Fast diffusion along mobile grain boundaries in calcite

Experimental measurements of grain boundary diffusion are usually conducted on static boundaries, despite the fact that grain boundaries deep in the Earth are frequently mobile. In order to explore the possible effect of boundary mobility on grain boundary diffusion rates we have measured the uptake of ⁴⁴Ca from a layer of ⁴⁴Ca-enriched calcite powder during the static recrystallization of a single crystal of calcite at 900°C. A region about 500 μm wide adjacent to the powder layer is heterogeneously enriched in ⁴⁴Ca, and complex zoning patterns, including sharp steps in composition and continuous increases and decreases in ⁴⁴Ca content, are developed. In metamorphic rocks, these would normally be interpreted in terms of changes in pressure or temperature, Rayleigh fractionation, or episodic fluid infiltration. These explanations cannot apply to our experiments, and instead the zoning patterns are interpreted as being due to variations in grain boundary migration rate. We have applied an analytical model which allows the product of grain boundary diffusion coefficient and grain boundary width (D _GB δ) to be calculated from the grain boundary migration rate and the compositional gradient away from the powder layer. The value of D _GB δ in the mobile grain boundaries is at least five orders of magnitude greater than the published value for static boundaries under the same conditions. In order to allow the scale of chemical equilibrium (and hence textural evolution) to be predicted under both experimental and geological conditions, we present quantitative diffusion-regime maps for static and mobile boundaries in calcite, using both published values and our new values for grain boundary diffusion in mobile boundaries. Enhanced diffusion in mobile boundaries has wide implications for the high temperature rheology of Earth materials, for geochronology, and for interpretations of the length- and time-scales of chemical mass-transport. Moreover, zones of anomalously high electrical conductivity in the crust and mantle could be regions undergoing recrystallization such as active shear zones, rather than regions of anomalous mineralogy, water- or melt-content as is generally suggested.     

Empirical geothermometers and geothermobarometers for spinel peridotite phase assemblages

Experimental synthesis of spinel peridotite phase assemblages for a range of compositions that mimic natural samples is used to derive a set of empirical geothermometers and geothermobarometers represented by multiple linear regression best-fit surfaces that link the variables of temperature, pressure, and composition. The calibrated geothermometers use reactions that govern the solubility of Al and Cr in both pyroxenes and the Mg–Fe exchange between silicates and spinel. Geothermobarometers map the Mg–Fe exchange between coexisting olivine and clinopyroxene and pyroxenes and Ca–Mg exchange between coexisting pyroxenes. Application of the geothermometers and geothermobarometers to suites of naturally occurring samples indicates that while reactions governing the Cr and Al solubility and solvus of orthopyroxene give useful estimates of ‘original’ mantle temperatures and pressures, respectively, comparable reactions for clinopyroxene yield estimates that are variably dependent on the transport phase of the sample suites. Temperature and pressure estimates from reactions governing Mg and Fe exchange between silicates and spinel and coexisting silicates are all sensitive to the later transport stage of the samples.     

Uranium in spinel peridotite inclusions in basalts from Sardinia

The uranium distribution in spinel peridotite inclusions and their host basalt from Sardinia, Italy, was determined by fission-track mapping. Whole-rock U concentrations range from 14 to 55 ppb. Although the partitioning of U among major silicate phases of the inclusions — olivine, orthopyroxene and clinopyroxene — remains roughly constant, the U content in the minerals is highly variable, e.g. ranging from 27 ppb to 177 ppb in clinopyroxene. The U variation in the minerals shows no apparent correlation with their major element chemistry. Liquid which equilibrated with the assemblages of inclusions with high U content, had U concentrations higher than those found in basaltic rocks. It is suggested that the inclusions were contaminated with a phase strongly enriched in U and subsequently recrystallized. The available data show that spinel peridotite inclusions of basaltic rocks frequently have complex multistage evolution and thus cannot provide a representative picture of the upper mantle radioactivity.     

新疆克拉玛依地区百口泉发现尖晶石橄榄岩

新疆克拉玛依地区出露的早古生代蛇绿混杂岩带规模巨大,岩石单元出露齐全。白碱滩地区的地幔橄榄岩相对比较新鲜,单斜辉石、斜方辉石、尖晶石和橄榄石保存完好。研究表明,白碱滩蛇绿岩就位前,地幔岩发生了大于50km的快速隆升,且没有发生部分熔融。百口泉地区发现的地幔岩普遍遭受了改造,辉石多发生了强烈蚀变(透闪石化),但尖晶石和橄榄石保存较好。百口泉地区出露的地幔岩和白碱滩地幔岩的矿物组成基本一致,表明它们属于同一蛇绿混杂岩带。百口泉蛇绿岩剖面的揭露,将该蛇绿混杂岩带的范围向NE方向延伸了35km。     

Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle

The effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and nickel-rich aqueous fluid was evaluated experimentally at 1.2 GPa and 1,200°C. The grain boundary (GB) migration caused nickel enrichment in the area swept by the GBs in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to ~80 times that was calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be 6.5 × 10⁻¹⁹–6.7 × 10⁻¹⁸ m³/s, which is 4–5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time scale of chemical homogenization in the upper mantle.     

新疆克拉玛依地区百口泉发现尖晶石橄榄岩

新疆克拉玛依地区出露的早古生代蛇绿混杂岩带规模巨大,岩石单元出露齐全。白碱滩地区的地幔橄榄岩相对比较新鲜,单斜辉石、斜方辉石、尖晶石和橄榄石保存完好。研究表明,白碱滩蛇绿岩就位前,地幔岩发生了大于50km的快速隆升,且没有发生部分熔融。百口泉地区发现的地幔岩普遍遭受了改造,辉石多发生了强烈蚀变(透闪石化),但尖晶石和橄榄石保存较好。百口泉地区出露的地幔岩和白碱滩地幔岩的矿物组成基本一致,表明它们属于同一蛇绿混杂岩带。百口泉蛇绿岩剖面的揭露,将该蛇绿混杂岩带的范围向NE方向延伸了35km。     

Cr and Al diffusion in chromite spinel: experimental determination and its implication for diffusion creep

Diffusion coefficients of Cr and Al in chromite spinel have been determined at pressures ranging from 3 to 7 GPa and temperatures ranging from 1,400 to 1,700°C by using the diffusion couple of natural single crystals of MgAl₂O₄ spinel and chromite. The interdiffusion coefficient of Cr–Al as a function of Cr# (=Cr/(Cr + Al)) was determined as D _Cr–Al = D ₀ exp {−(Q′ + PV*)/RT}, where D ₀ = exp{(10.3 ± 0.08) × Cr#^0.54±0.02} + (1170 ± 31.2) cm²/s, Q′ = 520 ± 81 kJ/mol at 3 GPa, and V* = 1.36 ± 0.25 cm³/mol at 1,600°C, which is applicable up to Cr# = 0.8. The estimation of the self-diffusion coefficients of Cr and Al from Cr–Al interdiffusion shows that the diffusivity of Cr is more than one order of magnitude smaller than that of Al. These results are in agreement with patterns of multipolar Cr–Al zoning observed in natural chromite spinel samples deformed by diffusion creep.     

Experimental modal metasomatism of a spinel lherzolite and the production of amphibole-bearing peridotite

Experiments have been done which simulate the modal metasomatism of spinel lherzolite by partial melts of the subducted slab. The experiments were designed so that the metasomatizing melts were generated during the experiments by partial melting of a slab analog (basaltic composition amphibolite). The melts are thought to be representative of hybridizing melts in that they are derived by high-pressure partial melting under conditions appropriate to a hot slab geotherm. During the experiments, the melts infiltrate into and metasomatize a model depleted peridotite. Chemical modifications to minerals in the peridotite are of the same nature and extent as those found in naturally metasomatized spinel lherzolites. Modal metasomatism produced pargasitic amphiboles in runs at 1.5 GPa and in all but the highest temperature run at 2.0 GPa. The amphiboles are indistinguishable from amphiboles found in amphibole-bearing peridotites from supra-subduction zone environments. Systematic variations in amphibole composition suggest that the melt infiltration process in the experiments involved continuous modification of the composition of the infiltrating melt as observed around inferred quenched melt (i.c., amphibolite or amphibolite/clinopyroxenite) veins in xenoliths and massif peridotites. The compositions of the initial and final mineral phases in the experiments and those of the metasomatizing melts are used to derive amphibole formation reactions at 1.5 and 2.0 GPa that are similar in form to those inferred in studies of natural amphibole-bearing peridotites. The metasomatism reactions show that the extent of amphibole formation in peridotite at 1.5 and 2.0 GPa will, in general, be limited by clinopyroxene and spinel abundance.     

Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.     

Hf isotope compositions and HREE variations in off-craton garnet and spinel peridotite xenoliths from central Asia

Garnet-facies continental mantle is poorly understood because the vast majority of mantle xenoliths in continental basalts are spinel peridotite. Peridotite xenoliths from Vitim (southern Siberia) and Mongolia provide some of the best samples of garnet and garnet-spinel facies off-craton lithospheric mantle. Garnets in those fertile to moderately depleted lherzolites show a surprisingly broad range of HREE abundances, which poorly correlate with modal and major oxide compositions. Some garnets are zoned and have Lu-rich cores. We argue that these features indicate HREE redistribution after the partial melting, possibly related to spinel-garnet phase transition on isobaric cooling. Most peridotites from Vitim have depleted to ultra-depleted Hf isotope compositions (calculated from mineral analyses: ε_Hf(0) = +17 to +45). HREE-rich garnets have the most radiogenic ε_Hf values and plot above the mantle Hf-Nd isotope array while xenoliths with normal HREE abundances usually fall within or near the depleted end of the MORB field. Model Hf isotope ages for the normal peridotites indicate an origin by ancient partial melt extraction from primitive mantle, most likely in the Proterozoic. By contrast, an HREE-rich peridotite yields a Phanerozoic model age, possibly reflecting overprinting of the ancient partial melting record with that related to a recent enrichment in Lu. Clinopyroxene-garnet Lu-Hf isochron ages (31-84 Ma) are higher than the likely eruption age of the host volcanic rocks (∼16 Ma). Garnet-controlled HREE migration during spinel-garnet and garnet-spinel phase transitions may be one explanation for extremely radiogenic ¹⁷⁶Hf/¹⁷⁷Hf reported for some mantle peridotites; it may also contribute to Hf isotope variations in sub-lithospheric source regions of mantle-derived magmas.     

Rare earth abundances and distribution in some spinel peridotite xenoliths from Assab (Ethiopia)

REE partition data between solid phases in nine spinel peridotite xenoliths from Assab (Ethiopia) are presented together with bulk-rock REE composition. REE partitioning between clinopyroxene and other coexisting solid phases varies over a relatively wide range. The largest ranges are for LREE in clinopyroxene/orthopyroxene and clinopyroxene/olivine pairs, while clinopyroxene/spinel partitioning of REE is more restricted. The range of REE partition values between coexisting phases is due to compositional dependency effects and is correlated with systematic variations in major element composition of the bulk rocks. The measured REE concentration in the Assab mantle harzburgites do not match with the compositions calculated by mass balance from the modal proportions and REE analyses in individual phases. Inconsistencies for HREE may be due to variable HREE amounts in the clinopyroxene, orthopyroxene phases within a single specimen, while the high LREE contents in the whole rocks are due to contamination during transport to the surface. A geochemical model based on theoretical treatment of the REE partition data suggests that the Assab harzburgites acquired their residual character during a batch melting episode in the upper mantle under the Afar region.     

Depleted and enriched mantle processes under the Rio Grande rift: spinel peridotite xenoliths

Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.     

Sc,Cr, Co and Ni partitioning between minerals from spinel peridotite xenoliths

The partitioning of divalent (Co, Ni) and trivalent (Sc, Cr) trace elements between olivine, ortho- and clinopyroxene and spinel from spinel peridotite xenoliths has been investigated. These peridotites cover a wide range in modal composition from dunite to primitive lherzolites and have equilibrated in the upper mantle between >900° C and <1,200° C.The distribution of Co and Ni shows only minor variation through the whole sequence. In contrast, Sc partitioning between ortho- and clinopyroxene and olivine and clinopyroxene as well as Cr partitioning between olivine and clinopyroxene or olivine and orthopyroxene display high but systematic variations which can be assigned to dependences upon equilibration temperatues. Empirical temperature calibrations are given for Sc-orthopyroxene/clinopyroxene, Sc-olivine/clinopyroxene and Cr-olivine/clinopyroxene which, in principle, may permit to estimate equilibration temperatures not only for lherzolites or harzburgites but for orthopyroxene-free peridotites, too.Sc and Ni partition coefficients between spinel and mantle silicate minerals are primarily dependent upon the major element composition of spinel (e.g. Cr and Al) although a temperature dependence can still be identified. Probably such compositional effects are not observed for trace element partitioning between pyroxenes and olivine or ortho- and clinopyroxene only for the reason that in normal spinel peridotites these minerals show much less variation in major element composition than their coexisting spinels.     

Geochemistry and mineralogy of two spinel peridotite suites from Dreiser Weiher,West Germany

Among the spinel peridotite nodules from Dreiser Weiher. West Germany which represent fragments of the earth's upper mantle two series may be distinguished. One group (Ib) is anhydrous while the second one (Ia) is characterized by the presence of amphibole and/or breakdown-products of amphibole. Both suites display a wide range in modal composition. Pyroxene geothermometry yields equilibration temperatures of ~ 1150°C for group Ib and ~950°C for group Ia.Rare earth element (REE) patterns reveal marked differences between both groups: nearly unfractionated, light REE depleted or slightly light REE enriched chondrite normalized patterns in nodules from group Ib, but a high relative light REE enrichment in the amphibole containing suite Ia. An inverse correlation between the clinopyroxene content (or CaO) and relative light REE enrichment is observed in group Ia only. Two sources of light REE enrichment of the nodules can be distinguished: The first is a contaminant on mineral surfaces and can be removed by acid leaching. The second is an integral part of the constituent minerals of the nodules. Trace element abundances and modelling show that nodules of group Ib cannot be derived from upper mantle pyrolite by a one-stage partial melting process. Nodules of type Ia are interpreted as being the result of a reaction between Ib-type mantle and a fluid or liquid which provides H₂O and incompatible elements. At Dreiser Weiher this ‘open system’ upper mantle metasomatism may be related to the young uplift of the Rhenish Shield in a similar way as previously proposed by Lloyd and Bailey (1975).A model for the lithospheric mantle below the Westeifel is different from the San Carlos model proposed by Frey and Prinz (1978) and more complex.     

Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton

We report mineralogical and chemical compositions of spinel peridotite xenoliths from two Tertiary alkali basalt localities on the Archean North China craton (Hannuoba, located in the central orogenic block, and Qixia, in the eastern block). The two peridotite suites have major element compositions that are indistinguishable from each other and reflect variable degrees (0–25%) of melt extraction from a primitive mantle source. Their compositions are markedly different from typical cratonic lithosphere, consistent with previous suggestions for removal of the Archean mantle lithosphere beneath this craton. Our previously published Os isotopic results for these samples [Earth Planet. Sci. Lett. 198 (2002) 307] show that lithosphere replacement occurred in the Paleoproterozoic beneath Hannuoba, but in the Phanerozoic beneath Qixia. Thus, we see no evidence for a compositional distinction between Proterozoic and Phanerozoic continental lithospheric mantle. The Hannuoba xenoliths equilibrated over a more extensive temperature (hence depth) interval than the Qixia xenoliths. Neither suite shows a correlation between equilibration temperature and major element composition, indicating that the lithosphere is not chemically stratified in either area. Trace element and Sr and Nd isotopic compositions of the Hannuoba xenoliths reflect recent metasomatic overprinting that is not related to the Tertiary magmatism in this area.     

Diffusion and partitioning of magnesium in quartz grain boundaries

Experiments were conducted to study Mg diffusion in quartz grain boundaries. A detector particle method was used to study grain-boundary diffusion because Mg was confined exclusively to the grain boundaries. Diffusion couples were assembled by placing a MgF₂ disk against a disk of quartzite, which was placed against a disk of quartzite that contained fayalite (Fe₂SiO₄) ‘detector particles.’ During diffusion experiments, Mg diffused along the grain boundaries of the central quartzite toward the fayalite quartzite where it was incorporated into fayalite detector particles. The only pathway for transport from the diffusant source to fayalite detector particles was through the grain boundaries in the central quartzite. The cross-sectional area of the grain boundaries that delivered Mg to the fayalite detector particles was determined from scanning electron microscope images. The Mg contents of the fayalite detector particles were used to calculate the mass of Mg that fluxed through the grain boundaries. During the diffusion experiments, pyroxene crystallites nucleated and grew in the central quartzites from Mg and Fe that was transported along quartz grain boundaries. The Mg contents of the crystallites vary linearly throughout the quartzites, suggesting that steady-state transport conditions were rapidly established in the quartz grain boundaries. Magnesium concentrations in the pyroxene crystallites are proportional to concentration gradients in the grain boundaries of the central quartzite. Grain-boundary fluxes and linear concentration gradients were used to calculate diffusion of Mg in grain boundaries of the central quartzite component in the diffusion couples.     

Clinopyroxene-orthopyroxene major and rare earth elements partitioning in spinel peridotite xenoliths from Assab (Ethiopia)

Major element and rare earth element (REE) partitioning among coexisting clinopyroxene-orthopyroxene pairs from mantle xenoliths of the Assab Range (Ethiopia) are discussed in terms of crystal-chemistry.Major element partitioning indicates relatively uniform conditions of subsolidus equilibration over a narrow range of temperatures (mean value about 1100 C) in the spinel peridotite stability field. Major element distributions and correlations, moreover, seem to indicate that the mantle material studied underwent slightly different depletions prior to the metamorphic equilibration.In spite of the rather homogeneous major element compositions for both cpx and opx, clinopyroxenes show chondrite-normalized REE patterns which are widely variable both in shape and absolute values, whereas orthopyroxenes exhibit more restricted ranges and concordant profiles.REE activity ratios have been investigated by applying Iiyama's (Bull. Soc. fr. Minéral. Christallogr.97, 143–151) thermodynamic model: the estimated activity patterns exhibit a good coherence for the different pyroxene pairs, in spite of the contrasting features of their REE concentration ratios. The wide ranges in the measured partition values for the same rare earth element in different pyroxene pairs have been related to coupled substitutions involving A1 in the Z site and REE in the M₂ site of clinopyroxene.     

Structural width of low-angle grain boundaries in olivine

Electron diffraction and imaging techniques have been used to examine the structure and thickness of low-angle grain boundaries in olivine. For edge-on grain boundaries, a periodic array of dislocations in a boundary gives rise to sets of periodic rods of diffracted intensity that are recorded as streaks in the diffraction pattern. The length of these streaks provides a measure of the structural width of a boundary. For olivine, the structural width of a boundary with a given dislocation spacing is three to four times smaller than that in metals where the boundary width is approximately equal to the spacing between dislocations in the boundary.     

Structure and chemistry of grain boundaries in deformed, olivine + basalt and partially molten lherzolite aggregates: evidence of melt-free grain boundaries

Olivine grain boundaries in deformed aggregates of olivine + basalt and partially molten lherzolite were analyzed with various electron microscopy techniques to test for the presence of thin (0.5-10 nm) intergranular melt films. High-resolution transmission electron microscopy (HREM) observations reveal that most of the boundaries do not contain a thin amorphous phase, although a small fraction of grains are separated by relatively thick (~1 µm) layers of melt. However, due to the anisotropy of the olivine-melt interfacial energy, melt often tapers from a triple junction into an adjoining grain boundary over a length of 1 to 2 µm, giving an effective dihedral angle of only ~2°. The chemistry of olivine-olivine grain boundaries was analyzed using energy dispersive X-ray (EDX) profiling by scanning transmission electron microscopy (STEM) with a probe size of <1.5 nm. Ca, Al and Ti segregate to grain boundaries forming enriched regions of <7 nm width. Although these elements are concentrated in the glass phases, the presence of glass films with the same chemical composition as the bulk glass phases cannot explain concentrations of other elements such as Si and Al at the boundaries. Combined with the HREM results, the STEM/EDX profiling demonstrates the existence of chemical segregation between solid grains but the absence of thin, grain boundary melt films. Additionally, if melt films exist along all of the grain boundaries, as reported for similar samples by other groups, the rock should be substantially weakened. Creep experiments on the partially molten rocks analyzed in this study reveal little weakening at small melt contents, consistent with our observations of melt-free grain boundaries.