Elsevier

Geochimica et Cosmochimica Acta

Volume 197, 15 January 2017, Pages 356-377
Geochimica et Cosmochimica Acta

Pressure-dependent compatibility of iron in garnet: Insights into the origin of ferropicritic melt

https://doi.org/10.1016/j.gca.2016.10.047Get rights and content

Abstract

Iron-rich silicate melts in the Earth’s deep mantle have been seismologically and geochemically inferred in recent years. The origin of local enrichments in iron and low-velocity seismic anomalies that have been detected in dense mantle domains are critical to understanding the mantle’s evolution, which has been canonically explained by long-term chemical reactions between the Earth’s silicate mantle and its liquid iron outer core. However, the Pleistocene alkaline ferropicrites (∼0.73 Ma) from Wudi, North China, show chemical and Sr–Nd–Os isotopic features that suggest derivation from the preferential melting of silica-deficient eclogite, a lithology of delaminated mafic lower continental crust that had stagnated at mid-upper mantle depths during the Mesozoic decratonization of the North China block. These rocks are characterized by substantial enrichment in iron (14.9–15.2 wt% Fe2O3), relative depletion in silica (40–41 wt% SiO2) and decoupled Y and heavy rare earth element (HREE) compositions. These ferropicrites have particularly higher Y/Yb ratios than the other Cenozoic basalts from North China. The pressure-dependent compatibility of Fe, Y and Yb in eclogitic garnet can adequately explain the Fe-enrichment and Y-HREE decoupling of the Wudi ferropicrites and indicates that the eclogites were melted at pressures of 5–8 GPa, as also constrained by previous high-P–T experiments. This melting depth ties together a seismically imaged high-velocity anomaly that extends from 150 km to 350 km in depth under the study area, which has been commonly interpreted as evidence for the stagnation of the missing, delaminated continental lithosphere. Our findings provide an alternative mechanism to produce an extremely iron-rich mantle reservoir in addition to core–mantle interaction. Iron-rich silicate melts that form by this process are likely to be denser than the ambient mantle peridotite (and therefore drive flow downward) and may play a more significant role in the deep-mantle geophysical and geochemical diversities than previously considered.

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

References (158)

  • A. Corgne et al.

    Trace element partitioning between majoritic garnet and silicate melt at 25 GPa

    Phys. Earth Planet. Inter.

    (2004)
  • A. Corgne et al.

    Trace element partitioning between majoritic garnet and silicate melt at 10–17 GPa: implications for deep mantle processes

    Lithos

    (2012)
  • R. Dasgupta et al.

    Major element chemistry of ocean island basalts – conditions of mantle melting and heterogeneity of mantle source

    Earth Planet. Sci. Lett.

    (2010)
  • J.M.D. Day et al.

    Evidence for distinct proportions of subducted oceanic crust and lithosphere in HIMU-type mantle beneath El Hierro and La Palma, Canary Islands

    Geochim. Cosmochim. Acta

    (2010)
  • D.S. Draper et al.

    Trace element partitioning between garnet and chondritic melt from 5 to 9 GPa: implications for the onset of the majorite transition in the martian mantle

    Phys. Earth Planet. Inter.

    (2003)
  • D.S. Draper et al.

    High-pressure phase equilibria and element partitioning experiments on Apollo 15 green C picritic glass: implications for the role of garnet in the deep lunar interior

    Geochim. Cosmochim. Acta

    (2006)
  • D. Francis et al.

    Picrite evidence for more Fe in Archean mantle reservoirs

    Earth Planet. Sci. Lett.

    (1999)
  • S. Gao et al.

    Chemical composition of the continental crust as revealed by studies in east China

    Geochim. Cosmochim. Acta

    (1998)
  • S. Gao et al.

    Poisson’s ratio of eclogite: the role of retrogression

    Earth Planet. Sci. Lett.

    (2001)
  • S. Gao et al.

    Recycling deep cratonic lithosphere and generation of intraplate magmatism in the North China Craton

    Earth Planet. Sci. Lett.

    (2008)
  • S.A. Gibson

    Major element heterogeneity in Archean to Recent mantle plume starting-heads

    Earth Planet. Sci. Lett.

    (2002)
  • S.A. Gibson et al.

    Ferropicrites: geochemical evidence for Fe-rich streaks in upwelling mantle plumes

    Earth Planet. Sci. Lett.

    (2000)
  • T.H. Green et al.

    SIMS determination of trace element partition coefficients between garnet, clinopyroxene and hydrous basaltic liquids at 2–7.5 GPa and 1080–1200 °C

    Lithos

    (2000)
  • W.L. Griffin et al.

    The origin and evolution of Archean lithospheric mantle

    Precambr. Res.

    (2003)
  • E.J. Hanski et al.

    Pechenga ferropicrites and other early Proterozoic picrites in the eastern part of the Baltic Shield

    Precambr. Res.

    (1989)
  • E.H. Hauri et al.

    Experimental and natural partitioning of Th, U, Pb and other trace elements between garnet, clinopyroxene and basaltic melts

    Chem. Geol.

    (1994)
  • J.S. Heinonen et al.

    Jurassic dikes of Vestfjella, western Dronning Maud Land, Antarctica: geochemical tracing of ferropicrite sources

    Lithos

    (2008)
  • J.S. Heinonen et al.

    Subduction-modified oceanic crust mixed with a depleted mantle reservoir in the sources of the Karoo continental flood basalt province

    Earth Planet. Sci. Lett.

    (2014)
  • C. Herzberg et al.

    Formation of cratonic lithosphere: an integrated thermal and petrological model

    Lithos

    (2012)
  • A.W. Hofmann et al.

    Nb and Pb in oceanic basalts: new constraints on mantle evolution

    Earth Planet. Sci. Lett.

    (1986)
  • T. Hou et al.

    Petrogenesis and metallogenesis of the Taihe gabbroic intrusion associated with Fe–Ti-oxide ores in the Panxi district, Emeishan Large Igneous Province, southwest China

    Ore Geol. Rev.

    (2012)
  • J. Huang et al.

    Origin of low δ26Mg Cenozoic basalts from South China Block and their geodynamic implications

    Geochim. Cosmochim. Acta

    (2015)
  • Y. Ichiyama et al.

    Geochemical and isotopic constraints on the genesis of the Permian ferropicritic rocks from the Mino-Tamba belt, SW Japan

    Lithos

    (2006)
  • M.G. Jackson et al.

    Compositions of HIMU, EM1, and EM2 from global trends between radiogenic isotopes and major elements in ocean island basalts

    Earth Planet. Sci. Lett.

    (2008)
  • C. Kato et al.

    Melting in the FeOSiO2 system to deep lower-mantle pressures: implications for subducted Banded Iron Formations

    Earth Planet. Sci. Lett.

    (2016)
  • M. Klein et al.

    Experimental partitioning of high field strength and rare earth elements between clinopyroxene and garnet in andesitic to tonalitic systems

    Geochim. Cosmochim. Acta

    (2000)
  • S. Klemme et al.

    Experimental constraints on major and trace element partitioning during partial melting of eclogite

    Geochim. Cosmochim. Acta

    (2002)
  • T. Kogiso et al.

    High-pressure partial melting of garnet pyroxenite: possible mafic lithologies in the source of ocean island basalts

    Earth Planet. Sci. Lett.

    (2003)
  • V. Le Roux et al.

    Zn/Fe systematics in mafic and ultramafic systems: implications for detecting major element heterogeneities in the Earth’s mantle

    Geochim. Cosmochim. Acta

    (2010)
  • C.-T.A. Lee

    Physics and chemistry of deep continental crust recycling

  • S.G. Li et al.

    Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: timing and processes

    Chem. Geol.

    (1993)
  • Y.-Q. Li et al.

    Recycling of oceanic crust from a stagnant slab in the mantle transition zone: evidence from Cenozoic continental basalts in Zhejiang Province, SE China

    Lithos

    (2015)
  • H.-Y. Li et al.

    Olivine and melt inclusion chemical constraints on the source of intracontinental basalts from the eastern North China Craton: discrimination of contributions from the subducted Pacific slab

    Geochim. Cosmochim. Acta

    (2016)
  • W.L. Ling et al.

    Contrasting geochemistry of the Cretaceous volcanic suites in Shandong province and its implications for the Mesozoic lower crust delamination in the eastern North China craton

    Lithos

    (2009)
  • C.Q. Liu et al.

    Major- and trace-element compositions of Cenozoic basalts in eastern China: petrogenesis and mantle source

    Chem. Geol.

    (1994)
  • Y.C. Liu et al.

    Geochemistry and geochronology of eclogites from the northern Dabie Mountains, central China

    J. Asian Earth Sci.

    (2005)
  • Y.-H. Liu et al.

    Compositions of high Fe–Ti eclogites from the Sulu UHP metamorphic terrane, China: HFSE decoupling and protolith characteristics

    Chem. Geol.

    (2007)
  • Y.S. Liu et al.

    Recycled crust controls contrasting source compositions of Mesozoic and Cenozoic basalts in the North China Craton

    Geochim. Cosmochim. Acta

    (2008)
  • Y.S. Liu et al.

    In situ analysis of major and trace elements of anhydrous minerals by LA-ICP-MS without applying an internal standard

    Chem. Geol.

    (2008)
  • Y.S. Liu et al.

    Geochemistry and magmatic history of eclogites and ultramafic rocks from the Chinese continental scientific drill hole: subduction and ultrahigh-pressure metamorphism of lower crustal cumulates

    Chem. Geol.

    (2008)
  • Cited by (31)

    • The slab gap-related Late Cretaceous-Paleocene magmatism of southern Patagonia

      2021, Journal of Geodynamics
      Citation Excerpt :

      Table 1). All the samples are depleted in heavy rare-earth elements (HREE) reflecting the existence of residual garnet in the mantle source at pressure less than 5 GPa since there are not positive anomalies of Y, whose partition coefficient (DY) in garnet drops remarkably with increasing pressure and temperature (Zhang et al., 2017). The sample 321 represent the more primitive magma because of its high Mg# (65) and higher contents of Ni, Cr, and Co (Table 1).

    View all citing articles on Scopus
    View full text