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Journal of Petrology Volume 42 Number 1 Pages 173-188 2001
© Oxford University Press 2001
Depletion Events, Nature of Metasomatizing Agent and Timing of Enrichment Processes in Lithospheric Mantle Xenoliths from the Veneto Volcanic Province
ISTITUTO DI MINERALOGIA, UNIVERSITÀ DI FERRARA, CORSO ERCOLE I° d’ESTE 32, 44100 FERRARA, ITALY
Received December 17, 1999; Revised typescript accepted June 27, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANALYTICAL METHODSPETROGRAPHY AND BULK-ROCK...MINERAL AND GLASS CHEMISTRYDEPLETION PROCESSESENRICHMENT PROCESSESREFERENCES |
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Mantle xenoliths included in the alkaline basic lavas from the Paleogene Veneto Volcanic Province (VVP) consist of predominant spinel lherzolites (21–6% clinopyroxene) and minor spinel harzburgites (4–2% clinopyroxene), mainly protogranular textured. Most of the xenoliths show superimposed textural evidence of metasomatic processes, consisting of reaction patches and spongy clinopyroxenes, variably associated with glass and secondary olivine, clinopyroxene, spinel and feldspar. Whole-rock and mineral major and trace element data indicate a complex history of depletion and enrichment processes undergone by the continental lithospheric mantle beneath a within-plate region. Protogranular-textured clinopyroxenes from lherzolites show heavy rare earth element (HREE) contents
10 times chondrite and strong light REE (LREE) depletion [(La/Yb)N = 0·002–0·025)], whereas those in harzburgites vary from slightly LREE depleted to LREE enriched [(La/Yb)N = 0·64–4·00], with much lower HREE contents (3 times chondrite). Most HREE patterns can be reproduced by a simple fractional melting model and extraction of 5–22% basic melts, starting from the most fertile VVP spinel lherzolite. The anomalously low HREE contents of clinopyroxenes in a single lherzolite sample, however, require more complex processes possibly involving multistage melting of garnet- to spinel-bearing sources, or diffusion-controlled melt–peridotite interactions. The slightly LREE-depleted to LREE-enriched patterns of protogranular-textured clinopyroxenes in harzburgites and spongy clinopyroxenes in lherzolites [(La/Yb)N = 0·27–5·67] can be accounted for by metasomatic enrichment events. Major and trace element mass balance calculations were successfully carried out to quantitatively model the metasomatic parageneses by the addition of 1–6% of Na-alkaline basic melt/s. Magmatic analogues of the modelled metasomatizing agent/s occur in the South Alpine domain as late Cretaceous lamprophyric dykes, as well as VVP alkaline basic lavas. An estimate of timing of the enrichment processes was achieved by a REE inward diffusion model in primary and spongy clinopyroxenes, assuming LREE-enriched secondary clinopyroxenes as the boundary surface composition. Model calculations imply a complete chemical rehomogenization of a 1 mm crystal in a time span of 4·8–16 kyr. Consequently, the observed REE zoning of clinopyroxenes, together with the presence of glassy patches, indicates that the most recent metasomatic processes occurred shortly before the entrainment of mantle xenoliths by the host lavas. KEY WORDS: mantle xenoliths; metasomatic melts; trace elements; clinopyroxenes; glasses
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INTRODUCTION |
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Metasomatic processes recorded in mantle xenoliths (Menzies et al., 1987
; Menzies, 1990; Siena et al., 1991; Hauri et al., 1993; Ionov et al., 1996) have been the object of increasing interest for earth scientists in recent decades, as they represent one of the most important mechanisms in modifying mantle compositions and, by implication, the sources of basic magmas. In this context, the quantitative evaluation of the metasomatic reactions and identification of their causative agents (Ionov et al., 1994; Coltorti et al., 1999a) represent important tools for understanding the nature of fluids migrating through the mantle.
Mantle xenoliths occurring in the Paleogene lavas of the Veneto Volcanic Province (VVP) are characterized by variably depleted mantle peridotites subsequently enriched by metasomatic event/s, recorded by widespread interstitial recrystallized patches and glasses (Morten, 1987; Siena & Coltorti, 1989). They are therefore suitable for investigating the compositional evolution related to depletion and enrichment processes affecting the continental lithospheric mantle beneath a within-plate region. A large amount of peridotite xenoliths (200 samples) was collected northward of Verona, from the Adige Valley in the west up to the Marostica Hills in the east, the quarries of Passo Buole, San Giovanni Ilarione and Monte Madarosa representing three of the most important sampling areas (Fig. 1a). The host lavas consist of alkali basalts, basanites and nephelinites, with incompatible element patterns similar to those of within-plate sodic magmas, showing a prevalent HIMU geochemical component (87Sr/86Sr 0·70322–0·70344, 144Nd/143Nd 0·51290–0·51298 and 206Pb/204Pb 19·20–19·79; Beccaluva et al., 2000). Detailed descriptions of the occurrence of xenoliths and host lavas have been reported by Siena & Coltorti (1989).
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New major and trace element whole-rock and phase analyses of xenoliths from this area are reported in the present paper. These data are used to constrain: (1) mantle depletion events related to the extraction of basic magmas; (2) types and timing of the metasomatic reactions and compositions of the metasomatizing agents.
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ANALYTICAL METHODS |
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Whole-rock major and trace element compositions were determined in duplicate on powder pellets using a Philips PW 1400 XR fluorescence spectrometer (Istituto di Mineralogia, Università di Ferrara). Rare earth elements (REE) and Y were analysed using an inductively coupled plasma mass spectrometer (ICP-MS) VG Plasma Quad2 Plus (Istituto di Mineralogia, Università di Ferrara), with a precision and accuracy better than 10% for all the elements well above the detection limit. The international standard JP-1 (peridotite at 0·1 chondritic REE) was analysed in quadruplicate for REE with a precision within 20%, and an average error of 15% with respect to the ICP-MS analyses carried out by Makishima & Nakamura (1997)
on the same reference material; for the international standard UB-N (serpentine at 1 chondritic REE) an average error of 7% was obtained with respect to recommended values (Govindaraju, 1989). Mineral and glass major elements were analysed with a Cameca-Camebax electron microprobe (CNR–Centro di Studio per la Geodinamica Alpina, Padova), using natural silicates and oxides as standards.
In situ REE, Y, Zr and Ti were analysed in clinopyroxenes, whereas the same dataset plus Rb, Sr and Ba were determined for feldspars and glasses, on polished gold-coated thin sections, using a Cameca IMF-4f ion microprobe (CNR–Centro di Studio per la Cristallochimica e Cristallografia, Pavia, Italy). Measurement conditions, analytical procedures, accuracy and precision have been described by Bottazzi et al. (1994).
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PETROGRAPHY AND BULK-ROCK CHEMISTRY |
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Peridotite xenoliths showing the most widespread and interesting metasomatic reactions were selected from a larger collection based on large (2–10 cm) samples lacking alteration and basaltic veins inside. The contacts between host lavas and peridotites are always sharp, and no significant reaction zones are present. The studied samples consist of predominant spinel lherzolites and minor harzburgites, according to their calculated modal compositions (Table 1).
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Most samples display primary protogranular textures, characterized by large crystals of olivine (ol) and orthopyroxene (opx) (up to 2 mm across), irregularly shaped smaller clinopyroxenes (cpx) (0·5–1·0 mm in size), and spinels (sp) (up to 0·5 mm across) with typical holly-leaf or lobate shape.
Several types of secondary pyrometamorphic textures are superimposed on the primary textures in both lherzolites and harzburgites, irrespective of their modal composition (Table 1). They consist of (1) reaction areas involving primary orthopyroxenes, clinopyroxenes and spinels, which include a secondary assemblage made up of small crystals of olivine, clinopyroxene, vermicular spinel, minor feldspar and rare glass; (2) glassy patches, brown to pale yellow in colour, containing secondary crystals of olivine, clinopyroxene, spinel and feldspar; relics of primary phases are absent; (3) spongy clinopyroxene crystals, sometimes almost completely replaced by secondary assemblages of clinopyroxene and glass. Glass is fairly common in all pyrometamorphic textures, although with highly variable modal abundance, ranging from traces to several percent (Table 1). Among the secondary minerals the occurrence of feldspar is peculiar: it is rare and is only reported in few xenolith suites (Xu et al., 1996; Ionov et al., 1999).
As already stated by Siena & Coltorti (1989), a continuous depletion trend is observed in the VVP mantle xenoliths, chemically reflected in the gradual decrease of the most fusible elements such as Al2O3, CaO and TiO2 and the parallel increase of Ni and mg-number [Mg/(Mg + Fe) x 100], from the clinopyroxene-rich lherzolites (cpx = 20·9%) through lherzolites (cpx = 16·6–6·0%) to harzburgites (cpx = 4·3–1·9%). Accordingly, the heavy REE (HREE) distribution varies from three times chondrite in cpx-rich lherzolites to 0·2 times chondrite in harzburgites (Fig. 2). On the other hand, both lherzolites and harzburgites are variably enriched in light REE (LREE) [(La/Yb)N up to 12·6 and 19·2, respectively], suggesting post-depletion enrichments related to metasomatic processes.
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MINERAL AND GLASS CHEMISTRY |
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Major elements
In primary olivines (ol1), mg-number varies from 86·9 to 90·4 in lherzolites with FeO contents up to 12·7 wt %, whereas in harzburgites it ranges from 87·8 to 91·4 with FeO contents up to 11·6 wt %. On the other hand, secondary olivines (ol2) show a systematic increase in iron content (FeO up to 17·2 wt % in lherzolites and up to 19 wt % in harzburgites), coupled with significant calcium enrichment (0·15–0·38 wt % CaO), with respect to the primary crystals (0·0–0·17 wt % CaO). Orthopyroxenes occur only as primary phases; they have never been found as reaction products in metasomatic assemblages.
Clinopyroxenes show a wide compositional range in relation to their textural positions. The large equilibrated crystals (cpx1), related to protogranular textures, are characterized by mg-number in the range 86·6–94·4, TiO2 0·05–0·74 wt %, Na2O 0·52–2·02 wt % and Al2O3 3·19–8·09 wt %, with those in harzburgites showing the highest mg-number values and lowest TiO2, Na2O and Al2O3 contents (Table 2). With respect to cpx1, secondary clinopyroxenes replacing either orthopyroxenes (cpx2-O) or former clinopyroxenes (cpx2-C) are depleted in sodium and aluminium at comparable mg-number values, whereas cpx2-O show comparatively higher silica and lower TiO2 contents (Table 2). Spongy clinopyroxenes (cpxSpo) vary considerably from one grain to another and from core to rim, and encompass the entire compositional range of primary and secondary clinopyroxenes, with the exception of TiO2, which is lower than cpx2-C (Table 2).
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Spinels, like clinopyroxenes, show a remarkably wide compositional range. Primary spinels equilibrated in protogranular textures (sp1) are Al rich and Cr poor, with cr-number [Cr/(Cr + Al) x 100] ranging from 8·27 to 23·89 in lherzolite and from 25·89 to 38·25 in harzburgites. Secondary spinels (sp2) are remarkably richer in Cr2O3 and TiO2 with respect to sp1.
Feldspars consist of plagioclase (An14–54) and alkali feldspar (Or18–44), the first predominating in lherzolites. The most An-rich plagioclases and Or-poor alkali feldspars occur intergrown with secondary spinel at the rim of primary spinels, whereas the reverse is found in the glassy patches (Table 2). It is to be noted that plagioclases from the host lavas are distinctly richer in anorthite (An66–72) than those in the reaction zones.
Glasses are characterized by high SiO2 (55·90–65·43 wt %), Al2O3 (14·77–22·44 wt %) and alkali contents (Na2O 4·07–9·67 wt %; K2O 4·08–8·28 wt %), and low CaO (0·37–3·38 wt %), MgO (0·17–3·89 wt %), FeO (0·70–3·78 wt %) and TiO2 (0·54–2·72 wt %) (Table 2). Their silica-saturation degree varies from strongly undersaturated (normative ne up to 26·8%) to oversaturated (normative qz up to 22·1%) in relation to the occurrence and extent of orthopyroxene incongruent melting. Within each xenolith, glasses present a rather narrow major element compositional range, and are constantly characterized by a sodic tendency with Na2O/K2O ratios varying from 0·64 to 2·05 (Table 2).
Thermobarometric evaluations by Siena & Coltorti (1989) provide equilibration conditions for the primary parageneses of the VVP xenoliths of 1130 ± 60°C, in the pressure range of 10–13 kbar. New calculations using orthopyroxene–clinopyroxene (Brey & Köhler, 1990) and olivine–spinel (O’Neill & Wall, 1987) thermometers give a temperature range of 990–1110°C with a mean value of 1050°C.
Trace elements
Representative in situ (SIMS) trace element analyses of clinopyroxenes, feldspars and glasses are reported in Table 2. The large protogranular-textured clinopyroxenes (cpx1) significantly differ from lherzolites to harzburgites, and from core to rim in harzburgites (Table 2). Cpx1 in lherzolites show HREE contents about 10 times chondrite and strong LREE depletion [(La/Yb)N = 0·002–0·025)], whereas cpx1 in harzburgites, which present much lower HREE contents (3 times chondrite), vary from slightly LREE depleted to LREE enriched [(La/Yb)N = 0·64–4·00]. In the chondrite-normalized incompatible element diagram (Fig. 3a), cpx1 display Ti and Zr negative anomalies, with Ti* {Ti/[(Eu + Gd)/2]} varying from 0·48 to 0·52 in lherzolites and from 0·23 to 0·34 in harzburgites, and Zr* {Zr/[(Nd + Sm)/2]} ranging between 0·24 and 0·48 in both lithologies. Spongy clinopyroxenes (cpxSpo), characterized by a wide compositional range, always present marked zoning from core to rim. In some cases the inner portions of the crystals, which do not appear to be involved in any reactions, show LREE-depleted patterns [(La/Yb)N = 0·01–0·08)] comparable with those of cpx1. In other cases the whole crystals show general LREE-enriched patterns [(La/Yb)N = 0·27–5·67], with the greatest enrichments in the marginal zones. Ti and Zr negative anomalies are generally comparable with those of cpx1 (Ti* 0·27–0·48 and Zr* 0·20–0·45) (Fig. 3b). Both secondary clinopyroxenes (cpx2-C and cpx2-O) are LREE enriched [(La/Yb)N = 3·36–5·38], and can be distinguished from cpx1 on the basis of the bulk enrichment in REE (cpx2-C: 
REE = 65·2–101·1; cpx2-O: REE = 43·7–62·0) (Fig. 3c).
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Figure 4 reports the chondrite-normalized incompatible element distributions of glasses and associated alkali feldspars. Within individual samples, glass compositions from different patches show moderate variations in incompatible element contents (Table 2). Glasses depict remarkable Ba, Sr and Eu negative anomalies and high Ti/Eu ratios (6800–12 500), whereas Ti and Zr negative anomalies are missing. The latter features are distinctive of mantle glasses, which are reaction products of alkali silicate metasomatic agents, in contrast to those related to carbonatite metasomatism (Coltorti et al., 1999a
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The antithetic Ba, Sr and Eu positive anomalies in alkali feldspars associated with glasses strongly suggest equilibrium relationships between the two phases. This can be quantitatively tested using the approach of Blundy & Wood (1991) for calculating the DSr and DBa between feldspar and silicate melts, and assuming DLa from the literature (Upton et al., 1999). Accordingly, the model calculations show that Ba, Sr and La contents of melt in equilibrium with the MA7 feldspar are remarkably close to those of the real MA7 glasses (inset of Fig. 4).
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DEPLETION PROCESSES |
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The depletion of fusible elements, such as Al, Ti, Y and HREE in bulk rocks and associated cpx1, together with the increase of both mg-number and Ni in ol1 and Cr/Al ratio in sp1 from lherzolites to harzburgites, are all characteristics of variably depleted mantle material.
The clinopyroxene trace element patterns can be used to place quantitative constraints on the relative importance of the partial melting processes undergone by mantle material; this approach has been successfully applied for oceanic and continental peridotites by several workers (e.g. Johnson et al., 1990; Niu, 1997; Norman, 1998; Xu et al., 2000). To constrain the amount of melt extraction in VVP mantle xenoliths, the HREE distribution of cpx1 and the inner portion of cpxSpo (assuming that diopside is the major HREE-bearing phase; Norman, 1998) was modelled using both the batch and fractional melting equations proposed by Johnson et al. (1990). LREE were disregarded, as their distribution was clearly controlled by metasomatic enrichment in most of the analysed cpx1 and cpxSpo (Fig. 3).
The best fit for the observed HREE patterns of residual clinopyroxenes was obtained using the fractional melting model, starting from an inferred 12 times chondrite primitive clinopyroxene of the undepleted and unmetasomatized spinel lherzolite SG3 (20·9% modal clinopyroxene: Table 1, Fig. 2). The results of this modelling show that cpx1 in samples SG34 and MA7, and the inner parts of cpxSpo in SG2 and SG7 underwent a depletion of 5% partial melting of the inferred primitive source (Fig. 5). It should be noted that, for such a low degree of melting, similar results are obtained for both fractional and batch melting (see also Johnson et al., 1990; Norman, 1998; Xu et al., 2000), whereas the batch melting model would require an unrealistically large degree of melting (>35%) to produce the extremely depleted HREE pattern of the residual cpx1 in MA3 harzburgite (YbN = 2·8 times chondrite). A more realistic degree of melting (22%) is obtained using the fractional melting equation (Fig. 5).
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A particular case is represented by the large spongy clinopyroxenes from lherzolite SG12, which show HREE patterns (YbN = 2·5 times chondrite) remarkably similar to those of harzburgite MA3. In fact, 22% fractional melting produces the observed low HREE patterns of clinopyroxene, but this would require its consumption up to 4%, in contrast to the 15·5% modal clinopyroxene of lherzolite SG12 (Table 1). The relatively undepleted nature of the bulk rock and the depleted composition of its large clinopyroxenes can be explained considering two different models. The first involves a multistage partial melting process whereby an initial 8% fractional melting of garnet lherzolite source is followed by 11% melting in the spinel lherzolite facies, using the partition coefficients and the equation of Takazawa et al. (1996). This results in 13% modal clinopyroxene in the calculated residuum, which reasonably approaches that observed in lherzolite SG12 (Table 1). The second model considers melting processes of a mantle column that undergoes continuous infiltration of a deep-seated melt at the base of the column (Vernières et al., 1997); thus the observed clinopyroxene REE pattern and its modal abundance depends on the composition of the infiltrated melts, the relative reactions with the peridotite matrix, as well as its permeability (Bodinier et al., 1990). If high porosity is assumed for the mantle portion represented by sample SG12, application of this model may result in significant crystallization of clinopyroxene with a slightly enriched LREE pattern, converting an original harzburgite to a lherzolite (Godard et al., 1995; Bedini et al., 1997; Xu et al.,1998).
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ENRICHMENT PROCESSES |
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The complex textural and chemical relationships described in the previous sections can be considered as the result of a multistage compositional evolution of the mantle portion represented by the VVP peridotite xenoliths. The last event recorded is represented by metasomatic reactions that increased the incompatible element contents of the mantle rocks and their constituent minerals. Several models have been proposed to account for these metasomatic processes, which consider either the chromatographic fractionation of metasomatic components infiltrating through veins and/or a porous peridotite system (Navon & Stolper, 1987
; Bodinier et al.,1990; Wulff-Pedersen et al., 1999) or else entrapment of metasomatizing melts reacting with primary peridotite minerals (O’Reilly et al., 1991; Neumann & Wulff-Pedersen, 1997; Ionov, 1998; Coltorti et al., 1999b). In the following sections, we attempt to constrain the metasomatic processes observed in VVP mantle xenoliths by (1) quantitative modelling of the metasomatic reactions occurring between the primary peridotite assemblage and the inferred metasomatizing agent/s; (2) application of a diffusion model in clinopyroxenes.
Modelling metasomatic reactions
It is now widely accepted that the most effective metasomatizing agents are represented by volatile-rich silicate or carbonatite melts (Schneider & Eggler, 1986; O’Reilly & Griffin, 1988; Yaxley et al., 1991; Wulff-Pedersen et al., 1996; Coltorti et al., 1999a), as a result of their inherent physical capability to permeate a peridotite matrix (Mysen et al., 1982; Watson et al., 1990).
To define the nature of these melts, experimental and/or theoretical solid–liquid partition coefficients have been used by several workers (i.e. Hauri et al., 1993; Vannucci et al., 1998; Mattielli et al., 1999). However, this approach is strongly limited by the fact that the coefficients vary over several orders of magnitude depending on the compositions, intensive properties (mainly temperature), and equilibrium conditions of the system (Wood & Blundy, 1997; Hirschmann et al., 1998).
A different approach has been followed by Coltorti et al. (1999b) for the Grande Comore metasomatized mantle xenoliths, where major and trace element mass balance calculations have proved to be successful in reconstructing a carbonatite metasomatizing agent. Accordingly, the secondary assemblage is considered as reaction products of the primary paragenesis with an inferred metasomatizing agent. For the VVP mantle xenoliths the appropriate equation to describe the metasomatic reactions represented in the pyrometamorphic textures may be as follows:
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Several melt compositions represented in the Phanerozoic magmatic events of the area, including the VVP Na-alkaline lavas, were tested (Table 3), and the best fit was obtained considering as metasomatizing agent the Late Cretaceous Na-alkaline lamprophyres from the Southern Alps (Galassi et al., 1994; Table 3). The most representative metasomatic reactions balanced on the major elements are summarized below:
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It is noteworthy that, despite the differences in the modal parageneses of the three samples, a common melt composition is required in calculations, which implies that no significant change in the metasomatic agent occurred during short-time percolation in the peridotite matrix.
Model results also imply that a considerable percentage of ol1 is consumed by the metasomatic reactions, although we do not generally observe spongy rims or destabilized olivine crystals. However, ol2 is often observed in optical continuity, and is hence considered to replace large primary crystals. On the whole, textural features of the VVP mantle xenoliths are remarkably coherent with the calculated model, as orthopyroxene, clinopyroxene and (to a lesser extent) spinel clearly show evidence of reactions, whereas secondary clinopyroxenes grew mainly at the expense of primary pyroxenes, with more pervasive recrystallization in harzburgites as compared with lherzolites.
On the basis of the above reaction coefficients, the calculated incompatible element compositional field of the hypothetical metasomatic melts is in good agreement with that of the alkaline lamprophyres previously considered in the major element mass balance reactions (Table 3; see Fig. 7, below). However, it should be noted that also the VVP Na-alkaline lavas (alkali basalt, basanite and nephelinite) reveal incompatible element patterns that remarkably overlap the calculated metasomatic melts (see Fig. 7); consequently, similar magmas belonging to the VVP magmatic system cannot be ruled out as a causative agent of the deep-seated mantle metasomatism. The above considerations constrain the nature of the metasomatizing agents to a rather restricted compositional range of small-volume alkaline basic melts with sodic affinity.
Diffusion model of clinopyroxene
The incompatible element zoning recorded in mantle minerals makes it possible to quantitatively model the chemical gradients of melt–peridotite interactions, and to put constraints on the timing and elemental mobility in metasomatic processes. The diffusion-controlled model of Bodinier et al. (1990) has been successfully applied by Witt-Eickschen et al. (1998) for evaluating the vein metasomatism of composite mantle xenoliths from the West Eifel. However, this approach cannot be directly applied for the VVP mantle xenoliths, as they are not composite (veined), and no significant compositional change of the metasomatic melt has been detected in the peridotite domains where the metasomatic reactions were balanced. The REE zoning of the investigated clinopyroxenes, particularly cpxSpo, characterized by LREE-depleted cores and LREE-enriched rims (Fig. 4), is consequently modelled only in terms of a solid–solid diffusion mechanism (Crank, 1975).
The simplified ‘inward diffusion’ model of Griffin et al. (1996) was therefore adopted. According to this model, zoning is formed by a mass transfer from the boundary surface with composition C1 (at constant concentration during the diffusion process) to the core with composition C0. In our modelling, C1 is represented by the most enriched compositions of cpx2-O and cpx2-C, considered in equilibrium with the metasomatic Na-alkaline basic melt, whereas C0 represents the depleted compositions of cpx1 and cpxSpo cores. The rather narrow major element compositional range of cpx1 and cpxSpo cores (Table 2), together with the restricted temperature interval estimated for both lherzolites and harzburgites, strongly suggests that the initial trace element distribution within a single crystal was homogeneous, as required by the model.
Two representative examples of ‘inward diffusion’ in clinopyroxenes are graphically modelled in Fig. 6, assuming C1 as the cpx2 compositions of lherzolites SG12 and MA7, and C0 as the LREE-depleted cpx patterns previously calculated by the partial melting model. Calculations indicate that the LREE-enriched pattern of the cpxSpo rim of lherzolite SG2 (Fig. 6a) can be reproduced in 800 yr, and a further 1000 yr would be necessary to increase the La content to reach that observed in the inner crystal portion, at a linear distance of 230 µm from the boundary surface. On the other hand, the reasonable homogeneity of REE patterns, from core to rim, of cpx1 crystals in harzburgite MA3 (Fig. 6b) can be attained in at least 400 yr (at a linear distance of 90 µm from the boundary surface). On the whole, model calculations imply that a crystal 1 mm in size will achieve complete chemical rehomogenization in a time span of 4·8–16 kyr.
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As a consequence, the observed REE zoning in cpx1 and cpxSpo restricts the occurrence of the metasomatic processes to a short time before the xenolith being entrained by the host lava, as already stated for composite mantle xenoliths (see Witt-Eickschen et al., 1998; Wulff-Pedersen et al., 1999). This is also in agreement with a recent suggestion by several workers, who consider the presence of glassy patches in mantle xenoliths as an indicator of metasomatic reactions occurring shortly before transport of the xenoliths to the surface (Jin et al., 1994; Draper & Green, 1997; Coltorti et al. 1999b; Wulff-Pedersen et al., 1999).
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ACKNOWLEDGEMENTS |
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The authors are grateful to R. Tassinari for carrying out the ICP-MS analyses, to A. Zanetti for performing ion-probe analyses, and R. Carampin for assistance with electron-microprobe analyses. The CNR–Centro di Studio per la Geodinamica Alpina (Padova) and the CNR–Centro di Studio per la Cristallochimica e Cristallografia (Pavia) are also acknowledged for the use of electron microprobe and ion-probe facilities, respectively. Thanks are also due to G. Yaxley and an anonymous reviewer for their constructive criticism. This work was funded by Italian CNR and MURST.
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FOOTNOTES |
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*Corresponding author: Telephone: +39-0532-293740. Fax: +39-0532-293760. E-mail: bdc{at}dns.unife.it
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REFERENCES |
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