Kinetics of the olivine→spinel transition: Implications to deep-focus earthquake genesis

The rate of the olivine→spinel transition at high overpressure increases with diminishing grain size, or increasing shear stress, temperature, and possibly pressure. The transition rate is higher in Fe-rich compositions than in Mg-rich compositions, and it can be greatly increased by adding water or other mineralizers. Of all variables controlling the kinetics of the olivine→spinel transition in the mantle, temperature is the most critical. The olivine→spinel transition can be suppressed below 500°C in Mg-rich compositions, even in geological period of time. Since the temperature within a downgoing slab varies greatly according to different models of calculation, it is not clear at this stage whether the temperature is low enough to suppress the olivine→spinel transition. If the olvine→spinel transition cannot be suppressed, it may not be responsible for the genesis of deep-focus earthquakes. However, the rise of the olivine-spinel boundary in the cold interior of downgoing slabs provides an additional driving force for the plunging of these slabs. The distortion of the olivine-spinel boundary may also control the stress distribution in downgoing slabs and may be responsible for the observed alignment of principal stress axes of deep-focus earthquakes.     

Homologous temperature of olivine: Implications for creep of the upper mantle and fabric transitions in olivine

The homologues temperature of a crystalline material is defined as T/T_m, where T is temperature and T_m is the melting(solidus) temperature in Kelvin. It has been widely used to compare the creep strength of crystalline materials. The melting temperature of olivine system,(Mg,Fe)_2SiO_4, decreases with increasing iron content and water content, and increases with confining pressure. At high pressure, phase transition will lead to a sharp change in the melting curve of olivine. After calibrating previous melting experiments on fayalite(Fe_2SiO_4), the triple point of fayalite-Fe_2SiO_4 spinel-liquid is determined to be at 6.4 GPa and 1793 K. Using the generalized means, the solidus and liquidus of dry olivine are described as a function of iron content and pressure up to 6.4 GPa. The change of T/T_m of olivine with depth allows us to compare the strength of the upper mantle with different thermal states and olivine composition. The transition from semi-brittle to ductile deformation in the upper mantle occurs at a depth where T/T_m of olivine equals 0.5. The lithospheric mantle beneath cratons shows much smaller T/T_m of olivine than orogens and extensional basins until the lithosphere-asthenosphere boundary where T/T_m 0.66, suggesting a stronger lithosphere beneath cratons. In addition, T/T_m is used to analyze deformation experiments on olivine. The results indicate that the effect of water on fabric transitions in olivine is closely related with pressure. The hydrogen-weakening effect and its relationship with T/T_m of olivine need further investigation. Below 6.4 GPa(200 km), T/T_m of olivine controls the transition of dislocation glide from [100] slip to [001] slip. Under the strain rate of 10~(-12)–10~(-15) s~(-1) and low stress in the upper mantle, the [100](010) slip system(A-type fabric) becomes dominant when T/T_m 0.55–0.60. When T/T_m 0.55–0.60, [001] slip is easier and low T/T_m favors the operation of [001](100) slip system(C-type fabric). This is consistent with the widely observed A-type olivine fabric in naturally deformed peridotites, and the C-type olivine fabric in peridotites that experienced deep subduction in ultrahigh-pressure metamorphic terranes. However, the B-type fabric will develop under high stress and relatively low T/T_m. Therefore, the homologues temperature of olivine established a bridge to extrapolate deformation experiments to rheology of the upper mantle. Seismic anisotropy of the upper mantle beneath cratons should be simulated using a four-layer model with the relic A-type fabric in the upper lithospheric mantle, the B-type fabric in the middle layer, the newly formed A- or B-type fabric near the lithosphere-asthenosphere boundary, and the asthenosphere dominated by diffusion creep below the Lehmann discontinuity. Knowledge about transition mechanisms of olivine fabrics is critical for tracing the water distribution and mantle flow from seismic anisotropy.     

Dose recovery tests using Ti-related ESR signals in quartz: First results

Experimental results from ESR dose recovery tests using Ti-related absorption lines in sedimentary quartz will be presented and discussed. The tests were performed on two Late Pleistocene Australian dune sands with a known burial dose based on OSL measurements. The regenerative dose technique (after thermal annealing) was used to detect any differences between ESR estimates of the natural dose, and ESR estimates of an equal laboratory gamma-dose. The studied absorption lines show consistent results with regard to the laboratory dose, but inconsistencies regarding the recovery of the natural dose. Line-shape changes between the natural and artificially induced ESR signals are a matter of concern. These changes are probably the result of the different relative abundance of Ti–H and Ti–Li centres in the naturally irradiated and artificially irradiated sample.     

上地幔橄榄石电导率影响因素及高压相变初探

人们对上地幔标志性矿物橄榄石的认识，基本上是经历一个从直流到交流，又从交流到阻抗谱的认识过程．本文主要通过对橄榄石电导率的影响因素出发，包括温度、压力、铁镁比值、氧逸度等因素的影响和制约．用目前国际高温高压学术界公认的阻抗谱技术探讨了橄榄石在高压下的相变，并在此基础上，进一步研究地球物理学家非常关心的热点问题-冷地幔的实验室的模拟，并作了充分的论证．     

Two Brazilian archaeological sites investigated by GPR: Serrano and Morro Grande

The application of the ground penetrating radar (GPR) at two archaeological sites, Serrano and Morro Grande, situated in Araruama County, Rio de Janeiro, Brazil, aids the study of a prehistoric indigenous culture, associated with the “Tupinambá” that inhabited the region during prehistoric times.The archaeological remains of the study area are mainly characterized by pottery artifacts for several uses, including funerary urns, which were buried within layers of sand and clay. Several profiles were acquired using a RAMAC system, with a 200 and 400 MHz frequency antennae. At the Serrano site, the profiles were acquired around some partially exposed pottery shards, due to sand exploitation. The resultant profiles provided a response model to guide the interpretation of new profiles acquired at other sites in the area, which present similar characteristics.The results showed the great importance of the dielectric permittivity contrast which exists between the targets and the host media, in order for possibly significant features to be identified in radar data.     

High-pressure recovery of olivine: implications for creep mechanisms and creep activation volume

High-temperature and high-pressure recovery experiments were made on experimentally deformed olivines at temperatures of 1613–1788 K and pressures of 0.1 MPa to 2.0 GPa. In the high-pressure experiments, a piston cylinder apparatus was used with BN and NaCl powder as the pressure medium, and the hydrostatic condition of the pressure was checked by test runs with low dislocation density samples. No dislocation multiplication was observed. The kinetics of the dislocation annihilation process were examined by different initial dislocation density runs and shown to be of second order, i.e.

$\text{d}\text{ρ}\text{d}\text{t}\text{= ?p}{}^2\text{K}{}_0\text{exp}\text{[?(}\text{E}{}^1\text{+PV}{}^1\text{RT}\text{]}$

where ρ is the dislocation density, k₀ is a constant, $\text{E}{}^1\text{and}\text{V}{}^1$ are the activation energy and volume respectively, and P, R and T are pressure, gas constant and temperature, respectively. Activation energy and volume were estimated from the temperature and pressure dependence of the dislocation annihilation rate as $\text{E}{}^1\text{=389±59}\text{kJ mol}{}^{\mathrm{?1}}$ and $\text{V}{}^1\text{=14±2}\text{cm}{}^3\text{mol}{}^{\mathrm{?1}}$, respectively.The diffusion constants relevant to the dislocation annihilation process were estimated from a theoretical relation k=αD where $\text{k=k}{}_0\text{exp}\text{[?(E}{}^1\text{+ PV}{}^1\text{)/RT], D}$ is the diffusion constant and α is a non-dimensional constant of ca. 300. The results agree well with the self-diffusion constant of oxygen in olivine. This suggests that the dislocation annihilation is rate-controlled by the (oxygen) diffusion-controlled dislocation climb.The mechanisms of creep in olivine and dry dunite are examined by using the experimental data of static recovery. It is suggested that the creep of dry dunite is rate-controlled by recovery at cell walls or at grain boundaries which is rate-controlled by oxygen diffusion. Creep activation volume is estimated to be 16±3 cm³ mol^?1.     

Experimental investigation of phase transformations of olivine and enstatite at the lower part of the mantle transition zone: Implications for structure of the 660 km seismic discontinuity

High-pressure polymorphs of olivine and enstatite are major constituent minerals in the mantle transition zone(MTZ).The phase transformations of olivine and enstatite at pressure and temperature conditions corresponding to the lower part of the MTZ are import for understanding the nature of the 660 km seismic discontinuity.In this study,we determine phase transformations of olivine(MgSi2O4) and enstatite(MgSiO3) systematiclly at pressures between 21.3 and 24.4 GPa and at a constant temperature of 1600℃.The most profound discrepancy between olivine and enstatite phase transformation is the occurency of perovskite.In the olivine system,the post-spinel transformation occures at 23.8 GPa,corresponding to a depth of 660 km.In contrast,perovskite appears at 23 GPa(640 km) in the enstatite system.The ~1 GPa gap could explain the uplifting and/or splitting of the 660 km seismic discountinuity under eastern China.     

Recuperated-OSL dating of quartz from Aegean (South Greece) raised Pleistocene marine sediments: current results

The purpose of this work is to provide an OSL dating framework for raised marine sequences in the South-West coast of Greece during Upper Quaternary. Paleontological investigations and Geographic Information System (GIS) analysis on elevated marine landforms have proved that a record of uplift and eustacy exists in South Greece since the Early Pleistocene (～1.6 Ma). Hereby, we test the suitability of the SAR methodology for recuperated-OSL (Re-OSL), proposed by Wang et al. (2007), on coarse-grained quartz aliquots from emerged nearshore outcrops. Protocol's performance is examined on the basis of signal characteristics, dose response, sensitivity changes, recovery of known doses and Re-OSL bleachability to sunlight. The accuracy of the Re-OSL dates calculated for natural samples is also discussed.     

Variability in quartz OSL signals caused by measurement uncertainties: Problems and solutions

We simulated the variability in measured quartz optically stimulated luminescence (OSL) signals and dose response curves (DRCs) caused by measurement uncertainties, including counting statistics and instrumental irreproducibility. We find that these measurement errors can give rise to large variations in the observed luminescence signal and contribute to among-aliquot or among-grain scatter in DRCs and equivalent dose (D_e) values. Different measurement systems (i.e., luminescence readers) may have different counting statistics properties and, hence, may exhibit differing extents of variation in the observed OSL signal, even for the same sample. Our simulation shows that the random measurement uncertainties may result in some grains or aliquots being ‘saturated’ (that is, the measured natural signal is consistent with, or lies above, the saturation level of the measured DRC) and that the rejection of these ‘saturated’ grains may result in a truncated D_e distribution, with D_e underestimation for samples with natural doses close to saturation (e.g., twice the characteristic saturation dose, D₀). We propose a new method to deal with this underestimation problem, in which standardised growth curves (SGCs) are established and the weighted-mean natural signal (L_n/T_n) from all measured grains is projected on to the corresponding SGCs to determine D_e. Our simulation results show that this method can produce reliable D_e estimates up to 5D₀, which is far beyond the conventional limit of ∼2D₀ using the standard SAR procedure.     

Luminescence characteristics of Scandinavian quartz,their connection to bedrock provenance and influence on dating results

The success of optically stimulated luminescence (OSL) dating relies to a large extent on suitable characteristics of the analysed mineral, in this case quartz. Previous OSL dating of Quaternary sediments in Scandinavia has shown that quartz characteristics vary widely across the region, resulting in dating studies with varied success. The aim of this study is to provide an overview of quartz luminescence characteristics in Sweden and Norway, evaluate their effect on dating results and discuss the underlying causes of their variability.A qualitative assessment of luminescence signal characteristics of quartz from Late Quaternary sediment deposits, from a range of geological and geographical settings, has been made by re-analysing data from samples previously dated at the Lund Luminescence Laboratory, Sweden. This allowed a general characterisation of signals and a study of the relationship of these properties to dating result ‘quality’. To quantify the results, selected samples were further analysed with single-grain measurements and with small aliquots.The results show that the average luminescence signal from quartz is fairly dim but dominated by a fast signal component and changes little during measurement. Dose determination precision is ∼4% for 8-mm aliquots and ∼6% for 2-mm aliquots. However, the luminescence signal characteristics have a spatial variation across Sweden and Norway, which appears to correlate with large-scale bedrock units. In areas of sedimentary bedrock outside the Scandinavian mountains and within the Blekinge-Bornholm province, the quartz is brighter and has a stronger fast signal component, while in the Caledonian orogenic belt, the signal is very weak and lacks a fast component. These differences lead to a range in precision of doses, from ∼2% to >40% (for doses in the order of 5–400 Gy), and in the number of rejected aliquots (0–100%) depending on location, but also implies that quartz luminescence can be used as a provenance indicator in part of Sweden and Norway.     

Kinetic modeling of the coesite–quartz transition in an elastic field and its implication for the exhumation of ultrahigh-pressure metamorphic rocks

The present paper examines a kinetic model of the coesite–quartz transition under an elastic field. This model is applied to discuss the possible exhumation path of ultrahigh-pressure (UHP) metamorphic rocks. By incorporating the model of transition kinetics into a three-shelled composite sphere model in linear elasticity, the internal stresses in coesite, quartz, and garnet shells were calculated for given external pressure ( P )–temperature ( T ) paths. The occurrence of rupture provides a constraint on the temperature and the amount of quartz inverted from coesite at the rupture for each P–T path. Comparison of calculated results and the natural occurrence of coesite inclusion from the Dora Maira Massif, containing ∼ 27% quartz at the rupture, enables us to constrain the possible exhumation path and possible transition kinetics. A steep decompression path with slow transition kinetics is most favorable, which is consistent with the estimated P–T path during exhumation for most UHP metamorphic rocks.     

Residual thermoluminescence for sun-bleached quartz: Dependence on pre-exposure radiation dose

Quartz grains zeroed by heat and subsequently irradiated—either in the laboratory, or naturally in volcanic deposits—were exposed to sunlight for several months prior to TL measurements. As a result, the residual TL, in either the Blue or the Red, appeared to grow significantly with the radiation-dose experienced by the grains since zeroing. In another experiment, unheated sedimentary quartz grains, sampled at the very surface of a coastal sand-dune, were irradiated and subsequently exposed to sunlight before TL measurements. The residual Red-TL still exhibited a strong dose-dependence, but at a level that was higher than the natural signal. Finally, it was concluded that the so-called “unbleachable TL” is not a constant, intrinsic quantity, for a given quartz sample.

Because the residual TL is strongly correlated with the radiation dose experienced since the last zeroing by heat, it can theoretically be quoted in terms of “dose”, as is sometimes done. However, this raises the question of its correct evaluation, and, on the other hand, there is a risk of misinterpreting its physical meaning in the case of unheated sediments.

In the context of sediment dating, the experimentally-estimated residual signal that is routinely subtracted from the total signal for palaeodose estimation is, therefore, higher than the signal that would have been measured prior to burial. Such an effect introduces an error that can be significant for Red TL. It can be corrected for, at least approximately, from experimental exploration of the dose-dependence of the residual TL.     

Vehicle-derived emissions and pollution on the road autovia 2 investigated by rock-magnetic parameters: A case study from Argentina

In this work, we carried out a preliminary study of traffic-derived pollutants from primary sources (vehicles), and on roads (paved area), road borders and surroundings areas. The study is focussed on the identification, distribution and concentration of pollutants and magnetic carriers. Magnetic parameters and their analyses suggest that the magnetic signal of vehicle-derived emissions is controlled by a magnetite-like phase. Magnetic grain size estimations reveal the presence of fine particles (0.1–5 μm) that can be inhaled and therefore are dangerous to human health. Magnetic susceptibility results (about 175 × 10⁻⁵ SI) show a higher magnetic concentration — magnetic enhancement — in the central area of the tollbooth line that is related to higher traffic. In addition, magnetic susceptibility was computed on several roadside soils along a length of 120 km and used to generate a 2-D contour map, which shows higher magnetic values (100–200 10⁻⁵ SI) near the edge of the road. The observed distribution of magnetic values indicates that magnetic particles emitted by vehicles are accumulated and mainly concentrated within a distance of several meters (1–2 m) from the edge of the road. In consequence, the magnetic susceptibility parameter seems to be a suitable indicator of traffic-related pollution. Non-magnetic studies show an enrichment of some trace elements, such as Ba, Cr, Cu, Zn and Pb, that are associated with traffic pollution. Furthermore, statistical correlations between the content of toxic trace metals and magnetic variables support the use of magnetic parameters as potential proxies for traffic-related pollution in this study area.     

Noble gas systematics for coexisting glass and olivine crystals in basalts and dunite xenoliths from Loihi Seamount

Noble gas isotopes including ³He/⁴He, ⁴⁰Ar/³⁶Ar and Xe isotope ratios were determined for coexisting glass and olivine crystals in tholeiitic and alkalic basalts and dunite xenoliths from Loihi Seamount.Glass and coexisting olivine crystals have similar ³He/⁴He ratios (2.8–3.4) × 10^?5, 20 to 24 times the atmospheric ratio (R_A), but different ⁴⁰Ar/³⁶Ar ratios (400–1000). Based on the results of noble gas isotope ratios and microscopic observation, some olivine crystals are xenocrysts. We conclude that He is equilibrated between glass and olivine xenocrysts, but Ar is not.The apparent high ³He/⁴He ratio (3 × 10^?5; = 21 R_A) coupled with a relatively high ⁴⁰Ar/³⁶Ar ratio (4200) for dunite xenoliths (KK 17-5) may be explained by equilibration of He between MORB-type cumulates and the host magma.Except for the dunite xenoliths, noble gas data for these Loihi samples are compatible with a model in which samples from hot spot areas may be explained by mixing between P (plume)-type and M (MORB)-type components with the addition of A (atmosphere)-type component.Excess ¹²⁹Xe has not been observed due to apparent large mass fractionation among Xe isotopes.     

Reflection and transmission coefficients for transition layers

Summary Formulae are derived for the reflection and transmission coeficients of plane elastic waves for a transition layer. Haskell's technique and the so-called delta matrices[5, 7] are used for this purpose. No problems are encountered in deriving the reflections and transmission coefficients from Haskell's matrices[3]. However, in some cases Haskell's matrices do not guarantee the accuracy required. For this reason attention is mainly devoted to deriving the reflection and transmission coefficients from the delta matrices. In deriving the transmission coefficients use is made of the fact that some3×3 subdeterminants of the delta matrices are squares of the3×3 subdeterminants of Haskell's matrices.     

华南中部地幔转换带厚度异常：海南地幔柱？

本文利用84个布设于华南中部地区的流动地震台站记录的239个远震地震事件的波形资料,通过接收函数方法得到了华南大陆中部地区的地幔410 km和660 km间断面起伏与地幔转换带厚度变化.结果表明在华夏块体下方转换带厚度明显减薄10~25 km,尤其是在南部靠近海岸线附近(23.7°N, 114.5°E)存在一个直径约200 km的转换带厚度异常区域(减薄约25 km),可能揭示海南地幔柱在该处上涌穿透660 km间断面进入地幔,在地幔转换带向周围扩散,热的地幔物质穿过410 km间断面继续上涌,造成了华夏块体下方上地幔大范围的低速异常,以及在雷州半岛和沿岸造成大范围的新生代玄武岩活动.

     

Comments on: “The partitioning of nickel between olivine and silicate melt” by S.R. Hart and K.E. Davis

We study a set of very high-quality records of first-order overtone Rayleigh waves from the deep-focus earthquake of September 29, 1973, in the Japan Sea. Standard surface wave techniques are used with these overtones, treated as individual seismic phases, to retrieve radiation pattern, Q, moment and phase velocity. A figure of M₀ = (6.7 ± 1.4) × 10²⁷dyn-cm is obtained, in total agreement with published values computed from either P waves, or fundamental Rayleigh waves. We also demonstrate the feasibility of using overtones as individual seismic phases in order to investigate their dispersion and attenuation properties.     

Cross-check of the dating results obtained by ESR and IR-OSL methods: Implication for the Pleistocene palaeoenvironmental reconstructions

The consistency of the dating results obtained by different methods, both absolute and relative, is investigated. The main absolute methods referred to in this article are the mollusc-based electron spin resonance (ESR) and feldspar-based infrared optically-stimulated luminescence (IR-OSL) dating methods used in the Research Laboratory for Quaternary Geochronology, Tallinn University of Technology. It was shown that the parallel comparative dating by these two methods yield, essentially, consistent results. U–Th age determinations were performed in some cases for better understanding of the uranium behaviour in the shells during their burial history. ESR and U–Th dating results obtained on one and the same shell-type, both marine and land snails, coincide well in the case if shells behaved as a close system during their burial history. It is also shown that the numerical dating results obtained on warm and cold climate-related deposits, correlate well with the corresponding palaeoclimate signals derived from continuous records of the climate and environmental evolution, constructed on the base of the most common of the relative dating methods used in Quaternary studies. Based on good consistency of the results obtained by all methods used in the present comparative study, we conclude that there is a good potential to improve our understanding of the middle to late Pleistocene palaeoenvironmental evolution in Northern Eurasia, with a special focus on the climatically highly sensitive Eurasian Arctic palaeo-shelf area.