Physical model of source region of subduction zone volcanics

The thermal structure of a generic subduction zone is investigated to elucidate the source region of subduction zone volcanics. The steady state thermal field is evaluated for a model subduction zone where the plates are prescribed by kinematic boundary conditions, such that the subducting slab induces a flow in the mantle wedge. The resulting model suggests that the oceanic crust of the downgoing slab is not melted extensively, if at all, and hence is not the source of subduction zone magmatism (with the possible exception of the special case of very young oceanic crust). The temperature in the mantle wedge is high enough to produce melting at the amphibole-buffered peridotite solidus. It is proposed that the combination of vertical motion of water as a free phase and the transport of hydrous phases (e.g., amphiboles) by the slab-induced mantle wedge flow lead to the net transport of water being horizontal, across the mantle wedge from the slab. Provided the subducting oceanic crust enters the asthenosphere at a velocity > 6(±2) cm/yr, the mantle wedge will be hot enough at the limit of the lateral water transport mechanism to generate melting at the amphibole-buffered solidus. The model was then extended to include the effect of localized sources of buoyancy (melt, residue, etc.) as a stationary body force, to investigate the possibility of reversing the slab-induced flow. Best estimates of the buoyancy sources and appropriate viscosity in the wedge suggest that there is likely to be only a weak modulation of the slab-induced flow unless the slab and wedge are locally decoupled, for instance by shear heating, the presence of water, or dehydration/hydration reactions. If there is decoupling, then it is possible for there to be an appreciable reversal of the slab-induced flow. Such an appreciable reversal of flow, if it persists, leads to cooling of the mantle wedge. Hence flow reversal cannot be a steady state mechanism. Instead it would lead to a cycle in the melting with a period of O(1 m.y.). The time dependence of a model with appreciable flow reversal would be reinforced by the need to clear the wedge of infertile material

Mantle melting as a function of water content beneath back-arc basins

Subduction zone magmas are characterized by high concentrations of H_(2)O, presumably derived from the subducted plate and ultimately responsible for melting at this tectonic setting. Previous studies of the role of water during mantle melting beneath back-arc basins found positive correlations between the H_(2)O concentration of the mantle (H_(2)O_o ) and the extent of melting (F), in contrast to the negative correlations observed at mid-ocean ridges. Here we examine data compiled from six back-arc basins and three mid-ocean ridge regions. We use TiO_2 as a proxy for F, then use F to calculate H_(2)O_o from measured H_(2)O concentrations of submarine basalts. Back-arc basins record up to 0.5 wt % H_(2)O or more in their mantle sources and define positive, approximately linear correlations between H_(2)O_o and F that vary regionally in slope and intercept. Ridge-like mantle potential temperatures at back-arc basins, constrained from Na-Fe systematics (1350°–1500°C), correlate with variations in axial depth and wet melt productivity (∼30–80% F/wt % H_(2)O_o ). Water concentrations in back-arc mantle sources increase toward the trench, and back-arc spreading segments with the highest mean H_(2)O_o are at anomalously shallow water depths, consistent with increases in crustal thickness and total melt production resulting from high H_(2)O. These results contrast with those from ridges, which record low H_(2)O_o (<0.05 wt %) and broadly negative correlations between H_(2)O_o and F that result from purely passive melting and efficient melt focusing, where water and melt distribution are governed by the solid flow field. Back-arc basin spreading combines ridge-like adiabatic melting with nonadiabatic mantle melting paths that may be independent of the solid flow field and derive from the H_(2)O supply from the subducting plate. These factors combine significant quantitative and qualitative differences in the integrated influence of water on melting phenomena in back-arc basin and mid-ocean ridge settings

Trace Element Partitioning in HP-LT Metamorphic Assemblages during Subduction-related Metamorphism, Ile de Groix, France: a Detailed LA-ICPMS Study

Devolatilization reactions and subsequent transfer of fluid from subducted oceanic crust into the overlying mantle wedge are important processes, which are responsible for the specific geochemical characteristics of subduction-related metamorphic rocks, as well as those of arc magmatism. To better understand the geochemical fingerprint induced by fluid mobilization during dehydration and rehydration processes related to subduction zone metamorphism, the trace element and rare earth element (REE) distribution patterns in HP-LT metamorphic assemblages in eclogite-, blueschist- and greenschist-facies rocks of the Ile de Groix were obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis. This study focuses on 10 massive basic rocks representing former hydrothermally altered mid-ocean ridge basalts (MORB), four banded basic rocks of volcano-sedimentary origin and one micaschist. The main hosts for incompatible trace elements are epidote (REE, Th, U, Pb, Sr), garnet [Y, heavy REE (HREE)], phengite (Cs, Rb, Ba, B), titanite [Ti, Nb, Ta, REE; HREE > LREE (light REE)], rutile (Ti, Nb, Ta) and apatite (REE, Sr). The trace element contents of omphacite, amphibole, albite and chlorite are low. The incompatible trace element contents of minerals are controlled by the stable metamorphic mineral assemblage and directly related to the appearance, disappearance and reappearance of minerals, especially epidote, garnet, titanite, rutile and phengite, during subduction zone metamorphism. Epidote is a key mineral in the trace element exchange process because of its large stability field, ranging from lower greenschist- to blueschist- and eclogite-facies conditions. Different generations of epidote are generally observed and related to the coexisting phases at different stages of the metamorphic cycle (e.g. lawsonite, garnet, titanite). Epidote thus controls most of the REE budget during the changing P-T conditions along the prograde and retrograde path. Phengite also plays an important role in determining the large ion lithophile element (LILE) budget, as it is stable to high P-T conditions. The breakdown of phengite causes the release of LILE during retrogression. A comparison of trace element abundances in whole-rocks and minerals shows that the HP-LT metamorphic rocks largely retain the geochemical characteristics of their basic, volcano-sedimentary and pelitic protoliths, including a hydrothermal alteration overprint before the subduction process. A large part of the incompatible trace elements remained trapped in the rocks and was recycled within the various metamorphic assemblages stable under changing metamorphic conditions during the subduction process, indicating that devolatilization reactions in massive basic rocks do not necessarily imply significant simultaneous trace element and REE releas

from oceanic to continental subduction implications for the geochemical and redox evolution of the supra subduction mantle

n/

Adakitic Dacites Formed by Intracrustal Crystal Fractionation of Water-rich Parent Magmas at Nevado de Longaví Volcano (36·2°S; Andean Southern Volcanic Zone, Central Chile)

The mid-Holocene eruptive products of Nevado de Longaví volcano (36·2°S, Chile) are the only reported occurrence of adakitic volcanic rocks in the Quaternary Andean Southern Volcanic Zone (33-46°S). Dacites of this volcano are chemically distinct from other evolved magmas of the region in that they have high La/Yb (15-20) and Sr/Y (60-90) ratios and systematically lower incompatible element contents. An origin by partial melting of high-pressure crustal sources seems unlikely from isotopic and trace element considerations. Mafic enclaves quenched into one of the dacites, on the other hand, constitute plausible parental magmas. Dacites and mafic enclaves share several characteristics such as mineral chemistry, whole-rock isotope and trace element ratios, highly oxidizing conditions (NNO + 1·5 to >NNO + 2, where NNO is the nickel-nickel oxide buffer), and elevated boron contents. A two-stage mass-balance crystal fractionation model that matches both major and trace elements is proposed to explain magmatic evolution from the least evolved mafic enclave to the dacites. Amphibole is the main ferromagnesian phase in both stages of this model, in agreement with the mineralogy of the magmas. We also describe cumulate-textured xenoliths that correspond very closely to the solid assemblages predicted by the model. We conclude that Nevado de Longaví adakitic dacites are the products of polybaric fractional crystallization from exceptionally water-rich parent magmas. These basaltic magmas are inferred to be related to an exceptionally high, but transient input of slab-derived fluids released from serpentinite bodies hosted in the oceanic Mocha Fracture Zone, which projects beneath Nevado de Longaví. Fractional crystallization that is modally dominated by amphibole, with very minor garnet extraction, is a mechanism for generating adakitic magmas in cold subduction zones where a high flux of slab-derived fluids is presen

Fluid flow in the subduction channel: Tremolite veins and associated blackwalls in antigoritite (Villa Clara serpentinite mélange, Cuba)

Exotic blocks of massive antigorite-serpentinite (antigoritite) document a deep-seated subduction channel in the Villa Clara serpentinite-matrix mélange, central Cuba. The petrological and geochemical characteristics of antigoritite allow distinguishing two types of rock: i) antigoritite and ii) dolomite-bearing antigoritite. Both types are intimately related in field exposures and represent deep peridotite infiltrated by H2O-CO2 fluid mixtures that triggered antigoritization and local carbonation. Fluid infiltration continued after antigoritization forming a vein network as a potential response to hydrofracturing that precipitated tremolitite in the veins and triggered fluid-antigoritite reaction forming blackwalls. The mineralogical and chemical zoning in the blackwalls (Atg + Chl + Tr adjacent to antigoritite and Chl + Tr adjacent to the tremolitite vein) attest for multi-step metasomatic processes during fluid-rock interaction characterized by advection of infiltrating fluid towards the blackwall and, possibly, by diffusion out of the blackwall towards the fluid-filled vein. Tentative thermodynamic modeling of the blackwall domain Atg + Chl + Tr points vein network formation at 400–500 °C and 5–10 kbar during exhumation in the subduction channel, suggesting the infiltration of deep-seated pressurized fluid that triggered hydrofracturing. The chemical compositions of antigoritites, veins and blackwalls indicate a LILE- and LREE-enriched fluid evolved from the subducting plate, while Srsingle bondNd isotope systematics are compatible with an external fluid composed of a mixture of fluids evolved from sediments and, probably to a lesser extent, altered oceanic crust.This research was funded by projects MICINN PID2019-105625RB-C21 (co-funded by Fondo Europeo de Desarrollo Regional, FEDER), Junta de Andalucía P20_00550, Catalonian project SGR 2014-1661 and the University of Granada. LD acknowledges PhD grant BES-2013-063205 of the Spanish Ministry of Economy and Competitiveness and scholarship of Fundació Universitària Agustí Pedro i Pons. Funding for open access charge: Universidad de Granada / CBUA

Lattice-Preferred Orientation and Microstructures of Minerals and Their Implications for Seismic Anisotropy

The lattice-preferred orientation (LPO) of minerals is important for interpreting seismic anisotropy, which occurs in the Earth’s crust and mantle, and for understanding the internal structure of the deep interior of the Earth. The characterization of microstructures, including LPO, grain size, grain shape, and misorientation, is important to determine the deformation conditions, deformation histories, kinematics, and seismic anisotropies in the crust and mantle The articles in this Special Issue prove that studies of LPO and microstructures of minerals and rocks are a major research area and provide a foundation for interpreting seismic anisotropy in the crust, mantle, and subduction zones. Therefore, the authors hope that this Special Issue encompassing recent advances in the measurement of LPOs of different minerals under various tectonic settings will be a fundamental and valuable resource for the readers and researchers interested in exploring the deformation conditions of minerals and rocks, as well as the interpretation of seismic anisotropy in the crust, mantle, and subduction zones

Halogen (F, Cl) concentrations and Sr-Nd-Pb-B isotopes of the basaltic andesites from the southern Okinawa Trough: implications for the recycling of subducted serpentinites

Author Posting. © American Geophysical Union, 2021. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Solid Earth 126(3), (2021): e2021JB021709, https://doi.org/10.1029/2021JB021709.Serpentinites are increasingly recognized as playing an important role in the global geochemical cycle. However, discriminating the contributions of serpentinites to arc magmas from those of other subduction components is challenging. The Okinawa Trough is a back-arc basin developed behind the Ryukyu subduction zone, where magmas are extensively affected by sediment subduction. In this study, we reported the F-Cl concentrations and Sr-Nd-Pb-B isotopes of basaltic andesites from the Yaeyama Graben, Yonaguni Graben, and Irabu Knoll in the southern Okinawa Trough. The Irabu Knoll lavas show the most enrichment of fluid-mobile elements and F ± Cl, and have the heaviest B isotopes (δ11B: +6.6 ± 1.5‰). They also have decoupled Sr-Nd isotopes: higher 87Sr/86Sr (∼0.7049) but have no obvious decrease of 143Nd/144Nd (∼0.5128). Results from slab dehydration modeling and mixing calculations suggest that the heavy δ11B in the Irabu Knoll lavas is not consistent with fluids derived from altered oceanic crust (AOC), sediments, or wedge serpentinites (formed in the mantle wedge), but rather from slab serpentinites (formed within the subducting plate); sediments control the subduction input of Nd, whereas the decoupled Sr-Nd isotopes are most likely due to the excess radiogenic Sr carried by AOC fluids. Our results imply that recycling of serpentinite fluids and AOC fluids are usually coupled in subduction zones, as the arc lavas influenced by subducted serpentinite generally show Sr-Nd isotopes decoupling. The large variation of Sr-Nd-B isotopes observed in a relatively localized area is consistent with a focused migration through the mantle wedge of components from multiple sources.This study was funded by the National Natural Science Foundation of China (91958213), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB42020402), the China Postdoctoral Science Foundation (2019M662454), the Shandong Provincial Natural Science Foundation, China (ZR2020QD068 and ZR2020MD068), the International Partnership Program of the Chinese Academy of Sciences (133137KYSB20170003), the Special Fund for the Taishan Scholar Program of Shandong Province (ts201511061), and the China Scholarship Council (201709410550).2021-09-1

청색편암, 에클로자이트, 그리고 감람암의 변형 미세구조와 암석조직: 섭입대의 지진파 비등방성에 대한 적용

학위논문 (박사)-- 서울대학교 대학원 : 지구환경과학부, 2014. 8. 정해명.과거 섭입대였던 두 지역(중국 North Qilian 조산운동대와 Qinling 조산운동대)에서 채취한 청색편암, 에클로자이트, 그리고 맨틀 감람암의 변형 미세구조와 암석구조 분석 및 지화학 분석을 통하여, 섭입대 환경하에서 지구의 동역학적 과정들과 지진파 비등방성을 이해하고자 하였다. 암질과 변형 정도가 각각 다른 청색편암과 에클로자이트에서 계산한 지진파 특성들은 다음과 같은 사실을 지시한다. (1) 청색편암과 엽리가 잘 발달된 에클로자이트들은 지진파 비등방성이 크며, 괴상의 에클로자이트와 감람암들과 비교할 때 대략 3-12% 낮은 지진파 속도를 가진다. 이를 통하여 상부 지진파 저속도층의 점진적인 감소와 섭입 깊이에 따른 지진파 비등방성의 소실을 설명할 수 있다. (2) 해구에 평행한 방향의 지진파 비등방성은 큰 각도로 (> 45‒60°) 섭입하는 해양지각에 의하여 야기될 수 있고, 그에 따른 지연시간은 0.03‒0.09초 (괴상 로소나이트 청색편암의 최저값) 에서 0.1‒0.3초 (엽리가 잘 발달된 녹렴석 청색편암의 최대값) 정도이다. (3) 청색편암과 에클로자이트의 지진파 반사 계수 차이가 매우 크기 때문에 (Rc = 0.04‒0.20), 얕은 깊이의 청색편암 위주의 부가체 안에 있는 거대한 에클로자이트 암괴가 고분해능 지진파 반사 실험을 통하여 관찰될 가능성이 있다. 맨틀 감람암의 경우 North Qilian 조산운동대와 Qinling 조산운동대에서의 전호 암석권 맨틀에서 ~25-30% 정도의 부분용융 이후에 남은 내열성의 잔여물들로, 하즈버자이트와 더나이트에 해당하며 B, C, A-/D-타입과 같은 다양한 감람석 암석조직이 관찰된다. 다수에 해당하는 A-/D-타입 감람석 암석조직은 해구에 평행한 방향의 지진파 비등방성을 약화시키고, 전호 맨틀에서의 사문석화 작용과 관련된 지진파 특성에 영향을 준다. 반면에, 물이 많은 환경에서 만들어지는 C-타입과 B-타입 감람석 암석조직의 경우 각각 저응력 고온 환경과 고응력 고온 환경에서 발달된다고 알려져있다. C-타입에서 B-타입으로의 감람석 암석조직의 변화는, 섭입대의 맨틀웨지에서 해구방향으로 생기는 삭박작용과 냉각 작용으로 인해 생길 수 있는, 빠른 지진전단파의 편광 방향이 해구에 수직한 방향에서 해구에 평행한 방향으로의 변화를 반영한다. 그러나 전호 맨틀의 발달과정 중에서 물이 많은 환경(B-, C-타입)과 물이 적은 환경(A-, D-타입)에서의 감람석 암석조직 사이의 포괄적인 관계에 대해서는 아직 모호한 부분이 있고, 앞으로 좀 더 실험실에서 암석의 미세구조 연구와 야외 지질조사가 추가되어야 할 것이다.CONTENTS CHAPTER 1: Introduction . 1 CHAPTER 2: Petro-fabrics and seismic properties of blueschist and eclogite in the North Qilian suture zone, NW China . 7 Abstract . 7 2.1. Introduction . 8 2.2. Geological setting . 12 2.3. Petrography . 17 2.3.1. Blueschist . 17 2.3.2. Eclogite . 20 2.4. Mineral chemical compositions and P-T estimations . 22 2.5. Lattice preferred orientations . 25 2.5.1. LPO of glaucophane . 27 2.5.2. LPO of epidote . 30 2.5.3. LPO of omphacite . 32 2.5.4. LPO of garnet . 32 2.5.5. LPO of phengite and quartz . 32 2.6. Rock seismic properties . 35 2.6.1. Glaucophane polycrystals . 37 2.6.2. Epidote polycrystals . 37 2.6.3. Omphacite polycrystals . 39 2.6.4. Garnet, phengite and quartz polycrystals . 42 2.6.5. Whole-rock . 44 2.7. Discussions . 49 2.7.1. Low velocity upper layer in subducting slab 49 2.7.2. Polarization anisotropy in subduction zone . 53 2.7.3. Detectability of eclogite in the subduction channel . 57 2.8. Conclusions . 62 CHAPTER 3: Microstructures and petro-fabrics of lawsonite blueschist in the North Qilian suture zone, NW China . 64 Abstract . 64 3.1. Introduction . 65 3.2. Geological setting . 68 3.3. Description of samples . 72 3.3.1. Massive lawsonite blueschist . 72 3.3.2. Foliated lawsonite blueschist . 74 3.4. Lattice preferred orientations . 76 3.4.1. LPO of glaucophane . 78 3.4.2. LPO of lawsonite . 80 3.5. Vorticity analysis . 83 3.6. Rock seismic properties . 86 3.6.1. Single crystal . 88 3.6.2. Glaucophane polycrystals . 89 3.6.3. Lawsonite polycrystals . 91 3.6.4. Whole rocks . 95 3.7. Discussions . 96 3.7.1. Deformation mechanisms of glaucophane and lawsonite . 96 3.7.2. LPO variations of glaucophane and lawsonite . 102 3.7.3. Seismic anisotropy of subducting oceanic crust . 104 3.8. Conclusions . 108 CHAPTER 4: Plastic deformation and seismic implications in the fore-arc mantle: Constrains from Yushigou harzburgites in North Qilian suture zone, NW China . 110 Abstract . 110 4.1. Introduction . 111 4.2. Geological setting . 114 4.3. Microstructures and petrology . 117 4.4. Determination of the dislocation slip systems . 121 4.4.1. Crystal preferred orientations . 121 4.4.2. Subgrain boundaries and rotation axes . 131 4.5. Water contents in olivine and orthopyroxene . 138 4.5.1. Analytical procedure . 138 4.5.2. IR spectra and water content . 140 4.6. Seismic properties . 144 4.6.1. Analytical method . 144 4.6.2. Seismic anisotropies and velocities . 145 4.7. Discussions . 148 4.7.1. Relations between petrological and deformational processes . 148 4.7.2. Distributions of olivine fabrics in subduction zone . 160 4.7.3. Seismic implications in the fore-arc mantle . 165 4.8. Conclusions . 172 CHAPTER 5: Partial melting, melt-rock interaction and plastic deformation in a fore-arc mantle: Constraints from Songshugou peridotites in Qinling orogenic belt, central China . 175 Abstract . 175 5.1. Introduction . 176 5.2. Geological setting . 178 5.3. Lithology and petrography . 180 5.3.1. Fine-grained and porphyroclastic dunite . 180 5.3.2. Coarse-grained dunite . 182 5.3.3. Harzburgite . 182 5.3.4. Olivine clinopyroxenite . 184 5.3.5. Mineral inclusions . 184 5.4. Mineral compositions and estimates of temperature and stress . 186 5.4.1. Mineral compositions . 186 5.4.2. Temperature estimates . 191 5.4.3. Stress estimates . 192 5.5. Whole-rock major and trace element compositions . 192 5.5.1. Major elements characteristics . 192 5.5.2. Trace element characteristics . 196 5.6. Lattice preferred orientations of olivine . 201 5.7. Discussions . 204 5.7.1. Highly refractory partial melting residue . 204 5.7.2. Melt-rock interactions . 208 5.7.3. Tectonic setting . 211 5.7.4. Tectonic evolutions . 216 5.7.5. Olivine fabric transition and tectonic evolution . 221 5.8. Conclusions . 229 CHAPTER 6: Summary . 232 REFERENCES . 236 ABSTRACT (IN KOREAN) . 270 ACKNOWLEDGEMENTS . 272Docto