Extensive impact melting on the H-chondrite parent asteroid during the cataclysmic bombardment of the early solar system: Evidence from the achondritic meteorite Dar al Gani 896

DaG 896 is an olivine-rich microporphyritic rock of komatiitic composition. Both the olivine composition (Fa_17.5±2.1, [Mn/Mg] = 0.0061) and the bulk oxygen isotopic composition (δ17O = +2.55, δ18O = +3.50) indicate that DaG 896 is a sample of the H-chondrite parent body. The bulk chemistry shows an H-chondritic distribution of lithophile elements, whereas chalcophile and siderophile elements are strongly depleted, indicating formation through whole-rock melting (or nearly so) of H-chondrite material, nearly complete loss of the metal plus sulfide component, and crystallization without significant igneous fractionation. Superheated, severely shocked chondritic relics (∼10 vol%), typically in the form of corroded lithic fragments <100 μm in size intimately distributed within the igneous lithology, indicate that melting was triggered by a highly energetic impact, which possibly induced shock pressures of ∼80-100 GPa. The relatively young 3.704 ± 0.035 Ga 40Ar-39Ar crystallization age is consistent with the impact melting origin, as magmatism in the asteroid belt was active only in the first hundred million years of solar system history.Based on textural data and thermodynamic crystallization modelling, we infer that DaG 896 crystallized from a liquidus temperature of ∼1630°C under relatively slow cooling rates (∼10°C h⁻¹) to ∼1300°C, before quenching. The two-stage cooling history indicates that a reasonable formation environment might be a dike intruding cooler basement below a crater floor. Metal-silicate fractionation may have been accomplished, at least at the centimeter-scale of the studied meteorite sample, through differential acceleration of immiscible liquids of different density during the intense flow regimes associated with the excavation and modification stages of the cratering mechanism. Alternatively, DaG 896 may represent a surface sample of a differentiated melt body at the floor of an impact crater, as gravitational settling appears to be an effective process at the surface of a chondritic parent asteroid: for metal particles 1 to 10 mm in size, typically observed in partially differentiated impact melt rocks, Stokes’ Law indicates a settling velocity >1 m h ⁻¹ during the first few hours of crystallization on asteroidal bodies of >25 km radius.The ∼3.7 Ga age of DaG 896 nearly overlaps with the slightly older resetting ages of H-chondrites (all impact melts) available from the literature, indicating that the H-chondrite parent asteroid underwent extensive impact melting at the enduring of the cataclysmic bombardment of the early solar system. Such an age overlap may also indicate early disruption of the initial H-chondrite parent asteroid.The close similarity between the bulk composition and degassing age of DaG 896 and silicate inclusions in IIE iron meteorites is further evidence in support of a common origin by impact melting and metal-silicate segregation on the H-chondrite parent asteroid. Our new high-precision oxygen isotopic measurements of H-chondrites (Δ17O = 0.77 ± 0.04) should be extended to IIEs to verify this possible petrogenetic link.     

Unraveling the evolution of chondrite parent asteroids by precise U-Pb dating and thermal modeling

U-Th-Pb isotopic data are reported for mineral fractions, individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite Richardton (H5). Chondrules and milligram-sized fractions of pyroxene-rich chondrule fragments contain highly radiogenic Pb and concordant or nearly concordant U-Th-Pb isotopic systems, and are suitable for precise Pb-Pb age determinations. Olivine and sulfide have low U concentrations and contain less radiogenic Pb. The ages of individual chondrules, pyroxene-rich and phosphate fractions are determined using U-Pb and Pb-Pb isochron and model date calculations. The Pb-Pb isochron date of 4562.7 ± 1.7 Ma of the Richardton chondrules and chondrule fragments is resolved from the Pb-Pb isochron date of 4550.7 ± 2.6 Ma obtained from multiple phosphate fractions. Possible biases of the isochron dates due to single-stage approximation of multi-stage evolution, contamination with modern common Pb, and disturbance to the system by reheating, are examined and are found to be insignificant. The chondrule and phosphate dates are interpreted as the timing of cessation of Pb diffusion during cooling following metamorphism in chondrite parent bodies. The difference in estimated closure temperatures, ∼950-1150 K for pyroxenes, and 700-800 K for phosphates (temperature estimates are based on published diffusion rates for Pb in pyroxenes and apatite), allows evaluation of the average cooling rate at 26 ± 13 K/million years for the Richardton parent body over the period of 4563-4551 my. Thermal modeling of the H-chondrite parent body (which is assumed to be asteroid 6 Hebe, heated by decay of ²⁶Al) suggests a scenario in which accretion initiated at 1.7 m.y. after formation of calcium-aluminum-rich inclusions and continued for 3.5 m.y.     

Iodine-xenon analysis of ordinary chondrite halide: implications for early solar system water

We report the results of iodine-xenon analyses of irradiated halide grains extracted from the H-chondrite Monahans (1998) and compare them with those from Zag (Whitby et al., 2000) to address the timing of aqueous processing on the H-chondrite parent body. Xe isotopic analyses were carried out using the RELAX mass spectrometer with laser stepped heating. The initial ¹²⁹I/¹²⁷I ratio in the Monahans halide was determined to be (9.37 ± 0.06) × 10⁻⁵ with an iodine concentration of ∼400 ppb. Significant scatter, especially in the Zag data, indicates that a simple interpretation as a formation age is unreliable. Instead we propose a model whereby halide minerals in both meteorites formed ∼5 Ma after the enstatite achondrite Shallowater (at an absolute age of 4559 Ma). This age is in agreement with the timing of aqueous alteration on the carbonaceous chondrite parent bodies and ordinary chondrite metamorphism and is consistent with the decay of ²⁶Al as a heat source for heating and mobilisation of brines on the H-chondrite parent body. Post accretion surface impact events may have also contributed to the heat source.     

Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif,Germany, on the Variscan continental collision orogeny

Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two‐ or multi‐plate setting during inter‐ or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir‐like body of high‐P granulite below from low‐P metasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high‐P amphibolite facies metamorphism in the mid‐ to late‐Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar–Ar biotite ages with published P–T–t data for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of ~8 mm/year and ~80°C/Ma, with a drop in exhumation rate from ~20 to ~2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag of c. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90°C/Ma when all units had assembled into the massif. A two‐plate model of the Variscan orogeny in which the above evolution is related to a short‐lived intra‐Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale of c. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem.     

类地行星热物理条件与热演化影响因素分析——以火星地幔热柱演化为例

太阳系内类地行星具有相似的岩石层包围金属核的圈层结构,在行星幔的热演化历史起源方面具有同时性和同源性,并且都在早期变形重力位能加热的基础上随放射性热能衰减而冷却。但是,由于半径、密度、粘度以及表层构造属性等物理条件的差异,其热演化历史各具特色。依据基本的热对流和热传导方程,我们计算分析了类地行星热物理条件差异对行星幔热演化历史的影响。计算表明,类地行星热演化的早期,行星幔热对流是主要的散热方式。半径较大的行星表面热流密度大,平均散热量也大。半径较小的行星内部温差小,粘滞系数高,对流能力低,提早进入传导散热状态,且传导散热的岩石层也比大行星厚。不同边界层热物理条件下,类地行星幔热演化历史会分别出现逐渐冷却的平稳式、包含热柱上涌的波动式、行星幔幕次翻转的周期式等特点不同的热演化过程。火星内部曾经存在的地幔热柱构造与火星地幔热动力学演化过程密切相关。我们从火星地幔热动力学演化模型出发,定量计算与地幔热柱构造演化相关的地幔热动力学演化特征,通过三维球壳数值模拟,研究了火星地幔热演化历史上可能存在的热柱活动造成的火星热演化历史的非单调变化,火星地幔对流环结构随时间的演变方式,以及与边界相关的地幔热柱对火星地形的影响。     

The thermal history of the Acapulco meteorite and its parent body deduced from U/Pb systematics in mineral separates and bulk rock fragments

U/Pb systematics of the Acapulco meteorite have been determined on phosphate and feldspar separates and on grain size fractions of bulk material. The latter show an enrichment of U and Th with respect to CI chondrites and a low (∼1) Th/U ratio. This is consistent with the model that the majority of U and Th was added early by a low temperature melt to the Acapulco precursor. The feldspar exhibits a Pb isotope composition that is close to the primordial Pb composition. Mineral separates and bulk fractions define a ²⁰⁷Pb/²⁰⁶Pb isochron. The age corresponds to 4555.9 ± 0.6 Ma. This age anchors the thermal evolution of the Acapulco parent body into an absolute time scale. Evaluation of the Hf/W and U/Pb records with the cooling rates deduced from mineralogical investigations confirms the idea that the Acapulco parent body was fragmented during its cooling. The U/Pb system precisely dates this break-up at 4556 ± 1 Ma.     

Pb-Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science297, 1679-1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene-olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from ²⁶Al and ⁶⁰Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10-50 km radius accreting within 1-3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.     

Cooling history of the Puttetti alkali syenite pluton,southern India

The Puttetti alkali syenite pluton in southern India belongs to the suite of felsic magmatic intrusives emplaced during the Late Neoprotrozoic-Cambrian time during the final phase of amalgamation of the Gondwana supercontinent. In this study, we evaluate the cooling history of this pluton based on various isotopic systems. We present whole-rock Pb-Pb data on the syenite which yields an isochron age of 508±25Ma. Three phlogopite separates from the syenite pluton give K-Ar ages of 454.0±9.0, 448.5±8.9 and 445.6±8.8 Ma indicating cooling age at temperatures of ∼415°C. U-Pb analyses of zircons from this syenite yielded an age of 572±2 Ma in a previous study. With U-Pb closure temperatures >800 o C, this age probably indicates the timing of emplacement of the Puttetti pluton. Collectively, we estimate from the isotopic age data and respective closure temperatures that the syenite body cooled at about 3.2 o C/Ma from about 800 o C to about 415 o C. The markedly low cooling rate of the syenite pluton, absence of chilled margin effects, and common occurrence of pyroxene, feldspar, phlogopite and zircons megacrysts in the rock indicate that the host granulites were at high temperatures during the emplacement of the syenite magma. The cooling history of Puttetti syenite estimated in this study is closely comparable with the 3–4 o C/Ma cooling rate estimated for a granite pluton in a previous study from Madagascar. Our study suggests protracted cooling rates for the late Pan-African intrusives emplaced within the Gondwana crust, with a long residence history in a hot crust bore they were exhumed to shallower levels.     

Cooling history of the Puttetti alkali syenite pluton, southern India

The Puttetti alkali syenite pluton in southern India belongs to the suite of felsic magmatic intrusives emplaced during the Late Neoprotrozoic-Cambrian time during the final phase of amalgamation of the Gondwana supercontinent. In this study, we evaluate the cooling history of this pluton based on various isotopic systems. We present whole-rock Pb-Pb data on the syenite which yields an isochron age of 508±25Ma. Three phlogopite separates from the syenite pluton give K-Ar ages of 454.0±9.0, 448.5±8.9 and 445.6±8.8 Ma indicating cooling age at temperatures of 415°C. U-Pb analyses of zircons from this syenite yielded an age of 572±2 Ma in a previous study. With U-Pb closure temperatures >800 o C, this age probably indicates the timing of emplacement of the Puttetti pluton. Collectively, we estimate from the isotopic age data and respective closure temperatures that the syenite body cooled at about 3.2 o C/Ma from about 800 o C to about 415 o C. The markedly low cooling rate of the syenite pluton, absence of chilled margin effects, and common occurrence of pyroxene, feldspar, phlogopite and zircons megacrysts in the rock indicate that the host granulites were at high temperatures during the emplacement of the syenite magma. The cooling history of Puttetti syenite estimated in this study is closely comparable with the 3–4 o C/Ma cooling rate estimated for a granite pluton in a previous study from Madagascar. Our study suggests protracted cooling rates for the late Pan-African intrusives emplaced within the Gondwana crust, with a long residence history in a hot crust bore they were exhumed to shallower levels.     

Exhumation history of the Peake and Denison Inliers: insights from low-temperature thermochronology

Multi-method thermochronology applied to the Peake and Denison Inliers (northern South Australia) reveals multiple low-temperature thermal events. Apatite fission track (AFT) data suggest two main time periods of basement cooling and/or reheating into AFT closure temperatures (～60–120°C); at ca 470–440 Ma and ca 340–300 Ma. We interpret the Ordovician pulse of rapid basement cooling as a result of post-orogenic cooling after the Delamerian Orogeny, followed by deformation related to the start of the Alice Springs Orogeny and orocline formation relating to the Benambran Orogeny. This is supported by a titanite U/Pb age of 479 ± 7 Ma. Our thermal history models indicate that subsequent denudation and sedimentary burial during the Devonian brought the basement rocks back to zircon U–Th–Sm/He (ZHe) closure temperatures (～200–150°C). This period was followed by a renewal of rapid cooling during the Carboniferous, likely as the result of the final pulses of the Alice Springs Orogeny, which exhumed the inlier to ambient surface temperatures. This thermal event is supported by the presence of the Mount Margaret erosion surface, which indicates that the inlier was exposed at the surface during the early Permian. During the Late Triassic–Early Jurassic, the inlier was subjected to minor reheating to AFT closure temperatures; however, the exact timing cannot be deduced from our dataset. Cretaceous apatite U–Th–Sm/He (AHe) ages coupled with the presence of contemporaneous coarse-grained terrigenous rocks suggest a temporally thermal perturbation related with shallow burial during this time, before late Cretaceous exhumation cooled the inliers back to ambient surface temperatures.     

Diffusion kinetics of Cr in olivine and 53Mn–53Cr thermochronology of early solar system objects

We have determined the diffusion coefficient of Cr in olivine as function of temperature, oxygen fugacity (fO₂), and crystallographic orientation and used these data to develop a quantitative understanding of the resetting of the short-lived ⁵³Mn–⁵³Cr decay system in olivine during cooling within meteorite parent body. The diffusion of Cr in olivine was found to be anisotropic, and effectively independent of fO₂ between wüstite–iron buffer and two orders of magnitude above this buffer. The diffusion data were used to calculate the spatially averaged mean closure temperature of the ⁵³Mn–⁵³Cr decay system in olivine as function of the initial temperature, cooling rate and grain size, and also the closure age profile of this system in olivine single crystal as function of radial distance and a dimensionless parameter that incorporates the effects of various parameters that affect the closure age. We also present a thermochronolgic formulation that permits retrieval of cooling rates from the extent of resetting of the bulk ⁵³Mn–⁵³Cr closure age of olivine during cooling. This method was applied to determine the cooling rate of the pallasite Omolon, which showed ⁵³Mn–⁵³Cr bulk age of olivine that is 10 Myr younger than the age of the solar system. The calculated cooling rate, which is 20–40 °C/Myr at ∼985–1000 °C, is in good agreement with the metallographic cooling rate at ∼500 °C, when the two results are considered in terms of a cooling model in which the reciprocal temperature increases linearly with time. The inferred cooling rate of Omolon, which seems to be a sample from the core-mantle boundary, yields a burial depth of ∼30 km in a parent body of at least ∼100 km radius.     

Lateral thinking: 2-D interpretation of thermochronology in convergent orogenic settings

Lateral motion of material relative to the regional thermal and kinematic frameworks is important in the interpretation of thermochronology in convergent orogens. Although cooling ages in denuded settings are commonly linked to exhumation, such data are not related to instantaneous behavior but rather to an integration of the exhumation rates experienced between the thermochronological ‘closure’ at depth and subsequent exposure at the surface. The short spatial wavelength variation of thermal structure and denudation rate typical of orogenic regions thus renders thermochronometers sensitive to lateral motion during exhumation. The significance of this lateral motion varies in proportion with closure temperature, which controls the depth at which isotopic closure occurs, and hence, the range of time and length scales over which such data integrate sample histories. Different chronometers thus vary in the fundamental aspects of the orogenic character to which they are sensitive. Isotopic systems with high closure temperature are more sensitive to exhumation paths and the variation in denudation and thermal structure across a region, while those of lower closure temperature constrain shorter-term behaviour and more local conditions.Discounting lateral motion through an orogenic region and interpreting cooling ages purely in terms of vertical exhumation can produce ambiguous results because variation in the cooling rate can result from either change in kinematics over time or the translation of samples through spatially varying conditions. Resolving this ambiguity requires explicit consideration of the physical and thermal framework experienced by samples during their exhumation. This can be best achieved through numerical simulations coupling kinematic deformation to thermal evolution. Such an approach allows the thermochronological implications of different kinematic scenarios to be tested, and thus provides an important means of assessing the contribution of lateral motion to orogenic evolution.     

Metallographic cooling rates and origin of IVA iron meteorites

We have determined the metallographic cooling rates for 13 IVA irons using the most recent and most accurate metallographic cooling rate model. Group IVA irons have cooling rates that vary from 6600 °C/Myr at the low-Ni end of the group to 100 °C/Myr at the high-Ni end of the group. This large cooling rate range is totally incompatible with cooling in a mantled core which should have a uniform cooling rate. Thermal and fractional crystallization models have been used to describe the cooling and solidification of the IVA asteroid. The thermal model indicates that a metallic body of 150 ± 50 km in radius with less than 1 km of silicate on the outside of the body has a range of cooling rates that match the metallographic cooling rates in IVA irons in the temperature range 700-400 °C where the Widmanstätten pattern formed. The fractional crystallization model for Ni with initial S contents between 3 and 9 wt% is consistent with the measured variation of cooling rate with bulk Ni and the thermal model. New models for impacts in the early solar system and the evolution of the primordial asteroid belt allow us to propose that the IVA irons crystallized and cooled in a metallic body that was derived from a differentiated protoplanet during a grazing impact. Other large magmatic iron groups, IIAB, IIIAB, and IVB, also show significant cooling rate ranges and are very likely to share a similar history.     

Cooling and uplift histories of the crystalline thrust stack of the Indian Plate internal zones west of Nanga Parbat, Pakistan Himalaya

Amphibole and mica K-Ar, Ar-Ar and Rb-Sr geochronology for the crystalline internal zones of the Indian Plate define both an extensive pre-Himalayan thermal history and a post-Himalayan metamorphism cooling history. South of the Main Mantle Thrust, near Besham, hornblende Ar-Ar ages from basement gneisses record an ca. 1850 Ma mid-Proterozoic thermal event. Hornblende, muscovite and biotite cooling ages from cover sequences metamorphosed during the Himalayan orogeny are 35 ± 4, 30 to 24, and 29 to 22 Ma respectively. The mica ages, together with those derived from zircon and apatite fission track data (Zeitler, 1985) demonstrate a rate of cooling, of about 30°C/Ma, during the late Oligocene to early Miocene that was greater than that either before or since. This rapid cooling was initiated during the post-metamorphic evolution of the Indian Plate south-verging crustal-scale thrust stack, during which cover sequences metamorphosed during the Himalayan orogeny were imbricated with basement rocks thermally unaffected during that event. Most of the cooling, which happened during the stripping of some 10 ± 2 km of overburden, reflects exhumation due to a combination of erosion, recorded in the Miocene molasse sediments of the foreland basin, and major crustal extension within the MMT zone. Both erosion and extension were the direct consequence of the evolution of the thrust stack.     

Supercontinent tectonics and biogeochemical cycle： A matter of ‘life and death＇

<正>The formation and disruption of supercontinents have significantly impacted mantle dynamics,solid earth processes,surface environments and the biogeochemical cycle.In the early history of the Earth,the collision of parallel intra-oceanic arcs was an important process in building embryonic continents.Superdownwelling along Y-shaped triple junctions might have been one of the important processes that aided in the rapid assembly of continental fragments into closely packed supercontinents. Various models have been proposed for the fragmentation of supercontinents including thermal blanket and superplume hypotheses.The reassembly of supercontinents after breakup and the ocean closure occurs through "introversion","extroversion" or a combination of both,and is characterized by either Pacific-type or Atlantic-type ocean closure.The breakup of supercontinents and development of hydrothermal system in rifts with granitic basement create anomalous chemical environments enriched in nutrients, which serve as the primary building blocks of the skeleton and bone of early modern life forms. A typical example is the rifting of the Rodinia supercontinent,which opened up an N—S oriented sea way along which nutrient enriched upwelling brought about a habitable geochemical environment.The assembly of supercontinents also had significant impact on life evolution.The role played by the Cambrian Gondwana assembly has been emphasized in many models,including the formation of 'Trans-gondwana Mountains' that might have provided an effective source of rich nutrients to the equatorial waters,thus aiding the rapid increase in biodiversity.The planet has witnessed several mass extinction events during its history,mostly connected with major climatic fluctuations including global cooling and warming events,major glaciations,fluctuations in sea level,global anoxia,volcanic eruptions, asteroid impacts and gamma radiation.Some recent models speculate a relationship between superplumes,supercontinent breakup and mass extinction.Upwelling plumes cause continental rifting and formation of large igneous provinces.Subsequent volcanic emissions and resultant plume-induced "winter" have catastrophic effect on the atmosphere that lead to mass extinctions and long term oceanic anoxia.The assembly and dispersal of continents appear to have influenced the biogeochemical cycle,but whether the individual stages of organic evolution and extinction on the planet are closely linked to Solid Earth processes remains to be investigated.     

Multi‐genetic origin of the continental Moho: insights from Lithoprobe

This paper reviews four hypotheses for the origin of the continental crust–mantle boundary and discusses seismic parameters with which these hypotheses might be tested. The relict Moho hypothesis posits that the oceanic Moho is preserved during continental assembly; the magmatic underplating hypothesis posits formation of a new Moho by episodic emplacement of sill‐like intrusive bodies; the metamorphic (or metasomatic) front hypothesis posits that the Moho is overprinted by a phase transformation; and the regional décollement hypothesis posits that the Moho behaves as a structural detachment. These hypotheses are not mutually exclusive, and examples from the Canadian Lithoprobe program suggest that all four may be applicable in different regions of North America. Comparison of seismic images from a fossil subduction zone with modern subduction at Cascadia suggests that serpentinization of the forearc mantle, a previously unrecognized mechanism for overprinting and erasing the reflection Moho, may have occurred in the Proterozoic.     

From emplacement to unroofing: thermal history of the Jiazishan gabbro, Sulu UHP terrane, China

On the eastern extremity of the Jiaodong peninsula, China, shoshonitic magmas have been injected into the supracrustal rocks of the Sulu ultra-high pressure (UHP) terrane during the crustal exhumation phase. These granitoids (collectively termed the Shidao igneous complex or Jiazishan alkaline complex) show geochemical and isotopic signatures of an enriched subcontinental lithospheric mantle and intruded soon after the subducted Yangtze crust had reached peak metamorphic pressure conditions (240–220 Ma). We have applied various geochronometers to an alkali-gabbro sample from the Jiazishan pluton and the results allow reconstruction of the Triassic-to-present thermal history. Initial rapid cooling of the gabbro at crustal depths is indicated by the close agreement between the Sm-Nd mineral isochron age (228?±?36 Ma) and the Rb-Sr biotite age (207?±?1) Ma. This interpretation is confirmed by previously published U-Pb zircon ages (225–209 Ma), and ⁴⁰Ar/³⁹Ar amphibole and K-feldspar ages (～214 Ma) from the Jiazishan syenites. A titanite fission-track age of 166?±?8 Ma (closure temperature range 285–240°C) records widespread Jurassic magmatism in the Jiaodong peninsula, indicating that the gabbro reached upper crustal levels before it was reheated by nearby Jurassic plutons. A subsequent cooling and reheating event is indicated by an apatite fission-track age of 106?±?6 Ma which coincides with the emplacement of the adjacent Weideshan pluton (108?±?2 Ma) and postdates a period of regional lithospheric thinning beneath eastern China. A period of slow cooling (or thermal stability) from late Cretaceous to early Tertiary, documented by an apatite (U-Th)/He age of 39?±?5 Ma, was followed by a final stage of more enhanced cooling since the late Eocene. Results of this work imply that the eastern Sulu terrane has experienced a complex cooling and reheating history. Our data are consistent with a model of initial rapid cooling (sudden exhumation) of the UHP terrane, driven by the release of buoyancy forces, followed by two progressively slower cooling intervals (both after renewed crustal reheating) during the Jurassic and Cretaceous.     

Stony-iron meteorites: History of the metal phase according to tungsten isotopes

The global composition of the early solar system is thought to be roughly chondritic in terms of refractory components, and this means that metal and silicate should be present together in early planetesimals. To fully understand the metal-silicate differentiation process within the eucrite parent body (EPB), it is important to try and identify the metal reservoir that is complementary to the silicate part. The isotope 182 of tungsten (W), a siderophile element, is partly formed from the decay of ¹⁸²Hf, and W isotopes are useful for examining metal-silicate segregation. The W isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites. However, mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities. Therefore, we undertook the analysis of the metal phase of these stony-irons. Here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body (bodies) and to assess possible relationships with eucrites.All stony-iron metals are depleted in radiogenic tungsten by −1.3 to −4.2 ε units, relative to the terrestrial standard, while chondrites, for comparison, are depleted by −1.9 ε units. In addition to W isotopic heterogeneity from one stony-iron to another, there is also W isotopic heterogeneity within individual meteorites. A formation model is tentatively proposed, where we show that mesosiderites, pallasites, and eucrites could possibly come from the same parent body. Several hypotheses are discussed to explain the isotopic heterogeneity: the production of cosmogenic tungsten, the in situ decay of hafnium present in inclusions, and tungsten diffusion processes after metal-silicate mixing during the cooling of the meteorites. The two latter hypotheses provide the best explanation of our data.     

晚三叠世-早侏罗世祁连山东部隆升的裂变径迹年代学证据及地质意义

祁连山东北部为青藏高原隆升和东扩的前锋带,新生代以来经历了快速隆升和强烈剥露改造过程,致使前新生代地层面目全非,中生代陆内构造演化事件研究仍较薄弱,缺乏年代学的约束.为揭示和分析祁连山东部中生代构造隆升时限与过程,进而探讨秦祁造山带中生代陆内构造演化特点及区域动力学环境.主要采用物源分析、碎屑沉积物及基岩磷灰石裂变径迹定年,并结合裂变径迹热史反演模拟技术开展研究.研究表明,研究区侏罗系龙凤山组为近源的断陷盆地沉积,物源主要来自其周邻前中生代地层;其碎屑磷灰石裂变径迹未发生重置,年龄、径迹长度特征表明其源区在晚三叠世（±215 Ma）出现了快速冷却事件,同时东北部基岩裂变径迹热史模拟结果亦显示其较好地记录了该期事件,这与前人利用40Ar-39Ar年代学所揭示的西秦岭地区中晚三叠世快速抬升事件具时空统一性.分析表明研究区晚三叠世-早侏罗世发生了快速抬升事件,并认为该构造隆升事件是对中晚三叠世勉略洋闭合、秦岭最终碰撞造山过程的响应.      

云南个旧锡铜多金属矿区新山和高峰山岩体中高温热演化史及其与成矿的关系

云南个旧是全球最大的锡铜多金属矿床,主要成矿作用是与燕山期花岗岩密切有关的岩浆–热液体系。本文依据锆石U-Pb测年和~(40)Ar/~(39)Ar年代学对矿区内新山和高峰山花岗岩体进行测试分析,数据揭示矿区内南部和北部的花岗岩体的高–中温阶段热演化史曲线具有相似的演化趋势,只是冷却时间存在3~6 Ma的间隔。南部新山岩体于89~85 Ma形成,此后岩体经历了快速冷却过程,冷却速率为58.70~62.08℃/Ma。之后进入中温400~250℃的缓慢冷却过程,冷却速率为17.39~19.32℃/Ma,并持续到68~69 Ma。北部岩体的热演化史曲线明显滞后于南部岩体,北部高峰山岩体于83~82 Ma形成,之后经历快速冷却过程,冷却速率为295.59℃/Ma和103.29℃/Ma,于80 Ma进入400~250℃,此后以冷却速率为7.14~5.69℃/Ma,进入极其缓慢冷却过程并持续至67 Ma。矿区内花岗岩体先遭受快速冷却后进入中温阶段的缓慢长时间冷却作用过程,为锡铜主期成矿作用提供了持续的热源和流体运移动力学过程,也与矿区南、北部的成矿作用差异相吻合。