Reaction Of Peridotite Research Articles

Two styles of melt-peridotite interactions in the upper mantle revealed by mantle xenoliths from northeastern China

To better constrain the mechanisms of interactions between different types of melts and mantle peridotites in the upper mantle of the big mantle wedge, we studied petrology, mineral chemistry, and temperatures recorded by major element and rare earth element (REE) for mantle xenoliths from the Wangqing area in the eastern part of northeastern China, and compared these data with those for xenoliths from the adjacent Jiaohe and Shuangliao areas. The Wangqing xenoliths, including olivine clinopyroxenite, wehrlite, orthopyroxenite, and websterite, generally contain clinopyroxene that is surrounding or replacing olivine. Mineral compositions of the xenoliths from northeastern China vary along melt–peridotite reaction trends, where the Wangqing xenoliths have lower mineral Mg#’s (77–85 in olivine and 81–87 in clinopyroxene) than the Jiaohe and Shuangliao peridotite and pyroxenite xenoliths. In addition, clinopyroxenes in the Wangqing samples have moderately elevated rare earth element contents and (La/Yb)N and 87Sr/86Sr ratios (0.7035–0.7046), within the range of those for clinopyroxenes in the Jiaohe and Shuangliao samples. The REE temperatures of the Wangqing samples (1024–1253 °C) are higher than those of the Jiaohe and Shuangliao samples (844–1120 °C). Based on the differences in mineral chemistry and temperature, we conclude that the Jiaohe and Shuangliao peridotites and pyroxenites were mainly derived from lithospheric mantle accreted from carbonated regions of the asthenosphere, whereas the Wangqing wehrlites and pyroxenites were formed by reaction of lithospheric mantle peridotites with hot carbonated silicate melts. These results provide evidence for two different types interactions between of carbonate-related melts and mantle peridotite, which are important for understanding the chemical evolution of the upper mantle in the big mantle wedge of northeastern Asia.

Deep segregation and crystallization of ultra-depleted melts in the sub-ridge mantle

Partial melting of mantle peridotite from which considerable amounts of melt have been extracted during prior melting episodes generates melts characterized by low incompatible element contents and very low ratios of highly to moderately incompatible elements, so-called ‘ultra-depleted’ melts. Reaction of peridotite with percolating ultra-depleted melts has been inferred from petrological-geochemical studies of abyssal peridotites and ophiolites. But so far, direct evidence for the existence of ultra-depleted melts, only comes from rare melt inclusions. Here, we show that a pyroxenite layer within abyssal peridotite from the Mid Atlantic Ridge (8°N, Doldrums Fracture Zone) formed by crystallization of a segregated melt that is highly depleted in incompatible elements, at >27 km-depth beneath the ridge axis (at T ∼ 1250 °C), and with little or no modification by interaction with the host harzburgite. During exhumation, the pyroxenite experienced decompression and partial re-equilibration under plagioclase-facies conditions (∼1060 °C and ∼ 15 km depth). The high Hf isotope ratio (εHf = 40.3) of the pyroxenite clinopyroxene is inherited from a melt sourced from an ultra-depleted peridotite that evolved with high Lu/Hf. The associated MORB-like Nd isotope ratios (εNd = 10.6), however, imply a long-term evolution of the source peridotite with composition moderately depleted in incompatible elements (rather low Sm/Nd). These compositions are different from the host harzburgite, but typical for peridotites that have melted and partially reacted with migrating melts in ancient times. Hence, the pyroxenite investigated here is a partial melt from an ultra-depleted peridotite that has become re-enriched in incompatible elements and clinopyroxene. This melt crystallized in the oceanic lithosphere, and partially re-equilibrated at low pressure during exhumation. Overall, our results show that renewed melting of ultra-depleted peridotites with a complex history of prior melting and melt-rock reaction occurs, and that such melts migrate through the sub-ridge mantle, and can thus contribute to mid ocean ridge magmatism, although to a still unknown extent.

Pyroxenite–harzburgite sequences in the Dazhuqu ophiolite (Southern Tibet) formed through hydrous melt infiltration and melt–peridotite reaction

Pyroxenite–harzburgite sequences in the Dazhuqu ophiolite (Southern Tibet) formed through hydrous melt infiltration and melt–peridotite reaction

Silica-rich melts originating from melt-hydrated peridotite reactions

Interactions between melts and hydrated peridotite may control production of silica-rich melts, fluids and magmas in the upper mantle and may be predominant mechanism for producing felsic crust in shallow ultramafic settings of rocky planets. This research investigated composition of olivine-hosted mineral, melt and multi-phase inclusions in dunite xenoliths from the 1800–1801 Kaupulehu flow of Hualalai Volcano. The host olivine (forsterite Fo 82–89 , mol. %) appears with mineral inclusions of crystalline chromite having compositions similar to those found in alkaline series rocks. Abundant silica-rich glasses (SiO 2 = 63 ± 2.9 wt%, Al 2 O 3 = 21 ± 1.2 wt%; K 2 O = 4.3 ± 0.5 wt%, Na 2 O = 3.6 ± 0.7 wt%, 38 analyses) found in the olivine-hosted melt and multiphase inclusions are interpreted as quenched silicate liquids entrapped due to nephelinite melt percolation in hydrated harzburgite in the presence of free aqueous fluid at pressures of 0.7 to 0.1 GPa and a melt-to-peridotite mass ratio of 0.2 to 0.3. This work also re-interprets the metasomatic origin of global examples of silica-rich glass inclusions in peridotite xenolith minerals according to results from recent experiments on melt-hydrated peridotite interaction processes. As a planetary process, production of silica-rich melts and fluids within series of associated magmas and their crystallized differentiates can occur due to reaction of primitive mafic (e.g., ocean island basalt, OIB) melts with hydrated peridotite (or serpentinite) in the presence of aqueous fluids at pressures below 1 GPa. • Si-rich glasses are produced due to reaction of mafic melts with hydrated peridotite.

Melt–rock interactions in a veined mantle: pyroxenite–peridotite reaction experiments at 2 GPa

Abstract. Interaction between peridotite and pyroxenite-derived melts can significantly modify the mineralogy and chemistry of the upper mantle, enhancing its heterogeneity, by creating re-fertilized peridotites and secondary-type pyroxenites. We experimentally investigated the reaction between a fertile lherzolite and MgO-rich basaltic andesite produced by partial melting of an olivine-free pyroxenite at 2 GPa and 1300–1450 ∘C. The aim was to constrain the rate and style of melt–peridotite reaction mostly as a function of temperature, i.e. assuming variable physical status of the host peridotite. Experiments juxtaposed pyroxenite on a synthesized fertile lherzolite to evaluate the modal and mineral compositional changes in the fertile lherzolite resulting from the reaction with pyroxenite-derived melt. At 1300 and 1350 ∘C, the reaction produces a thin orthopyroxene-rich reaction zone confined between partially molten pyroxenite and modally unmodified subsolidus lherzolite. Chemical changes in minerals of the pyroxenite crystal mush suggest that element diffusion across the pyroxenite–peridotite interface, coupled with orthopyroxene precipitation, plays a role in the reactive crystallization of mantle pyroxenite veins. At 1380 and 1400 ∘C, infiltration of pyroxenite-derived melt significantly modifies the mineralogy and chemistry of the host peridotite by creating orthopyroxene-rich websterites and pyroxene-rich lherzolite. At 1450 ∘C, pyroxenitic melt fluxes into molten peridotite, enhancing peridotite melting and creating a melt-bearing dunite associated with a refractory harzburgite. At a given pressure, bulk compositions of hybrid rocks originating through melt–peridotite interaction are mostly controlled by the chemistry of the reacting melt. Interaction between pyroxenitic melt and peridotite causes XMg[XMg=Mg/(Mg+Fetot)] and XCr[XCr=Cr/(Cr+Al)] decrease and TiO2 increase in pyroxenes and spinel across the pyroxenite–peridotite boundary. Similar chemical gradients in minerals are observed in pyroxenite–peridotite associations from natural mantle sequences. The comparison with mineral chemistry variations derived by reaction experiments potentially represents a petrologic tool to discriminate between low- versus high-temperature melt–peridotite reactions.

Mineralogical and Re-Os isotope constraints on fluid– and melt–rock interactions and the origin of mantle peridotites from the Amdo ophiolite, northern Tibet

Ophiolites from the Bangong–Nujiang Suture Zone in the central part of the Tibetan Plateau represent relics of Neo-Tethyan oceanic lithosphere. Here we present the results of a detailed mineralogical and Os isotopic study of mantle peridotites of the Amdo ophiolite, which provide new insights into the origin of the peridotites and their diverse melt/fluid–peridotite reactions. The Amdo ophiolite comprises mantle peridotites, mafic–ultramafic cumulates, massive gabbros, diabases, and pillow basalts. The mantle peridotites are mainly refractory harzburgites, composed of olivine (Ol, 65%–85%), orthopyroxene (Opx, 12%–35%), clinopyroxene (Cpx, <4%), and Cr-spinel (Cr-Spl, ~2%). Most olivine grains (Ol-I) are protogranular, locally enclosed in orthopyroxene and Cr-spinel, and have high Fo values (91.1–92.9) and NiO contents (0.29–0.44 wt%), typical of residual origin. In contrast, interstitial olivines (Ol-II) have low Fo values (67.2–84.7) and NiO contents (<0.05 wt%), but extremely high MnO contents (0.28–1.65 wt%), suggesting metasomatism by carbonate-bearing fluids. Hypidiomorphic-granular clinopyroxene grains (Cpx-I) have high Mg# values (~94.5) and low Al2O3 (average 1.26 wt%) and Na2O (<0.19 wt%) contents, typical of residual phases after melting. However, clinopyroxene coronas (Cpx-II) around orthopyroxene have higher Na2O contents (0.24–1.06 wt%), indicative of mantle metasomatism by melts. Osmium isotopic compositions in the harzburgites vary from subchondritic to suprachondritic (187Os/188Os ratios: 0.11716–0.13284). The subchondritic Os isotopic compositions (0.11716–0.12367) correspond to TRD model ages of 0.76–1.56 Ga, suggesting Mesoproterozoic mantle depletion events. On the other hand, several harzburgites with low Os (0.29–0.86 ppb) and suprachondritic 187Os/188Os ratios (0.12823–0.13284) reflect the overprinting of original Os budgets and Os isotopic compositions by percolating melts or fluids. The combined mineralogical and Os isotopic compositions suggest that the Amdo peridotites represent a mixture of ancient subcontinental lithospheric mantle (SCLM) and Mesozoic oceanic lithosphere, which were subsequently infiltrated by carbonate-bearing fluids and silicate melts in a supra-subduction zone.

Highly refractory dunite formation at Gibbs Island and Bruce Bank, and its role in the evolution of the circum-Antarctic continent

ABSTRACT The continental margin is of profound importance as it records continental growth by accretion of orogenic magmas and following continental rifting. A high degree of mantle melting due to hydrous fluid input is expected to simultaneously stimulate continental growth and lower the intrinsic density of the mantle than more fertile mantle, which in turn isolates the continental lithosphere from the convective mantle. The mantle peridotites from Gibbs Island (South Shetland Islands) and Bruce Bank in the Drake Passage provide us an insight into the tectonic history in the circum-Antarctic region. To elucidate the continental growth of Antarctica, we present geochemical data of eight dunites from Gibbs Island and one dunite from Bruce Bank, including Re–Os isotope and highly siderophile element compositions. The dunites are severely affected by serpentinization as evidenced by antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt.%) but contain primary euhedral to subhedral chromites with or without spherical inclusions. The chromites rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture in the chromite with low LOI (3 wt.%), indicating a deserpentinization origin from a precursor serpentinized dunite. The dunites show highly depleted bulk-rock major element compositions (Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for Gibbs Island dunites, Mg/Si = 0.66 and Al/Si = 0.008 for Bruce Bank dunite), overlapping a compositional field defined by forearc peridotites. The positive correlation in Re/Ir–LOI space corroborates Re input during the later serpentinization process. The 187Os/188Os ratios of the dunites range from 0.11907 to 0.14493. Phanerozoic Re-depletion (melt depletion) ages of ca. 535–129 Ma are recorded in the Gibbs Island dunites, except for one with a Mesoproterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, the ages provide minimum time estimates for melt depletion events. The early Paleozoic melt depletion is inferred to have occurred at a very early stage of Antarctic Peninsula formation in response to plate convergence along the margin of Gondwana, whereas the Mesoproterozoic Re-depletion age reflects convecting mantle heterogeneity unrelated to any nearby crust-forming events. The petrographic characteristics of the chromites and highly depleted nature of the dunites are attributed to melt–peridotite reaction in a subduction zone setting. A feasible interpretation for the dunite formation is that the mantle had experienced two stages of melting with the final stage occurring along the Gondwana continental margin in the subduction zone setting. Resultant highly refractory lithospheric mantle was later displaced and dispersed during the Gondwana breakup. Widespread existence of the dunite may be attributed to multi-stage melt depletion along the continental margin.

The Chicken and Egg Dilemma Linking Dunites and Chromitites in the Mantle–Crust Transition Zone beneath Oceanic Spreading Centres: a Case Study of Chromite-hosted Silicate Inclusions in Dunites Formed at the Top of a Mantle Diapir (Oman Ophiolite)

AbstractThe mantle–crust boundary beneath oceanic spreading centres is a major chemical and thermal interface on Earth. Observations in ophiolites reveal that it is underlined by a dunitic transition zone (DTZ) that can reach a few hundred meters in thickness and host abundant chromitite ore bodies. The dunites have been deciphered as essentially mantle-derived in most ophiolitic massifs; that is, reactional residues of interactions between peridotite and percolating melt(s). Although both dunite and chromitite in ophiolites have been the focus of many studies, the reasons for their systematic association remain unclear. In this study we have explored the inclusion content of the chromite grains disseminated in the dunites from the DTZ exposed in the Maqsad area of the Oman ophiolite where a former asthenospheric diapir is exposed. Similarly to chromite in chromitite ore bodies, disseminated chromite grains in dunites contain a great diversity of silicate inclusions. Based on the major and minor element composition of 1794 single silicate inclusions in chromites from 285 samples of dunite and associated rocks in the DTZ, we infer that the disseminated chromites formed by a similar ‘metallogenic’ process to the chromitites, and that, as a whole, dunites from the DTZ actually represent the low-grade end-member of a single, giant ore body. The nature of the silicate inclusions (amphibole and mica among others) enclosed in chromite grains in dunites from the Maqsad DTZ precludes their crystallization from an anhydrous primitive basaltic melt, and rather calls for a crystallization from a melt hybrid between common mafic melts and more exotic Si-, Na- and volatile-rich fluids. The hybrid parent medium of both dunites and chromitites results from the interaction between an asthenospheric diapir (the mid-ocean ridge basalt source), and a colder, altered lithospheric lid and hydrothermal fluids responsible for this alteration. The excess silica in the hybrid melt is provided by the incongruent dissolution of enstatite from mantle harzburgite and/or from moderate degree of partial melting of the altered gabbroic crust. The chemical composition of the silicate inclusions is more variable when enclosed in the disseminated chromites than in the chromitites, suggesting a greater variability of melt and/or fluid fractions involved in the genesis of dunites than of chromite ores. Finally, the DTZ can be viewed as a metamorphic contact aureole between episodically rising asthenospheric diapirs and formerly accreted axial lithospheric lids. Our conclusion about the chicken and egg dilemma linking dunites and chromitites beneath oceanic spreading centres (i.e. do the chromitites form in response to the formation of dunites or conversely?) is that the mantle dunitization itself is a potential process for the release of Cr and its re-concentration as chromite ores, and that in turn the competition between orthopyroxene (± plagioclase) and chromite fractionation during this fluid–melt–peridotite reaction process is responsible for the great mineralogical and chemical variability of the DTZ dunites.

Lithospheric modification by carbonatitic to alkaline melts and deep carbon cycle: Insights from peridotite xenoliths of eastern China

Carbonates in subducting oceanic slabs can survive beyond slab dehydration and be transferred into the deep mantle. Such deep carbon cycling plays a critical role in generating carbonatitic to alkaline melts. However, whether and how this process has influenced the lithospheric mantle still remains enigmatic. To address these issues, here we provide a detailed petrographic, in-situ chemical and Sr isotopic study on two mantle xenoliths (a wehrlite and a melt pocket-bearing peridotite) entrained by the Changle Miocene basalts from the eastern China. The Changle wehrlite contains carbonate melt inclusions and apatites and is merely enriched in clinopyroxene relative to the Changle lherzolites. The clinopyroxenes are characterized by high (La/Yb)N (4.7–41) and low Ti/Eu (873–2292) ratios and equilibrated with carbonated silicate melt-like compositions. These petrographic and chemical features indicate that the wehrlite was formed by reaction between peridotite and carbonated silicate melts. On the other hand, the Changle melt pocket-bearing peridotite is suggested to have been produced by in-situ melting/breakdown of amphiboles of an amphibole-rich dunite. Low olivine Fo (~89), presence of amphiboles with high (La/Yb)N (~50) and low Ti/Eu (~1070) ratios suggest that such amphibole-rich dunite would have been formed by reaction of peridotite with hydrous alkaline basaltic melts from a carbonated mantle. Our data, combined with previously reported data of the Changle lherzolite xenoliths, unravel a series of mantle metasomatisms by carbonatitic to alkaline melts from carbonated mantle sources. The consistently high 87Sr/86Sr ratios (up to 0.7036) of the clinopyroxenes in both the wehrlites and lherzolites indicate the carbonate components in the mantle sources were derived from the stagnant Pacific slab within the Mantle Transition Zone. This study provides a fresh perspective on the role of deep carbon cycling from subducted oceanic slabs in chemical modification of intracontinental lithospheric mantle through reaction with different types of melts.

Diffusion-driven Ca-Fe isotope fractionations in the upper mantle: Implications for mantle cooling and melt infiltration

This study reports Ca-Fe isotope compositions of mineral separates in a set of peridotite xenoliths including ten lherzolites, one harzburgite, and one wehrlite from Hainan Island, south China. Iron isotope compositions show a small variation (<0.15‰ of δ56/54Fe) among olivine (ol), orthopyroxene (opx), and clinopyroxene (cpx). Δ44/40Caopx-cpx (δ44/40Caopx-δ44/40Cacpx) show a large range from 0.00‰ to 1.23‰. Together with data in previous studies, Δ44/40Caopx-cpx in mantle xenoliths worldwide show a negative correlation with the apparent temperatures, which cannot be explained by equilibrium fractionation. Instead, the observed inter-mineral Ca isotope fractionation between opx and cpx (>2‰) in previous studies should be dominated by disequilibrium sub-solidus Ca redistribution between cpx and opx, which is driven by the changes of mantle temperature and chemical composition in pyroxene. The wehrlite, which was formed by reaction of peridotite with an evolved, silica-undersaturated melt, has δ44/40Ca of 3.22‰ and δ56/54Fe of 0.22‰, significantly heavier than the value of fertile mantle. Such heavy isotope signatures in the wehrlite are best explained by diffusion during the metasomatic melt percolating the refractory mantle. In summary, this study reveals that significant kinetic Ca isotope fractionations could occur in the mantle by inter-mineral Ca diffusion during re-equilibration upon cooling and Ca diffusion from melt into peridotite during melt percolation.

Post-collisional ultramafic complex in the northern North China Craton: Implications for crust-mantle interaction

The post-collisional ultramafic intrusions within the northern margin of the North China Craton (NCC) preserve important imprints of crust-mantle interaction. Here we investigate ultramafic intrusions from the Luojianggou complex composed of pyroxenite and hornblendite associated with serpentine, with a view to gain insights into the nature of orogenic lithospheric mantle in this major continental collision zone. Zircon UPb data from the ultramafic suite define different age populations, with Paleoproterozoic ages (>2.2 Ga and 1.82 Ga) representing xenocrystic grains captured from the basement. The magmatic zircon grains range in age from 872 Ma to 458 Ma, and are possibly related to multiple magma emplacement associated with the Paleo-Asian Ocean closure. The youngest age of ca. 230 Ma is related to a period of post-collisional extension in the northern margin of the NCC, and this inference is further supported by apatite UPb ages of ∼207 Ma. Phase equilibrium modeling suggests temperatures of 700–800 °C and pressures of 11–13 Kbar. The pyroxenite and hornblendite show similar geochemical features and REE patterns, indicating the same magma source and formation through differentiation and accumulation. The arc-like geochemical features of the rocks with enrichment of LILE (Rb, Th and La) and LREE, but depletion of HFSE (Nb, Zr and Hf), possibly formed at the boundary of sub-continental lithospheric mantle (SCLM) and crust through metasomatic reaction of peridotite with felsic melts derived from subduction-related components. The arc-like features, zircon rare earth element patterns and obvious positive Pb anomaly in primitive mantle-normalized trace element spidergrams also indicate the mixing of continental materials in the magma source. The post-collisional extensional setting is correlated to the tectonics associated with the assembly of the Mongolian arc terranes within the northern NCC during the Triassic.

Lithosphere tearing along STEP faults and synkinematic formation of lherzolite and wehrlite in the shallow subcontinental mantle

Abstract. Subduction-transform edge propagator (STEP) faults are the locus of continual lithospheric tearing at slab edges, resulting in sharp changes in the lithospheric and crustal thickness and triggering lateral and/or near-vertical mantle flow. However, the mechanisms at the lithospheric mantle scale are still poorly understood. Here, we present the microstructural study of olivine-rich lherzolite, harzburgite and wehrlite mantle xenoliths from the Oran volcanic field (Tell Atlas, northwest Algeria). This alkali volcanic field occurs along a major STEP fault responsible for the Miocene westward slab retreat in the westernmost Mediterranean. Mantle xenoliths provide a unique opportunity to investigate the microstructures in the mantle section of a STEP fault system. The microstructures of mantle xenoliths show a variable grain size ranging from coarse granular to fine-grained equigranular textures uncorrelated with lithology. The major element composition of the mantle peridotites provides temperature estimates in a wide range (790–1165 ∘C) but in general, the coarse-grained and fine-grained peridotites suggest deeper and shallower provenance depth, respectively. Olivine grain size in the fine-grained peridotites depends on the size and volume fraction of the pyroxene grains, which is consistent with pinning of olivine grain growth by pyroxenes as second-phase particles. In the coarse-grained peridotites, well-developed olivine crystal-preferred orientation (CPO) is characterized by orthorhombic and [100]-fiber symmetries, and orthopyroxene has a coherent CPO with that of olivine, suggesting their coeval deformation by dislocation creep at high temperature. In the fine-grained microstructures, along with the weakening of the fabric strength, olivine CPO symmetry exhibits a shift towards [010] fiber and the [010] and [001] axes of orthopyroxene are generally distributed subparallel to those of olivine. These data are consistent with deformation of olivine in the presence of low amounts of melts and the precipitation of orthopyroxenes from a melt phase. The bulk CPO of clinopyroxene mimics that of orthopyroxene via a topotaxial relationship of the two pyroxenes. This observation points to a melt-related origin of most clinopyroxenes in the Oran mantle xenoliths. The textural and geochemical record of the peridotites are consistent with interaction of a refractory harzburgite protolith with a high-Mg no. melt at depth (resulting in the formation of coarse-grained clinopyroxene-rich lherzolite and wehrlite) and with a low-Mg no. evolved melt in the shallow subcontinental lithospheric mantle (forming fine-grained harzburgite). We propose that pervasive melt–peridotite reaction – promoted by lateral and/or near-vertical mantle flow associated with lithospheric tearing – resulted in the synkinematic crystallization of secondary lherzolite and wehrlite and had a key effect on grain size reduction during the operation of the Tell–Rif STEP fault. Melt–rock reaction and secondary formation of lherzolite and wehrlite may be widespread in other STEP fault systems worldwide.

Heterogeneous Oceanic Arc Volcanic Rocks in the South Qilian Accretionary Belt (Qilian Orogen, NW China)

Primitive arc magmas in oceanic island arcs are probes of sub-arc magmatic processes and are crucial for understanding oceanic subduction. We report data for an Early Paleozoic oceanic arc volcanic complex in the Lajishan–Yongjing terrane, South Qilian Accretionary Belt (SQAB), Qilian Orogen, including zircon U–Pb dating and Hf–O isotopes, mineral and whole-rock geochemistry, and Sr–Nd isotope compositions. New zircon ages of ∼455–440 Ma constrain the timing of arc volcanism and the subduction of the Qilian Ocean. Based on petrography and bulk-rock composition, five lithological types have been identified: ankaramite; high-Mg basaltic andesite; high-Al andesite; boninite; sanukite. The volcanic sequence thus is one of the few island arcs where three types of near-primitive arc rocks including boninite, ankaramite and sanukite have been simultaneously produced. All these rocks have variably enriched Sr–Nd isotopic compositions, positive to slight negative zircon εHf(t) values and elevated zircon δ18O values. Boninites, ankaramites and sanukites are interpreted as contemporary near-primitive melts generated from different sources and conditions within an island arc setting. Boninites are characterized by low Ti and REE concentrations and high Cr# chrome spinel, and are interpreted as melts of refractory, Cpx-poor, spinel lherzolite or harzburgite at >25% partial melting. Anomalous zircon δ18O values of 6·57–7·61‰ and Sr–Nd mixing calculations suggest less than 2% incorporation of subducted oceanic sediments into the mantle source of the magmas. The ankaramites are characterized by low SiO2 and high MgO (Mg#), Cr, Ni and La/Yb ratios, and have similar isotopic ratios to tectonically adjacent ocean island basalt (OIB) lavas. The ankaramite lavas are likely to have derived from mantle sources similar to those of OIB; that is, pyroxenite-bearing garnet peridotite enriched in incompatible elements. High-Mg basaltic andesites and high-Al andesites may be derived from parental ankaramite magmas. Sr–Nd–Hf isotopic mixing modeling constrains the amount of silicic melt to ∼1–4% for ankaramite magma. Sanukites are of andesitic–dacitic composition with high Mg#, Cr and Ni, and enriched large ion lithophile elements and high La/Yb ratios. They are interpreted as having been generated by reaction of mantle peridotite with a silicic melt, itself derived from subducted sediments. Enriched Sr–Nd–Hf isotopic compositions constrain the amount of silicic melt to ∼10–15% for sanukite. Large compositional variations among the volcanic rocks from the same arc reflect heterogeneous mantle sources and variable degrees of mantle metasomatism by sediment-derived hydrous fluids or silicic melts, accompanied by secondary assimilation–fractional crystallization processes during magma ascent to the surface. The generation of the island arc volcanic sequence in the Lajishan–Yongjing Terrane is a response to the collision between the Lajishan–Yongjing Oceanic Plateau (recorded by the Lajishan–Yongjing Ophiolite) and the pre-existing trench–continental margin. Evolution from a continental margin in the North Qilian Accretionary Belt to an oceanic island arc in the SQAB records subduction advance and retreat in the history of the Qilian Ocean.

The effect of P–T on the reaction between tonalitic melt and mantle lherzolite at 2–4 GPa and implications for evolution of North China Cratonic Lithosphere and generation of High Mg# andesite

To assess the effects of reaction factors (mainly P and T) on the mineral assemblage and chemical composition of residual minerals and reacted melt in the reaction between andesitic melt and mantle peridotite, we conducted experimental reactions between tonalitic melt and mantle lherzolite using a DS-3600 six-anvil apparatus at 2–4 GPa and 1250 °C–1400 °C. The reaction products contain two types of reaction vein: orthopyroxene(r) [r means this mineral generated in melt-peridotite reaction and located at reaction rim] + garnet(r) and orthopyroxene(r). The mineral assemblages of the reaction veins are controlled by the reaction P and T, which can be determined via the empirical discriminant function: Y = 3.33 × 10−6 T2–2.17 × 10−3 T − 0.5 − P (T in °C, P in GPa; Y ≥ 0: Opx veins form; Y < 0: Opx + Grt veins form). These two types of reaction vein match those found in mantle xenoliths in Mesozoic and Cenozoic magmas from the North China Craton (NCC), indicating that they were formed by similar processes. The melt–peridotite reaction (including P and T) also influences the chemical composition of the residual minerals in the experimental products, with the major element variations being similar to those of NCC lithospheric mantle evolution from Palaeozoic to Cenozoic. Based on the experimental results, olivine in xenoliths from depths of ~100 km (~3 GPa) has higher SiO2 and lower NiO and MnO contents than olivine in xenoliths from depths of ~130 km (~4 GPa), and orthopyroxene in xenoliths from depths of ~100 km has higher Mg#, higher CaO content, and lower NiO content than orthopyroxene in xenoliths from depths of ~130 km. The reacted-mantle xenoliths contain clinopyroxene that originated from cold (<1300 °C) and deep (~135 km) lithospheric mantle. The P–T conditions also control the chemical composition of the reacted melt. An increasing reaction pressure increases the SiO2 content, whereas an increasing reaction temperature reduces the SiO2 content. In combination with the varying Na2O and K2O contents of the parental magma, the reacted melt can yield almost all types of intermediate magma with high Mg# (Mg# = mol[Mg/(Mg + ∑Fe)], Mg# > 0.45) character. This variation in major element of the reacted melt agrees with field observations of Mesozoic high-Mg# andesitic rocks from the NCC, indicating similar causes of the variation in the major element contents. With respect to the trace element content of the reacted melt, a garnet content of 15%–20% produces a right deviation of the heavy rare earth element partition curve (chondrite-normalized) and substantially elevates the Sr/Y and LaN/YbN ratios, indicating an adakite-like reacted melt.

Hybridization Melting Between Continent‐Derived Sediment and Depleted Peridotite in Subduction Zones

AbstractTo better constrain the processes by which subducted continent‐derived sediment and the overlying mantle wedge react to produce hybrid magmas, we have performed a series of high‐pressure interaction experiments involving a phyllitic metasediment and a depleted peridotite (dunite) from the Mediterranean section of the Alpine‐Himalayan orogenic belt at 2–3 GPa. Two different experimental methods were compared, namely, the reaction of separate, juxtaposed peridotite and metasediment blocks (reaction experiments) and capsules in which the two rocks were included as an intimately mixed powder before the experiments (mixed experiments). In reaction experiments, only the metasediment partially melted, and a marked, thin reaction zone dominated by orthopyroxene formed between the dunite and phyllite blocks. In contrast, a hybrid melt phase was found in all mixed experiments. Trace element analyses indicated that Cs, Rb, Ba, Th, U, Nb, Ta, Zr, and Hf were strongly incompatible, together with marked light rare earth element/heavy rare earth element fractionation. The reaction with peridotite can significantly affect the mobility of medium rare earth element, heavy rare earth element, and transition elements during sediment‐mantle interaction, enriching them in hybrid melts. In addition, the nature of the sedimentary starting materials can considerably affect trace element behavior in resulting hybrid melts. There is a significant resemblance between the trace element distribution patterns of hybrid melts produced in our experiments and those of natural postcollisional potassium‐rich volcanic rocks from the eastern Mediterranean region. The experiments show that phlogopite is not necessarily required in the source to produce the SiO2‐richer end of the range of K‐rich magmas.

Deep komatiite signature in cratonic mantle pyroxenite

AbstractWe present new and compiled whole‐rock modal mineral, major and trace element data from extremely melt depleted but pyroxenite and garnet(‐ite)‐bearing Palaeoarchean East Greenland cratonic mantle, exposed as three isolated, tectonically strained orogenic peridotite bodies (Ugelvik, Raudhaugene and Midsundvatnet) in western Norway. The studied lithologies comprise besides spinel‐ and/or garnet‐bearing peridotite (dunite, harzburgite, lherzolite) garnet‐clinopyroxenite and partially olivine‐bearing garnet‐orthopyroxenite and ‐websterite. Chemical and modal data and spatial relationships between different rock types suggest deformation to have triggered mechanical mixing of garnet‐free dunite with garnet‐bearing enclosures that formed garnet‐peridotite. Inclusions of olivine in porphyroclastic minerals of pyroxenite show a primary origin of olivine in olivine‐bearing variants. Major element oxide abundances and ratios of websterite differ to those in rocks expected to form by reaction of peridotite with basaltic melts or silica‐rich fluids, but resemble those of Archean Al‐enriched komatiite (AEK) flows from Barberton and Commondale greenstone belts, South Africa. Websterite GdN/YbN, 0.49–0.65 (olivine‐free) and 0.73–0.85 (olivine‐bearing), overlaps that of two subgroups of AEK, GdN/YbN 0.25–0.55 and 0.77–0.90, with each of them being nearly indistinguishable from one another in not only rare earth element fractionation but also concentration. Websterite MgO content is high, 22.7–29.0 wt%, and Zr/Y is very low, 0.1–1.0. The other, non‐websteritic pyroxenites overlap—when mechanically mixed together with garnetite—in chemistry with that of AEK. It follows an origin of websterite and likely all pyroxenite that involves melting of a garnet‐bearing depleted mantle source. Pyroxene exsolution lamellae in the inferred solidus garnet in all lithological varieties require the pyroxenites to have crystallized in the majorite garnet stability field, at 3–4 GPa (90–120 km depth) at minimum 1,600°C. Consequently, we interpret the websterites to represent the first recognized deep plutonic crystallization products that formed from komatiite melts. The other pyroxenitic rocks are likely fragments of such crystallization products. An implication is that a mantle plume environment contributed to the formation of (one of) the worldwide oldest lithospheric mantle underneath the eastern Rae craton.

Constant Cu/Ag in upper mantle and oceanic crust: Implications for the role of cumulates during the formation of continental crust

Delamination of dense, sulfide-bearing pyroxenitic cumulates was proposed to explain the low Cu content and the evolved major element composition of the continental crust, yet evidence for this hypothesis has been circumstantial. In this study, we present Cu and Ag contents of mantle pyroxenites and associated peridotites from the Balmuccia peridotite massif (Italian Alps) and mantle xenoliths from Hannuoba (North China Craton) to constrain the fractionation behavior of Cu and Ag during melt–peridotite reaction and magmatic accumulation in the mantle. Furthermore, we reexamine the behavior of these elements during arc magmatism and in other reservoirs to highlight the fractionation of Cu and Ag as an important tracer for processes that led to the formation of continental crust.Melt–peridotite reaction and magmatic accumulation led to highly variable sulfide abundances, Cu and Ag contents in the mantle pyroxenites (e.g., Balmuccia, 87–484 μg/g Cu; Hannuoba, 15–116 μg/g Cu). The mean Cu/Ag of the pyroxenite suites (Balmuccia: 3800 ± 1100; Hannuoba: 3100 ± 900, 1s) are indistinguishable from those of their host peridotites and other lherzolites (3500 ± 1200), MORBs (3600 ± 400), and Hawaiian basalts (3200 ± 100). These results reflect the limited fractionation of Cu from Ag during magmatic processes in the upper mantle and during the formation of oceanic crust, and indicate a similar mean Cu/Ag (3500 ± 1000, 1s) in these reservoirs. Experimental data indicate that similar partitioning of Cu and Ag between sulfide melt and silicate melt is responsible for the limited variation of Cu/Ag.Magmatic processes in the convecting mantle and in the oceanic crust, however, cannot explain the low Cu/Ag ratio of the continental crust (about 500). Experimental constraints on sulfide stability in oxidized mantle and data from back-arc magmatic series suggest that formation of primitive back-arc and island arc basalts inherit mantle-like Cu/Ag and thus also cannot explain the low Cu/Ag of the continental crust. In contrast, previous data on evolved back-arc magmas suggest that the low Cu/Ag coupled with a considerable depletion of Cu in the continental crust might be ascribed to the segregation of monosulfide solid solution (MSS) during fractional crystallization of evolving arc magmas. These results thus support the model that accumulation of sulfide-bearing mafic cumulates in the lower crust of magmatic arcs has been a critical process in defining some of the chemical characteristics of the continental crust.

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative e Nd( t ) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive e Nd( t ) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.

Crystallization of high-Ca chromium garnet upon interaction of serpentine, chromite, and Ca-bearing hydrous fluid

The results of experimental modeling of the conditions of crystallization of high-Ca chromium garnets in the system serpentine–chromite–Ca-Cr-bearing hydrous fluid at a pressure of 5 GPa and temperature of 1300°С are reported. The mineral association including quantitatively predominant high-Mg olivine and diopside-rich clinopyroxene, bright-green garnet, and newly formed chrome spinel was formed. Garnet mostly crystallized around primary chromite grains and was characterized by a high concentration of CaO and Cr2O3. According to the chemical composition, garnets obtained are close to the uvarovite–pyrope varieties, which enter the composition of relatively rare natural paragenesis of garnet wehrlite. The experimental data obtained clearly show that high-Ca chromium garnets are formed in the reaction of chromite-bearing peridotite and Ca-rich fluid at high P–T parameters.

A petrographic study of carbonate phases in the Ulten Zone ultramafic rocks: insights into carbonation in the mantle wedge and exhumation-related decarbonation

We present a comprehensive petrographic study on carbonates in orogenic mantle wedge-derived peridotites from the Ulten Zone (UZ) in the Eastern Italian Alps representing a fragment of the Variscan belt. These peridotites are found in ultramafic bodies incorporated in high-grade crustal rocks from a former continental slab and are characterized by highly variable mineral assemblages and microstructures, which reflect their evolution from spinel peridotite in a hot mantle wedge, to garnet peridotite after incorporation into the subducting slab and finally exhumation accompanied by retrogression. Carbonate phases in UZ peridotites are observed in diverse textural sites and can be related to particular petrographic peridotite types. Inclusions of dolomite and dolomite-breakdown products in primary spinel from coarse-grained protogranular peridotites indicate that carbon-bearing liquids were introduced into the mantle wedge at a time before achieving garnet stability. Discrete dolomite grains occur in fine-grained deformed garnet-bearing peridotite and are suggested to have formed simultaneously with hydrous phases (amphibole, apatite) from a carbon-bearing aqueous crustal slab-derived fluid during garnet stability. Intergrowths of calcite and brucite occur mainly in serpentinized fine-grained garnet-amphibole peridotites and are interpreted to be products of dedolomitization (CaMg(CO 3 ) 2 + H 2 O → CaCO 3 + Mg(OH) 2 + CO 2 ), thus decarbonation, during exhumation. Veins of dolomite and calcite-brucite indicate secondary dolomite formation from carbon-bearing fluids due to interaction with slab-derived crustal fluids during retrogression on the exhumation path of the UZ peridotite. Magnesite veins and calcite veins that are texturally linked to alteration features are probably of low-temperature origin. In summary, this petrographic study reveals multi-stage carbonation of the UZ peridotites during residence in the mantle wedge and decarbonation with the release of a carbon species during low-temperature peridotite reaction with aqueous fluids leading to serpentinization accompanied by dolomite breakdown, thus providing important constraints on the carbon budget and carbon cycling in collisional settings. Overall, the ubiquity of carbonates in a variety of textural settings in the UZ peridotites suggests that the supra-subduction zone mantle in continental settings represents an efficient carbon trap.