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Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier


Convergent margin volcanism originates with partial melting, primarily of the upper mantle, into which the subducting slab descends1,2. Melting of this material can occur in one of two ways. The flow induced in the mantle by the slab can result in upwelling and melting through adiabatic decompression1,3. Alternatively, fluids released from the descending slab through dehydration reactions can migrate into the hot mantle wedge, inducing melting by lowering the solidus temperature2,4. The two mechanisms are not mutually exclusive1. In either case, the buoyant melts make their way towards the surface to reside in the crust or to be extruded as lava. Here we use magnetotelluric data collected across the central state of Washington, USA, to image the complete pathway for the fluid–melt phase. By incorporating constraints from a collocated seismic study5 into the magnetotelluric inversion process, we obtain superior constraints on the fluids and melt in a subduction setting. Specifically, we are able to identify and connect fluid release at or near the top of the slab, migration of fluids into the overlying mantle wedge, melting in the wedge, and transport of the melt/fluid phase to a reservoir in the crust beneath Mt Rainier.

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Figure 1: Map showing station locations for the CAFE seismic and magnetotelluric stations (wideband and long-period) across central Washington state, USA.
Figure 2: Primary seismic (a) and magnetotelluric (b) models.
Figure 3: The resistivity of peridotite for a given melt fraction and water content (within the melt) at a temperature of 1,150 °C.

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We thank V. Maris, M. Brown, A. Kelbert, and Quantec Geoscience, Inc. for their part in data acquisition. We also thank A. Pommier, D. Lizarralde, A. Malcolm, A. Shaw, H. Marschall and J. P. Canales for critical discussions and input on early versions of the manuscript. Finally, we thank P. van Keken for use of his thermal overlay in the magnetotelluric figure. This work was supported by US National Science Foundation grant EAR08-44041 (Principal Investigator R.L.E.) and US National Science Foundation grant EAR08-43725 (Principal Investigator P.E.W.), both through the Earthscope programme. R.S.M. was supported by a National Defense Science and Engineering Graduate (NDSEG) fellowship.

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Authors and Affiliations



R.L.E. and P.E.W. conceived the experiment. R.S.M. participated in data collection and was primarily responsible for the processing and inversion work and analysis. R.L.E. was involved in all aspects of the development of the magnetotelluric models. P.E.W. coordinated and led the data collection, and also performed some of the processing and analysis of the broadband data. J.E. assisted with data reduction and processing. S.R. was involved in the production of the seismic image. All authors contributed to the understanding of the results and editing of the manuscript.

Corresponding author

Correspondence to R. Shane McGary.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Rose diagram showing overall strike directions.

The colour code reflects the Bahr skew as determined using the STRIKE algorithm15 for the CAFE data set.

Extended Data Figure 2 Primary standard inversion images for the CAFE data.

These magnetotelluric images were generated without incorporating a tear zone on top of the slab or setting the initial resistivity for the slab. The top image was generated using a combination of the TM mode and tipper, whereas the bottom image was produced using the TM and TE modes along with the tipper.

Extended Data Figure 3 The TM (a) and TE (b) pseudo-sections for the CAFE magnetotelluric data.

The two upper panels in a and the two upper panels in b show apparent resistivity and phase for the data. The two lower panels in a and the two lower panels in b show apparent resistivity and phase for the model. Both models are limited horizontally to correspond with the surface covered by the CAFE magnetotelluric array.

Extended Data Figure 4 Plot of root-mean-square misfit against the 60 CAFE magnetotelluric stations for the TM/TE/tipper models.

The blue line shows root-mean-square misfit by station for the halfspace model (without a tear or initial resistivity set for the upper slab), and the green line shows the same for the augmented model (using a tear at the top of the slab and imposing initial resistivity for the upper part of the slab). The overall root-mean-square misfit values were 3.08 for the halfspace model, and 1.89 for the augmented model.

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McGary, R., Evans, R., Wannamaker, P. et al. Pathway from subducting slab to surface for melt and fluids beneath Mount Rainier. Nature 511, 338–340 (2014).

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