Journal of Petrology Volume 42 Number 1 Pages 117-130 2001
© Oxford University Press 2001
Relict Majoritic Garnet Microstructures from Ultra-Deep Orogenic Peridotites in Western Norway
VENING MEINESZ RESEARCH SCHOOL OF GEODYNAMICS, FACULTY OF EARTH SCIENCES, UTRECHT UNIVERSITY, 3508 TA UTRECHT, THE NETHERLANDS
Received December 17, 1999; Revised typescript accepted June 28, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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Protogranular, porphyroclastic and equigranular (or equant-polygonal) garnet microstructures from Mg–Cr type orogenic garnet peridotites, Otrøy, Western Gneiss Region, Norway, have been studied using naked eye, light-optical, electron-optical and confocal laser (fluorescence) microscopy techniques. Protogranular and porphyroclastic garnets contain microstructural evidence for the former existence of majoritic (or super-silicic) garnet. The microstructural evidence consists of exsolution textures involving pyroxene. Two types of exsolution microstructures occur—needles parallel to <111>grt and interstitial grains. The maximum volume percentage for intra-crystalline pyroxene exsolution is 2·7, and 3·6 for inter-crystalline pyroxene exsolution. The maximum pyroxene total volume percentage measured in one single protogranular or porphyroclastic garnet is 4·0. This value, at 1200°C, corresponds to minimum pressures of 6·4 GPa (
200 km). Excluding exsolved pyroxene needles and interstitials, protogranular and porphyroclastic garnets also contain coarser-grained silicate inclusions of orthopyroxene, clinopyroxene and olivine. However, orthopyroxene is by far the most common solid inclusion and often is the only solid silicate inclusion present. Garnet porphyroclasts with only orthopyroxene inclusions may have interstitial-like orthopyroxene microstructures. The maximum volume percentage of the latter adds up to 6·7, whereas the maximum volume percentage of normal coarser-grained orthopyroxene inclusions in garnet is eight. If all pyroxene inclusions in garnet are due to exsolution of a majoritic garnet component, this implies a minimum pressure of 8 GPa at 1200°C (246 km). In contrast, the dominant matrix-type around protogranular and porphyroclastic garnet is dunitic to harzburgitic in composition with ol >>opx>cpx. This matrix composition thus puts severe constraints on the origin of garnet microstructures characterized by pyroxene inclusions alone. KEY WORDS: majoritic garnet; relict majoritic garnet microstructures; orogenic garnet peridotite; Scandinavian Caledonides
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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Orogenic ‘root-zone’ peridotites (Den Tex, 1969
) are ubiquitous features of mountain belt systems (Wyllie, 1967; Medaris & Carswell, 1990). Most are spinel-bearing peridotites or their retrograde, hydrous equivalents (chlorite peridotites or serpentinites). Garnet-bearing peridotites are much less common, but where they do occur, they are associated with eclogites and other high-grade crustal rocks in high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic terranes. One terrane in which garnet peridotites and (U)HP metamorphic rocks occur together is the Western Gneiss Region (WGR) in southwest Norway (Bryhni & Sturt, 1985; Koenemann, 1993; Andersen, 1998; Carswell et al., 1999). The WGR is well known for its spectacular HP metamorphic rocks [Eskola, 1921; Griffin et al., 1985, Griffin, 1987 (and references therein)]. In addition, UHP metamorphic rocks (Coleman & Wang, 1995) have also been recognized, including external opx eclogites (Lappin & Smith, 1978, 1981), garnet peridotites (O’Hara & Mercy, 1963; Bryhni, 1966; Carswell, 1968, 1973, 1986; Lappin, 1973, 1974; Brueckner, 1977; Medaris, 1980, 1984; Jamtveit, 1984), coesite eclogite–gneiss (Smith, 1984; Wain, 1997; Cuthbert et al., 2000) and diamond-bearing gneiss (Dobrzhinetskaya et al., 1995). Recently also relicts of majoritic garnet, indicative of pressures >4·5 GPa at 1200°C, have been found within WGR garnet peridotites (Van Roermund & Drury, 1998; Terry et al., 1999).
The spatial distribution of known occurrences of Mg–Cr type garnet peridotites (Carswell et al., 1983) in the WGR, ranging in size from lenses a few metres thick to large tabular bodies several kilometres in areal extent, is illustrated in Fig. 1. Most researchers working with WGR garnet peridotites have treated all Mg–Cr type garnet peridotites as a uniform group (Griffin et al., 1985; Medaris & Carswell, 1990; Krogh & Carswell, 1995; Brueckner & Medaris, 1998). Carswell (1986), followed by Medaris & Carswell (1990) and Krogh & Carswell (1995), established for the WGR Mg–Cr type garnet peridotites the seven-stage metamorphic evolution illustrated in Fig. 2a. Modifications to this general scheme have been made by Van Roermund et al. (2000; Fig. 2b).
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The spatial distribution of Mg–Cr type garnet peridotites in the WGR (Fig. 1) can be subdivided into a northern and a southern domain separated by a gneissic terrane with approximately ENE–WSW vertical foliations (Bryhni, 1989
; Koenemann, 1993; Robinson, 1995). Garnet peridotites from the northern domain strikingly contrast with their southern equivalents in that relicts of majoritic garnet occur only in the northern domain (Fig. 1) on the islands of Otrøy (Van Roermund & Drury, 1998; Van Roermund et al., 2000), Fjørtoft and Flemsøy (Terry et al., 1999). The southernmost Mg–Cr type garnet peridotite occurrences are restricted to isolated ‘spots’, often ‘shielded’ by thick garnet pyroxenite layers, and within otherwise extensively recrystallized chlorite peridotites. The latter are iso-facial with the surrounding amphibolite-facies country-rock gneisses (Medaris, 1980, 1984) and share with them the same structural elements (Brueckner, 1969, 1977; Medaris, 1980; Cordellier et al., 1981). In contrast, the amphibolite facies overprint of garnet peridotites in the northernmost localities at Otrøy, although present, is far less pronounced. Independent of rock-type, primary garnet (stage 2 in Fig. 2a) is still present as relicts throughout most of the Otrøy peridotite bodies. Moreover, garnet microstructures characteristically define three different garnet generations (stages 1, 2 and 3; Fig. 2b). Large single garnet porphyroclasts (
14 cm) and recrystallized polycrystalline garnet aggregates (12 cm) define early microstructures that are ‘embedded’ within a normal (stage 2) garnet peridotite matrix (olivine, opx, ±cpx, grt). Independent of garnet chemistry, this recrystallized garnet microstructure in the northern domain contrasts with the single, stage 2 (Fig. 2a), garnet–olivine (without spinel) mineral assemblage as defined by Carswell (1986), Medaris & Carswell (1990) and Krogh & Carswell (1995; Fig. 2a) for all WGR Mg–Cr type garnet peridotites. Recent research has demonstrated, however, that the majority of the larger garnets in the Otrøy peridotites do not contain microstructural evidence of the former existence of majoritic garnet. However, a few of them do. The reason for this discrepancy is unknown, but it implies that either the large garnets belong to two different garnet generations, or alternatively that all large (+ small?) garnets were once majoritic in composition but their microstructures and chemistries have subsequently been (fully) reset. This forms the topic of the present paper.
Given the geodynamic importance of majoritic garnet in orogenic peridotites (see also Drury et al., 2001) we present in this paper an overview of relict majoritic garnet microstructures found within the Otrøy garnet peridotites, WGR, Norway. Van Roermund & Drury (1998) described, for the first time, relict majoritic garnet microstructures from the Otrøy garnet peridotites, and Van Roermund et al. (2000) presented a more detailed field, petrographic and mineral-chemical study. The samples described by Van Roermund & Drury (1998) and Van Roermund et al. (2000) were collected during a reconnaissance field study when only large garnet grains were sampled. More detailed field studies were conducted in 1998–1999 after the discovery of relict majoritic garnet. Special attention was paid to the relationship between silicate garnet inclusions and bulk-rock chemistry. In this paper we present the microstructural results (supported by some mineral-chemical data) from this more extensive garnet sample-set from the Otrøy garnet peridotites. In addition, we present garnet microstructures that, in our opinion, also represent relicts of majoritic garnet but their microstructures are already in a more advanced state of final transformation towards normal garnet. We hope these descriptions might facilitate recognition elsewhere. After all, within the Otrøy garnet peridotites only a few large garnets reveal the relict majoritic garnet microstructure; most of them do not.
Finally, within the northern domain of the WGR (Fig. 1) relict majoritic garnet microstructures were found within two different garnet peridotite settings. Van Roermund & Drury (1998) described relict majoritic garnet microstructures from nodular and porphyroclastic garnets within peridotites. In contrast, Terry et al. (1999) described relict majoritic garnet microstructures from garnet pyroxenite lenses characterized by extremely coarse-grained high Ca–Al orthopyroxenite crystals (see also Drury et al., 2001). The present paper is restricted to the first type.
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METHODS AND TECHNIQUES |
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TOPABSTRACTINTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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For field and other petrographic characteristics of the Otrøy peridotites the reader is referred to Carswell (1968
, 1974, 1986), Van Roermund & Drury (1998) and Van Roermund et al. (2000); and for geochronological and geochemical characteristics of WGR garnet peridotites to Jamtveit et al. (1991) and Brueckner & Medaris (1998). For this study we have used naked-eye, light-optical, electron-optical and confocal-laser (fluorescence) microscopy techniques. Garnets were cut out of the rock and because of their often heavily fractured nature, impregnated, sliced up in sheets (1–2 mm thick) and polished on both sides. Garnets were then inspected for inclusions and microstructures by optical microscopy techniques using transmitted, reflected and incident light sources. Non-transparent garnets were sliced up again in thinner sheets. A five-axes universal stage, mounted on an optical microscope, was used to measure the orientation of spindle-shaped pyroxene needles with respect to the garnet host. Because of the (often) large grain size of garnet, ‘geographical’ reference-maps for scanning electron microscopy (SEM) work had to be made. The easiest, as well as the most accurate, method was to scan the polished rock surface with a laser-scanner. Subsequently, suitable areas of the garnet were scanned using the lowest magnification of a scanning electron microscope equipped with a back-scattered detector. Suitable areas consisted of up to 65 digital micrographs, which were merged using PC software. Finally, detailed areas within the photomontage were analysed using semi-quantitative energy dispersive spectroscopy (EDS) techniques on the scanning electron microscope, wavelength dispersive (WD) techniques on a microprobe or by confocal laser (fluorescence) microscopy.
In addition to the four samples (A–D) described by Van Roermund & Drury (1998) and Van Roermund et al. (2000), more than 60 new samples were investigated. After final selection the following samples have been studied in detail:
- Ugelvik: 2W1,WH-1,GH-7, 10S, 22S, 23S, 24S, 25S, 2P5-2, N1A and N1B;
- Raudhaugene: R-10, NRa-1, NRa-2, NRa-98-1.
The garnet samples represent a range of rock types. Most large garnets occur in harzburgites containing minor or no clinopyroxene. NRa-98-1, N1A and N1B are garnet pyroxenites. Dark green to black garnets (23S, 24S, 25S, R-10) were sampled from garnet-rich bands of 5–40 cm thickness that occur in layers of garnet- and clinopyroxene-free harzburgites and dunites of 2–30 m thickness.
Two techniques have been used to quantify the amount of majorite component present in garnet before pyroxene exsolution took place. Back-scattered SEM in combination with image analysis programs were used to measure the area fraction. For non-equidimensional inclusions an area fraction cannot be directly used to calculate volume fraction if there is any shape-preferred orientation of the inclusions. This problem can be circumvented using 3D confocal laser (fluorescence) microscopy; this was applied only for the area revealing the highest 2D pyroxene volume percentage. The technique can be visualized as a series of 2D pictures taken at different depth levels throughout the sample. A computer program transforms the data into a 3D picture that can be rotated and viewed from various directions. Finally, the relative phase volume percentages are calculated from the 3D images using image-analysing facilities.
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GARNET MICROSTRUCTURE |
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TOPABSTRACTINTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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On the island of Otrøy two garnet peridotite bodies occur: Ugelvik (UTM 6954700N, 0382500E) and Raudhaugene (UTM 6955500N, 0382700E). Only the Ugelvik garnet peridotite (UGP) has been studied in detail previously (Carswell, 1968
, 1973; Van Roermund & Drury, 1998; Van Roermund et al., 2000). Both garnet peridotites consist of deformed, inter-banded, garnet-bearing and garnet-free peridotite (Carswell, 1968; Van Roermund et al., 2000). In addition, both bodies consist of variable amounts of dunite, harzburgite, lherzolite, websterite and pyroxenite. The compositional banding is sub-parallel to a foliation defined by elongated trails of garnet, pyroxene and/or late amphibole. Tight to isoclinal folds of the compositional banding occur with axial planes sub-parallel to the foliation. Observed fold wavelengths are 
5–10 cm in the Ugelvik body and up to 5 m in the Raudhaugene body. In the Raudhaugene body, complex fold interference patterns are also present. Most of the peridotite rock volume, however, is defined by an equigranular annealed texture of strain-free olivine grains having straight or gently curved grain boundaries that meet in 120° triple junctions (Fig. 3a, bottom and top). Sporadically larger olivine porphyroclasts occur up to several centimetres across (Fig. 3a, centre). These olivine porphyroclasts reveal a distinct deformation microstructure defined by narrowly spaced (001) kinks (not visible in Fig. 3a). In contrast, the surrounding equigranular olivine foam texture is strain free. In addition, the olivine porphyroclasts contain ilmenite and spinel rods (Fig. 3b) equivalent to those described from Alpe Arami (Dobrzhinetskaya et al., 1996; Green et al., 1997) and Chijiadian (Hacker et al., 1997) garnet peridotites (Van Roermund et al., in preparation).
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Like olivine, garnet occurs also as two distinct microstructural types (Fig. 4a–c). Garnet is predominantly found as ‘small’ normal grains (
5 mm) dispersed in the ol ± opx ± cpx ± sp matrix. Garnet occurs also, however, in subordinate amounts as ‘large’, nodular garnets (14 cm; Fig. 4a). Individual garnet nodules can be mono- or polycrystalline. Single garnet grains can have a grain size up to 10 cm. Individual garnets in polycrystalline garnet nodules have grain sizes of 2–8 mm. In addition, large garnet nodules can be stretched out into lenticular shapes revealing central garnet porphyroclasts with symmetrical recrystallized tails parallel to the foliation (Fig. 4b); others have been (partly or fully) recrystallized into new garnet grains with the same grain size as normal matrix garnet elsewhere (Fig. 4c).
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By analogy with the commonly used terminology of microstructures in mantle xenoliths the terms nodular, porphyroclastic and recrystallized garnets will be replaced by protogranular, porphyroclastic and equigranular mosaic, respectively, as defined by Nicolas & Poirier (1976)
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The recrystallized garnet grain size in the porphyroclastic garnet microstructure is roughly similar to that of normal matrix garnet (Fig. 4b and c), with spinel being absent. This porphyroclastic garnet microstructure is thus distinctly different from the five-phase, stage 3 (Fig. 2a), mineral assemblage defined by Carswell (1986), which refers to normal matrix garnets (stage 2, Fig. 2a) partly recrystallized into a much finer-grained (<0·5 µm) garnet–olivine–spinel assemblage (stage 3, Fig. 2a and b). Finally, the porphyroclastic garnet microstructure was found in peridotites as well as pyroxenites.
Majoritic garnet
Microstructural evidence for the former existence of majoritic (or super-silicic) garnet was presented from coarse-grained polycrystalline garnet nodules (Van Roermund & Drury, 1998; Van Roermund et al., 2000). The microstructural evidence consisted of (two) pyroxene exsolution microstructures from garnet. In crystal-chemical terms the reaction can be described as
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n 1·0. Reaction (1) is insensitive to changes in redox state. The amount of octahedral silicon in garnet is a direct function of pressure (Akaogi & Akimoto, 1977; Irifune, 1987; Fig. 5a) with a much smaller temperature dependence (Fig. 5b; Gasparik, 1990; Terry et al., 1999; Drury et al., 2001). Taking the relative mineral densities into account, the formation pressures of super-silicic garnets can also be expressed as a function of the relative volume percentage between exsolved pyroxene and ‘normal’ garnet (Fig. 5c). Using back-scattered electron microscopy and 3D confocal laser (fluorescence) microscopy we have used the latter technique to make a detailed estimate of the original ‘super-silicic’ component of garnet. The latter component can also loosely be referred to as the majoritic component of garnet. The pressure can be estimated from the volume percentage of exsolved pyroxene by the following two methods:
- the majoritic garnet composition is calculated from microprobe analyses of the individual crystalline phases in combination with the relative volume percentage and mineral densities. Subsequently, the reconstructed super-silicic garnet composition can be used to estimate pressure from experimental data on natural compositions for the dependence of garnet silica content on pressure (Akaogi & Akimoto, 1977; Irifune, 1987; Fig. 5a). This is the method used by Van Roermund & Drury (1998), following Sautter et al. (1991). It allows for calculations including natural multi-component systems. The method is, however, sensitive to inaccuracy in silica measurements obtained by electron microprobe (EMP).
- The inverse method: by using equation (1) with various numbers for n for simple end-member compositions with known mineral densities, the relative volume percentages of exsolved pyroxene from garnet can be calculated for each super-silicic garnet composition (Fig. 5c).
Both methods of estimating the original pre-exsolution garnet composition give similar results but once the databank of method (2) has been calculated that method is much more practical.
Extensive experimental data are available only for natural compositions at 1200°C. Gasparik (1990) has constructed a petrogenetic P–T grid for the CMAS system, which shows the octahedral co-ordination of Si in garnet to be significantly temperature dependent. Some limited data on natural compositions (Canil, 1991; Walters, 1998; Fig. 5b) can be used to estimate the temperature dependence, although many of the higher-temperature data are scattered and there is an urgent need for more studies on the variations of majoritic garnet compositions with P and T. Pressures, calculated from the octahedral co-ordinated Si content in garnet, have been converted into depths using a pressure–depth calibration (for depths between 50 and 400 km) based on the Preliminary Reference Earth Model (PREM) given by Anderson (1989): P(kbar)= -3·263 + 0·34032d, where d is depth in kilometres.
Pyroxene exsolution lamellae, indicative of super-silicic (or majoritic) garnet, occur in a few of the polycrystalline garnet nodules and some coarse-grained single garnet grains. Relicts of majoritic garnets have not been found within normal matrix garnet. It is important to note that the majority of protogranular or porphyroclastic garnets are devoid of any clear microstructural evidence of pyroxene exsolution from garnet.
In the following we will briefly summarize the microstructural evidence for the former existence of majoritic garnet within some polycrystalline garnet nodules (from now on we will refer to this microstructure as the majoritic garnet microstructure). Subsequently, newly discovered majoritic garnet microstructures in coarse-grained single garnet crystals will be described. This will be followed by other relict majoritic garnet micructures that are interpreted to be already in a more advanced state of transformation into normal garnet microstructures. Finally, in the discussion, the age relationship between majoritic, protogranular, porphyroclastic and normal garnet will be discussed.
Majoritic garnet microstructures in polycrystalline garnet nodules
The majoritic garnet microstructures in polycrystalline garnet nodules have been described in detail by Van Roermund & Drury (1998). The four characteristic microstructural elements can be summarized as follows (Fig. 6):
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- (a) majoritic garnet nodules consist of polycrystalline garnets ranging in size from 2 to 8 mm. Garnet–garnet grain boundaries are straight or gently curved; triple-point junctions are common.
- (b) Interstitial orthopyroxenes decorate garnet grain boundaries. Interstitial orthopyroxene grain shapes vary from triangular (at triple junctions) to elongate. Elongated crystals always have thickness <0·5 mm. The latter is equivalent to the size of triangular interstitial orthopyroxenes.
- (c) In the larger garnet grain cores two-pyroxene exsolution needles occur, which are 5–10 µm thick and oriented parallel to <111>grt. From back-scattered electron images the relative volume percentage between exsolved cpx and opx was determined to be 1:9.
- (d) Two-pyroxene-bearing garnet cores are surrounded by precipitation-free rims of 2 mm thickness.
The pyroxene exsolution lamellae [element (c)] are crucial in the recognition of the super-silicic garnet microstructure. Without element (c) the nature of the microstructure cannot be recognized. However, to fully appreciate the exsolution microstructure each of the above-quoted characteristic four elements are required.
Interstitial orthopyroxenes at garnet grain boundaries comprise up to 3·6 vol. %. In the garnet grain cores the maximum pyroxene content is 1 vol. %. Both pyroxene types have been interpreted to be the result of exsolution. Consequently, the maximum amount of exsolved pyroxene was estimated to be 3·6–4·0 vol. % which (from Fig. 5) is equivalent, at 1200°C, to depths of 190–200 km. Representative EMP mineral analyses were presented by Van Roermund et al. (2000). Microprobe stepscan profiles across garnet from majoritic garnet microstructures in polycrystalline garnet nodules illustrate that the individual garnets are homogeneous in terms of chemistry (Van Roermund et al., 2000). Representative garnet core compositions are given in Table 1.
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Majoritic garnet microstructures in coarse-grained single garnet crystals
Majoritic garnet microstructures were recently also discovered within coarse-grained, single garnet crystals (sample 2W1; Fig. 7). The samples were found close to each other within a harzburgitic to dunitic bulk-rock composition. The single garnet grains, heavily fractured, contain several coarse-grained orthopyroxene inclusions (Fig. 7b). In addition, at various, apparently unrelated, isolated spots within the garnet grain, clusters of oriented orthopyroxene needles occur distinctly smaller in size than the isolated normal coarser-grained orthopyroxene inclusions (Fig. 7c and d). The orthopyroxene needles are 10–20 µm thick and oriented parallel to <111>grt. The spatial distribution of the clusters of oriented orthopyroxene needles within the single garnet grains is cloudy (Fig. 7b) and prohibits recognition of classical precipitation-free rims [element (d) in Fig. 6]. As a result of the absence of garnet–garnet grain boundaries interstitial orthopyroxene microstructures are absent [element (b) of Fig. 6].
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In terms of chemistry coarser-grained orthopyroxene inclusions and small pyroxene needles were altered by later re-equilibrations. Consequently, representative EMP analyses could not be obtained. The 2W1 garnet core composition is indicated in Table 1 and Fig. 11 (below). Towards the rim the garnet chemistry was modified.
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The local volume percentage of exsolved pyroxene needles, measured by confocal microscopy, adds up to 2·7 and contrasts markedly with the 1 vol. % of exsolved pyroxene needles in polycrystalline majoritic garnet relicts. The relative volume percentage of coarser-grained opx inclusions is 4·5.
Silicate inclusions in porphyroclastic and normal garnets
Like the garnets in all other WGR peridotites, the Otrøy garnets contain coarse-grained inclusions of olivine, orthopyroxene and clinopyroxene (Fig. 8). Single and composite inclusions occur. Composite silicate inclusions consist of various combinations of olivine, orthopyroxene and/or clinopyroxene. All coarse-grained solid-phase silicate inclusions have gently curved grain boundaries. There is no clear crystallographic relationship between host and inclusion. Mineral chemistries of coarse-grained, single and composite silicate inclusions have not been studied in detail. For references the reader is referred to Carswell (1986), Medaris & Carswell (1990) and Krogh & Carswell (1995).
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There is, however, a very strong argument against interpreting the normal coarse-grained solid-phase silicate inclusions in (coarse-grained) garnet as being captured stage 1 or 2 (Fig. 2b) matrix grains during primary garnet growth. Our systematic studies on garnet inclusions revealed that the relative volume percentage between solid silicate inclusions in garnet usually is opx olivine > cpx. In many cases orthopyroxene was the only inclusion present in garnet. Some representative microstructures are illustrated in Fig. 9. The maximum volume percentage of coarse-grained opx inclusion in garnet is 8 (measured in back-scattered electron-micrographs). The microstructural observation that opx is by far the most dominant garnet inclusion, and often the only silicate inclusion present, is in strong contrast to the composition of the matrix directly surrounding the garnet, which, depending on host rock chemistry, can be taken as olivine >> opx > cpx.
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Clinopyroxene inclusions are scarce, but do occur (Carswell, 1968). They have been found only in very large garnets from harzburgite and in garnets from pyroxenite layers within otherwise garnet-free harzburgite.
Interstitial orthopyroxene microstructure
The interstitial orthopyroxene microstructure [element (b) in Fig. 6] has been found within some polycrystalline porphyroclastic garnets (Fig. 10). The interstitial orthopyroxene microstructure is characterized by a systematic arrangement of elongated to triangular-shaped orthopyroxene crystals outlining garnet–garnet grain boundaries and triple junctions. In plan view the orthopyroxene crystals decorate garnet–garnet grain boundaries or precipitate at garnet triple junctions. Garnet–garnet grain boundaries are straight or gently curved. Elongated orthopyroxene crystals have limited thicknesses (0·5 mm) but variable lengths. The interstitial orthopyroxene microstructure is strongly reminiscent of element (b) of the relict majoritic garnet microstructure found in polycrystalline garnet nodules (Fig. 6); however, the exsolved pyroxene needles [element (c)] in garnet grain cores are missing. The maximum volume percentage of exsolved pyroxene in interstitial orthopyroxene microstructures was estimated from back-scattered electron micrographs to be 6·2. In our samples containing the interstitial orthopyroxene microstructures the orthopyroxene mineral chemistry has been affected by later re-equilibrations. Garnet grain core EMP analyses are indicated in Table 1 and Fig. 11.
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GARNET CHEMISTRY |
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TOPABSTRACTINTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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Garnet core compositions of the investigated samples, measured with WDS on an EMP (20 kV, 10·0 nA; standards used were pyrope and almandine), are illustrated in Table 1 and Fig. 11. In the field, protogranular and porphyroclastic garnets occur in two colour types: pink to purple vs dark green to black. Both garnet colour types occur in harzburgites and dunites. However, pink to purple garnets are by far the most general and have CaO
7·0 wt % and Cr2O3 7·0 wt % (Fig. 11a). Pink to purple, protogranular and porphyroclastic garnets define a linear trend that generally corresponds to the lherzolite field of Sobolev et al. (1986) or the lherzolite–Ca-harzburgite field of Gurney (1984). Pink to purple garnet chemistries generally overlap with CaO–Cr2O3 compositions given by Brueckner & Medaris (1998) for WGR garnet peridotites (Fig. 11b). However, within the Otrøy peridotite bodies alone a much wider trend towards higher Cr2O3 wt % can be recognized (Fig. 11a).
Relicts of majoritic garnet microstructures have been found only in pink to purple garnets. Core compositions are given in Table 1. In addition, in Fig. 11a garnet compositions of relict majoritic garnet microstructures were plotted with separate symbols. CaO–Cr2O3 compositions of relict majoritic garnet microstructures overlap with CaO–Cr2O3 compositions given by Brueckner & Medaris (1998) for WGR garnet peridotites (Fig. 11b). From Fig. 11, however, it can be seen that the garnet chemistry within the Otrøy peridotites is also variable in CaO wt %. Dark green to black garnets from layers containing higher garnet contents in otherwise garnet-free harzburgites and dunites have higher CaO contents and in some cases higher Cr2O3 contents than pink to purple garnet. Dark green to black garnets consistently plot in the calcic field and clearly define a second trend that thus far has not been recognized within peridotites from the WGR; their origin is currently under investigation. Most important, it can be seen from Fig. 11 that the composition of garnets containing relict majoritic garnet microstructures is equivalent to other protogranular, porphyroclastic or equigranular mosaic garnet compositions from the WGR, and consistently plots in the lherzolite trend field of Sobolev et al. (1986). This indicates that garnets containing majoritic garnet microstructures have been fully re-equilibrated. This is in full agreement with results obtained from thermobarometry (Van Roermund et al., 2000).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODS AND TECHNIQUESGARNET MICROSTRUCTUREGARNET CHEMISTRYDISCUSSIONREFERENCES |
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The crystallographically controlled pyroxene lamellae in the cores of garnet grains [element (c) in Fig. 6] have clearly formed by exsolution from a supersaturated majoritic garnet. Our latest volume fraction measurement of pyroxene needles formed by exsolution using confocal laser microscopy is up to 2·7 %. This is similar to values of 1% pyroxene exsolution lamellae in the samples described by Van Roermund & Drury (1998)
and 2% pyroxene exsolution reported by Terry et al. (1999). Van Roermund & Drury (1998) also interpreted the interstitial orthopyroxenes [element (b) in Fig. 6] as being the result of exsolution from garnet. Such an interpretation was justified by the intimate spatial relationship between the two microstructural elements. In some of the garnet microstructures described in the present paper there is, however, no obvious relationship between garnet domains containing exsolved pyroxene rods or needles and areas that contain larger pyroxene inclusions or interstitial pyroxene grains. This hampers a straightforward interpretation. The following three models can be applied:
- in the case of a closed system the orthopyroxene interstitials and inclusions in garnet may be explained by
- The inclusions might be explained by growth of garnet blasts including some matrix mineral grains during growth. The matrix of harzburgitic to dunitic bulk-rock compositions consists of ol >> opx > cpx. This strikingly contrasts with the relative volume percentage between the solid-phase silicate inclusions found within protogranular and porphyroclastic garnet. Many of these garnets contain only pyroxene inclusions (Fig. 9); others have opx >> olivine. Therefore, to explain the dominance of opx inclusions in garnet, olivine would have to be consumed, or transformed into opx, during garnet growth, otherwise olivine would be the dominant inclusion type. Few solid-state or high-pressure melting reactions involve garnet growth and olivine breakdown (Gasparik, 1990; Walters, 1998). The reaction of ol + amp to form garnet + opx + cpx + vapour (Medaris, 1984) might result in the formation of opx and cpx inclusions in garnet overgrowing an olivine-rich matrix. Because of the lack of amphibole in stage 1 assemblages (Fig. 2b) this reaction can be regarded as unlikely. Transformation of olivine into orthopyroxene requires the addition of SiO2. In the rocks studied we have found no support for such an interpretation.
- The dominance of pyroxene inclusions in garnet can also be explained by assuming that the garnets have been derived from disrupted garnet-rich pyroxenite layers or melt segregations, although such an interpretation is inconsistent with our field observations. In this respect, it should be noted, however, that garnet microstructures (with normal pyroxene inclusions) in some pyroxenite layers in the Otrøy peridotites are similar to that of the garnet nodules. However, this pyroxene–garnet microstructure in pyroxenite layers can also be explained by exsolution. A striking feature of some pyroxenites is the presence of a garnetite core surrounded by thin clinopyroxene-rich rims. This geometry could be explained by exsolution of clinopyroxene along the margins of a majoritic garnet layer. Furthermore, it should be noted that D. A. Carswell, Sheffield University, UK (personal communication, 2000), recently found relict majoritic garnet microstructures within some garnets from garnet pyroxenites of the Raudhaugene peridotite body.
Taking into account the overall similarity between relict majoritic and non-majoritic garnet compositions (Fig. 11; Table 1) we prefer the interpretation that most (ortho)pyroxene inclusions and interstitials originated by precipitation and subsequent coarsening from a supersaturated majoritic garnet.
The minimum pyroxene volume fraction in garnets in the form of exsolution lamellae is 2·7%. This corresponds to a Si content of 3·025 per 12 oxygens in garnet and a pressure of 5·8 GPa at 1200°C. If the pyroxene interstitials and inclusions have also formed by exsolution from majoritic garnet then a pyroxene content of up to 8% corresponds to a Si content up to 3·074 per 12 oxygens in garnet and a pressure of 8·1 GPa at 1200°C (248 km).
We have no constraints on the temperatures for the stability of the majoritic garnets. The presence of high-Al–Ca pyroxenite lenses (Carswell, 1973; Terry et al., 1999; Van Roermund et al., 2000; Drury et al., 2001) indicates that the Otrøy peridotites have had an early very high temperature history. The subordinate temperature dependence of the Si content in garnet (Canil, 1991; Walters, 1998), causes majoritic garnet with a ‘natural’ composition and a Si content <3·03 to be stable between 1500–1600°C at 5–5·5 GPa, corresponding to depths of 150 km. For such high temperatures the minimum depth of origin of the Otrøy peridotites may be less than originally proposed. [Alternatively, Terry et al. (1999) obtained, using the simple petrogenetic CMAS grid of Gasparik (1990), an estimate of 3·4 GPa and 1490°C for relict majoritic garnet microstructures, containing 2 vol. % exsolved pyroxene, for garnet-bearing high-Ca–Al orthopyroxenite lenses within garnet peridotites from Fjørtoft (Fig. 1).]
Even if the minimum depth of origin of the Otrøy peridotites is ‘only’ 110–150 km the combination of very high temperatures and ultra-high pressures indicates that the peridotites originated in the asthenosphere and were emplaced into the lithosphere by asthenosphere diapirism (Van Roermund & Drury, 1998). Numerical models of asthenosphere diapirism (De Smet et al., 1998, 2000) show that material in such diapirs originates from the base of the convection cells in the asthenosphere, so peridotites emplaced by this process probably were present in the deep upper mantle or transition zone before diapiric emplacement into the lithosphere (Drury et al., 2001).
The sequence of garnet microstructures ranging from protogranular to equigranular implies that the garnets did not behave passively during deformation of the peridotite. Larger garnets contain intracrystalline deformation features (Van Roermund et al., 2000). The microstructures suggest that the garnets were plastically deforming and that recrystallization occurred during deformation. Experimental deformation studies and innumerable descriptions of deformed peridotites show that garnet has a much higher creep resistance than olivine under normal mantle conditions (Karato et al., 1995). Extensive deformation of garnet in an olivine matrix may be explained by very high temperatures. If the main deformation occurred during upwelling of hot 1500–1600°C asthenosphere then under these conditions garnet may be ductile.
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ACKNOWLEDGEMENTS |
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This project was supported by an NWO (Netherlands Organization for Scientific Research) PIONIER subsidy. Electron microscopy was performed at EMSA, the Utrecht University Centre for Electron Microscopy and Structure Analysis and EMP analyses at the IVA microprobe laboratory. The manuscript benefited from constructive reviews by Tony Carswell and Ben Harte. Izaak Santoe, audiovisuele diest UU, is thanked for his help in making the figures.
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FOOTNOTES |
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*Corresponding author. Telephone: 0031-30-2535053. Fax: 0031-30-2537725. E-mail: HermanvR{at}geo.uu.nl
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REFERENCES |
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