compositions of eruptive rocks in arcs is crucial to development of valid petrogenetic models for arc magmas. Plots of tetrahedrally coordinated Al vs. octahedral Ti in augite of gabbroic and ultramafic cumulates show that augite data arrays for arc-related cumulates have a trend of substantially higher Al/Ti in clinopyroxene than do those from rift-related tholeiites. The Al/Ti ratio in clinopyroxene may be utilized to discriminate ophiolitic from nonophiolitic ultramafic-mafic allochthons in orogenic belts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.