Lithospheric controls on magma composition along Earth’s longest continental hotspot track

Abstract

Hotspots are anomalous regions of volcanism at Earth’s surface that show no obvious association with tectonic plate boundaries. Classic examples include the Hawaiian–Emperor chain and the Yellowstone–Snake River Plain province. The majority are believed to form as Earth’s tectonic plates move over long-lived mantle plumes: buoyant upwellings that bring hot material from Earth’s deep mantle to its surface1. It has long been recognized that lithospheric thickness limits the rise height of plumes2,3,4 and, thereby, their minimum melting pressure. It should, therefore, have a controlling influence on the geochemistry of plume-related magmas, although unambiguous evidence of this has, so far, been lacking. Here we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain plume melting depths beneath Earth’s longest continental hotspot track, a 2,000-kilometre-long track in eastern Australia that displays a record of volcanic activity between 33 and 9 million years ago5,6, which we call the Cosgrove track. Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, with: (1) standard basaltic compositions in regions where lithospheric thickness is less than 110 kilometres; (2) volcanic gaps in regions where lithospheric thickness exceeds 150 kilometres; and (3) low-volume, leucitite-bearing volcanism in regions of intermediate lithospheric thickness. Trace-element concentrations from samples along this track support the notion that these compositional variations result from different degrees of partial melting, which is controlled by the thickness of overlying lithosphere. Our results place the first observational constraints on the sub-continental melting depth of mantle plumes and provide direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas.

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Figure 1: The distribution and classification of eastern Australian Cenozoic volcanic centres and their relationship to regional lithospheric thickness variations.
Figure 2: Trace-element abundances of volcanic samples along the Cosgrove hotspot track.

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Acknowledgements

D.R.D. is funded by an Australian Research Council Future Fellowship (FT140101262). G.I. acknowledges support from the Ringwood Fellowship at the Australian National University. Digital geological data were provided by Geosciences Australia.

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Authors

Contributions

D.R.D. conceived this study and integrated all interdisciplinary observational constraints. N.R. created the lithospheric thickness map by combining constraints from the AuSREM reference model and body-wave data from the WOMBAT array. He also devised and implemented the method for estimating uncertainty in lithospheric thickness. G.I. performed the hotspot-track reconstruction and estimated the associated uncertainties. D.R.D. and I.H.C. undertook the geochemical synthesis. D.R.D. wrote the paper, following discussion with, and contributions from, all authors.

Corresponding author

Correspondence to D. R. Davies.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Locations of the 15 volcanic centres used in our reconstruction of the Cosgrove hotspot track.

The hotspot track is indicated with a dashed line.

Extended Data Figure 2 The Cosgrove hotspot track.

As in Fig. 1a but incorporating all 15 dated volcanic complexes and extended southwards to show the predicted present-day location of the underlying mantle plume (green square to the northwest of Tasmania). The approximate location of the East Australia Plume System, imaged previously using finite frequency tomography30, is marked by the dotted green line.

Extended Data Figure 3 Reconstruction score map.

The number of predicted volcanic centre locations, from a total of 15 (listed in Extended Data Table 1), that fall within the uncertainty circles surrounding the dated volcanic centres, for a range of plume drift velocities and melt region diameters. Note that the reconstructions illustrated in Fig. 1a and Extended Data Fig. 2 assume a plume drift velocity of 1 cm yr−1 and a melt region diameter of 100 km (black square).

Extended Data Figure 4 Location of WOMBAT array stations used to create the three-dimensional P-wave velocity model from which our lithospheric thickness estimate was derived.

Station spacing is 50 km, which roughly equates to the maximum horizontal resolution of the three-dimensional velocity model.

Extended Data Figure 5 Depth slice at 120 km, through the three-dimensional P-wave velocity model.

North of 28° S, the model reverts to the AuSREM mantle model, owing to a lack of additional data coverage in this region (see Extended Data Fig. 4).

Extended Data Figure 6 Lithospheric thickness estimate and associated uncertainty.

a, b, Lithosphere thickness model illustrated in Fig. 1b (a), alongside an estimate of its uncertainty (b), given by the standard deviation (σ) of an ensemble of 540 plausible models examined. Note that south of 28 °S, the lithospheric thickness estimate is constrained by high-resolution body-wave tomography (50 km horizontal resolution), whereas north of this latitude it is constrained entirely by the AuSREM mantle model (200–250 km horizontal resolution)24.

Extended Data Table 1 Age estimates, derived via 40Ar–39Ar geochronology, for the volcanic centres considered in this study
Extended Data Table 2 Rock type, sample locations, sample numbers and data source, from previously analysed samples along the Cosgrove track
Extended Data Table 3 Trace-element concentrations for a number of samples along the Cosgrove track

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Davies, D., Rawlinson, N., Iaffaldano, G. et al. Lithospheric controls on magma composition along Earth’s longest continental hotspot track. Nature 525, 511–514 (2015). https://doi.org/10.1038/nature14903

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