Formation of pyroxenite layers in the Totalp ultramafic massif (Swiss Alps) – Insights from highly siderophile elements and Os isotopes

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Abstract

Pyroxenitic layers are a minor constituent of ultramafic mantle massifs, but are considered important for basalt generation and mantle refertilization. Mafic spinel websterite and garnet–spinel clinopyroxenite layers within Jurassic ocean floor peridotites from the Totalp ultramafic massif (eastern Swiss Alps) were analyzed for their highly siderophile element (HSE) and Os isotope composition.

Aluminum-poor pyroxenites (websterites) display chondritic to suprachondritic initial γOs (160 Ma) of −2 to +27. Osmium, Ir and Ru abundances are depleted in websterites relative to the associated peridotites and to mantle lherzolites worldwide, but relative abundances (Os/Ir, Ru/Ir) are similar. Conversely, Pt/Ir, Pd/Ir and Re/Ir are elevated.

Aluminum-rich pyroxenites (clinopyroxenites) are characterized by highly radiogenic 187Os/188Os with initial γOs (160 Ma) between +20 and +1700. Their HSE composition is similar to that of basalts, as they are more depleted in Os, Ir and Ru compared to Totalp websterites, along with even higher Pt/Ir, Pd/Ir and Re/Ir. The data are most consistent with multiple episodes of reaction of mafic pyroxenite precursor melts with surrounding peridotites, with the highest degree of interaction recorded in the websterites, which typically occur in direct contact to peridotites. Clinopyroxenites, in contrast, represent melt-dominated systems, which retained the precursor melt characteristics to a large extent. The melts may have been derived from a sublithospheric mantle source with high Pd/Ir, Pt/Ir and Re/Os, coupled with highly radiogenic 187Os/188Os compositions. Modeling indicates that partial melting of subducted, old oceanic crust in the asthenosphere could be a possible source for such melts.

Pentlandite and godlevskite are identified in both types of pyroxenites as the predominant sulfide minerals and HSE carriers. Heterogeneous HSE abundances within these sulfide grains likely reflect subsolidus processes. In contrast, large grain-to-grain variations, and correlated variations of HSE ratios, indicate chemical disequilibrium under high-temperature conditions. This likely reflects multiple events of melt–rock interaction and sulfide precipitation. Notably, sulfides from the same thick section for the pyroxenites may display both residual-peridotite and melt-like HSE signatures. Because Totalp pyroxenites are enriched in Pt and Re, and depleted in Os, they will develop excess radiogenic 187Os and 186Os, compared to ambient mantle. These enrichments, however, do not possess the requisite Pt–Re–Os composition to account for the coupled suprachondritic 186Os–187Os signatures observed in some Hawaiian picrites, Gorgona komatiites, or the Siberian plume.

Introduction

Clinopyroxenites and websterites occur as layers or dykes in peridotite massifs, mantle tectonites associated with ophiolites, and mantle xenoliths. They comprise about 1–5% of peridotite massifs (e.g. Kornprobst, 1969, Pearson and Nowell, 2004, Downes, 2007), but are rarely found in association with abyssal peridotites (Dantas et al., 2007). Past studies have suggested that pyroxenite layers in the mantle may play an important role during the genesis of basaltic magmas at mid-ocean ridges (Hirschmann and Stolper, 1996) and intra-plate settings (Lassiter et al., 2000, Hirschmann et al., 2003, Sobolev et al., 2005). In order to estimate the influence pyroxenites may have on basalt genesis and upper mantle refertilization, the formation of pyroxenite layers must be better understood.

A number of processes might lead to the formation of pyroxenites in the mantle, including: (a) formation as tectonically emplaced slices of subducted eclogitic crust, or residues of in situ partial melting of such eclogites (Polvé and Allègre, 1980, Loubet and Allègre, 1982, Allègre and Turcotte, 1986, Blichert-Toft et al., 1999, Morishita et al., 2003, Obata et al., 2006), (b) crystal accumulation at high pressures from asthenosphere-derived magmas passing through the lithosphere (Obata, 1980, Irving, 1980, Sinigoi et al., 1983, Bodinier et al., 1987, Bodinier et al., 1990, Takahashi, 1992, Vaselli et al., 1995, Becker, 1996, Kumar et al., 1996, Garrido and Bodinier, 1999), whereby the magmas may be derived from partial melting of subducted crust (Davies et al., 1993, Pearson et al., 1993), (c) in situ metamorphic segregation of pyroxene from the host peridotite (Dick and Sinton, 1979) or in situ crystallization of partial melts from peridotite wall rock (Sinigoi et al., 1983, Voshage et al., 1988), (d) melt–rock reaction between existing pyroxenite, host peridotite and percolating melt (Garrido and Bodinier, 1999), or reaction of melt derived from subducted eclogitic oceanic crust with peridotite in the asthenosphere (Yaxley and Green, 1998).

Pyroxenites and associated peridotites often show evidence for a depleted mantle origin. Depletion of light rare earth elements (REE), compared to heavy REE, is commonly seen in both pyroxenites and associated peridotites (Bodinier et al., 1987, Bodinier, 1988, Garrido and Bodinier, 1999, Bodinier and Godard, 2003). Initial ratios of 87Sr/86Sr and 143Nd/144Nd show considerable variation, and overlap with data for mantle peridotites, forming a cluster in the depleted mantle field (e.g. Voshage et al., 1988, Downes et al., 1991, Mukasa et al., 1991, Pearson et al., 1993, Downes, 2007). Because of these broad similarities, lithophile incompatible trace elements and isotope systems may provide only limited information about pyroxenite formation processes. Highly siderophile elements (HSE; including Os, Ir, Ru, Pt, Pd and Re) and the long-lived Re–Os isotope system, however, provide a different perspective on pyroxenite formation. Under upper mantle conditions, these elements mostly show chalcophile behavior and a wide range of compatibility during mantle melting. While Os, Ir and Ru are considered compatible during partial mantle melting in the presence of sulfides, Pt and Pd can behave compatibly as well as incompatibly, whereas Re is moderately incompatible (Morgan and Lovering, 1967, Morgan et al., 1981, Roy-Barman and Allègre, 1994). Peridotites as mantle residues often show depletion in Pt, Pd and Re (e.g. Pearson et al., 2004, Luguet et al., 2007), whereas mantle melts, represented by basalts, are enriched in those elements compared to the more compatible Os, Ir and Ru (Rehkämper et al., 1999a, Bézos et al., 2005, Dale et al., 2008).

Because of the large difference in partitioning behavior between Os and Re, partial melts with high Re/Os develop radiogenic 187Os/188Os over time, while mantle residues often display unradiogenic 187Os/188Os. The large differences in Os isotopic composition and HSE signatures between melts and residues allow the use of HSE for the study of pyroxenite formation and melt–rock interaction in the Earth’s mantle.

Field and petrographic observations from the Jurassic Totalp massif, eastern Switzerland, combined with major element, Sm–Nd and Re–Os isotopic data for the host peridotites provide evidence for refertilization of the host spinel lherzolites by melts related to the pyroxenite layers (Peters and Stettler, 1987, Müntener et al., 2004, van Acken et al., 2008). Refertilization of the massif may have occurred in the spinel lherzolite–spinel–garnet pyroxenite facies of the former oceanic lithosphere beneath the Tethys ocean basin in a regime, transitional between lithosphere and asthenosphere. In the present study, abundances of Os, Ir, Ru, Pt, Pd and Re and Os isotope compositions from websterite and clinopyroxenite samples of the Totalp massif were obtained, to constrain melt–rock interaction, pyroxenite formation and the origin of the infiltrating mafic melts.

Section snippets

Geology and petrology

The Totalp ultramafic massif in eastern Switzerland consists of serpentinized spinel lherzolites associated with layered and folded spinel and spinel–garnet clinopyroxenites, spinel websterites, and more rarely spinel orthopyroxenites. The massif forms part of the Arosa imbricate zone, which separates the Penninic and Austroalpine units (e.g. Schmid et al., 2004). It was emplaced on the ocean floor of the Jurassic Piedmont-Liguria branch of the Tethys Ocean. The Piedmont-Liguria ocean basin

Methods

Sixteen websterites, spinel- and spinel–garnet pyroxenites from pyroxenitic layers from the Totalp massif were studied for major element compositions, HSE concentrations (Re, Os, Ir, Ru, Pt and Pd) and Os isotopic composition. Several samples were collected in association with adjacent peridotites or comprise profiles across modally layered pyroxenites. Layer thickness varies from few mm up to ca. 0.5 m. Detailed descriptions of Totalp pyroxenite samples from this study are given in the

Whole rock compositions

Volatile-free calculated major element and HSE abundances are listed in Table 1, with original major element data included in the Electronic annex. Major element, Os and Re abundances as well as Os isotopic compositions of samples TA11A2, TA13B, TA13D and TA61 have previously been reported by van Acken et al. (2008). In the subsequent discussion, volatile-free calculated major element data are used.

Major element concentrations for Totalp samples are within the range reported for websterites and

Modeling Totalp pyroxenite formation with HSE

Pyroxenites have been interpreted to represent residues of melting of eclogitic layers or to represent precipitates from melts related or unrelated to surrounding peridotites. Like many other pyroxenites and associated peridotites the Totalp samples display broadly linear major element covariations. These variations do not provide much information that might help in distinguishing the aforementioned formation models. Formation of the pyroxenites by in situ crystallization of partial melts of

Conclusions

Modeling results suggest that pyroxenites of the Totalp ultramafic massif formed as cumulates from melts reacting to various extents with mantle peridotites in the spinel lherzolite facies, as suggested for other pyroxenite suites (Obata, 1980, Sinigoi et al., 1983, Bodinier et al., 1987, Bodinier et al., 1990, Takahashi, 1992, Davies et al., 1993, Pearson et al., 1993, Vaselli et al., 1995, Becker, 1996, Kumar et al., 1996, Garrido and Bodinier, 1999). Cumulate precipitation of sulfides

Acknowledgments

We thank C. Behr, M. Feth, H. Frohna-Binder, A. Gottsche, K. Hammerschmidt, W. Michaelis, R. Milke, R. Naumann, B. Pracejus and I. Puchtel for technical assistance. Sulfur analyses at the University of Leicester were provided by the late Dr. T.S. Brewer. The electron microprobe analyses were performed in the NispLab and we acknowledge the support of the Maryland NanoCenter and its NispLab. Discussions with J.-P. Lorand, S.J. Barnes, V. Le Roux, A.J.V. Riches and O. Müntener are gratefully

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