Majoritic Garnet Research Articles

Exsolution textures in majoritic garnets from the Mir kimberlitic pipe (Yakutia)

Exsolution textures in majoritic garnets from the Mir kimberlitic pipe (Yakutia)

Pyroxene exsolution topotaxy in majoritic garnet from 250 to 300 km depth

We report here the detailed microstructure and chemistry of pyroxene exsolution from a polycrystalline garnet porphyroblast of the Western Gneiss Region (WGR) garnet peridotite, Otrøy, Norway. For both clinopyroxene (Cpx) and orthopyroxene (Opx), the same basic crystallographic relationship is found with the host garnet: (100)py//{112}grt, (010)px//{110}grt and (001)px//{111}grt for the majority (>90%) of its intracrystalline pyroxene rods. In addition, this pattern is also exhibited by some interstitial Opx and a subpopulation of both pyroxenes shows a different pattern or no discernible pattern. The results provide quantitative microstructural evidence demonstrating an exsolution (precipitation) origin of both the intracrystalline Cpx and Opx and the small interstitial Opx crystals. The reconstructed precursor majoritic garnet, taking into account both the intracrystalline pyroxenes and interstitial Opx, was characterized by Si = ∼3.07 cation per formula unit that corresponds to a minimum pressure of 7.7 GPa (∼250 km depth). We also deduce from the observation of Opx being the majority of intracrystalline precipitates and 100% of the interstitial ones that the precursor majoritic garnet probably originated from a pressure less than ∼10 GPa (∼300 km depth). A multistage decomposition hypothesis is proposed for this WGR majoritic garnet during exhumation of the peridotite from 250 to 300 km depth to explain the topotaxy and chemistry of the exsolved pyroxenes.

Dislocation microstructures in majorite garnet experimentally deformed in the multi-anvil apparatus

Majorite garnets with the composition Mj70Py30 have been deformed in the multi-anvil apparatus at 17 GPa and 2000 °C. The microstructure has been characterized by transmission electron microscopy. It is shown that under these conditions majorite garnet is ductile. Dislocations with <100> and ½<111> Burgers vectors are observed with a density 1-5 × 1012 m−2. The absence of clear glide planes and the occurrence of subgrain boundaries suggest the importance of diffusion and climb in the plasticity of majorite garnets in mantle transition zone conditions.

Crystal chemistry of sodium in the Earth's interior: The structure of Na2MgSi5O12 synthesized at 17.5 GPa and 1700 C

The crystal structure and chemical composition of a crystal of Na 2 MgSi 5 O 12 garnet synthesized in the model system Mg 3 Al 2 Si 3 O 12 –Na 2 MgSi 5 O 12 at 17.5 GPa and 1700 °C have been investigated. Quantitative analysis leads to the following formula: Na 1.98 Mg 1.00 Si 5.01 O 12 . Na 2 MgSi 5 O 12 garnet was found to be tetragonal, space group I 4 1 / acd , with lattice parameters a = 11.3966(6), c = 11.3369(5) A, V = 1472.5(1) A 3 . The structure was refined to R = 5.13% using 771 independent reflections. Sodium and Mg are disordered at the X sites (with a mean bond distance of 2.308 A for both the sites), whereas Si is ordered at both the Y (mean: 1.793 A) and Z sites (means: 1.630 and 1.624 A). Na-bearing majoritic garnet may be an important potential sodium concentrator in the lower parts of the upper mantle and transition zone. The successful synthesis of the Na 2 MgSi 5 O 12 end-member and its structural characterization is of key importance because the study of its thermodynamic constants combined with the data of computer modeling provides new constraints on thermobarometry of majorite garnet assemblages.

Experimental Evidence for a Reduced Metal-saturated Upper Mantle

The uppermost mantle as sampled by xenoliths, peridotite massifs and primitive basaltic melts appears to be relatively oxidized, with oxygen fugacities between the magnetite^wu« stite and fayalite^ferrosilite^magnetite equilibria.Whether this range in oxygen fugacity is a shallow mantle signature or representative of the entire upper mantle still is unclear and a matter of debate because mantle regions deeper than 200 km are not well sampled. To constrain the redox state of the deeper upper mantle, we performed experiments from 1 to 14 GPa and 1220 to 16508C on a model peridotite composition, encompassing the convecting asthenospheric mantle down to the Transition Zone at 410 km depth.The experiments were run in iron metal capsules to buffer fO2 close to an oxygen fugacity about 0·5 log units below the iron^wu« stite equilibrium. Analysis of the experimental phases for ferric iron using electron energy loss spectroscopy reveals that at pressures higher than 7 GPa, subcalcic pyroxene and majoritic garnet incorporate appreciable amounts of ferric iron, even though at the experimental conditions they were in redox equilibrium with metallic iron. The major ferric iron carrier in the upper mantle is majoritic garnet, followed by subcalcic pyroxene. At around 8 1GPa, corresponding to 250 30 km depth in the upper mantle, sufficient quantities of subcalcic pyroxene and majoritic garnet are stabilized that all the ferric iron thought to be present in fertile upper mantle (i.e. 2000 ppm) can be accommodated in solid solution in these phases, even though they were synthesized in redox equilibrium with metallic Fe. Based on the results of the experiments, it can be stated that, on a global scale, an oxidized upper mantle near the fayalite^ferrosilite^magnetite equilibrium is the exception rather than the rule. More than 75 vol. % of the Earth’s present-day mantle is likely to be saturated with metallic iron.

The oxidation state of mantle wedge majoritic garnet websterites metasomatised by C-bearing subduction fluids

The majoritic garnet-bearing websterites from Bardane (Western Gneiss Region, Norway) are unique examples of metasomatised mantle wedge that interacted with C-saturated COH subduction fluid phases at 200 km depth. These samples represent slices of former Archean transition zone mantle that upwelled, melted and accreted to a thick cratonic lithosphere, where it cooled until the Middle Proterozoic (stages M1–M2). During the subsequent Caledonian to Scandian subduction cycle (stage M3) these depleted mantle rocks were dragged into deep portions of the supra-subduction mantle wedge, where the episodic/progressive infiltration of crustal fluids at increasing depth enhanced crystallisation of diamond and of majoritic garnet. In these rocks microdiamond occurs as daughter phase in polyphase, fluid-related, inclusions and majoritic garnet is present as vein mineral. We studied the peak (stage M3) mineral assemblage garnet + orthopyroxene + olivine ± clinopyroxene ± phlogopite, where garnet composition becomes majoritic with pressure increase from 3 to 6.5 GPa and 800–1000 °C temperature. Majoritic garnet contains polyphase inclusions with daughter Cr-spinel + phlogopite/K-amphibole + dolomite/magnesite + graphite/diamond. They witness COH fluid/mineral interaction closely related to the oxidation state of the rock and responsible for diamond formation. We determined the fO 2 in the M3 assemblage starting from Fe 3+ analyses in majoritic garnet, which is characterised by the progressive enrichment in Fe 3+/ΣFe up to 0.15. The fO 2 values obtained for the Bardane mantle wedge garnet websterites are up to −2 log units lower than the fayalite–quartz–magnetite (FMQ) buffer along a trend from arc lavas (FMQ + 1.5–FMQ + 3) to mantle wedge garnet peridotites equilibrated at 4–5 GPa (FMQ–FMQ + 2). The determination of oxygen fugacity of the hydrate–carbonate-bearing M3 mineral association enabled us to estimate the speciation of slab-derived metasomatic COH fluids responsible for polyphase inclusions precipitation. Such fluids are mixtures of silicates and H 2O + CO 2, where water is the dominant species of the COH component. The peculiar composition of majorite-hosted diamond-bearing polyphase inclusions from Bardane and the speciation of its COH component point to an “oxidised” silicate-rich aqueous fluid contaminant from the subducted slab to the mantle wedge.

Elasticity of diopside to 8 GPa and 1073 K and implications for the upper mantle

Simultaneous measurements of elastic wave velocities and density have been conducted on diopside by a combined ultrasonic interferometry and X-ray diffraction methods at pressure and temperature conditions relevant to the Earth's upper mantle. The current study not only yielded the first direct measurement of the shear modulus at high pressures, but also enabled a simultaneous determination of the bulk and shear properties and their pressure and temperature derivatives from the measured density and velocities. Finite strain analysis of the experimental data results in K S0 = 116.4(7) GPa, K ′ S 0 = 4.9 ( 1 ) , ( ∂K S/ ∂T) P = −0.012(1) GPa GPa, G ′ 0 = 1.6 ( 1 ) and ( ∂G/ ∂T) P = −0.011(1) GPa/K. With these results and the same finite strain equations, the P and S wave velocities of diopsidic mantle clinopyroxene were calculated along a 1600 K adiabatic geotherm. In comparison with other mantle minerals, the seismic velocities of diopsidic clinopyroxene at upper mantle depths are 1–3% higher than those of orthopyroxene, 1–2% and 6–8% lower than those of olivine and majoritic garnet, respectively.

カンラン石、メージャライトガーネット/ペリドタイトメルト間における分配係数の圧力依存性

カンラン石、メージャライトガーネット/ペリドタイトメルト間における分配係数の圧力依存性

Majoritic garnet: A new approach to pressure estimation of shock events in meteorites and the encapsulation of sub-lithospheric inclusions in diamond

A means for estimating pressures in natural samples based on both the coupled substitution (Na+) [1+] (Ti + [VI]Si) [4+] = (M) [2+] (Al + Cr) [3+], and the classic pyroxene-stoichiometry majorite-substitution into garnet at high-pressure, is derived for garnets with majoritic chemistry. The technique is based on a compilation of experimental data for different bulk compositions. It is compositionally and thermally robust and can be used to estimate pressures experienced by natural materials during formation of majoritic garnet. In addition, it can be used either retrospectively, or in new experimental studies to establish the pressures of crystallization of reaction products, and determine if disequilibrium is recorded by the chemistries of majoritic garnets. Pressures are calculated based on majoritic chemistries in chondritic meteorites and diamond inclusions. Majoritic garnets associated with Mg perovskite in shocked L chondrites ( n = 4) yield uniform pressures of 23.8 ± 0.2 GPa that are slightly higher than pressures recorded by majoritic garnet in shock-derived melt veins in L chondrites (22.4 ± 0.6 GPa; n = 5). Similar pressures are also exhibited by shock-derived majoritic garnets in H chondrites (22.2 ± 1.1 GPa; n = 3). Diamond inclusions with eclogitic and peridotitic majoritic garnet chemistries exhibit mean pressures of 10.7 ± 2.7 GPa ( n = 30) and 8.3 ± 1.6 GPa ( n = 15) respectively, consistent with a sub-lithospheric origin. However, pressures defined by majoritic diamond inclusions from Jagersfontein (22.3 ± 0.8 GPa and 16.9 ± 1 GPa), Monastery (15.7 ± 7 GPa) and Kankan (15.5 ± 0.2 GPa) show that these inclusions originated from the mantle transition zone. Thus, this new single-phase method for pressure estimation has unmatched potential to map the depth of formation of garnets with majoritic chemistries that occur as diamond inclusions in all parageneses except those that include Ca silicate perovskite. The derived pressures confirm the sub-lithospheric origin of eclogitic majoritic diamond inclusions, and thus provide a more comprehensive picture of the important role of storage of oceanic lithosphere in the transition zone.

Majoritic garnets in diamonds from placers of the Northeastern Siberian Platform

Majoritic garnets in diamonds from placers of the Northeastern Siberian Platform

Evidence of former majoritic garnet in Himalayan eclogite points to 200-km-deep subduction of Indian continental crust

Relict majorite, recognized petrographically and confirmed by in situ laser-ablation microprobe analyses, is reported from eclogites of the Tso Morari Complex, India, and represents the first record of relict majorite in the Himalaya. Previous studies have established that the Tso Morari Complex eclogites represent continental metamorphosed basic rocks within the sequence of paragneisses and metapelites interbedded with granitic augen gneisses. Our study provides evidence for subduction of the Indian continental crust to majorite-forming depths (∼200 km), and even more significant, its exhumation from such depth without being completely overprinted. This estimated depth is almost double the previous depth approximations and hence has important implications on the estimation of the angle of subduction and average rates of exhumation of the Indian plate in the Himalayan collisional zone.

Survival of majoritic garnet in diamond by direct kimberlite ascent from deep mantle

Majoritic garnets discovered as diamond inclusion in several localities originated from the deep upper mantle or the mantle transition zone. Some of them show evidence of partial transformation to low‐pressure phases of pyropic garnet and clinopyroxene during transportation to the Earth's surface. The degree of partial transformation can be used potentially as a unique speedometer on the average ascent rate of diamond through the entire upper mantle. Here we present experimental results on the back transformation kinetics of majoritic garnet into pyropic garnet and clinopyroxene by time‐resolved in‐situ synchrotron X‐ray diffraction measurements. We observed that back transformation occurs by grain‐boundary nucleation and diffusion‐controlled growth at 6.5–7.5 GPa and 1020–1300°C. The transformation mechanisms are similar to those observed in natural diamond. The temperature dependence of diffusion‐controlled growth rates were determined from the kinetic data obtained, which was used to constrain the survival time of majoritic garnet in diamond. We found that the ascending velocity in the normal mantle convection is far too slow to satisfy the time limitation for the survival of majoritic garnet, indicating that such diamonds have been transported directly from the deep mantle by the rapid kimberlite magma within a few days.

Petrology and Raman spectroscopy of high pressure phases in the Gujba CB chondrite and the shock history of the CB parent body

Abstract– High pressure phases majorite, possibly majorite‐pyropess, wadsleyite, and coesite are present in the matrix and in barred olivine fragments in the Gujba CB chondrite. Grossular‐pyrope was also observed in some small inclusions. The CB chondrites are metal‐rich meteorites with characteristics that sharply distinguish them from other chondrite groups. All of the CB chondrites contain impact melt regions interstitial to their chondrules, fragments and metal and a major impact event (or events), on the CB chondrite parent body is clearly a significant stage in its history. We studied three areas interstitial to chondrules and metal in the Gujba CBa chondrite. From Raman spectra, the barred olivine fragments and matrix in these regions have various combinations of olivine and low‐Ca pyroxene, as well as majorite garnet (Mg4Si4O12), a phase that forms by high‐pressure transformation of low‐Ca pyroxene and wadsleyite, a high pressure product of olivine. Compositions of the majorite suggest both majorite and majorite‐pyrope solid solution may be present. The mineral assemblage of majorite and wadsleyite suggest minimum shock pressures and temperatures of ∼19 GPa and ∼2000 °C, respectively. The occurrences of high pressure phases are variable from one area to another, on the scale of millimeters or less, suggesting heterogeneous distribution of shock and/or back transformation to low pressure polymorphs throughout the meteorite. The high pressure phases record a high temperature–pressure impact event that is superimposed onto, and thus postdates formation of, the chondrules and other components in the CB chondrites. The barred chondrules and metal in the CB chondrites are primary materials formed prior to the impact event either generated in an earlier planetesimal scale impact event or in the nebula.

Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones

AbstractStudies of the inclusions contained in natural diamonds have shown the occurrence of minerals which must have formed at depths below the lithosphere and which may be closely matched with the silicate mineral assemblages determined by high pressure and temperature experimental studies for depths of 300 to 800 km in the Earth's mantle. The inclusions come principally from two main depth zones: (1) the lower asthenosphere and upper transition zone; (2) the Upper Mantle/Lower Mantle (UM/LM) boundary region and the uppermost LM. The inclusions from zone 1 are very largely majoritic garnets (with or without clinopyroxene) which indicate bulk compositions of eclogitic/metabasic affinity. The minerals from zone 2 include Ca-Si and Mg-Si perovskites and ferropericlase and are dominantly of metaperidotitic bulk composition, but include some possible metabasite assemblages. In many of these natural assemblages, the tetragonal almandine pyrope phase occurs rather than the garnet found in experiments.As natural diamonds are believed to crystallize in fluids/melts, the hypothesis is developed that the restriction of diamonds and inclusions of particular compositions to the above two depth intervals is because they are controlled by loci of fluid/melt occurrence. Attention is focused on subduction zones because both suites of inclusions show some evidence of subducted protoliths. The lower zone (600–800 km) coincides with the region where dehydration may be expected for hydrous ringwoodite and dense hydrous Mg-silicates formed in subducted peridotites. The dehydration of lawsonite in subducted metabasites provides a particular location for melt formation and the inclusion of the shallower (~ 300 km) majoritic inclusions. For the deeper majoritic inclusions in the region of the upper transition zone, melt development may occur as a consequence of the hydrous wadsleyite-to-olivine transformation, and such melt may then interact with the upper crustal portion of a subducting slab. These suggestions offer an explanation of the depth restrictions and the compositional restrictions of the inclusions. The differences in δ13C values in the host diamonds for the two suites of inclusions may also be explained on this basis.

The continental lithosphere–asthenosphere boundary: Can we sample it?

The lithosphere–asthenosphere boundary (LAB) represents the base of the Earth's lithosphere, the rigid and relatively cool outer shell characterised by a conductive thermal regime, isolated from the convecting asthenosphere. Chemically, the LAB should divide a lithospheric mantle that is variably depleted in basaltic components from a more fertile asthenosphere. In xenolith suites from cratonic areas, the bottom of the depleted lithosphere is marked by a rapid downward increase in elements such as Fe, Ca, Al, Ti, Zr and Y, and a rapid decrease in the median Mg# of olivine, reflecting the infiltration of mafic melts and related fluids. Eclogites and related mafic and carbonatitic crystallisation products are concentrated at the same depths as the maximum degrees of metasomatism, and may represent the melts responsible for this refertilisation of the lithosphere. This refertilised zone, at depths of ca 170–220 km beneath Archean and Proterozoic cratons, is unlikely to represent a true LAB. Re–Os isotopic studies of the deepest fertile peridotite xenoliths show that they retain evidence of ancient depletion events; seismic tomography data show high-velocity material extending to much greater depths beneath cratons. The cratonic “LAB” probably represents a level where asthenospheric melts have ponded and refertilised the lithosphere, rather than marking a transition to the convecting asthenosphere. Our only deeper samples are rare diamond inclusions and some xenoliths inverted from majoritic garnet, which are unlikely to represent the bulk composition of the asthenosphere. In younger continental regions the lithosphere–asthenosphere boundary is shallower (commonly at about 80–100 km). In regions of extension and lithosphere thinning (e.g. eastern China, eastern Australia, Mongolia), upwelling asthenosphere may cool to form the lowermost lithosphere, and may be represented by xenoliths of fertile garnet peridotites in alkali basalts. The LAB is a movable boundary. It may become shallower due to thermal and chemical erosion of the lithosphere, assisted by extension. Refertilised lithospheric sections, especially where peridotites are intermixed with eclogite, may be capable of gravitational delamination. The lithosphere–asthenosphere boundary may also be deepened by subcretion of upwelling hot mantle (e.g. plumes). This process may be recorded in the strongly layered lithospheric mantle sections seen in the Slave Craton (Canada), northern Michigan (USA) and the Gawler Craton (Australia).

Subduction zone metamorphism – pioneering contributions from the Alps

Circum-Pacific-type high-pressure (HP) metamorphic belts occupy contractional sutures marked by intensely deformed, imbricated nappes that verge seaward. Mineral assemblages reflect recovered depths of ∼25–50 km for mafic and felsic crustal units carried down on an underlying oceanic plate; such units recrystallized prior to disengagement and exhumation of slices of chiefly low-density rock. Low-grade parts of Alpine-type subduction complexes are similar to regurgitated circum-Pacific-type HP belts, but coesite ± microdiamond inclusions in tough, unreactive host minerals show that lithospheric underflow and continental collision involve partial recovery of ultra-high-pressure (UHP) rocks from depths of 100–140 km. Associated garnet peridotites from the Central Alps and several other collision sutures display exsolution lamellae and intergrowths suggesting the former existence of majoritic garnet and additional phases requiring formation at >300 km, attesting to earlier, unrelated mantle upwelling, or to the resurrection of profoundly subducted and back-reacted complexes. HP and UHP terranes return to mid-crustal levels mainly driven by buoyancy. Times of deep-seated storage and rates of exhumation are short; mineral parageneses, intervals of HP–UHP recrystallization and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. More fully than any other areas, integrated mineralogic-geochemical-structural studies of the Alps have illuminated relationships between metamorphic petrology and convergent plate tectonics. HP–UHP transformations of mafic and quartzofeldspathic rocks initially formed at, or near, the Earth's surface, imbricate structures of subduction complexes including their prograde metamorphic zonations + retrograde parageneses, association with deep-seated UHP mantle rocks and speedy ascent were first documented in the Alps.

Mantle wedge peridotites: Fossil reservoirs of deep subduction zone processes: Inferences from high and ultrahigh-pressure rocks from Bardane (Western Norway) and Ulten (Italian Alps)

The garnet websterites from Bardane (Western Gneiss Region, Norway) derive from cold Archean subcontinental lithosphere involved in Scandian continental subduction to ultrahigh-pressures. Subduction zone metamorphism was promoted by slab fluid infiltration into the cold overlying mantle wedge. The earliest subduction transformation (M3-1) consists of garnet/clinopyroxene exsolution from old pre-subduction orthopyroxene. This stage was likely coeval with fluid input and formation of phlogopite and dolomite rods in the exsolution structures. Magnesite formation after dolomite and entrapment of fluid-related diamond-bearing polyphase inclusions in corona structures around the exsolved orthopyroxenes point to pressure increase to 4.5 GPa (M3-2). Peak pressures of 6.5–7 GPa (c.a. 200 km depth) are witnessed by crystallization of majoritic garnet (M3-3), mostly in veins cutting all the above microstructures. When such veins infiltrate the corona domains, formation of majoritic garnet in coronas is enhanced. This multistage evolution thus envisages episodic fluid influx, favouring rock recrystallization and formation of microdiamond-bearing inclusions and of majoritic garnet veins. These mantle rocks thus record fluid circulation along grain boundaries and microfractures down to 200 km depth in subduction environments. The Ulten Zone peridotites are slices of Variscan mantle wedge. Infiltration of metasomatic subduction fluids favoured transition from spinel-facies to garnet + amphibole ± dolomite parageneses at pressures below 3 GPa. Formation of metasomatized garnet-bearing peridotite mylonites suggest channelled influx of subduction fluids. The incompatible element-enriched signature of all subduction minerals in Bardane indicate that previously depleted websterites have been refertilized by COH subduction fluids. Comparison with the Ulten Zone garnet + amphibole ± dolomite peridotites outlines striking similarities in the metasomatic style and in the COH fluid phase involved. Comparable geochemical fingerprints observed in HP and UHP Ulten and Bardane mantle rocks call for concomitant subduction of the continental crust, that would provide the incompatible element-enriched fluids. For Bardane this implies subduction of crustal slabs to 200 km depth, to release the fluid phases necessary for UHP metasomatism and for majoritic garnet crystallization. Mantle refertilization by crust-derived COH subduction fluids thus operates over a large depth range, down to 200 km depth. The textural features of the Bardane and Ulten Zone mantle rocks indicate that rock recrystallization and refertilization is concomitant with fluid infiltration along channelways, outside which the rocks preserve pre-subduction, long-lasting stories and pristine geochemical characteristics. Comparably with uprising magmas, the subducted continental crust appears to be a quite efficient carrier to the surface of ultradeep mantle tectonic ‘xenoliths’ representing major observatories of diverse metamorphic and geodynamic histories recorded by the Earth's mantle at variable depths.

The mantle and crustal evolution of two garnet peridotite suites from the Western Gneiss Region, Norwegian Caledonides: An isotopic investigation

A compilation of published and unpublished geochronological and isotopic data from garnet-bearing orogenic peridotites in the HP/UHP Western Gneiss Region (WGR) of the Norwegian Caledonides indicate a common origin for all WGR peridotites, followed by different, though related, Proterozoic and Phanerozoic histories for those in the northwestern WGR (NW peridotites) compared to those in the central and western WGR (CW peridotites). All peridotites are refractory fragments of the subcontinental lithosphere generated by Archean melt extraction, which produced strongly depleted dunites and harzburgites with relict orthopyroxene and majoritic garnet megacrysts (M 1NW) within the NW peridotites. The Archean history is preserved by Re–Os sulfide and whole-rock ages from several WGR bodies and by Sm–Nd ages from the M 1NW megacrysts. Subsequently the CW peridotites were re-fertilized within the lithospheric mantle by mid-Proterozoic or older silicate melts that generated M 2CW garnet pyroxenites and adjacent garnet peridotites. Clinopyroxenes from these bodies show large variation in 143Nd/ 144Nd, but nearly constant 87Sr/ 86Sr, suggesting autometasomatism of depleted mantle by LREE-enriched, Rb-poor melts derived from equally depleted mantle. NW peridotites lack mid-Proterozoic garnet pyroxenite intrusions, but M 2NW garnet-rich assemblages that exsolved from relict M 1 megacrysts may have equilibrated at the same time as the M 2CW refertilization. Sm–Nd and Lu–Hf mineral apparent isochron ages from both suites range from 1.75 to ca. 0.87 Ga. The age spectrum suggests continuous diffusion among M 2 minerals that formed ≥ 1.75 Ga ago punctuated by partial re-equilibration during a 1.0 Ga thermal event. Much later the NW peridotites were transferred from the mantle wedge into the crust as the WGR was subducted into the mantle during the ca 400 Ma Scandian Orogeny. Further subduction heterogeneously metasomatized and recrystallized the NW peridotites to form M 3NW garnet, clinopyroxene and, where metasomatism was pervasive, new M 3NW radiogenic ( 87Sr/ 86Sr > 0.715), LIL-enriched minerals (phlogopite, amphibole) and microdiamond consistent with invasion by hydrous fluids from the enclosing Proterozoic gneisses. Nine young apparent ages (672 to 424 Ma), all from exsolved or recrystallized garnets within NW peridotites, represent mixed (M 2NW and M 3NW) apparent ages. The three youngest ages (weighted mean of 429.5 ± 3.1 Ma; 2 σ) may date M 3NW prograde re-equilibration during earliest Scandian subduction. The CW peridotites show no evidence of prograde M 3 re-equilibration, suggesting derivation from a different part of the Laurentian mantle wedge during the exhumation of the WGR from the mantle.

Mantle-wedge garnet peridotites from the northernmost ultra-high pressure domain of the Western Gneiss Region, SW Norway

Garnet peridotites exposed on the islands of Otroy, Flemsoy and Fjortoft form an integral part of the northernmost ultra- high pressure terrane of the Western Gneiss Region, SW Norway. Their mantle and crustal evolution is fundamentally different from similar Western Gneiss Region garnet peridotites exposed further south. For this reason the occurrences and interpretation of the northernmost garnet peridotites is reviewed here emphasizing the progress made in recent years. Re-Os analyses of whole-rock powders of peridotites and garnetites, Sm-Nd analyses of megacrystic garnets and a 5 point Sm-Nd whole-rock errorchron of garnet pyroxenites suggest an Archean origin (M1), involving majoritic (20 %) garnet growth, decom- pression melting at high temperature, accretion to the Laurentian subcontinental mantle with simultaneous growth of a second generation of majoritic garnet (1 %) followed by cooling towards pressure-temperature conditions indicated by a stable cratonic geotherm. Minimal mid Proterozoic Sm-Nd garnet-pyroxene ages are interpreted to reflect intense dynamic recrystallisation of garnet megacrysts (M2). Subsequent crustal emplacement of the garnet peridotites (from around 120 km depth) took place during the early Scandian (� 430 Ma) when the continental crust of Baltica subducted beneath Laurentia. A prograde, Scandian, subduction event (M3) can be recognised in the peridotites as recrystallised M3 orthopyroxene crystals with markedly lower Al2O3 contents than M2 orthopyroxene crystals in garnet websterite protoliths. M3 is contemporaneous with the introduction of a supercritical, COH-rich, dense subduction zone fluid that formed the source of carbon that gave rise to diamond crystallisation during subsequent continued continental subduction. Scandian peak metamorphic conditions are indicated by the growth of a ''third'' generation of majoritic garnet indicating pressures on the order of 5.5-6.5 GPa. Roughly similar PT conditions can be calculated for opx-grt mineral- chemistries using classical geothermobarometric techniques.

Relative strength of the pyrope–majorite solid solution and the flow-law of majorite containing garnets

Even though the garnet phase is the second most abundant phase in the upper-mantle and transition-zone, no previous studies have directly measured the effect of majorite content on the strength of garnet under mantle conditions. Here we report the results of constant strain-rate and stress-relaxation experiments on garnets in the pyrope–majorite solid solution which constrain the strength of majoritic containing garnets relative to pyrope as a function of majorite content and temperature. We find that at temperatures below 650 ° C both pure pyrope and majoritic garnets have the same strength. Conversely, above 650 ° C we find that majoritic garnets are initially stronger than pure pyrope but weaken with increasing temperature and majorite content and with significant majorite contents are weaker than pyrope above approximately 800 ° C. We develop a flow law for the entire pyrope–majorite solid solution as a function of temperature and majorite content.