Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P–T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.     

Fossil Association from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges Area, Hubei, South China

Apart from previously reported Small Shelly Fossils (SSFs), a macroscopic fossil assemblage, comprising abundant algae, cone-shaped tubular fossil forms, and probable impressions of a megascopic metazoan, comes from the Lower Cambrian Yanjiahe Formation in the Yangtze Gorges area of western Hubei Province, south China. The visible fossils are preserved in thin-laminated siltstone or muddy siltstone intercalated between 8–15 mm-thick carbonate deposits, probably representing sedimentary settings of a constrained local depression in the shallow water carbonate platform during the Early Cambrian Meishucunian Stage. The macroscopic fossil association provides significant fossil evidence about the evolution of life from the late Precambrian to the ‘Cambrian explosion’ interval.     

Numerical modeling of thermal structure, circulation of H2O, and magmatism–metamorphism in subduction zones: Implications for evolution of arcs

Numerical models on thermal structure, convective flow of solid, generation and transportation of H₂O-rich fluid in subduction zones are consolidated to have a comprehensive view of the subduction zone processes: heat balance, circulation of H₂O magmatism–metamorphism, growth of arcs and continental margins. A large scale convection model with steady subduction of a cold old slab (130 Myr old) predicts rapid ( 100 Myr) cooling of subduction zones, resulting in cessation of magmatism. The model also predicts that the mantle temperature beneath arcs and continental margins is greatly affected by the effective temperature of the subducting slab, i.e., the age of the subducting slab. If subduction of a young hot slab, including ridge subduction, occurs every 60 to 120 Myr as is suggested for eastern Asia, the average temperature beneath arcs is increased by about 300 °C, which may explain the long-lasting magmatism in eastern Asia. Associated with subduction of young slabs and ridges, thermal structure and circulation of H₂O are greatly modified to cause a transition from (1) normal arc magmatism, (2) forearc mantle melting, to (3) slab melting to produce a significant amount (100 km³) of granitic melts, associated with both high-P/T and low-P/T type metamorphism. The last stage of (3) can result in formation of a granitic batholith belt and a paired metamorphic belts. Synthesis of the numerical models and observations suggest that episodic subduction of young slabs and ridges can explain heat source for generating a large amount of granitic magmas of batholiths, synchronous formation of batholith and regional metamorphic belts, and P–T conditions of the paired metamorphism. Even the high-P/T metamorphism requires an elevated geothermal structure in the forearc region, associated with ridge subduction. Although the emplacement of the batholiths and the regional metamorphic belts, and the mass balance in subduction zones are not well constrained at present, the episodic event associated with ridge subduction is thought to be essential for net growth of arcs and continental margins, as well as for the long-term heat balance in subduction zones.     

Kurosegawa zone and its bearing on the development of the Japanese Islands

The Kurosegawa zone in southwest Japan is a 600 km long serpentinite mélange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone:

1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma,

2. (2) Siluro-Devonian formations,

3. (3) Upper Carboniferous to Jurassic formations,

4. (4) Upper Jurassic to Lower Cretaceous formations,

5. (5) serpentinite and

6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar).

The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units.Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.     

A generalization of the minimum variance analysis method

In order to determine the normal direction of the magnetopause, the minimum variance analysis technique is frequently used: it is applied to the magnetic field data of a magnetopause crossing observed by a satellite, and provides the direction along which the magnetic field variation is minimum. In this study we propose a method to extend naturally the framework of the minimum variance analysis so that it includes all continuity conditions across a planar magnetopause, so that we could use all physical quantities observed by the spacecraft. We first extend it to the electric field: we discuss how to determine the normal direction and the speed of the magnetopause along the normal by using both the magnetic and electric field data. We next discuss the full extension to all continuity conditions across the magnetopause. Finally, we discuss how to extend our method further so that it can deal with the magnetopause which is accelerating. A discussion of application to observations will appear separately.     

Determination of the Drag Coefficient over the Tibetan Plateau

In this paper,a preliminary study is given on the drag (i.e.bulk transfer for momentum) coefficient,on the basis of data from four sets of AWS in Tibet during the first observational year from July 1993 to July 1994 according to China Japan Asian Monsoon Cooperative Research Program.The results show that the drag coefficient over the Tibetan Plateau is 3.3 to 4.4×103.In addition,monthly and diurnal variations of drag coefficient and the relationship among the drag coefficients and the bulk Richardson number,surface roughness length and wind speed at 10 m height are discussed in detail.     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.     

Nine requirements for the origin of Earth's life:Not at the hydrothermal vent,but in a nuclear geyser system

The origin of life on Earth remains enigmatic with diverse models and debates.Here we discuss essential requirements for the first emergence of life on our planet and propose the following nine requirements:(1)an energy source(ionizing radiation and thermal energy);(2)a supply of nutrients(P.K.REE.etc.);(3)a supply of life-constituting major elements;(4)a high concentration of reduced gases such as CH_4,HCN and NH_3;(5)dry-wet cycles to create membranes and polymerize RNA;(6)a non-toxic aqueous environment;(7)Na-poor water;(8)highly diversified environments,and(9)cyclic conditions,such as dayto-night,hot-to-cold etc.Based on these nine requirements,we evaluate previously proposed locations for the origin of Earth's life,including:(1)Darwin's "warm little pond",leading to a "prebiotic soup" for life;(2)panspermia or Neo-panspermia(succession model of panspermia);(3)transportation from/through Mars;(4)a deepsea hydrothermal system;(5)an on-land subduct ion-zone hot spring,and(6)a geyser systems driven by a natural nuclear reactor.We conclude that location(6)is the most ideal candidate for the o rigin point for Earth's life because of its efficiency in continuously supplying both the energy and the necessary materials for life,thereby maintaining the essential "cradle" for its initial development.We also emphasize that falsifiable working hypothesis provides an important tool to evaluate one of the biggest mysteries of the universe-the origin of life.