Peridotite Minerals Research Articles

Rates and Mechanisms of Mineral Carbonation in Peridotite: Natural Processes and Recipes for Enhanced, in situ CO2 Capture and Storage

Near-surface reaction of CO2-bearing fluids with silicate minerals in peridotite and basalt forms solid carbonate minerals. Such processes form abundant veins and travertine deposits, particularly in association with tectonically exposed mantle peridotite. This is important in the global carbon cycle, in weathering, and in understanding physical-chemical interaction during retrograde metamorphism. Enhancing the rate of such reactions is a proposed method for geologic CO2 storage, and perhaps for direct capture of CO2 from near-surface fluids. We review, synthesize, and extend inferences from a variety of sources. We include data from studies on natural peridotite carbonation processes, carbonation kinetics, feedback between permeability and volume change via reaction-driven cracking, and proposed methods for enhancing the rate of natural mineral carbonation via in situ processes (“at the outcrop”) rather than ex situ processes (“at the smokestack”).

In-situ trace element analysis of clinopyroxene on thin section by using LA-ICP-MS

Among the common minerals of the spinel peridotites, clinopyroxene is the main carrier of trace elements and serves as a tracer of geological mantle processes. The trace element composition of clinopyroxenes on a thin section 30 μm thick was measured using LA-ICP-MS. The repeatability of thin section analysis was better than 3% at the 2 sigma level. When using a 213 nm laser with a 55 μm beam size, optimal analytical conditions for the LA-ICP-MS were determined to be a 55% energy level (9 J/cm2) and 10 Hz repetition rate, giving a penetration speed of clinopyroxene of 0.17 μm/pulse. Overall, the LA-ICP-MS technique can be used to determine the trace element composition of clinopyroxenes on thin sections with reasonable precision, accuracy, and limit of detection.

Iron and magnesium isotopic compositions of peridotite xenoliths from Eastern China

We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ 56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ 26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ 56Fe; ±0.10‰ for δ 26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ 56Fe ol–cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ 56Fe ol–opx = −0.05 ± 0.11‰; Δ 26Mg ol–opx = −0.09 ± 0.12‰; Δ 26Mg ol–cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ 56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ 26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ 56Fe and δ 26Mg. The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ 56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral–mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ 56Fe, the larger deviations from average δ 56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ 56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ 56Fe and slightly heavy δ 26Mg, which may reflect Fe–Mg inter-diffusion between peridotite and percolating melt.

First-principles estimates of equilibrium magnesium isotope fractionation in silicate, oxide, carbonate and hexaaquamagnesium(2+) crystals

Equilibrium mass-dependent magnesium isotope fractionation factors are estimated for a range of crystalline compounds including oxides, silicates, carbonates, and salts containing the Mg ( H 2 O ) 6 2 + solvation complex. Fractionation factors for the gas-phase species Mg and MgO are also presented. Fractionation factors are calculated with density functional perturbation theory (DFPT), using norm-conserving pseudopotentials. The results suggest that there will be substantial inter-mineral fractionation, particularly between tetrahedrally coordinated Mg 2+ in spinel (MgAl 2O 4) and the more common octahedrally coordinated Mg 2+-sites in silicate and carbonate minerals. Isotope fractionations calculated for Mg 2+ in hexaaquamagnesium(2+) salts are in good agreement with previous fractionation models of Mg aq 2 + based on large molecular clusters ( Black et al., 2007), but show possibly more significant disagreement with a more recent study ( Rustad et al., 2010). These models further suggest that solvated Mg aq 2 + , in the form of Mg ( H 2 O ) 6 2 + , will have higher 26Mg/ 24Mg than coexisting magnesite and dolomite. Calculated fractionations are consistent with Mg-isotope fractionations observed in peridotite mineral separates and inorganic carbonate precipitates. Predicted large, temperature-sensitive spinel-silicate fractionations, in particular, may find use in determining equilibration temperatures of peridotites and other high-temperature rock types.

Multistage melt/fluid-peridotite interactions in the refertilized lithospheric mantle beneath the North China Craton: constraints from the Li–Sr–Nd isotopic disequilibrium between minerals of peridotite xenoliths

Elemental and Li-Sr-Nd isotopic data of minerals in spinel peridotites hosted by Cenozoic basalts allow us to refine the existing models for Li isotopic frac- tionation in mantle peridotites and constrain the melt/fluid- peridotite interaction in the lithospheric mantle beneath the North China Craton. Highly elevated Li concentrations in cpx (up to 24 ppm) relative to coexisting opx and olivine (\4 ppm) indicate that the peridotites experienced metasomatism by mafic silicate melts and/or fluids. The mineral d 7 Li vary greatly, with olivine (?0.7 to ?5.4%) being isotopically heavier than coexisting opx (-4.4 to -25.9%) and cpx (-3.3 to -21.4%) in most samples. The d 7 Li in pyroxenes are considerably lower than the normal mantle values and show negative correlation with their Li abundances, likely due to recent Li ingress attended by diffusive fractionation of Li isotopes. Two exceptional samples have olivine d 7 Li of -3.0 and -7.9%, indicating the existence of low d 7 Li domains in the mantle, which could be transient and gener- ated by meter-scale diffusion of Li during melt/fluid-peri- dotite interaction. The 143 Nd/ 144 Nd (0.5123-0.5139) and 87 Sr/ 86 Sr (0.7018-0.7062) in the pyroxenes also show a large variation, in which the cpx are apparently lower in 87 Sr/ 86 Sr and slightly higher in 143 Nd/ 144 Nd than coexisting opx, implying an intermineral Sr-Nd isotopic disequilib- rium. This is observed more apparently in peridotites having low 87 Sr/ 86 Sr and high 143 Nd/ 144 Nd ratios than in those with high 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, suggesting that a rela- tively recent interaction existed between an ancient me- tasomatized lithospheric mantle and asthenospheric melt, which transformed the refractory peridotites with highly radiogenic Sr and unradiogenic Nd isotopic compositions to the fertile lherzolites with unradiogenic Sr and radiogenic Nd isotopic compositions. Therefore, we argue that the litho- spheric mantle represented by the peridotites has been het- erogeneously refertilized by multistage melt/fluid-peridotite interactions.

Post-deformational impregnation of depleted MORB in Nain lherzolite (Central Iran)

Plagioclase lherzolites of Nain melange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin.

A review of water contents of nominally anhydrous natural minerals in the mantles of Earth, Mars and the Moon

Olivine, pyroxene and garnet are nominally anhydrous but can accommodate tens to hundreds of parts per million (ppm) H 2O or “water” in the form of protons incorporated in defects in their mineral structure. This review concerns the amount of water in nominally anhydrous minerals from mantle and mantle-derived rocks: peridotites, eclogites, megacrysts, basalts and kimberlites. Trends between internal and external parameters and water content in olivine, pyroxene, and garnet of mantle rocks allow us to identify what controls their H intake. The water content of pyroxenes and garnets in peridotites appears to depend primarily on mineral chemistry, while that of olivines may more readily reflect water activity and water fugacity conditions in the mantle. In eclogites, both mineral chemistry and metamorphic pressure control the water intake of pyroxene and garnet. The water content of minerals in crystallized melts (basalt and kimberlite phenocrysts, xenocrysts, and megacrysts) is determined by the degree of differentiation, the amount of water in the parent melt, and degassing. Basalt and cumulate minerals from Martian meteorites may be as water-rich as their Earth equivalents. No water has been detected at present in Moon minerals but low amounts in degassed basaltic glasses signify that deep Moon reservoirs may still retain water. The presence of water in mantle minerals, typically amounts of tens to hundreds of ppm, enhances their deformation properties. Water contents of peridotite minerals in the oceanic upper mantle, in and around cratons, and in subduction zones may have tremendous influence on Earth's geodynamics.

Serpentinization of peridotites from the southern Mariana forearc

A detailed petrologic and mineralogic study was carried out on serpentinized peridotites dredged from the southern landward slopes of the Mariana Trench, in order to reveal the serpentinization process of these unusual rocks and to identify the sole presence of the mineral lizardite. The constituent minerals of these southern Mariana forearc peridotites are olivine, amphibole and spinel, as well as serpentine, chlorite and talc. Compared with serpentinite seamounts, the serpentinized peridotites from the southern Mariana forearc are characterized by the absence of magnetite and brucite, and the common presence of talc; besides, the serpentine mineral variety is simplex, only lizardite. Combining mineral chemistry and mineral phase relationships, we conclude that (1) the absence of magnetite in the serpentinized peridotites is due to incomplete serpentinization, other than magnetite, the iron end-member in olivine forms Fe-rich brucite and Fe-rich serpentine; (2) brucite is not stable with high silica activity, reacting with later SiO 2-rich fluid and then forming lizardite, leading to a lack of brucite in these serpentinized peridotites; (3) the occurrence of talc is the result of later SiO 2-rich fluid reactions with lizardite; and (4) the reason for the sole occurrence of lizardite is that the temperature condition of our study area was not high enough for the formation of antigorite (which is stable at >500 °C). Despite the broad overlap of lizardite and chrysotile in growth temperature, differences in the modes of occurrence of lizardite and chrysotile, such as the scarcity of H 2O, low porosity and permeability, as well as the actual situation of initial serpentinization in the study area, result in the absolute prevalence of lizardite over chrysotile in the area.

The Oxidation State of Metasomatized Mantle Wedge: Insights from C-O-H-bearing Garnet Peridotite

Oxygen fugacity (fO2) is an important parameter in determining the relative stabilities of phase assemblages. Whereas a number of studies have been devoted to determining the redox state of low-pressure assemblages in the mantle system, the fO2 of the supra-subduction mantle wedge is still poorly known. An essential input for fO2 estimates is the determination of the ferric–ferrous iron content of key mantle minerals such as garnet, which can be measured using the ‘flank method’ technique on an electron microprobe. We selected samples of orogenic peridotites from the ultrahigh-pressure Sulu belt (Eastern China) and from the Ulten Zone (Italian Alps) for detailed case studies; these correspond to slices of metasomatized mantle wedge sampled at different depths. They are characterized by the assemblage phlogopite + magnesite +amphibole in equilibrium with olivine, orthopyroxene and Fe3+-bearing garnet. The ‘flank method’ measurements indicate that these pyrope-rich garnets contain Fe3+/ΣFe up to 0·12–0·14. For peridotite mineral assemblages fO2 can be evaluated from equilibria involving the Fe3+-garnet component skiagite (Fe2+3Fe3+2Si3O12) on the basis of Fe3+–Al substitution on the octahedral site, which is sensitive to the garnet oxidation state. We modelled non-ideal mixing of Al and Fe3+ on the octahedral site and non-ideal mixing on the dodecahedral site, with a symmetric regular solution model for reciprocal solid solutions of Ca–Fe2+–Mg–Al–Fe3+-garnet. This allowed us to calculate garnet-peridotite fO2, given the presence of Fe3+ in garnet. Our results indicate that the Sulu and Ulten peridotites record high oxygen fugacities (FMQ to FMQ + 2) compared with garnet peridotite xenoliths from the sub-cratonic mantle equilibrated at similar pressures. The determination of the oxygen fugacity of these hydrate–carbonate-bearing garnet peridotites allowed us to estimate the speciation of C–O–H metasomatic fluids derived from the subducting slab, which are enriched in CO2. The fO2 evaluation of the metasomatized mantle-wedge peridotites, representing the oxygen chemical potential μO2, provides a first step in unravelling the relationship between μO2 and the metasomatic phase assemblage.

Isla Isabel (Nayarit, México): Quaternary alkalic basalts with mantle xenoliths erupted in the mouth of the Gulf of California

Isla Isabel is a small island (~ 1.06 km 2) off the coast of Nayarit where intraplate-type alkali-basaltic volcanic rocks erupted during the Quaternary, and one of two Mexican islands (along with Isla Guadalupe) where it is known that the alkali basalts have transported peridotite xenoliths to the surface. The volume of the island is estimated at approximately 4.2 km 3; the volume above sea level is estimated to be approximately 0.03 km 3. The dominant volcanic rock type exposed above sea level is analcime-cemented basaltic tuff-breccia, the rapidly lithified products of hydrovolcanic eruptions; however, subaerial scoria-fall deposits and lava flows are also present. Ten analyzed Isla Isabel volcanic rocks are relatively homogeneous alkali basalts with 4.5–6.4 wt.% nepheline in the CIPW norm. Compared to other analyzed Mexican intraplate-type volcanic rocks from oceanic and continental localities, at the same MgO content, the Isabel samples are depleted in TiO 2, FeO total, and Na 2O, and moderately incompatible trace elements (e.g., Sm, Eu, Tb, and Zr). On the other hand, they are enriched in CaO and Al 2O 3, and relatively compatible trace elements (e.g., Lu and Sc). It is suggested that relative to other Mexican intraplate alkaline lavas the Isabel lavas were derived by higher degrees of melting at lower pressures of mantle that is characterized by ancient HIMU-like isotope systematics. Isabel olivine phenocrysts and their spinel inclusions range to Mg#s that overlap with the peridotite minerals in the accompanying mantle xenoliths, which has not been reported at other Mexican xenolith localities. Olivine-hosted glass inclusions in rapidly quenched scoria yielded the first estimates for volatile contents in Mexican intraplate-type mafic melts; maximum values are: 1.3 wt.% H 2O, 2241 ppm CO 2, 1500 ppm F, 1300 ppm S, and 1200 ppm Cl. Isabel peridotites (Ol ± Hy ± Di + Sp ± Pl) include dunites, harzburgites, and rare plagioclase–harzburgite, and are depleted in clinopyroxene in comparison to other Mexican peridotite localities where lherzolites dominate; Isabel peridotite clinopyroxenes are also depleted in Na 2O compared to other Mexican localities, another reflection of their refractory nature. Pressure estimates for Isabel xenoliths based on olivine–clinopyroxene Ca exchange indicate that they equilibrated at relatively low pressures (5.9–15.5 kb) compared to other Mexican peridotites (~ 10–20 kb), which is consistent with the presence of a single plagioclase–harzburgite xenolith. The refractory character of the Isabel xenoliths is consistent with the results of the Alarcon segment of the PESCADOR seismic experiment, and Lizarralde et al.'s [Lizarralde, D., Axen, G.J., Brown, H.E., Fletcher, J.M., Gonzáles-Fernández, A., Harding, A.J., Holbrook, W.S., Kent, G.M., Paramo, P., Sutherland, F., Umhoefer, P.J., 2007. Variation in styles of rifting in the Gulf of California. Nature 448: 466–469] hypothesis that wide, magma-poor rifted margins result from rifting of depleted mantle.

Nanoinclusions in microdiamonds from Neogenic sands of the Ukraine (Samotkan’ placer): A TEM study

Mineral and fluid inclusions in seven microdiamonds (ca. 0.2 mm in diameter) from the Neogene, Samotkan’ placer of the Ukrainian shield were investigated by TEM. Various types of submicron- or nanometre-sized inclusions such as olivine and orthopyroxene, Fe,Sn oxide and assemblages of mineral multi-phases were observed.In an octahedral microdiamond crystal numerous olivine and enstatite micro- and nanocrystals were observed. Their composition is close to that of other olivine and enstatite inclusions found in kimberlitic diamonds worldwide. An unusual feature of these minerals in the samples studied, however, is a relatively high NiO content: 0.50–0.80 wt.% in olivine (Fo91.3–92.1) and 0.40–0.70 wt.% in enstatite (Fo94.1–95.4). These peridotitic mineral associations represent the host (possibly lherzolitic mantle) in which the diamonds grew. The mean Mg# values of olivine and enstatite inclusions in Samotkan’s microdiamond are, respectively, 91.43 and 94.83, which are closer to Mg#-values of these minerals in lherzolite, rather than in harzburgite assemblages (included in diamond). In addition, enstatite–clinoenstatite, K-richterite, graphite, and Fe,Sn oxide were also identified as nanoinclusions in Samotkan’ diamond; the Fe,Sn oxides and magnetite were found in two microdiamonds of the transitional {111}+{110} form. The K-richterite and graphite have been attributed to epigenetic inclusions. The origin of Fe,Sn oxide is not clear.Assemblages of multi-phase, minerals were observed in four microdiamonds of variable morphology: octahedron, transitional {111}+{110} form, cube and cube-coated diamond. These phases belong to fluid-bearing inclusions and are enclosed in cavities which, as a rule, are not filled completely by the solid phases. The remainder of the volume is occupied by a fluid that for the most part has been released during sample preparation. In the majority of cases the multi-phase assemblages consist of carbonate, mica, rutile, ilmenite, apatite and sylvite. Carbonates are the most abundant phases in the fluid inclusions; they are calcium-rich carbonates, frequently with admixtures of Fe, Mg and Sr. The mica inclusions are enriched in silicon, with Si varying from ca. 6.7 to 7.0 a.p.f.u. The multi-phase mineral assemblages present in Samotkan’s microdiamond are similar to fluid-bearing, microinclusions found previously in other diamond types (e.g., fibrous diamonds, the fibrous coat of coated diamonds or the internal clouds of octahedral diamonds) from kimberlites of different provinces worldwide. They represent the carbonate-, alkali-, and chlorine-rich mantle fluid composition present during crystallisation of the Samotkan’s microdiamond. Judging from the composition of the multi-phase assemblages it may be assumed that the Samotkan’s microdiamond grew from a carbonatitic to a slightly silicic melt, rich in alkali and volatile components.

Metal-saturated peridotite in the mantle wedge inferred from metal-bearing peridotite xenoliths from Avacha volcano, Kamchatka

Lithospheric mantle is inferred to be more oxidized than the asthenosphere, and mantle-wedge peridotites are characterized by high oxidation state relative to abyssal and continental peridotites due to addition of slab-derived fluids or melts. We found metals (native Ni, Fe silicides, native Fe and possible native Ti) from otherwise oxidized sub-arc mantle peridotite xenoliths from Avacha volcano, Kamchatka. This is contrary to the consensus and experimental results that the metals are stable only in deeper parts of the mantle (> 250 km). The metals from Avacha are different in chemistry and petrography from those in serpentinized peridotites. The Avacha metals are characteristically out of chemical equilibrium between individual grains as well as with surrounding peridotite minerals. This indicates their independent formation from different fluids. Some of the Avacha metals form inclusion trails with fluids and pyroxenes, leading to the inference that very local metal saturation resulted from rapid supply (‘flashing’) of reducing fluids from deeper levels. The fluids, possibly rich in H 2, are formed by serpentinization at the cold base of the mantle wedge just above the slab, and they reduce overlying peridotites. We propose a metal-saturated peridotite layer, underlying the main oxidized portion, within the mantle wedge beneath the volcanic front to fore-arc region.

Metasomatic mantle origin for Mbuji-Mayi and Kundelungu garnet and clinopyroxene megacrysts (Democratic Republic of Congo)

Mbuji-Mayi (east Kasai province) and Kundelungu (Shaba province) are the two kimberlite fields known for a long time in the Democratic Republic of Congo (DRC). Mbuji-Mayi intrudes the Archean basement (Congo–Kasai Craton) and is diamond-rich, whereas Kundelungu cuts across Paleoproterozoic basement (Bangweulu block) and is diamond-poor. The megacryst suites (or discrete nodule suites) of both fields include garnet and clinopyroxene megacrysts. The pyrope-rich megacrysts can be subdivided in three groups on the basis of their Cr contents: low-Cr (0.00–1.79 wt.%Cr 2O 3; Mg #: 72.8–84.0); medium-Cr (1.93–5.16 wt.%Cr 2O 3; Mg #: 76.2–86.3) and high-Cr (5.42–7.10 wt.%Cr 2O 3; Mg #: 79.2–84.6). There are no significant geochemical differences between the garnets from Mbuji-Mayi and from Kundelungu. Polymineral inclusions composed of K-rich hydrated phases (phlogopite and amphibole), fresh glass and Cr-spinels are identified in garnets from all three groups, in both localities, which suggest a common origin. Two groups of diopside megacrysts from Mbuji-Mayi are distinguished on the basis of their Ca content: low-Ca (Ca #: 39.5–42.1; 0.61–0.92 wt.%Cr 2O 3) and medium-Ca (Ca #: 44.1–48.5; 0.41–1.09 wt.%Cr 2O 3); they differ from a third group of high-Cr diopsides (Ca #: 47.1–49.4; 1.31–2.77 wt.%Cr 2O 3). The major element compositions of DRC megacrysts are distinct from those of many other megacryst suites worldwide: the clinopyroxenes are lower in Fe and Ti and higher in Mg and the garnets contain more Cr and significantly less Ti, Fe and Al. These DRC megacryst compositions are intermediate between those of peridotite minerals and those of kimberlite megacrysts from other localities. Most garnets have “normal” REE profiles ((La/Yb) N = 0.003–0.027), whereas clinopyroxenes display relative LREE enrichment ((La/Yb) N = 5.1–43.2). The REE patterns of garnet and clinopyroxene megacrysts are similar to those from metasomatized South African mantle lherzolites. The differences in composition between DRC megacrysts and those from other kimberlites might reflect different modes of formation. Some megacryst suites are related by fractional crystallization processes, the DRC garnet and clinopyroxene megacrysts display geochemical similarities with peridotites and may originate by metasomatic transformation and recrystallization of mantle peridotites.

Significance of serpentinization of wedge mantle peridotites beneath Mariana forearc, western Pacific

In the Mariana forearc, horst and graben structures are well developed in the outer forearc basement, which is composed of both island arc and oceanic crust-mantle rocks. A zone of dome-shaped diapiric seamounts, which are composed mainly of serpentinized peridotites, formed on the basement in the outer forearc regions. Serpentine minerals in peridotites from both diapiric seamounts and basement are mostly chrysotile and/or lizardite. Antigorite, however, is rarely found in peridotites recovered from Conical, Big Blue, Celestial, and South Chamorro Seamounts. Antigorite-bearing peridotites always contain secondary iron-rich olivine and metamorphic clinopyroxene, and antigorite seems to coexist stably with them. Iron-rich secondary olivine (Fo 86–90 ) occurs as overgrowth on the rim or along the cleavage traces of primary olivine (Fo 90–92 ). The assemblage shows high-temperature conditions of serpentinization at ~450–550 °C, whereas chrysotile- and/or lizardite-bearing assemblages occur at ~200–300 °C. In antigorite-bearing samples, chrysotile and/or lizardite veins both predating and postdating antigorite formation are recognized. This may reflect a complex process of tectonic cycling of shallow mantle wedge serpentinized peridotites to depth and then back again to the surface.

A snapshot of mantle metasomatism: Trace element analysis of coexisting fluid (LA-ICP-MS) and silicate (SIMS) inclusions in fibrous diamonds

We have determined the trace element compositions of coexisting fluid (carbonate–K-chloride–H 2O) and single-phase mineral inclusions in peridotitic (Cr-diopside) and eclogitic (omphacite, garnet) inclusions in fibrous diamonds from the Panda kimberlite (Slave craton, Canada). These diamonds provide a unique insight into the nature of the metasomatic agent, the metasomatised minerals and the pre-metasomatic protolith. The fluid component is strongly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE). Co-existing peridotitic minerals record a melt extraction event (high Cr and Ni) in the protolith prior to the influx of the trapped metasomatic fluid. The silicate minerals are also strongly enriched in LREE. Calculated partition coefficients agree with experimentally determined values in the literature, despite the complex composition of the natural fluid. This indicates that the minerals have re-equilibrated with the metasomatic fluid. The trace element compositions of the mineral inclusions are comparable to many equivalent phases in monocrystalline diamonds. This suggests that the metasomatic fluid and the process recorded in these samples may also be responsible for the growth of some types of monocrystalline diamonds.

A new SIMS study of Li, Be, B and δ7Li in mantle xenoliths from Harrat Uwayrid (Saudi Arabia)

Concentrations of Li, Be and B and the isotopic composition of lithium have been measured in minerals from a suite of dry and metasomatised spinel peridotite and pyroxenite xenoliths from Harrat Uwayrid, a Pliocene–Quaternary volcanic field in the NW part of the Arabian peninsula, developed during formation of the Red Sea. In most peridotites, Li concentrations in pyroxene and olivine grains show significant systematic zoning, with a core-to-rim decrease in pyroxene but a minor increase in olivine. δ 7Li tends to show a rim-ward increase in clinopyroxene, while it is constant or decreases slightly in olivine. These variations indicate a diffusive transfer of Li from clinopyroxene to olivine during cooling, implying that the partition coefficients K Li Ol/Cpx and K Li Ol/Opx are temperature-dependent and increase with falling temperature. K Li Cpx/Opx, stays nearly constant at ∼ 1. δ 7Li values measured in virtually unmodified cores of olivine, orthopyroxene and clinopyroxene from three peridotites are similar within individual xenoliths but vary among the xenoliths, with individual values ranging from + 1.1 to + 9.7‰. In marked contrast to Li, the concentrations of Be and B are nearly constant in mineral grains, and apparent inter-mineral partition coefficients for these elements are independent of concentrations and xenolith equilibration temperature. Concentrations of Li, Be and B in minerals of un-metasomatized peridotites are relatively low and are similar to values reported previously. Relative abundances of the three light elements in minerals of xenoliths affected by metasomatism have been used to constrain the nature of metasomatic agents. A preferential enrichment of both Li and B relative to Be that is observed in some xenoliths suggests metasomatism by a hydrous fluid, well below the critical point of the solvus between hydrous fluid and silicate melt. A uniform enrichment of all three light elements shown by other xenoliths points to either a silicate melt at subcritical conditions or a supercritical liquid as metasomatic agent.

Chemical composition and ultrahigh-P metamorphism of garnet peridotites from the Sulu UHP terrane, China: Investigation of major, trace elements and Hf isotopes of minerals

The Zhimafang peridotite body recovered by a pre-pilot borehole (PP1) of the Chinese Continental Scientific Drilling Project occurs in gneiss from the southern Sulu ultrahigh-P (UHP) terrane, east-central China. Detailed petrographic observation of the freshest lherzolites reveals the existence of pre-peak mantle relics, rare enstatite inclusions (Opx1) in garnet, and coarse-grained diopside (Cpx1) in the peridotites. Matrix phases Ol + Grt + Opx + Cpx ± Phl represent the assemblage of the peak-stage. At least > 1.5 GPa pressure increase is recognized from the mantle setting to the peak-stage of subduction-zone UHP metamorphism at ~ 800 °C and 6.8 GPa for the peridotites. All peridotitic minerals have high Mg# (Ol, 91–93, Cpx, 93–94 and Opx, 92–94). Garnets are depleted in LREE, and have increased abundances from Pr through the MREE to a plateau of heavy REE (HREE) at about 10 times chondrite. In contrast, clinopyroxenes display humped REE patterns with high (La/Yb) N values of 27–68. Phlogopite is a major carrier of Ba (5205 ppm) and Th (5.5 ppm). Partition coefficients ( D Cpx/Grt) for REE between clinopyroxene and garnet decrease from LREE (~ 100) to HREE (< 1 but > 0.1), suggesting that clinopyroxene and garnet equilibrated at low- T (< 1100 °C). Zircons from three phlogopite and/or magnesite-bearing peridotites show very low 176Lu/ 177Hf values (between 10 − 3 and 10 − 6 ), and a wide range in 176Hf/ 177Hf ratios (0.28221–0.28250). The zircons with low 176Hf/ 177Hf and relatively high 176Lu/ 177Hf document that metasomatism affected the peridotite protolith. The model ages ( T DM) of zircons mostly cluster around 1.3–1.4 Ga, which probably reflects the metasomatic source. These chemical data combined with previous published bulk-rock major and trace element data indicate that the Zhimafang peridotites probably represent depleted and metasomatized subcontinental lithospheric mantle (SCLM) beneath the Sino-Korean craton. Investigation of the Sulu UHP metamorphic garnet peridotites provides additional insights into the Earth's geochemical and tectonic evolution, and enlarges our understanding of the exhumation of orogenic peridotites.

An integrated study of microstructural, geochemical, and seismic properties of the lithospheric mantle above the Kerguelen plume (Indian Ocean)

Peridotite xenoliths brought up to the surface by the volcanism of the Kerguelen Islands represent a mantle that has been affected by a high degree of partial melting followed by intense melt percolation above the Kerguelen plume. These xenoliths are therefore particularly suitable to investigate effects of melt‐rock interaction on crystallographic fabrics (lattice‐preferred orientation (LPO)) of peridotite minerals and on the LPO‐induced seismic properties of peridotites above a mantle plume. We have studied a suite of 16 ultramafic samples representative of different degrees of partial melting and magma‐rock interaction among which the protogranular harzburgites are the least metasomatised xenoliths and dunites are the ultimate stage of metasomatism. Olivine LPO is characterized by high concentration of [010] axes perpendicular to the foliation and [100] axes close to the lineation or distributed in the foliation plane in harzburgites, whereas the high concentration of [100] axes is parallel to the lineation and [010] axes is perpendicular to the assumed foliation in dunites. Olivine LPO in harzburgites is interpreted as being due to a deformation regime in axial compression or transpression. The fabric strength of olivine decreases progressively from protogranular to poikilitic harzburgites and finally to dunites, for which it remains nevertheless significant (J index 3.8). Seismic properties calculated from LPO of minerals indicate that metasomatism at higher melt/rock ratio lowers the P wave velocities. The most significant difference between harzburgites and dunites corresponds to the distribution of S wave anisotropy. Harzburgites display the maximum of anisotropy within the foliation plane and the minimum of anisotropy perpendicular to the foliation plane, whereas the lowest anisotropy is parallel to the lineation for dunites. These modifications of seismic properties as a result of metasomatic processes may induce seismic heterogeneities in the mantle above the Kerguelen plume. In addition, assuming a lithospheric mantle primarily harzburgitic and structured with a horizontal foliation, the seismic properties calculated for the Kerguelen xenoliths reconcile the rather high anisotropy evidenced by the horizontally propagating surface waves with the apparent isotropy revealed by the absence of splitting of vertically propagating teleseismic SKS waves recorded by the GEOSCOPE Kerguelen station.

Origin of Pyroxenite-Peridotite Veined Mantle by Refertilization Reactions: Evidence from the Ronda Peridotite (Southern Spain)

The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclase^spinel websterites. The ‘’ part of the Ronda peridotite contains abundant layered websterites (‘group C’pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitionstowardstheirhost peridotites. Previousstudieshave suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modellingstudy ofa layered outcrop ofgroup C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization melt^ rock reactions. Mg-number [¼ Mg/(Mg þ Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (� 0� 65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (40� 1and41, respectively) to balance Mg^Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front.The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convexupward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)N vs (Sm/Yb)N diagram (where N indicates chondrite normalized). Based on (Ce/Nd)N values, two transient, enriched domains between the light REE (LREE)depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxenite^peridotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions.The latterare probably volatile-rich, small-volume melt fractions residual after the refertilization reactionsthatgeneratedgroup C pyroxenites, which migrated throughout the massifcincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the ‘layered granular’subdomain and ‘plagioclase-tectonite’domain) are on average morefertilethan the residual,‘coarsegranular’subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.

EVOLUTION OF RODINGITIC DYKES: METASOMATISM AND METAMORPHISM IN THE MOUNT AVIC SERPENTINITES (ALPINE OPHIOLITES, SOUTHERN AOSTA VALLEY)

The Mount Avic ophiolite mainly consists of a large mass of serpentinites, which constitutes the base of a subunit of the Piemonte ophiolitic nappe. Serpentinites represent the mantle portion of the oceanic lithosphere of the Mesozoic Tethys and consist of antigorite-titanian clinohumite-diopside schists as products of oceanic metasomatism and tectono-metamorphic evolution of the Alpine orogeny, at the expense of abyssal peridotite mineral assemblage. The serpentinite mass includes metagabbro pods (without metasomatic alteration) and associated rodingitic dykes. Various rodingitic dykes can be distinguished on the basis of their mineralogical assemblages. The main assemblage consists of ugranditic garnet, chlorite ± diopside ± epidote ± vesuvianite. We observed also some other peculiar rodingites such as vesuvianite-chlorite-, diopside-epidote-chlorite-, and diopside-chlorite-bearing rodingites, as distinctive of the Mount Avic massif, as well as rodingitic reaction zones and foliated rodingites with chlorite, diopside, or epidote. These mineral assemblages are strictly related to the chemistry of the protolith (probably mafic dykes within serpentinite), as well as to the oceanic rodingitization during the serpentinization event and to the Alpine evolution that affected the Mount Avic massif.