Haemyeong Jung

Water, partial melting and the origin of the seismic low velocity and high attenuation zone in the upper mantle

Abstract The common belief that the seismic low velocity and high attenuation zone (the asthenosphere) is caused by the presence of a small amount of melt is not supported by recent mineral physics and seismological observations. A review of recent mineral physics observations suggests that water significantly reduces seismic wave velocities through anelastic relaxation and hence, at a small melt fraction expected in most of the Earths upper mantle, partial melting will increase seismic wave velocities through the removal of water from minerals such as olivine. Therefore the asthenosphere, in this model, is a layer where no significant partial melting occurs and hence a high water content is retained. We apply this model to calculate seismic wave velocities and attenuation in the upper mantle with a range of water contents. The seismic structures calculated from this model depend on geotherm, the mode of partial melting (batch or fractional melting) and the geometry of upwelling flow (passive flow or dynamic upwelling). The sharp velocity change around 60–80 km (the Gutenberg discontinuity) can be attributed to a sharp change in water content due to partial melting, if the temperature there is relatively high as implied by the plate model and if melting occurs as fractional melting but not by batch melting. However, the significant increase in seismic wave velocity with age in young oceanic upper mantle suggests rapid cooling as predicted by a cooling half-space model. Thus, the present model suggests fast cooling in the early stage but slow cooling in the later stage of evolution of the oceanic upper mantle, the latter being caused presumably by some additional heat in the old oceanic upper mantle. The seismic structures of typical oceanic upper mantle with a fast spreading rate (e.g., the Pacific) is consistent with passive spreading, whereas the greater depth of the G-discontinuity and the weaker seismic anisotropy in back-arc regions (e.g., the Philippine Sea) suggest dynamic upwelling caused presumably by a higher degree of melting due to a larger amount of water.

New type of olivine fabric from deformation experiments at modest water content and low stress

A new type of olivine fabric was found by high-strain simple-shear deformation experiments in the presence of trace amounts of water at ;0.5‐2.2 GPa and ;1470‐1570 K. In this fabric, called E-type fabric, the olivine [100] axis is subparallel to the shear direction, and the (001) plane is parallel to the shear plane; this geometry suggests that the [100](001) slip system makes the dominant contribution to total strain. This fabric is dominant at a modest water content, 200 , COH , 1000 H/10 6 Si at low stresses and high temperatures. Some mylonites from peridotite massifs show this type of olivine fabric, which suggests the presence of water during the shear localization. The seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric (A type), but the magnitudes of anisotropy are different between the two types: for horizontal flow, the amplitude of VSH/VSV anisotropy is weaker, but the amplitude of shear-wave splitting is stronger for the E-type fabric than for the A-type dry fabric. Seismic anisotropy in the oceanic upper mantle may be due to the olivine E-type fabric.

Intermediate-depth earthquake faulting by dehydration embrittlement with negative volume change

Earthquakes are observed to occur in subduction zones to depths of approximately 680 km, even though unassisted brittle failure is inhibited at depths greater than about 50 km, owing to the high pressures and temperatures. It is thought that such earthquakes (particularly those at intermediate depths of 50–300 km) may instead be triggered by embrittlement accompanying dehydration of hydrous minerals, principally serpentine. A problem with failure by serpentine dehydration is that the volume change accompanying dehydration becomes negative at pressures of 2–4 GPa (60–120 km depth), above which brittle fracture mechanics predicts that the instability should be quenched. Here we show that dehydration of antigorite serpentinite under stress results in faults delineated by ultrafine-grained solid reaction products formed during dehydration. This phenomenon was observed under all conditions tested (pressures of 1–6 GPa; temperatures of 650–820 °C), independent of the sign of the volume change of reaction. Although this result contradicts expectations from fracture mechanics, it can be explained by separation of fluid from solid residue before and during faulting, a hypothesis supported by our observations. These observations confirm that dehydration embrittlement is a viable mechanism for nucleating earthquakes independent of depth, as long as there are hydrous minerals breaking down under a differential stress.

Crystal preferred orientation of an amphibole experimentally deformed by simple shear

Seismic anisotropy has been widely observed in crust and mantle materials and plays a key role in the understanding of structure and flow patterns. Although seismic anisotropy can be explained by the crystal preferred orientation (CPO) of highly anisotropic minerals in the crust, that is, amphibole, experimental studies on the CPO of amphibole are limited. Here we present the results of novel experiments on simple shear deformation of amphibolite at high pressure and temperatures (1 GPa, 480–700 °C). Depending on the temperature and stress, the deformed amphibole produced three types of CPOs and resulted in a strong seismic anisotropy. Our data provide a new understanding of the observed seismic anisotropy. The seismic data obtained from the amphibole CPOs revealed that anomalous seismic anisotropy observed in the deep crust, subducting slab and mantle wedge can be attributed to the CPO of amphibole.

Deformation Microstructures of Olivine and Chlorite in Chlorite Peridotites from Almklovdalen in the Western Gneiss Region, SW Norway and Implications for Seismic Anisotropy

Chlorite peridotites from Almklovdalen in southwest Norway were studied to understand the deformation processes and seismic anisotropy in the upper mantle. The lattice preferred orientation (LPO) of olivine and chlorite was determined using electron backscattered diffraction (EBSD)/scanning electron microscopy. A sample with abundant garnet showed [100] axes of olivine aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: A-type LPO. Samples rich in chlorite showed different olivine LPOs. Two samples showed [001] axes aligned sub-parallel to lineation, and [010] axes aligned subnormal to foliation: B-type LPO. Two other samples showed [100] axes aligned sub-parallel to lineation, and [001] axes aligned subnormal to foliation: E-type LPO. Chlorite showed a strong LPO characterized by [001] axes aligned subnormal to foliation with a weak girdle subnormal to lineation. Fourier transform infrared (FTIR) spectroscopy of the specimens revealed that the olivines with A-type LPO contain a small amount (170 ppm H/Si) of water. In contrast, the olivines with B-type LPOs contain a large amount (340 ppm H/Si) of water. The seismic anisotropy of the olivine and chlorite was calculated. Olivine showed Vp anisotropy of up to 3.8% and a maximum Vs anisotropy of up to 2.7%. However, the chlorite showed a much stronger Vp anisotropy, up to 21.1%, and a maximum Vs anisotropy of up to 31.7%. A sample with a mixture of 25% of olivine and 75% of chlorite can produce a Vp anisotropy of 14.2% and a maximum Vs anisotropy of 22.9%. Because chlorite has a wide stability field at high pressure and high temperature in the subduction zone, the strong LPO of chlorite can be a source of the observed trench-normal or trench-parallel seismic anisotropy in the mantle wedge as well as in subducting slabs depending on the dipping angle of slab in a subduction zone where chlorite is stable.

Deformation microstructures of olivine and pyroxene in mantle xenoliths in Shanwang, eastern China, near the convergent plate margin, and implications for seismic anisotropy

Deformation microstructures, including lattice-preferred orientations (LPOs) of olivine, enstatite, and diopside, in mantle xenoliths at Shanwang, eastern China, were studied to understand the deformation mechanism and seismic anisotropy of the upper mantle. The Shanwang is located across the Tan-Lu fault zone, which was formed due to the collision between the Sino-Korean and South China cratons. All samples are spinel lherzolites and wehrlites, and LPOs of minerals were determined using scanning electron microscope/electron backscattered diffraction. We found two types of olivine LPO: type-B in spinel lherzolites and type-E in wehrlites. Enstatite showed two types of LPO (types BC and AC), and diopside showed four different types of LPO. Observations of strong LPOs and numerous dislocations in olivine suggest that samples showing both type-B and -E LPOs were deformed in dislocation creep. The seismic anisotropy of the P-wave was in the range of 2.2–11.6% for olivine, 1.2–2.3% for enstatite, and 2.1–6.4% for diopside. The maximum seismic anisotropy of the shear wave was in the range 1.93–7.53% for olivine, 1.53–2.46% for enstatite, and 1.81–6.57% for diopside. Furthermore, the thickness of the anisotropic layer was calculated for four geodynamic models to understand the origin of seismic anisotropy under the study area by using delay time from shear wave splitting, and S-wave velocity and anisotropy from mineral LPOs. We suggest that the seismic anisotropy under the study area can be most likely explained by two deformation modes that might have occurred at different times: one of deformed lherzolites with a type-B olivine LPO by lateral shear during/after the period of the Mesozoic continental collision between the Sino-Korean and South China cratons; and the other deformed the wehrlites with a type-E olivine LPO by horizontal extension during the period of change in absolute plate motion in relation to the westward-subducting Pacific plate.

Development of a compact helicon ion source for neutron generators

A compact helicon ion source has been designed for a neutron generator. Energetic deuterium beams of 120 keV from this ion source will be directed onto a Ti-coated copper target where D–D nuclear fusion reactions take place and generate 108 n/s of neutrons. High-density radio frequency (rf) plasma sources such as a helicon plasma source, known for its highest efficiency of generating high-density plasmas, are chosen for the development of a high-current density compact ion source. Highest plasma densities for hydrogen plasmas are obtained at relatively low magnetic fields of 100–300 G although plasma densities of helicon plasmas are well known to be almost linearly dependent on magnetic field strength. With Nagoya type III antenna, plasma densities of up to 2×1011 cm−3 are obtained with 1.1 kW rf power at the frequency of 13.56 MHz. With the prototype helicon ion source, hydrogen beam currents of up to 44 mA at 23 kV have been extracted in continuous wave operations with the beam current density of 28.5 m...

System for detecting acoustic emissions in multianvil experiments: Application to deep seismicity in the Earth

One of the major goals in the experimental study of deep earthquakes is to identify slip instabilities at high pressure and high temperature (HPHT) that might be responsible for the occurrence of earthquakes. Detecting acoustic emissions from a specimen during faulting provides unique constraints on the instability process. There are few experimental studies reporting acoustic emissions under HPHT conditions, due to technical challenges. And those studies have used only one or at most two acoustic sensors during the experiments. Such techniques preclude the accurate location of the acoustic emission source region and thus the ability to distinguish real signal from noise that may be coming from outside the sample. We have developed a system for detecting acoustic emissions at HPHT. Here we present a four-channel acoustic emission detecting system working in the HPHT octahedral multianvil apparatus. Each channel has high resolution (12 bits) and a sampling rate of 30 MHz. In experiments at the pressures up...

Olivine fabrics and tectonic evolution of fore-arc mantles: A natural perspective from the Songshugou dunite and harzburgite in the Qinling orogenic belt, central China

To advance our understanding of deformation characteristics, rheological behaviors and tectonic evolution of the forearc lithospheric mantle, we analyzed mineral fabrics for a large spinel-bearing ultramafic massif in the Songshugou area in the Qinling orogenic belt, central China. In the spinel-poor coarse-grained dunite, stronger A-/D-type and weaker C-type-like fabrics were found, whereas the spinel-rich coarse-grained dunite displayed a comparatively stronger B-type-like fabric. These olivine fabrics are high-T fabrics influenced by the presence of melt, in which B- and C-type-like fabrics are inferred to be produced by melt-assisted grain boundary sliding during synkinematic high-T melt‒rock reactions. In contrast, the spinel-poor porphyroclastic and fine-grained dunites present weak AG- and B-type-like fabrics, respectively. Their olivine fabrics (low-T fabrics) are inferred to transform from A-/D-type fabric in their coarse-grained counterparts possibly through mylonitization process assisted by low-T fluid‒rock reactions, during which strain was accommodated by the fluid-enhanced dislocation slip and/or fluid-assisted grain boundary sliding processes. Combined with the tectonic results of our previous work [Cao et al., 2016], the high-T olivine fabrics are probably related to a young and warm forearc mantle where intense partial melting and high-T boninitic melt‒rock reactions prevalently occurred, whereas the low-T olivine fabrics likely reflect the evolving tectonic settings through the cooling forearc mantle to a continental lower crust in a collisional orogeny where low-T fluid‒rock reactions were pervasively activated. These low-T olivine fabrics imply that, though cold, the forearc lithospheric mantle may be locally weak (∼20‒30 MPa), allowing ductile deformation to occur at a geologically significant strain rate. This article is protected by copyright. All rights reserved.

LA-ICP-MS analysis of natural rock samples using XRF glass beads

Glass beads that remain after measuring major element compositions using XRF can also be used to measure trace element compositions using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The JA-1 and JB-3 reference glass beads were used to evaluate the homogeneity of trace elements contained within them as well as the data quality of LA-ICP-MS measurements. JA-1 has a homogenous trace element composition with less than 10% relative standard deviation, and JB-3 has an average relative standard deviation and average limit of detection of the trace elements of 4% and 0.94%, respectively. The measured and reference values of JB-3 have the highest correlation (R2 = 0.996). Natural rock samples with intermediate to acidic compositions, such as Hwasun andesite, Mudeungsan dacite tuff, and Dogok rhyolite from Mudeungsan National Park, South Korea, are also used to evaluate the LA-ICP- MS data quality. Trace elements in these natural rock samples are plotted in a reasonable area of the geochemical discrimination diagram with sufficient precision. Accordingly, glass beads made from natural rock samples can be used to measure their trace element compositions using fast in situ and low-cost LA-ICP-MS analysis with similar data quality as achieved with normal solution ICP-MS analysis.