A composite, isotopically-depleted peridotite and enriched pyroxenite source for Madeira magmas: Insights from olivine
Introduction
Identification of source components and determination of their relative contribution to the origin of mafic, mantle-derived magmas represent one of the main petrologic problems in understanding the evolution of the Earth. The existing systematic difference in radiogenic isotope composition between mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) is traditionally explained by the presence of isotopically different mantle components. These include high time-integrated U/Pb mantle (HIMU), enriched mantle one (EM1) and two (EM2) (Hart, 1988, White, 1985, Zindler and Hart, 1986, Zindler et al., 1982), whose origin is ascribed to the recycling of oceanic crust, marine sediments and continental lithosphere through the deep mantle back to the surface (Allègre and Turcotte, 1986, Hofmann and White, 1982, White and Hofmann, 1982), in addition to the depleted N-MORB source (DMM) and ubiquitous (FOZO or C) mantle components (Galer and O'Nions, 1985, Hanan and Graham, 1996, Hart et al., 1992). An open question concerns the lithologies (e.g., pyroxenite, eclogite and/or peridotite) that host these endmembers in the Earth's mantle.
A number of studies (e.g., Dasgupta et al., 2007, Hirschmann and Stolper, 1996, Ito and Mahoney, 2005a, Ito and Mahoney, 2005b, Kawabata et al., 2011, Mallik and Dasgupta, 2012, Phipps Morgan and Morgan, 1999, Sims and DePaolo, 1997, Sobolev et al., 2005, Sobolev et al., 2007, Sobolev et al., 2011, Stracke and Bourdon, 2009, among others) have investigated the effects of two- (and more) component melting of the ascending mantle, dealing with mineralogically, chemically and isotopically distinct lithologies. Recently, Sobolev et al., 2005, Sobolev et al., 2007 proposed a method for the quantitative assessment of the presumed recycled material in the source of intraplate magmas. The basic assumption of this approach is that quartz-normative melts, which are generated by partial melting of eclogite (the high-P and -T manifestation of subducted ocean crust in the mantle), react with ambient mantle peridotite and convert it to olivine-free “reaction pyroxenite”. If the newly formed reaction pyroxenite subsequently melts along with ambient peridotite due to further upwelling and adiabatic decompression, the pyroxenite-derived melts should be enriched in Ni and depleted in Mn, as compared to melts generated by peridotite alone (because Ni is less and Mn is more compatible in pyroxenite than in peridotite). Mixing and pooling of both (i.e., peridotite- and pyroxenite-derived) melts lead to a “hybrid” parental magma, from which olivine will eventually crystallize at shallower depth. As demonstrated by Sobolev et al., 2005, Sobolev et al., 2007, the Ni and Mn contents of this olivine can be used to determine the original proportions of the contributing parental lithologies and, thus, to infer the amount of recycled material in the magma source.
Most of the lavas analyzed by Sobolev et al., 2005, Sobolev et al., 2007 were tholeiitic or transitional basalts from large oceanic islands and continental lava flows. Therefore, the proposed model best explains the origin of silica-saturated mantle-derived magmas. The question as to whether a similar mechanism can produce silica-undersaturated alkali basaltic lavas, which are common in the late shield and rejuvenated volcanic stages of almost all oceanic island volcanoes (e.g., Hawaii, Azores, Madeira, Cape Verde and Canary Islands) was shortly reviewed by Sobolev et al. (2007) and then in the later studies by Pilet et al. (2008) and Aulinas et al. (2010) among others. In particular, Sobolev et al. (2007) proposed several possible scenarios for generating silica-undersaturated melts within the framework of the “reaction pyroxenite” model. These are: (1) a CO2-trigerred melting of peridotite at temperatures below the solidus of hybrid pyroxenite melts (e.g., Gudfinnsson and Presnall, 2005); (2) melting of hybrid pyroxenite in contact with peridotite at temperatures lower than the solidus of pyroxenite alone (e.g., Herzberg, 2006); (3) melting of silica-deficient mafic garnet pyroxenite or bimineralic eclogite (no free silica phase is present) possibly formed from silica-undersaturated recycled crust (e.g., Hirschmann et al., 2003, Kogiso and Hirschmann, 2006), and (4) possible retention of low-degree, highly viscous, silica-rich eclogite-derived melts not in contact with peridotite, preventing the formation of reaction pyroxenite (Sobolev et al., 2005). Alternatively, Pilet et al. (2008) suggested a model where the recycled components in the sources of ocean island alkali basalts can be derived from veined/metasomatized oceanic or continental lithosphere, which is in fact very similar to the model previously proposed for Canary Islands by Lundstrom et al. (2003).
Since the behavior of Ni and Mn is source-indicative in the “hybrid” pyroxenite model and the Earth's peridotitic mantle and subducted/recycled crustal material have substantially different radiogenic isotope signatures (e.g., Hart, 1988, Zindler and Hart, 1986), coherent relationships are expected between the Ni and Mn concentrations of olivine phenocrysts and the isotopic composition of their host lavas, if the “reaction pyroxenite” model is correct. Indeed, significant correlations between Ni, Mn (and also Ca) contents of olivine phenocrysts and Sr, Nd, Pb and Os isotope ratios in their host lavas have been described for Iceland, the Hawaiian and the Canary Islands, allowing the chemical (isotopic) compositions of their recycled components to be constrained (Gurenko et al., 2009, Gurenko et al., 2010, Sobolev et al., 2008).
Here, we report a new dataset for the elemental composition of olivine phenocrysts and demonstrate their links to the Sr–Nd–Pb isotopic compositions of the host lavas from the Madeira Archipelago. Madeira is well-suited to further test the relationship between olivine composition and bulk radiogenic isotope ratios, because the geochemistry of its lavas is interpreted to reflect progressive melting of a heterogeneous mantle plume source containing both fertile (recycled pyroxenite/eclogite) and depleted (peridotite) lithologies derived from recycled oceanic lithosphere (Geldmacher and Hoernle, 2000, Geldmacher et al., 2006), but without a significant contribution of DMM from the upper mantle. Furthermore, the Madeira Archipelago is located only ~ 500 km north of the Canary Islands, also a hotspot system, but the lavas from both archipelagos have distinct Sr–Nd–Pb, Os and Hf isotope signatures (e.g., Geldmacher et al., 2001, Geldmacher et al., 2005, Geldmacher et al., 2011, Hoernle and Tilton, 1991, Hoernle et al., 1991, Hoernle et al., 1995, Widom et al., 1999). The purpose of this study was to investigate (1) if similar coherent relationships between olivine composition and radiogenic isotopes are present in Madeira lavas, as were found in the neighboring Canary Island lavas (Gurenko et al., 2009, Gurenko et al., 2010); (2) if independent modeling of two-component source melting using trace elements in the host lavas confirms the results obtained from the olivine studies; and (3) if the composition of recycled components in the Madeira and Canary magma sources matches that proposed by Geldmacher and Hoernle (2000). Finally, this work aims to place additional constraints on the origin of the isotopically depleted peridotitic component in the Madeira plume and to estimate the possible amount of recycled crust in the magma source.
Section snippets
Geological setting
The Madeira Archipelago in the eastern North Atlantic is located on 140 Ma old oceanic crust at the end of a slightly curved NE–SW oriented age-progressive chain of islands and seamounts interpreted as a > 70 Ma-old hotspot track (Fig. 1). It consists of five islands: (1) the island of Madeira (with the age of volcanic rocks ranging from 0 to > 5.2 Ma), (2) Desertas Islands (Ilhéu Chão, Deserta Grande and Bugio; 1.9–5.1 Ma), which are situated on a submarine ridge extending more than 60 km SSE from
Mineral chemistry
Olivine phenocrysts were analyzed for major (Si, Fe, Mg) and minor (Ti, Al, Cr, Mn, Ca, Ni, Co) elements, using the JEOL Superprobe JXA-8200 electron microprobe (Max Planck Institute for Chemistry, Mainz, Germany) and the technique described by Sobolev et al. (2007) and Gurenko et al. (2009). Forsterite (Fo) contents, Ni, Mn and Ca concentrations and Ni × FeO/MgO and Mn/FeO ratios of olivine (Ol) phenocrysts together with bulk-rock radiogenic isotope data are listed in Table 1, Table 2. The
Amount of the recycled crustal component in the source of Madeira magmas
The original weight fractions of reaction pyroxenite, peridotite and eclogite restite in the mantle source, and thus the amount of recycled material in the mantle source can be computed using mass balance, if the following parameters are defined (Sobolev et al., 2005):
- 1.
The proportion of pyroxenite-derived melt in the parental magma (Xpxm), which can be directly acquired from Ni × FeO/MgO and Mn/FeO ratios in olivine using the parameterization of Sobolev et al. (2008) (Table 2).
- 2.
The respective
Summary and conclusions
As previously shown for Iceland (Sobolev et al., 2008), the Canary Islands (Gurenko et al., 2009, Gurenko et al., 2010) and presently demonstrated for Madeira, the concentrations of Ni, Mn and Ca in early-crystallized olivine phenocrysts represent powerful tracers in unraveling deep mantle processes. We observe strong linear relationships of Ni and Mn concentrations normalized to FeO and MgO contents and absolute Ca concentrations in olivine with the Sr–Nd–Pb radiogenic isotope ratios of their
Acknowledgments
We thank the Museum of Natural History (Washington, DC, USA) for providing us with the standards for electron microprobe analysis. We thank Meritxell Aulinas and one anonymous referee for their thorough reviews and constructive comments that helped us to improve the manuscript. Editorial handling by Andrew Kerr is gratefully acknowledged. We acknowledge financial support from the Max Planck Society, the DFG grants HO1833/5 and HO1833/8 to KAH and the Chair of Excellence grant ANR-09-CEXC-003-01
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