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Journal of Petrology Volume 42 Number 1 Pages 131-140 2001
© Oxford University Press 2001
Emplacement of Deep Upper-Mantle Rocks into Cratonic Lithosphere by Convection and Diapiric Upwelling
1VENING MEINESZ RESEARCH SCHOOL OF GEODYNAMICS, FACULTY OF EARTH SCIENCES, UTRECHT UNIVERSITY, PO BOX 80.021, 3508 TA UTRECHT, NETHERLANDS
2ENVIRONMENTAL AND GEOLOGICAL SCIENCES, UNIVERSITY OF SHEFFIELD, DAINTON BUILDING, BROOKHILL, SHEFFIELD S3 7HF, UK
Received December 17, 1999; Revised typescript accepted June 26, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Rocks containing breakdown products of majoritic garnet, derived from the deep upper mantle, occur in kimberlite xenoliths and in orogenic peridotites from Otrøy in Norway. The Otrøy peridotites are banded harzburgites and dunites with similar compositions to mantle xenoliths from Precambrian cratons and Phanerozoic supra-subduction-zone peridotites. Pressure–temperature (P–T) paths deduced for the Otrøy peridotites and kimberlite xenoliths from South Africa are consistent with emplacement of deep mantle peridotites into cratonic lithosphere by asthenosphere diapirism. Numerical thermo-convection models provide insight into the possible P–T histories of deep upper-mantle rocks. In the models, material from the base of the convecting system is transported to depths of 60–100 km by convection and small (50–100 km) diapirs. Diapir intrusion induces small-scale convection in the low-viscosity deeper part of the thermochemically defined lithosphere. Small-scale convection in the craton root can produce complex P–T paths, complex recurrent melting histories and complex compositional structure in the craton. P–T paths derived from the numerical models for asthenosphere diapirism in a hot upper mantle are consistent with the sequence of sub-solidus P–T conditions deduced for the cratonic peridotites.
KEY WORDS: asthenosphere diapirs; cratonic lithosphere; deep upper mantle; majoritic garnet
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Some mantle xenoliths (Haggerty & Sautter, 1990
) and diamond inclusion suites (Moore & Gurney, 1985; Moore et al., 1991) are derived from the deep upper mantle (150–410 km), transition zone (410–670 km) (Sautter et al., 1991) and even the lower mantle (Scott-Smith et al., 1984; Kesson & Fitz Gerald, 1991; Harte et al., 1999). A few orogenic peridotite bodies may also be derived from the deep upper mantle (Pearson et al., 1989; Dobrzhinetskaya et al., 1996; Van Roermund & Drury, 1998; Bozhilov et al., 1999), although some cases are controversial (Pfiffner et al., 1998; Bozhilov et al., 1999).
Peridotites from the deep upper mantle probably consist of olivine, clinopyroxene and majoritic garnet (Herzberg, 1995). Majoritic garnet has Si in tetrahedral and octahedral sites. Octahedral co-ordination for Si is significant only above 4·5 GPa (at 1200°C) and the amount of Si increases with pressure. If majoritic garnet is decompressed it breaks down to garnet with pyroxene exsolution. Breakdown products of majoritic garnet were first found in kimberlite xenoliths by Haggerty & Sautter (1990) and similar relict microstructures have recently been found in orogenic peridotites from the Western Gneiss Region (WGR) in Norway (Van Roermund & Drury, 1998; Terry et al., 1999).
In this contribution we consider the role of asthenosphere diapirism in the emplacement of deep upper-mantle peridotites into the cratonic lithosphere. The occurrence of majoritic garnet relicts in the Otrøy peridotites has been described by Van Roermund & Drury (1998) and Van Roermund et al. (2000). Here we present new data on rock compositions in the Ugelvik peridotite. A new interpretation of the P–T history of the Otrøy peridotites and peridotites from kimberlite xenoliths is also presented. We compare possible P–T paths of cratonic peridotites with those predicted from numerical models of mantle convection and asthenosphere diapirism beneath a cratonic root (De Smet et al., 1998, 1999, 2000). Finally, we discuss the implications of the numerical modelling results for the history and emplacement processes of deep upper-mantle rocks.
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UPPER-MANTLE PERIDOTITES WITH MAJORITIC GARNET RELICTS |
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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Orogenic peridotites, WGR, Norway
Many orogenic peridotite bodies occur in the WGR of Norway, which is a terrane of mainly Proterozoic basement rocks and infolded pre-Caledonian sediments reworked during the Caledonian orogeny. The dominant lithology is amphibolite-facies gneiss with bodies of higher-pressure eclogites and garnet peridotites (Krogh & Carswell, 1995
). Breakdown products of majoritic garnet occur in garnet peridotites on the islands of Otrøy (Van Roermund & Drury, 1998), Flemsøy and Fjortøft (Terry et al., 1999). Published isotope data from WGR peridotites suggest an age of depletion around 2·5–3·0 Ga whereas Sm–Nd dates range from early Proterozoic for early mineral assemblages to Caledonian ages for some recrystallized assemblages (Jamtveit et al., 1991; Brueckner & Medaris, 1998).
The Otrøy peridotites have a clear compositional banding (Carswell, 1968; Van Roermund et al., 2000) with bands of cpx-bearing garnet harzburgite (with minor lherzolites), garnet harzburgite, garnet-free harzburgite and dunites. Interlayered with the peridotites are lenses and layers of pyroxenites, garnet- and pyroxene-rich peridotites (Carswell, 1968, 1973) and some garnetites. The average composition of the Ugelvik body is harzburgite to dunite (Van Roermund et al., 2000). The Raudhaugene body has less dunite and possibly more lherzolite than Ugelvik. The results from a detailed mapping study of the metre-scale average compositions in the eastern part of the Ugelvik peridotite are shown in Fig. 1. Compositional variations occur in a systematic and symmetric sequence of fertile to depleted compositions of cpx harzburgite, cpx-free harzburgite, opx dunite and dunite (Fig. 1).
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Data on mineral and whole-rock chemistry from Carswell (1968, and unpublished data) are presented in Figs 2 and 3. Otrøy peridotites commonly have high modal olivine (75–95%) and high Fo (92–94) olivine (Medaris, 1984; Carswell, 1986; Van Roermund et al., 2000). In terms of olivine composition and modal olivine (Fig. 2) the dominant Otrøy compositions are similar to some cratonic xenoliths (Bernstein et al., 1998; McDonough & Rudnick, 1998; MacKensie & Canil, 1999; Schmidberger & Francis, 1999) and some Phanerozoic peridotites from supra-subduction settings (Bonetti & Micheal, 1989; Menzies, 1990; Bernstein et al., 1998). Data for whole-rock chemistry (Fig. 3) show that the dominant depleted Otrøy rocks are similar to low-SiO2 Kaapvaal xenoliths and Wiedemann Fjord xenoliths (Bernstein et al., 1998). Compared with most Kaapvaal rocks, however, the Otrøy peridotites have lower whole-rock SiO2 and modal orthopyroxene. Brueckner & Medaris (1998) showed that garnets in WGR peridotites have similar compositions to garnets from Precambrian cratonic xenoliths.
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The Otrøy peridotites are similar to kimberlite xenoliths, as first emphasized by Carswell (1968). Both rock suites are dominated by depleted compositions and have an equigranular olivine microstructure. The P–T conditions estimated from mineral chemistry in kimberlite xenoliths define a cratonic array (Nixon & Boyd, 1973). The most recent P–T estimates (Brueckner & Medaris, 1998), for nine WGR peridotite bodies, fall along a similar array from 805°C and 3·4 GPa to 975°C and 5·4 GPa (Fig. 4). Megacrysts occur in both rock suites. At Otrøy garnet and olivine megacrysts occur as porphyroclasts in the peridotites whereas orthopyroxene megacrysts occur in websterite layers. Compared with the Kaapvaal xenoliths the Otrøy peridotites show a lack of hot sheared peridotites, lower modal orthopyroxene and overprinting of lower-pressure metamorphic reactions.
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Brueckner & Medaris (1998) have shown that the Norwegian peridotites represent fragments of Precambrian cratonic lithosphere, which was present in a static cold environment a long time before being incorporated into the Caledonian orogeny. They concluded that the WGR peridotites are an ‘excellent example of cold, melt depleted, buoyant lithosphere of the type that would be expected beneath the Baltic or Laurentian shield prior to Caledonian collision’. As noted by Brueckner & Medaris (1998) and Jamtveit et al. (1991), age data from WGR peridotites and crustal rocks indicate that this portion of continental lithosphere was possibly first formed in the Archaean and was later possibly involved in two Proterozoic orogenies and an episode of Mid-Proterozoic anorogenic magmatism.
P–T history of WGR peridotites
Van Roermund & Drury (1998) suggested that the P–T path of the Otrøy peridotites involves two main stages: (1) adiabatic ascent from the stability field of majoritic garnet to the stability field of high-Al–Ca orthopyroxene (Carswell, 1973); (2) cooling to 800°C at pressures around 3 GPa. The stability field estimated for the high-Ca–Al orthopyroxene depends on the proportion of primary and exsolved garnet in the megacrystal orthopyroxene within the garnet websterite lens reported by Carswell (1973). Similar garnet peridotites and exsolved megacrystal orthopyroxene-dominant garnet websterite occur on the island of Fjortøft close to Otrøy (Terry et al., 1999). The Fjortøft garnet websterite contains interstitial garnet with pyroxene exsolution lamellae between the orthopyroxene megacrysts with their internal exsolved garnet and clinopyroxene. This demonstrates that the high-Al–Ca orthopyroxenes were stable with majoritic garnet (Terry et al., 1999). The P–T conditions for an assemblage of high-Al–Ca opx (4–3% Al2O3) and majoritic garnet (with 1–3% pyroxene exsolution) range from 3·4 GPa and 1490°C (Terry et al., 1999) to 4·5 GPa and 1600°C based on the P–T grid of Fig. 4. It is not clear if the Otrøy high-Ca–Al megacrystal orthopyroxene was stable with majoritic garnet. Hence a range of P–T conditions are possible (Fig. 4) ranging from minimum conditions of around 2·7 GPa and 1350°C (point 1a in Fig. 4) to conditions of 4·5 GPa and 1600°C (point 1b in Fig. 4).
Megacrystal garnets containing pyroxene exsolution lamellae occur in the Otrøy peridotites as centimetre-scale aggregates and single grains. Some large garnets are associated with clinopyroxene (Carswell, 1973) although most garnets investigated by Van Roermund et al. (2001) contain interstitial orthopyroxene and inclusions of orthopyroxene, subordinate olivine and minor amounts of cpx. The large majoritic garnet relicts in the peridotites could be porphyroclasts from a coarse early assemblage or they may have been formed from distributed melt segregations or disrupted websterite layers in the peridotite. Aggregates of garnet and clinopyroxene in otherwise highly depleted peridotites may be derived from some form of melt segregation whereas many garnet aggregates have compositions consistent with an origin as porphyroclasts. If the interstitial orthopyroxene and the orthopyroxene inclusions in the garnet aggregates formed by exsolution then original pressures of more than 6·5–8 GPa are implied, depending on temperature (Van Roermund et al., 2001).
Extremely high temperatures of 1350–1600°C at depths of 80–130 km, suggestive of asthenosphere upwelling, are based on the stability of high-Al–Ca megacrystal orthopyroxene, which occurs in a discrete garnet websterite lens within the peridotites (Carswell, 1973). The P–T conditions inferred from a bulk integrated analysis of the exsolved megacrystal orthopyroxene given by Carswell (1973) provide a minimum estimate for the potential temperature of the upwelling mantle. The Otrøy peridotites could be derived from the upwelling deep mantle and possible P–T paths (a and b) are shown in Fig. 4. The original megacrystal orthopyroxenites probably formed by fractional crystallization from an intruding melt so the peridotites may not have experienced temperatures as high as the pyroxenites. The coarse exsolution microstructure in the orthopyroxene megacrysts (Carswell, 1973) suggests a slow cooling rate consistent with a small temperature difference of <200°C between the orthopyroxenite ‘intrusion’ and surrounding peridotite (Sautter & Fabriès, 1990). If the peridotites and pyroxenites had different P–T paths then it is possible that the peridotites could have been derived from deep lithospheric mantle (at 180–200 km) entrained and heated by upwelling asthenosphere (path c, Fig. 4).
The depleted composition of the Otrøy peridotites implies extensive melting and a substantial supersolidus P–T path at some stage in their history. Bernstein et al. (1998) concluded that such compositions (Fig. 3) can be produced either by dissolution of pyroxene from previously depleted peridotites, perhaps in an arc setting, or by extraction of Munro-type komatiite at pressures of 2–3 GPa. The compositional data (Fig. 3) of the depleted Otrøy peridotites lie on a trend consistent with komatiite extraction. Extensive melting at <4·5 GPa, however, would result in the complete consumption of garnet (Herzberg & O’Hara, 1998; Walter, 1998) and majoritic garnet porphyroclasts would not be preserved in the residual peridotite.
Melting of the Otrøy rocks could have occurred during emplacement into Proterozoic lithosphere (path a, Fig. 4). The crystallization of megacrystal orthopyroxenites could mark the end of melting at 3·4–5 GPa during upwelling from the deeper mantle. The preservation of centimetre-scale relict majoritic garnet porphyroclasts is possible if melting occurred above 4·5 GPa (Walter, 1998). Alternatively, the depleted compositions and preservation of relict majoritic garnet porphyroclasts could be explained by a more complex history of early shallow melting and tectonic transport to the deep upper mantle, followed by upwelling and emplacement into Proterozoic lithosphere (path b or c, Fig. 4).
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P–T history of kimberlite xenoliths
Many studies (e.g. Nixon & Boyd, 1973
) have used the P–T estimates from kimberlite xenoliths to estimate the geotherm at the time of kimberlite eruption. The earlier P–T history can also be deduced (Fig. 5) from the reconstruction of early mineral compositions based on exsolution relationships. Xenoliths with breakdown products of majoritic garnet (Sautter et al., 1991) imply original depths in the deep upper mantle to transition zone. Cox et al. (1987) and Boyd & Mertzman (1987) presented evidence that garnet and clinopyroxene in both cold undeformed and hot deformed xenoliths exsolved from a high-Ca–Al pyroxene precursor, implying cooling from temperatures of 1200–1600°C down to 800–1000°C at depths around 125–150 km. Eclogite xenoliths containing megacrystal pyroxene grains with exsolution provide independent evidence for this cooling (Lappin & Dawson, 1975; Harte & Gurney, 1975). P–T paths can be inferred for the cold undeformed kimberlite xenoliths (Fig. 5) with high-temperature decompression from the stability field of majoritic garnet to the stability field of the high-Ca–Al pyroxenes followed by dominant cooling from 1200–1600°C down to a normal cratonic geotherm (Fig. 5). For hot sheared kimberlite xenoliths a P–T path involving cooling and increasing pressure is implied (Fig. 5) (Cox et al., 1987; Canil, 1991).
The depleted compositions of kimberlite xenoliths suggest that extensive melting occurred at some time in their history. Melting and depletion could have occurred during asthenosphere upwelling into cratonic lithosphere or in an earlier stage of a more complex history involving early shallow melting, tectonic transport to deeper levels and finally upwelling with limited high-pressure melting during emplacement of the peridotites into cratonic lithosphere. Kelemen et al. (1998) have argued that a correlation between CaO and Yb contents in cratonic garnet peridotites implies that most melting occurred at pressures <3 GPa, suggesting that a multi-stage history may be applicable to kimberlite xenoliths.
The P–T paths derived for the Otrøy peridotites and kimberlite xenoliths are qualitatively consistent with upwelling of hot asthenosphere to lithospheric depths (Kornprobst, 1969; Nicolas, 1986). This type of P–T path has also been obtained for asthenosphere diapirs in a numerical model for continental lithosphere formation (De Smet et al., 2000). We now describe the results from the modelling study of De Smet et al. (1998, 1999, 2000).
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NUMERICAL THERMOCHEMICAL CONVECTION MODELS |
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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De Smet et al. (1998
, 1999, 2000) studied the evolution of the continental lithosphere using a thermochemical convection model of the upper mantle below stable continental crust. The model considers only sub-continental cratonic mantle and does not include the effects of subduction. The lower mantle is treated as a heat reservoir with no material transfer from the lower mantle to upper mantle via plumes or mantle overturns. The key petrological features are (1) the effect of melting on peridotite density and (2) a simplified phase diagram with linear solidus and liquidus, which is a good approximation at depths <150 km but less realistic at deeper levels. Use of a linear solidus reduces the amount of deep melting and the extent of mantle differentiation (De Smet et al., 1999). Partial melting and melt extraction results in a density decrease of the residual peridotite (O’Hara, 1975); a relationship based on Jordan (1979) was used. The results show that a depleted lithosphere 150–200 km thick forms quickly, in 50–100 my, and remains stable for billions of years (De Smet et al., 1998), which is consistent with evidence from Archaean cratons (Richardson et al., 1984; Boyd et al., 1985; Menzies et al., 1987; Pearson, 1999). Subsequent lithosphere growth occurs by accretion of small diapirs, of 50–100 km diameter (Fig. 6), which originate from instabilities near the base of the cratonic root (De Smet et al., 1998). Material can rise from the transition zone in an upwelling limb of a larger-scale mantle convection cell, become included in a small-scale diapir and then rise into the lithospheric root. As the diapirs rise, melting occurs and continues until the diapirs stop at a depth of 60–100 km (Fig. 6) beneath the high-viscosity mechanical boundary layer (MBL). Diapirs form as relatively small-scale instabilities near the base of the lithosphere. They do not originate as instabilities from a hot boundary layer at the base of the upper mantle. Intrusion of diapirs results in local, small-scale convection in the lower part of the cratonic lithosphere. This small-scale circulation stretches and attenuates rock volumes with different depletion and produces a compositional banding in the lithosphere (De Smet et al., 2000).
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In a model run for a simulated period of 4 by, three episodes of craton growth occurred (De Smet et al., 2000). Lithosphere growth by episodes of asthenosphere diapirism occurred only in the first 3 by, when the horizontally averaged mantle temperature was >1550°C.
P–T–t paths for individual tracers in the model (De Smet et al., 2000) are compared in Fig. 7 with the P–T estimates for cratonic peridotites. The P–T paths from the numerical model are for tracer particles in diapirs from three different stages of craton growth. These tracers started in the deep upper mantle. The paths show adiabatic decompression during ascent to the base of the lithosphere and during intrusion of the diapir into the lithosphere. Approximately isobaric cooling occurs once the diapir stagnates below the high-viscosity MBL at depths of 60–100 km and cools to the ambient cratonic geotherm. Some parts of diapirs (or earlier hotter diapirs) can become involved in the small-scale circulation in the lower part of the thermochemical lithosphere and descend to the base of the lithosphere. One tracer shown in Fig. 7 rose to 75 km but ended up at depths of 150 km. De Smet et al. (2000) have shown that continued circulation in the root can produce a remarkable spiral P–T path with episodes of recurrent melting.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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The subsolidus P–T paths deduced for kimberlite xenoliths and some of the possible paths (paths a and b, Fig. 4) for the Otrøy peridotites are similar to P–T paths obtained from the numerical model (Fig. 7). Thus, the P–T paths of deep upper-mantle peridotites are consistent with transport to shallow depths by convection and asthenosphere diapirism in a hot upper mantle. As there are limited constraints on the timing and conditions of melting in the Otrøy peridotites the P–T paths are also consistent with diapiric upwelling of previously depleted mantle from the transition zone (e.g. Ringwood, 1989
) or lower mantle (Haggerty, 1994).
Very high potential temperatures for melting or mantle upwelling are often attributed to mantle plumes. It is important to distinguish between diapiric instabilities, which form close to the base of the lithosphere, and plumes, which originate as instabilities from thermal boundary layers deeper in the mantle. The high temperatures inferred for early mineral assemblages in cratonic peridotites (Figs 4 and 5) can be explained either by asthenosphere diapirism in a hot upper mantle or possibly by episodes of plume activity (Haggerty & Sautter, 1990; Herzberg, 1993; Haggerty, 1994). A hot upper mantle may occur beneath cratons in the Precambrian (De Smet et al., 1998) or may arise from mantle overturn events (Tackley et al., 1993; Griffin et al., 1998).
The model P–T paths have a substantial supersolidus path with melting over a large pressure range. P–T paths like this may be possible for the WGR peridotites but extensive deep melting may be inconsistent with the CaO–Yb correlation in the kimberlite xenoliths (Kelemen et al., 1998). As shown in Figs 4 and 5 cratonic peridotites may have P–T paths with limited melting during upwelling. If this is the case then earlier melting followed by tectonic transport to deeper levels is required before the upwelling event (e.g. Ringwood, 1989). The dynamics of upwelling diapirs derived from deep thermochemical boundary layers may be very different from that of diapirs forming near the base of the cratonic lithosphere.
The P–T path for hot deformed kimberlite xenoliths suggested by Canil (1991) is similar to model P–T paths where material becomes involved in small-scale convection in the cratonic root. Thus this type of P–T path is plausible and can be explained by small-scale convection. It is uncertain, however, if the hot deformed xenoliths have such a P–T history. The stage of equilibrium at high temperatures and shallow depths (3 GPa) is based on evidence that the garnet and pyroxene in these xenoliths were exsolved from a high-Al–Ca opx precursor (Cox et al., 1987). In some sheared xenoliths, however, 40–70% of the garnet was formed during later metasomatism (Griffin et al., 1989; Smith & Boyd, 1989). Boyd & Mertzman (1987) and Walter (1998) have shown that the compositions of hot sheared xenoliths are consistent with melting at relatively low to moderate pressures, suggesting that some form of tectonic transport to deeper levels is required.
In the numerical models small-scale convection in the lower part of the cratonic root is active during episodes of craton growth and diapir intrusion (Fig. 6). Stirring produced by convection in the craton root will modify the simple variation of depletion with depth expected from polybaric melting (Kelemen et al., 1998). The variation of peridotite depletion with depth derived from the numerical models shows an overall trend of decreasing depletion with depth (Fig. 6) but on a smaller scale the compositional variation is complex (De Smet et al., 1998, 2000). The compositional banding in the models is produced by stretching of diapir-scale (10–100 km diameter) heterogeneities produced by variations in melt extraction. Studies on stirring and mixing in fluid flows show that outcrop-scale compositional banding in cratonic peridotites (Fig. 1) can be formed by convective stirring of heterogeneities in the craton root (Allègre & Turcotte, 1986; Kellog & Turcotte, 1986).
Small-scale convection in the cratonic root can account for the transport of material to deeper levels in the cratonic root without invoking subduction. Within one episode of craton growth, cratonic peridotites can be involved in several stages of diapirism, upwelling, melting and recirculation in the dynamic cratonic root. Thus, melting and depletion of a peridotite body may occur in an earlier stage of evolution than the upwelling event that resulted in the final emplacement in the lithosphere. Small-scale convection can transport material to deeper levels but a subduction-like process is needed to transport near-surface material to the deep upper mantle and transition zone. Eclogite xenoliths in kimberlites may be derived from oceanic crust (e.g. Jagoutz et al., 1984; Jacob & Foley, 1999). In addition, some eclogite xenoliths bear evidence for residence at transition zone depth (Haggerty & Sautter, 1990). Other eclogites contain high-temperature exsolution microstructures (Harte & Gurney, 1975), suggesting high temperatures during lithosphere emplacement. The combined evidence from eclogites is consistent with a shallow origin and deep tectonic transport, followed by high-temperature upwelling and emplacement into cratonic lithosphere (MacDougall & Haggerty, 1999).
In the numerical models of De Smet et al. (1998, 1999, 2000) episodes of asthenosphere diapirism and emplacement of deep upper-mantle material occurred in the simulated Precambrian when the potential temperature is above 1550°C. Deep mantle rocks may also be emplaced in the lithosphere in the Phanerozoic by upwelling of previously depleted material from the transition zone (Ringwood, 1989; Ringwood et al., 1992) or from the core–mantle boundary (Haggerty, 1994). In a study on Nd and Sr isotopes, MacDougall & Haggerty (1999) found evidence that deep mantle rocks were emplaced into the cratonic lithosphere over time scales of many hundreds of million years during episodes of plume- or kimberlite-related magmatism in the Precambrian and Phanerozoic.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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- In terms of mineral and whole-rock chemistry the Otrøy orogenic peridotites in the Western Gneiss Region of Norway are similar to Precambrian cratonic xenoliths and Phanerozoic supra-subduction-zone peridotites.
- Mineral compositions and reconstructed early mineral compositions in the Otrøy peridotites and kimberlite xenoliths suggest a P–T path with high-temperature decompression from depths of >200 km to 100–150 km, followed by cooling to conditions on a cratonic geotherm.
- Outcrop-scale compositional banding in craton peridotites can be produced by small-scale convection associated with diapirism in the deeper, low-viscosity part of the thermochemically defined cratonic lithosphere.
- Complex P–T histories and multi-stage metamorphic–igneous histories can be produced during craton growth by asthenosphere diapirism in a hot upper mantle.
- P–T paths predicted from models of asthenosphere diapirism are consistent with the subsolidus P–T history deduced for the Otrøy orogenic peridotites and Kaapvaal kimberlite xenoliths.
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ACKNOWLEDGEMENTS |
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This work was supported by an NWO (Netherlands Organization for Scientific Research) PIONIER subsidy. Martin Menzies and two anonymous referees are thanked for comprehensive and constructive reviews.
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FOOTNOTES |
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*Corresponding author. Telephone: +31-30-2535108. Fax: +31-30-2537725. E-mail: martynd{at}geo.uu.nl
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TOPABSTRACTINTRODUCTIONUPPER-MANTLE PERIDOTITES WITH...NUMERICAL THERMOCHEMICAL...DISCUSSIONCONCLUSIONSREFERENCES |
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