Majoritic Garnet Research Articles

Structure of a new Al-rich phase, [K, Na]0.9[Mg, Fe]2[Mg, Fe, Al, Si]6O12, synthesized at 24 GPa

A new Al-rich phase, ![Formula][1] , was synthesized at 24 GPa, 1700 °C, and its structure was determined by single-crystal X-ray diffraction. The unit cell is hexagonal, space group P 63/ m , a = 8.830(1), c = 2.779(1) A, V = 187.65(5) A3, Z = 1, formula weight = 448.62, calculated density = 3.970 g/cm3. The previously unknown structure consists of alkali- and vacancy-bearing M3 sites in tunnels along c , mostly Mg-bearing linear arrays of trigonal prismatic M2 sites, very unusual for mantle phases, and a framework of edge-linked distorted octahedral M1 sites filled mostly with Al and Si. The observed range of compositional variations and high density suggest complex solid solution behavior at the lower-mantle pressures similar to pyroxenes at the upper-mantle and majorite garnet at the transition-zone pressures. The stability of the Al-rich phase appears to expand to lower Al contents with increasing pressure, suggesting that a chondritic lower mantle could contain up to 24% of the new phase on the molecular basis. [1]: /embed/tex-math-1.gif

A high-temperature and high-pressure Raman spectroscopic study of CaGeO 3 garnet

High-pressure and high-temperature Raman spectra of CaGeO3 tetragonal garnet have been collected to 11.5 GPa and 1225 K, respectively, in order to investigate possible intrinsic anharmonic behaviour in this phase. The Raman peak positions were observed to vary linearly with pressure and temperature within the ranges studied, with the higher-energy peaks showing larger P- and T-induced shifts than the low energy modes. The observed T-induced shifts are similar to those reported for grossular and andradite, while the observed P-induced shifts are generally larger than those of aluminosilicate and MgSiO3 majorite garnets (Gillet et al. 1992; Rauch et al. 1996) due to the larger bulk modulus of CaGeO3 garnet. The observed mode shifts of CaGeO3 garnet were used to determine the isothermal and isobaric mode Gruneisen parameters for this phase. These parameters are similar in value to those reported previously for grossular and andradite (Gillet et al. 1992). The calculated intrinsic anharmonic parameters, a i , for CaGeO3 garnet were determined to be nonzero, indicating significant anharmonic behaviour for this phase. These values, which range from −3.8 × 10−5 K−1 to −1.3 × 10−5 K−1, are also similar to those reported for andradite and grossular, but smaller than those determined for pyrope (Gillet et al. 1992). Hence, we expect MgSiO3 majorite to show greater anharmonicity than the germanate analogue studied by us. The anharmonic parameters determined for CaGeO3 tetragonal garnet may now be introduced into quasiharmonic vibrational heat capacity models to account for the observed anharmonic behaviour.

Subsolidus and melting experiments of K‐doped peridotite KLB‐1 to 27 GPa: Its geophysical and geochemical implications

Subsolidus and melting experiments with a K‐doped peridotite (K2O 0.64 wt %) were performed at 10–27 GPa in order to investigate the behavior of potassium in the deep mantle, and the element partitioning between Mg‐perovskite (Mg‐pv) and magnesiowustite (mw). Three new K‐rich phases were discovered, K phase I [K2FeMg4Si5O16] in coexistence with wadsleyite, pyroxene and garnet at 15 GPa, K phase II [(K,Mg)(Al,Si)O4] with CaSiO3‐rich perovskite (Capv), ringwoodite and majoritic garnet at 20 GPa, and K phase III [(K,Na,Ca)(Mg,Fe,Cr,Al,Ni,Ti,Mn)4(Si,Al)4O12] in coexistence with Mg‐pv, mw, and Ca‐pv at 25.5 GPa. These three phases are the main K‐bearing phases in the deep peridotite mantle, rather than K‐hollandite [KAlSi3O8]. With the exception of pyroxene at relatively low pressure, the solubilities of K2O in other mantle phases, such as wadsleyite, ringwoodite, and garnet are very low (<200 ppm), even under the conditions saturated with one of the K phases. At 25.5 GPa, the contents of K2O in Mg‐pv, mw, and Ca‐pv are also very low and decrease with decreasing temperature. The K‐rich phases should appear in low concentrations in the transition zone and the lower mantle, assuming a pyrolitic mantle composition (K2O, ∼300 ppm). In contrast to basaltic composition, K‐phases in peridotite are the first to melt at pressures up to 27 GPa, and thus K is incompatible in the whole experimental pressure range. The solidus temperature of K‐rich peridotite increases drastically from about 1600°C at 20 GPa to 2300°C at 25.5 GPa. Portions of the upper mantle could be partially molten even at the present mantle temperatures. Ascending lower mantle material will partially melt to a very low degree around the transition zone/lower mantle boundary, forming a K‐rich melt. This melt would strongly affect the geophysical properties of the transition zone, and may be directly related to the worldwide mantle metasomatism and K‐rich magmatism. For partitioning of Fe/Mg, Na2O, NiO, Cr2O3, and MnO between Mg‐pv and mw, no evident variation with temperature or pressure was detected. The results show that most of the reported (Mg,Fe)O and MgSiO3 inclusions in diamonds were not in equilibrium.

Major Rock-Forming Minerals in UHP Metamorphic Rocks

This paper reviews chemical characteristics of common minerals in ultrahigh-pressure (UHP) metamorphic rocks. Garnet in UHP metamorphic rocks belongs to the almandine-pyrope-grossular series and has a wide compositional range in Xalm (up to 0.73), Xprp (0.98), and Xgrs (0.92). Garnet with pyroxene exsolution microstructures reported from the Western Gneiss Region, Norway, shows relicts of majoritic garnet formed under P = 6 to 7 GPa. Most clinopyroxenes are Na-Ca groups and their Xjd ranges from 0.0 to 0.89. Members of the clinoenstatite-clinohypersthene (P21/c) were reported from peridotites in UHP terranes, and are considered to have been originally the high-pressure polymorph (C2/c) formed under P > 7 to 8 GPa. The presence of sodium in garnet and of potassium in clinopyroxene characterizes UHP equilibrium. Attention is drawn to minerals of the epidote group as strontium and REE containers, to amphibole as a sensor of metamorphic fluid composition, and to phengite as a geobarometer.

New Ca-silicate inclusions in diamonds — tracers from the lower mantle

Diamonds from the Kankan district of Guinea frequently contain majoritic garnet and ferropericlase mineral inclusions similar to those from Sao Luiz in Brazil [1–3]. Besides these ultra-high pressure phases in the Guinea diamonds, we have now identified, initially by in-situ single crystal X-ray diffraction, a new inclusion paragenesis of Ca-silicates. One diamond contained walstromite-structured CaSiO 3 and three others the mineral assemblage CaSi 2O 5 (titanite-structured) with larnite (p-Ca 2SiO 4). The first two phases represent new minerals not described before from natural occurrences. From the phase diagram for mantle CaSiO 3 [4], there are indications that primary CaSiO 3-perovskite underwent successive retrograde phase transformations. Development of equilibrium textures suggests slow exhumation. Ca-silicates are possibly important carriers of strontium, phosphorus and potassium and thereby contain part of the inventory of radioactive elements in the transition zone and the lower mantle. The presence of coesite (formerly stishovite) in two of the diamonds containing Ca-silicates indicates that these diamonds belong to an ‘eclogitic’ suite [5,6], whereas the presence of ferropericlase together with CaSiO 3 and MgSiO 3 in a third diamond may imply a ‘peridotitic’ environment.

The Nd and Hf isotopic evolution of the mantle through the Archean. results from the Isua supracrustals, West Greenland, and from the Birimian terranes of West Africa

Evidence for early differentiation events in the Earth’s mantle similar to those that demonstrably affected the Moon and Mars seems to be embedded in the existence of largely super-chondritic 143Nd/ 144Nd ratios in the Early Archean terrestrial mantle. Samples from the ca. 3.8 Ga old rocks from the Isua supracrustal belt, Greenland, and from the ca. 2.1 Ga old juvenile segment of Birimian crust, West Africa, were measured for Nd and Hf isotopic compositions in order to evaluate whether Hf systematics may help enlighten this debate. The determination of Nd and Hf initial isotopic compositions of these terranes is entangled with the uncertainty of which geological events isochron ages and ion probe zircon U-Pb ages actually record, and by metasomatic fractionation of parent/daughter element ratios. It is shown that Nd and Hf mobility has affected the samples for which the Sm/Nd and Lu/Hf ratios depart from the range of modern volcanic rocks. The most conspicuous effect is a preferential leaching of Hf—presumably accompanied by Zr—from mafic metavolcanic rocks. Precipitation of Zr from metasomatic fluids may explain the significantly younger overgrowth zones on zircons from the felsic rocks. For Isua, the pooled whole-rock isochron ages are 3.712 ± 26 Ga (Nd) and 3.593 ± 15 Ga (Hf), while for the Birimian the respective ages are 2.135 ± 29 Ga (Nd) and 2.093 ± 16 Ga (Hf). Elemental mobility and the unsupported assumption of minimum variance underlying the isochron model, however, render the significance of these ages questionable. In an attempt to restore possible ancient mantle-crust arrays, the trajectory of each sample in the (ε Nd[T], ε Hf[T]) space for variable values of the age T are reconstructed. Our best estimate for the isotopic properties of the mantle protolith is ε Nd[3.85] ≈ +2 ± 2 and ε Hf[3.85] ≈ +4 ± 2 for Isua and ε Nd[2.15] ≈ +3 ± 1 and ε Hf[2.15] ≈ +6 ± 2 for the Birimian. We therefore agree with the earlier suggestion that some samples from the Isua supracrustal belt were extracted from a mantle that went through strong parent/daughter fractionation very early in the Earth’s history. This event may be the original differentiation of a largely molten mantle in the presence of majorite garnet. In addition, the contrast between the isotopically inferred long-term Lu/Hf fractionation and the actual Lu/Hf ratio of the rocks requires that garnet was present as a residual mineral (the ‘Hf paradox’). It indicates that melting must have started in the Archean in plumes at a very deep level, thereby providing a mechanism to create thick buoyant oceanic plateaux that would act as nuclei for proto-continents.

Geochemical diversity in oceanic komatiites and basalts from the late Archean Wawa greenstone belts, Superior Province, Canada: trace element and Nd isotope evidence for a heterogeneous mantle

Diverse types of komatiites and basalts are present in the 2.7 Ga Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Wawa subprovince. Al-undepleted and Al-depleted komatiites coexist. The least altered Al-undepleted komatiites are characterized by variably depleted LREE and flat to slightly fractionated HREE patterns (La/Sm n=0.5–1.2, Gd/Yb n=0.90–1.3). They have small positive Zr (Hf) anomalies relative to MREE. Al-depleted komatiites and komatiitic basalts have fractionated REE (La/Sm n=2.2–2.4), and variable Nb and Zr (Hf) anomalies (Nb/La pm=0.8–1.4; Zr/Zr*=0.8–1.0). They feature pronounced Fe-enrichment in comparison with Al-undepleted komatiites (Fe 2O 3=15–22 vs. 11–16 wt%). Tholeiitic basalts are the prevalent volcanic rock type. Mg# varies from 65 to 33 over a relatively narrow range of SiO 2 (47–53 wt%). Chromium and Ni decrease erratically with Mg#, whereas Fe, Ti, Zr and all REE increase. Compositionally, these volcanic rocks are similar to Phanerozoic ocean plateau basalts such as at Ontong Java. In suites of basalts from each of four areas, Nb and Th anomalies relative to La vary continuously from negative to positive (Nb/La pm=0.56–1.10, Th/La pm=0.32–1.23). Mantle melting processes, crustal assimilation, alteration and metamorphism, or fractional crystallization can all be ruled out as the cause of anomalies. Rather, the anomalies are interpreted to be recycling of lithosphere into the mantle: recycled ocean crust processed though a subduction zone having positive Nb anomalies, and complementary recycled subarc mantle or continental crust having negative anomalies. Alkali basalts show strong REE fractionation (La/Yb n=14–16), and large positive Nb anomalies (Nb/La pm=1.32–1.42) like Phanerozoic HIMU ocean island basalts, whereas transitional counterparts have less fractionated REE (La/Yb n=2.6–5.3), and higher Al 2O 3 contents with convex-up patterns through La–Nb–Th. Both variants share the negative Zr and Hf anomalies with Al-depleted komatiites and komatiitic basalts, and generally have high Zr/Hf ratios. Al-depleted komatiites and associated alkali basalts tend to have more positive ε Nd (+3.3 to +3.5) than Al-undepleted komatiites and associated tholeiites (+0.7 to +0.5). The former association has generally flat trends of Nb/La pm and Th/Nb pm with ε Nd consistent with mixing of more and less depleted mantle components. The latter association shows negative trends of Th/La pm and Th/Nb pm with ε Nd, consistent with mixing between a depleted mantle component and a less depleted or enriched component of recycled crust. The komatiites and spatially associated tholeiitic to alkaline basalts are interpreted to have been an ocean plateau derived from a heterogeneous multi-component plume. Negative Zr (Hf) anomalies are consistent with melt segregation with residual majorite garnet at a depth of >400 km, whereas the tholeiitic basalts melted at shallower depths either in the plume or in entrained upper mantle. The plateau was fragmented in a subduction–accretion complex and tectonically mixed with primitive arc tholeiites.

Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa; implication for the behavior of potassium in the mantle

Subsolidus and melting experiments of a K-rich basaltic composition have been performed to 27 GPa. Pyroxene and majoritic garnet contain significant amounts of K 2 O (1- 2 wt%) at 6-12.5 GPa and above 22.5 GPa, respectively. Both pyroxene and garnet may act as main reservoirs for K in the mantle. Sanidine (KAlSi 3 O 8 ) or its polymorph potassium hollandite is the first phase to be melted up to 20 GPa, but potassium hollandite becomes a liquidus phase at P ≤ 22.5 GPa. In contrast to its typical incompatible behavior at low pressures, potassium is a compatible element in the deep mantle, at least in basaltic systems. This compatible behavior implies that a K-rich melt like kimberlite is difficult to generate at lower mantle conditions.

GARNET MICROSTRUCTURES FROM AN ULTRA-DEEP (>185 KM) OROGENIC PERIDOTITE

Relic majoritic (or super-silicic) garnets were recently discovered within orogenic garnet peridotites at the island of Otroy, Western Gneiss Region, W. Norway (Van Roermund and Drury, 1998; Van Roermund et al., 1999). However within the Otroy garnet peridotites relic majoritic garnet microstructures are rare and the petrogenetic relationship( s) between relic majoritic- and “normal” garnet is unknown. For this reason we have made a detailed SEM study of microstructures and solid-phase inclusions present exclusively within all garnets from the Otroy garnet peridotites. MAJORITIC GARNET MICROSTRUCTURES AND CHEMISTRY The super-silicic garnet microstructure is characterised by the following microstructural elements: a) Majoritic garnet nodules consist of polycrystalline garnets ranging in size from 2 to 8 mm. Garnet-garnet grain boundaries are straight or gently curved, triple point junctions are common, interstitial orthopyroxenes decorate some garnet-grain boundaries. b) In the larger garnet grain cores two pyroxene exsolution needles occur 5-10 mm thick and oriented // grt. The relative vol% between exsolved cpx and opx is 1: 9. c) Two-pyroxene-bearing garnet-cores are surrounded by 2 mm thick precipitation-free rims. Backscattered scanning electron microscopy was used to estimate the amount of majorite component present in garnet nodules before exsolution. Large interstitial orthopyroxenes at garnet grain boundaries comprise up to 3.6 vol.%. In the garnet grain cores the maximum pyroxene content is 1 vol.%. Including both types of pyroxene-exsolution the maximum amount of exsolved pyroxene is 3.6-4.0 vol.%. This corresponds to pressure estimates around 6-6.5 GPa and a minimum depth of origin for the Otroy garnet peridotites in the range of 185-200 km. Microprobe analyses of exsolved pyroxenes and garnethost reveal homogeneous mineral compositions, except for Al2O3 wt% in opx. The most dominant Al2O3 wt% value in opx is 0.7-0.8. Estimated PT conditions for mineral-chemical equilibration, using standard geothermobarometric methods, indicate upper mantle conditions around 805 ± 40°C and 3.2 ± 0.2 GPa. These PT estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory extends backwards to twice as great a depth-range than can be obtained by thermobarometric methods. In addition to the exsolved two pyroxenes we have found native nickel-particles as solid inclusions in relic majoritic garnets. This is indicative for relatively reduced oxygen fugacity conditions. The nickel particles are surrounded by complex symplectitic coronas involving Ni-Al oxide, Orthopyroxene, SiO2 and Ni-bearing garnet. NON-MAJORITIC GARNET MICROSTRUCTURES AND CHEMISTRIES The following solid-phases occur as inclusions in “normal” pyrope-rich garnets: a) Ni-Fe-Cu sulfides. Two types occur: 1) co-linear trails of rounded particles less than 1 mm in size and interpreted to represent precipitation along healed cracks. 2) Coarsened irregular-shaped inclusions up to 50 mm in size. The latter are often present in clusters. b) (Fe-) Ti (oxide??) needles (oriented // grt). c) (Fe-) Ti -oxides randomly precipitated or decorating dislocations. d) Isolated spinel needles and precipitates. With respect to the garnet host two types occur: oriented and randomly distributed crystals. e) Recrystallised garnet 2 - spinel - cpx/amphibole - orthopyroxene (± Ni-Fe-Cu sulfides) mineral assemblages. In addition to the various solid-garnet-inclusions garnetdeformation microstructures are present including naturally decorated dislocations. In thin-section garnet is often heavily fractured, and a penetrative crack-spacing down to the micron-scale is not uncommon. More widely-spaced or more heterogeneously fractured garnets exhibit the following microstructural domains: i) in between the cracks garnet is geometrically strained and deformation-induced microstructures such as undulous extinction and cross-hatched deformation bands are visible. This is recognised by the local non-isotropic nature of garnet. Within some of the strained domains dislocations are identified due to the fact that micron-sized particles have been precipitated along the dislocation cores. TEM and attached analytical facilities demonstrate that the solid inclusions are titanium-oxides, most likely rutile. ii) In between the cracks garnet is fully isotropic and strainfree. Within such domains randomly oriented solid inclusions are recognised one order of magnitude larger than the precipitates that decorate dislocations in strained garnet domains. All solid garnet inclusions and deformation microstructures predate the well-known two-pyroxene-spinel corona’s that surround garnet and form due to the instability of the mineral assemblage garnet-olivine. All garnet microstructures are consistent with a polycyclic exhumation model involving diapiric upwelling of deep-mantle peridotites to lithospheric depths followed by isobaric-cooling down to normal continental lithosphere conditions. Crustal incorporation occurs during renewedsubduction of continental crust during continental collision subsequently followed by final exhumation.

Plume magmatism and crustal growth at 2.9 to 3.0 Ga in the Steep Rock and Lumby Lake area, Western Superior Province

The greenstone belts of the western Superior Province are predominantly 2.78 to 2.69 Ga and provide evidence of oceanic and arc volcanism during the accretionary phase of development of the Superior Province. There is also scattered evidence of Meso-Archean crust (predominantly 2.9 to 3.0 Ga) within the western Superior Province. The Meso-Archean greenstone belts commonly contain platformal sediments and unconformably overlie granitoid basement. The platformal sediments occur associated with komatiitic and tholeiitic volcanic rocks that suggest a history of magmatism associated with rifting during the Meso-Archean. The central Wabigoon Subprovince is a key area of Meso-Archean crust and in its southern portion comprises the Steep Rock, Finlayson and Lumby Lake greenstone belts. The Steep Rock greenstone belt unconformably overlies 3 Ga continental basement and contains platformal sediments succeeded by komatiitic and tholeiitic volcanic rocks. The Lumby Lake greenstone belt contains thick sequences of mafic volcanics, a number of komatiite horizons, and thin platformal sedimentary units. The two belts are joined by the predominantly mafic volcanic Finlayson greenstone belt. The volcanics throughout these three greenstone belts may be correlated to some extent and a range of basaltic and komatiite types is present. Al-undepleted komatiites present in the Lumby Lake greenstone belt have an Al 2O 3/TiO 2 ratio ranging from 14 to 27 and (Gd/Yb) N from 0.7 to 1.3. These are divided into basaltic komatiites with generally unfractionated mantle-normalised multi-element profiles, and spinifex-textured high-Mg basalts with slightly light REE enriched multi-element profiles and small negative Nb and Ta anomalies. The unfractionated basaltic komatiites represent high degree partial melts of the upper mantle whereas the spinifex-textured high-Mg basalts represent evolutionary products of the komatiite liquids following olivine and chromite fractionation and crustal contamination. Al-depleted komatiites are present in both the Lumby Lake and Steep Rock belts and have Al 2O 3/TiO 2 ratio ranges from 2.5 to 5. These display strong enrichment in the light REE and Nb and strong depletion in the heavy REE and Y ((Gd/Yb) N=2–4). They represent a deep mantle plume source generated from a high degree of partial melting in the majorite garnet stability field. The basaltic flows in all three greenstone belts are predominantly slightly light REE depleted and represent a slightly depleted upper mantle source. Basalts spatially associated with the unfractionated basaltic komatiites and the slightly light REE enriched spinifex-textured high-Mg basalts are also slightly enriched in light REE and have negative Nb and Ta anomalies. These basalts represent evolved products of the primitive basaltic komatiites and enriched spinifex-textured high-Mg basalts after further crustal contamination and olivine and clinopyroxene fractionation. The geochemical stratigraphy in the Lumby Lake belt is consistent with an ascending mantle plume model. The light REE depleted basalts were derived from upper mantle melted by an ascending mantle plume. These are overlain by the unfractionated basaltic komatiites and their evolutionary products which represent hotter plume head material derived from a mixture of plume mantle and entrained depleted upper mantle. In turn, these are overlain by strongly light REE and HFSE enriched komatiites that represent a deep plume source that has not been mixed with depleted mantle and are, therefore, likely to have been derived from a plume core or tail. Volcanism was protracted in these three greenstone belts lasting ca. 70 Ma and combined stratigraphic evidence from the Lumby Lake and Steep Rock belts suggests that more than one plume may have ascended and tapped the same mantle sources, over time, within the area. Plume magmatism and rifting of continental platforms thus appears to have been an important feature of crustal development in the Meso-Archean.

Ultra‐high pressure (P > 6 GPa) garnet peridotites in Western Norway: exhumation of mantle rocks from > 185 km depth

We report here for the first time the occurrence of relics of majoritic garnet within orogenic garnet peridotites from Otrøy, Western Gneiss Region, Norway. The microstructural evidence consists of two‐pyroxene exsolution from garnet. Majoritic garnets are only stable at depths greater than 150 km. Estimates of the initial composition of the majoritic garnets imply pressures of 6–6.5 GPa indicating that the Otrøy peridotites were derived from depths > 185 km. Mineral‐chemical data indicate that the present mineral compositions equilibrated at mantle conditions around 805 ± 40 °C and 3.2 ± 0.2 GPa. Estimates of the initial pressure temperature (PT) conditions and PTtime (t) path are consistent with a multistage, multiorogenic exhumation history with upwelling of hot asthenosphere up to ≈ 100 km in the Pre‐Cambrian followed by subsequent crustal emplacement and exhumation during the Caledonian orogeny.

Majorite Destabilisation on Decompression: Constrains from Natural Samples on Plume Velocity

Majorite garnet hosting pyroxene component had been synthesized by Ringwood in 1967, prior to its discovery in nature at pressure in excess of 8G Pa within the MgO-FeO-CaO-Na20-AI203 system. The presence of natural majorite garnet in xenoliths from Jagersfontein kimberlite (garnet xenocrysts: Haggerty and Sautter 1990; garnet lherzolite xenolith: Sautter et al., 1991) and as syngenetic inclusions in diamonds from Monastery (Moore and Gumey, 1985) and Sao Luis (Wilding et al., 1989) evidence thus depths of sampling whithin the Transition Zone (TZ in between 400 and 670 km). Consequently the question of depth of kimberlite magma generation has been reassessed from those ultradeep samples. According to Ringwood et al, 1992, kimberlites result from partial melting in the TZ thus transport directly to the surface ultradeep xenoliths, garnet xenocrysts and diamonds with their majorite inclusion. A different scenario would be a two stages ascent: slow entrainment into convective system from more than 400 km depth to the base of the lithosphere relayed by final explosive sampling by the kimberlite throughout the thickness of the lithosphere. Another kind of two stages ascent would be an initial transportation at the head of rising plume followed by final kimberlite capture. In this paper, we use the various degree of majorite destabilisation exsolving its pyroxene component on decompression, to infer exhumation history of those ultradeep samples. In the high-Si majoritic garnet (3.x Si cations for 12 O anions), excess Si is in octahedral site and some Na occupies cubic sites. As pressure decreases, octahedral site expansion releases this extra-Si (0.x) that combines with Na to form jadeiterich clinopyroxene coexisting now with a normal garnet (3Si cations for 12 O anions). Such a reaction involves structural and chemical changes. It is a reconstructive process that proceeds by heterogeneous nuclation on defects and growth by diffusion and interface migration. If pressure decrease is the driving force of this transformation, its depends as well on temperature as exsolution is also a thermally activated process. During uplift pressure and temperature will thus have opposite effects.

Thermal equation of state of garnets along the pyrope-majorite join

P-V-T relations of two garnet samples along the pyrope (Py)-majorite (Mj) join (Py n Mj 1− n , where subscripts indicate molar percent) were measured at pressure and temperature conditions up to 11 GPa and 1163 K, respectively, with energy-dispersive synchrotron X-ray diffraction in a cubic-anvil, DIA-6 type apparatus (SAM-85). For each volume measurement, non-hydrostatic stress was determined from relative shifts of the diffraction lines of NaCl, within which the sample was embedded. Heating to ∼ 1100 K reduced the strength of NaCl below 0.1 GPa, making the measurements nearly hydrostatic. The recovered samples were examined by transmission electron microscopy (TEM) to detect irreversible changes that may affect data quality. For the cubic garnet Py 62Mj 38, no irreversible changes took place during the experiment. A fit using the third-order Birch-Murnaghan equation of state yielded ambient cell volume V 0 = 1509.2(3) Å 3 (113.62 cc/mol), isothermal bulk modulus K T0 = 160(3) GPa, and its pressure derivative K′ T0 = 4.9(5). Various thermal equations-of-state analyses gave consistent results for the temperature derivative of bulk modulus ( ∂K T ∂T ) P = −0.020(1) GPa K −1 pressure derivative for thermal expansion ( ∂α ∂P ) T = −7.8 × 10 −7 GPa −1 K −1 , with the ambient volumetric thermal expansion α 0 = 2.5 × 10 −5 K −1. For the tetragonal sample Py 21Mj 79, TEM indicated that micro-twin domains were significantly coarsened after the P-V-T experiment. Stress relaxation may have occurred during the coarsening, thereby compromising the P-V-T data. In addition, the tetragonal distortion in this sample was too small to be resolved by the energy-dispersive technique. Therefore, data for the latter sample were not suitable for accurate equation-of-state analysis. Previous data on pyrope were compiled, analyzed, and compared with the new data on the majoritic garnet. The entire data set enabled us to examine systematics of the thermoelastic properties of garnets along the majorite-pyrope join. These data will have important applications for modeling the transition zone of the mantle. Published by Elsevier Science B.V.

Variationally induced breathing equations of state of pyrope, grossular, and majorite garnets

We have completed the first non‐empirical fully optimized calculations of the equations of state for pyrope, grossular, and MgSiO3 majorite garnets. The static Gibbs free energy of the 160‐atom unit cell was minimized relative to all 27 structural parameters for the tetragonal structure. Zero point vibration and thermal effects were estimated with a Debye‐like model and Debye temperatures obtained from the equations of state. Calculated zero‐pressure densities differ from experimental values by less than 1%, while the bulk‐moduli are systematically larger than those obtained spectroscopically. Nevertheless, the calculations strongly support experimental evidence that the bulk modulus of MgSiO3 majorite is close to that of pyrope and further indicate that its (∂KT/∂P)o is not distinctly larger than that of other silicate garnets. If these properties are close to those of real majorite, the steep gradient of the bulk modulus in the transition zone is at best marginally consistent with a Iherzolitic composition.

Optical absorption spectra of transition zone minerals and implications for radiative heat transport

Optical and near-infrared absorption spectra of synthetic crystals of wadsleyite of bulk composition (Mg0.80Fe0.20)2SiO4 and polycrystalline samples of majorite garnet with composition Mg0.85Fe0.15SiO3 were measured from 4,000 to 25,000 cm−1. Unpolarised spectra obtained from wadsleyite display a continuous band of absorption peaks from the near-infrared through most of the visible region. A minimum of four Gaussian components is required to reproduce the spectra. Two of these bands, at 8,902 cm−1 and 11,136 cm−1, are assigned to crystal field transitions 5T2g→5Eg of Fe2+ in the octahedral sites, and two peaks centred at 13,384 cm−1 and 15,921 cm−1 are attributed to Fe2+→Fe3+ intervalence charge transfer facilitated by the edge-sharing octahedra in the structure. The optical absorption spectrum of majorite also has contributions from crystal field transitions and intervalence charge transfer bands. The peaks at 5,561, 6,294, and 8,439 cm−1 are assigned to 5Eg→5T2g transitions of Fe2+ in the dodecahedral site, and the peak at 9,428 cm−1 is due to 5T2g→5Eg transition of Fe2+ in the octahedral site. The peak at 22,663 cm−1 is assigned to the 6A1g→4A1g (4Eg) transition of Fe3+, probably in an octahedral site. The broad peaks at 16,844 and 20,225 cm−1 are attributed to Fe2+→Fe3+ intervalence charge transfer between edge-shared dodecahedral and octahedral sites in the structure. These results indicate that radiative transport of heat in the Earth's transition zone will be effectively blocked by the strong absorption bands of wadsleyite and majorite that cover the near-infrared and visible range. On the other hand, if the total iron content in the Earth's interior is high enough to generate a continuous network of Fe2+ and Fe3+ polyhedra in these phases, then Fe2+→Fe3+ charge transfer could provide a mechanism increasing the electrical conductivity of the Earth's transition zone.

The stability and equation of state of majoritic garnet synthesized from natural basalt at mantle conditions

The high‐pressure stability and equation of state of majoritic garnet synthesized from natural basalt have been determined using the laser‐heated diamond cell and x‐ray diffraction. Natural mid‐ocean ridge basalt (MORB) and tholeiitic basalt transform from a garnet‐dominated mineral assemblage to a silicate perovskite‐dominated mineralogy near 42 (±3) GPa. Static compression data (at 300 K) for the majoritic garnet synthesized from the natural tholeiitic basalt yields an isothermal bulk modulus, K0T, of 226.2 (±9.3) GPa with dK0T/dP constrained to be 4. An equally acceptable fit to the data is obtained if K0T is constrained to be 180 GPa and dK0T/dP is 7 (±1). Using our new equation of state and phase stability results for basaltic (majorite‐structured) garnet, we calculate density models for the subducting oceanic crust at mantle pressures and temperatures, and find that the basaltic crust is 0.4 g/cm³ (9%) less dense than the lower mantle at 660 km depth. If we assume that the subducted crust can delaminate from the underlying lithosphere, a simple buoyancy model for the crustal portion of subducting slabs indicates that basaltic crust may not sink below 660 km in depth.

An experimental study of the garnet-perovskite transformation in the system MgSiO 3Mg 3Al 2Si 3O 12

Phase transformations in the system MgSiO 3Mg 3Al 2Si 3O 12 have been studied at pressures between 22 and 28 GPa, at 1500°C, using a multi-anvil apparatus. Majorite garnet with less than about 10 wt.% Al 2O 3 was found to completely transform into an orthorhombic perovskite structure over a pressure interval between 24 and 27 GPa, whereas pyrope garnet disproportionated into sub-aluminous perovskite plus corundum solid solutions at pressures greater than 26.5 GPa. Alumina contents in the perovskite structure are dependent on temperature, and increase from 11 wt. % at 1500°C to 17 wt. % at 1800°C, at a pressure of 27 GPa. The alumina content of perovskite also in creases with increasing pressure, such that aluminous perovskite with the pyrope composition (25 mol % Al 2O 3) should be formed by about 30–40 GPa. These observations are consistent with our recent experimental results on a pyrolite composition, which demonstrated that no separate aluminous phase exists in the upper part of the lower mantle. The entire inventory of Al 2O 3 in pyrolite can be accommodated in solid solution in magnesium silicate perovskite.

Elasticity of stishovite at high pressure

The elastic-wave velocities of stishovite, the rutile-structured polymorph of SiO 2, were measured to 3 GPa at room temperature in a piston cylinder apparatus using ultrasonic interferometry on polycrystalline samples. These polycrystalline samples (2–3 mm in length and diameter) were hot-pressed at 14 GPa and 1050°C in a 2000 ton uniaxial split-sphere apparatus (USSA-2000) using fused silica rods as starting material. They were characterized as low porosity (less than 1%), single phase, fine grained, free of cracks and preferred orientation, and acoustically isotropic by using density measurement, X-ray diffraction, scanning electron microscopy, and bench-top velocity measurements. On the basis of subsequent in situ X-ray diffraction study at high P and T on peak broadening on similar specimens, it is evident that the single crystal grains within these polycrystalline aggregates are well equilibrated and that these specimens are free of residual strain. P- and S-wave velocities measured at 1 atm are within 1.5% of the Hashin-Shtrikman bounds calculated from single-crystal elastic moduli. Measured pressure derivatives of the bulk and shear moduli, K′ 0 = 5.3 ± 0.1 and G′ 0 = 1.8 ± 0.1, are not unusual compared with values measured for other transition zone phases such as silicate spinel and majorite garnet. Isothermal compression curves calculated with the measured values of K 0 and K′ 0 agree well with experimental P-V data to 16 GPa. The experimental value of dG dP is in excellent agreement with predictions based on elasticity systematics. Theoretical models are not yet able to replicate the measured values of K′ 0 and G′ 0.

Compositional change of majoritic garnet in a MORB composition from 7 to 17 GPa and 1400 to 1600°C

Experimental phase relations of an average normal mid-ocean ridge basalt (N-MORB) composition from 7 GPa to 17 GPa and 1400 to 1600°C reveal the compositional variation of garnet in equilibrium with pyroxene. The Na 2O content of majoritic garnet in equilibrium with pyroxene and stishovite increases up to 2.5 wt.% with increasing pressure and temperature. Na enters the garnet structure via the substitution (Mg,Fe,Ca) + Al = Na + (Si,Ti). The pyroxene-garnet partition coefficient ( D cp-ga) for na decreases with increasing pressure. However, clinopyroxene coexisting with garnet becomes more Na rich as pressure increases. These relationships are independent of the FeO content in garnet. Our experiments confirm that rare majoritic and Na-rich garnet inclusions in diamonds have formed at transition zone pressure, in lithologies of basaltic bulk composition.

Secular trends in the melting depths of mantle plumes: Evidence from HFSE/REE systematics of Archean high-Mg lavas and modern oceanic basalts

Al-depleted komatiites and associated Fe-tholeiites from Boston Township, Abitibi belt (∼ 2.7 Ga), Canada, are characterized by the conjunction of low Al 2O 3/TiO 2 (4–5) ratios, low Sc and Y contents, and HREE depletion [(Gd/Yb) n = 1.7–2.3], with negative P, Zr and Hf anomalies relative to MREE on primitive-mantle-normalized diagrams (P/Nd = 23–36; Zr/Sm = 13–18; Hf/Sm = 0.3–0.5). Enriched high-Mg tholeiites (EHMT) from the Norseman area ( ∼ 2.7 Ga), Western Australia, are characterized by low Al 2O 3/TiO 2 (4.0–4.7), fractionated HREE [(Gd/Yb) n = 2.3–2.5], and negative P anomalies (P/Nd = 21–26), but near-chondritic Zr/Sm (23–27 vs. 25) and Hf/Sm (0.6–0.8 vs. 0.7) ratios. In both areas there are tholeiites with flat REE patterns derived from a distinct mantle source than the komatiites and EHMT. Different HFSE and REE systematics between the two late Archean high-Mg lavas are interpreted as partial melting of mantle plumes with different residual minerals at different depths; Boston Township Al-depleted komatiites may have been derived from partial melting of a plume with majorite garnet in the residue at pressures between 14 and 24 GPa, whereas Norseman EHMT may have been generated either by low-degree partial melting with a olivine + orthopyroxene + clinopyroxene + garnet residue at pressures < 6 GPa, or more advanced melting with pyrope garnet in the residue at pressures between 6 and 14 GPa. Comparison of HFSE/REE systematics between Archean high-Mg lavas and modern OIB indicates that most modern OIB are featured by low Al 2O 3/TiO 2 (4–7), P/Nd (50–60) and fractionated HREE [(Gd/Yb) n = 2–3], but near-chondritic Zr/Sm (24–25) and Hf/Sm (0.6–0.7) ratios, similar to Norseman EHMT, consistent with low-degree partial melting with garnet in the residue at low pressures. Partial melting of Archean plumes which generated Al-depleted komatiites may have occurred at greater depths than that of modern plumes. HFSE/REE systematics of plume-derived volcanic rocks records decreasing melting depths of mantle plume over time, consistent with the secular cooling of the Earth.