Search

This study reports the first geochemical and Pb isotopic data for mantle xenoliths from beneath the Neoproterozoic Ribeira Belt, southeastern Brazil. The cm-sized spinel peridotite xenoliths are hosted by a Cretaceous lamprophyre dike that intruded high-grade metamorphic rocks. Major- and trace-element compositions of the main minerals indicate that the xenoliths derive from a shallow fertile mantle that has undergone a low degree of melt extraction (2–9% partial melting). On the basis of modeled isochemical phase diagrams for lherzolites, pressure and temperature conditions are inferred to vary from 1300–1350 °C and 17–19 kbar (fertile composition) to 1330–1430 °C and 17–23 kbar (relatively depleted composition), which correspond to high geothermal gradients of 65–80 mW/m2. Temperatures of last equilibration calculated based on the average REE content of pyroxenes for the same lherzolite samples vary from 1233 ± 56 °C to 1085 ± 42 °C, while conventional thermometry (TBKN) yields average values of 807 and 755 °C, indicating re-equilibration at lower temperatures. Pb isotope ratios of clinopyroxene define a mixing line that intercepts the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 200 Ma. Linear regressions yield two errorchrons of 56 ± 75 Ma and 571 ± 99 Ma (95% confidence level). These results combined with the ages and tectonic settings of host rocks are suggestive of an overprint of a younger tectono-thermal event, most likely related to the opening of the South Atlantic Ocean, over a mantle previously equilibrated during the Precambrian development of the Ribeira Belt. [ABSTRACT FROM AUTHOR]

This study presents new secondary ion mass spectrometry (SIMS) reference materials (RMs) for measuring water contents in nominally anhydrous orthopyroxenes from upper mantle peridotites. The enstatitic reference orthopyroxenes from spinel peridotite xenoliths have Mg#s between 0.83 and 0.86, Al2O3 ranges between 4.02 and 5.56 wt%, and Cr2O3 ranges between 0.21 and 0.69 wt%. Based on Fourier-transform infrared spectroscopy (FTIR) characterizations, the water contents of the eleven reference orthopyroxenes vary from dry to 249 ± 6 µg/g H2O. Using these reference grains, a set of orthopyroxene samples obtained from variably altered abyssal spinel peridotites from the Atlantic and Arctic Ridges as well as from the Izu-Bonin-Mariana forearc region was analyzed by SIMS and FTIR regarding their incorporation of water. The major element composition of the sample orthopyroxenes is typical of spinel peridotites from the upper mantle, characterized by Mg#s between 0.90 and 0.92, Al2O3 between 1.66 and 5.34 wt%, and Cr2O3 between 0.62 and 0.96 wt%. Water contents as measured by SIMS range from 68 ± 7 to 261 ± 11 µg/g H2O and correlate well with Al2O3 contents (r = 0.80) and Cr#s (r. = -0.89). We also describe in detail an optimized strategy, employing both SIMS and FTIR, for quantifying structural water in highly altered samples such as abyssal peridotite. This approach first analyzes individual oriented grains by polarized FTIR, which provides an overview of alteration. Subsequently, the same grain along with others of the same sample is measured using SIMS, thereby gaining information about homogeneity at the hand sample scale, which is key for understanding the geological history of these rocks. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Spinel peridotites from Ia Bang reveal variations in lherzolites and harzburgites. • The peridotites have undergone 1–20% fractional melting and cryptic metasomatism. • The equilibrium temperatures of the peridotites range from 841 °C to 1131 °C. • The peridotites are present as the Proterozoic harzburgites and the Phanerozoic lherzolites. Abstract Mantle xenoliths are scattered in alkali basalt in central and southern Vietnam. The xenoliths from Ia Bang, Pleiku plateau, consist of fertile spinel lherzolites and refractory spinel harzburgites. The spinel peridotites display varying textures from protogranular through porphyroclastic to equigranular. Primitive mantle-normalized rare earth elements (REE) of clinopyroxene in the peridotites reveal depleted, spoon-shaped, and enriched patterns, suggesting a depletion event followed by various enrichment processes. The correlation between (Yb)n and (Y)n of clinopyroxene in spinel peridotites indicates that the subcontinental lithospheric mantle (SCLM) beneath Ia Bang has been affected by 1–20% fractional melting. The presence of glass and secondary clinopyroxene, coupled with the enrichment of light REEs (LREEs), high field strength elements (HFSEs), and large ion lithophile elements (LILEs) without introductions of new minerals suggests that the xenoliths underwent cryptic metasomatism. Metasomatic agents were identified as Na-alkali silicate, carbonate-rich melts, and H 2 O-CO 2 fluids. The peridotites were at equilibrium temperatures of 841–1131 °C before being brought up by alkali basalt from depths of 36 to 50 km. The SCLM beneath Ia Bang experienced a thinning process in which the refractory lithosphere was thinned and replaced by asthenosphere during the Phanerozoic, following the asthenosphere-lithospheric interaction. [ABSTRACT FROM AUTHOR]

Our study of Colorado Plateau and Rio Grande rift spinel peridotite xenoliths determined bulk Lu–Hf budgets and cpx, opx, and whole rock 176 Hf/ 177 Hf to evaluate the potential age significance of Lu/Hf–Hf isotope correlations in sub-continental lithospheric mantle (SCLM) derived xenoliths. The samples have fertilities (spinel Cr# range from 0.1 to 0.5), and equilibration temperatures (950–1050 °C) that overlap with those of spinel peridotites commonly used in Lu–Hf dating studies. The Lu/Hf of clinopyroxene (cpx) and its associated whole rock (WR) are similar in fertile samples, but cpx has a lower Lu/Hf than that of the WR in refractory samples. Orthopyroxene (opx) has systematically higher Lu/Hf than cpx, but in many samples, the opx and cpx have identical 176 Hf/ 177 Hf, suggesting that the Hf isotope composition of these minerals is supported by the Lu/Hf of the WR rather than that of the individual minerals. Many opx fractions and whole rocks have less radiogenic 176 Hf/ 177 Hf than their corresponding cpx (but similar to that of the host magma), reflecting contamination. Because the 176 Hf/ 177 Hf of cpx is supported by the WR Lu/Hf, it is necessary to use the latter rather than the cpx Lu/Hf for isochron dating. However, because the actual WR Lu/Hf values are susceptible to secondary overprinting, we recommend using model WR Lu/Hf values (determined from mineral Lu and Hf concentrations and modal abundances) to construct external cpx isochrons. Model WR and measured cpx 176 Hf/ 177 Hf are correlated (r 2 = 0.98) in a suite of eastern Colorado Plateau xenoliths and yield an apparent age of 1.2 Ga (versus 1.5 Ga when – incorrectly – using cpx Lu/Hf). Despite the good correlation between Lu/Hf and 176 Hf/ 177 Hf, we do not find correlations between indicators of melt depletion and Lu/Hf or 176 Hf/ 177 Hf. This suggests that Lu/Hf– 176 Hf/ 177 Hf correlations are not the result of melt depletion. Instead, we propose that Lu/Hf– 176 Hf/ 177 Hf correlations are the result of mixing between depleted and enriched components (e.g. metasomatism of depleted lithospheric mantle). We emphasize that additional data should be utilized (e.g. whole rock Lu–Hf budget, degree of metasomatism, opx 176 Hf/ 177 Hf) when interpreting apparent Lu–Hf isochrons for age significance. [ABSTRACT FROM AUTHOR]

Clinopyroxenes in some fresh anhydrous spinel peridotite mantle xenoliths from the northern Massif Central (France) and Lower Silesia (Poland), analysed for a range of incompatible trace elements by laser ablation inductively coupled plasma mass spectrometry, show unusually strong negative anomalies in Hf and Zr relative to adjacent elements Sm and Nd, on primitive mantle-normalised diagrams. Similar Zr-Hf anomalies have only rarely been reported from clinopyroxene in spinel peridotite mantle xenoliths worldwide, and most are not as strong as the examples reported here. Low Hf contents give rise to a wide range of Lu/Hf ratios, which over geological time would result in highly radiogenic εHf values, decoupling them from εNd ratios. The high Lu/Hf could in theory produce an isochronous relationship with Hf/Hf over time; an errorchron is shown by clinopyroxene from mantle xenoliths from the northern Massif Central. However, in a review of the literature, we show that most mantle spinel peridotites do not show such high Lu/Hf ratios in their constituent clinopyroxenes, because they lack the distinctive Zr-Hf anomaly, and this limits the usefulness of the application of the Lu-Hf system of dating to garnet-free mantle rocks. Nevertheless, some mantle xenoliths from Poland or the Czech Republic may be amenable to Hf-isotope dating in the future. [ABSTRACT FROM AUTHOR]

Traces of water in mantle minerals strongly influence mantle melting and viscosity that, in turn, governs large-scale processes like mantle convection, plate tectonics, and the stabilization of cratons. One way of estimating the mantle's water content is by analyzing mantle xenoliths brought to the Earth's surface. A major problem in estimating the mantle's water budget from xenoliths arises from decompression-induced water loss during uplift. Mantle-derived xenoliths from numerous occurrences worldwide have been investigated with respect to water. However, little is known about water in the mantle beneath most parts of Europe and Asia. This study presents water contents for mantle olivine from Germany, Austria, Mongolia, and Nigeria and suggests a possibility to assess water loss. It also addresses the question whether or not water contents are related to olivine composition and/or the presence of coexisting amphibole. The highest water concentrations are present in olivine from the Eifel, Germany (up to 21 ppm H2O), whereas the Fichtelgebirge xenoliths, Germany, reveal the lowest contents (<1 ppm). All water-bearing olivines show three dominant infrared absorption bands: bands at 3572, 3525, and 3560 cm-1 and weaker bands at 3485, 3542, and 3597 cm-1. The peaks at 3572 and 3525 cm-1 are ascribed to Ti-related substitutions of H. Additional peaks, related to H substitutions involving trivalent cations, occur in the 3300-3400 cm-1 range. However, their intensity does not correlate with the content of trivalent cations. The olivines show a pronounced correlation of Al + Cr and Ca implying that incorporation of Al and Cr is governed by pressure and temperature and primarily attributed to Tschermak's substitution. This study confirms that a coupled substitution involving Ti is the most important mode of water storage in shallow upper mantle olivine. The Ti content and the fraction of water bound to "Ti defects" are related and support the substitution model Ti4+ + 2H+ = Mg2+ + Si4+. Hence, the Ti content is a useful proxy to estimate the maximum amount of water incorporated by this substitution--providing a tool to approximate the degree of water loss. Five of the investigated olivines provide evidence for no or very little water loss. Their water content of 16-21 ppm is presumably typical for the depleted uppermost mantle. Twelve samples with <1-15 ppm may have lost between 36 and >80% of their original water. Olivine from amphibole-bearing spinel peridotite has relatively low water contents as well as low amounts of Ca, Al, Cr, Ti, and V. Particularly low Ca, Al, and Cr contents suggest fluid infiltration, amphibole formation, and re-equilibration of the whole assemblage at comparably low pressure and/or temperature and may explain the low water content of olivine. Infrared spectra with dominant peaks in the 3200-3300 cm-1 range (spectrum type E) are confined to olivine from amphibole-bearing peridotite. [ABSTRACT FROM AUTHOR]

Abstract: We determined the Lu–Hf and Sm–Nd isotope compositions of spinel peridotite xenoliths in alkali basalts from Baengnyeong and Jeju islands, South Korea, to constrain the timing of melt-depletion events and stabilization of the lithospheric mantle beneath the region. Equilibration temperatures estimated by two-pyroxene thermometry range from 780 to 950°C, and from 960 to 1010°C for Baengnyeong and Jeju peridotites, respectively. The Baengnyeong peridotite clinopyroxenes are characterized by extremely radiogenic Hf in association with isotopically less extreme Nd, resulting in strong Nd–Hf decoupling referenced to the mantle array. This is in stark contrast to the well-correlated isotopic compositions of Hf and Nd in the Jeju peridotite clinopyroxenes, which plot along the Nd–Hf mantle array. The Hf abundances and isotopic compositions of the Baengnyeong clinopyroxenes were less affected by relatively recent secondary enrichments that overprinted the light rare earth element abundances and Nd isotopes, causing the decoupling of the Nd–Hf isotopes. The Nd–Hf isotopic compositions of the Jeju peridotites appear to have been re-equilibrated, probably as a result of efficient diffusion at the relatively higher temperatures of the Jeju peridotites. Lu–Hf tie lines for clinopyroxene and orthopyroxene from four of the Korean peridotites have negative slopes on a Lu–Hf isochron diagram, yielding negative ages. This is interpreted to indicate recent isotopic exchange in orthopyroxene by reaction with metasomatic agents having low 176Hf/177Hf components. Secondary overprinting in orthopyroxene was facilitated by the considerably lower Hf concentrations than in co-located clinopyroxene. Baengnyeong lherzolite clinopyroxenes yield a Lu–Hf errorchron age of 1.9±0.1Ga, which is independently supported by a model Os age (based on Re depletion or TRD) of 1.8Ga on a refractory Baengnyeong peridotite. We interpret this age range to mark the time of stabilization of the mantle section beneath this area by major melt extraction. This Proterozoic melt removal coincided in time with widespread ca. 2.1 to 1.8Ga tectonothermal events documented throughout the Korean Peninsula. These observations indicate that the Lu–Hf system can be used to date mantle melting events recorded in lherzolite xenoliths. [Copyright &y& Elsevier]

Abstract: We present new data on spinel peridotite xenoliths hosted in Agardag alkaline lamprophyres from the Sangilen Plateau (Tuva, South Siberia, Russia), sampling at ~450Ma the subcontinental lithospheric mantle of the Tuva-Mongolian micro-continent that belongs to the accretionary Central Asian orogenic belt at the southern edge of the Siberian craton. Xenoliths are spinel lherzolites principally showing poikilitic and subordinately coarse granular and coarse equigranular textures. Geothermobarometric calculations for pyroxene yield a narrow range of equilibration temperature (ca. 1000–1100°C) that corresponds to lithospheric depths from 43 to 53km (1.3–1.6GPa) along a hot intracontinental geotherm. Variation of mean Mg# [100*Mg/(Mg+Fe)] of olivine (87.9–90.9) with mean Cr# [100*Cr/(Cr+Al)] of spinel (9.5–45.7) indicates that spinel lherzolites are mostly residues of up to 10% melting of a fertile peridotite source. In terms of normalized REE (Rare Earth Element) and incompatible trace element patterns of clinopyroxene, the Sangilen xenoliths can be classified into three types: Type I characterized by convex-upward REE patterns depleted in LREE (0.10≤La/YbN ≤0.49), and with relative negative anomalies of Rb, Pb, Hf, Zr and Ti and positive spikes of U and Sr; Type II displaying variable LREE/HREE ratios (0.53

Abstract: Orthopyroxene in oceanic spinel peridotite from the Mid-Atlantic Ridge near 15°N, ODP-Leg 209, contains about 15wt.-ppm water that is bound as hydroxyl in the mineral structure. This value is one order of magnitude lower than the water contents reported for (i) orthopyroxene in peridotite from the Mid-Atlantic Ridge at 23°N (270wt.-ppm), ODP-Leg 153, and (ii) sub-continental spinel peridotite xenoliths. Orthopyroxene from both latter occurrences contains the maximum amount of water that can be incorporated in enstatitic orthopyroxene at pressures of the spinel peridotite facies (c. 200–300wt.-ppm). The water contents in orthopyroxene from the sub-oceanic mantle at the Mid-Atlantic Ridge, ranging from about 15 (=highly depleted) to nearly 300wt.-ppm (=water saturation in orthopyroxene), compare well with the highly variable water contents of Atlantic MORB glass that also scatter over one order of magnitude. Mineral trace element data indicate different degrees of partial melting: 18% for peridotite samples of Leg 209 and 12% for those from Leg 153. The mantle equilibrium temperatures differ by 200°C (Leg 209: 1150–1200°C, Leg 153: 950–1000°C). The orthopyroxene water contents of Leg 209 are compatible with residual values after c. 18% partial melting of a peridotitic source with a water content of c. 120–130wt.-ppm in the bulk rock and of c. 300wt.-ppm in the original orthopyroxene, respectively. In contrast, the water content of 270wt.-ppm in orthopyroxene of Leg 153 peridotite cannot be reconciled with residual amounts to be expected after 12% partial melting. This is because an unrealistically high initial water content of c. 3500wt.-ppm would be required for the original orthopyroxene — exceeding the water storage capacity of orthopyroxene at an ambient pressure of c. 20kbar by one order of magnitude. We conclude that – following melt depletion – the water contents of Leg 153 peridotite re-equilibrated in the spinel-peridotite facies. This may have been facilitated by a longer mantle residence time of Leg 153 peridotite (compared to Leg 209) after partial melting and prior to tectonic exhumation from c. 60km depth. An extended mantle residence time is compatible with isobaric cooling from >1200°C to 950–1000°C (at c. 20kbar pressure) as documented by Leg 153 peridotite, and it facilitates re-introduction of water from less or non-depleted mantle regions due to the high diffusivity of H. [Copyright &y& Elsevier]

Abstract: Serpentinites from the Hero Fracture Zone, Drake Passage, Antarctica, contain serpentine, amphibole, talc, chlorite, and magnetite as well as relict olivine, pyroxene, and spinel from a spinel peridotite precursor. Two episodes of serpentinization are recognized. The first event produced lizardite and antigorite, whereas the second produced only lizardite. The initial and subsequent serpentinization events occurred at about 178–283°C and 170–298°C, respectively. Calculated water/rock ratios of the serpentinization events are less than 5 for the initial event and greater than 5 for the subsequent event. A model involving deep seawater infiltration in the fracture zone, followed by serpentinization of mantle peridotite and buoyancy-driven uplift controlled by relatively small-scale faults and not by a large-scale detachment fault, is invoked to explain the emplacement of serpentinite on the surface. [Copyright &y& Elsevier]

The Mohelno peridotite is a medium-sized ultramafic body (2 km × 4 km in size) enclosed in the Gföhl granulites in the eastern part of the Bohemian Massif. It consists mainly of coarse spinel peridotite (harzburgite and dunite); garnet peridotite occurs only in the sheared and deformed margins of the body. To decipher the origin and history of this mantle-derived peridotite, we determined the mineral chemistry by electron microprobe analysis and olivine fabric patterns by the electron backscattered diffraction method for each rock type. We found two distinct types of olivine fabric (crystal-preferred orientation; CPO) in the peridotite, which can be correlated with the mineralogy and thermal history of each. The olivine CPO in coarse-grained spinel peridotite shows a strong concentration of [100] slightly oblique to the lineation and [010] and [001] girdles normal to the lineation (which is the so-called {0kl}[100] pattern typical of medium-temperature deformation). Olivine in coarse-grained garnet peridotite, on the other hand, shows a strong concentration of [010] normal to the foliation and a concentration of [100] parallel to the lineation (which is the so-called (010)[100] pattern typical of high-temperature deformation). These fabric patterns become diffuse as the grain size is reduced for each mineralogical type. We interpret the development of these contrasting fabric patterns and mineralogical types based on the pressure–temperature history of each rock type determined by applying published geothermometers and geobarometers to the constituent minerals. Starting from a high-temperature (>1200°C) spinel peridotite, during exhumation and cooling in contact with surrounding granulites, the marginal part of the body was transformed to garnet peridotite, whereas the interior remained in the spinel-peridotite facies because cooling was slower inside the body. Because of the slow cooling and continuous deformation in the interior of the body, the original high-temperature fabric pattern in the spinel peridotite was converted to a lower-temperature type. The high-temperature fabric was preserved only at the margin of the body where cooling was more rapid. Reduction of grain size that occurred during later, low-temperature, deformation partly obliterated the high-temperature fabric patterns for both garnet and spinel peridotites. The initial rapid cooling at high temperatures associated with deformation probably occurred after the mantle peridotite was emplaced within the crustal granulites, which implies that the spinel- to garnet-peridotite transformation took place in the continental crust. [ABSTRACT FROM PUBLISHER]

We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80–100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ∊Hf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ∊Hf=65, at nearly constant 143Nd/144Nd ratios (∊Nd=7–8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd–Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80–100 Ma depleted oceanic lithosphere by melts products of extensive mantle–melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere. [Copyright &y& Elsevier]