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Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust

Nature volume 530, pages 81–84 (2016)Cite this article

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Abstract

Intermediate-depth earthquakes in cold subduction zones are observed within the subducting oceanic crust, as well as the mantle1,2. In contrast, intermediate-depth earthquakes in hot subduction zones predominantly occur just below the Mohorovičić discontinuity1. These observations have stimulated interest in relationships between blueschist-facies metamorphism and seismicity, particularly through dehydration reactions involving the mineral lawsonite1,2. Here we conducted deformation experiments on lawsonite, while monitoring acoustic emissions, in a Griggs-type deformation apparatus. The temperature was increased above the thermal stability of lawsonite, while the sample was deforming, to test whether the lawsonite dehydration reaction induces unstable fault slip. In contrast to similar tests on antigorite, unstable fault slip (that is, stick–slip) occurred during dehydration reactions in the lawsonite and acoustic emission signals were continuously observed. Microstructural observations indicate that strain is highly localized along the fault (R1 and B shears), and that the fault surface develops slickensides (very smooth fault surfaces polished by frictional sliding). The unloading slope during the unstable slip follows the stiffness of the apparatus at all experimental conditions, regardless of the strain rate and temperature ramping rate. A thermomechanical scaling factor3 for the experiments is within the range estimated for natural subduction zones, indicating the potential for unstable frictional sliding within natural lawsonite layers.

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Figure 1: Distribution of intermediate-depth earthquakes, pressure–temperature paths for cold and hot subduction zones, and major dehydration reactions of lawsonite blueschist.
Figure 2: Differential stress, axial displacement and acoustic emissions counts as a function of time during temperature ramping experiments on lawsonite and antigorite.
Figure 3: Stress–displacement curves and unloading slopes during temperature ramping experiments on lawsonite and antigorite for a range of the thermomechanical factor .
Figure 4: Lawsonite sample from a temperature ramping experiment.

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Acknowledgements

We thank A. Schubnel, D. Forsyth, T. Altman and T. Togo for technical advice on the acoustic emissions measurement, B. Proctor for discussion and assistance in the laboratory, J. Boesenberg for technical assistance with the electron probe microanalyser, R. Milliken and K. Robertson for helping with XRD measurements, and H. Marschall and Terry E. Tullis for discussions. This study was supported by the US National Science Foundation (EAR-1049582, EAR-1315784).

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Authors and Affiliations

  1. 1Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street Box 1846, Providence, Rhode Island 02906, USA,

    Keishi Okazaki & Greg Hirth

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  1. Keishi Okazaki
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Contributions

G.H. proposed the project. K.O. conducted experiments and microstructural and mechanical analyses. K.O. took all the photographs. Both authors contributed to developing the main ideas, designing the experiments, interpreting the results and writing the manuscript.

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Correspondence to Keishi Okazaki.

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Extended data figures and tables

Extended Data Figure 1 Lawsonite vein sample and XRD spectra for samples.

a, The lawsonite vein in a lawsonite–blueschist block from the Franciscan Complex in California, USA, used for the starting material for the temperature ramp experiments. b and c, XRD spectra for the starting material (SM) and recovered samples (W1946 and W1965) from experiments on simulated lawsonite gouge. Most of the peaks in the SM are lawsonite (lw); there are minor peaks of staurolite (st) and glaucophane (gl). Spectra for the recovered samples from a temperature ramping experiment (W1965) show a weaker peak in lawsonite than in the SM and small peaks for anorthite (an). Anorthite, which is formed by lawsonite break down (Fig. 1 and Extended Data Fig. 3), is evidence of lawsonite dehydration. Corundum peaks (cr) are contamination from the alumina shear piston. We did not find any reaction products in the recovered sample deformed within the lawsonite stability field with no temperature ramping (W1946). d, f, Cross-polarized light micrographs of a hot-pressed sample (W1943) and a sample from a temperature ramping experiment (W1971), respectively. e, g, Same regions as d and f, but with the gypsum plate inserted under cross-polarized light. cps, counts per second.

Extended Data Figure 2 Stick–slip behaviour of lawsonite gouge without temperature ramping.

a, A violent stick–slip of lawsonite was observed in an experiment (W1946) conducted with no temperature ramping at 400 °C, at a confining pressure of 1.0 GPa, and an axial displacement rate of 0.18 μm s−1 (Extended Data Table 1). Because of the enormously large stress drop and fast slip, the thermocouple failed, and the load point displacement was not measured correctly during the stick–slip event, so the unloading slope for this run could not be estimated. b, Enlargement of the stick–slip event.

Extended Data Figure 3 Sample assembly used in this study (a) and pressure–temperature paths for the temperature ramping experiments (b).

Pink circle and black triangle show pressure–temperature paths for lawsonite and for antigorite, respectively. Blue lines show the lawsonite phase diagram for the CaO–SiO2–H2O system13 applicable for the experimental system in this study. Grey lines show the lawsonite stability limit in the MORB system30 applicable for the natural subduction zone setting. Green lines show the phase diagram for antigorite in the MgO–SiO2–H2O system14,34. Atg = antigorite, Zo = zoisite, Ky = kyanite, Qz = quartz, An = anorthite, Fo = forsterite, Tlc = talc, En = enstatite, Tlc-like = talc-like phase.

Extended Data Table 1 Experimental details on the temperature ramping experiment on lawsonite
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Okazaki, K., Hirth, G. Dehydration of lawsonite could directly trigger earthquakes in subducting oceanic crust. Nature 530, 81–84 (2016). https://doi.org/10.1038/nature16501

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