Pressure-dependent compatibility of iron in garnet: Insights into the origin of ferropicritic melt
Introduction
Ferropicrites, a type of picritic rock with unusually high iron content (typically Fe2O3 >14 wt%), were first discovered in the Paleoproterozoic Pechenga Group, NW Russia (Hanski and Smolkin, 1989). Compositionally, ferropicrites are characterized by an extreme FeO-rich/SiO2-poor composition compared to ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) (Fig. 1a). Although ferropicrites that range from Archean age to geologically recent have been reported, Phanerozoic-age ferropicrites are relatively rare (Gibson, 2002). Notably, Archean ferropicrites occur in ancient cratons (e.g., Onverwacht Group, S. Africa and Superior Province, Canada) (Francis et al., 1999, Gibson et al., 2000, Goldstein and Francis, 2008), whereas Phanerozoic-age high-Fe magnesian magmas typically occur in Large Igneous Provinces (e.g., Siberia, Parana-Etendeka, Afar, and the North Atlantic Igneous Province) and appear to represent some of the earliest magmas to be generated during the melting of mantle plume starting-heads (Gibson et al., 2000, Gibson, 2002, Ichiyama et al., 2006, Heinonen and Luttinen, 2008, Rogers et al., 2010). The survival of iron-rich melts in the Earth’s deep mantle carries vital consequences for its exceptional chemical and physical evolution and geodynamics (Labrosse et al., 2007, Lee et al., 2010, Nomura et al., 2011, Andrault et al., 2012). The shear modulus would decrease markedly with increasing iron content, and the bulk density would increase, thereby considerably reducing the shear-wave velocities (Jacobsen et al., 2004, Matsukage et al., 2005, Sakamaki et al., 2006, Lee et al., 2010). This observation provides a possible explanation for the seismically observed low-velocity zones in the mantle. Therefore, identifying the origin of ferropicritic melts in more detail may lead to a better understanding of the dynamics of the Earth’s deep mantle.
An excess iron mantle source has been proposed to be incorporated into high-density ferropicritic liquid (ca. 3.33 g/cm3; Goldstein and Francis, 2008), but its source mineralogy, initial melting depths, mantle potential temperatures, and the extent of melting remain highly controversial (e.g., Gibson et al., 2000, Gibson, 2002, Hirschmann et al., 2003, Tuff et al., 2005, Ichiyama et al., 2006, Heinonen and Luttinen, 2008, Rogers et al., 2010). One proposed hypothesis is that some Archean and early Proterozoic ferropicrites that preferentially occur in cratons could have resulted from the high-degree partial melting of the early primitive mantle that left a complementary iron-poor Archean cratonic mantle root (Francis et al., 1999, Gibson et al., 2000, Goldstein and Francis, 2008, Milidragovic and Francis, 2016; Fig. 1b) because the mantle potential temperature during the Archean and early Proterozoic was higher than today (Griffin et al., 2003, Herzberg and Rudnick, 2012, Johnson et al., 2014). Alternatives suggest that the chemical interaction of the Earth’s iron-poor silicate mantle with the liquid outer core strongly affects the deep mantle’s iron (and perhaps iron-loving elements) content and can create an iron-rich source for mantle plumes that originate at the core–mantle boundary (Knittle and Jeanloz, 1989, Brandon et al., 1998, Humayun et al., 2004, Herzberg et al., 2013). In addition, recycled dense, iron-rich crustal materials (e.g., banded iron formations) could have contributed to the formation of chemical heterogeneities that are composed of potentially iron-rich regions at the base of the mantle (Kaneshima and Helffrich, 1999, Dobson and Brodholt, 2005, Nebel et al., 2010, Kato et al., 2016).
As a whole, the average iron content of Archean basalts is higher than post-Archean basalts and modern MORBs (Fig. 1a). This observation implies that abundant iron could have been transported from the primitive upper mantle to the primary crust during the early stages of mantle melting. Archean metabasalts and their eclogite-facies metamorphic equivalents, in which almanditic garnet can accommodate abundant iron (Fig. 1a), have a density that is 10–16% higher than ambient mantle peridotite (Fig. 1b–c). Once these iron-rich eclogites sink into the base of the convecting layer at subduction zones or via density foundering (Foley et al., 2003, Johnson et al., 2014) and ultimately reach the solidus because of internal heating, the decomposition of almanditic garnets could release massive amounts of iron to form a locally iron-rich mantle domain.
We turn to a suite of Pleistocene ferropicrites from Wudi, North China to further explore the above hypothesis (Fig. 2a). By combining geochemical data from the Wudi ferropicrites (plus Qixia iron-rich basalts; Fig. 2a) and high-resolution seismic tomographic images (Fig. 2b–d), we propose that these ferropicritic melts were derived from the preferential melting of recycled dense continental eclogite that stagnated at the mid-upper mantle, whereas their extreme iron enrichment and silica deficiency are attributed to the pressure-dependent compatibility of iron in garnet and breakdown of eclogitic garnet (the major carrier of iron in eclogite). Our findings provide a model to explain the seismically observed low-velocity anomalies above the mantle transition zone, where dense, iron-rich silicate melts that formed by partial melting at >150 km become dynamically unstable and hence migrate downward, thereby implying no requirement for core–mantle interactions to produce these iron-rich deep mantle reservoirs.
Section snippets
Geologic setting and samples
The North China Craton (NCC) is one of the world’s oldest cratons. The NCC comprises two different tectonic domains (East and West Blocks) which were melded together during the Paleoproterozoic along the Trans-North China orogen (Zhao et al., 2001; Fig. 2a). The presence of ⩾3.6 Ga crustal remnants that are exposed at the surface and in lower crustal xenoliths in both the northern and southern parts of the craton suggests that the NCC has remained partially intact since the early Archean (Liu et
Bulk major and trace element analyses
Whole-rock samples were crushed in a corundum jaw crusher (to 60 mesh). Approximately 60 g was powdered in an agate ring mill to less than 200 mesh. The major elements were determined by using a Rigaku RIX 2100 X-ray fluorescence spectrometer (XRF) at the State Key Laboratory of Continental Dynamics, Northwest University in Xi’an, China. Analyses of reference materials and duplicate analyses suggest that the analytical precision and accuracy are better than 5% (Rudnick et al., 2004, Gao et al.,
Whole-rock chemical and Sr–Nd–Os isotopic compositions
The major and trace element and Sr–Nd–Os isotopic compositions of the Wudi ferropicrites are listed in Table 1. The most striking feature of the Wudi ferropicrites is their high Fe2O3 (14.91–15.23 wt%) contents and low SiO2 (40–41 wt%) (Fig. 3; Fig. S1). For a given MgO and MnO range, the rocks show more pronounced iron enrichment and invariably higher Fe/Mn ratios (73.6–75.2) relative to MORBs (Fe/Mn = 55–58) and Hawaiian picrites (Fe/Mn = 65.6–70.9) (Humayun et al., 2004) (Figs. 1 and 3a–b; Table 1
Discussion
Recently, both P- and S-wave velocity models revealed that an obvious N–S trending, narrow low-velocity region was confined atop the mantle transition zone beneath the central NCC (Fig. 2). Such a low-velocity region has been regarded as a thermal anomaly, possibly because of a mantle plume (Zhao et al., 2009), which has been interpreted to explain the two parallel volcanic chains in Shandong, North China (Fig. 2; Zeng et al., 2011). However, a petrological record of high-temperature mantle
Conclusions
A suite of Pleistocene intraplate silica-poor ferropicrites in Wudi, North China was studied. Combined with seismic observations, our findings provide evidence for the melting of recycled continental silica-poor/iron-rich eclogitic residues that stagnated in the mid-upper mantle. Iron (plus Y, Yb and Lu) partitioning in eclogitic garnet critically depends on pressure: both Fe and Y become simultaneously incompatible in eclogitic garnet at pressures of ⩾5 GPa, while Yb and Lu remain compatible at
Acknowledgements
We are grateful to Claude Herzberg, Sally Gibson, Yuan-Bao Wu and Liang Zhao for provocative discussions and helpful suggestions, and to Zi-Wan Chen, Xian-Jun Xie, Yong-Juan Gao, Ying-Hua Zhang and Meng Cheng for field work and elemental analyses. Yigang Xu and an anonymous reviewer are thanked for their constructive reviews and Associate Editor Weidong Sun is thanked for editorial handling. This research was supported by the National Nature Science Foundation of China (Grants 41125013, 41503020
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