Mantle Xenoliths Research Articles

Noble gas isotopic compositions and abundance ratios of mantle xenoliths from Honolulu series volcanism, Oahu, Hawaii

We report new noble gas isotopic compositions and abundance ratios (helium, neon, and argon) extracted from six lherzolite xenoliths on Oahu Island, Hawaii, by vacuum crushing. Considering the equilibrium temperature of the xenoliths and the fluid density of their fluid inclusions, they originated from a pressure of 0.98–1.04 GPa, corresponding to 33–34 km depth. The Moho depth beneath Oahu Island is around 15 km, suggesting that the xenoliths are part of the sub-oceanic lithospheric mantle (SOLM), 90 million years old, after generation of mid-oceanic ridge basalts (MORB). The samples have 3He/4He of 3.0–9.5 times atmosphere (Ra). The neon isotope compositions overlap with the data of previous studies for lava flow samples of the Hawaiian Islands, MORB, and air. The 40Ar/36Ar range from atmospheric to 3100. They show very low 4He/40Ar* and 4He/21Ne*, where asterisks denote correction for atmospheric contributions. The low 4He/40Ar* and 4He/21Ne* are characteristic of small fluid inclusions, based on step crushing experiments, indicating that they record kinetic fractionation that occurred in the SOLM most likely due to melt infiltration. Using 4He/40Ar* and 20Ne/22Ne respectively as indices of the kinetic fractionation and atmospheric contribution, we corrected for these effects. The corrected noble gas isotopic compositions are best explained by interaction between plume-derived melts and SOLM. The kinetic fractionation yields a decrease in 3He/238U in SOLM, which should result in a rapid decrease in 3He/4He with time. The slightly elevated 3He/4He (above 8 times atmospheric), as found in the present xenoliths cannot be maintained for more than one million years in the fractionated SOLM with 3He/U molar ratio of ∼1 × 10−8, which is calculated assuming that the 3He/U has the same degree of kinetic fractionation as 4He/40Ar* and 4He/21Ne*. Therefore, the fractionation event is likely to be recent, and may be related to recent magmatism in the SOLM beneath Oahu Island. Such igneous activity is limited to Honolulu series volcanism (0.35–0.80 Ma). Therefore, the xenoliths contain a record of magmas with plume-derived noble gases, most likely associated with the Honolulu series volcanism.

The heterogeneity of continental mantle: A perspective from the Hannuoba basalts and mantle xenoliths

了解大陆地幔的不均一性对于理解地壳成分再循环、壳-幔相互作用等过程至关重要。本文通过对华北克拉通北缘汉诺坝地区来自不同地幔深度幔源岩石（玄武岩及橄榄岩和辉石岩包体）的研究进行综述，探讨了地幔在垂向上成分的不均一性特征。汉诺坝玄武岩的研究揭示了汉诺坝地区软流圈顶部存在再循环的碳酸盐化榴辉岩以及岩石圈底部具有富集的古老洋壳和沉积物成分。另外，玄武岩携带的橄榄岩和多类型辉石岩包体记录了不同来源（蚀变洋壳、沉积碳酸盐岩、碎屑沉积物、拆沉下地壳、软流圈）且不同成分（硅酸盐、碳酸盐）的熔/流体活动。富集地壳来源的熔/流体加入明显影响了汉诺坝地区陆下地幔成分的均质性，使得陆下地幔在微米至千米尺度存在明显的成分不均一特征。汉诺坝地区陆下地幔不均一性的认识为了解地壳物质再循环提供了绝佳的窗口。

Large-scale replacement of ancient mantle lithosphere during supercontinent assembly: Evidence from the South China Craton

The modification and destruction of the sub-continental lithospheric mantle (SCLM) has been identified in numerous studies. However, what principally controls the susceptibility and extent of destruction of the SCLM and how this leads to the corresponding variation of the overlying crust remain long-standing issues. Here we provide an integrated study of petrology, mineralogy, and geochemistry, including highly siderophile element (HSE) abundances and whole-rock and/or mineral Os-Hf-Sr-Nd isotopic compositions of mantle peridotite xenoliths from Hainan Island, together with a compiled dataset from the eastern South China Craton (SCC) to investigate the mechanism and timing of replacement and destruction of the SCLM. The mantle xenoliths from the eastern SCC are overall more fertile than cratonic lithospheric mantle, and have generally experienced various extents of metasomatic enrichment, including carbonate and silicate melt metasomatism. In addition, the absence of Archean ages and compositional or temporal stratification of the SCLM beneath the eastern SCC indicates that the Archean lithospheric mantle may have been replaced/reworked and has essentially vanished, hence the present underlying lithospheric mantle is a relatively new and young lithospheric mantle. The formation time of the SCLM underneath Hainan Island constrained by both the ReOs and LuHf isotopic systems is late Mesoproterozoic (∼1.2 Ga), largely consistent with the Meso- to Neoproterozoic peak of whole-rock TRD ages (0.9–1.3 Ga) of the refractory mantle peridotites in the compiled dataset. This age distribution can be reconciled with the assembly time of the Rodinia supercontinent, indicating a close linkage between the replacement of large-scale lithospheric mantle via mantle convection erosion and regional continental amalgamation. Meanwhile, the relative location to the plate margins influences the preservation of the cratonic mantle. The SCLM located at the plate margin (e.g., eastern SCC compared to western SCC), where destructive processes frequently occur, is susceptible to destruction, whereas that within the plate interior is likely to remain relatively intact.

Cenozoic thermal-rheological evolution of the coastal cathaysia block of East Asia: geodynamic implications

ABSTRACT The thermal-rheological structure of the continental lithosphere is an important indicator of geodynamic processes. Tectonic compression and extension may change thermal-rheological structure of lithosphere. However, the thermal-rheological evolution over geological time may be difficult to determine because of overprints of subsequent geological events. Mantle xenolith temperature and pressure conditions can differentiate the lithospheric thermal structure over geological time and indicate the lithospheric rheological strength. Chronological and temperature-pressure condition data from 11 mantle xenoliths in Cenozoic basalts from the Wuyishan terrane (WYT) and coastal terrane (CT) on both sides of the Zhenghe-Dapu Fault (ZDF) in the Cathaysia Block indicate that these terranes have experienced distinct thermal-rheological evolution since the Eocene. Moreover, the thermal-rheological evolution of the CT and WTY were differently influenced by the West Pacific tectonic domain, respectively, because the ZDF has been relatively active since 23 Ma and thus has partly accommodated and thus reduced the tectonic impact of Eurasia-Pacific convergence. Therefore, we classified the thermal-rheological structural evolution of the Cathaysia Block into three stages (35.5–20 Ma, 20–6.5 Ma, and 6.5–0 Ma) that sequentially recorded the opening and closing dynamics of the northeastern margin of the South China Sea and uplift of Taiwan Island on the eastern margin of Eurasia.

Subduction-related mantle accretion and makeover revealed by mantle xenoliths at the Pacific margin of NE Eurasia

Subduction is thought to be a major factor in the origin and makeover of continental lithospheric mantle, but essential aspects of these processes continue to be debated. Here, we report textural, chemical and Re Os isotope data on mantle xenoliths from far eastern Russia located at different distances from the Asian Pacific coast to examine how the proximity to subduction zones affects lithospheric mantle compositions. The xenoliths come from the Khanka block close to the border with China and several sites in the Sikhote-Alin orogenic belt (SAOB) near the Japan Sea coast. The inboard Khanka xenoliths are spinel lherzolites with chemical compositions, suggesting that they are residues of low- to moderate-degrees of melt extraction from fertile mantle and essentially unaffected by later events for 1.9 Ga. Such rocks are also common among peridotites in the SAOB, located closer to the subduction margins, but the SAOB suite also includes: a) heterogeneous rocks with veins and patches of pyroxenites or hydrous minerals, and b) rocks enriched in iron (8.8–13.5 wt%), Al 2 O 3 and incompatible minor elements. The positive correlation between elevated 187 Os/ 188 Os ratios (>0.135) and FeO contents in some Fe-rich SAOB xenoliths is interpreted to indicate that the metasomatic melts were derived from the asthenosphere hybridized with recycled materials from the Pacific slab. The lack of correlation between Re -depletion Os model ages (T RD ) and depletion indices, as well as the similarity of Os isotopic compositions between the SAOB peridotites and abyssal peridotites are interpreted to show that the SAOB mantle possibly formed and accreted to Eurasia from the asthenosphere during eastward migration of the Paleo-Pacific subduction trench. Based these results, we infer that the SAOB lithospheric mantle experienced multi-stage metasomatism after its formation, the most significant of which is localized infiltration of subduction-related basaltic melts with recycled characteristics induced by the extension of Japan Sea and release of volatiles from the slab. By contrast, the inboard Khanka lithospheric mantle shows little, if any, evidence for metasomatism. Such contrasting metasomatic modification implies different degrees and styles of makeover processes at different distances relative to the subduction margins. • The Khanka mantle was little affected by metasomatism since it was formed at ~1.9 Ga. • The Sikhote-Alin lithospheric mantle was formed by subduction and accretion. • Mantle metasomatism beneath Sikhote-Alin was caused by melts with recycled signature. • The nature and degrees of metasomatism are related to the distance from subduction front.

He–Ne–Ar–N2–CO2 Systematics of Fernando de Noronha Mantle Xenoliths: Confirmation of Mantle Plume Origin

He–Ne–Ar–N2–CO2 Systematics of Fernando de Noronha Mantle Xenoliths: Confirmation of Mantle Plume Origin

Aragonite in the Interstitial Space of a Mantle Xenolith from the Udachnaya Kimberlite Pipe (Siberian Craton): Direct Evidence for the Presence of Carbonatite Melts in the Deep Lithospheric Mantle

Aragonite in the Interstitial Space of a Mantle Xenolith from the Udachnaya Kimberlite Pipe (Siberian Craton): Direct Evidence for the Presence of Carbonatite Melts in the Deep Lithospheric Mantle

Baddeleyite U-Pb age and Hf isotopes, and constraints on genesis of the Panzhihua carbonatite in SW China

The Panzhihua district is well-known for its magmatic V-Ti-magnetite deposit, with plenty of carbonatite intrusions and extrusives (total length: over 20 km), making it one of the world’s largest carbonatitic belts. Mantle-derived xenoliths (dunite, lherzolite and olivine lamprophyre) of different sizes are the most direct and important evidence of magmatic carbonatite. The carbonatite consists mainly of calcite, dolomite, olivine, clinopyroxene, orthopyroxene and biotite, almost without felsic minerals. We performed baddeleyite U-Pb dating on the four largest carbonatite intrusions at Panzhihua, which yielded weighted mean 206Pb/238U ages (from NE to SW: Zhujiabaobao, Jianbao, Gongshan and Lanaqing) of 261.8 ± 3.8 Ma (MSWD = 0.084, n = 28), 263.7 ± 2.3 Ma (MSWD = 0.10, n = 27), 260.6 ± 2.3 Ma (MSWD = 0.27, n = 22), and 262.3 ± 1.8 Ma (MSWD = 0.44, n = 34). Samples of three extrusive carbonatites and one carbonatitic breccia yielded weighted average baddeleyite 206Pb/238U ages of 261.7 ± 3.0 Ma (MSWD = 0.095, n = 33), 263.8 ± 2.5 Ma (MSWD = 0.21, n = 30), 263.2 ± 2.9 Ma (MSWD = 0.13, n = 28), and 259.1 ± 3.6 Ma (MSWD = 0.12, n = 28), respectively. The weighted 206Pb/238U ages of baddeleyites for all intrusions and extrusives are 262.1 ± 1.2 Ma (MSWD = 0.26, n = 111) and 262.4 ± 1.5 Ma (MSWD = 0.18, n = 119), respectively. Baddeleyites from different intrusions and extrusives have varying Hf isotope compositions, with the weighted average εHf(t) of −0.8 ± 0.5 (Zhujiabaobao), −3.2 ± 0.8 (Jianbaobao), −5.5 ± 1.5 (Gongshan) and −11.9 ± 0.5 (Lanaqing), which are largely negative and decrease progressively from northeast to southwest. εHf(t) values of the two extrusive carbonatite and one carbonatitic breccia samples from the southwest of Panzhihua are −14.5 ± 0.7, −15.1 ± 0.8 and −7.8 ± 1.3, and one extrusive sample from the Lanjian ore-block has weighted mean εHf(t) of −7.8 ± 1.3. δ13CVPDB and δ18OV-SMOW values of the Panzhihua carbonatite range, respectively, −13.4 to +0.4 ‰ (avg. −3.7 ‰) and +15.4 to +21.8 ‰ (avg. +19.5 ‰), similar to those of most global carbonatites. εHf(t) of baddeleyites in Panzhihua carbonatite are dominantly negative, and analogous to that of the standard Phalaborwa baddeleyite, indicating that carbonatitic magma may have been derived from partial melting of the metasomatized lithospheric mantle or an enriched asthenospheric component during the Permian intracontinental rifting (Emeishan mantle plume-related). The hyperthermal carbonatitic magma has partially or completely assimilated mantle xenoliths in the chamber and during its ascent, implying it was of high temperature greater than the solidus curve of peridotite (≥1100℃). Assimilation of xenoliths and wallrocks may have altered the Hf isotope compositions and yielded the distinctive alternating dark peridotite and white carbonatite layers around each xenolith. As a newly identified rock type, the Panzhihua may be one of the global largest carbonatite belts, which also enrich the rock sequences of Emeishan large igneous province.

Seismic anisotropy in the mantle of a tectonically inverted extensional basin: A shear-wave splitting and mantle xenolith study on the western Carpathian-Pannonian region

Information on seismic anisotropy in the Earth's mantle can be obtained from (1) shear-wave splitting analyses which allow to distinguish single or multi-layered anisotropy and delay time of the fast and slow polarized wave can indicate its thickness, and (2) studying mantle peridotites where seismic properties can be inferred from lattice preferred orientation of deformed minerals. We provide a detailed shear-wave splitting map of the western part of the Carpathian-Pannonian region (CPR), an extensional basin recently experiencing tectonic inversion, using splitting data. We then compare the results with seismic properties reported from mantle xenoliths to characterize the depth, thickness, and regional differences of the anisotropic layer in the mantle.Mantle anisotropy is different in the northern and the central/southern part of the western CPR. In the northern part, the lack of azimuthal dependence of the fast split S-wave indicates a single anisotropic layer, which agrees with xenolith data from the Nógrád-Gömör volcanic field. Systematic azimuthal variations in several stations in the central areas point to multiple anisotropic layers, which may be explained by two distinct xenolith subgroups described in the Bakony-Balaton Highland. The shallower layer probably has a ‘fossilized’ lithospheric structure, representing former asthenospheric flow, whereas the deeper one reflects structures attributed to present-day convergent tectonics, also observed in the regional NW-SE fast S-wave orientations. In the Styrian Basin at the western rim of the CPR, results are ambiguous as shear-wave splitting data hint at the presence of multiple anisotropic layers. Spatial coherency analysis of the splitting parameters places the center of the anisotropic layer at ~140–150 km depth under the Western Carpathians, which implies a total thickness of ~220–240 km. Thicknesses estimated from seismic properties of xenoliths give lower values, pointing to heterogeneously distributed anisotropy or different orientation of the mineral deformation structures.

Lithological structure of western Pacific lithosphere reconstructed from mantle xenoliths in a petit-spot volcano

The lithospheric mantle, formed at the mid-ocean ridge as a residue of crustal production, comprises theoretically depleted peridotite, but more fertile components (e.g., lherzolite and pyroxenite) have been reported, creating an enigmatic picture of the lithosphere. The oceanic lithosphere has also been found to be locally modified by intraplate magmatism as proposed from geochemistry of mantle xenolith. Petit-spot xenoliths are particularly notable as direct evidence of old lithospheric mantle and expected to retain essential information about oceanic lithosphere prior to its subduction. In this study, we report on the lithological structure of Pacific lithosphere aged at 160 Ma, just subducting into Mariana Trench, based on petrology and chemistry of ultramafic xenoliths from a petit-spot knoll, and then, we suggest the occurrence of petit-spot melt infiltration resulting in mantle metasomatism and formation of pyroxene-rich vein. Our petit-spot ultramafic xenoliths can be divided into three main types: a depleted peridotite as a residue of crust production, an enriched peridotite, and fertile pyroxenites as the product of melt–rock interactions prior to entrapment. Geothermobarometry also suggests that the depleted peridotite was derived from the uppermost lithospheric mantle, whereas the enriched peridotite and Al-augite pyroxenites were obtained from deeper layers of the lithosphere. Moreover, thermal gradient of the lithosphere estimated from these data is considerably hotter than pristine geotherm estimated on the basis of plate age. Hence, we could illustrate that the oldest portion of the Pacific lithosphere (160 Ma), which was not observed before, was locally fertilized and heated by prior multiple petit-spot magmatic events, and pyroxene-rich metasomatic veins penetrated from the base to the middle/upper lithosphere. Such local lithospheric fertilization is plausible at the plate-bending field, and the nature of Pacific Plate subducting into Mariana Trench may be partly different from what has been assumed so far.

Spinel harzburgite–derived silicate melts forming sulfide-bearing orthopyroxenite in the lithosphere. Part 2: Sulfide compositions and their chalcophile and highly siderophile trace element signatures

In the first article, we have reported petrological data for a new, glass-bearing orthopyroxenite vein cutting a sub-arc mantle xenolith from Kamchatka. As similar veins from the West Bismarck arc, this orthopyroxenite is sulfide-rich and formed by cooling of parental melts derived by partial melting of spinel harzburgite sources. Here, I report new data for the abundances of major base metals and chalcophile and highly siderophile trace elements in vein sulfides from the two localities. Kamchatka vein sulfides are all Cu-poor monosulfide solid solution (MSS). West Bismarck veins contain MSS and a ternary (Fe, Cu, Ni)S solid solution (“xSS”), which ranges between MSS and intermediate solid solution (ISS) in composition. Sulfides follow Ni and Cu enrichment trends and have chondrite-normalized platinum-group element (PGE) patterns with elevated Pt relative to Os, Ir, Ru, and Rh. Pt alloys are frequently associated with sulfides and vugs formed from hydrothermal fluids, which also contain metallic Fe and wüstite. Vein sulfides, ranging from Fe-rich MSS (ca. 1,050–1,100°C) to xSS (≤850°C) through Ni-rich MSS, were formed in a sulfide liquid line of descent under oxygen and sulfur fugacity conditions (fO2 and fS2) down to one log unit below the fayalite–magnetite–quartz and close to the Pt-PtS buffers, respectively. The Ni and Cu enrichment trends in MSS are consistent with cooling and fractionation of Ni-rich and Cu-poor sulfide liquids (original atomic ∑metal/S∼0.9), which will finally solidify as xSS or ISS. Chondrite-normalized Pt/Pd>1 in some of the sulfides is a signature of spinel harzburgite sources. Because it occurs at relatively low fS2, the crystallization sequence of these sulfide liquids is accompanied by the formation of abundant PGE alloys and other metallic phases. Melts derived from spinel harzburgite sources can be originally oxidized to carry up to ∼2,600 ppm S (predominantly as S6+) and follow a sulfide-undersaturated evolution trend, until they are rapidly cooled to crystallize as orthopyroxenite dykes or sills. There, S6+-Fe2+ redox reactions with host rocks, together with the production of high-Mg# andesite derivatives with low S solubility and high-temperature, hydrothermal fluids at decreasing fO2 and fS2, will lead to the local precipitation of abundant sulfides and alloys.

Serpentinization of mantle xenoliths in Kerguelen archipelago: A first petrographic and geochemical study

The spinel harzburgites xenoliths from the Lac Michèle outcrop, Kerguelen archipelago (South Indian Ocean), have the particularity to show unique and unusual evidences of fluids circulation and especially of serpentinization. In this paper, in situ and whole-rock petrographic and geochemical data on a selection of variously serpentinized samples are presented and attest that several episodes of fluid-rock interaction occurred. Serpentinization processes affected notably and with variable degrees the spinel harzburgites leading to changes in their mineralogy and chemical compositions. Degree of serpentinization ranges from very slight (LOI < 3 wt% and Fe 3+ /Fe tot < 0.1) to moderate (4 wt% < LOI < 6 wt% and 0.21 < Fe 3+ /Fe tot < 0.33). Most serpentinized samples show a preferential petrographic direction of serpentinization, forming a subparallel serpentine network, which is sometimes intersected by serpentine veins. As a result, at least two serpentinization episodes were identified within the Lac Michèle samples, without any chemical differences between the two generations except for Cl content. This suggests very few chemical evolution of the system during serpentinization. Due to the significant amount of Cl measured in serpentine minerals (601 ppm on average), the nature of the fluids that interacted with the spinel harzburgites xenoliths during the serpentinization events can be hypothesized to be seawater-like or seawater-derived fluids. • Mantle xenoliths in Loranchet (Kerguelen) are slightly to moderately serpentinized. • Several episodes of fluid-rock interactions affected the peridotite xenoliths. • Fluids are mainly seawater-like or seawater-derived fluids.

Correlations between olivine composition and groundmass mineralogy in Sierra Leone kimberlites provide constraints on craton-specific melt-lithosphere interactions

Sierra Leone contains two Jurassic-aged diamondiferous kimberlite clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes and NE-SW trending dikes. In this study, a combination of detailed petrography, and phlogopite, spinel, and olivine compositions in hypabyssal samples is presented to classify and constrain the petrogenesis of these kimberlites. Both the Koidu and Tongo rocks are predominantly macrocrystic and highly micaceous with phlogopite abundances, normalised to olivine-free, of 36–65 vol% in the groundmass. These phlogopite contents are comparable to those of some cratonic lamproites and significantly higher than any other kimberlites. Other groundmass minerals include spinel, perovskite, and apatite set in a base of carbonate and serpentine. Phlogopite and spinel have similar compositions in the Koidu and Tongo samples, displaying evolutionary trends consistent with those observed in worldwide kimberlites. Olivine macrocrysts and microcrysts display complex zoning with distinct cores, internal zones, and rims. The core compositions display a range in Mg# (81–95) and are interpreted to be derived from the disaggregation of lithospheric mantle xenoliths and proto-kimberlite related megacrysts. The Tongo olivine rims, interpreted to be primary magmatic crystallisation products, have similar compositions from various locations within the cluster whereas the Koidu samples display a range in rim compositions (Mg# 87–89) from different locations within the cluster. The average Koidu rim compositions display a strong positive correlation with the average core compositions, consistent with the trend formed by kimberlites worldwide and indicative of a strong control by melt-lithosphere interaction on melt compositions. Previously, it has been shown that the Mg# of olivine rim's negatively correlates with the abundance of groundmass phlogopite (± oxide minerals) in kimberlites. However, the Koidu and Tongo kimberlites are exceptionally phlogopite-rich given their olivine Mg#, fall outside of the worldwide kimberlite array and have olivine compositions and phlogopite abundances like some cratonic lamproites. This leads us to suggest that Koidu and Tongo represent a rare style of highly micaceous kimberlite magmatism, not previously reported in other cratonic regions, and are genetically linked to the assimilation of K 2 O-rich, metasomatic mantle lithologies. We further suggest that the K 2 O content, reflected by groundmass phlogopite abundances, of worldwide kimberlite and cratonic lamproite parent magmas ascending to the surface may be related to assimilation of craton-specific styles of metasomatic lithologies in the SCLM. • Unique highly micaceous kimberlites from Sierra Leone. • Olivine compositions constraining melt-SCLM interactions. • Craton-specific melt-SCLM assimilation and K2O enrichment.

Magnetotelluric Evidence for Lithospheric Alteration Beneath the Wuyi‐Yunkai Orogen: Implications for Thermal Structure of South China

AbstractThe Wuyi‐Yunkai Orogen experienced a polyphase tectonomagmatism and is a key region for deciphering the alteration and thermal structure of the South China Block lithosphere. Herein, an electrical resistivity model of the lithosphere is presented via the three‐dimensional inversion of broadband (0.003–3600 s) magnetotelluric (MT) data collected along a 380‐km‐long profile comprising 62 MT sites across the Wuyi‐Yunkai Orogen, and the robustness of this model is critically evaluated through a series of sensitivity tests. The resistivity model reveals that the upper crust of the Cathaysia Block and the Wuyi‐Yunkai Orogen is dominated by high‐resistivity sedimentary cover interposed with low‐resistivity features, mainly along fault zones. High‐resistivity bodies and strong conductors in the upper crust are interpreted as magmatic rocks and tectonic mélanges, respectively. Another feature of this resistivity model is the presence of zones featuring enhanced electrical conductivity (&lt;30 Ωm) extending from the lower crust to the upper mantle beneath the Wuyi‐Yunkai Orogen. The conductors in the lower crust are attributed to saline fluids from either the dehydration of the subducting Paleo‐Pacific slab or the regional metamorphism‐induced dehydration of sandy argillaceous rocks. In contrast, the conductors in the upper mantle are attributed to 4%–7% partial melt, which corresponds to the analyses of mantle xenoliths in South China. These conductors in the upper crust and upper mantle supply volatiles and heat to shallow geothermal systems. This work ultimately shows that the lithospheric thinning of South China is controlled mainly by mantle upwelling caused by the retreat of the subducting Paleo‐Pacific slab.

Kelyphite Textures Experimentally Reproduced through Garnet Breakdown in the Presence of a Melt Phase

Abstract Complex multiphase reaction rims that form during garnet breakdown are known as kelyphite coronae and are common amongst exhumed mantle xenoliths. It has long been established that a reaction of garnet and olivine produces kelyphite corona consisting of spinel and pyroxenes, and that preservation of high-pressure garnet cores requires sufficiently rapid uplift of material through the spinel lherzolite stability field from depths of at least 60 km. We present new high-pressure, high-temperature experiments of garnet breakdown in the spinel–lherzolite stability field demonstrating that a series of cascading reactions can reproduce the multilayer, multiphase kelyphites seen in nature. In all experiments where breakdown occurred, a melt appears to have moderated the reactions towards equilibrium; we believe this to be the first experimental confirmation of the importance of such melts in garnet breakdown reactions. In our experiments at least three distinct zones of concentric kelyphite growth can occur at a single pressure, temperature condition; we suggest, therefore, that such kelyphites seen in natural samples do not have to be caused by a multistage uplift path as is often assumed. Kelyphitic coronae surrounding garnet have previously been used to estimate uplift rates; however, the lack of kinetic data for relevant exhumation reactions has limited their use for PTt pathway estimations and the understanding of emplacement mechanisms. In order to constrain accurate PTt pathways we use reaction rim thickness as a proxy for reaction progress and present preliminary results for the kinetics of garnet breakdown.

An insight into the evolution of the lithospheric mantle of south Saharan metacraton: Mantle xenoliths from Jabal Eghei Volcanic Complex, Libya

Mantle xenoliths from a Pliocene (∼2 Ma) pyroclastic cone of the Jabal Eghei Volcanic Complex (south Libya) are studied for unravelling depletion-, deformation- and metasomatic events within this part of the Saharan Metacraton. The xenoliths are fertile Cpx-rich spinel lherzolites texturally distinguished into protogranular (hereafter: undeformed – UX) and porphyroclastic/equigranular (deformed – DX) types. The extent of depletion is estimated at 5–10% (olivine-spinel compositions) and 3–13% (HREE contents in clinopyroxene) of melt extraction; all parameters of depletion indicate that the UX are slightly more depleted than the DX. Calculated equilibrium temperatures range 800–1100 °C and generally overlap with temperatures obtained for other xenolith suites in Libya and for other off-cratonic xenoliths worldwide. The temperature estimates for the UX are slightly higher (857–1091 °C) than those for the DX (800–923 °C) and the former xenolith types are better equilibrated. The studied xenoliths exhibit evidence of stealth (high abundances of modal Cpx in some samples) and cryptic metasomatism (LREE-enriched patterns on whole-rock and Cpx REE diagrams) with metasomatic agents most likely akin to alkaline mafic melts similar in composition to host magmas. The study provides a simplified 2D model assuming that: a) the Jabal Eghei lithospheric mantle was roughly stratified with more depleted undeformed peridotite in deeper and less depleted deformed peridotite in shallower regions, b) in the Miocene, normal faulting caused ductile and brittle deformations in the lower and the upper parts of the lithosphere, respectively, and produced small-scale mafic alkaline melts that moved upward and metasomatized the overlying mantle, and c) the advancing extension produced mafic alkaline magma that captured xenoliths and brought them to the surface.

MINERAL COMPOSITION AND PT PARAMETERS OF CRYSTALLIZATION OF MANTLE ROCKS UNDER KIMBERLITE FIELDS OF THE ANABAR REGION

The composition of barophilic minerals from mantle xenoliths and Cpx from concentrates of the Kuranakh, Luchakan, Dyuken, and Ary-Mastakh fields of the Anabar region has been studied. Under these fields, the lithospheric mantle compositions vary significantly. The PT parameters of crystallization were calculated using the composition of clinopyroxenes from xenoliths and heavy fraction of kimberlites. The lithospheric mantle rocks under the northern fields have higher values of Mg# of minerals and calculated mantle geotherm (35–48 mW/m2) compared to the parameters for the southern diamond fields. The pipes from the southwestern part of the Ary-Mastakh field are promising for the diamond potential, presenting Grt-bearing lherzolites and harzburgites with a high content of Cr2O3 (to 8.5 wt. %).

XENOLITH GARNETS FROM MIR KIMBERLITE PIPE: CHEMICAL COMPOSITION AND EVIDENCE OF METASOMATIC PROCESSES IN THE LITHOSPHERE MANTLE

This paper reports the results on the composition of lithosphere mantle under the Mirny kimberlite field. The authors investigated 57 samples of the mantle xenoliths collected from the Mir pipe. The samples were represented by peridotites (Grt lherzolites) and pyroxenites (Grt websterite, Grt clinopyroxenite and eclogite). The composition of minerals (garnet, clinopyroxene) and various rocks in the lithosphere mantle under the Mirny kimberlite field were analyzed based on petrographic features and chemical data. Besides, PT conditions of rock crystallization were calculated using different geothermobarometers. Garnets from peridotites and websterites show relatively high Mg# (75–83) and low TiO2 contents (up to 0.2 wt. %). Since the eclogite has high-Ca (3.78–9.46 wt. %) and high-Fe (7.77–17.20 wt. %) garnet composition, it lies in the area of wehrlite paragenesis. In general, garnets from the lithosphere mantle under the Mirny kimberlite field have low-Ti garnet composition (up to 0.7 wt. %). Thus, the lithosphere mantle under the Mirny kimberlite field differs from the lithosphere mantle under other diamondiferous fields in a widespread development of eclogite and pyroxenite (up to 50 %), low-Ti composition of rocks, as well as virtual absence of deformed lherzolites. These signs probably indicate minor alteration of silicate metasomatism in the lithospheric mantle under the Mirny field (in contrast to the center of the Siberian craton).

Alkaline rocks of the Bobaomby volcanic field point to a petrogenetic link between Comoros and northern Madagascar lithosphere

The Bobaomby volcanic field (10–11 Ma) is the northernmost volcanic area of Madagascar, and is a monogenetic volcanic field comprising outcrops of lava flows, dykes, scoria cones, tuff rings and plugs, widely scattered over an area of roughly 500 km2. The volcanic rocks range in composition from nephelinite, basanite and tephrite, through tephritic phonolite, to F- and Cl-rich peralkaline phonolite (MgO from 13 to 0.01 wt%), and the serial affinity varies from sodic to potassic. A few mica-amphibole-rich lamprophyric dykes have tephritic composition and ultrapotassic affinity. The mafic lavas host intrusive xenoliths with evident cumulate features (wehrlites, composite olivine gabbros s.l., amphibole clinopyroxenites and “kaersutitites”), as well as various types of mantle-derived xenoliths and xenocrysts in the most primitive rocks. The very wide compositional variations of the observed phases (olivine, clinopyroxene, amphibole, oxides, feldspars, feldspathoids, apatite, titanite, aenigmatite and other accessories) in lavas, dykes and cognate xenoliths are fully consistent with the variable degree of differentiation of the host lavas/dykes, and pointing out to limited open-system or polybaric crystallization. The mafic lavas have marked enrichment in incompatible elements and light rare-earth element (LREE) (e.g., Lan/Ybn = 19–27), whereas concave REE patterns are found in the peralkaline phonolites, as a result of removal of accessory titanite starting from tephritic phonolite magmas. The gabbroic/ultramafic xenoliths are interpreted as crustal cumulates of basanitic and tephritic magmas. Several liquid lines of descent in the basanites and tephrites are evident from the trace-element distribution, and from the differing geochemistry of the evolved rocks. The isotopic compositions reach extreme values (e.g., 206Pb/204Pb = 20.065 in the ultrapotassic lamprophyre) when compared to the rest of the Cenozoic/Recent Madagascan volcanic rocks, but similar to those of the Comoros archipelago, suggesting analogies of mantle sources and enrichment processes in the lithosphere of this volcanic archipelago. The origin of the Bobaomby mafic rocks is compatible from a derivation from low degree partial melting of an incompatible element-enriched peridotite source (possibly located in the lowermost lithospheric mantle) rich in volatile-rich phases (pargasite, locally also phlogopite and possibly carbonates), matching the sources of other Cenozoic volcanic areas throughout Madagascar, and perhaps Comoros.

Heat Transfer and Production in Cratonic Continental Crust: U‐Pb Thermochronology of Xenoliths From the Siberian Craton

AbstractCoupled U‐Pb and trace‐element analyses of accessory phases in crustal xenoliths from the Late Devonian Udachnaya kimberlite (Siberian craton, Russia) are used to constrain Moho temperature and crustal heat production at the time of kimberlite eruption. Rutile and apatite in lower‐crustal garnet granulites record U‐Pb dates that extend from 1.8 Ga to 360 Ma (timing of kimberlite eruption). This contrasts with upper‐crustal tonalites and amphibolites that contain solely Paleoproterozoic apatite. Depth profiling of rutile from the lower‐crustal xenoliths show that U‐Pb dates increase gradually from rim to core over μm‐scale distances, with slower‐diffusing elements (e.g., Al) increasing in concentration across similar length‐scales. The U‐Pb and trace element gradients in rutile are incompatible with partial Pb loss during slow cooling, but are consistent with neocrystallization and re‐heating of the lower crust for &lt;1 Myr prior to eruption. Because Paleoproterozoic rutile and apatite dates are preserved, we infer that long‐term ambient lower‐crustal temperatures before this thermal perturbation were cooler than the Pb closure temperature of rutile and probably apatite (&lt;400°C). The lower‐crustal temperature bounds from these data are consistent with pressure‐temperature arrays of Udachnaya peridotite xenoliths that suggest relatively cool geothermal gradients, signifying that the mantle xenoliths accurately capture the thermal state of the lithosphere prior to eruption. Combined, the xenolith data imply low crustal heat production for the Siberian craton (∼0.3 μW/m3). Nevertheless, such values produce surface heat flow values of 20–40 mW/m2, higher than measured around Udachnaya (average 19 mW/m2), suggesting that the surface heat flow measurements are inaccurate.