Abstract: The presence of intergranular glassy layers and pockets along mineral interfaces, on micro fractures and as inclusions in minerals in mantle peridotite xenoliths from different locations is revealed by optical microscopy and transmission electron microscopy (TEM). All these glasses represent former melts, that is confirmed by electron diffraction as well as their typical geochemical signatures. Often very thin glass layers are present on grain boundaries and show characteristic chemical compositions that strongly depend on the adjacent minerals. The composition of these layers differs distinctly from the bulk melt composition of partial melt experiments or from the compositions of wider melt pools of glasses observed in other xenoliths. Furthermore, a relation of these glasses to the adjacent host basalt can be excluded by the distinctly different geochemistry of the melts. The chemical composition of the melt changes with increasing thickness of the glass layers, which is due to mixing processes of the different types of glasses in the xenoliths. Wider melt films (> I Ilm) are more similar to glasses observed in large melt pools and veins given in the literature as well as partial melting experiments. Thus, the chemical composition varies from that of the very first melt at different interfaces to the bulk composition of partial melts created by experiments depending on the melt film thickness. Melts are probably formed by grain boundary melting due to lattice mismatch and impurity segregation in the xenolith triggered by decompression processes during the uplift of the xenolith. This point is consistent with the corrosion textures and the absence of chemical equilibrium between melt and adjacent olivine crystals. Chemical equilibrium is only found for very few melt films along olivine boundaries and melt inclusions in olivine neoblasts. These early melts were generated during
thermal overprint and dynamic recrystallisation of the xenolith in the mantle. The occurrence of melt on grain boundaries has important geological and petrological implications. Intergranular layers give an insight into the very first melting processes and the development of melt composition with time and degree of partial melting. Furthermore, melt films on interfaces are suggested to have an important significance for the rheology of the mantle by distinctly increasing the creep rate of the rock. Finally, diffusion processes may be distinctly enhanced by the presence of melt and may give way for a very fast reequilibration of the mineral chemistry.
1. INTRODUCTION
Mantle xenoliths can provide insight into structure and chemical composition of the earth's mantle and the evolution of mantle derived magmas. A widespread feature of mantle xenoliths is the occurrence of glass which is generally accepted to represent a trapped former melt. Some xenoliths are surrounded by a thin glassy layer called jacket glass [Edgar et aI., 1989; Heinrich and Besch, 1992; Zinngrebe and Foley, 1995]. In other xenoliths healed fractures are filled with glass [White, 1966; Zinngrebe and Foley, 1995; Wulff-Pedersen et aI., 1996; Franz and Wirth, 1997; Klugel, 1998]. Most of the xenoliths include glass patches, pockets or veinlets. The dimension of these patches or melt pools is in the range of hundreds of nanometers up to a few hundred microns. Microlites such as olivine, spinel, clinopyroxene, and plagioclase frequently occur in larger melt pockets [White, 1966; Frey and Green, 1974; Maal¢e and Printzlau, 1978; Jones et aI., 1983; Edgar et aI., 1989; Zinngrebe and Foley, 1995; Szabo et al., 1996, Wirth, 1996; Franz and Wirth, 1997; Yaxley et aI., 1997]. Grain boundaries or phase boundaries exhibit intergranular glassy layers ranging from 1-2 nanometer up to a few hundred nanometer [Wirth, 1996; Franz and Wirth, 1997]. Thicker glassy layers along interfaces are often called veins or veinlets and their widths range from 1 to more than 20 !-lm [Frey and Green, 1974; Frey and Prinz, 1978; Heinrich and Besch, 1992; Zinngrebe and Foley, 1995; Wulff-Pedersen et al.,1996; Yaxley et al., 1997]. Inclusions of glass are very common in the mineral assemblage of xenoliths mostly in olivine and clinopyroxene but also in orthopyroxene and even in spinel [Gamble and Kyle, 1987; Edgar et at., 1989; Zinngrebe and Foley, 1995; Schiano et at., 1995; Wulff-Pedersen et al., 1996; Szabo et al., 1996; Franz and Wirth, 1997; Yaxley et al., 1997].
The origin of glasses in xenoliths is controversial, and probably in some cases more than one single mechanism generate the glass. Some glasses observed along cracks in xenoliths or in minerals are very likely melt from the host magma, sometimes slightly changed in composition by
8. Amorphous Intergranular Layers and Inteiface Melting 231
reaction with the minerals of the xenolith [Wuljf-Pedersen et al., 1996; Franz and Wirth, 1997; Klugel, 1989]. Another common mechanism for melt generation is decompression melting of xenoliths, especially in the presence of hydrous minerals like amphibole or phlogopite generating disequilibrium melt composition [Frey and Prinz, 1978; Girod et al., 1981]. In the literature a widely accepted origin of glasses in mantle xenoliths is metasomatism. Partial melting of the xenolith in the upper mantle may be triggered due to the presence of a fluid phase or a percolating melt from an underlying hydrous subducted slab [lones et al., 1983; Edgar et al., 1989; Schiano et al., 1995; Szabo et al., 1996; Xu et al., 1996; Wiechert et al., 1997]. Grain boundary or phase boundary melting as a mechanism for generation of thin amorphous intergranular melt layers (below the resolution limit of optical microscopy) along mineral interfaces in xenoliths is discussed by Wirth [1996] and Franz and Wirth [1997].
The occurrence of glass patches, glassy veins and glass films in mantle derived xenoliths poses some fundamental questions: 1. Where does the very first melt form and why? 2. What is the chemical composition of the first melt and how does it
change with increasing degree of partial melting? 3. How does melt migrate? 4. What is the influence of hydrous fluids on the degree of partial melting?
Some of these questions are already answered by the results of appropriate experiments. Melting experiments, mostly with natural mantle peridotite samples, have been carried out to study the influence of pressure, temperature and water on the degree of partial melting and the chemical composition of the partial melts [e.g., Green, 1973; Mysen and Boettcher, 1975; Takahashi, 1986; Herzberg et at., 1990; Hirose and Kushiro, 1993]. Water added to melting experiments with dry natural lherzolite resulted in an increase of the degree of partial melting with increasing H20 content and a lowering of the solidus temperature of lherzolite from 1250 °C to 1100 °C [Hirose and Kawamoto, 1995; Gaetani and Grove, 1998].
Experimental studies with partially molten natural or synthetic mantle rocks exhibit thin melt films along mineral interfaces when melting occurred simultaneously with deformation [lin et at., 1994; Drury and Fitz Gerald, 1996; Dimanov et al., 1998]. In a very recent paper, Odling et al., 1997 presented an experimental method of melt inclusion synthesis within olivine crystals to determine the composition of the melt generated in a partially molten peridotite. In this experiment the partial melt is preserved as homogeneous glass inclusions up to 50 !lm in size in olivine.
Intracrystalline glass droplets of 0.1 - 0.2 !lm within pyroxenes, olivine grains and intergranular isolated glass pockets have been found in annealed natural spinellherzolites deformed in H20 saturated conditions. A common feature of the intracrystalline glass droplets is the enrichment in silica (about
232 R. WIRTH and L. FRANZ: Chapter 8
70 wt% Si02). The intergranular pockets are also silica enriched (54-66 wt% Si02). This phenomenon is called early partial melting (EPM; cpo Raterron et al. [1997]).
Thin melt films along interfaces on a submicroscopic scale are reported for the first time from natural untreated Iherzolites from San Carlos, Arizona and the RhOn area, Germany [Wirth, 1996; Franz and Wirth, 1997]. These films are regarded as a key tool to solve the problem of the formation of the very first melt and to determine its chemical composition. The thin amorphous intergranular layers observed in natural lherzolites exhibit a chemical composition which depends on the nature of the interface. Different grain or phase boundaries generate different melt compositions; e.g. an olivine grain boundary produces one distinct melt composition and an orthopyroxene-olivine phase boundary creates another. Melting is suggested to start at the interface due to lattice distortion in the vicinity of grain and phase boundaries and due to impurity segregation into the interface including the migration of fluid phase along interfaces.
Thin melt films along grain or phase boundaries have been reported from systems other than basalts or peridotites e.g. in the system quartz-albiteorthoclase-H20 and quartz-anorthite-H20 [Laporte, 1994]. At temperatures > 1400 K melt films along some grain boundaries have been observed in deformed synthetic german ate samples [Dupas, 1994].
Grain boundary melt films with a thickness of about 1-3 nm are reported from covalent ceramics, especially Si3N4 [Clarke, 1987; Das Chowdhury et al., 1995; Pezzotti et al., 1996], in alumina/silicon carbide composites [Ohje et al., 1996] and in polycrystalline alumina sintered with additions of calcium silicate [Brydson et al., 1998].
In contrast to melt pockets or wide veins, thin melt films (submicroscopic) do not show microlites. Why are these melt films stable and do not devitrify? Theoretical approaches to these problems have been presented by several investigators: thermodynamic calculations postulate the existence of thin liquid films along mineral interfaces [Hess, 1994]. An approach based on interfacial energies and on force balance normal to the boundary indicates stable intergranular films in ceramic materials in the range of 1 nm [Clarke, 1987]. Another theoretical investigation on the thermodynamic origin and stability of amorphous grain boundary films in pure silicon was performed by molecular dynamics calculations [Keblinski et al., 1997]. It is a question whether results from ceramic systems can be transferred to natural systems and silicate minerals. However, the presence and the stability of thin amorphous layers along mineral interfaces in xenoliths seems to be a very common phenomenon but not universal [see Vaughan et al., 1982; Kohlstedt, 1992] and may be of fundamental importance to melt formation, rheology and mass transport processes.
8. Amorphous Intergranular Layers and Interface Melting 233
Other striking aspects of melt patches and melt films in xenoliths are melt geometry, dihedral angles, wetting behaviour of partial melts and melt movement. Static melting experiments with basaltic systems indicate that the liquid forms an interconnected network along intergranular edge intersections but does not wet grain boundaries [Waif and Bulau, 1979]. The anisotropic behaviour of glass in triple junctions was observed by Vaughan et al. [1982]. The existence of crystallographically controlled, flat crystalline interfaces demonstrates that the dihedral angle is not single valued governing the distribution of melt in a crystalline matrix [Waif and Faul, 1992]. Or more specifically, that wetting angle is a function of boundary orientation. Melt geometry, movement of melt and wetting angles are considered in several other papers [Laporte, 1994; Laporte et al., 1997; Cmfral et al., 1998]. A special chapter of this volume is dedicated to this topic.
The present paper reports on TEM investigations of thin amorphous intergranular layers from mantle derived xenoliths from Victoria, Australia, a xenolith from Saudi Arabia, a spinel lherzolite from San Carlos, Arizona and a spinel harzburgite from the Rhon, Germany. For thin melt films it is shown that specific grain or phase boundaries produce characteristic melt compositlons (e.g., olivine (ol)-olivine grain boundaries; olivineclinopyroxene (cpx) phase boundaries). It is argued that those melts have been formed at the interfaces by grain boundary melting and represent the very first melt composition under given melt conditions. The thin melt films or small melt patches are not accessible for chemical and structural analyses by electron microprobe analysis (EMP). Therefore methods with better spatial resolution like transmission electron microscopy (TEM), analytical electron microscopy (AEM) or electron energy-loss spectroscopy (EELS) were applied. A careful study of thin sections by optical microscopy and a subsequent chemical analysis of the interesting minerals by EMP prior to the use of TEM is essential.
2. SAMPLE PREPARATION AND ANALYTICAL METHODS
2.1 Electron microprobe analysis EMP
Minerals and glass inclusions in the rim section of clinopyroxene were analysed on a CAMECA SX 50 microprobe with four spectrometers at the GFZ Potsdam. Major and minor elements were determined at 15 kV acceleration voltage and a beam current of 20 nA with counting times of 20 s for Si, AI, Mg, Ca and K, and 30 s for Fe, Ni, Na, Cr, Mn and Ti. The
234 R. WIRTH and L. FRANZ: Chapter 8
CAMECA standard set was used for reference and the PAP program for matrix correction. Ca in olivine was determined using the trace element program of CAMECA with wollastonite as a standard. Counting time was 300 s, acceleration voltage 20 kV and beam current 55 nA. The accuracy of the analyses was checked with the SClKA olivine standard [Kohler and Brey, 1990], which contains 524 (±5) ppm Ca. The microprobe reproduced this value within an error range of ±15 ppm.
2.2 Sample preparation, transmission electron microscopy (TEM), analytical electron microscopy (A EM) and EELS.
Suitable specimens for TEM were selected from petrographic thin
sections by optical microscopy. Prior to the extraction of the selected area of
the specimen from the glass support it is impregnated with a resin
(Technovit ®). The resin protects the specimen and especially the grain
boundaries from damage prior to ion beam thinning. Cracks along the grain
boundaries would accelerate the thinning process in this area thus making
the specimen unusable. The resin is dissolved in acetone and the specimen
can be fixed to a copper grid. Ion beam thinning by Ar-ions occurred in a
Gatan Duo Mill at 5 kV with a tilt angle of 11 0. Subsequent coating of the
sample with carbon prevents charging in the TEM. TEM and AEM were carried out in a Philips CM200 electron
microscope with twin lens configuration at 200 kV. Imaging of thin amorphous layers at grain or phase boundaries requires that the lattices on either side of the interface are exactly aligned along a low-index zone axis and that the interface is oriented parallel to the electron beam. Only thin areas near the hole edge are suitable for lattice imaging and allow a reasonable interpretation of the lattice fringe image without image simulation.
AEM measurements were carried out at 200 kV with an EDAX energy dispersive X-ray spectrometer with a Be-window and later with an ultra thin window UTW. The minimum diameter of the electron beam used for analysis was about 4 nm under focused condition in the scanning transmission mode (STEM). The electron source was a LaB6 filament and counting time 60 seconds at a take-off angle of 20 degrees. AEM is best performed on interfaces parallel to the electron beam which extend from the hole to thicker regions of the sample. One has to assure that a profile is measured along a line of constant thickness. For medium atomic number materials (Ca-Cu) and typical foil thickness (about 100 nm) spatial resolution for microanalysis is about 10-40 nanometers, depending on the
8. Amorphous Intergranular Layers and Inteiface Melting 235
initial probe size and acceleration voltage [Hall, 1994]. Spatial resolution is improved in low atomic number materials and in thinner areas. Using a low initial probe size (4 nm), the thin area of the specimen and the interfaces oriented parallel to the beam exclude an overlap of the excited volume in the specimen. In thin specimens secondary fluorescence can be excluded. In contrast to electron microprobe, where the characteristic X-rays come from a pear-shaped volume of the specimen below the surface, in thin specimens the characteristic X-rays come exclusively from a volume which corresponds to the diameter of the electron beam. With increasing thickness a beam broadening can be observed.
Most of the measurements were carried out in the scanning transmission mode (STEM). In cases of serious beam shift or sample drift during counting, the TEM mode with nanoprobe allowed a manual compensation of the movement. Quantitative analyses have been carried out using well-known olivine, opx and cpx standards. The computer program calculates the necessary kAB-factors from the measured intensities and the element concentrations of the standard at measurement conditions (see Goldstein and Williams [1989]). Local thickness determination by EELS using the total intensity reaching the spectrometer and the intensity of the zero-loss peak allows a correction of the results for absorption and fluorescence. The quantitative results are normalised to 100%. This procedure may cause some uncertainty in analysing samples with a significant Na-content, as Na evaporates during the measurement under focused beam conditions. The true change in concentration of the chemical components can be easily corrected by observing the background subtracted count-rate values. By this method it can be excluded that observed changes in oxide concentrations are a total sum effect. Evaporation can be minimized by scanning the electron beam over the area of interest. The most significant error in AEM-analysis is due to poor X-ray counting statistics. At the 3s confidence level the error in the number of accumulated counts N would be 3x-N· The relative error in the number of counts is (3x-N /N)x100. The minimum accumulated count rate in the measurements reported here was in the range of 500, according to a relative error of about 13%. Assuming a sufficient count rate (10000 cts.), the error in X-ray analysis in the AEM is < 5 % reI. [Goldstein and Williams, 1989].
2.3 Electron Energy - Loss Spectroscopy EELS
A basic description of the EELS method, which can be used for qualitative and quantitative analysis of very thin areas with high spatial resolution, is given by Egerton [1996]. The EELS technique can be applied for thickness determinations of the foil which is necessary to correct the
236 R. WIRTH and L. FRANZ: Chapter 8
EDX analyses for absorption and fluorescence [Goldstein and Williams, 1989; Egerton, 1996]. The minimum mass detection limit depends on the scattering cross section of the elements. It is similar to that of X-ray spectroscopy and better for light elements. Another possible application of the technique is element mapping with high spatial resolution.
SIMS and laser ablation ICPMS are useful techniques for studying compositions of glasses but have not been used by the authors.
3. SAMPLE DESCRIPTION
For microprobe and TEM investigations, samples of mantle peridotite from four different localities were chosen: The first sample is a spinel lherzolite xenolith (sample EI0l) from the San Carlos basanites (Arizona). The selected peridotite belongs to the so-called Group I xenoliths of this area (see Frey and Prinz [1978]) and predominantly contains olivine, orthoand clinopyroxene, and minor, Cr-rich spinel. A detailed petrological and geochemical description of this group of xenoliths is given by Frey and Prinz [1978] who estimated reequilibration temperatures of lIOO-l300°C at pressures between 9 and 25 kbar. Pyroxene thermometry using the TBKN and the TCaOpx calibrations of Brey and Kohler [1990] on sample EIOl yields lI50-1200°C in the pressure range listed above thus confirming the results of Frey and Prinz [1978]. Sample EI0l displays a granular near equilibrium texture ("coarse equant" in the sense of Harte [1977]) with 120° angles at triple junctions and long, straight grain boundaries. Wavy extinction of olivine is ubiquitous. At mineral interfaces, the formation of olivine subgrains with low-angle grain boundaries is observed. Small isotropic glass dots or rod-like glass bodies with a size of a few microns are observed at these grain boundaries. Furthermore, fine glass layers are present on cracks in olivine and, like the other occurrences of glass, these films are also near the resolution limit of the light microscope [cpo Wirth, 1996].
The second investigated xenolith was taken from Tertiary basalts of the RhOn area (Variscides of Central Germany). The RhOn is part of the MidGerman Crystalline Rise (MGCR), a suture zone in the Variscan fold belt, where a collision of two crustal segments took place in Lower Carboniferous (see Franke and Oncken [1990]; Behrmann et al. [1991]). The RhOn area, which forms part of the Tertiary-Quarternary European alkaline volcanic province, experienced a period of prominent volcanism about 25-11 Ma ago [Lippolt, 1978]. Especially the alkali-basalts and basanites transported numerous mantle xenoliths to the surface. For TEM and microprobe investigations, a spinel harzburgite (sample BKS22) was
8. Amorphous Intergranular Layers and Interface Melting 237
selected from the basalt quarry Kellerstein in the central RhOn area. The sample displays a partly sheared texture ("porphyroclastic" following the classification of Harte [1977] with dynamic recrystallisation of about 30% of the olivine. Wavy extinction is only present in the olivine porphyroclasts and pyroxenes while it is completely absent in the neoblasts. Along grain boundaries and on the triple junctions of olivine neoblasts, fine irregular melt pockets and droplets of glass are recognised microscopically. Clinopyroxene shows a discontinuous zonation with rims spongy in appearance full of fine glass inclusions. Microprobe analyses reveal - with the exception of clinopyroxene rims and the outermost rim sections of the olivine - rather homogeneous mineral chemistry without exsolution features. The application of different geothermobarometers on this sample yields rather consistent P-T conditions of 1270 (±30)OC and 26.1 (±4) kbar [Franz et ai., 1997]. This high-temperature event as well as the shearing of this xenolith is considered to have occurred during lithospheric stretching in the mantle in the course of the Tertiary magmatic event. This is evident because any signs of retrograde mineral zoning (e.g. Ca in olivine) are missing and the porphyroclastic texture would soon have recrystallized to a granular fabric under such elevated temperatures in the mantle [cpo Franz et al., 1996; 1997].
The third specimen is a spinel lherzolite from Mt. Pordon, Victoria, Australia. Lherzolite inclusions are common in basanite from the Newer Volcanics in Western Victoria. The basalt hosts of Quarternary age are described by Ollier and Joyce [1973], Irving and Green [1976]. Like the xenoliths from San Carlos and the Rh6n area, the Mt. Pordon xenoliths are not genetically related to the host magma [Frey and Green, 1974]. The mineral assemblage is olivine-opx-cpx and minor spinel. Olivine is coarse grained with equilibrium angles (120°) at triple junctions. Undulous extinction of the olivine grains indicates the presence of low-angle grain boundaries and thus recovery under mantle conditions. Hydrous phases like phlogopite or amphibole are not present in the investigated thin section.
The last specimen is a thin section from a mantle xenolith from Saudi Arabia donated by A. Nicolas. It is a coarse grained xenolith composed of olivine, opx, cpx and minor spinel. Glass patches and glass dots along interfaces are frequently observed by optical microscopy.
4. DEFINITIONS
interface: A term synonymously used with boundary [Sutton and Balluffi, 1995]
grain boundary: A homophase interface involving a misorientation between the adjoining crystal lattices [Sutton and Balluffi, 1995]
238 R. WIRTH and L. FRANZ: Chapter 8
phase boundary: An interface between two crystals of different crystal structure
large-angle grain boundary: A grain boundary for which the misorientation angle is larger than 15°. In the large-angle regime the distortions of the structure are sufficiently large that the core structures of intrinsic dislocations are topologically distinct from those of isolated dislocations [Sutton and Balluffi, 1995]
intergranular layers: dense, amorphous or non-crystalline layers without porosity along grain or phase boundaries
Figure 1. Optical micrograph of an olivine grain boundary with melt inclusions along the interface. The interface is inclined with respect the surface with dark dots and rods (arrows) which represent glass (scale bar = 30 flm).
5. RESULTS
5.1 Glass along grain or phase boundaries
Thin sections of xenoliths frequently reveal small dot-like or rod-shaped inclusions along the grain boundaries of olivine and along phase boundaries
8. Amorphous Intergranular Layers and Inteiface Melting 239
(Fig. I) . Especially, at inclined interfaces a more detailed structure of the inclusion is visible. The inclusions are optically isotropic which indicates a non-crystalline structure and sometimes form a rather complicated pattern. The size of the inclusions is in the range of a few microns down to the resolution limit of optical microscopy. The optically visible inclusions at the mineral interfaces are linked by amorphous films of various thickness that are not optically visible (Fig.2). The width of the glassy intergranular layers is in the range of a few nanometers (Fig.3) up to several hundred nm. Thin amorphous layers are visible in lattice fringe images with both crystals in suitable orientation and the interface parallel to the electron beam as lattice fringe-free areas. The areas between the two crystals without lattice fringes are non-crystalline which is confirmed by electron diffraction (selected area diffraction SAD or convergent beam electron diffraction CBED). The width of the amorphous layers varies. Thin melt films below the resolution of optical microscopy never show devitrification or microlites. Microlites occur in larger melt patches (20 - 40 11m in diameter) containing crystallites of olivine, orthopyroxene and clinopyroxene with sizes up to 5 11m. The micro lites are embedded in a highly silica enriched amorphous matrix (up to 90 - 95 wt% Si02).
Figure 2. TEM micrograph at low magnification which shows an olivine grain boundary with glass dots and rods. The dots are linked by a thin amorphous layer which is not resolved on the optical micrograph. The different contrast in the glass areas is due to differences in local foil thickness.
240 R. WIRTH and L. FRANZ: Chapter 8
Figure 3. TEM micrograph of an olivine grain boundary at high magnification. The lattice fringe image shows an intergranular amorphous layer which is characterised by the absence of lattice fringes. Spacing of the lattice fringes of the upper crystal is 0.38 nm. The width of the amorphous layer varies along the interface.
Regarding the chemical composition of the melt films without devitrification we distinguish between film thickness < 1000 nm and amorphous layers > 1000 nm. In Table 1 the chemical composition of amorphous intergranular layers along different interfaces are given as a function of thickness of the film. The chemical composition of melt films < 1000 nm depends on the type of interface. An olivine grain boundary shows a distinct chemical signature; an opx-grain boundary a different one. The same holds for the other investigated grain or phase boundaries. In Fig. 4 the FeO-, MgO-, A1203- and CaO-contents are plotted versus the Si02-contents of melt films at interfaces of different types of xenoliths. The FeO- Si02 plot for olivine grain boundaries shows FeO concentrations between 8-15 wt% for most of the melt films. However, the Rhon specimens exhibit an additional type of melt film with about 30 wt% FeO at much lower Si02
8. Amorphous Intergranular Layers and Interface Melting 241
concentrations. This observation indicates the existence of two different types of melt composition along olivine grain boundaries in the same xenolith. For the Rhon specimen this observation is similar for MgO- Si02, A1203- Si02 and CaO- Si02 (cp. Fig. 4a).
50 a) olivine grain boundaries
_40 ~ ~ 30
0 20 Q) LL 10
50
_40 i 30
020
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_40
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o 102030405060708090100 50 Si02 wt.%
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Figure 4. Chemical compositions of melt films at different interfaces in xenoliths from San Carlos (SC), Saudi Arabia (S.A.), Australia (Austr.) and the Rhon. (a) Olivine grain boundaries, (b) opx-grain boundaries, (c) ol-opx-boundaries. (open symbols: width of grain boundary < 1000 nm , filled symbols: width of grain boundary> 1000 nm; 0 • = SC , 'iT .... = Rhon, O. = Austr., 1]' = S.A. )
The chemical composition of films along the opx grain boundaries is completely different. The CaO concentration in two of the specimens is nearly doubled compared with that of the olivine grain boundaries. Another striking feature is the Si02 concentration of the melt films of opx grain boundaries. With the exception of two specimens (olbl2 SC, olb2a SC) the Si02 concentration is in the range 55-81 wt%, in contrast to that of olivine grain boundaries (38-54 wt% ; see Fig. 4b).
For melt films wider than 1000 nm the differences between opx/ol and 01101 interfaces disappear (Fig. 4a and c). In both types of melt films the Si02 concentrations are in the range of 60-80 wt% . FeO concentration in
242 R. WIRTH and L. FRANZ: Chapter 8
olivine grain boundary melt films is 2-6 wt%, in opx grain boundary films 1-3 wt%. MgO concentration in olivine grain boundary melt films is 1-7 wt%, in opx amorphous intergranular layers 1-5.5 wt%. Al203 concentration in olivine grain boundary melt films is 4-23 wt%, in opx interface layers 11-20 wt% . Grain boundary melt films> 1000 nm show no significant differences in chemical composition in contrast to that of thinner me I t films.
Olivine-opx phase boundaries exhibit a different pattern of chemical composition of intergranular layers (Fig. 4c). Si02 concentrations are in the range of 30-50 wt% for layers < 1000 nm. FeO concentrations between 8-20 wt% are higher than in opx or olivine grain boundaries. CaO concentrations are also slightly increased compared with opx and olivine grain boundaries. However, for thicker melt films (> 1000 nm) there are no pronounced differences.
The chemical composition of cpx grain boundary melt films and olivine -cpx phase boundary layers show other different patterns (Table 1).The cpx grain boundary melt films of the RhOn specimen are characterised by high CaO concentrations (34-37 wt%) whereas in the case of San Carlos xenoliths CaO is only in the range of 6-7 wt%. For the Al203 concentration the observation is reversed; about 32 wt% in San Carlos samples and 0.6-2 wt% in RhOn specimens. It should be mentioned that the investigated intergranular layers of the San Carlos sample are much thinner than those of the RhOn xenolith (200 nm and 800 - 1000 nm). A similar distribution of CaO and Al203 is visible in the data from ol-cpx phase boundaries of the Rhon and San Carlos specimen.
Generally, it is observed that the intergranular amorphous layers are inhomogeneous in chemical composition across the melt film as well as parallel to the interface.
The glassy layers along the interfaces of the minerals distinctly weaken the cohesion between the minerals of the San Carlos xenolith. The resulting single grains, mainly olivine, are very often completely covered with a continuous glassy layer several microns thick. Fig. 5 shows an olivine grain where the glassy layer, which is highly enriched in A1203, is partially removed from the minerals interface. The thickness of the layer is about 5 microns. A qualitative X-ray analysis indicates mainly A1203, Si02, MgO and Na20. This observation is important because it shows that the interfaces can be totally covered with melt.
5.2 Concentration profiles across melt films
Profiles across the amorphous intergranular layers reveal two features. First, the melt composition is inhomogeneous across the melt film.
8. Amorphous Intergranular Layers and Interface Melting 243
Secondly, in a very narrow region adjacent to the interface melt/crystal (100-500 nm) depending on the width of the melt film the chemical composition of the crystals is slightly changed. This point will be demonstrated with two examples. The experimental requirement for such profiles is that both interfaces have to be parallel to the electron beam. The diameter of the electron beam used is about 4 nrn. The first example is an olivine grain boundary (RhOn specimen) with a melt film of about 800 nm thickness (Fig. 6a). MgO concentration in olivine decreases towards the interfaces olivine/glass while the Si02 concentration remains nearly constant. Only in the very vicinity of the interface a slight increase can be observed. FeO concentrations show irregular changes approaching the interface, i.e. decreasing towards the left olivine contact and increasing towards the right one (see Fig. 6a). Similarly different zonation patterns of FeO are recorded in different olivine grains. Al203 concentration decreases in both olivine crystals over a distance of about 1000 nm from more than 8 wt% down to traces towards the core of the olivine grain.
Figure 5. SEM micrograph (SE) of an isolated olivine grain from a mantle xenolith, San Carlos (Arizona). The grain is covered by a glass layer which is enriched in A1203.
244 R. WIRTH and L. FRANZ: Chapter 8
70 ) olivine glass olivine
70 b)
60 60
50 50
~ 40 SIO
i 30
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10 10 AI203
o o.~~~~~~~~~~~ -4000-3000-2000-1000 0 1000200030004000 -2500-2000-1500-1000 -500 0 500 100015002000
distance in nanometer distance in nanometer
Figure 6. Concentration profiles across melt films and the adjacent grains. (a) olivine- glassolivine (Rh6n specimen); (b) opx-glass-olivine (San Carlos). Both profiles show slight changes in major components like A1203, MgO, FeO and Si02 in the vicinity of melt films.
Figure 7. TEM micrograph showing a melt film between olivine and opx. Opx I is the original grain which has formed the phase boundary olivine-opx prior to melt formation. Opx I has grown into the glass forming opx II which has the same orientation like opx I. In cases where the migrating phase boundary opx-glass meets sufficient high CaO concentration in the glass cpx has formed.
8. Amorphous Intergranular Layers and Inteiface Melting 245
The second example is a melt film along an opxlolivine interface of about 1500 nm thickness (San Carlos specimen; see Fig. 6b). The observation is basically the same. In a very narrow rim in both crystals adjacent to the melt film slight changes in chemical composition are observed. MgO decreases slightly towards the rim sections of both opx and olivine whereas FeO only reveals a decrease towards the rim of olivine. Al203 concentration in opx gradually decreases over a distance of about 1500 nm towards the opx/glass interface. In the olivine the Al203 concentration increases over a distance of about 200-300 nm towards the interface melt/olivine.
8. Amorphous Intergranular Layers and Inteiface Melting 249
Figure 8. TEM lattice fringe image (magnification of Figure 7) showing the formation of clinopyroxene. (lOO)opx with a lattice fringe spacing of 1.8 nm is parallel to (200)cpx with 0.46 nm.
5.3 Crystal growth into glassy intergranular layers
In one case, growth of opx into a melt film of the San Carlos specimen is observed. This melt film is located between an olivine and an opx grain boundary and has a thickness of about 2000 nm. Fig. 7 presents a TEM bright field image of the primary opx crystal (opx I) and a diagonally running glassy film (uniformly grey area without diffraction contrast). The adjacent olivine crystal is not visible on that image. The wedge shaped newly grown orthopyroxene (opx II) shows the same crystallographic orientation as opx I. This is confirmed by (1 OO)opx lattice fringes running through opx I and opx II. Due to preferential thinning the original phase boundary opx IIglass is still visible. The maximum width of opx II is about 200-300 nm. Fig. 8 is an enlargement of the previous figure and shows lattice fringes of opx II and a thin rim of clinopyroxene with the orientation relationship (lOO)opx II II (200)cpx. The spacing of the lattice fringes is 1.8 nm for (lOO)opx II and 0.46 nm for (200)cpx, The growth of opx II into the glass occurred by moving the interface opx/glass in steps (Fig. 8). Chemical analysis of the glassy phase exhibits a relatively high CaO concentration thus initiating the formation of clinopyroxene. The CaO concentration right in front of the clinopyroxene/glass interface is significantly higher than in the centre of the glass film. This is confirmed by X-ray analysis. Inhomogeneity of the glass composition is a characteristic feature of the intergranular layers.
Figure 9. shows a comparison of the chemical composition of melt inclusions at triple junctions with melt composition of the adjacent grains (V = melt films Rhon < 1000 nm; ... = melt films RhOn > 1000 nm; ~ = melt group II at triple junctions Rhon; ~ = melt group I triple junction RhOn; t = melt at cpx-cpx-cpx triple junctions; # = melt at ol-opx-cpx triple junctions).
5.4 Melt pockets at three grain junctions
At three grain junctions of olivine neoblasts in the Rh6n xenolith melt pockets several microns thick are observed. They exhibit different wetting angles depending on the orientation relationship of the grains which form the grain boundaries. At a three grain junction of olivine e.g. three different wetting angles are observed: 81 = 180 ,82 = 35 0 and 83 = 43 0 • The chemical composition of glasses at triple junctions in the Rh6n xenolith is described by Franz and Wirth [1997]. In Fig. 9 the composition of the melt at triple junctions of grains of the same type (olivine) and opx-cpx and 01-opx-cpx three grain junctions are compared with glass compositions from thin films « 1000 nm) and thick films (> 1000 nm) along olivine grain boundaries. Two different melt compositions are present at the olivine interfaces. Melt type I is low in Si02 concentration (about 33 wt%) and high in FeO concentration (about 27 wt%). Melt type II resembles the melt composition of most of the melt along olivine grain boundaries (Fig. 4a). The chemical composition of the melt at olivine triple junctions is equivalent either to that of melt type I or melt type II on thin intergranular layers but it shows prominent differences to the composition of the wide melt films.
8. Amorphous Intergranular Layers and Inteiface Melting 251
60
50
~ 40
i 30 o
melt inclusions in minerals 50
If 20
10 ~
O~~~mn~~mmmm~~ o 1020304050607080 90100
Si02 wt.% 50
o I!~ Ef I!I! ...
O~~~~~~~~~~~ o 10 20 30 40 50 60 70 80 90 100
Si02wt.%
:::!! 40 <III 0
~30 CO)
020 0 ('II .A'" :;:;: 10 D~
O~~~~mri~mmnm~~~
20
~ 15 ..: ~ 10 o III (,) 5
o 102030405060708090100 Si02wt. %
o
o 0'" I! ~ ~
O~~~~~~~~~~~ o 10203040 50 60708090100
Si02wt.%
Figure 10. Comparison of the chemical composition of melt inclusions in different minerals (0 = opx, SC; 0 = cpx, Austr.; .... = olivine Rhein; ~ = cpx, Rhon group I; ~ = cpx, Rhon group II; T = cpx, Rhon group III).
Melt pockets at three grain junctions composed of opx and cpx grains show a comparatively high CaO concentration (about 30 wt. %) but low Al203 concentration « 2 wt%). The melt composition at triple junctions basically depends on the grains that form the three grain junctions. This observation is in accordance with the observed differences in chemical composition of melt films along different interfaces. Melt pockets at triple junctions have only been observed in the porphyroclastic spinel harzburgite from the Rhon area which, however, may be due to a cutting effect in the thin sections. In case of the Rhon xenolith there are numerous olivine neoblasts with small grain sizes which makes it more likely to cut through a triple junction. The other specimens have a coarse equant texture with mmsized olivine grains which makes it rather unlikely to cut through three grain junctions even if they were really present. Triple junctions with melt exhibit either similar composition patterns like melt films of melt group 1« 1000 nm) or like melt films of melt group II « 1000 nm). In all cases, the composition of melt at triple junctions, independent of which the three minerals olivine, cpx and opx create the triple junction, is different from the composition of wide melt films.
Figure 11. Comparison of the chemical composition of melt in cracks in olivine with that of the host basanites (. = crack with melt in olivine, SC; D = host basanite SC, Frey and Prinz [1978];.A. = crack with melt in olivine, Rhon; ~ = host basalt, Rhon [Ficke, 1961].
5.5 Melt inclusions in minerals
The occurrence of melt inclusions in minerals of mantle xenoliths is common. Melt inclusions considered here are isolated inclusions i.e., they are not melt inclusions along inclusion trails. Inclusion trails can be observed predominantly in olivine grains and the optical pattern looks similar to that of the melt inclusions along interfaces. The melt inclusion trails are along cracks which were filled with melt right after the opening of the crack. Isolated melt inclusion are described in opx (San Carlos), in olivine (RhOn) and in cpx (RhOn). Similar to the melt films along different types of grain boundaries, melt inclusions in different minerals have different chemical compositions. A common feature of the melt inclusions is the strong variability in chemical composition even from inclusions in the same crystal like the inclusions in olivine from the RhOn xenolith (cp. Fig. 10). Another remarkable signature of the composition is the presence of significant amounts of Na20 (cpx, Australia; cpx RhOn ) and K20 (see Table 2). Melt inclusions in cpx crystals from Australia contain even 1 wt. % of chlorine. Chemical composition of melt inclusions in cpx is similar for the Rhon specimen and for the Victorian xenolith. The composition pattern of the melt inclusions in olivine, opc and cpx is most discriminating for FeO-Si02 and A1203-Si02 (Fig. 10 ).
8. Amorphous Intergranular Layers and Inteiface Melting 253
5.6 Cracks filled with melt
In two xenoliths cracks in olivine a few hundred nanometers wide and filled with melt have been investigated. These amorphous fillings do not show devitrification. The boundaries of the cracks are straight at an optical scale. However, they show facetting of the olivine wall material at an atomic scale. Lattice fringe images exhibit a scalloped interface glass/olivine thus indicating a reaction between melt and wall or possibly facetting of the surface caused by interface anisotropy. The chemical composition of the glasses and the corresponding host rocks is given in Table 3 and Fig.l1. The variation of the chemical composition between host rock and glass is too large that the glass could be derived from intruded basaltic melt. Both glass fillings in olivine from completely different origins (San Carlos SC, RhOn) exhibit an enrichment of Si02 up to 13 wt% in the glass. The crack in the olivine from the Rhon specimen is extremely low in CaO concentration (0.35 wt%) in contrast to the host basalt with 10.8 wt%.
6. DISCUSSION
6.1 Why are the amorphous intergranular layers former melt films?
Amorphous layers are frequently observed along mineral interfaces in mantle xenoliths and in cracks through minerals. Occasionally, amorphous inclusions are found inside minerals like olivine, opx and cpx. The amorphous state is confirmed by electron diffraction technique or by isotropic behaviour in the optical microscope. The width of the layers covers a wide range from a few nanometers to several microns. These layers are not alteration products because typical alteration phases like sheet silicates are never observed. Sheet silicates can be identified very easily be TEM. The intergranular layers are no gels because they only show a homogeneous density contrast in TEM, completely different to gels with density fluctuations resulting in inhomogeneo}ls density contrast. The intergranular layers exhibit flat glasslcrystal interfaces and no signs of corrosion. A rough surface as a result of corrosion is observed only at the walls of melt-filled cracks. Devitrification of thin amorphous intergranular layers is never observed in contrast to thicker layers (> 1000 nm). Microlites are observed only in wide layers or melt pools with newly crystallised olivine, orthopyroxene, cpx and spinel. Microlites in melt veins or melt pockets are also described in literature [White, 1966; Frey and Green, 1974; Maal¢e and Printzlau, 1978; Jones et al., 1983; Edgar et ai, 1989; Szab6 et aI.,
254 R. WIRTH and L. FRANZ: Chapter 8
1996; Yaxley et al., 1997]. The observation of orthopyroxene growing into an amorphous intergranular layer, followed by the formation of cpx due to a local CaO enrichment in the glass, proves that these layers are not gels. The most striking argument against alteration is the observation that distinct grain or phase boundaries produce characteristic melt compositions [Wirth, 1996; Franz and Wirth, 1997].
It can be also excluded that the melt films are due to thinning. Ion beam thinning can never produce two different glass compositions along an interface of the same kind like an olivine grain boundary. It is well known that ion beam thinning produces a thin amorphous area right at the edge of the specimen, due to the destruction of the lattice by ion bombardment resulting in displacement damage of the crystal [Hobbs and Pascucci, 1980; Carter and Kohlstedt, 1981; Inui et ai., 1990; Martin et al., 1996]. This area is extremely narrow, less than 100 nm, and shows the chemical composition of the irradiated crystal. The strongest argument against a thinning artefact is the observation of the layers at the surfaces of isolated olivine grains which have detached from xenolith nodules (Fig. 5).
6.2 Formation of melt along the mineral interfaces
Accepting the idea that amorphous intergranular layers are former melt films, the question arises "where was the melt created?" Following the wetting angle criteria, observed wetting angles at olivine triple junctions ( e.g. 81 = 18°, 82 = 35°, 83 = 43°) suggest the formation of an interconnected network of melt channels along the grain edges because the wetting angles are 0°< 8< 60° [Cm{ral et ai., 1998; Laporte, 1994; Harte et al., 1993; Waf! and Faul, 1992; Jurewicz and Jurewicz, 1986; Cooper and Kohlstedt, 1982]. An interconnected network of melt channels is never observed in the investigated xenoliths. The measured wetting angles do not allow wetting of the olivine interface which only occurs at a wetting angle of 0°. Of course, melting will also occur at triple junctions but the melt will spread out over the interfaces only if the wetting criteria are satisfied. Nevertheless, melt films are present at the interfaces of mantle xenoliths. From the existence of very thin melt films (ca. 1-2 nm, Wirth [1996]) it is concluded that melting starts along grain boundaries. The very thin melt films show some characteristic features. The thickness of the film changes along the interface. The chemical composition of the melt film is inhomogeneous along and across the interface and changes with film thickness [Wirth, 1996]. The migration ability of a thin fluid film should be very limited because of the strong interaction of the fluid surface with the crystal surface. Thin films have properties which allow a fluid to exist along planar boundaries even for systems characterised by non zero dihedral
8. Amorphous Intergranular Layers and Inteiface Melting 255
angles [Clarke, 1987; Hess, 1994]. That means that a thin melt film will be created along a grain or phase boundary and will not migrate, at least under hydrostatic conditions. Partial melt experiments under non-hydrostatic conditions have shown that melt can spread out over the interfaces and wets grain boundaries [lin et al., 1994]. In the case of the RhOn xenolith, where the neoblasts indicate dynamic conditions like deformation and recrystallization, the presence of melt along the interfaces may be due to shearing or grain boundary sliding. The presence of melt films along interfaces of large grains, which exhibit a fabric of static annealing conditions like low-angle grain boundaries, equilibrium angles at three grain junctions and last not least the large grain size, is more likely explained by grain boundary melting. However, the most striking argument for the formation of thin melt films and the beginning of melting along grain or phase boundaries is the characteristic melt composition for distinct grain or phase boundaries.
The following model is suggested for grain or phase boundary melting. At first, it is necessary to have a closer look at the characteristic features of grain boundaries. Different models for grain boundary structures exist, and nearly all of them are derived from metals or metal oxides. The idea of modelling a high-angle grain boundary in terms of an array of closely spaced dislocations is an extension of the well established structure of low-angle grain boundaries [Gleiter, 1983]. The island model describes grain boundaries in terms of regions (islands) of good and poor atomic fit and was first proposed by Mott [1948]. The coincidence model correlates the crystallographic parameters of a boundary (lattice of the crystals forming the interface, the orientation relationship between the two crystals and the inclination of the boundary) with the actual atomic arrangement in the interface [Kronberg and Wilson, 1949]. Reviews of grain boundary structures are given by Gleiter and Chalmers [1972] and by Sutton and Balluffi [1995]. The energy of a grain boundary is defined as the free energy per unit area of the boundary surface (e.g., Gleiter and Chalmers [1972]). Since a grain boundary is the interface between two single crystals of different orientations, most atoms at the boundary are displaced from the positions which they would occupy in the perfect crystal lattice. Hence, their free energy is higher than it would be for the atoms in the undisturbed lattice. Grain boundary melting means the preferential melting in the grain boundary region. The presence of interfaces lowers the equilibrium melting temperature of the surface of the crystal, as will any other structural feature which increases the free energy of the grain boundary. Depending on the orientation relationship of two adjacent grains and the inclination of the boundary plane, the bonds of the atoms in the grain boundary are more or less distorted with respect to the bulk crystal. Grain boundaries act as sinks for impurities due to the less dense packing of atoms in the grain boundary
256 R. WIRTH and L. FRANZ: Chapter 8
[Gleiter and Chalmers, 1972]. From measurements in alloys it is known that an enhancement of the solute concentration at grain boundaries by a factor of 10-1000 depending on the alloy system is realistic. Segregation of impurities may change the Gibbs free energy, the specific volume and the entropy of the boundary. The influence of impurities on the depression of the melting point of grain boundaries in metals and alloys is discussed in Gleiter and Charmers [1972] and Sutton and Balluffi [1995]. In grain boundaries of magnesium aluminate spinel solid solutions Mg*nA1203 segregation of impurity Ca and Si is common and observed at almost all grain boundaries [Chiang and Kingery, 1990]. It is likely that impurity atoms will cause an additional distortion of the lattices in the grain boundary thus decreasing the melting temperature. A fluid phase that is present would certainly enhance effects caused by segregation of impurity atoms. Cahn and Hillard [1958] proposed a transition region where the concentration changes continuously between the enhanced concentration in the segregated phase and the concentration in the matrix far away from the boundary. The width of the transition region is in the order of 10-30 lattice constants and will increase with rising temperature. Chiang and Kingery [1990] observed strong deviations in stoichiometry at grain boundaries of magnesium aluminate spinels which supports the idea of a transition region. The idea of a transition region in the crystal adjacent to the grain boundary is also supported by an enrichment of oxygen and nitrogen in a zone of about 80 nm width parallel to the interface in SiC/Si3N4 interfaces and Si3N4 grain boundaries [Das Chowdhury et al., 1995]. The transition zone is called the chemical width of a grain boundary.
An enrichment of impurities - in this case A1203, Ti02, CaO - was observed in thin amorphous layers along olivine grain boundaries [Wirth, 1996]. These oxides are considered as impurities because their concentration in olivine usually is very low « 0.1 wt%). Concentration profiles across the intergranular amorphous layer and the adjacent olivine grains show the enrichment of Al203 (Fig. 6a). In other melt films elevated concentration of Na20 and K20 additional to the above mentioned impurities are observed. Once melting has started, the impurities dissolve in the melt. The observed heterogeneity of the chemical composition of the melt films may be due to primary inhomogeneous distribution of impurities in the interface and the transition zone, and it shows that thermodynamic equilibrium was not achieved. The wall-regions of cracks in olivine, filled with an amorphous layer, do not show an enrichment of impurities in a narrow transition zone. Distortion of the crystal lattices in the grain boundary due to differences in the orientation relationship of two grains, the more open structure of the interface region, dangling bonds without charge balance and stressed bonds between the atoms cause a lowering of the melting temperature. This effect is enhanced by segregation of impurity
8. Amorphous Intergranular Layers and Inteiface Melting 257
atoms into the distorted region and by metasomatic fluids. Metasomatic origin for the formation of glass has been suggested by several authors [Jones et al., 1983; Schiano et al., 1995; Szab6 et al., 1996].
6.3 Stability of amorphous intergranular layers
Why are the glassy layers along interfaces stable or metastable? To answer this question it is useful to consider the very thin films « 10 nm) separately from the wider layers « 1000 nm). The stability of very thin films has been investigated for liquid-phase-sintered ceramics because they have strong effect on the high temperature mechanical properties of these materials. Ceramic materials are certainly different from mantle peridotite but physics of electrostatic forces (attraction or repulsion), van der Waals forces should be basically the same. Although the larger glass pockets often can be crystallised by subsequent heat treatment, these thin layers are highly stable. They remain stable even under compressive load [Cinibulk et ai." 1992; Bonell et al., 1987]. Lange [1982] suggested that liquids remain on grain boundaries after sintering because of kinetic limitations. However, this fails to explain the apparent stability of the amorphous intergranular layers under creep conditions. The most convincing explanation of this phenomenon is given by Clarke [1987]. It is based on the development of a thermodynamically stable glass phase on the interface between two crystals. The equilibrium thickness is a result of two competing forces, an attractive van der Waals-dispersion interaction between the grain on either side of the boundary acting to thin the film and a repulsive term, due to the structure of the intergranular liquid, opposing this attraction. Both of these interactions are of short range « 10 nm), therefore the equilibrium thickness is of the order of 1 nm. The calculated equilibrium thickness for an AI203-Si02-Al203 ceramic system is 8 nm. In an extension of this model electric double-layer forces have been proposed as an additional repulsive force at the interface [Clarke et al., 1993]
The thickness of the amorphous layers is constant and independent of grain boundary orientation. The addition of CaO to a high-purity Si3N4 leads to a systematic increase of film thickness. This observation is consistent with the idea of an electrical double-layer effect. It is therefore concluded that Si3N4 grain boundaries contain an equilibrium amorphous film, the thickness of which depends on local chemistry [Wilkinson, 1998]. The nonuniform width of the melt film at the olivine grain boundary (Fig. 3) can be explained by the inhomogeneous distribution of impurities along the interface. Theoretical considerations of the stability problem of thin films lead to the result that thin films have properties distinct from that of thick layers or the bulk fluid. Consequently, it is possible for fluid to exist along
258 R. WIRTH and L. FRANZ: Chapter 8
planar boundaries even for systems characterised by non zero dihedral angles [Hess, 1994].
Figure 12. (a) Comparison of the chemical composition of intergranular melt films with melt compositions from partial melt experiments (0 = 01 grain boundary < 1000 nm; • = 01 grain boundary > 1000 nm; f,. = opx grain boundary < 1000 nm; ... = opx grain boundary > 1000 nm; 0 = opx-ol phase boundary < 1000 nm;. = opx-ol phase boundary> 1000 nm; • = Hirose and Kushiro [1993]; 1i' = Hirose and Kawamoto [1995]; # = Mysen and Boettcher [1975]; * = Drury and Fitz Gerald [1996]; 0 = Green [1973]).
(b) Comparison of the chemical composition of melt mms with naturally occurring melts (literature data). (0 = 01 grain boundary < 1000 nm; • = 01 grain boundary> 1000 nm; f,. = opx grain boundary < 1000 nm; ... = opx grain boundary> 1000 nm; 0 = opx-ol phase boundary < 1000 nm;. = opx-ol phase boundary> 1000 nm; ffi = literature data: Frey and Green [1974], Edgar et al. [1989]; Heinrich and Besch [1992], Szabo et al. [1996]. Schiano et al. [1995], Zinngrebe and Foley [1995], Yaxley et al. [1997]).
8. Amorphous Intergranular Layers and Inteiface Melting 259
The thermodynamic origin, structure and stability of thin amorphous intergranular layers commonly found in grain boundaries in covalent ceramics have been investigated by molecular-dynamics simulation. Calculations with pure silicon, a single component covalent model material, have shown that all high energy boundaries exhibit an amorphous structure with a width of about 0.25 nm [Keblinski et al., 1997]. Another conclusion from this work is that amorphous intergranular layers do not require impurities to their stabilisation. This assumption is in contrast to the model proposed by Clarke [1987].
There are good arguments for the existence and the stability of thin intergranular amorphous layers as mentioned above. However, models which can explain the observed stability of thicker melt films (less than 1000 nm) are still lacking. Rapidly quenched melt films caused by decompression melting during the ascent of the xenolith could explain the metastable preservation of the observed glassy layers, but the origin of the intergranular layers is not decompression melting in all cases [Maal¢e and Printzlau, 1978; Jones et al., 1983; Edgar et ai., 1989; Schiano et ai., 1995; Szabo et al., 1996].
6.4 Development of the chemical composition with degree of partial melting
In the following chapter the chemical compOSItlOn of melts found in natural xenoliths of this study are compared with those of partial melt experiments with peridotites [Mysen and Boettcher, 1975, Hirose and Kushiro, 1993, Hirose and Kawamoto, 1995] and synthetic mantle rocks [Green, 1973, Drury and Fitz Gerald, 1996]. Comparing the data, it is necessary to emphasise that the plots show data from glasses from distinct grain or phase boundaries and bulk glass compositions from melt experiments. In the Harker-diagrams of Fig. 12a FeO, MgO, Al203 and CaO are plotted versus Si02. The open symbols (square, triangle, circle) represent melt films along olivine grain boundaries, opx grain boundaries and olivine-opx phase boundaries with thickness < 1000 nm. The same symbols filled show the corresponding melt films > 1000 nm. Data from partial melt experiments are labelled by filled crosses [Hirose and Kushiro, 1993], open crosses 'll' [Hirose and Kawamoto, 1995], # [Mysen and Boettcher, 1975], * [Drury and Fitz Gerald, 1996] and 0 [Green, 1973].
The plots show that the composition of the glasses from narrow melt films « 1000 nm) are distinctly different from the chemical composition of the wider melt films. Comparing data on natural samples with those of the experiments shows that the FeO concentrations of melts from experiments are split into two groups. One is similar to a part of the narrow melt films,
260 R. WIRTH and L. FRANZ: Chapter 8
the other is close to the composition of the wide melt films. In any case the FeO content of partial melt experiments covers the range of the natural interface glasses. The splitting of the experimental results may be due to the differences in run duration because the pressure and temperature ranges are similar. Hirose and Kushiro [1993] and Hirose and Kawamoto [1995] used run duration of 12-56 hours resulting in intermediate Si02 concentrations (45-54 wt%) and FeO concentrations between 8-14 wt%. The experiments of Mysen and Boettcher [1975] and Green [1973] exhibit Si02 values of 58-68 wt% and FeO concentrations in the range of 1-4 wt% using run duration of 1-16 hours. Only the CaO concentrations of the experimentally achieved melts deviate significantly from the naturally occurring melt films with only a small overlap with the wide melt films. The main contributor for CaO is cpx which, however, shows only a small modal amount in the investigated peridotites (about 2 %). Consequently, the composition of wide melt layers can only show elevated CaO concentrations, if a clinopyroxene grain is in the vicinity of the investigated volume.
The plots in Fig. 12a allow the following conclusion: 1. The chemical composition of thin melt films along grain or phase
boundaries is different from the bulk melt composition of partial melting experiments, which is due to the specific chemical signature of thin intergranular melt films.
2. The chemical composition of the wider melt films is determined by mixing of melt from different grain boundaries and resembles that of experimentally created partial melts.
Concluding, a sufficient number of investigated intergranular amorphous layers of different width could show a transition from the chemical composition of the very first melt at different interfaces in mantle xenoliths to the bulk composition of melts from partial melt experiments and probably to natural melt compositions.
6.5 Comparison of intergranular melt composition with glass composition from literature
A comparison of melt composition from literature data, including lherzolite, harzburgite, melt inclusions in olivine or other minerals, melt from olivine grain boundaries and so called interstitial melt, is only possible with melt films wider than 1000 nm, because there is no data from the literature from smaller melt inclusions or melt films with exception of the investigation of Drury and Fitz Gerald [1996]. In Fig. 12b Harker-diagrams of FeO, MgO, Al203 and CaO versus Si02 are presented which show that chemical compositions of melt films wider than 1000 nm plot together with the glass composition data from the literature. However, the composition of
8. Amorphous Intergranular Layers and Inteiface Melting 261
narrow melt films « 1000 nm) exhibit different chemical characteristics, depending on the nature of interface. The plots of Fig. 12b support the idea that specific types of interfaces produce characteristic melt compositions as long as the melt films are narrow and not interconnected. With increasing width of the melt film melt mixing occurs and the chemical composition approaches that of the large melt pools and veins given in literature. The composition of the experimentally created partial melts deviates significantly from the composition of large natural melt pools or veins for FeO, Al203 and CaO. The reason may be that even in cases where the glass is generated by decompression melting, the experimental run times are much shorter than in nature. A strong influence of the amount and composition of the fluid phase on the melt production can be assumed.
Further investigations of intergranular amorphous layers in mantle xenoliths will provide a data base which allows us to relate specific compositions to the origin of the melt. This point is illustrated by an example of two melt films different in chemical composition along olivine grain boundaries in a xenolith from the RhOn area. There are two types of melt with a specific chemical signature present on olivine grain boundaries [Franz and Wirth, 1997]. Melt group I is characterised by low Si02 (37 wt%), Al203 (5 wt%), but high FeO (22 wt%) and MgO (31 wt%) concentrations. Melt group IT, which is the most common type of melt in xenolith, exhibits high Si02 (48 wt%) and Al203 (17 wt%) concentrations but lower contents of MgO (20 wt%) and FeO (11 wt%). Due to textural features and its chemical equilibrium with the adjacent olivine, melt group I is suggested to have formed in the upper mantle during the thermal overprint and dynamic recrystallization of the xenolith. The chemical equilibrium between melt and adjacent olivine neoblasts reflects a transient event of thermal disturbance coupled with intensive deformation in the mantle during Tertiary magmatic episode [Franz et al., 1997].
The second melt type, which is evidently not in equilibrium with the neighbouring olivine crystals, is attributed to decompression melting during uplift with the magma in Tertiary [cpo Franz and Wirth, 1997]. Future investigations need more attention to sodium and potassium and trace elements, depending on the analytical possibilities, because these elements may represent metasomatic events. Sodium and potassium can be analysed by AEM with a high confidence level by scanning the beam over the area of interest, avoiding evaporation or mass loss by irradiation damage.
Why do we observe thin melt films along interfaces? Even under conditions of decompression melting we can assume several hours or days with conditions for melt formation and an ongoing melt process. Partial melt experiments show that large degrees of partial melting can be achieved in
262 R. WIRTH and L. FRANZ: Chapter 8
run times of only several hours. That means, once melting has started on interfaces the melt films should become wider and wider. Melts that have formed deep in the mantle like melt type I mentioned above, also show thin melt films and the conditions for melt formation must have lasted much longer. If it is true that melt forms on the interfaces due to the specific structure of the interface and the presence of impurities then the amount of melt which can form is limited to the amount of impurities and the distorted grain boundary region. However, additional components like fluid phase or alkalies supplied by metasomatism would increase the amount of melt. It is concluded that the thickness of the very first melt layer is determined by the initial state of the interface and the amount of impurities present. This argument would explain the observed inhomogeneity and non-equilibrium state of the melt films. Extracting the melt from the grain boundary, which seems to be possible for thin films only if the melt was squeezed out by deformation processes, would not necessarily continue the melt process, because it needs the presence of impurities or defects in the interface.
6.6 Geological implications
There are some geological implications related with melt films and melt inclusions in mantle xenoliths.
1. Intergranular amorphous layers give an insight into the very first melt processes and the development of melt composition with time and degree of partial melting. They show us where the first melt occurs in mantle rocks and which chemical composition the melt has. The melt composition can indicate the origin of the melting process (metasomatism, melt percolation, decompression melting). More generally, mantle xenoliths with glass layers along the interfaces are a natural partial melting experiment where we can study the beginning of melting under natural conditions.
2. Deformation experiments in the presence of melt indicate that a significant change of the rheological behaviour of partially molten aggregates occurs at larger melt fractions [Hirth and Kohlstedt, 1995]. At a melt fraction of 0.04 the strain rate of melt-added samples is enhanced only by a factor of 3 relative to melt-free aggregates. Melt fraction of 0.07 enhances the strain rate by a factor of 25 compared to melt-free specimens. Consequently, thin melt films should not influence the rheology of the mantle. This topic is covered in this volume in a contribution by Kohlstedt. Deformation experiments with synthetic labradorite crystallized from glass show that a melt fraction of about 1 vol % enhances the strain rate about half an order of magnitude. In this
8. Amorphous Intergranular Layers and Inteiface Melting 263
case melt was formed along the interfaces due to hydrous melting [Dimanov et al., 1998].
3. Experiments with Si3N4 suggest that the film thickness and the viscosity of the glass is influenced by the composition of the melt especially the anion content [Kleebe and Pezzotti, 1998]. Similar influence can be expected for natural melts and glasses because there is a variety of glassmodifying impurities which can act as network-formers increasing the viscosity (Si, AI, Ti, B, P) or others can form non-bridging bonds thus decreasing viscosity (network-modifiers like Na, K and Ca, Mg).
4. The presence of melt films on interfaces has a strong effect on diffusion processes in mantle rocks. Compared to self-diffusion processes in neighbouring grains, the mobility of elements is significantly enhanced by the highly mobile melt on mineral interfaces, especially when melt generation on grain boundaries is coupled with ductile shearing processes in the mantle as observed in the porphyroclastic Rhon xenolith (sample BKS22). As a consequence, perfect mineral equilibria could be established in this xenolith during the thermal overprint by the Tertiary magmatic event, which is not the case for the un strained, melt free xenoliths from the same region [cpo Franz et al., 1997]. These observations show that width, structure and chemical composition of the melt film are the critical parameters for grain boundary diffusion and not the structure of the grain boundary itself. The influence of structure and composition of melt on diffusion properties is discussed in a review article by Chakraborty [1995]. The chemical mixing of different thin films on interfaces can only occur by diffusion.
The observations presented in this study, which were made in different kinds of mantle peridotites from different tectonic settings, show that the presence of submicroscopic melt films, layers and inclusions may be a widespread but not universal phenomenon in mantle peridotites. Further studies using TEM and AEM analyses in connection with detailed petrologic studies have to be performed to collect a wider database on melt occurrences and chemical compositions in mantle rocks. The findings gained from these studies may help to get access to small scale melting processes, which are the key to an understanding of melting in the inaccessible depth of the mantle.
ACKNOWLEDGMENTS
The authors would like to thank Karin Peach for her excellent work in TEM specimens preparation. The donation of xenolith samples by Adolphe
264 R. WIRTH and L. FRANZ: Chapter 8
Nicolas and Anke Wendt is greatly appreciated. The reading of an early version of the manuscript by Brian Evans is thankfully acknowledged. Thoughtful reviews by David L. Kohlstedt and Alan B. Thompson sharpened this paper.
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