Metamorphic consequences of secular changes in oceanic crust composition and implications for uniformitarianism in the geological record

Cooling of the Earth＇s mantle since the Meso-Archean is predicted by thermal and petrological models to have induced a secular change in the composition of primary mantle-derived magmas-and thus bulk oceanic crust; in particular, suggesting a decrease in maficity over time. This hypothesis underpins several recent studies that have addressed key geological questions concerning evolving plate tectonic styles, the rates and timing of continental crust formation, comparative planetology, and the emergence of complex life on Earth. Major, minor, and trace element geochemical analyses of（meta）mafic rocks preserved in the geological record allows exploration of this theory, although no consensus currently exists about the magnitude of this change and what compositions-if anything-constitute representative examples of Paleo-, Meso-, or Neo-Archean primitive oceanic crust. In this work, we review the current state of understanding of this issue, and use phase equilibria to examine the different mineral assemblages and rock types that would form during metamorphism of basalt of varying maficity in subduction zone environments. The presence（or absence） of such metamorphic products in the geological record is often used as evidence for（or against） the operation of modern-day subductiondriven plate tectonics on Earth at particular time periods; however, the control that secular changes in composition have on the stability of mineral assemblages diagnostic of subduction-zone metamorphism weakens such uniformitarianistic approaches. Geodynamic interpretations of the Archean metamorphic rock record must therefore employ a different set of petrological criteria for determining tectonothermal histories than those applied to Proterozoic or Phanerozoic equivalents.

Oceanic crustal structure and tectonic origin of the southern Kyushu-Palau Ridge in the Philippine Sea

A new high-resolution velocity model of the southern Kyushu-Palau Ridge(KPR) was derived from an activesource wide-angle seismic reflection/refraction profile. The result shows that the KPR crust can be divided into the upper crust with the P-wave velocity less than 6.1 m/s, and lower crust with P-wave velocity between 6.1 km/s and 7.2 km/s. The crustal thickness of the KPR reaches 12.0 km in the center, which gradually decreases to 5.0–6.0 km at sides. The velocity structure of the KPR is similar to the structures of the adjacent West Philippine Basin and Parece Vela Basin(PVB), indicating a typical oceanic crust. Isostatic analysis shows that some regional compensation occurs during the loading of the KPR, which implies that the KPR was built mainly by magmatism during the splitting of the Izu-Bonin-Mariana arc and the following back-arc seafloor spreading of the PVB during30–28 Ma BP. The absence of the thick middle crust(6.0–6.5 km/s) and high velocity lower-crustal layers(7.2–7.6 km/s) suggest that arc magmatism plays a less important role in the KPR formation.

Sheeted Dike Complexes in Contemporary Oceanic Crust: Implications for Spreading Processes and the Interpretation of Ophiolites

As anticipated from studies of ophiolite complexes,direct investigations of the oceanic crust confirm that basaltic dikes are an integral part of the upper 2 km of the oceanic crust.Currently available information suggests

Sulfide Aggregation in Ophiolitic Dunite Channels Explains Os-Isotope Mismatch between Oceanic Crust and Mantle

The osmium-isotope mismatch commonly reported between mid-ocean-ridge basalts(MORBs) and residual mantle might reflect evolution of the MORB Re-Os system after extraction from the asthenosphere, or preferential contribution of radiogenic Os components from mantle. However, in a MOR system, the role of dunite melt channels from the upper mantle and Moho transition zone in regulating isotopic systems between mantle and crust has rarely been evaluated. We report new Re-Os isotopic compositions of base-metal sulfides(BMS), chromites and dunites from dunite lenses with low spinel Cr# [Cr3+/(Cr3++Al3+) ≤ 0.66](products of interaction between MORB-like melts and upper-mantle harzburgites) from the Zedang ophiolite(South Tibet). Re-Os isotopic compositions of low-Cr# dunites from the Oman ophiolite are also shown for comparison. Mineralogical evidence suggests that the Zedang sulfides were originally precipitated as monosulfide solid solutions. The highly variable 187Os/188Os initial ratios(0.1191-0.1702) and low 187Re/188Os(<0.22) of the sulfides suggest that the chromite acted as a sink for Os-bearing sulfides, aggregating discrete Os components with heterogeneous isotopic signatures from asthenospheric or lithospheric mantle into dunite channels. The Zedang chromites and dunites show 187Os/188Os ratios similar to the primitive upper mantle(PUM), except for two dunites with sub-PUM ratios, reflecting the contribution of Os balanced by smaller volumes of Os-rich, unradiogenic sulfides(likely nucleating on Os nanoparticles) and larger volumes of Os-poor radiogenic BMS. Such isotopic heterogeneity, despite with less variation, has been observed in dunite channels from the Oman ophiolite and present-day mid-ocean ridges. Formation of dunite channels in the upper mantle thus can aggregate Os-bearing sulfides with chromite, leaving high Re/Os components into the residual melts. Once such channel systems were built up at the crust-mantle transition zone, the newly incoming MOR magmas would preferentially melt and dissolve the volumetrically abundant radiogenic BMS and retain Os-rich nanoparticles in the channels, further amplifying the Os-isotope mismatch between oceanic crust and mantle. This study sheds new light on the multistage evolution and small-scale behaviors of chalcophile and siderophile elements(e.g., Re-Os) and their isotopes(e.g., 187Re-187Os) with sulfides and chromites in a silicate-dominated melt plumbing system beneath mid-ocean ridges.

Relic Oceanic Crust at Sub-Arc Depth: An Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan, China

Ultra-high pressure（UHP）eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation（Hermann et al.,2000;Guillot et al.,2001）.Under normal subduction geotherms,serpentinites break down just before UHP conditions are reached and therefore most of the exhumed eclogites representing subducted oceanic crust formed under fore-arc conditions.We investigated eclogite blocks enclosed into serpentinites that occur in the southwestern Tianshan oceanic subduction,China.A previous study proved that the serpentinites derive from altered oceanic crust and experienced UHP metamorphism at low temperatures of 510-530°C（Shen et al.,2015）.Three relatively fresh eclogite samples were studied in detail.Sample 129-7 shows the retrograde mineral assemblage of amphibole+biotite+albite+chlorite+minor titanite and peak metamorphic relics of omphacite+garnet±chlorite.Sample C107-23 is mainly composed of amphibole+albite+chlorite+zoisite+muscovite+minor titanite as a retrograde assemblage and garnet+phengite as the peak metamorphic relics with omphacite only found as inclusions in garnet.Similar to sample C107-23,sample C11066 preserves large-grained euhedral to subhedral garnet relics with omphacite inclusions,and epidote,diopside,amphibole,muscovite,chlorite,albite and biotite are in the matrix belong to the retrograde assemblage.These three retrograde eclogite samples were modelled using thermodynamic calculations in the Mn NCKFMSHO（Mn O-Na;O-Ca O-K;O-FeO-Mg O-Al;O;-SiO;-H;O-Fe;O;）system.Based on the peak assemblage of omphacite+garnet and the crossing of the grossular and pyrope isopleths in garnet,peak P-T conditions of;60-470oC,28-29 kbar（129-7）,450-500oC,28-35 kbar（C107-23）,;75-505oC,26-29 kbar（C11066）were calculated.The retrograde assemblages indicate near isothermal decompression resulting in a clockwise P-T evolution of these eclogites.The peak metamorphic pressures at 500°C are well within UHP conditions（coesite stability field）and are within error the same as peak conditions of the host serpentinites（Shen et al.,2015）.This provides evidence that eclogites and serpentinites shared the same evolution.We infer that the subducted low-density serpentinites were assembled with the high-density eclogites during subdution and helped the latter to exhume back to the surface.The studied eclogites thus represent rare examples of relics of oceanic crust that was subducted to sub-arc depth.

Relic Oceanic Crust at Sub-arc Depth: an Example from UHP Eclogites Enclosed in Serpentinites from the Southwestern Tianshan Mountains, China

Ultra-high pressure(UHP)eclogites that derive from subducted oceanic crust are rarely found at the Earth’s surface because they need to be enclosed in a buoyant host rock such as serpentinites that facilitate exhumation

Regarding an Oceanic Crust/Upper Mantle Geochemical Signature at the KT Boundary:If not from Chicxulub Crater,then Where Did it Come from?

Evidence for a mantle and/or basaltic component in KT boundary distal ejecta is apparently inconsistent with ejection from Chicxulub Crater since it is located on;5km thick continental crust（De Paolo et al.,1983;Montanari et al.,1983;Hildebrand and Boynton,1988,1990）.This evidence,along with ejected terrestrial chromites（Olds et al.,2016）suggest the impact sampled terrestrial mafic and/or ultramafic target rocks which are not known to exist in the Chicxulub target area.Possible resolutions to the paradox are:1)the existence of an unmapped/unknown suture in Yucatan Platform basement,2)an additional small unmapped/unknown impact site on oceanic lithosphere,or 3)an additional large impact on oceanic lithosphere or continental margin transitional to oceanic lithosphere.The third hypothesis is elaborated here since:1)Ophiolites nearest to Chicxulub crater are found in Cuba and apparently were obducted in latest Cretaceous/earliest Danian times（García-Casco,2008）,inconsistent with the documented Eocene collision of Cuba with the Bahamas platform;and 2)Cuba hosts the world’s thickest known KT boundary deposits（Iturralde-Vinent,1992;Kiyokawa et al.,2002;Tada et al.,2003）.These and geometric considerations suggest oceanic crust and upper mantle rock,exposed as ophiolite in the Greater Antilles island chain,marks the rim of a roughly 700 km diameter impact basin deformed and dismembered from an originally circular form by at least 50 million years of left-lateral shear displacement along the North American-Caribbeantransform plate boundary.

High-pressure metamorphic rocks in the Borborema Province, Northeast Brazil: Reworking of Archean oceanic crust during proterozoic orogenies

We present the first evidence of Archean oceanic crust submitted to Proterozoic high-pressure(HP)metamorphism in the South American Platform.Sm-Nd and Lu-Hf isotopic data combined with U-Pb geochronological data from the Campo Grande area,Rio Grande do Norte domain,in the Northern Borborema Province,reflect a complex Archean(2.9 Ga and 2.6 Ga)and Paleoproterozoic(2.0 Ga)evolution,culminating in the Neoproterozoic Brasiliano/Pan-African orogeny(ca.600 Ma).The preserved mafic rocks contain massive poikiloblastic garnet and granoblastic amphibole with variable proportions of plagioclase+diopside in symplectitic texture,typical of high-pressure rocks.These clinopyroxene-garnet amphibolites and the more common garnet amphibolites from the Campo Grande area are exposed as rare lenses within an Archean migmatite complex.The amphibolite lenses represent 2.65 Ga juvenile tholeiitic magmatism derived from depleted mantle sources(positive values of+3.81 to+30.66)later enriched by mantle metasomatism(negative εNd(t)values of-7.97).Chondrite and Primitive Mantle-normalized REE of analyzed samples and discriminant diagrams define two different oceanic affinities,with E-MORB and OIB signature.Negative Eu anomalies(Eu/Eu*=0.75-0.95)indicate depletion of plagioclase in the source.Inherited zircon cores of 3.0-2.9 Ga in analyzed samples indicate that the Neoarchean tholeiitic magmatism was emplaced into 2923±14 Ma old Mesoarchean crust(εNd(t)--2.58 and Nd TDM=3.2 Ga)of the Rio Grande do Norte domain.The age of retro-eclogite facies metamorphism is not yet completely understood.We suggest that two high-grade metamorphic events are recognized in the mafic rocks:the first at 2.0 Ga,recorded in some samples,and the second,at ca.600 Ma,stronger and more pervasive and recorded in several of the mafic rock samples.The Neoproterozoic zircon grains are found in symplectite texture as inclusions in the garnet grains and represent the age of HP conditions in the area.These zircon grains show a younger cluster of concordant analyses between 623±3 Ma and 592±5 Ma withεHf(t)values of+0.74 to-65.88.Thus,the Campo Grande rock assemblage is composed of Archean units that were amalgamated to West Gondwana during Neoproterozoic Brasiliano orogeny continent-continent collision and crustal reworking.

Petrology of an Arc-Oceanic Crust Contact Zone in the Laohushan Back-arc Basin, the Eastern Section of the North Qilian Mountains, NW China

A contact zone sandwiched between an arc and an oceanic crust was discoveredin the Laohushan area in the present study. It consists of a series of north-dipping imbricatedthrust sheets and is exposed on the surface as a narrow arcuate belt, which extends for about 30 kmin an E-W direction and measures about 1-3 km wide. Lithologically, it can be divided into foursubzones. Subzone 1 consists of meta-andesite and metasandstone; subzone 2, psammitic schists;subzone 3, psammitic and pelitic schists, quartz diorite and hornfelses; and subzone 4, metagabbro,epidote amphibolite and pelitic schists. The metamorphism has the following grading sequence: lowgreenschist facies in subzone 1 - > high greenschist facies in subzone 2 - > low amphibolite fadesin subzone 3 - > epidote amphibolite facies in subzone 4. Petrographic and geochemical evidenceshows that rocks in subzones 1, 2 and 3 are arc rocks, whereas those of subzone 4 are oceaniccrustal rocks. The metamorphic mineral assemblages and especially mineral chemistry of the bluishgreen amphibole from the pelitic schists and epidote amphibolite of subzone 4 suggest that the rocksof the contact zone were metamorphosed at a pressure of up to 0.69 GPa. It is thought that theLate-Mid Ordovician oceanic lithosphere of a back-arc basin was underthrust northerly beneath an arcto a depth of 20-23 km, where the basaltic rocks and gabbro were converted to epidote amphiboliteand metagabbro respectively. Then, the root rocks of the arc and these metamorphosed oceanic rockswere brought up to shallower depths by thrust faults to form a contact zone between the arc and theoceanic crust in the Laohushan area.

Typical Oxygen Isotope Profile of Altered Oceanic Crust Recorded in Continental Intraplate Basalts

Recycled oceanic crust （ROC） has long been suggested to be a candidate introducing en- riched geochemical signatures into the mantle source of intraplate basalts. The different parts of oceanic crust are characterized by variable oxygen isotope compositions （δ^18O=3.7‰ to 13.6‰）. To trace the sig- natures of ROC in the mantle source of intraplate basalts, we measured the δ^18O values of clinopyroxene （cpx） phenocrysts in the Cenozoic basalts from the Shuangliao volcanic field, NE China using secondary ion mass spectrometer （SIMS）. The δ^18O values of the Shuangliao cpx phenocrysts in four basalts ranging from 4.10‰ to 6.73‰ （with average values 5.93‰±0.36‰, 5.95‰±0.30‰0, 5.58‰±=0.66‰e, and 4.55‰±= 0.38‰, respectively） apparently exceed those of normal mantle-derived cpx （5.6‰±0.2‰） and fall in the typical oxygen isotope range of altered oceanic crust. The δ^18O values display the negative correlations with the Eu, Sr anomalies of whole rocks and erupted ages, demonstrating that （1） the ROC is the main enriched component in the mantle source of the Shuangliao basalts and （2） the contributions of ROC var- ied with time. The basalt with the lowest δ^18O value is characterized by a significant K positive anomaly, highest H2O/Ce and Ba/Th ratios, suggesting that the mantle source of basalts with low δ^18O can also in- clude a water-rich sediment component that may be the trigger for partial melting. Considering the continuous subduction of the Pacific slab, the temporal heterogeneity of the source components is likely to be caused by the Pacific slab subduction.

Crustal Structure of the Jurassic Quiet Zone in the West Pacific Ocean:Insights from 2D Multichannel Seismic Reflection Profiles

The Jurassic oceanic crust is the oldest existing oceanic crust on earth,and although distributed sparsely,carries essential information about the earth's evolution.The area around the Pigafetta Basin in the west Pacific Ocean(also known as the Jurassic Quiet Zone,JQZ)is one of a few areas where the Jurassic oceanic crust is present.This study takes full advantage of high-resolution multichannel seismic reflection profiles in combination with bathymetry,magnetic,and gravity data from the JQZ to examine the structure,deformation,and morphology of the Jurassic oceanic crust.Our results show the following insights:1)The Moho lies at 2–3 s in two-way travel time beneath the seafloor with the segmented feature.The gaps between the Moho segments well correspond to the seamounts on the seafloor,suggesting the upward migration of magma from the mantle has interrupted the pre-existing Moho.2)The oceanic crust is predominantly deformed by crustal-scale thrust faults,normal faults cutting through the top of basement,and vertical seismic disturbance zones in association with migration of thermal fluids.The thrust faults are locally found and interpreted as the results of tectonic inversion.3)Seafloor morphology in the JQZ is characterized by fault scarps,fold scarps,seamounts,and small hills,indicating the occurrence of active faults.4)The oceanic crust in the JQZ and East Pacific Rise has many structural and geometrical variations,such as the thickness of sediments,seafloor topography,basement morphology,fault size and type.

The Junggar Immature Continental Crust Province and Its Mineralization

According to the study on the peripheral orogenic belts of the Junggar basin and combined with the interpretation of geophysical data, this paper points out that there is an Early Paleozoic basement of immature continental crust in the Junggar area, which is mainly composed of Neoproterozoic-Ordovician oceanic crust and weakly metamorphosed covering sedimentary rocks. The Late Paleozoic tectonism and mineralization were developed on the basement of the Early Paleozoic immature continental crust. The Junggar metallogenic province is dominated by Cr, Cu, Ni and Au mineralization. Those large and medium-scale deposits are mainly distributed along the deep faults and particularly near the ophiolitic melange zones, and formed in the Late Paleozoic with the peak of mineralization occurring in the Carboniferous-Permian post-collisional stage. The intrusions related to Cu, Ni and Au mineralization generally have low Is, and positive εNd(t) values. The δ34S values of the ore deposits are mostly near zero, and the lead isotopes are mostly of normal lead. All these indicate that the ore-forming material comes either directly from the mantle-derived magma (for chromite and Cu-Ni deposits) or from recirculation of the basement material of the Early Paleozoic immature crust (for most Cu and Au deposits).

Chronology and Geochemistry of the Nadingcuo Volcanic Rocks in the Southern Qiangtang Region of the Tibetan Plateau:Partial Melting of Remnant Ocean Crust along the Bangong-Nujiang Suture

The Nadingcuo high-K calc-alkaline rocks mainly composed of trachyte and trachyandesite are the largest outcrop area of volcanic rocks in southern Qiangtang terrane in the Tibetan plateau. However,their exact source and peterogenesis are still debated.^（40）Ar-^（39）Ar and LAM-ICPMS zircon U-Pb isotopic dating confirm that these rocks erupted in Eocene.In addition,the Nadingcuo volcanic rocks are characterized by high Sr/Y content ratios,similar with the adakite derived from partial melting of oceanic crust.They can be further classified as high Mg~#（Mg~#=48-57） and low Mg~# （Mg~#=33-42） subtypes.The Nadingcuo adakitic rocks have relatively low（^（87）Sr/^（86）Sr）_i and highε_（Nd）（t）, showing a trend of similarity to the Dongcuo ophiolite present in the Bangong-Nujiang oceanic crust. Simple modeling indicates that the Nadingcuo adakitic rocks are a mix resulting from the basalt of Bangong-Nujiang Ocean with 10%-20%crustal material of Lhasa terrane.On these bases we suggest that the low Mg~# Nadingcuo adakitic rocks are the product of partial melting of remnant oceanic crust with small sediment,and the high Mg~# rocks are the result of reaction between rising melt of remnant oceanic crust with subducted sediment and mantle wedge.Therefore,the origin of Nadingcuo adakitic rocks may be related to intracontinental subduction triggered by collision of India-Asia during Cenozoic.

Faulting, magmatism and crustal oceanization of the Okinawa Trough

The paper focuses on the characteristics of faulting and magmatism of the Okinawa Trough and the relation between them. En-echelon grabens are ranked oblique to the continental shelf edge uplift, and the Longwang uplift, the rifting block ridge in the northern segment and the ＂Mianhua uplift＂ in the southern segment have possibly preserved characteristics of volcanism and magmatism occurring with those rifting phases. The clockwise rotation of the southern Ryukyu Islands, driven by collision between Luzon and Taiwan, has played a key role in the crustal oceanization, enhancing the crustal extension of the southern segment and inducing volcanic magmatism in those grabens, among which the Yaeyama graben is a typical example of the presence of oceanic crust. Faulting and magmatism were mainly migrating towards the island arc asymmetrically. The crustal oceanization of the Okinawa Trough is difficultly interpreted by the linear magnetic anomaly model, which is fit for the symmetric spreading of the mid-oceanic ridges.

Five-Stage Model of the Palaeozoic Crustal Evolution in Xinjiang

Abstract: The great majority of the Palaeozoic orogenic belts of Central Asia are of the intercontinental type, whose evolution always follows a five-stage model, i.e. the basal continental crust-extensional transitional crust-oceanic crust-convergent transitional crust-new continental crust model. The stage for the extensional transitional crust is a pretty long, independent and inevitable phase. The dismembering mechanism of the basal continental crust becoming an extensional continental crust is delineated by the simple shear model put forward by Wernike (1981). The continental margins on the sides of a gently dipping detachment zone and moving along it are asymmetric: one side is of the nonmagmatic type and the other of the magmatic type with a typical bimodal volcanic formation. In the latter case, however, they were often confused with island arcs. This paper discusses the five-stage process of the crustal evolution of some typical orogenic belts in Xinjiang.

Scholte wave dispersion and particle motion mode in ocean and ocean crust

The dispersion equation of the Scholte wave was reviewed using the homogeneous elastic half-space covered by a liquid layer,and the range of the Scholte wave propagation velocity was examined using the dispersion equation.The displacement expressions of the Scholte waves in liquid and solid were derived.Additionally,the mode of motion of Scholte waves in liquid and solid and their variation with depth was studied.The following results were obtained:The dispersion equation shows that the propagation velocity of the fundamental Scholte wave was greater than the P-wave in liquid and less than that of the Scholte wave in homogeneous elastic half-space.In contrast,the velocity of higher-order Scholte waves was greater than that of P waves in liquid and S-waves in solid.Only the fundamental Scholte wave has no cutoff frequency.The Scholte wave at the liquid surface moved only vertically,while the particles inside the liquid medium moved elliptically.The amplitude variation with depth in the solid medium caused the particle motion to change from a retrograde ellipse to a prograde ellipse.The above results imply the study of Scholte waves in the ocean and oceanic crust and help estimate ocean depths.

A Global Perspective on Crustal Structure and Evolution based on A New Crustal Classification

Knowledge of the crustal structure is the key for understanding physical and chemical conditions of its formation and later modification by geodynamic processes.It has long been recognized that crustal structure is controlled by tectonic settings,and that the crustal thickness is one of the most important parameters that reflects the geodynamic origin of the crust.A long tectonic life of continental crust leads to its significant reworking by plate tectonics processes and crust-mantle interaction,which include mechanical extension.

Composition and Seismic Properties of the Oceanic Lithosphere: A Synthesis of Ophiolites and Core Samples of the IODP

Our knowledge of the oceanic lithosphere largely comes from analogy with ophiolite complexes and the direct scientific drilling of the present-day oceanic crust(e.g.,Christensen and Salisbury,1975,1989;Smith and Vine,1989;Dilek and Furnes,2011,2014).In this study,we summarized previous experimental results on seismic properties of oceanic lower crust and upper mantle according to different tectonic settings.The results are used to highlight the compositional heterogeneity and the nature of the oceanic Moho.Observation in different ophiolites reveal an ideal oceanic lithosphere profile with ideal petrologic units and seismic units(Dilek and Furnes,2011,2014).The lithospheric mantle beneath ocean basins is composed of tectonized peridotites,which include layered lherzolites and harzburgites and lenses of dunites with chromitites and nearly correspond to the seismic Layer 4.The overlying layered gabbros and mafic sheeted dike complex equal to the seismic Layer 3,as a result of crystallization from a magma chamber.The transitional unit between the former two petrologic units consists of layered ultramafic and mafic rocks,corresponds to the petrological Moho.The seismic Layer 2 and 1 are well defined by pillow lavas and massive flows,and the overlying abyssal sediments,respectively.Compared these results with the refraction seismic profiles,the oceanic crust and upper mantle show different composition and structure.The Pwave velocities of the Layer 3 gabbros varies from 6.7 to 7.0 km s-1 and have low velocity gradients of<0.1 km s-1.Although the gradual increase of P-and S-wave velocities with depth can be attributed to the increasing proportion of mafic minerals from the top to the bottom,prehnite-pumpellyite facies alteration of basalts,greenschist-faces metamorphism to epidote-amphibolite facies metamorphism of gabbros will decrease the velocities of the Layer 2 and Layer 3(Christensen and Salisbury,1975,1989),because the P-wave velocities of chlorite and hornblende are 6.00 and 7.00 km s-1,respectively,lower than those of plagioclase and pyroxene,respectively(Carlson,2004).In addition,local velocity anomalies near the petrologic Moho can be related serpentinization of ultramafic rocks(Salisbury and Christensen,1978;Carlson et al.,2009).In the Layer 4,the characteristic P-wave velocities of the upper mantle should fall in the range of 7.8 to 8.2 km s-1.Poisson’s ratios of chrysotile and lizardite,which are stable in oceanic crustal environments according to the phase diagram,is 0.267 and 0.359,respectively,higher than those of olivine and pyroxene(Wang et al.,2013).Serpentinization will significantly decreased velocities and densities of peridotites and is the main reason for the variation of the Moho reflectivity beneath oceans.

Lunar surface mineralogy using hyperspectral data： Implications for primordial crust in the Earth-Moon system

Mineralogy of the Lunar surface provides important clues for understanding the composition and evo- lution of the primordial crust in the Earth-Moon system. The primary rock forming minerals on the Moon such as pyroxene, olivine and plagioclase are potential tools to evaluate the Lunar Magma Ocean （LMO） hypothesis. Here we use the data from Moon Mineralogy Mapper （M3） onboard the Chandrayaan- 1 project of India, which provides Visible/Near Infra Red （NIR） spectral data （hyperspectral data） of the Lunar surface to gain insights on the surface mineralogy. Band shaping and spectral profiling methods are used for identifying minerals in five sites： the Moscoviense basin, Orientale basin, Apollo basin, Wegener crater-highland, and Hertzsprung basin. The common presence of plagioclase in these sites is in conformity with the anorthositic composition of the Lunar crust. Pyroxenes, olivine and Fe-Mg-spinel from the sample sites indicate the presence of gabbroic and basaltic components. The compositional difference in pyroxenes suggests magmatic differentiation on the Lunar surface. Olivine contains OH/H20 band, indicating hydrous phase in the primordial magmas.

Modern-style Subduction Processes in the Archean:Evidence from the Shangyi Complex in North China Craton

Three fragments of the Archean oceanic crust have been found between the Archean granulite belt and the Paleo-Proterozoic Hongqiyingzi group in North China craton, which spread along the Shangyi-Chicheng ancient fault. This paper presents integrated field, petrology, geochemistry and geochronology evidence of the ancient oceanic fragments. The magma crystallizing age of the tonalite in the Shangyi complex is 2512±19 Ma and the geochemical characteristics suggest that the Nbenriched basalts may be related to crustal contamination and formed in the intra-oceanic arc of the supra subduction zone setting.