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.     

Thin amorphous films (1–2 nm) at olivine grain boundaries in mantle xenoliths from San Carlos,Arizona

 Olivine grain boundaries and phase boundaries in xenoliths from San Carlos have been investigated by high-resolution transmission electron microscopy (HREM) and analytical electron microscopy (AEM). Thin amorphous intergranular layers with variable width (1–2 nm) were detected along olivine grain boundaries. The Al₂O₃, TiO₂ and CaO concentrations of the amorphous layers increase with increasing width of the layer. The composition of the amorphous intergranular layers depends on the interface type – grain or phase boundary. Morphology, amorphous state and chemical composition of the intergranular layer suggest the presence of a melt film at olivine grain boundaries. Since the composition of the amorphous phase strongly depends on the type of interface, the melt must have been generated at the grain boundary. Also, the melt chemistry is different from the composition of partial melts produced from possible hydrous phases, such as phlogopite or amphibole, and from the host basanite. The mobility of very thin melt films is assumed to be very limited due to the strong interface forces between the melt and the grain boundary. It is concluded that grain boundary melting occurred at the interfaces due to decompression during uplift. The melt wetted olivine grain boundaries as well as olivine-opx phase boundaries. The thin amorphous layers formed melt microsystems. Mixing of melts from different microsystems is suggested to occur in wider melt films, melt veins or melt pockets thus creating a magmatic melt that could be extracted from its source. Received: 6 November 1995 / Accepted: 24 January 1996     

Distribution of brine in grain boundaries during static recrystallization in wet, synthetic halite: insight from broad ion beam sectioning and SEM observation at cryogenic temperature

We report observations from room temperature static recrystallization experiments (annealing times from minutes to year) of cold-pressed, synthetic, coarse-grained, wet sodium chloride, prepared by broad ion beam polishing and SEM observations at cryogenic temperature to observe directly the brine in grain boundaries. At all stages of annealing, the majority of the brine in the samples is connected in 2D sections along grain boundaries. Another part of the brine is in isolated brine inclusion arrays along grain boundaries and in brine inclusions left behind by migrating brine-filled grain boundaries. Most of these boundaries are mobile because the aggregate is coarsening. We interpret that the boundaries without observable brine films (<15 nm) and brine inclusion arrays are healed and immobile. Evolution of grain boundary structure involves three major processes. First, dissolution on one side of the grain boundary and precipitation on the other side, resulting in grain boundary migration. Second, the development of facets formed by low-index crystallographic planes of the grains bounding the grain boundary brine. When both sides of a grain boundary are able to develop low-index facets in a thick brine film, the resulting impingement boundary is interpreted to be immobile and may prevent the new grain from migrating into a deformed neighbor. When one side of a faceted boundary consists of low-index crystallographic planes and the other side passively follows this faceted shape along irrational surfaces, the boundary is mobile. Third, the healing of grain boundary brine films, producing solid–solid grain boundaries without resolvable brine films.     

Fluid Assisted Recrystallization in Upper Mantle Peridotite Xenoliths from Kimberlites

The development of recrystallization microstructures has beenstudied in some ‘hot deformed’ peridotite xenolithsfrom the Thaba Putsoa Kimberlite pipe in S. Africa. The xenolithswere deformed to high strains by dislocation creep in the uppermantle and then annealed as they were uplifted by the kimberlitefluid. Static recrystallization occurs during annealing producingeuhedral shaped ‘tablet’ grains. Tablet grain boundariesare sub-parallel to crystal growth habits in olivine and orthopyroxene.This microstructure is characteristic of recrystallization byfluid-assisted grain boundary migration, where a thin fluidfilm is present along the boundary. There is microstructural evidence for a complex fluid infiltrationhistory involving an early Fe-Ti rich metasomatic silicate fluidand later kimberlite fluids. Minor partial melting of clinopyroxenecan also be inferred, which, is consistent with infiltrationof a kimberlite-derived C-H-O rich fluid into the xenoliths.Any of these fluids could have been present along the tabletgrain boundaries during static recrystallization. The occurrenceof tablet grains in ‘cold deformed’ xenoliths, whichhave a simple infiitration history, suggests that a C-H-O richfluid derived from kimberlite is the most probable boundaryfluid in both the hot and cold deformed xenoliths. The occurrence of dynamically stable semi-continuous grain boundaryfluid films during re crystallization indicates that mechanismsof fluid segregation and transport in the upper mantle are likelyto be dependent upon the type of deformation and recrystallizationmechanisms operating. In addition the destabilization of thestatic fluid distribution by grain boundary migration and deformationwill also influence the rheology of the upper mantle where fluidsare present. ^*Present address: Mineralogy Research Centre, Research School of Chemistry, Australian National University Canberra ACT 2601, Australia.     

A close look at dihedral angles and melt geometry in olivine-basalt aggregates: a TEM study

Olivine-basalt aggregates sintered at high P/T have been used as a simplest approximation of partially molten upper mantle peridotite. In the past, geometry of partial melt in polycrystalline olivine (and other materials) has been characterised by dihedral (wetting) angles which depend upon surface free energy. However, since olivine (like most other crystalline materials) is distinctively anisotropic, the simple surface energy balance defining the dihedral angles cos(Θ/2)=_gb/2_sl is not valid and melt geometry is more complicated than can be expressed by a single dihedral angle value. We examine in detail melt geometry in aggregates held at high temperature and pressure for very long times (240–612 h). We show the simple dihedral angle concept to be invalid via transmission electron microscope images. Olivine-basalt interfaces are frequently planar crystal faces (F-faces) which are controlled by the crystal structure rather than the surface area minimisation used in the simple dihedral angle concept. Nevertheless, the dihedral angles may provide useful insights in some situations. They may give a rough estimation of the wetting behaviour of a system, or be used to approximate the melt distribution if F-faces are not present (possibly at large grain size and very low melt fraction). Our measurements, excluding F-faces, give a range of dihedral angle values from 0 to 10° which is significantly lower than reported previously (20–50°). The nature of 0° angles (films and layers up to 1 μm in thickness) is unclear but their frequency compared to dry grain boundaries depends on grain size and melt fraction (e.g. 70% for grain size 43 μm and melt fraction 2%). Received: 13 April 1997 / Accepted: 2 October 1997     

压力对辉长岩矿物反应和部分熔融影响的实验研究

本文在高温高压条件下,开展了辉长岩矿物反应与部分熔融实验,利用偏光显微镜与扫描电镜对实验样品微观结构观察,研究实验中的新生矿物与熔体的分布;通过电子探针分析熔体成分特征。实验结果表明,在低压(300MPa)条件下,静压和塑性变形实验样品中,单斜辉石以固体反应方式生成橄榄石,在高压(1300MPa)塑性实验中所有实验样品都没有发现新生矿物颗粒,这与相图中低压条件下斜长石与橄榄石稳定共存,而高压下斜长石-辉石稳定共存相吻合。高压塑性变形条件下,单斜辉石和黑云母首先发生部分熔融,随着温度增高,斜长石逐渐参与熔融,熔体呈薄膜状分布在矿物颗粒边界,熔体成分依赖于参与熔融的矿物成分,表明出现的熔体为非平衡熔融结果。     

High-temperature creep in a Ni2GeO4: a contribution to creep systematics in spinel

 High-temperature creep behavior in Ni₂GeO₄ spinel was investigated using synthetic polycrystalline aggregates with average grain sizes ranging from submicron to 7.4 microns. Cylindrical samples were deformed at constant load in a gas-medium apparatus at temperatures ranging from 1223 to 1523 K and stresses ranging from 40 to 320 MPa. Two deformation mechanisms were identified, characterized by the following flow laws: where σ is in MPa, d is in μm and T is in Kelvin. These flow laws suggest that deformation was accommodated by dislocation creep and grain-boundary diffusion (Coble) creep, respectively. A comparison with other spinels shows that an isomechanical group can be defined for spinels although some differences between normal and inverse spinels can be identified. When creep data for olivine and spinel are normalized and extrapolated to Earth-like conditions, spinel (ringwoodite) has a strength similar to olivine in the dislocation creep regime and is considerably stronger than olivine in the diffusion creep regime at coarse grain size. However, when grain-size reduction occurs, spinel can become weaker than olivine due to its high grain-size sensitivity (Coble creep behavior). Analysis of normalized diffusion creep data for olivine and spinel indicate that spinel is weaker than olivine at grain sizes less than 2 μm. Received: 18 June 2000 / Accepted: 3 April 2001     

The determination of partial melt compositions of peridotitic systems by melt inclusion synthesis

An experimental method of melt inclusion synthesis within olivine crystals has been developed to determine the composition of the melt present in a partially molten peridotite assemblage. Trace element doped peridotite was equilibrated with 5 wt% of a C-O-H volatile source at 20 kbar/1175 °C in a piston-cylinder apparatus under buffered oxygen and sulphur fugacity conditions [log(f _O2) ∼ IW +1 log unit, log (f _S2) ∼ Fe/FeS > +1 log unit]. A single crystal of olivine, which had been cut to a disc shape, was included in the sample capsule. At run conditions the peridotite charge formed olivine, orthopyroxene, clinopyroxene, Fe-Ni sulphide and a volatile-bearing melt. The melt phase is preserved as homogeneous glass inclusions up to 50 μm in size, trapped in situ in the olivine disc. The major element composition of the glass inclusions showed them to be of broadly basaltic character, but with a low Mg/(Mg + ΣFe), which is associated with precipitation of olivine from the melt inclusion onto the walls of the olivine disc during quenching. Thus the equilibrium melt composition has been calculated from the glass inclusion composition by addition of olivine component using the Fe/Mg exchange coefficient of Roeder and Emslie (1970); the desired Mg/(Mg + ΣFe) being determined from the composition of olivine formed at run conditions in the peridotite section of the charge. The melt composition obtained is close to the trend for dry melting established by Falloon and Green (1988), and it is evident that although the reduced volatiles in this case have induced a liquidus depression of some 250 °C, there has been only a small shift in melt composition. Trace element, carbon and hydrogen contents of thirteen melt inclusions have been determined by secondary ion mass spectrometry (SIMS). The trace element signature is consistent with ∼29% melting in equilibrium with a lherzolitic assemblage. The equilibrium melt has a C/H of 0.48 by weight. Carbon solubility in partial melts is thus significant under reducing conditions in the presence of dissolved “water components” and establishes a major melt fluxing role for carbon in the upper mantle. The ubiquitous presence of carbon and hydrogen in basaltic magmas underscores the importance of determining both the position of vapour-present solidi and the composition of melts generated, when developing petrogenetic models. Received: 1 July 1996 / Accepted: 25 June 1997     

Compositional zoning of calcite in a high-pressure metamorphic calc-schist: clues to heterogeneous grain-scale fluid distribution during exhumation

Calcite in former aragonite–dolomite-bearing calc-schists from the ultrahigh-pressure metamorphic (UHPM) oceanic complex at Lago di Cignana, Valtournanche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries. These patterns are interpreted in terms of lattice diffusion and interfacial mass transport linked with a heterogeneous distribution of fluid and its response to a changing state of stress. The succession of events that occurred during exhumation is as follows: As the rocks entered the calcite stability field at T=530–550 °C, P ca. 1.2 GPa, aragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite. Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca–Kα radiation intensity patterns) indicates that transformation rates have been much higher than rates of compositional equilibration of calcite (involving resorption of dolomite and grain boundary transport of Mg, Fe and Ca). This rendered the phase transformation an isochemical process. During subsequent cooling to T ca. 490 °C (where lattice diffusion effectively closed), grains of matrix calcite have developed diffusion-zoned rims, a few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles across concentrically zoned, large grains in geometrically simple surroundings can be successfully modelled with an error function describing diffusion into a semi-infinite medium from a source of constant composition. The diffusion rims in matrix calcite are continuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport on phase boundaries over a distance of several hundred micrometres, if matrix dolomite has supplied the Mg and Fe needed for incorporation in calcite. In contrast, diffusion rims are lacking at calcite–calcite and most calcite–garnet boundaries, implying that only very minor mass transport has occurred on these interfaces over the same T–t interval. From available grain boundary diffusion data and experimentally determined fluid–solid grain boundary structures, inferred large differences in transport rates can be best explained by the discontinuous distribution of aqueous fluid along grain/phase boundaries. Observed patterns of diffusion zoning indicate that fluid was distributed not only along grain-edge channels, but spread out along most calcite–white mica and calcite–quartz two-grain junctions. On the other hand, the inferred non-wetting of calcite grain boundaries in carbonate-rich domains is compatible with fluid–calcite–calcite dihedral angles >60° determined by Holness and Graham (1995) for a wide range of fluid compositions under the P–T conditions of interest. Whereas differential stress has been very low at the stage of diffusion zoning (T > 490 °C), it increased as the rocks were cooling below 440 °C (at 0.3–0.5 GPa). Dislocation creep and the concomitant increase of strain energy in matrix calcite induced migration recrystallisation of high-angle grain boundaries. For that stage, the compositional microstructure of recrystallised calcite grain boundary domains indicates significant mass transport along calcite two-grain junctions, which at the established low temperatures is likely to have been accomplished by ionic diffusion within a hydrous grain boundary fluid film (“dynamic wetting” of migrating grain boundaries). Received: 10 January 2000 / Accepted: 10 April 2000     

Thin intergranular melt films and melt pockets in spinel peridotite xenoliths from the Rhön area (Germany): early stage of melt generation by grain boundary melting

Optical microscopy and transmission electron microscopy (TEM) on a porphyroclastic high temperature spinel peridotite from the Rhön area reveal fine, irregular glass layers and pockets along mineral interfaces, cracks in olivine, inside olivine crystals and in spongy rims of clinopyroxene. The chemical composition of the glass deviates significantly from the composition of the host basanite. Electron diffraction technique confirms the amorphous nature of the glass, thus classifying it as a former melt. Every grain or phase boundary shows amorphous intergranular glass layers of variable thickness and characteristic chemical composition with distinct chemical inhomogeneities. Olivine grain boundaries, as the most common type of interfaces, exhibit two different types of melt glasses: (1) Type I melt at olivine grain boundaries, which is characterized by low contents of SiO₂ (～37?wt%) and Al₂O₃ (～5?wt%) and elevated contents of MgO (～31?wt%) and FeO (～22?wt%), is supposed to have formed prior to or during the thermal overprint and the dynamic recrystallisation of the xenolith in the mantle. Melt inclusions inside olivine grains with an average composition of type I melt are suggested to be earlier melt droplets at olivine interfaces, overgrown by migrating olivine grain boundaries during recrystallization in the mantle prior to the uplift of the xenolith. (2) Type II melt, the most common type of melt in the xenolith, shows higher contents of SiO₂ (～48?wt%) and Al₂O₃ (～17?wt%) but lower contents of MgO (～20?wt%) and FeO (～11?wt%). The observation of different types of glass within a single xenolith indicates the development of different chemical melt equilibria at interfaces or triple junctions in the xenolith. The absence of geochemical trends in bivariate plots excludes a unifying process for the genesis of these glasses. Melt inclusions in the spongy rims of clinopyroxene are interpreted to be the product of a potassium-rich metasomatism. The formation of most amorphous intergranular melt layers and pockets at the mineral interfaces including type II melt at olivine grain boundaries is suggested to result from decompression melting during the uplift with the basalt magma. We suggest that these glasses were produced by grain boundary melting due to lattice mismatch and impurity segregation. The observed intergranular amorphous layers or melts represent the very beginning of mineral melting by grain boundary melting.     

Dislocation recovery in fine-grained polycrystalline olivine

The rate of static dislocation recovery in Fo₉₀ olivine has been studied under conditions of high temperature and controlled atmosphere in compressively deformed polycrystals hot-pressed from synthetic (sol–gel) and natural (San Carlos) precursor powders. The sol–gel olivine, containing a small fraction of orthopyroxene, was deformed to a final strain of 19% with a maximum differential stress of 266 MPa whereas the San Carlos specimen was deformed to 15% strain and 260 MPa differential stress. Small samples cut from these deformed materials were annealed under high-temperature, controlled atmosphere conditions, for different durations to allow partial recovery of the dislocation sub-structures. Oxidative-decoration of the microstructural features, followed by backscattered electron imaging at 5 kV and image analysis, was used to determine dislocation density. The variation of dislocation density ρ with time t at absolute temperature T was fitted to a second-order rate equation, in integral form, 1/ρ(t) − 1/ρ(0) = kt with k = k ₀ exp(−E _a/RT). The activation energy E _a of the recovery process is 240 ± 43 and 355 ± 81 kJ mol⁻¹ for sol–gel and San Carlos olivine polycrystals, respectively. The measured rates are one to two orders of magnitude lower than those reported in previous studies on natural single crystal olivine. The difference may be explained by several factors such as high dislocation densities measurable from large areas at high magnification for the SEM and the technique used to estimate dislocation densities. Comparison between fine-grained sol–gel olivine and the coarser-grained San Carlos olivine aggregate did not indicate that grain boundaries play an important role in dislocation recovery, but the absence of grain boundaries might also have contributed to the high dislocation recovery rates previously measured for single crystals.     

Distribution of the glass phase in hot-pressed,olivine-basalt aggregates: An electron microscopy study

Samples of olivine mixed with small amounts of tholeiitic basalt which were hot-pressed above the solidus temperature were examined by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques. Two sets of samples were compared. One set was hot-pressed for approximately 1 h near 1,300° C at 0.2 GPa, and the other set was held for approximately 200 h near 1,250° C at 1.0 GPa. SEM observations reveal that, in samples which were hot-pressed for 1 h, the glass phase occurs in irregular pockets surrounded by four or five olivine grains as well as in triple junctions. The crystal-glass interfaces show both positive and negative curvature. These observations and the presence of voids suggest that the microstructure is far from textural equilibrium. In contrast, in samples which were hot-pressed for 200 h, glass is largely confined to triple junctions of uniform size and the crystal-glass interfaces have uniform curvature indicating a much greater degree of textural equilibrium. TEM images reveal layers of glass 10–50 nm thick along most of the grain boundaries in the samples annealed for short times. However, within the limit of resolution, 2 nm, virtually all of the grain boundaries in the samples hot-pressed for long times appear to contain no glass. These observations indicate that segregation of melt from grain boundaries to triple junctions is an integral part of the process of textural equilibration.     

Mechanisms and orientation of melt segregation paths during pure shearing of a partially molten rock analog (norcamphor–benzamide)

In-situ deformation experiments were performed on partially molten analog materials (norcamphor in the presence of a benzamide–norcamphor melt) undergoing pure shearing at a constant melt fraction of 0.13. Melt in the samples induces a strain-dependent transition from purely dislocation creep to dislocation creep associated with minor intergranular fracturing and grain boundary sliding (GBS). Intergranular fractures drain the melt from initially isotropic melt pockets to grain boundaries. Along such boundaries, grain-boundary migration recrystallization is inhibited, while GBS occurs. Intergranular melt pockets occur along grain boundaries oriented subparallel to the shortening direction, but melt must have migrated parallel to the elongation direction of the samples, as indicated by melt accumulations at both extruding ends of the sample. Intergranular melt pockets parallel to the elongation direction were only rarely observed, because melt was rapidly expelled from these sites. Nevertheless, these grain boundaries are the pathways of melt segregation in the samples.     

The dunite bodies, websterite and orthopyroxenite dikes of the Leka ophiolite complex, Norway

Summary The investigated mantle section of the Leka ophiolite complex extends 1.4 km from and 1.1 km along the exposed Moho. The foliated peridotite contains numerous tabular and elongated dunite bodies, orthopyroxenite dikes, websterite veins, and dikes. The foliation of the peridotite is inclined by about 45° to the Moho. The dunite bodies and the dikes cut the foliation at low angles. The dunite bodies vary in width from 0.1 to 50 m and in length from 10 m to more than 1 km. Wider dunite bodies are commonly surrounded by 0 to 1.0 m wide margins of dunitized peridotite. Websterite veins may be present outside these margins. Apart from sporadic chromite layers the dunite is very homogenous. The dunite bodies are considered to have formed by deposition of olivine along the walls of dikes originally containing tholeiitic melt. The tholeiitic melt at first heated the peridotitic sidewalls so that they became partially molten and dunitized. The ascending magma then eroded the sidewalls and removed olivine as xenocrysts. When the ascent rate decreased, the temperature of the sidewalls decreased, so that olivine (Fo_89–92) began to crystallize along the dike walls. There is also evidence for percolative melt migration along foliation planes, however, the largest proportion of the melts intruded along dikes. The websterite dikes are mostly 1 to 4 cm wide and 3 to 20 m long and dispersed with mutual distances of 20–50 m. The websterite veins and dikes probably originated from melts that were generated along the heated sidewalls of the dunite bodies. The 0.02 to 10 m wide orthopyroxenite dikes have exceptionally high MgO contents for their SiO₂ contents; about 36 wt.% MgO and 50 wt.% SiO₂. They may have formed as segregates from a SiO₂-rich magma, although the parent magma does not appear to have been boninitic. The parent magma may instead have formed by second stage partial melting of depleted lherzolite.     

Hydrogen diffusion in spinel grain boundaries and consequences for chemical homogenization in hydrous peridotite

Hydrogen can be stored in the structure of nominally anhydrous minerals as point defects, and these impurities substantially modify many physical properties of Earth’s mantle minerals. However, mantle rocks are composed of mineral grains separated by grain boundaries and interphase grains boundaries. Therefore, as a potential hydrogen reservoir, grain boundaries should be given proper attention. Here, I report an experimental investigation into hydrogen diffusion through grain boundaries in polycrystalline aggregates. Sintering and diffusion experiments were performed using a gas-medium high-pressure vessel at under pressure of 300 MPa and over a temperature range of 900–1,250°C. The diffusion assembly consisted of a polycrystalline cylinder of aluminous spinel + olivine crystals with a talc cylinder as the main hydrogen source. A Ni capsule was used to buffer the oxygen fugacity at Ni–NiO. Experimental durations varied from 5 min to 5 h. The presence of hydrogen in the crystals was measured by Fourier-transform infrared spectroscopy. The calculation of the diffusion coefficients was based on the estimation of the characteristic distance. The absence or presence of hydrogen recorded by the ‘hydrogen sensor’ olivines embedded in the aggregate allows the estimation of bounds on this characteristic distance. Results presented here suggest that hydrogen effective diffusion coefficients are only one order of magnitude faster (~10⁻⁹ m²s⁻¹ at 1,000°C) than in an olivine single crystal along the [100] axis. Resulting diffusion coefficients for hydrogen in grain boundary are four orders of magnitude faster than in a single crystal, but this diffusivity is not fast enough to affect hydrogen mobility in mantle rocks with grain sizes greater than ~1 mm. Thus, very limited chemical homogenization would occur using grain boundaries diffusion in mantle hydrous peridotite for incompatible and volatile element, such as hydrogen.     

Experimental melting of a modally heterogeneous mantle

Summary Melting of a spinel lherzolite with a spinel clinopyroxenite layer was investigated experimentally from 3.5 to 20 kbar and from 1200 to 1450 °C. The melt fraction in the spinel pyroxenite layer increases rapidly, and clinopyroxene disappears leaving olivine-spinel residua according to the reaction Cpx + Sp = Ol + Liq. The melt in the pyroxenite layer reacts with the surrounding lherzolite resulting in the formation of an essentially monomineral (olivine) zone with interstitial melt near the former pyroxenite. Melt compositions in the central melt pool are similar to those produced by other authors in melting experiments with peridotites similar to the bulk compositions of our samples. It is suggested that similar small-scale mantle heterogeneities (i.e. thin pyroxenite layers in lherzolite) may exert significant influence on mantle rheology and melt segregation, whereas melt compositions are not strongly affected and controlled by the dominating lherzolite lithology. Received October 10, 2000; revised version accepted August 31, 2001     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

Water solubility in nominally anhydrous minerals measured by FTIR and 1H MAS NMR: the effect of sample preparation

Samples of enstatite and forsterite were crystallized in the presence of a hydrous fluid at 15 kbar and 1100 °C. Water contents in quenched samples were measured by ¹H MAS NMR and by FTIR. If the samples were prepared in the same way, similar water concentrations were obtained by both methods. There is no evidence that one or the other method would severely over or underestimate water contents in nominally anhydrous minerals. However, measured water contents vary by orders of magnitude depending on sample preparation. The lowest water contents are measured by polarized FTIR spectroscopy on clear, inclusion-free single crystals. These water contents probably reflect the real point defect solubility in the crystals. Polycrystalline material shows much higher total water concentrations, presumably due to hydrous species on grain boundaries, growth defects, and in submicroscopic fluid inclusions. Grinding the sample in air further increases water concentration. This effect is even more pronounced if the sample is ground in water and subsequently dried at 150 °C. Polarized FTIR measurements on clear single crystals of enstatite saturated at 15 kbar and 1100 °C give 199 ± 25 ppm by weight of water. The spectra show sharp and strongly polarized bands. These bands are also present in spectra measured through turbid, polycrystalline aggregates of enstatite. However, in these spectra, they are superimposed on much broader, nearly isotropic bands resulting from hydrous species in grain boundaries, growth defects, and submicroscopic fluid or melt inclusions. Total water contents for these polycrystalline aggregates are between 2000 and 4000 ppm. Water contents measured by FTIR on enstatite powders are 5300 ppm after grinding in air and 12 600 ppm after grinding under water und subsequent drying at 150 °C. Received: 25 June 1999 / Revised, accepted: 4 October 1999     

K-feldspar–quartz and K-feldspar–plagioclase phase boundary interactions in garnet–orthopyroxene gneiss's from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy

A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz = orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H₂O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this. These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures >500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 10⁹ to 3 × 10⁹/cm²) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet, orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and H₂O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade, low H₂O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for the Mafic Formation; and that this dehydration event did not involve partial melting. Received: 15 February 2000 / Accepted: 26 June 2000     

Transport properties of olivine grain boundaries from electrical conductivity experiments

Grain boundary processes contribute significantly to electronic and ionic transports in materials within Earth’s interior. We report a novel experimental study of grain boundary conductivity in highly strained olivine aggregates that demonstrates the importance of misorientation angle between adjacent grains on aggregate transport properties. We performed electrical conductivity measurements of melt-free polycrystalline olivine (Fo₉₀) samples that had been previously deformed at 1200 °C and 0.3 GPa to shear strains up to γ?=?7.3. The electrical conductivity and anisotropy were measured at 2.8 GPa over the temperature range 700–1400 °C. We observed that (1) the electrical conductivity of samples with a small grain size (3–6 µm) and strong crystallographic preferred orientation produced by dynamic recrystallization during large-strain shear deformation is a factor of 10 or more larger than that measured on coarse-grained samples, (2) the sample deformed to the highest strain is the most conductive even though it does not have the smallest grain size, and (3) conductivity is up to a factor of ~?4 larger in the direction of shear than normal to the shear plane. Based on these results combined with electrical conductivity data for coarse-grained, polycrystalline olivine and for single crystals, we propose that the electrical conductivity of our fine-grained samples is dominated by grain boundary paths. In addition, the electrical anisotropy results from preferential alignment of higher-conductivity grain boundaries associated with the development of a strong crystallographic preferred orientation of the grains.