Mantle Xenoliths Research Articles

Helium isotopes and olivine geochemistry of basalts and mantle xenoliths in Jeju Island, South Korea: Evaluation of role of SCLM on the Cenozoic intraplate volcanism in East Asia

Jeju Island, South Korea, is a Cenozoic intraplate volcano located in East Asia. The Jeju basalts display ocean island basalt (OIB)-like trace element patterns and enriched mantle type 2 (EM2) radiogenic isotope compositions, but the source that enriched the mantle remains unclear. Here we report new geochemical compositions of basalts and xenoliths, including the first helium isotope and element analysis of olivine phenocrysts to constrain the source of basalt. Olivines in the Jeju basalts have 3He/4He ratios of 3.5 to 7.3 Ra while olivines in the mantle xenoliths are 2.9 to 6.3 Ra. They provide no evidence of a lower mantle plume but overlap the range of the regional subcontinental lithospheric mantle (SCLM) and OIBs with low 3He/4He values. Olivine phenocrysts included in the basalts show variations in Mn, Ni, and Ca concentrations with forsterite (Fo) contents, similar to the trend for olivines crystallized from the pyroxenite-derived melts. Elemental ratios of Ca-Fe-Ni-Mg-Mn for the olivines also indicate the pyroxenite contribution. Considering our results, it is suggested that the main source of the Jeju basalts is the SCLM containing pyroxenite. In addition, the tomography image shows the low-velocity zone extending to the asthenosphere mantle beneath the central Jeju Island. This implies that the localized asthenospheric upwelling is caused by edge-driven convection, and the high-velocity zones exist around Jeju island indicating a thick cratonic lithosphere. Therefore, we propose that the interaction between the SCLM component containing pyroxenite and the rising asthenosphere is the main mechanism to generate enriched basaltic magmas for the Cenozoic intraplate volcanism in East Asia, including Jeju Island.

Zinc isotopic behavior of mafic rocks during continental deep subduction

Zinc isotopes may act as a new tool of tracking recycling of crustal materials that causes compositional heterogeneity of the mantle. This application relies on an investigation of Zn isotopic behaviors during slab subduction. In this study, we report Zn isotopic compositions for a suite of metabasalts (greenschists, amphibolites, and coesite-bearing eclogites) from the Dabie Orogen (China), which were formed via the subduction of mafic rocks into different depths and up to > 200 km. Three out of eight greenschists are characterized by lighter δ66ZnJMC-Lyon (0.10‰–0.16‰) than those of global basalts (0.28‰ ± 0.05‰), which may be caused by crustal assimilation of the protoliths by sedimentary rocks due to their extremely high 87Sr/86Sr (up to 0.7130) and low εNd values (down to −12.3). The remaining greenschists have relatively low 87Sr/86Sr and their δ66Zn values (0.21‰–0.38‰) overlap the ranges of amphibolites (0.18‰–0.32‰) and coesite-bearing eclogites (0.18‰–0.36‰). There is no correlation between δ66Zn and sensitive indicators of dehydration (Rb/TiO2, Ba/Yb, and H2O+), suggesting that no detectable Zn isotope fractionation has occurred during the deep subduction of mafic rocks even into > 200 km, which is attributed to the limited loss of Zn during prograde metamorphism and dehydration. Thus, Zn isotopic compositions of the deeply subducted mafic rocks are inherited from their protoliths. Considering that these metamorphosed rocks have higher δ66Zn than that of the mantle value by up to 0.2‰, the recycled/subducted mafic crust can incorporate isotopically heavy Zn into the mantle. The subducted slabs may partially melt and generate a metasomatized mantle, resulting in changes of Zn isotopic composition of the hybridized mantle as have been observed in some mantle xenoliths and basaltic lavas.

Trace elements in olivine: Proxies for petrogenesis, mineralization and discrimination of mafic-ultramafic rocks

Olivine is a ubiquitous mineral in mafic-ultramafic rocks and has been widely used as a mineral marker in various geological processes. However, its development of trace elements is limited. Here we present newly-obtained trace element data 7Li, 27Al, 29Si, 31P, 43Ca, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 59Co, 60Ni, and 66Zn of olivine in typical mantle xenoliths, mantle peridotites in ophiolites, and plutonic rocks from layered and Alaskan-type intrusions to develop trace element proxies for the petrogenesis, mineralization and discrimination of various mafic-ultramafic rocks. Residual olivine grains in mantle xenoliths and ophiolitic peridotites, which represent residues of mantle melting, have higher Ni/Co (>20) and Ni/Mn (>2) ratios than magmatic olivine (Ni/Co < 20, Ni/Mn < 2), which are consistent with the compatibilities of these elements during partial melting and magma differentiation. Lower Ni content, and lower Ni/Co and Ni/Mn ratios at a given Fo content can distinguish olivine in Alaskan-type intrusions from layered intrusions, reflecting the nature of their mantle sources. The V and Sc contents and V/Sc ratios in olivine can distinguish mantle xenoliths (V > 2 ppm, V/Sc > 0.5) from ophiolitic peridotites (V < 2 ppm, V/Sc < 0.5), indicating a more reduced state of continental lithospheric mantle compared to the oceanic lithospheric mantle. As a consequence, the four occurrences of mafic-ultramafic rocks can be distinguished by olivine with (Sc × 10)-(Ti × 2)-Zn and V/Sc-(Co/Ni × 2)-(Zn/Mn × 5) ternary diagrams. In addition, Li, Ti and P contents in olivine are good tracers of melt/fluid metasomatism, whereas Ni/Co, Ni/Mn and Mn/Zn ratios are indicators of chromite mineralization. Therefore, trace elements in olivine can be used as chemical proxies to distinguish the origin of various mafic-ultramafic rocks, as well as the processes by which they evolved.

What can we learn from REE abundances in clinopyroxene and orthopyroxene in residual mantle peridotites?

Clinopyroxene and orthopyroxene are the two major repositories of rare-earth elements (REE) in spinel peridotites. Most geochemical studies of REE in mantle samples focus on clinopyroxene. Recent advances in in situ trace element analysis have made it possible to measure REE abundance in orthopyroxene. The purpose of this study is to determine what additional information one can learn about mantle processes from REE abundances in orthopyroxene coexisting with clinopyroxene in residual spinel peridotites. To address this question, we select a group of spinel peridotite xenoliths (9 samples) and a group of abyssal peridotites (12 samples) that are considered residues of mantle melting and that have major element and REE compositions in the two pyroxenes reported in the literature. We use a disequilibrium double-porosity melting model and the Markov chain Monte Carlo method to invert melting parameters from REE abundance in the bulk sample. We then use a subsolidus reequilibration model to calculate REE redistribution between clinopyroxene and orthopyroxene at the extent of melting inferred from the bulk REE data and at the closure temperature of REE in the two pyroxenes. We compare the calculated results with those observed in clinopyroxene and orthopyroxene in the selected peridotitic samples. Results from our two-step melting followed by subsolidus reequilibration modeling show that it is more reliable to deduce melting parameters from REE abundance in the bulk peridotite than in clinopyroxene. We do not recommend the use of REE in clinopyroxene alone to infer the degree of melting experienced by the mantle xenolith. In general, HREE in clinopyroxene and LREE in orthopyroxene are more susceptible to subsolidus redistribution. The extent of redistribution depends on the modes of clinopyroxene and orthopyroxene in the sample and thermal history experienced by the peridotite. By modeling subsolidus redistribution of REE between orthopyroxene and clinopyroxene after melting, we show that it is possible to discriminate mineral mode of the starting mantle and cooling rate experienced by the peridotitic sample. We conclude that endmembers of the depleted MORB mantle and the primitive mantle are not homogeneous in mineral mode. A modally heterogeneous peridotitic starting mantle provides a simple explanation for the large variations of mineral mode observed in mantle xenoliths and abyssal peridotites. Finally, using different starting mantle compositions in our simulations, we show that composition of the primitive mantle is more suitable for modeling REE depletion in cratonic mantle xenoliths than the composition of the depleted MORB mantle.

Nb–Zr–Y Systematics and Thermal Regimes of Subcontinental Lithospheric Mantle in the Archaean: Data from Mantle Xenoliths

Nb–Zr–Y Systematics and Thermal Regimes of Subcontinental Lithospheric Mantle in the Archaean: Data from Mantle Xenoliths

Formation of Amphibole‐Bearing Peridotite and Amphibole‐Bearing Pyroxenite Through Hydrous Melt‐Peridotite Reaction and In Situ Crystallization: An Experimental Study

AbstractAmphibole is a common hydrous mineral in mantle rocks. To better understand processes leading to the formation of amphibole‐bearing peridotites and pyroxenites in the lithospheric mantle, we conducted experiments by juxtaposing a lherzolite against hydrous basaltic melts in Au‐Pd capsules. Two melts were examined, a basaltic andesite and a basalt, each containing 4 wt% of water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C produced a melt‐bearing orthopyroxenite‐dunite sequence. Crystallization of the partially reacted melts during cooling lead to the formation of an amphibole‐bearing gabbronorite‐orthopyroxenite‐peridotite sequence. Orthopyroxene in the peridotite and orthopyroxenite has a poikilitic texture enclosing olivines and spinels. Amphibole in the peridotite occurs interstitial to olivine, orthopyroxene, clinopyroxene, and spinel. Comparisons of texture and mineral compositions in the experimental products with those from field observations allow a better understanding of hydrous melt‐rock reaction in the lithospheric mantle. Amphibole‐bearing pyroxenite veins (or dikes) can be formed in the lithospheric mantle or at the crust‐mantle boundary by interaction between hydrous melt and peridotite and subsequent crystallization. Hornblendite or amphibole gabbronorite can be formed in the veins when the flux of hydrous melt is high. Differences in reacting melt and peridotite compositions are responsible for the variation in amphibole composition in mantle xenoliths from different tectonic settings. The extent of melt‐rock reaction is a factor that control amphibole composition across the amphibole‐bearing vein and the host peridotite.

Crust–Mantle Xenoliths from the Kharchinsky Volcano (Central Kamchatka Depression): Mineralogy and Petrogenesis

Abstract —We present results of a study of plutonic-rock xenoliths from the Kharchinsky Volcano (Central Kamchatka depression). The studied xenolith collection comprises nine samples of peridotites and clinopyroxenites. The peridotites are identified as wehrlites, dunites, and harzburgites composed of olivine, clino- and orthopyroxenes, amphibole, and chromite in varying amounts. The clinopyroxenites consist mostly of clinopyroxene and often contain subordinate olivine, amphibole, hercynite, and magnetite. The xenoliths have interstitial segregations and veins composed of chlorite, plagioclase, K-feldspar, orthopyroxene, barite, fluorapatite, ilmenite, and, more seldom, anhydrite, phlogopite, and some other minerals. The study has revealed that veinlet minerals sometimes replace primary minerals and form pseudomorphs, thus indicating the metasomatic origin of interstitial and vein mineral assemblages. The thermobarometric calculations for minerals have shown that peridotites formed at ~1140 °C and ≤10 kbar in the intermediate chambers at the depths from the spinel stability field to the Moho. Interstitial metasomatic alterations of rocks took place at ~400–850 °C.

Hydrogen diffusion mechanism in the mantle deduced from H-D interdiffusion in wadsleyite

Classical view has interpreted deuterons in nominally anhydrous minerals (NAMs) diffuse slightly slower than protons by the Knudsen limit of 2, whereas here we report deuteron diffusion is about 3∼7 times faster than proton one in Mg2SiO4 wadsleyite based on the H-D interdiffusion couple experiments using a pair of wadsleyite single crystal at 16 GPa and 900-1300 K. Our results indicate hydrogen diffusion in wadsleyite is affected by multiple diffusion mechanisms, of which, proton (deuteron) migrating along interstitial defects is particularly efficient to separate hydrogen isotopes in crystalline network probably owing to a quantum sieving effect. The resulting diffusion-driven H/D fractionation can explain common observations of deuterium depletion of NAMs in mantle xenoliths, suggesting that MORB inclusions hosted in xenoliths are more reliable for indicating the hydrogen isotope composition of the mantle rather than the xenolithic NAMs.

Evolution of lithospheric mantle beneath the Maguan region, southwestern margin of the South China block based on mantle xenoliths in Miocene alkaline volcanic rocks

The Maguan (MG) region in the southwestern part of the South China Block contains many Neogene potassic basaltic volcanic rocks along the Ailao Shan–Red River (ASRR) shear zone. These alkaline volcanic rocks contain abundant peridotite xenoliths which are mostly spinel (Spl)-facies lherzolite. We examined xenoliths in young, ~12 Ma, alkaline basalts. Most contain clinopyroxene (Cpx), >15 vol%, and minor xenoliths contain low Cpx, ~10 vol%. Olivine (Ol) shows forsterite components ranging from 88.6 to 90.1 in most lherzolite and from 90.4 to 90.8 in depleted lherzolite, Spl Cr# (=Cr/(Cr + Al)) between 0.08–0.11 for lherzolite and 0.17–0.22 for depleted lherzolite. Platinum group elements (PGE) show a near flat primitive mantle-normalized pattern; the ratios of IPGE/PPGE are 1.3–1.8 for lherzolite and 2.1 for depleted lherzolite. The xenoliths do not contain metasomatic minerals, but Cpx in depleted lherzolite samples is enriched in light rare earth elements (LREE; (La/Sm)N = 0.97–1.74). Two pyroxene geothermometry gives the temperatures ranging from 1029 °C to 1148 °C (average 1087 °C, n = 13). Spl–Ol geothermobarometry yields the oxidation state ranging from FMQ −1.5 to −0.25 with an average value of FMQ −0.59 (n = 13), which is comparable to those of abyssal peridotite and the asthenospheric mantle. The calculated temperatures and oxidation state are similar among samples independent of mineralogy and mineral chemistry, suggesting that partial melting and the cryptic metasomatism did not change the oxidation conditions. The data suggest that the subcontinental lithospheric mantle (SCLM) beneath the region formed from the asthenosphere mantle. Since the South China Block is generally underlain by Proterozoic SCLM, the data obtained this study suggest that the ancient lithosphere was replaced by the asthenospheric mantle in the local extension along the ASRR shear zone.

Geochemical and O–C–Sr–Nd Isotopic Constraints on the Petrogenetic Link between Aillikites and Carbonatites in the Tarim Large Igneous Province

Abstract Aillikites are carbonate-rich ultramafic lamprophyres often associated with carbonatites. Despite their common field relationships, the petrogenetic links, if any, between aillikites and carbonatites remain controversial. To address this question, this study reports the results of a detailed geochemical and isotopic examination of the Permian Wajilitag aillikites in the northwestern Tarim large igneous province, including bulk-rock major- and trace-element and Sr–Nd isotope compositions, olivine major- and trace-element and (in situ secondary ion mass spectrometry) oxygen isotope compositions, oxygen isotope data for clinopyroxene separates, and bulk-carbonate C–O isotopic analyses. Olivine in the aillikites occurs in two textural types: (1) microcrysts, 0·3–5 mm; (2) macrocrysts, 0·5–2·5 cm. The microcrysts exhibit well-defined linear correlations between Fo (79–89) and minor and trace elements (e.g. Ni = 1304–3764 μg g–1 and Mn = 1363–3042 μg g–1). In contrast, the olivine macrocrysts show low Fo (79–81), Ni (5·3–442 μg g–1) and Ca (477–1018 μg g–1) and very high Mn (3418–5123 μg g–1) contents, and are displaced from the compositional trend of the microcrysts. The microcrysts are phenocrysts crystallized from the host aillikite magmas. Conversely, the lack of mantle-derived xenoliths in these aillikites suggests that the macrocrysts probably represent cognate crystals (i.e. antecrysts) that formed from earlier, evolved aillikite melts. Olivine phenocrysts in the more primitive aillikite dykes (Dyke 1) have relatively higher Fo (Fo82–89) and mantle-like oxygen isotope values, whereas those in the more evolved dykes (Dykes 2 and 3) exhibit lower Fo (Fo79–86) and oxygen isotope values that trend toward lower than mantle δ18O values. The decreasing δ13C values of carbonate from Dyke 1 to Dykes 2 and 3, coupled with the indistinguishable Sr–Nd isotopes of these dykes, suggest that the low δ18O values of olivine phenocrysts in Dykes 2 and 3 resulted from carbonate melt/fluid exsolution from a common progenitor melt. These lines of evidence combined with the overlapping emplacement ages and Sr–Nd isotope compositions of the aillikites and carbonatites in this area suggest that these exsolved carbonate melts probably contributed to the formation of the Tarim carbonatites, thus supporting a close petrogenetic relationship between aillikites and carbonatites.

Halogen heterogeneity in the subcontinental lithospheric mantle revealed by I/Br ratios in kimberlites and their mantle xenoliths from South Africa, Greenland, China, Siberia, Canada, and Brazil

Abstract To investigate halogen heterogeneity in the subcontinental lithospheric mantle (SCLM), we measured the concentrations of Cl, Br, and I in kimberlites and their mantle xenoliths from South Africa, Greenland, China, Siberia, Canada, and Brazil. The samples can be classified into two groups based on halogen ratios: a high-I/Br group (South Africa, Greenland, Brazil, and Canada) and a low-I/Br group (China and Siberia). The halogen compositions were examined with the indices of crustal contamination using Sr and Nd isotopes and incompatible trace elements. The results indicate that the difference between the two groups was not due to different degrees of crustal contamination but from the contributions of different mantle sources. The low-I/Br group has a similar halogen composition to seawater-influenced materials such as fluids in altered oceanic basalts and eclogites and fluids associated with halite precipitation from seawater. We conclude that the halogens of the high-I/Br group are most likely derived from a SCLM source metasomatized by a fluid derived from subducted serpentinite, whereas those of the low-I/Br group are derived from a SCLM source metasomatized by a fluid derived from seawater-altered oceanic crust. The SCLM beneath Siberia and China could be an important reservoir of subducted, seawater-derived halogens, while such role of SCLM beneath South Africa, Greenland, Canada, and Brazil seems limited.

Hales Discontinuity in the Southern Indian Continental Lithosphere: Seismological and Petrological Models

AbstractWe model the depth and Vs structure of the Hales discontinuity (H‐D) beneath Eastern Dharwar Craton (EDC) and Southern Granulite Terrain (SGT) using P wave receiver function (P‐RF) analysis and joint inversion with Rayleigh wave phase velocity dispersion. We calculate P‐RFs at higher frequency (fmax = 0.46 Hz), compared to previous studies, to show that the H‐D P‐to‐S converted phase (Phs) is distinct from crustal reverberations. Phs at stations Hyderabad (HYB) and Gauribidanur (GBA), in the EDC, arrive at ∼11.5 s and ∼12 s, respectively. From joint inversion, the H‐D is modeled at 107 ± 5 km and 113 ± 4 km depth, with ∼3% and ∼4% Vs increase, beneath HYB and GBA, respectively. For station Kodaikanal (KOD), in SGT, the Phs destructively interferes with the negative midcrustal reverberation at most ray‐parameters, which explains its apparent absence in previous studies. We isolated P‐RFs where Phs is distinct at ∼11 s and model it at 102 ± 3 km depth. Common conversion point stack profiles constructed by depth migrating P‐RFs through the Vs model show an undulatory nature of the H‐D. From data of mantle xenoliths (Wajrakarur kimberlite field), we calculate Vs of mantle peridotite and eclogite, using published bulk rock compositions. At the H‐D depth and temperature derived from Indian shield geotherm, we observed a good match to the Vs structure of the H‐D. Our results support the geodynamic model of the H‐D being an interface of paleosubducted eclogitic oceanic crust embedded within the upper mantle peridotite. Global observations of mantle reflectors within the continental lithosphere, at depths similar to H‐D, have been related to relict subduction and independently support our model.

Nature and evolution of lithospheric mantle beneath the western North China Craton: Constraints from peridotite and pyroxenite xenoliths in the Sanyitang basalts

This paper reports petrology, major and trace element abundances in bulk-rocks and minerals, and clinopyroxene Sr-Nd-Hf compositions from a suite of mantle xenoliths entrained in Cenozoic Sanyitang alkali basalts, Jining County in the Inner Mongolia Suture Zone. The aim is to constrain the nature and evolution of the lithospheric mantle beneath the western North China Craton. The Sanyitang mantle xenoliths are mainly anhydrous spinel lherzolites with subordinate websterites. Petrological and geochemical features of the spinel lherzolites suggest that they are refractory mantle residues formed by variable degrees of melt extraction (3–14%), followed by silicate and carbonatite metasomatism. The Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7031–0.7037, εNd = +5.2 − +11.2 and εHf = +8.4 − +29.3) of clinopyroxene separates from the majority of the spinel lherzolites lie on, or close to, the mantle array defined by the composition of ocean-island basalts and mid-ocean ridge basalts. On the other hand, clinopyroxene separates from a minority of spinel lherzolites (SYT09-01, SYT09-09, SYT11-01) have high 87Sr/86Sr (0.7040–0.7049), εNd (+12.4 − +21.9) and εHf (+58.2 − +67.8), above the mantle array, reflecting long-term depletion of the mantle. The websterites are characterized by lower MgO (18–22 wt%) and higher Al2O3 contents (4.5–5.6 wt%), and lower Mg# (86.2–87.3) than the spinel lherzolites (Mg# = 89.3–90.8; Al2O3 = 0.9–3.6 wt%). Their convex upward rare earth element patterns, along with lower Ni contents (334–389 ppm) indicate that they were cumulates fractionated from basaltic magmas circulating through the upper mantle. The trace element patterns of clinopyroxenes from three phlogopite-free websterites show enrichment in large ion lithophile and negative anomalies for the high field strength elements. In addition, the phlogopite-free websterite clinopyroxene Sr-Nd-Hf isotope ratios plot in the enriched lithospheric field (87Sr/86Sr = 0.7058–0.7061, εNd = −9.2 to −8.4 and εHf = −5.2 to −3.7 respectively), which implies the contribution of recycled lower crustal materials to their source. The presence of phlogopite in some Sanyitang websterites, coupled with strong enrichments in both light rare earth elements and large ion lithophile elements, as well as high 87Sr/86Sr (0.7065–0.7068) and negative εNd (−1.5 to −0.3) and εHf (−6.1 to −4.8) values of clinopyroxene, provide strong evidence for the existence of subduction-related hydrous fluids/melts metasomatism. This study demonstrates that the Sanyitang mantle xenoliths and their component minerals record the complex petrological history and local heterogeneity of the subcontinental mantle lithosphere under the western North China Craton.

Multistage Metasomatized Mantle Beneath Alakit: Evidence from Mantle Xenoliths from Komsomolskaya Kimberlite Pipe, Yakutia

Multistage Metasomatized Mantle Beneath Alakit: Evidence from Mantle Xenoliths from Komsomolskaya Kimberlite Pipe, Yakutia

The Late Cenozoic volcanic groups in the South Daxing'anling, NE China: geology, geochemistry, and chronology

Abstract During the late Cenozoic, the extensional tectonic setting in northeastern China caused large-scale block uplifts and depressions, and thus a large amount of magma erupted along structural fractures in the eastern Inner Mongolia, NE China. The Abaga, Beilike, Dalinor and Wulanhada (ABDW) volcanic rocks along the Daxing'anling–Taihangshan Gravity Lineament in the southern section of the Daxing'anling are characterized by their extensive distribution, numerous volcanic cones and various eruption types. Each volcanic group has distinctive volcanic landforms and geochemical characteristics. The geochronological data have revealed that the volcanism spanned Miocene to late Pleistocene. The ABDW volcanic rocks contain primary alkaline basalts and subordinate tholeiites. The trace element curve pattern is similar to that of ocean island basalt, but completely different from that of mid ocean ridge basalt, while the light rare earth elements are more enriched than the heavy rare earth elements. The geochemical features of the volcanic rocks and the entrained mantle xenoliths reveal the broad heterogeneities of the lithospheric mantle and varieties of the volcanic rock evolution in the south Daxing'anling. The Cenozoic volcanism in eastern Inner Mongolia, and even within the east Asian plate, is attributed to the westward subduction and rolling backward of the Pacific slab, as well as the trench retreat.

Petrological features of picrobasaltic melts on Lanzarote, Canary Islands

The paper discusses issues related to the petrology of picrobasaltic melts forming lava flows on Lanzarote, Canary Islands. During ascientific and educational expedition on the aforementioned island, lava flows of picro basalt of the third phase of the 1731–1732 eruption were studied and tested. Preference was given to the well-preserved streams in the west and south of the island thatare accessible for direct study. Volcanogenic-sedimentary associations were identified in the studied outcropsand described, the structural and textural features of the flows were characterized, and samples with mantle rock xenoliths were taken for petrological reconstructions. New geochemical data onrocks and minerals are presented. It was established that the temperature of the initial melt was 1100–1180°C. Large harzburgitephenocrysts in picro basalt are xenocrysts formed as a result of melting and disintegration of mantle xenoliths.

Pre‐Alpine Fault Fabrics in Mantle Xenoliths From East Otago, South Island, New Zealand

AbstractWe present a quantitative microstructural and seismic property analysis of harzburgitic and lherzolitic xenoliths from East Otago, New Zealand. Olivine crystallographic preferred orientations (CPOs) are dominated by orthorhombic CPOs with a tight maxima of [100] orthogonal to (010). Orthopyroxene showed the [001] axes aligned (sub)parallel to the [100] axes of olivine and the (100) plane subparallel to the (010) plane of olivine. Misorientations of olivine subgrain boundaries indicate that dislocations with [100] slip vectors are common and CPOs are interpreted as olivine [100] representing the flow direction and (010) the shear plane. Olivine grains are coarse with lobate grain boundaries, indicative of grain boundary migration at high temperatures. Olivine contains numerous subgrains whilst orthopyroxene is mostly strain‐free and it is possible that orthopyroxene crystallized from melt during deformation. Clinopyroxene lamellae in orthopyroxene, relate to decompressional cooling prior to Alpine Fault movement. Seismic properties derived from the measured CPOs, indicate P‐wave and S‐wave anisotropies at mantle conditions of 4%–10% and 3%–9%, respectively. Calculated shear wave splitting delay times of 0.1–0.9 s, for the current lithospheric mantle thickness, fit seismological observations, suggesting that the xenoliths may be representative of the lithospheric mantle that lies below East Otago today. To explain the seismic data, olivine [100] needs to be subhorizontal and parallel to NNW‐SSE fast polarization directions with [010] subhorizontal and perpendicular to these directions. This is consistent with the East Otago lithospheric mantle containing a fossil vertical shear zone with NNW‐SSE strike slip motion.

Experiment constraints on orthopyroxene enrichment in the Kaapvaal craton lithospheric mantle

Peridotite xenoliths from the Kaapvaal cratonic lithosphere are characterized by high modal orthopyroxene (Opx), which is inconsistent with the mineralogy of residues of partial melting of primitive mantle. The high orthopyroxene content has been explained by the reaction between Opx-poor peridotite and silica-rich melts. To assess and further constrain this model, we conducted a series of experiments on the interaction of Si-rich melt with Opx-poor harzburgite at 1.5 GPa and 1100 to 1250 °C. Orthopyroxene is enriched in all experiments. In the low temperature run at 1100 °C, melt is consumed and phlogopite is precipitated with the generation of orthopyroxene. Mg# (atomic 100*Mg/(Mg+Fe)) values in olivines also exhibit a decrease compared with the starting olivines. At temperatures of ≥1200 °C, phlogopite is absent as the temperatures are higher than its upper stability field. The residues are composed of high modal orthopyroxene and high Mg# olivine. Only the residues in the high temperature runs are consistent with Kaapvaal cratonic mantle xenoliths. Combined our experiments with the distribution, geochronological and geochemical data of the xenoliths, we propose a two-stage ‘metasomatism-depletion’ model to explain the generation of Opx-rich xenoliths in Kaapvaal craton. In the early Archean, high-degree melting of primitive mantle produced a highly depleted lithospheric mantle, which was Opx-poor. Subsequently, Si-rich melts/fluids were released from the subducting oceanic slabs to interact with the Opx-poor mantle, generating an Opx-rich, but hydrous and fertile mantle. The metasomatic mantle is similar to the result of the run at 1100 °C. In the late Archean, the metasomatic mantle was re-melted in post-collisional extensional environments, which produced voluminous lavas with residues of highly depleted, Opx-rich peridotites. The Opx-rich residual peridotites are consistent with the runs at ≥1200 °C. Thus, Opx enrichment in the Kaapvaal craton lithospheric mantle can be well explained by the two-stage ‘metasomatism-depletion’ model.

Constraining the density evolution during destruction of the lithospheric mantle in the eastern North China Craton

The thermoelastic properties of minerals in mantle xenoliths combined with the thermal states can provide an integrated understanding of the petrophysics of the lithospheric mantle. Here, we conducted high-pressure and high-temperature experiments on the main minerals (e.g. olivine, orthopyroxene, clinopyroxene, spinel, and garnet) in peridotite xenoliths from basalt of the eastern North China Craton (NCC) using in situ synchrotron single-crystal X-ray diffraction combined with diamond anvil cells. The pressure-temperature-volume data were fitted to the third-order Birch-Murnaghan equations of state and yielded the thermoelastic parameters that included bulk modulus, pressure and temperature derivatives, and thermal expansion coefficients. The density profiles of the eastern NCC during the destruction process since the Mesozoic are presented from the temporal and spatial aspects. The lithospheric density dramatically decreased during destruction, and high heat flow may have been a trigger. The spatially distributed density profile also provides firm evidence for lateral heterogeneities in the eastern NCC. This may suggest that the present mantle is characterized by heterogeneous destruction of the NCC.

The role of celadonite on mantle metasomatism in subduction settings

Celadonite is the most abundant K-rich hydrated mineral formed during basalt alteration. In convergent margins, celadonite from the altered oceanic crust sinks into the mantle and contributes to the mantle wedge metasomatism, recorded by incompatible element enrichment in lavas and mantle xenoliths samples from these settings. However, experimental data on the role of celadonite in subduction zones as a function of P-T are still scarce. Therefore, celadonite stability under “hot” and “cold” subduction settings is investigated experimentally in this study. Results show an early silica loss, with quartz/coesite replacing celadonite from 400 to 600 °C and 2.5–7.7 GPa. At 700 °C and 2.5 GPa, celadonite is partially converted to phlogopite-annite, and at 800 °C and 4.0 GPa celadonite is absent. At temperatures above 800 °C, enstatite and phlogopite are the stable silicate phases from 2.5 to 7.7 GPa. Mössbauer spectroscopy and X-ray diffraction analyses show that the reaction of celadonite to phlogopite-annite is progressive and controlled by the Fe3+ to Fe2+ reduction in the octahedral site. Coupled to this, there is a transition of pyrite to pyrrhotite and the release of reducing fluids enriched in H2S, SiO2, OH− and K+ in the mantle wedge. These fluids have the potential to mobilize chalcophile elements (e.g. Cu, Zn) and act as mineralizing agents in shallower and more oxidizing crustal environments. Data in this study demonstrate that celadonite-phlogopite-annite series minerals are stable down to at least 230 km in the mantle, contributing to the sources of mantle metasomatism, arc-related magma genesis and possibly chalcophile mineralization.