Mantle metasomatism

Introduction: The Simlipal Basin consists of a 2-3 kmthick sequence of basaltic lavas and pyroclastics interbedded with three quartzite units, deposited on 3.44 Ga TTG gneisses, and 3.3-3.12 Ga granites and greenstone belts of the Archaean Singhbhum Craton, in NE Odisha, India [1,2]. The detrital zircons in the lower quartzite give provenance U-Pb ages of 3.55 to 3.08 Ga [3]. The basin outcrops in the form of a concentric ring structure (centered on 21°52’07.66” N; 86°20’08.75” E), with a diameter of c. 45 km, situated within a NNESSW trending plateau (Fig. 1). Three inward-dipping topographic rings are defined by the resistant quartzites. The centre of the Simlipal Structure is occupied by an 800m-thick differentiated ultramafic-mafic intrusion, the Amjori Sill, covering an area of 130 km2 [4]. The basalts of the Simlipal ring structure are intruded by the Mayurbhanj suite of gabbros, granophyres, and pyroxene granites, dated at c. 3.08 Ga [2]. Later events are dated at 2.37, 2.08 ...

This work investigates the role of mantle metasomatism in the genesis of calc-alkaline magmas, using a geochemical, petrological, and geodynamic approach. Mantle metasomatism, defined as the chemical modification of the mantle by fluids or melts derived from subducted slabs, occurs predominantly in subduction zones. Two forms are recognized: cryptic metasomatism, which alters mantle chemistry without forming new minerals, and modal metasomatism, which introduces hydrous phases such as amphibole and phlogopite.
The nature of metasomatic agents depends on pressure-temperature conditions, ranging from aqueous fluids at low temperatures to silicate melts and supercritical fluids at higher pressures. Their interaction with peridotites modifies mantle mineralogy and trace element composition, lowers the solidus, and facilitates partial melting. Resulting calc-alkaline magmas are enriched in large ion lithophile elements and light rare earth elements, with characteristic negative anomalies in high field strength elements. Isotopic ratios of Sr, Nd, and Pb further reflect slab-derived contributions.
This study highlights mantle metasomatism as a key process in arc magmatism, explaining the geochemical and isotopic signatures of calc-alkaline rocks and offering insights into the geodynamic evolution of convergent margins.

Experiments were run to investigate Cr-bearing, Mg-, Al-spinel assemblages that are predicted to form during condensation in dust-enriched systems. The spinels are compared to natural Cr-rich spinels found in the carbonaceous chondrite, Allende.

This research investigates the Marginal basalts in the Asal-Ghoubbet rift, a poorly understood geological feature, with the aim of clarifying magma sources and identifying the mantle reservoir responsible for forming basaltic rocks in the region. The study involved analysing major geochemical elements, rare earth elements and radiogenic isotopes of Sr, Nd and Pb from basalt samples collected in the Marginal basalts of the rift. Additionally, the mineral composition of plagioclase, olivine, pyroxene and Fe-Ti oxides was examined using electron probe microanalysis (EPMA) on polished thin sections. Geochemical results indicate a tholeiitic composition for the Marginal basalts, with evidence of fractional crystallization driven initially by clinopyroxene and olivine, followed by plagioclase crystallization. Thermobarometric and MELTS modelling estimated magma storage depths ranging from the Moho to the upper crust. The basalt is slightly enriched in light rare earth elements (LREEs) and depleted in Heavy Rare Earth Elements (HREEs), similar to the characteristics of enriched Mid-Ocean Ridge Basalt (E-MORB) and Ocean Island Basalt (OIB). Isotopic data of Sr and Nd reveal sources that vary between the high μ mantle source (HIMU) and the dominant prevalent mantle source (PREMA). Furthermore, Pb isotopic signatures suggest an influence from an enriched mantle (EM) component, plotting between the EMI and EMII mantle reservoirs fields. Forward-melting modelling and isotopic evidence suggest that the Afar mantle plume is a significant contributor to the magma source of Marginal basalts in the Asal-Ghoubbet rift. Overall, the study enhances understanding of the magma plumbing system in this area, elucidating the process governing magma chamber evolution and the nature of the mantle reservoirs that generated these basaltic rocks.

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental arc setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua^Pacaya and Tecuamburro^Moyuta. The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral^aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb) N ratios of V1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression

The Mesoarchean Na-granitoids exposed in the Rio Maria domain, southeastern Amazonian craton, are represented by tonalite-trondhjemite and leucogranodiorite-granite suites. The 2.98-2.92 Ga tonalite-trondhjemites are the most voluminous rock type in the Rio Maria domain and host 2.86 Ga leucogranodiorite-granite plutons. These rocks share common geochemical characteristics, such as relatively high Al 2 O 3 and Na 2 O and low Yb and Y contents, as well as the behavior of the REE. However, based on an extensive geochemical data set, it is possible to show that the leucogranodiorite-granites have higher K 2 O, Ba, Sr, and Rb and lower CaO contents than the tonalite-trondhjemites. The latter are compositionally similar to typical tonalite-trondhjemite-granodiorite (TTG) series and probably originated from partial melting of garnet amphibolites, derived from tholeiitic rocks or from metabasalts of the Identidade greenstone belt, at pressure conditions suitable to produce high, medium, and low La/Yb tonalite-trondhjemite groups. The leucogranodiorite-granites show geochemical affinity with the Transitional TTG of the Yilgarn craton and are related to the Hybrid granitoid group. The ambiguous geochemical character of the Rio Maria leucogranodiorite-granite suite, which shares some characteristics that are typical of the tonalite-trondhjemite rocks and others more commonly observed in the sanukitoid suites, may be related to complex processes involving TTG and sanukitoid magmas. The discrimination of these two Na-granitoid groups helps us estimate the true volume of TTG magmatism in the Rio Maria domain and in understanding the dynamics of petrogenetic processes in the terrane at the end of the Archean.

In the Mesoarchean Rio Maria granite-greenstone terrane, the leucogranodiorite-granite (LGdG) suites are mostly represented by the 2.87 Ga Guarantã suite which includes the Trairão and Azulona granodiorite and the Guarantã granite plutons. These granites originated around 50 m. y. after the 2.93 ± 0.1 Ga major TTG magmatic event and were approximately coeval with the sanukitoid suites (~2.87 Ga), potassic leucogranites (~2.86 Ga) and the younger TTGs (Água Fria trondhjemite; ~2.86 Ga). The rocks of the LGdG suites are characterised by strongly fractionated REE patterns, with either no Eu anomaly or slight negative Eu anomalies, and show enrichment in Ba and, to a lesser degree, in Sr compared with the other Archean granitoids of the RMGGT. The LGdG have strong geochemical affinity with the high-Ca granites or Transitional TTG of the Yilgarn craton. The different models proposed to explain the origin of these granites are still controversial and apparently are not able to explain the genetic processes and evolution of the LGdG of the RMGGT. The dual geochemical character of the LGdG suites that share some features typical of the TTGs with others more commonly observed in the sanukitoid suites, suggest a complex evolution for these granites and a possible genetic link between them and the mentioned suites. On the basis of modelling and geochemical data we deduced that the LGdG suites could be possibly derived from mixing between a Ba-and Sr-enriched granite magma and trondhjemitic liquids. The granite magmas participating in the mixture originated by fractional crystallization of 35% of a sanukitoid magma of granodioritic composition leaving a residue consisting of plagioclase (46.72%), hornblende (39.05%), clinopyroxene (10.36%), magnetite (3.12%), ilmenite (0.7%) and allanite (0.06%). The large compositional variations observed in the Guarantã suite can be apparently explained by mixing in different proportions between the granite and trondhjemitic liquids. This large spectrum of variation could be augmented if it is assumed that fractional crystallization played a significant role in the evolution of the liquids formed by mixing processes.

The rocks of 2.87 Ga Rio Maria sanukitoid suite of the eastern Amazonian craton include granodiorites, intermediate rocks, layered rocks and mafic enclaves. Their REE patterns and the behavior of Rb, Ba, Sr, and Y allowed the distinction of a granodioritic (granodiorite and intermediate rocks) and a monzonitic (mafic enclaves) sanukitoid series. Petrogenetic modeling indicated that the granodiorites and intermediate rocks are not related by fractional crystallization. The internal evolution of the intermediate rocks were leaded by fractionation of amphibole + biotite ± apatite, whereas the granodiorites evolved by fractionation of plagioclase + amphibole ± biotite. The layered rocks were probably derived from the granodiorite magma by an accumulation of 50% of amphibole (dark layer) and of 30% of amphibole ± plagioclase (gray layer). The petrogenesis of the Rio Maria suite required melting of a modified mantle extensively metasomatized by addition of about 30% TTG-like melt to generate the granodiorite (11% of melt) and intermediate magmas (14% of melt), and ~20% TTG-like melt in the case of mafic enclave magma (9% of melt). Modeling and geochemical data, particularly the behavior of Sr and Y, suggest that mafic enclave and granodiorite magmas were originated at different depths and should have mingled during their ascent and final emplacement. The modal and geochemical differences observed between the granodioritic and monzonitic sanukitoid series of Rio Maria are apparently a general feature of the Archean sanukitoids. This indicates the existence of at least two distinct sanukitoid series and suggests that the nature of the sanukitoid series is strongly dependent of the pressure of magma generation. Our results indicate that the sanukitoid magmas were originated in a two stage process. The envisaged model admits an active subduction tectonic setting in the Rio Maria terrane in between 2.98 and 2.92 Ga when the TTG magmas responsible by the mantle metasomatism were generated (first stage). At ~2.87 Ga, a tectonothermal event, possibly related to slab-break-off or due to the action of a mantle plume, induced the partial melting of the metasomatized mantle and generated the Rio Maria sanukitoid magmas (second stage).

The Archean granites exposed in the Mesorchean Rio Maria granite-greenstone terrane (RMGGT), southeastern Amazonian craton can be divided into three groups on the basis of petrographic and geochemical data. (1) Potassic leucogranites (Xinguara and Mata Surrão granites), composed dominantly of biotite monzogranites that have high SiO 2 , K 2 O, and Rb contents and show fractionated REE patterns with moderate to pronounced negative Eu anomalies. These granites share many features with the low-Ca granite group of the Yilgarn craton and CA2-type of Archean calc-alkaline granites. These granites result from the partial melting of rocks similar to the older TTG of the RMGGT. (2) Leucogranodioritegranite group (Guarantã suite, Grotão granodiorite, and similar rocks), which is composed of Ba-and Sr-rich rocks which display fractionated REE patterns without significant Eu anomalies and show geochemical affinity with the high-Ca granite group or Transitional TTG of the Yilgarn craton and the CA1-type of Archean calc-alkaline granites. These rocks appear to have been originated from mixing between a Ba-and Sr-enriched granite magma and trondhjemitic liquids or alternatively product of interaction between fluids enriched in K, Sr, and Ba, derived from a metasomatized mantle with older TTG rocks. (3) Amphibole-biotite monzogranites (Rancho de Deus granite) associated with sanukitoid suites. These granites were probably generated by fractional crystallization and differentiation of sanukitoid magmas enriched in Ba and Sr. The emplacement of the granites of the RMGGT occurred during the Mesoarchean (2.87e2.86 Ga). They are approximately coeval with the sanukitoid suites (w2.87 Ga) and post-dated the main timing of TTG suites formation (2.98e2.92 Ga). The crust of Rio Maria was probably still quite warm at the time when the granite magmas were produced. In these conditions, the underplating in the lower crust of large volumes of sanukitoid magmas may have also contributed with heat inducing the partial melting of crustal protoliths and opening the possibility of complex interactions between different kinds of magmas.

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The lower continental crust section of the Ivrea-Verbano Zone (Italian Alps) was intruded by a ∼ 8 km thick gabbroic-dioritic body (Ivrea Mafic Complex) in the Upper Carboniferous-Lower Permian, in conjunction with the post-collisional transtensional regime related to the Variscan orogeny. In the deepest levels of the Ivrea Mafic Complex, several peridotite-pyroxenite sequences considered of magmatic origin are exposed. We present here a petrological-geochemical investigation of the peridotites from the largest magmatic ultramafic sequence of the Ivrea Mafic Complex, locally called Rocca d'Argimonia. In spite of the widespread subsolidus re-equilibration under granulite facies conditions, most likely reflecting a slow cooling evolution in the lower continental crust, the Rocca d'Argimonia peridotites (dunites to harzburgites and minor clinopyroxene-poor lherzolites) typically retain structures and microstructures of magmatic origin. In particular, the harzburgites and the lherzolites typically show poikilitic orthopyroxenes enclosing partially dissolved olivine and minor spinel. Olivine has forsterite proportion diminishing from the dunites to the harzburgites and the lherzolites (90 mol % to 85 mol %) and negatively correlating with δ 18 O (+5.8 ‰ to +6.6 ‰). Gabbronorite dykes locally crosscut the peridotites and show millimetre-scale thick, orthopyroxenite to websterite reaction zones along the contact with host rocks. We propose that the Rocca d'Argimonia peridotites record a process of reactive melt flow through a melt-poor olivine-rich crystal mush or a pre-existing dunite. This process was most likely responsible for the olivine dissolution shown by the poikilitic orthopyroxenes in the harzburgites-lherzolites. We infer that the reactively migrating melts possessed a substantial crustal component and operated at least at the scale of ∼ 100 m.

The Eocene Rasht-Abad volcanic rocks are located in the Alborz-Azerbaijan magmatic belt (including the Tarom-Hashtjin province) of NW Iran. Those are mainly mafic to intermediate with calc-alkaline affinities, comprising andesite, andesitebasalt, trachy-andesite, and dacite. Clinopyroxene ranging in composition from diopside to augite is the most significant mafic mineral of the basic rocks. Aluminum partitioning between tetrahedral and octahedral sites shows that those crystalized at low pressure. Ferric iron of clinopyroxene also indicates high oxygen fugacity for formation of crystal. Geothermobarometry using clinopyroxene-melt equilibrium calculations constrains the crystallization temperature and pressure of this mineral as 1100-1200 °C and 2-6 kbar. Ce/Pb values of the mafic lavas are lower than values expected for mantle-derived melts but do not support crustal contamination. Co-existing basalt to trachyandesite lavas display parallel and tight REE patterns that suggest these rocks originated from a common mantle source or parental magma. Tectonomagmatic discrimination diagrams and mineral compositions are consistent with the host volcanic rocks having the characteristics of continental margin arcs. Geochemical data are consistent with the parental magma of mafic-intermediate rocks of Rasht-Abad area being derived from a typical subcontinental lithospheric mantle which was enriched by subducted slab-derived fluids and melts during tectonic events in the active continental margin. The data support a model of Eocene flare-up magmatism associated with rollback of a flattened slab.

The Eocene Rasht-Abad volcanic rocks are located in the Alborz-Azerbaijan magmatic belt (including the Tarom-Hashtjin province) of NW Iran. Those are mainly mafic to intermediate with calc-alkaline affinities, comprising andesite, andesitebasalt, trachy-andesite, and dacite. Clinopyroxene ranging in composition from diopside to augite is the most significant mafic mineral of the basic rocks. Aluminum partitioning between tetrahedral and octahedral sites shows that those crystalized at low pressure. Ferric iron of clinopyroxene also indicates high oxygen fugacity for formation of crystal. Geothermobarometry using clinopyroxene-melt equilibrium calculations constrains the crystallization temperature and pressure of this mineral as 1100-1200 °C and 2-6 kbar. Ce/Pb values of the mafic lavas are lower than values expected for mantle-derived melts but do not support crustal contamination. Co-existing basalt to trachyandesite lavas display parallel and tight REE patterns that suggest these rocks originated from a common mantle source or parental magma. Tectonomagmatic discrimination diagrams and mineral compositions are consistent with the host volcanic rocks having the characteristics of continental margin arcs. Geochemical data are consistent with the parental magma of mafic-intermediate rocks of Rasht-Abad area being derived from a typical subcontinental lithospheric mantle which was enriched by subducted slab-derived fluids and melts during tectonic events in the active continental margin. The data support a model of Eocene flare-up magmatism associated with rollback of a flattened slab.

Carbon dioxide solubilities in H20-free hydrous silicate melts of natural andesite (CA), tholeiite (K 1921), and olivine nephelinite (O M 1) compositions have been determined employing carbon-14 beta-track mapping techniques. The CO2 solubility increases with increasing pressure, temperature, and degree of silica-undersaturation of the silicate melt. At 1650 ~ C, CO2 solubility in CA increases from 1.48• wt % at 15 kbar to 1.95• wt % at 30 kbar. The respective solubilities in OM1 are 3.41 • 0.08 wt % and 7.11 • wt %. The CO2 solubility in K1921 is intermediate between those of CA and OM 1 compositions. At lower temperatures, the CO2 contents of these silicate melts are lower, and the pressure dependence of the solubility is less pronounced. The presence of H/O also affects the CO2 solubility (20-30% more CO2 dissolves in hydrous than in H20-free silicate melts); the solubility curves pass through an isothermal, isobaric maximum at an intermediate CO2/(CO2 + H20 ) composition of the volatile phase. Under conditions within the upper mantle where carbonate minerals are not stable and CO2 and H20 are present a vapor phase must exist. Because the solubility of CO2 in silicate melts is lower than that of H20, volatiles must fractionate between the melt and vapor during partial melting of peridorite. Initial low-temperature melts will be more HzO-rich than later hightemperature melts, provided vapor is present during the melting.

Diamonds from George Creek, Colorado, preserve complex intergrowth textures between two major growth generations: homogeneous diamond with yellow-buff cathodoluminescence (CL); and diamond with blue-green CL and local growth zonation. Secondary-ion mass spectrometry (SIMS) has revealed large variations in N concentration and C isotope composition within these diamonds. Even within single stones, N contents and δ13C values can vary from 0 to 750 ppm and 0 to −20‰ respectively. These variations are similar to those recorded elsewhere for entire diamond suites. The CL characteristics correlate directly with N: diamond with yellow-buff CL has uniform N contents, whereas the zoned diamond has bright blue CL bands with high N (50–750 ppm) and dark blue or green CL bands with low N (0–20 ppm). These bands are too narrow (10–5 µm) for analysis by IR spectroscopy. δ13C also varies between the two growth generations in any one diamond plate, and to a lesser extent within these generations, bu...

Mantle xenoliths from Hainan and Qilin, South China have been studied to constrain the nature of the upper mantle and mantle processes beneath a continental margin. The extremely low Ti (160±245 ppm) contents in clinopyroxenes from some spinel lherzolites, indicative of high degrees of partial melting are inconsistent with the relatively high clinopyroxene modes (7.4±13%) in these samples. This inconsistency could be due to polybaric melting that started in the garnet stability ®eld, then, after the breakdown of garnet to pyroxene and spinel, continued in the spinel stability ®eld. Polybaric melting, due to adiabatic decompression of upwelling mantle, would leave a residual mantle in which the degree of depletion decreases with depth. The predicted strati®ed lithospheric mantle is evidenced by the negative correlation between the forsterite content in olivine and the equilibration temperature, proportional to the depth in the lithosphere from which the xenolith was derived. The lower part of the lithospheric mantle beneath South China consists predominantly of fertile and moderately depleted peridotites, which are either devoid of LREE enrichment, or show the trace element signature of incipient metasomatism, and plot within the Phanerozoic mantle domain. In contrast, the upper part of the mantle contains harzburgite and cpx-poor lherzolite, which are strongly affected by metasomatism of melt/¯uid of highly variable composition. The anomalously high orthopyroxene mode (up to 47%) makes some of these refractory samples compositionally similar to the Proterozoic/Archean mantle. Their low equilibrium temperature (800±900 8C) points to the presence of old lithospheric relicts in the uppermost mantle beneath South China. Such lithosphere architecture may have resulted from partial replacement of Archean±Proterozoic lithosphere by asthenosphere that rose adiabatically subsequent to lithospheric thinning during the Cenozoic.

Petrologic, trace element and Sr-Nd isotopic studies of mantle xenoliths in Quaternary basalts from Huinan, NE China provide constraints on the origin of coarse-grained harzburgites and the nature of lithosphere-asthenosphere interaction during lithospheric thinning. The Huinan harzburgites have a secondary recrystallized texture and their composition deviates from the partial melting trend of residual peridotites. The convex-upward REE pattern and a positive Cr-Yb correlation in clinopyroxene imply an interaction with basaltic melts at a high melt/rock ratio. The Huinan harzburgites are therefore not simple residues of partial melting, but likely resulted from melt-rock interaction during which the percolating melts preferentially dissolved pyroxenes by precipitation of olivine, transforming lherzolite to harzburgite. The melt percolationreaction enhanced grain boundary diffusion kinetics, and gave rise to the characteristic texture of these mantle rocks. These "reactive" harzburgites were eventually metasomatized by compositionally distinct small volume volatile-rich melts, which may be derived from the main harzburgite-forming event as a result of meltconsuming reaction. Most likely the formation of the Huinan harzburgites was coeval with thermo-tectonic erosion of the continental lithosphere by upwelling asthenospheric melts. Thermometric considerations suggest a relatively long time interval between lithospheric thinning and eruption of the host basalts, consistent with the contention that lithospheric thinning in eastern China may have peaked in the late Cretaceous.

Petrographic investigations and electron microprobe analyses have been performed on a rare aggregate of clinopyroxene megacrysts collected from Cenozoic basalts in Yinfengling, Leizhou Peninsula of South China. The aggregate, composed of several clinopyroxene megacrysts, shows abundant exsolution lamellae of garnet (Grt) and orthopyroxene (Opx), and granular texture. Cr-and Ti-poor spinels are also present in this sample. They occur predominantly as Sp-Opx-Grt clusters (Cr# = 0.025-0.034) at the interspace between different megacrysts, and subordinately as bleb-shaped (Cr# = 0.025-0.034) or thin-lamella crystals (Cr# = 0.006-0.021) in clinopyroxene. Three different assemblages of exsolution are identified, namely (1) Sp (high Cr/Al) and Opx; (2) Grt-Opx; (3) Sp (low Cr/Al) and Opx. In addition, some garnets were likely developed as response to breakdown of the high-Cr/Al Sp. The homogeneous compositions in all constituent minerals and the good agreement between calculated Cpx/Grt partition coefficients (K d 's) for trace elements and reference data strongly suggest a chemical equilibrium among coexisting minerals, probably attained by diffusion after the exsolution. Thermobarometric calculation based on exsolved assemblage yields a temperature of 900 ± 30°C and a pressure of 12 ± 2.2 kbar, corresponding to the present-day thermal gradient in the region. Much higher P-T estimates (T = 1,210 ± 30°C, P = 16.2 ± 3.5 kbar) are obtained for the reconstructed composition of cpx prior to exsolution. The contrast in thermal state before and after the exsolution might reflect the thermal evolution of the lithosphere beneath South China during the Cenozoic. Communicated by J. Hoefs.

The 40 Ar-39 Ar dating reveals three episodes of basaltic volcanism in eastern Guangdong of SE China since the late Eocene (i.e., 35.5, *20 and 6.6 Ma). The Miocene alkali olivine basalts (*20 and 6.6 Ma) have OIB-like trace element characteristics, which is coupled with low ( 87 Sr/ 86 Sr) i , high e Nd (t), and high e Hf (t). In contrast, the late Eocene basalts (35.5 Ma) have overall characteristics of ''Island Arc'' basalts with strong negative Ta-Nb-Ti anomalies in the primitive mantle-normalized multi-element diagram with high ( 87 Sr/ 86 Sr) i , negative e Nd (t), and relatively low e Hf (t). All basalts have unexpectedly high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, delineating a DUPAL signature in the sources. The late Eocene Arc-like basalts may reflect contributions of relict ancient metasomatized mantle lithosphere that melted as the result of extension-induced asthenospheric upwelling and heating, whereas the Miocene OIB-like basalts may represent partial melting of the asthenospheric mantle beneath the thickened lithosphere. We propose that the Cenozoic basaltic volcanism in eastern Guangdong records an overall lithospheric thickening process beneath SE China, that is, a continental rift system from its maximum extension in the late Eocene to its waning in the Miocene. This interpretation is consistent with the evolution of the South China Sea, whose origin is most consistent with the development of a passive continental margin. The seafloor spreading of the South China Sea during * 32-16 Ma may not result from the effect of the ''Hainan'' mantle plume, but rather played a positive role in allowing the mantle plume to express on the surface.

The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS: pre-spreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puzzling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better understanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogenesis, but also is heavily dependent on systematic sampling of seafloor rocks. volcanism, mantle melting, rifting continental margin, opening of the South China Sea, Hainan mantle plume

The volcanoes of the northern lzusouth-cental Honshu and northeastem Honshu arcs, Japan, occur in a region where vmiables such as age, conposition, and nature ofthe sediment cover oftle subducting plate are relatively uniform. In this study, v/e attempt to isolate some ofthe variables associated with suprasubduction zone (SSZ) magma genesis, in order to provide insight into ttre nature of sources and petrogenetic processes. The chemical compositions of selected parental and evolved magmas in fifteeu Plio-Pleistocene centers along the strike ofthe volcanic front allow us to assess tle significance of over-riding lithospheric input for SSZ magmatism. These volcanoes are distributed across a number of major terrane boundaries, including the lithosphere-penetrating Tanakura Tectonic Line. Conpositions range from basalt to rhyolite, with a preponderance of basaltic andesite and andesite; an exception is basalt-dominated Fuji. Most basalts are signiffcmfly fractionate4 with low MgO (<7 wtTo), Mg# $0, and low Ni (d0 ppm). Significant differences among volcanic suites are established on the basis of isotopic @b, Sr, and Nd), majorand trace-element data-The predominant source of these ma€mas is a relatively refractory (compared to mid-oceaa ridge basalt) mantle-wedge are4 with subchondritic MZr and ZrlIIf. A number of volcanoes also havelow ZrN (< MORB), but several have both elevated hN (>3) and Dy/Yb, indicative of gamet involvement in their gercsis. The Nd and Sr isotope ratios also indicate regional variations in chemical composition of the source(s) (ong-term depleted to enriched). Pb isotopic data are distinctive in northern Honshu, with elevated 48Pb/2I/.Pb afi,NTPbPePb at specific 2n6Pb2oaPb (south versas north of the Tanakura Tectonic Line), are collectively elevated above the Northem Hemisphere Reference Line (and lzu-Mariana arcs), appear similax to portions of the Philippines and Tertiary basalts of eastern Asia and probably reflect an Asian lithospheric heritage. The combined trace-element and isotope geochemistry of parental basalts distinguish different domains within the Izu-Bonin, central and northeastern Honshu arcs. The ultimate cause of these is most likely multicomponent, and some crustal assimilation is higbly probable. However, tlese domains may in some paxt represent nagma interaction with a subcontinental lithosphere dominated by Indian-Ocean-type mantle, or alt€matively, derivation from an Indian-Ocean-type asthenosphere. The correlation of some magma compositional characteristics with nature of the basement terrane shows that the lithosphere plays a subtle but significant role in determining parental island-arc basalt chaxacteristics. The longevity and character ofthe continental lithosphere in Asian margin arc systems, and its interaction with asthenosphere-derived melts, is a1 imFcntant process in island-arc basalt genesis.

Oxygen isotope data are reported for clinopyroxene phenocrysts from volcanic rock samples from Filicudi island, Italy, with the aim of investigating mechanisms of magma evolution and mantle source characteristics in a continental arc volcano. Filicudi rocks range from calc-alkaline basalt to high-K andesite and dacite. Mafic rocks have MgO 6-7 wt%, Mg# 60, Ni < 50 ppm and Cr < 200 ppm, suggesting they represent evolved melts from more primitive mantle-derived parents. Variations of Sr-Nd isotope ratios against silica and MgO suggest magma evolution under opensystem. Oxygen isotope ratios on clinopyroxene phenocrysts from representative rocks are in the range 18 O cpx = + 5.37 to + 6.20 ‰ SMOW, corresponding to 18 O melt = + 5.6 to + 6.4 ‰ and display an overall, but poorly defined, positive correlation with 87 Sr/ 86 Sr and MgO and a rough negative correlation with SiO 2 . Basalts exhibit the highest 18 O cpx values for the volcanic series studied. Together, major, trace element and isotopic data indicate complex, multistage polybaric evolutionary processes for the Filicudi magmas. A process of fractional crystallisation, accompanied by variable degrees of crustal assimilation, best explains the isotopic and petrological data of the Filicudi volcanics. Evolution processes are inferred to have occurred in distinct magma reservoirs sited at different depths. Magma contamination affected basalts more extensively than andesitic and dacitic magmas. This was a consequence of the higher temperature and lower viscosity of mafic melts that were able to assimilate higher amounts of crustal wall rocks. Therefore, andesites and dacites preserved stable and radiogenic isotopic compositions more closely than basalts. Combined trace element and isotopic data suggest that primitive magmas at Filicudi were generated in a heterogeneous and metasomatised mantle source, which underwent contamination by slab-derived fluids. When compared with the nearby islands, isotopic variations of Filicudi rocks resemble more closely those found at Alicudi than at Salina and Vulcano. This suggests that the same kind of evolutionary processes occurred in the western island of Filicudi and Alicudi, whereas at Salina and Vulcano, in the central arc, magmas typically evolved by AFC processes, generating derivative melts that display higher Sr-O isotope compositions compared to parent basalts. These contrasting styles of magma evolution may be related to different structures of the plumbing systems of volcanic islands that were constructed in different sectors of the arc.

Infrared spectra of C-O-H micro-inclusions were collected from a microinclusion bearing diamond during step-heating and freezing experiments to examine fluid speciation as a function of pressure and temperature. The inclusions contain H 2 O, CO 2 , carbonate, apatite, quartz and mica, which together represent the oxidising remnant mantle fluid composition after diamond crystallisation. The internal pressure of the inclusions, measured from calibrated shifts of the quartz peaks, increases from 1.3 GPa at ambient temperature, to approximately 4-5 GPa at 737 o C, close to the conditions of crystallisation of the host diamond in the mantle.

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from *700 to 1,400°C and pressures (P) [ 1.6-8.9 GPa, corresponding to depths of *52-270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64-86 mol% pyrope; 0-10 wt% Cr 2 O 3 ), Cr-rich diopside (0.5-3.5 wt% Cr 2 O 3 ), Al-poor orthopyroxene (0-5 wt% Al 2 O 3 ), high-Cr spinel (Cr/(Cr ? Al) 9 100 atomic ratio = 2-86) and olivine (88-94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from *700 to 950°C and pressure [3.5-5.0 GPa, corresponding to depths of [110-150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mgnumber of garnet for orogenic is less than that of offcratonic and on-cratonic xenolith, (4) Al 2 O 3 , Cr 2 O 3 , CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crustmantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, oncratonic and orogenic garnet peridotite.

In Central China, long-distance effects from collision between the North China and Yangtze cratons, uplift of the Tibetan Plateau and subduction of the Pacific Ocean are believed to converge in the Western Qinling. Mantle xenoliths from Baiguan and Haoti kamafugites in the Western Qinling were investigated to understand the lithospheric structure and mantle metasomatism beneath the orogenic belt. The Western Qinling lithosphere with depths of at least 120 km is geothermally hot and compositionally stratified, companied by a step-wise decrease in fertility with depth. The shallower portion of the lithospheric mantle is represented by type 2 xenoliths which lack alteration and deformation, and have fertile characteristics in compositions. The dominant rock type in the mantle section is the type 1 peridotites, which have had undergone relatively higher melt extraction and subsequent metasomatism. In-situ trace element data for minerals in the Baiguan and Haoti xenoliths are presented in this study. Baiguan clinopyroxenes have high LREE and show Ba, Th, U, Pb, Sr enrichment and negative Hf and Y anomalies, but the Haoti clinopyroxenes have large variations in trace elements and complicated distributions in LILE, and show negative Ti, Zr and Hf anomalies. Clinopyroxenes from both xenolith types have Ti/Eu and (La/Yb) N ratios in large ranges of 902-15,813 and 0-20, respectively. The petrological and geochemical characteristics suggest that the Western Qinling peridotites had been subjected to silicate and subsequent carbonatite metasomatism, and that carbonatite metasomatism was predominant in Baiguan but lesser in Haoti. Comparisons with the Western Qinling carbonatites and world-wide carbonatites indicate that carbonatite melts involved in the metasomatism originated from deep mantle and most likely related to carbonatite magma, with contributions from subducted oceanic crust. The type 2 peridotites, comprising clinopyroxene megacrysts and clinopyroxenite display similar trace element abundances and patterns, suggesting a close affinity in petrogenesis.

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO 2 contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb-Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO 2 ) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO 2 , which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in d 57 Fe are in a range of -0.25 to 0.14% for olivine, -0.17 to 0.17% for orthopyroxene, -0.21 to 0.27% for clinopyroxene, and -0.16 to 0.26% for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on ''d-d'' plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb) N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.

The Lar igneous suite (LIS), in southeastern Iran, is part of post collisional alkaline magmatism in Sistan suture zone. Shonkinite and kersantite are the only two high-Mg, K-rich olivine bearing rocks in the LIS. We study major and some compatible trace elements in the Lar shonkinite and kersantite (LSK) olivines to define mantle mineralogy and metasomatic processes. Olivines in shonkinite have higher Fo (83-90), compared with those in kersantite (Fo76-80). Ca and Ni contents in the olivines are relatively low, whereas their Mn and Ti contents are high and variable, respectively. Low Ni contents exhibit olivine crystallization at igneous conditions from a magma originated by partial melting of an olivine-rich mantle source. Geochemical date reveal that magma evolution is responsible for high-Mn and low Fo contents in kersantitic olivines. In contrast, high Mn, Mn/Fe and Fo contents in shonkinitic olivines indicate an existence of Mn-rich and Ca-Si-poor metasomatic agents in the source. So, considering the Middle Oligocene-Miocene post-collision nature of the Lar igneous suite, melts or fluids derived from upwelling asthenosphere in the form of magnesitic-carbonatite melts, had great potential in metasomatism of subcontinental lithospheric mantle. This CO2 and K-rich liquid then reacts with peridotite to produce new mineral assemblages including low-Ca clinopyroxene, olivine and phlogopite. Partial melting of such metasomatized source region was responsible for producing the undersaturated, K-rich shonkinite and kersantite in the LIS.

The 40 Ar-39 Ar dating reveals three episodes of basaltic volcanism in eastern Guangdong of SE China since the late Eocene (i.e., 35.5, *20 and 6.6 Ma). The Miocene alkali olivine basalts (*20 and 6.6 Ma) have OIB-like trace element characteristics, which is coupled with low ( 87 Sr/ 86 Sr) i , high e Nd (t), and high e Hf (t). In contrast, the late Eocene basalts (35.5 Ma) have overall characteristics of ''Island Arc'' basalts with strong negative Ta-Nb-Ti anomalies in the primitive mantle-normalized multi-element diagram with high ( 87 Sr/ 86 Sr) i , negative e Nd (t), and relatively low e Hf (t). All basalts have unexpectedly high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, delineating a DUPAL signature in the sources. The late Eocene Arc-like basalts may reflect contributions of relict ancient metasomatized mantle lithosphere that melted as the result of extension-induced asthenospheric upwelling and heating, whereas the Miocene OIB-like basalts may represent partial melting of the asthenospheric mantle beneath the thickened lithosphere. We propose that the Cenozoic basaltic volcanism in eastern Guangdong records an overall lithospheric thickening process beneath SE China, that is, a continental rift system from its maximum extension in the late Eocene to its waning in the Miocene. This interpretation is consistent with the evolution of the South China Sea, whose origin is most consistent with the development of a passive continental margin. The seafloor spreading of the South China Sea during * 32-16 Ma may not result from the effect of the ''Hainan'' mantle plume, but rather played a positive role in allowing the mantle plume to express on the surface.

Based on Moho and Curie depth, heat flow, and upper mantle S-wave velocity anomaly, we infer the thermo-chemical structure of the lithospheres in Africa and surrounding oceans. The Moho depth is derived from gravity anomaly using the Parker-Oldenburg method, with constraints from seismic Moho. Crustal stratification defined by Curie-Moho depth difference shows that thermal and strong compositional processes may have shaped the lithospheric architecture of the African continental plate. Moho and Curie depths indicate the southern and eastern African cratons have thermochemical structures different from the West African Craton. Large Curie-Moho depth difference in southern and eastern Africa aligns with the low velocity anomaly originated from the core-mantle boundary. Mantle upwelling from the African low-velocity anomaly presumably induced partial melting at great depth, and the release of mineral-rich fluid and large amounts of volatile components facilitates a regional metasomatism, and results in a depleted, predominantly felsic, low-density paramagnetic crust. Mantle xenolith in kimberlites and volcanic rocks supports metasomatism by melts transmitted through narrow conduits as an intermittent or continuous upward flux of mineral-rich fluid. Alignment of the Curie-Moho depth difference at the intra-plate volcanic province correlates with weak lithospheric strength along the corridor connecting the intra-plate volcanic province with the Ethiopian plateau, suggesting a pathway for thermochemical asthenospheric flow. Crustal stratification and compositional-driven density layering support crustal buoyancy and uplift in the Hoggar, and southern and eastern Africa. A magnetized uppermost mantle is prevalent in the entire oceanic region, except at large igneous provinces (LIPs), volcanic seamounts, and oceanic plateaus, which have partial paramagnetic crusts. Our results support thermochemical upwelling related to the low velocity anomaly beneath the African plate.

4 Abstract: POLYGONAL JOINTING IN SANDSTONES FROM EASTERN PARAGUAY Columnar joints, resembling those in ig- neous rocks, are present in quarz sandstones of various formations (Mesozoic to Cenozoic) in Eastern Paraguay. New data concerning the field relationships and the geochemical characteristics constrain the processes responsible for the generation of columnar fracture patterns in sedimentary rocks. The columns are mostly normal to bedding and occur at several different stratigraphic levels. The host rocks are cut by volcanic necks or plugs, acting as heat sources and the development of columnar jointing may be controlled by the presence of magmatic rocks. Alternatively, thermal waters, circulating through fractures and favorable sedimentary layers, are responsible for the remplacement of the sandstones by opal. As a result of the strain originated by the contraction of opal, a system of joints was developed. It is also possible that the local tectonic stresses have been develope...

Angola-Etendeka (PAE) system, at the westernmost fringe of the Early Cretaceous flood tholeiites (Serra Geral Formation, SGF), was the site of repeated and different magmatic activity from Mesozoic to Paleocene times. In the early Middle Triassic, sodic alkaline magmatism occurred along the belt represented by the Paraguay River, at the boundaries between Brazil and the Chaco-Pantanal basin. During the Early Cretaceous, potassic alkaline magmatism (where are present also alkaline-carbonatitic complexes) pre-and post-dates SGF. Further alkaline sodic magmatism occurred in Eastern Paraguay during late Early Cretaceous and Paleocene. The latter are characterized by the presence of abundant mantle xenoliths (spinel facies) in the ultramafic rock-types in the Misiones and Asuncio  n provinces. Geological, petrological, mineralogical, geochemical and isotopic results suggest that almost two main mantle components have been involved in the genesis of the magmatism in Eastern Paraguay: an extreme and heterogeneous EM and a HIMU component. The EM-like component appear prevalent in the Early Cretaceous potassic alkaline magmatism, whereas the HIMU was important in the late Early Cretaceous and Paleocene sodic magmatism. Different contributions of mantle component similar to EM and HIMU could also explain the geochemical heterogeneity of the Early Cretaceous flood tholeiites in Eastern Paraguay (i.e. high-and low-Ti types, with high and low contents of incompatible elements, respectively). In the light of these facts, the mantle plume/hotspot hypothesis for the origin of the magmatism in Eastern Paraguay must be reviewed, at least regarding which plumes are most likely to have been active at the right place and right time for a specific province. In fact, in order to explain the widespread distribution of South American (Angolan and Namibian) Early Cretaceous tholeiitic and alkaline magmatism, a hypothetical mantle plume head is not compelling. We support rifting processes which resulted in different lithospheric thickness beneath the edge of cratonic shields, inducing small-scale convection cells. In addition, the presence of long-lived thermal anomalies in the mantle has already been demonstrated by seismic velocity distribution models based on tomographic techniques using both P-and S-waves. On the whole, the geophysical evidence and geochemical results, combined with new well controlled ages (mainly 40 Ar/ 39 Ar ages) for the magmatic events in Eastern Paraguay, indicate that any model proposed for the evolution of the PAE in terms of HIMU and EM end-members must satisfy the following constraints: (a) HIMU and EMI-II are not restricted to the oceanic environment; (b) end-members are variously associated in space as a function of the various protoliths; (c) mantle regions with HIMU and EMI isotope compositions are capable of generating melts that can lead to the formation of a wide variety of silicate rocks, including melts enriched in CO 2 (including alkaline-carbonatitic complexes, i.e. mantle metasomatism); (d) the geochemical features of the sodic alkaline rock-types cluster together, well distinct in comparison with the potassic alkaline from Paraguay, but consistent with the potassic alkaline-carbonatite rocks from Angola and Namibia; (e) the paleomagnetic results indicate that any mantle plume hypothesis is in disagreement with the fixed and mobile plume models. Finally, it is suggested that a more complete fluidodynamic models must be developed for the plate-tectonic models.

Mantle xenoliths from Kimberley, South Africa, contain texturally distinct peridotites; coarse granular to extensively sheared peridotites. Mineral chemistries of the textually distinct peridotite xenoliths indicate that these mantle rocks were equilibrated at similar P-T conditions: 44-54 kbar and 970-1130 o C for granular peridotites, and 41-50 kbar and 950-1080 o C for sheared peridotites. The Kimberley peridotites showing various deformation textures are restricted to a relatively narrow depth range (120-160km). They display no systematic correlation between texture and estimated equilibrium pressure and temperature, whereas sheared peridotites in the Lesotho kimberlites commonly originated from greater depths and higher temperature. The large local variations in the degree of deformation in the cratonic lithosphere beneath Kimberley require pronounced weakening due to localized high water flux that can be associated with mantle metasomatism. Sheared peridotites have more depleted compositions in FeO and CaO compositions than those of granular peridotites, but have a higher orthopyroxene modal abundance and contain sodium-rich clinopyroxene, which are consistent with that extensive deformations associated with Si-rich fluid/melt metasomatism. We therefore conclude that mantle metasomatism has an important role in facilitating deformation locally within the cratonic lithosphere, and is responsible for the textural variations of peridotite xenoliths from the cratonic roots.

INTRODUCTION GEOLOGICAL AND TECTONIC SETTING Geography and surface topography of the EARS Geology of the EARS and its basement Continental rifting events in Africa Volcanism and the 'mantle plume' issue Previous studies on mantle xenoliths from the EARS THE IMPORTANCE OF MARSABIT AIMS AND ORGANISATION OF THE STUDY

Most eclogitic mantle xenoliths brought to the surface exhibit a certain degree of enrichment with incompatible elements, usually attributed to the effect of mantle metasomatism by a putative metasomatic fluid. The metasomatic overprint is represented mainly by enrichments in Na, K, Ba, Ti and LREE and the original source of this fluid remains unknown. In this paper, we present a detailed petrological study of a typical eclogitic mantle xenolith from the Roberts Victor kimberlite mine in South Africa. We find that its textural and mineralogical features present strong evidence for incipient melting. The melting assemblage we observe did not necessarily require introduction of additional components, that is: in-situ melting alone could produce highly incompatible element enriched melt without involvement of a hypothetical and speculative "metasomatic event". Due to the higher abundance in incompatible elements and lower solidus temperature than peridotites, mantle eclogites, some of which represent previously subducted oceanic crust, are much more plausible sources of mantle metasomatism, but on the other hand, they can be considered as highly metasomatised themselves. This brings us to the "chicken or egg" dilemmawas the secondary mineral assemblage in mantle lithologies a result or a source of mantle metasomatism?

We constrain the lithospheric mantle density of the North China Craton (NCC) at both in situ and standard temperature-pressure (STP) conditions from gravity data. The lithosphere-asthenosphere boundary (LAB) depth is constrained by our new thermal model, which is based on a new regional heat flow data set and a recent regional crustal model NCcrust. The new thermal model shows that the thermal lithosphere thickness is <120 km in most of the NCC, except for the northern and southern parts with the maximum depth of 170 km. The gravity calculations reveal a highly heterogeneous density structure of the lithospheric mantle with in situ and STP values of 3.22-3.29 and 3.32-3.40 g/cm 3 , respectively. Thick and reduced-density cratonic-type lithosphere is preserved mostly in the southern NCC. Most of the Eastern Block has a thin (90-140 km) and high-density lithospheric mantle. Most of the Western Block has a high-density lithospheric mantle and a thin (80-110 km) lithosphere typical of Phanerozoic regions, which suggests that the Archean lithosphere is no longer present there. We conclude that in almost the entire NCC the lithosphere has lost its cratonic characteristics by geodynamic processes that include, but are not limited to, the Paleozoic closure of the Paleo-Asian Ocean in the north, the Mesozoic Yangtze Craton flat subduction in the south, the Mesozoic Pacific subduction in the east, the Cenozoic remote response to the Indian-Eurasian collision in the west, and the Cenozoic extensional tectonics (possibly associated with the slab roll-back) in the center.

We have examined Highly Siderophile Element (HSE) and Os-isotope variations in suites of variably fractionated lavas from the Late Cenozoic mafic alkaline volcanic province of NW Turkey in order to evaluate the source characteristics and possible effects of post melting differentiation processes on HSE signatures of these lavas. The results reveal that the behavior of Os and other HSEs changes during magma differentiation. The concentrations of Os, Ir and Ru (IPGE) strongly decrease with increasing fractionation for melts with MgO&gt;10 wt.%, suggesting that these elements are sited in olivine and associated HSE retaining trace phases and behave compatibly during olivine-dominated fractional crystallization. Fractionation trends indicate significantly lower bulk partition coefficients for IPGE in lavas with less than 10 wt.% MgO when compared to values for more primitive lavas with MgO&gt;10 wt.%, possibly reflecting a change in the fractionation assemblages. Platinum, Pd and Re, on...

Petrological studies play a significant role in the debate regarding the origin of intraplate magmas by providing unequivocal constraints about the source(s) composition and melting processes related to basalt formation. Two major hypotheses are currently in debate: first, intraplate magmas are produced at depth (i.e. within the asthenosphere) by low-degrees melting of an enriched peridotitic source in the presence of CO2 [1]; second, alkaline magmas are produced by the melting of metasomatic hydrous veins present within the lithospheric mantle . If the existence of metasomatic veins in the continental lithospheric mantle is well documented, their existence and the mechanism of their formation in an oceanic setting are still mostly unconstrained. Here we report new petrological data demonstrating that metasomatic veins can be produced within the oceanic lithosphere by percolation and differentiation of low-degree melts initially located in the low velocity zone .