Elsevier

Lithos

Volumes 268–271, January 2017, Pages 364-382
Lithos

Invited review article
Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

https://doi.org/10.1016/j.lithos.2016.11.004Get rights and content

Highlights

  • Review of petrologically plausible explanations for cratonic mantle discontinuities

  • Melt at cratonic LAB depth (≥ 200 km) due to anomalous sublithospheric X-T, retarded drainage and/or resupply

  • Conflicting seismic-petrologic LAB signatures due to thermochemical-mechanical erosion

  • MLDs result mostly from past deposition of metasomes at mechanical boundary layers.

  • Omnipresent metasomes documented by alkaline melts, underestimated due to non-representative sampling

Abstract

Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) beneath cratons have received much attention over recent years, but a consensus on their origin has not yet emerged. Cratonic lithosphere composition and origin is peculiar due to its ultra-depletion during plume or accretionary tectonics, cool present-day geothermal gradients, compositional and rheological stratification and multiple metasomatic overprints. Bearing this in mind, we integrate current knowledge on the physical properties, chemical composition, mineralogy and fabric of cratonic mantle with experimental and thermodynamic constraints on the formation and migration of melts, both below and within cratonic lithosphere, in order to find petrologically viable explanations for cratonic mantle discontinuities.

LABs characterised by strong seismic velocity gradients and increased conductivity require the presence of melts, which can form beneath intact cratonic roots reaching to ~ 200–250 km depth only in exceptionally warm and/or volatile-rich mantle, thus explaining the paucity of seismical LAB observations beneath cratons. When present, pervasive interaction of these - typically carbonated - melts with the deep lithosphere leads to densification and thermochemical erosion, which generates topography at the LAB and results in intermittent seismic LAB signals or conflicting seismic, petrologic and thermal LAB depths. In rare cases (e.g. Tanzanian craton), the tops of live melt percolation fronts may appear as MLDs and, after complete lithosphere rejuvenation, may be sites of future, shallower LABs (e.g. North China craton).

Since intact cratons are presently tectonomagmatically quiescent, and since MLDs produce both positive and negative velocity gradients, in some cases with anisotropy, most MLDs may be best explained by accumulations (metasomes) of seismically slow minerals (pyroxenes, phlogopite, amphibole, carbonates) deposited during past magmatic-metasomatic activity, or fabric inherited from cratonisation. They may accumulate as layers at, or as subvertical veins above, the depth at which melt flow transitions from pervasive to focussed flow at the mechanical boundary layer, causing azimuthal and radial anisotropy. Thermodynamic calculations investigating the depth range in which small-volume melts can be produced relative to the field of phlogopite stability and the presence of MLDs show that phlogopite precipitates at various pressures as a function of age-dependent thermal state of the cratonic mantle, thus explaining variable MLD depths. Even if not directly observed, such metasomes have been shown to be important ingredients in small-volume volatile-rich melts typically penetrating cratonic lithospheres. The apparent sparseness of evidence for phlogopite-rich assemblages in the mantle xenolith record at geophysically imaged MLD depths, if not due to preferential disaggregation in the kimberlite or alteration, may relate to vagaries of both kimberlite and human sampling.

Introduction

Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphere-asthenosphere boundaries (LAB) in Earth's ancient continental masses (cratons; Fig. 1) have received much attention over recent years. A plethora of papers has been published on the topic of both, with a focus on geophysical observables (e.g. Hopper and Fischer, 2015, Rader et al., 2015, Selway et al., 2015), and on petrologic-geochemical data (e.g. O’Reilly and Griffin, 2010). The overarching goal of the various lines of investigating cratonic lithosphere, including current efforts to find and explain cratonic mantle discontinuities, is to elucidate the origin and evolution of continents, which bears on early terrestrial dynamics, such as the nature of crust and mantle differentiation or the viability of plate tectonics (Gerya, 2014).

Xenoliths, xenocrysts and magmas forming in or passing through cratonic mantle provide temporally and spatially punctuated information on the composition and fabric of cratonic roots at the time of magmatism, and on the nature and timing of events that formed them. Geophysical observations can provide regional or craton-scale insights, but require validation from actual sample properties, and must rely on background information afforded by mantle samples for correct interpretation. Although these different modes of lithosphere interrogation do not operate on the same spatial or temporal scales, they offer the opportunity to investigate lithospheric discontinuities through the joint consideration of all available physical, dynamic, chemical and petrologic constraints.

Perhaps not surprisingly, there is little agreement on the origins of cratonic mantle discontinuities, and the increasing integration of multiple geophysical observables with the mineralogy and composition of mantle samples has led to the recognition that universally applicable mechanisms are unlikely to explain all observations (e.g. Fischer et al., 2010, Selway et al., 2015). It is clear, however, that the peculiarities of cratonic lithosphere composition and construction - cool geothermal gradients, compositional and rheological stratification, multiple metasomatic overprints in the course of their long existence - offer myriad possibilities for the generation, modification, translation and even obliteration of such discontinuities in ancient lithospheric keels.

In this review, we first provide a brief introduction into the cratonic lithosphere, its uniqueness compared to younger lithosphere, and the nature and shortcomings of the cratonic mantle sample (Section 2). We then present the different lines of investigation that can be combined to make inferences on the nature and origin of cratonic lithosphere discontinuities (Section 3): Petrographic-geochemical observations (microstructures, mineral modes, elemental and isotopic composition), geothermobarometry (to locate mantle samples in the lithosphere column), thermodynamic modelling and geophysical observations (seismic velocity and anisotropy, resistivity, reflectivity). We subsequently discuss possible origins of the two major mantle lithospheric discontinuities - LABs and MLDs (Section 4). We conclude with a summary of the most important and, in some cases, novel insights gained from this exercise. In particular, we would like to highlight that the combined perspective from petrology, geochemistry and thermodynamic modelling indicates that most MLDs may be best explained by accumulations of seismically slow minerals deposited during past magmatic-metasomatic activity. Finally, we suggest that the sparseness of seismic LAB observation beneath stable cratons relate to the difficulty to form and retain melt beneath these typically thick (200–250 km) lithospheres.

Section snippets

Craton definition, craton types and nature of the cratonic mantle sample

Cratons are often defined as the cores of tectonomagmatically quiescent continents where the last major tectonothermal event occurred in the Archaean or Palaeoproterozoic and which were amalgamated by Neoarchaean time (≥ 2.5 Ga); larger continental regions that arose by later collisions, as evidenced by Proterozoic mobile belts, are also sometimes referred to as cratons (Fig. 1). These mobile belts shielded the cratonic cores from later tectonomagmatic activity, ensuring their long-term stability

Petrology, fabric, geochemistry and thermodynamic modelling

Over its long existence and subsequent to the partial melting event leading to its depletion, the cratonic mantle has been multiply refertilised and metasomatised (e.g. O'Reilly and Griffin, 2013, Pearson and Wittig, 2014). It is known that fertile peridotite changes its modal composition with increasing degree of melt extraction from lherzolite (olivine, orthopyroxene, clinopyroxene plus feldspar or Al-spinel or garnet, depending on pressure) to harzburgite (≤ 5 vol.% clinopyroxene) and dunite (≤

Lithosphere-asthenosphere boundary (LAB)

Various definitions exist for the continental LAB (chemical, thermal, mechanical, seismic, electrical; Artemieva and Mooney, 2001, Eaton et al., 2009), implying that the thickness of cratonic roots can be measured by a variety of geochemical, petrologic and geophysical means. Compositionally, the LAB is typified by a transition from depleted to fertile (or refertilised) peridotite that has been mapped using xenoliths and xenocrysts (Gaul et al., 2000, Griffin et al., 2002, Griffin et al., 2004,

Summary and conclusions

We have integrated the physical properties, chemical composition, mineralogy and fabric of cratonic mantle samples (xenoliths, xenocrysts, melts) with experimental and thermodynamic constraints on the formation and migration of melts below and within cratons in order to isolate the petrologically most viable mechanisms for geophysically observed cratonic mantle discontinuities, namely the LAB and MLDs. Bearing in mind that geophysical and petrologic modes of lithosphere interrogation do not

Acknowledgments

Andrea Tommasi and Ingo Wölbern are thanked for providing invaluable informal feed-back on the manuscript. Insightful and constructive formal reviews by Gumer Galán and Michel Grégoire are highly appreciated, as are the seamless editorial handling by Andrew Kerr and the positivity, patience and swiftness with which Tim Horscroft has supported this work throughout the process. SA gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft under grant DFG AU386/8. F.G. and M.M.

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