Peridotite Layer Research Articles

ПРИРОДА КРИСТАЛЛОМОРФОЛОГИИ АЛМАЗОВ В КИМБЕРЛИТАХ

The obtained evidences indicates that kimberlites were formed from high-pressure (8 GPa) residual liquors of the fractionated peridotitic layer of magmatic ocean. Diamonds originated as a result of an increase in the concentration of carbon due to a decrease in the amount of residual liquor during the crystallization of magmas. Due to the still high temperature of the upper parts of the early magmatic ocean, the cooling and crystallization of its bottom layer at first occurred very slowly and for a long time. This caused the formation of giant diamonds and explains their predominantly octahedral cut.

Origin of large diamonds in kimberlite

The evidence of the existence and fractionation of the global magmatic ocean on the Earth allows us to distinguish two stages of the formation of diamonds of increased size. The largest giant diamonds arose at an early stage of crystallization and fractionation of the peridotite layer of the magmatic ocean, since its bottom layer at that time was blocked by still hot upper ones and therefore cooled and crystallized very slowly and long. The mafic composition of this layer and the low viscosity of its magmas caused mainly octahedral cutting of the formed giant diamonds. Later crystallized mainly rhombododecahedral diamonds have increased coarseness due to the formation mainly by the growth of early octahedral crystals.

The microstructure of layered ultramafic cumulates: Case study of the Bear Creek intrusion, Trinity ophiolite, California, USA

In the Trinity ophiolite, California, USA, several mafic-ultramafic plutons intruded a peridotitic host 435 to 405 m.y. ago in a tectonic setting interpreted as an arc-related spreading centre. One of these intrusions, in the Bear Creek area, exposes basal ultramafic cumulates with igneous layering comprising an alternation of uncommonly thin (down to a few mm) layers of dunite, peridotite and pyroxenite that might be specific to this tectonic setting. These layers offer an excellent opportunity to characterise the microstructure of uncommon cumulates from the lower crust using EBSD (Electron Backscatter Diffraction). This “high-frequency” layering rests on underlying lherzolites and grades upward to more massive pyroxenites (i.e. clinopyroxenites with minor olivine-rich layers). Our field observations and data from the Bear Creek cumulates together with the preservation of magmatic features suggest the environment was tectonically stable after the emplacement of the cumulates. A detailed microstructural investigation of all minerals from the Bear Creek cumulates allows us to decipher their magmatic and plastic deformation history. In a structural reference frame defined by the compositional layering and the elongation direction of the surrounding host peridotites, olivine in the cumulates presents a [010]-fibre fabric and rarely a [001](010) fabric. Clinopyroxene shows a concentration of [010] axes normal to the layering plane with [100] and [001] defining girdles. Orthopyroxene mostly has a fabric with [100] and/or [010] subnormal to the layering plane and [001] scattered along a girdle in the plane of layering. All minerals show a strong fabric. We interpret the formation of the developed planar microstructures as a result of magmatic processes, with high contribution of crystal settling. To a lesser extent, compaction could have been operating and may be linked to the rare evidence of plastic deformation. Clusters of axes within the girdles of olivine and pyroxene CPOs preferentially appear close to the direction of elongation of the surrounding peridotites (i.e. N115°). EBSD analysis of the shape-preferred orientation of Bear Creek's minerals revealed a preferential alignment of the olivine and cpx long axis with the N°115 direction. This magmatic lineation and the preferred direction in the CPOs girdles are both consistent with the stretching lineation acquired during solid-state deformation by the mantle peridotite of the Trinity ophiolite. We suggest that a weak magma flux early on and/or an ongoing but limited regional stress could be responsible for these clusters. Although a direct coupling between asthenospheric flow and magmatic flow cannot be invoked in this context of melt intrusion in the lithosphere, this result highlights that the stress field applied on the mantle could have been still active and similar during the formation of Bear Creek intrusion.Our new field and microstructural data, together with previously presented petrological data, fit a scenario for the evolution of the Trinity ophiolite in which a mantle segment was intruded by a single large batch of primitive boninitic-andesitic melt. Our results emphasise the importance of considering the initial magmatic microstructures and the original shape anisotropy when investigating later deformation in ultramafic rocks.

Nature of the Features of Kimberlite Placement

The obtained evidence of hot heterogeneous accretion of the Earth leads to a fundamentally new solution to genetic problems. According to these data, the kimberlites were formed as a result of the rise of the last residual melts of the bottom peridotite layer of the magmatic ocean, which arose as a result of impact heat release during accretion. The diamond crystallized due to the accumulation of carbon in the residual melts during fractionation. The absence of kimberlites in oceanic and collision regions is due to the expansion of the fractionation products of the magmatic ocean by surfaced mantle plumes during the formation of these regions. The all-earth distribution of the magmatic ocean explains the presence of kimberlites on all the studied ancient platforms. A very high degree of crystallization of the peridotite layer is the reason for the small volume of kimberlite residual melts and the bodies formed by them. The low temperature of kimberlite magmas caused their decompression solidification after boiling at the shallow stage of ascent and explosion under the influence of the high pressure of the fluid phase preserved by solidification. This is the reason for the formation of kimberlite pipes and the absence of kimberlite lavas.

Природа парадоксальных особенностей кимберлитов

Based on the obtained evidence of hot heterogeneous accretion of the Earth and the calculated quantitative model of kimberlite magmas, the nature of the main features of kimberlites is explained. Their presence only within ancient cratons is conditioned by the crystallization of the magma ocean. The formation of kimberlites from residual melts in the bottom peridotite layer resulted in the enrichment of kimberlites with melt-like components, the presence of diamonds in them, the small volume of bodies and the scattered distribution. Decompression solidification of magmas at the shallow stage of ascent is the cause of the explosion under the influence of the preserved pressure of the gas phase and the absence of kimberlite lavas.

Subduction sediment-lherzolite interaction at 2.9 GPa: effects of metasomatism and partial melting

We present the results of analogue experiments carried out in a piston–cylinder apparatus at 750–900°C and 2.9 GPa aimed to simulate metasomatic transformation of the fertile mantle caused by fluids and melts released from the subducting sediment. A synthetic H2O- and CO2-bearing mixture that corresponds to the average subducting sediment (GLOSS, Plank, Langmuir, 1998) and mineral fractions of natural lherzolite (analogue of a mantle wedge) were used as starting materials. Experiments demonstrate that the mineral growth in capsules is controlled by ascending fluid and hydrous melt (from 850°C) flows. Migration of the liquids and dissolved components develops three horizontal zones in the sedimentary layer with different mineral parageneses that slightly changed from run to run. In the general case, however, the contents of omphacite and garnet increase towards the upper boundary of the layer. Magnesite and omphacite (± garnet ± melt ± kyanite ± phengite) are widespread in the central zone of the sedimentary layer, whereas SiO2 polymorph (± kyanite ± phengite ± biotite ± omphacite ± melt) occurs in the lower zone. Clinopyroxene disappears at the base of lherzolite layer and the initial olivine is partially replaced by orthopyroxene (± magnesite) in all experiments. In addition, talc is formed in this zone at 750°C, whereas melt appears at 850°C. In the remaining volume of the lherzolite layer, metasomatic transformations affect only grain boundaries where orthopyroxene (± melt ± carbonate) is developed. The described transformations are mainly related to a pervasive flow of liquids. Mineral growth in the narrow wall sides of the capsules is probably caused by a focused flow: omphacite grows up in the sedimentary layer, and talc or omphacite with the melt grow up in the lherzolite layer. Experiments show that metasomatism of peridotite related to a subducting sediment, unlike the metasomatism related to metabasites, does not lead to the formation of garnet-bearing paragenesis. In addition, uprising liquid flows (fluid, melt) do not remove significant amounts of carbon from the metasedimentary layer to the peridotite layer. It is assumed that either more powerful fluxes of aqueous fluid or migration of carbonate-bearing rocks in subduction melanges are necessary for more efficient transfer of crustal carbon from metasediments to a mantle in subduction zones.

Природа разнообразия алмазов в кимберлитах

The one-way evolution of inclusions in diamonds and its ultra-long duration (more than 2.8 Ga) provide evidence that the crystallization of diamonds took place from the fractionated peridotitic layer in the global magmatic ocean. Diamonds were formed as a result of accumulation of carbon in residual kimberlitic melts. The increment of SiO 2 concentration in the melts caused increase in its viscosity, decrease of diffusion velocity and degree of carbon oversaturation. These resulted that the layered diamond crystallization mode was changed by the radial one, diamond octahedrons transformed into rhombic dodecahedrons and cubes, varied sculptures were created on diamond crystals faces.

Three types of element fluxes from metabasite into peridotite in analogue experiments: Insights into subduction-zone processes

Piston-cylinder experiments with natural rocks and mineral separates were carried out at 750–900 °C and 2.9 GPa, conditions relevant to hot subduction zones, to study the mechanisms of metasomatic alteration of mantle-wedge rocks such as dunite and lherzolite, and the transfer of trace elements released from a carbonate-bearing amphibolite during its eclogitization. Element transfer from the slab to the mantle lithologies occurred in porous-, focused- and diffusive-flow regimes that remove melt and carbon, and partially water, from the metabasite layer. Porous flow is recorded by dissolution of clinopyroxene and growth of orthopyroxene ± garnet ± magnesite ± chlorite along grain boundaries in the peridotite layers, but is invisible in the metabasite layers. Porous flow of the same fluids/melts produces harzburgite mineralogy in both dunite and lherzolite. The transformation of lherzolite to harzburgite reflects breakdown of clinopyroxene in the lherzolite and diffusion of the liberated calcium into the metabasite layer, i.e. against the direction of major fluid/melt flow. Focused flow develops along the side walls of the capsules, producing a melt-free omphacite ± phengite ± quartz paragenesis in the metabasite, and melt segregations, separated from the host peridotite layers by newly-formed omphacite ± garnet ± phlogopite + orthopyroxene + magnesite. Diffusive flow leads to the formation of orthopyroxene ± magnesite ± garnet reaction zones at the metabasite-peridotite interface and some melt-peridotite interfaces.Melt segregations in the peridotite layers at 850–900 °C are rich in LREE and LILE, strongly depleted in Y and HREE, and have higher Sr/Y and La/Yb ratios than island arc andesites, dacites and rhyolites. These features, and negative anomalies in Nb–Ta and low Nb/Ta, resemble those of high-silica adakites and TTGs, but K2O is high compared to TTGs. Metasomatism in the dunite layer changes the REE patterns of dunite, recording chromatographic fractionation during porous melt flow. During metabasite-lherzolite interaction, the metabasite layer becomes mildly enriched in LREE; the lherzolite layer, in contrast, is generally depleted in LREE relative to the initial composition. This also indicates element transfer against the direction of fluid flow. Trace-element profiling reveals the development of Eu anomalies in the peridotite layers and the diffusion of many trace elements out of both layers toward the contact zone. The documented processes may be applicable to both Phanerozoic and Precambrian subduction zones.

Subduction factory in an ampoule: Experiments on sediment–peridotite interaction under temperature gradient conditions

To better understand processes above subducted oceanic slabs, we have undertaken experiments with juxtaposed sediment and peridotite layers at pressures of 7.5 and 10.5 GPa at a controlled temperature gradient from ∼100 to ∼500 °C per a sample length of ∼3 mm. The sediment starting material contains H2O (6.9 wt%) and CO2 (5.9 wt%) and has a major-element composition similar to GLOSS (Plank and Langmuir, 1998) doped with trace elements at 10–100 ppm levels. Several experiments were conducted with ∼0.5 wt% Cl or F. The peridotite layer is composed of natural olivine (66 wt%), orthopyroxene (27 wt%) and garnet (7 wt%) mixed with ∼15 wt% graphite. Several experimental configurations were investigated, but the “basic” setup has the sediment layer at the bottom in the cold zone (400–1200 °C) overlain by peridotite at 900–1500 °C. The temperature distribution was determined by two thermocouples and orthopyroxene–garnet thermometry.Features common to many experiments are (1) the development of multiple layers of various lithologies and a pool of hydrous silicate or carbonate–silicate melt in the hottest part of the capsule; (2) replacement of olivine by orthopyroxene in the metaperidotite; (3) preservation and growth of garnet and local development of magnesite in the metaperidotite layer; (4) enrichment in garnet within the metasediment layer at the contact with the metaperidotite; (5) formation of a clinopyroxene–garnet assemblage at the bottom (the coldest part); (6) presence of K-bearing phases (phlogopite or phengite) and carbonates in the metasediment layer only at temperatures <700 °C; and (7) occurrence of accessory zircon, rutile and phosphates in the coldest regions. In terms of element redistribution, the peridotite becomes strongly enriched in SiO2 compared to the starting composition, and the sediment gains MgO, FeO and Cr2O3. Potassium is fully extracted into the melt, while Na and Ca are largely retained in the coldest part of the metasediment layer in clinopyroxene, Ca-rich garnet and aragonite. The melt is a product of interaction between partial melt or fluid from the sediment and peridotite. It has a silico-carbonatite composition with variable SiO2, MgO, FeO and CaO contents and low Al2O3. The addition of Cl has almost no effect on element distribution, whereas the addition of F results in the appearance of humite-group minerals containing significant amounts of Ti. Trace-element distribution is controlled by pressure, temperature and mineral assemblages. At low temperatures in the sediment layer (<700 °C) Ba, Rb, Sr and Li are much more mobile than REE and HFSE, which results in high Ba/La, Ba/Nb, Sr/Nb etc. (fluid metasomatism). At higher temperatures in the sediment layer, the melt is markedly enriched in Ba, Rb, Sr, LREE and U relative to Ti, MREE and HREE. Negative Nb–Ta and Zr–Hf anomalies in melts are caused by the retention of rutile, zircon and humite-group minerals in the solid residue. Thermodiffusion may affect the ratios of some highly incompatible elements (e.g., Ta/La). Possible applications of the results to natural deep subduction are discussed in view of variations in mineral assemblages and trace element ratios.

Modeling chemical geodynamics of subduction zones using the Arc Basalt Simulator version 5

Arc Basalt Simulator version 5 (ABS5) is a forward geochemical and petrological model that can be used to examine the element mass balance in primary arc magmas including the source and nature of slab materials and flux melting of the mantle-wedge peridotite. The inverse problem approach using ABS5 allows the estimation of intensive and extensive geophysical variables in arc magma genesis. The intensive variables are slab dehydration depth ( P SS ) and temperature ( T SS ) and mantle melting pressure ( P perid ) and temperature ( T perid ). The extensive variables are the amount of slab liquid added to the mantle ( F slab liq ) and the degree of melting of the mantle ( F perid ) along with the amounts of water in the slab liquid ( XH 2 O slab liq ), mantle ( XH 2 O perid ), and magma ( XH 2 O melt ). Subordinate geochemical variables that also can be estimated using ABS5 include the degree of chemical reaction between slab liquids and the solid slab ( %R ); slab liquid fractions derived from igneous oceanic crust [ F liq ( AOC )], sediment [ F liq ( SED )], and metasomatized mantle peridotite layers [ F liq ( DMM )]; and the degree of depletion of the mantle wedge ( %MORB ext ). The mass balances for 26 incompatible elements, six major elements including H 2 O and Sr, Nd, Hf, and Pb isotopes are calculated based on the same scheme. Monte Carlo calculations are used to estimate the aforementioned variables by fitting the calculated magma composition to observed values. This paper describes the ABS5 calculation scheme and presents examples of its successful use. The geophysical variables determined for these example cases are compared with those estimated by other methods. The spatial variations of the magma productivity and implications for the location of the volcanic front are also discussed.

High-Pressure Melting Experiments on Basalt-Peridotite Layered Source (KLB-1/N-MORB): Implications for Magma Genesis in Hawaii

In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite; a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.

Elasticity of single‐crystal superhydrous phase B at simultaneous high pressure‐temperature conditions

AbstractWe investigated the combined effect of pressure and temperature on the elasticity of single‐crystal superhydrous phase B (Shy‐B) using Brillouin scattering and X‐ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy‐B along slab geotherms, showing that Shy‐B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy‐B show that Shy‐B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy‐B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy‐B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

Applications and limitations of U–Th disequilibria systematics for determining ages of carbonate alteration minerals in peridotite

238U–234U–230Th dating was conducted on carbonate alteration minerals in the peridotite layer of the Samail Ophiolite, Sultanate of Oman, in order to assess the applicability of U-series dating techniques to these types of Quaternary terrestrial carbonates and also to further constrain natural rates of carbonation of the peridotite. Due to their low U concentrations and relatively high Th/U ratios, Samail carbonates are challenging to date with the 230Th technique because of the sensitivity of ages to corrections for initial 230Th. Uncorrected 230Th ages for Ca-rich travertines are consistently older than previously obtained 14C ages. However, geologically reasonable initial 230Th corrections bring the two sets of ages into concordance. This age concordance suggests that the travertines are generally closed systems, adding a level of credence to the reliability of previously obtained 14C ages. In contrast, uncorrected 230Th ages for Mg-rich carbonate veins are generally younger than previously obtained 14C ages. These young ages are interpreted in terms of remobilization of hexavalent U, which is subsequently deposited as tetravalent U by reduced serpentinization fluids. Two Mg-rich carbonate veins sampled at a roadcut have near-equilibrium (230Th/238U) and (234U/238U) values, which indicate that these veins are >375,000years in age, consistent with their “14C dead” (>50,000yearsBP) ages. The variable young and old ages for these Mg-rich carbonate veins indicate that carbonation of the peridotite layer of the Samail Ophiolite is an ongoing process and that there have been multiple generations of subsurface carbonate vein formation. Overall, this study provides insights into some of the challenges associated with applying U-series dating methods to Quaternary terrestrial carbonates, in particular carbonate alteration minerals in peridotites, and highlights some areas where there is room for improvement, such as obtaining better constraints on the isotopic composition of admixed detritus, and also some advantages, such as the ability to identify open system behavior not apparent from 14C dating and stable C and O isotopic analysis alone.

Record of multiple stage channelized fluid and melt activities in deeply subducted slab from zircon U–Pb age and Hf–O isotope compositions

Geochemical features of island arc rocks require two slab-derived components of a melt and a fluid added to their sources. These two liquids may unite to an intermediate aqueous melt at the base of volcanic arc. Direct evidence of this process has been scarce yet. Here we present an integrated study of in situ U–Pb age, trace element, and O–Hf isotope for zircons from a suite of a quartz vein, a felsic vein, their host eclogite, and surrounding gneiss at the Xitieshan UHP terrane of the North Qaidam metamorphic belt (NQMB). Oscillatory zoned zircons from the quartz vein are enriched in U, Li, Nb, and REE, with a formation age of 442±6Ma, consistent with the timing of the peak UHP metamorphism. This demonstrates there is no temporal gap between the UHP metamorphism and fluid flow in the Xitieshan terrane. In particular, the oscillatory zoned zircons of the quartz vein have similar Hf isotope compositions to zircons from the surrounding gneiss, but variable O isotope compositions between the host eclogite and surrounding gneiss. The fluid from surrounded gneiss was thus an aqueous melt with high transport ability for Lu and Hf and O, while the fluid from the host eclogite was an aqueous fluid only with the ability to transport O. According to the similar behaviors of granitic rock and sediment, it is inferred that a channelized hydrous melt can be formed in the downgoing sediment layer, where an aqueous fluid can be formed from underlying basalt and/or peridotite layers at the sub-arc depth. The two types of fluid are fed to the overlying mantle wedge from subducting slab to form island arc rock. By contrast, the zircons in the felsic vein yield a weighted mean 206Pb/238U age of 420±4Ma, which can be regard as the timing of partial melting during the exhumed stage for the Xitieshan UHP rocks. The zircon O and Hf isotopic features of the felsic vein are between the country gneiss and host eclogite, compatible with their emanating from hydrous melt of both the eclogite and gneiss during the exhumation stage of the UHP rocks. This indicates that hydrous melt would be released from both the sediment and basalt or peridotite layers of the subducting slabs at relatively shallow depth.

Effects of pressure and temperature on the silicon diffusivity of pyrope-rich garnet

We have determined the pressure and temperature dependencies of Si volume diffusion rates in a single crystal of Pyp75Alm15Gr10 garnet at 6–25GPa and 1400–1800°C by the 29Si tracer diffusion method. High-pressure experiments were conducted using the Kawai-type multi-anvil high-pressure apparatus. Diffusion profiles were obtained using secondary ion mass spectrometry in the depth-profiling mode. The Si diffusion coefficient in garnet (Dgt) is expressed by the Arrhenius equation: Dgt=D0 exp(−(E∗+PV∗)/RT), with log10D0=−7.1±1.7m2/s, E∗=345±56kJ/mol, and V∗=4.7±0.5cm3/mol. Si diffusion seems to be the slowest in the major constituent elements in a pyrope-rich garnet and control rates of plastic deformation. The comparison between Si diffusion rates in garnet and wadsleyite/ringwoodite suggests that garnet has similar or slightly lower strength (at most ∼2times) compared with wadsleyite, and has similar or slightly higher strength (at most ∼3times) compared with ringwoodite at temperatures ranging from 900 to 1300°C. Thus, strength contrast between subducted oceanic crust and underlying peridotite layer may be small at the mantle transition zone conditions. This result suggests that the separation of subducted oceanic crust from an underlying peridotite layer may not occur.

Carbonation rates of peridotite in the Samail Ophiolite, Sultanate of Oman, constrained through 14C dating and stable isotopes

Detailed 14C dating as well as stable C and O isotope analyses were conducted on carbonates formed during alteration of the peridotite layer of the Samail Ophiolite, Sultanate of Oman. 14C results obtained in this and previous studies indicate that surface travertines range in age from modern to >45,000yr BP, indicating long-term deposition and preservation. Travertine deposition rates in two localities were ∼0.1 to 0.3mm/yr between ∼30,000 and 45,000yr BP. Using an estimate of total travertine area, this would result in a maximum of ∼1000 to 3000m3/yr of travertine being deposited throughout the ophiolite during this time period. This travertine deposition would have sequestered a maximum of ∼1 to 3×106kgCO2/yr. Ca-rich carbonate veins that are associated with the surface travertine deposits have ages ranging from ∼4000 to 36,000yr BP (average: 15,000yr BP). Mg-rich carbonate veins exposed in outcrops have ages ranging from ∼8000 to 45,000yr BP (average: 35,000yr BP). Detailed sampling from numerous locations (3 locations in this study and 10 locations in the previous studies) indicates that no carbonate veins from the natural peridotite weathering surface are older than the ∼50,000 yr BP dating limit of 14C. However, 14C dating of Mg-rich carbonate veins from three roadcut exposures (Qafeefah, Fanja, and Al-Wuqbah) indicates that a significant number of roadcut veins are 14C dead (>50,000yr BP). A location weighted average indicates that ∼40% of veins sampled at the three roadcuts are 14C dead. An average including veins sampled at both roadcuts and outcrops indicates that overall ∼8% of Mg-rich carbonate veins are 14C dead. Mg-rich carbonate veins are estimated to sequester on the order of 107kgCO2/yr throughout the ophiolite.

The lithosphere–asthenosphere system beneath Ireland from integrated geophysical–petrological modeling — I: Observations, 1D and 2D hypothesis testing and modeling

Modeling the continental lithosphere's physical properties, especially its depth extent, must be done within a self-consistent petrological–geophysical framework; modeling using only one or two data types may easily lead to inconsistencies and erroneous interpretations. Using the LitMod approach for hypothesis testing and first-order modeling, we show how assumptions made about crustal information and the probable compositions of the lithospheric and sub-lithospheric mantle affect particular observables, particularly especially surface topographic elevation. The critical crustal parameter is density, leading to ca. 600m error in topography for 50kgm−3 imprecision. The next key parameter is crustal thickness, and uncertainties in its definition lead to around ca. 4km uncertainty in LAB for every 1km of variation in Moho depth. Possible errors in the other assumed crustal parameters introduce a few kilometers of uncertainty in the depth to the LAB.We use Ireland as a natural laboratory to demonstrate the approach. From first-order arguments and given reasonable assumptions, a topographic elevation in the range of 50–100m, which is the average across Ireland, requires that the lithosphere–asthenosphere boundary (LAB) beneath most of Ireland must lie in the range 90–115km. A somewhat shallower (to 85km) LAB is permitted, but the crust must be thinned (<29km) to compensate.The observations, especially topography, are inconsistent with suggestions, based on interpretation of S-to-P receiver functions, that the LAB thins from 85km in southern Ireland to 55km in central northern Ireland over a distance of <150km. Such a thin lithosphere would result in over 1000m of uplift, and such rapid thinning by 30km over less than 150km would yield significant north–south variations in topographic elevation, Bouguer anomaly, and geoid height, none of which are observed. Even juxtaposing the most extreme probable depleted composition for the lithospheric mantle beneath southern Ireland against the most extreme fertile composition beneath northern Ireland only allows some 20km of LAB variation; any further variations would produce effects that are well beyond those observed.One model that satisfies almost all the extant data to first order includes a spinel-peridotite upper lithospheric mantle layer to 85km in southern Ireland and to 55km in northern Ireland, thinning over a lateral distance of 150km. Below this in southern Ireland is a garnet peridotite layer extending down to 115km, and in northern Ireland a refertilized layer down to 95km. The mid-lithospheric chemical discontinuity (MLD) at the base of the Spinel Peridotite zone may explain the observed discontinuity in S-to-P (Sp) receiver functions.

Experimental modeling of mantle metasomatism coupled with eclogitization of crustal material in a subduction zone

Alteration of mantle wedge rocks under the influence of fluids and melts is a poorly known subduction-zone process. It was experimentally modeled using various materials analogous to the crust (glaucophane schist and amphibolite) and mantle (olivine and olivine + orthopyroxene) under the P-T conditions (800°C and 29 kbar) corresponding to a hot subduction zone. Schist or amphibolite was loaded into the lower part of a capsule and underwent partial (10–90%) eclogitization during the experiment with the formation of omphacite, garnet, and quartz, sometimes coexisting with Ca-Na amphibole and orthopyroxene. The eclogitization was accompanied by the release of aqueous fluid, which dissolved minerals and products of partial melting of the schist. Ascending fluid flows transported major components into the overlying peridotite. This resulted in the formation of a garnet-phlogopite-orthopyroxene reaction zone at the base of the peridotite layer; this zone accumulated Si and K, which was practically absent in the starting materials. The gain of Si, Al, and CO2 and loss of Mg resulted in the growth of new minerals in the olivine material: garnet, orthopyroxene, and magnesite. Under natural conditions, such a change would have been described as dunite transformation to garnet-bearing harzburgite. The experiments showed that the mineral and chemical composition of the suprasubduction mantle strongly depends on the transfer of components from a downgoing lithospheric slab.

In-situ trace elements and Li and Sr isotopes in peridotite xenoliths from Kuandian, North China Craton: Insights into Pacific slab subduction-related mantle modification

The trace element, Li and Sr isotopic compositions of major minerals in peridotite xenoliths from Kuandian, North China Craton (NCC) were analyzed in situ to investigate Pacific slab subduction-related mantle modification beneath the eastern NCC. The 87Sr/86Sr ratios (0.70282–0.70553) are positively correlated with the La/Nb and U/Nb ratios for clinopyroxenes (Cpx). Based on the trace element distribution patterns, three types of Cpx were identified. Type 1 Cpx are characterized by significant Nb–Ta–Ti depletions and the highest Sr isotopic ratios (up to 0.70553); Type 2 Cpx display Nb (or Ta) depletions and highly variable Nb and Ta fractionation; and Type 3 Cpx show no significant Nb or Ta depletion or even weak Ta enrichment and have the lowest Sr isotopic ratios (0.70282–0.70323). Some Cpx display a pattern of increasing Ca+Mg±Si from the cores to the rims, indicating addition of Ca+Mg±Si-rich fluids derived from a serpentinized peridotite layer above the subducting slab. These features indicate an ancient mantle metasomatic event associated with the subduction of the altered Pacific oceanic crust, which may contribute substantially to the destruction of the eastern NCC.The variations in the Li contents and isotopic compositions of major minerals in the Kuandian peridotite xenoliths encode both the melt/fluid metasomatism and the cooling process. The Li contents in the Cpx (up to 34.8ppm) and orthopyroxenes (Opx) (up to 28.0ppm) are typically higher than those in the coexisting olivines (Ol) (<9.19ppm), suggesting silicate melt metasomatism. Considering the rapid diffusion rate of Li in minerals at mantle temperature, the Li enrichment and low δ7Li values in the rims of most Ol, Cpx and Opx may have arisen from diffusive fractionation due to faster diffusion of 6Li compared to 7Li during a latest Li-rich melt/fluid metasomatic process shortly before the host magma eruption. On the other hand, a few Ol grains show decreased Li contents and higher δ7Li values from the cores to the rims. This could imply Li isotope fractionation caused by cooling-induced Li redistribution during and/or after the host magma eruption, because DLiOl/Cpx decreases significantly with decreasing temperature and 6Li diffuses faster than 7Li.

Sound velocity measurements of CaSiO3 perovskite to 133 GPa and implications for lowermost mantle seismic anomalies

We report the measurements of aggregate shear velocity (VS) of CaSiO3 perovskite (CaPv) at high pressure (P) between 32 and 133GPa and room temperature (T) on the basis of Brillouin spectroscopy. The sample had a tetragonal perovskite structure throughout the experiments. The measured P–VS data show the shear modulus and its pressure derivative at ambient condition to be G0=115.8GPa and G'=1.20, respectively. The zero-pressure shear velocity is determined to be VS0=5.23km/s, in good agreement with the previous estimate inferred from the ultrasonic measurements on Ca(Si,Ti)O3 perovskite at 1bar. Our experimental results are broadly consistent with the earlier calculations on tetragonal CaPv but exhibit lower velocity at equivalent pressure. Such tetragonal CaPv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle. While primitive mantle includes certain amount of CaPv, a depleted peridotite (former harzburgite) layer in subducted oceanic lithosphere is deficient in CaPv and enriched in ferropericlase in the lower mantle. Such harzburgite exhibits 0.9% faster VS and 0.7% slower bulk sound velocity (VΦ) at the lowermost mantle P–T conditions if CaPv is present in the tetragonal form in the surrounding mantle. The observed fast VS and slow VΦ anomalies in the D” layer underneath the circum-Pacific region might be attributed in large part in the presence of subducted harzburgitic materials.