Magnesioferrite from the Cretaceous-Tertiary boundary,Caravaca, Spain

Magnesioferrite grading toward magnetite has been identified as a very small but meaningful constituent of the basal iron-rich portion of the Cretaceous-Tertiary (K-T) boundary clay at the Barranco del Gredero section, Caravaca, Spain. This spinel-type phase and others of the spinel group, found in K-T boundary clays at many widely separated sites, have been proposed as representing unaltered remnants of ejecta deposited from an earth-girdling dust cloud formed from the impact of an asteroid or other large bolide at the end of the Cretaceous period. The magnesioferrite occurs as euhedral, frequently skeletal, micron-sized octahedral crystals. The magnesioferrite contains29 ± 11 ppb Ir, which accounts for only part of the Ir anomaly at this K-T boundary layer(52 ± 1 ppb Ir). Major element analyses of the magnesioferrite show variable compositions. Some minor solid solution exists toward hercynite-spinel and chromite-magnesiochromite. A trevorite-nichromite (NiFe₂O₄-NiCr₂O₄) component is also present. The analyses are very similar to those reported for sites at Furlo and Petriccio, Umbria, Italy.On the basis of the morphology and general composition of the magnesioferrite grains, rapid crystallization at high temperature is indicated, most likely directly from a vapor phase and in an environment of moderate oxygen fugacity. Elemental similarity with metallic alloy injected into rocks beneath two known impact craters suggests that part of the magnesioferrite may be derived from the vaporized chondritic bolide itself, or from the mantle; there is no supporting evidence for its derivation from crustal target rocks.     

A test of terminal Mesozoic “catastrophe”

Terminal Mesozoic “catastrophe”-type extinction models that advocate synchronous marine and terrestrial extinctions spanning short time intervals (a few days up to a few millennia) have a common foundation: the simultaneous terminations of geological ranges of some taxa of marine CaCO₃-producing microplankton (and possibly the dinosaurs) at the end of the Cretaceous. Gartner and McGuirk [1] propose a new catastrophe theory that at the end of the Cretaceous fresh-brackish water from the Arctic Ocean spread over the surface of the world's oceans, causing global cooling, aridity, and the extinctions. Like other catastrophe models, this one also fails to address the possibility of hiatus control of ranges at the end of the Cretaceous; a well documented, seemingly nearly universal hiatus of variable and unknown duration separates Cretaceous and Tertiary strata. Documented terminal Cretaceous marine regression (perhaps 10 times more rapid than a typical regression according to Cooper [8] would have caused terrestrial erosion and stripping away of the latest Cretaceous stratigraphic record, thus truncating geological ranges along a seemingly planar datum. The terminal Cretaceous marine CaCO₃ dissolution event would have had the same effect on ranges of marine planktonic CaCO₃-producing microplankton (the event was a shallow-water phenomenon). The simultaneous terminations of geological ranges is thus possibly the result of hiatus control, and the terminal Cretaceous “catastrophe” an illusion. Attempts to use Cretaceous-Tertiary transition floras to support global cooling at the time of the extinctions are not based on sound stratigraphic foundations; realistic paleobotanical-climatic inferences can only be based on the precise correlation of the Cretaceous-Tertiary contact in marine and terrestrial stratigraphic sections, and these correlations have not been made with sufficient precision to support catastrophe theory. The much used “across the Cretaceous-Tertiary boundary” glosses over ignorance of the true terminal Cretaceous scenario, lost forever in most places by the destruction of the terminal Cretaceous stratigraphic record. For now, stable isotope paleotemperature data from marine strata that can be dated radiometrically provide the most reliable estimates of the Cretaceous-Tertiary transition climate; Boersma et al. [5] indicate global warming of deep and shallow oceans “across” the contact (and not surficial cooling only as is required by the spillover model). Older much-cited climate inferences based on leaf physiognomy are suspect in light of Dolph and Dilcher's [23] work that shows little correlation between leaf physiognomy and climate.     

Effects of cation disordering in a natural MgAl2O4 spinel observed by rectangular parallelepiped ultrasonic resonance and Raman measurements

At moderate temperatures, the elastic properties of natural MgAl₂O₄ spinel differ in several significant ways from properties of synthetic spinels. Below 1000 K, the ultrasonic resonant frequencies of an ordered natural spinel change significantly after heat treatment; at higher temperatures, both types of spinels have similar resonant responses. The temperature derivatives of the elastic constants of an ordered spinel also differ from those of disordered spinels at moderate temperatures; again, at higher temperatures, both types of spinels have similar behaviors. The Raman spectra also differ below 1000 K for ordered natural and disordered spinels and are similar at higher temperatures and after cooling to ambient temperature. We associate these changes in ultrasonic resonance and Raman spectra of spinel with cation disordering at high temperature which may be quenched by cooling. We deduce estimates of the inversion parameter from the relative intensities of the two A_1g Raman modes in very good agreement with estimates made from other measurements. We find thatC ₁₁ andC ₁₂ decrease by 4 and 8%, respectively, with 20% inversion in spinel;C ₄₄ is less sensitive to cation order. These results imply that previous measurements of the adiabatic elastic constants of spinels at ambient conditions have been affected by the state of cation disorder of the specimen.     

镍锗尖晶石位错域的高温蠕变实验研究

利用人工合成的多晶材料研究了镍锗尖晶石在位错域的高温蠕变性质.多晶材料的颗粒尺寸约8μm.单轴压缩试件为圆柱状,使用气体介质围压筒.常压蠕变试验过程中,围压为300MPa,温度为1373-1523K,应力在55-330MPa范围内.从实验结果得出了镍锗尖晶石在位错域的流动律,应力指数n=29±01,表明流动的微观机制为位错蠕变.与其他尖晶石进行对比可以发现一个力学同构群,虽然在正尖晶石和反尖晶石之间存在一定的差异.在位错蠕变域,尖晶石与橄榄石归一化的强度类似.由于尖晶石的剪切模量比橄榄石高50%,其实际强度也比橄榄石高.将橄榄石和尖晶石的蠕变数据外推到地球内部条件时,由于其高应力指数,橄榄石则有可能比尖晶石的强度高.     

Three thousand years of flank and central vent eruptions of the San Salvador volcanic complex (El Salvador) and their effects on El Cambio archeological site: a review based on tephrostratigraphy

The volcanic events of the last 3,000?years at San Salvador volcanic complex are reviewed using detailed stratigraphic records exposed in new excavations between 2005 and 2007, at El Cambio archeological site (Zapotitán Valley, El Salvador), and in other outcrops on the northern and northwestern sectors of the complex. The sequences that overlie Tierra Blanca Joven (cal. 429?±?107?ad), from the Ilopango caldera, comprise the Loma Caldera (cal. 590?±?90?ad) and El Playón (1658?C1671) deposits and the San Andrés Tuff (cal. 1031?±?29?ad), related to El Boquerón Volcano. The surge deposits within the El Playón, San Andrés Tuff and overlying Talpetate II sequences indicate the significance of phreatomagmatic phases in both central vent and flank eruptions during the last 1,600?years. Newly identified volcanic deposits underlying Tierra Blanca Joven at El Cambio extend the stratigraphic record of the area to 3,000?years?bp. Paleosols interstratified with those deposits contain cultural artifacts which could be associated with the Middle Preclassic period (900?C400?bc). If correct, human occupation of the site during the Preclassic period was more intense than previously known and volcanic eruptions must have affected prehistoric settlements. The archeological findings provide information on how prehistoric populations dealt with volcanic hazards, thousands of years ago in the eastern Zapotitán Valley, where several housing projects are currently being developed. The new stratigraphic and volcanological data can be used as a basis for local and regional hazard assessment related to future secondary vent activity in the San Salvador Volcanic Complex.     

Elemental and stable isotope variations of organic matter from a terrestrial sequence containing the Cretaceous/Tertiary boundary at York Canyon,New Mexico

Elemental and isotopic composition of organic matter from a terrestrial sequence including the palynological Cretaceous/Tertiary (K-T) boundary together with an Ir anomaly at York Canyon, New Mexico, record information about paleoclimatological and environmental conditions. Six layers of coal, carbonaceous shale and mudstone with high contents of organic material were selected for analysis. A Late Cretaceous coal bed 10 m below the K-T boundary and an Early Tertiary coal bed containing the K-T iridium anomaly at its base were sampled intensively. In the lower bed, the isotopic ratios¹³C/¹²C,¹⁵N/¹⁴N,andD/H and the elemental ratiosC/N andN/H, all varied sympathetically with one another over depth. In contrast, in the upper coal layer, only theD/H,C/N,andN/H ratios showed some coupling. Immediately above the K-T boundary, theδ¹³C values displayed a long-term shift of 1.8‰ to more negative values, while the hydrogen isotope ratios in these samples did not change significantly. We interpret the covariations in both coal layers as sympathetic responses of the isotopic and elemental ratios to climatic and environmental changes, as have been observed for younger sedimentary organic matter. The long-termδ¹³C shift during the early Tertiary is similar to the trend observed forδ¹³C values of marine carbonates. Our data thus support the proposal that the carbon cycle was perturbed globally by the effects of a drastically decreased marine bioproductivity along the K-T transition. The uncoupling of variations in the climatically sensitive isotopic and elemental ratios seen in the Early Tertiary coal bed provides evidence for major geochemical and environmental changes in the York Canyon area at the end of the Mesozoic. On the other hand, the constancy of δD values in the organic matter deposited at and above the K-T boundary gives no indication of significant changes in the hydrologic/climatic regimes as recorded in theD/H ratios at the site for several thousand years following the event which produced the high Ir concentrations. Our results provide constraints on models that have been advanced to explain that event and its consequences.     

Explosive felsic volcanism on El Hierro (Canary Islands)

The Canary Islands consist of seven basaltic shield volcanoes whose submerged portion is much more voluminous than the subaerial part of each island. Like so many other volcanic oceanic islands, the indicative deposits of explosive felsic volcanism are not a common feature on the Canary archipelago. Hitherto, they have only been documented from the central islands of Gran Canaria and Tenerife, which are the largest volcanic complexes of the islands. On the other Canary Islands, the presence of felsic rocks is mostly restricted to intrusions and a few lava flows, generally within the succession in the oldest parts of individual islands. In this paper, we present a detailed stratigraphic, lithological and sedimentological study of a significant felsic pumice deposit on the island of El Hierro, referred here as the Malpaso Member, which represents the only explosive episode of felsic volcanism found on the Canary Islands (outside of Gran Canaria and Tenerife). The products of the eruption indicate a single eruptive event and cover an area of about 15 km². This work provides a detailed stratigraphic and chronological framework for El Hierro, and four subunits are identified within the member on the basis of lithological and granulometric characteristics. The results of this study demonstrate the importance of an explosive eruption in a setting where the activity is typified by effusive basaltic events. Given the style and the spatial distribution of the Malpaso eruption and its products, a future event with similar characteristics could have a serious impact on the population, infrastructure and economy of the island of El Hierro.     

Effects of melt percolation on the Re-Os systematics of continental mantle lithosphere: A case study of spinel peridotite xenoliths from Heilongjiang,NE China

Os isotope ratios of mantle peridotites have been considered to be largely immune to recent melt-rock interaction. However, Os isotope ratios and PGE (Platinum group elements) concentrations of the Yong’an xenoliths have been significantly modified by melt percolation, and are not suitable for determining the formation age of lithosphere mantle in Yong’an. In this study, the Yong’an spinel peridotite xenoliths are divided into two groups: N-Type and E-Type. The N-Type group including cpx (clinopyroxene)-poor lherzolite and harzburgite, shows a large variation of Cr#(sp) (13.2-48) and sulfur contents (from 171 ppm to below detection limit), whereas the E-Type peridotites are mainly refractory harzburgites and are characterized by high Cr#(sp) (35.3-42.2) and overall low sulfur contents (below 51 ppm). Both types show similar major and REE (rare earth element) patterns. Furthermore, the N-Type peridotites display a restricted range of iridium-group PGE (IPGE), Os/Ir and Ru/Ir ratios (Os/Ir = 0.64-1.12, Ru/Ir = 1.52-1.79) and variable palladium-group PGE (PPGE) contents (3.4-14.9 ppb), whereas the E-Type peridotites show a large variation of Os/Ir and Ru/Ir ratios (Os/Ir = 0.33-0.84, Ru/Ir = 0.94-1.6), and a restricted range of PPGE (4.3-6.9 ppb). 187Os/188Os ratios of E-Type peridotites are higher than those of N-Type peridotites at comparable fertility levels. These results suggest that N-Type peridotites may have been overprinted by metasomatism via small melt fractions, in which the percolation of the volatile-rich, small melt fractions only resulted in LILE (large ion lithophile element) enrichment of clinopyroxene, and their whole rock PGE contents and Re-Os isotope values were little changed. Moreover, E-Type peridotites may have been modified by melt-rock reaction involving relatively large melt fractions, which may result in the formation of secondary cpx and olivine and the removal of IPGE-bearing minerals such as Ru-Os-(Ir) alloys or laurite, followed by precipitation of secondary sulfides from melt with radiogenic isotopic signature.     

Experimental crystallization of chrome spinel in FAMOUS basalt 527-1-1

FAMOUS basalt 527-1-1 (a high-Mg oceanic pillow basalt) has three generations of spinel which can be distinguished petrographically and chemically. The first generation (Group I) have reaction coronas and are high in Al₂O₃. The second generation (Group II) have no reaction coronas and are high in Cr₂O₃ and the third generation (Group III) are small, late-stage spinels with intermediate Al₂O₃ and Cr₂O₃. Experimental synthesis of spinels from fused rock powder of this basalt was carried out at temperatures of 1175–1270°C and oxygen fugacities of 10^?5.5 to 10^?10 atm at 1 atm pressure. Spinel is the liquidus phase at oxygen fugacities of 10^?8.5 atm and higher but it does not crystallize at any temperature at oxygen fugacities less than 10^?9.5. The composition of our spinels synthesized at 1230–1250°C and 10^?9 atmf_O2 are most similar to the high-Cr spinels (Group II) found in the rock. Spinels synthesized at 1200°C and 10^?8.5 atm_O2 are chemically similar to the Group III spinels in 527-1-1. We did not synthesize spinel at any temperature or oxygen fugacity that are similar to the high-Al (Group I) spinel found in 527-1-1. These results indicate that the high-Cr (Group II) spinel is the liquidus phase in 527-1-1 at low pressure and Group III spinel crystallize below the liquidus (～1200°C) after eruption of the basalt on the sea floor. The high-Al spinel (Group I) could have crystallized at high pressure or from a magma enriched in Al and perhaps Mg compared to 527-1-1.     

Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence

Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO^*/FeO^* + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.     

A multi-isotopic and trace element investigation of the Cretaceous-Tertiary boundary layer at Stevns Klint, Denmark - inferences for the origin and nature of siderophile and lithophile element geochemical anomalies

Os, Sr, Nd and Pb isotope data were collected from a profile across the Cretaceous-Tertiary (K-T) boundary layer at Stevns Klint, Denmark. ?_Nd [T=65 Ma] values from within the boundary layer (Fish Clay) are lower by ∼1 ? unit than those of the underlying Maastrichtian limestone and the overlying Danian chalk sequences. Systematic profile-upward changes of Pb, Sr and Os isotopic compositions and concentrations in the boundary layer cannot be accounted for by in situ growth of daughter products since the sedimentation of the Fish Clay. While Os, Nd and Pb isotopes indicate the admixing of less radiogenic components to the Fish Clay, Sr isotopes show elevated radiogenic values in the boundary layer, relative to the carbonate sequences beneath and above it. The sudden change in lithophile (e.g., Sr, Pb and Nd) isotope compositions at the base of the Fish Clay and profile-upward trends of ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios towards those of the overlying Danian chalk are interpreted to reflect recovery from enhanced, acid rain-induced continental (local?) weathering input to the seawater. However, a continental crustal source is invalid for the siderophile element Os. In the light of evidence from chromium isotopes for a cosmic origin of the platinum group elements (PGEs) and certain moderately siderophile elements (Cr, Ni, Co, V) in K-T boundary sediments, including Stevns Klint [Shukolyukov and Lugmair, Science 282 (1998) 927-929], and supported by the finding of projectile debris [Bauluz et al., Earth Planet. Sci. Lett. 182 (2000) 127-136] and the occurrence of abundant Ni-rich spinel at many K-T sites [Robin et al., Nature 363 (1993) 615-617; Kyte, Nature 396 (1998) 237-239], we favor to explain the sudden drop of ¹⁸⁷Os/¹⁸⁸Os ratios from 0.210 to 0.160 at the K-T boundary to derive from global fall-out of extraterrestrial matter. The present ¹⁸⁶Os/¹⁸⁸Os ratio of 0.119836±0.000004 measured in the basal layer of the Fish Clay is within the uncertainty a chondritic value. We therefore exclude the possibility of a major contribution of PGEs to the sediment from iron meteorites. Chondrite-normalized (Ru/Ir)_N ratios of ∼0.95±0.14 and (Os/Ir)_N ratios of ∼0.93±0.14 in the Fish Clay cannot distinguish between abundance ratios of different types of chondrites, and strongly sub-chondritic (Pt/Ir)_N ratios of ∼0.62±0.09 (2σ) suggest differential PGE remobilization through the sedimentary column (and consequently the alteration of inter-element ratios). PGEs and the moderately siderophile elements Cr, Ni, V, and Co form an elemental association with systematically upward-decreasing concentrations in the Fish Clay. Low Co/Ni ratios of ∼0.12 in the Fish Clay relative to values of ∼0.35 in the over- and underlying carbonate sequences support mixing of meteorite-derived (Co/Ni ∼0.05) and terrestrial upper mantle/crustal (Co/Ni >∼0.3) sources. While lithophile element isotope data indicate an increased continental crustal input to the Fish Clay at the K-T transition, the uncertainty with respect to possible post-depositional alteration of abundance patterns of siderophile and moderately siderophile elements - though not affecting the chondritic isotopic composition of Os - does not allow confirmation of indications from chromium isotopes for a carbonaceous (CV-type) meteorite as the preferred K-T impactor type by Shukolyukov and Lugmair [Science 282 (1998) 927-929].     

Untangling the Palaeocene climatic rhythm: an astronomically calibrated Early Palaeocene magnetostratigraphy and biostratigraphy at Zumaia (Basque basin, northern Spain)

The magnetostratigraphy of a 54-m-long section above the Cretaceous-Tertiary boundary at the sea-cliff section of Zumaia in the Basque basin (northern Spain) has been established. The section encompasses the entire Danian and the lower part of the Selandian stages as indicated by calcareous plankton biostratigraphy. The studied interval consists of (hemi)pelagic limestone-marl alternations in the form of couplets and bundles, which range from centimetre/decimetre to metre scale respectively and a few thin-bedded calcareous turbidites. The magnetostratigraphy, based on samples from about 200 stratigraphic levels, allows the identification of six reversal boundaries from chron C29r to C26r at a bed level. The spatial (or temporal) evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique. A preliminary age model based on the standard CK95 GPTS indicates that the basic lithologic carbonate-marl couplet corresponds to the 19-23-kyr precession cycle (21-31-cm cycle in the depth domain) and that a bundle cycle (usually groups of four to six basic couplets) with global periodicity centred at 1.22 m corresponds to the ∼110-kyr eccentricity cycle. We have tuned the bundle cycles to the Va03_R7 eccentricity orbital solution [Astrophys. J. 592 (2003) 620-630] following an initial match of a node of the ∼2.4-Ma eccentricity modulatory cycle in the target time series to particularly carbonate-rich bundles from the upper part of the Zumaia section that displays significant power of a 4.4-m-period cycle corresponding to the ∼404-kyr eccentricity cycle. Consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the ∼404-kyr eccentricity cycle is not persistent throughout. The tuning, however, appears robust as it brings the age of the K/T boundary at ∼65.8 Ma. It is argued that a sea-level signal (tectonically driven?) is superimposed on the climatic forcing at the Milankovitch band masking the full expression of the low-frequency astronomical periods. We provide a cycle-tuned duration for all intervening Early Palaeocene polarity chrons and estimate relative ages for bioevents. The cycle-tuned chronology indicates that the CK95 GPTS overestimates the duration of chrons C28 and C27 by 20 and 26% respectively. Our data may prove useful to better constrain Early Palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the Danian and Selandian stages.     

厄尔尼诺持续时间与大气环流异常形势

针对不同持续时间的El Nio事件,进行了大尺度大气环流及其演变的合成分析研究.其结果清楚地表明,不同持续时间的El Nio事件的发生、发展和消亡过程,对流层低层风场和对流层高层速度势场的距平都有极为显著差异.分析得到了对El Nio事件的发生和消亡起着重要作用的大气环流异常形势.还发现对于持续时间较长的El Nio事件,东北太平洋上850hPa异常气旋性环流减弱和西北太平洋上异常反气旋性环流增强较慢,因此赤道太平洋异常西风维持的时间也较长,而与澳大利亚冬季风加强相关联的南半球西太平洋的速度势正距平的维持,对El Nio的持续也起一定作用;对应持续时间较短的El Nio事件,西太平洋上200hPa速度势正距平的迅速东移,对El Nio的迅速消亡起重要作用.     

Chemical characteristics of chromian spinel in plutonic rocks: Implications for deep magma processes and discrimination of tectonic setting

We summarize chemical characteristics of chromian spinels from ultramafic to mafic plutonic rocks (lherzolites, harzburgites, dunites, wehrlites, troctolites, olivine gabbros) with regard to three tectonic settings (mid‐ocean ridge, arc, oceanic hotspot). The chemical range of spinels is distinguishable between the three settings in terms of Cr# (= Cr/(Cr + Al) atomic ratio) and Ti content. The relationships are almost parallel with those of chromian spinels in volcanic rocks, but the Ti content is slightly lower in plutonics than in volcanics at a given tectonic environment. The Cr# of spinels in plutonic rocks is highly diverse; its ranges overlap between the three settings, but extend to higher values (up to 0.8) in arc and oceanic hotspot environments. The Ti content of spinels in plutonics increases, for a given lithology, from the arc to oceanic hotspot settings by mid‐ocean ridge on average. This chemical diversity is consistent with that of erupted magmas from the three settings. If we systematically know the chemistry of chromian spinels from a series of plutonic rocks, we can estimate their tectonic environments of formation. The spinel chemistry is especially useful in dunitic rocks, in which chromian spinel is the only discriminating mineral. Applying this, discordant dunites cutting mantle peridotites were possibly precipitated from arc‐related magmas in the Oman ophiolite, and from an intraplate tholeiite in the Lizard ophiolite, Cornwall.     

On steps and methods for the establishment of global boundary stratotype section and point (GSSP) from the viewpoint of integrated stratigraphy

The steps and methods for the establishment of the global boundary stratotype section and point (GSSP) are summed up briefly as follows. (i) Select rock sequences of approximately the same age duration in a region, make a thorough study of their properties or attributes in order to establish the high-resolution stratigraphic units reflecting the natural rhythms in Earth's history, and proceed by multidisciplinary comprehensive studies to reveal the relationship, including time-space relation and possible mutual causality, among the various stratigraphic units and the different natural rhythms established. (ii) Seek for the "natural break" that represents the "major natural changes in the historical development of the Earth" in shallow marine facies areas, which is frequently the third-order sequence boundaries. (iii) Trace from shallow marine facies areas toward the continental slope and bathyal areas to seek for a continuous depositional sequence that corresponds in time span to the "natural break" of shallow marine facies areas. (iv) Seek for a horizon within the continuous depositional sequence that approximately coincides with the maximum regressive point in the "natural break". This horizon is commonly within a lowerstand systems tract (LST) or a shelf margin systems tract (SMST) of the relevant third-order sequence. (v) Seek immediately above this horizon of maximum marine regression for an organic radiation or explosion event closely related to the natural boundary, which is generally the first flooding surface (FFS) of the relevant third-order sequence. (vi) Select within the organic event deposits closely related to the FFS the base boundary of a fossil taxon with widest geographical range as the Leading Group biozone for designation of the stratigraphic boundary. (vii) Select from among the sections with continuous depositional sequence formed under similar sedimentary palaeogeographic background (in general continental slope or bathyal environments) the section which possesses the shortest distance between the base boundary of the Leading Group biozone and the immediately underlying FFS of the relevant third-order sequence as the global stratotype section. The first appearance datum (FAD) of the Leading Group biozone species in the section may be regarded as the ideal GSSP. The steps suggested above are a supplement and improvement of the currently used procedures and methods for establishing GSSP. The GSSP established by using the steps and methods suggested here would make the stratigraphic boundaries better reflect the "major natural changes in the historical development of the Earth", more readily distinguishable and easily operable in recognition and correlation, and at the same time also make the designation of boundaries more objective.     

Chemical composition of boulder-2 rocks and soils,Apollo 17, station 2

Forty-two elements have been measured via INAA and RNAA in six samples of five rocks from a 2-m tan-gray boulder-2 breccia (South Massif), in four soils from the South Massif and a valley soil. The chemical composition of the four metaclastic rocks corresponds to “high alumina” (52% Pl) and medium-K KREEP-type rocks. Rock 72335,2 is a medium K anorthositic gabbro (74% Pl). Both the North and South Massifs appear to be medium-K KREEP in composition and thus may represent a single stratigraphic unit of the Serenitatis basin event. Four soils of the South Massif are identical in composition to medium-K KREEP; they are more feldspathic and lower in LIL trace elements relative to the boulder-2 rocks. The valley soil 75081 is like the 10084 soil; both soils are high in TiO₂ and both are deficient in KREEP. Th and U give a sharp distinction between the valley and highland soils. The South Massif rocks and soils contain siderophiles at the 2–4% Cl level and show an ancient meteoritic pattern. Five samples of the four rocks have Ir/Au ratios of ± 0.02 which we assign to the Serenitatis basis planetesimal. The valley soil at Camelot Crater has low siderophiles (1% Cl). Our systematic study of four shadowed and exposed soils does not support the labile hypothesis for Cd, In, Tl and Zn. We observe no volatile (atmophile) movement from the South Massif highland soils to the valley soil 75081. The volatiles Cs and Tl appear to have been fractionated in the boulder-2 rocks during cratering, brecciation and metamorphic processes. The uniform ratio of FeO/MnO = 80–85, observed for all previous mare and highland sites, also holds for the Taurus-Littrow site.     

Noble gas isotopic abundances and noble metal concentrations in sediments from the Cretaceous-Tertiary boundary

Noble gases in four sediment samples from the Cretaceous-Tertiary boundary collected near Stevns in Denmark were investigated to test the possibility of the presence of noble gases indicative for meteorites. All samples were also analyzed for the noble metals Os and Ir and twelve other elements. The observed enrichments of³He,⁴He and of Ar, Kr, and Xe relative to atmospheric abundances can be explained without invoking the addition of extraterrestrial material. The ²⁰Ne/²²Ne ratio and the Kr and Xe isotopic compositions are identical with the isotopic ratios in the terrestrial atmosphere. In contrast, the high noble metal concentrations indicate the presence of material with elemental abundances similar to chondritic matter. Compared to noble metals the noble gases are less sensitive tracers of an admixture of extraterrestrial matter in sediments.     

Anomalous iridium enrichment at the Triassic–Jurassic boundary, Blomidon Formation, Fundy basin, Canada

We present new analyses that confirm Ir enrichment (up to 0.31 ng/g) in close proximity to the palynological Triassic–Jurassic boundary in strata near the top of the Blomidon Formation at Partridge Island, Nova Scotia. High Ir concentrations have been found in at least two samples within the uppermost 70 cm of the formation. Ratios of other PGEs and Au to Ir are generally higher by an order of magnitude than in ordinary chondrites. No impact-related materials have been identified at this horizon in the Blomidon Formation, therefore we cannot confirm an extraterrestrial source for the anomalous Ir levels. We consider, however, the possibility that regional basaltic volcanism is a potential source for the Ir in these sediments. The elevated Ir concentrations are found in reduced, grey colored mudstones, so redox concentration is a possible explanation for the distribution of Ir in these strata.     

由包体推导的河北汉诺坝下地壳-上地幔地温线及其地质意义

通过对采自河北汉诺坝玄武岩中的下地壳和上地幔包体的详细研究 ,建立了本区下地壳—上地幔地温线。该地温线高于大洋地温线和古老地盾地温线 ,接近克拉通边缘的地温线 ,符合该区的大地构造环境。由该地温线建立的下地壳—上地幔地质结构剖面表明 ,该区下地壳主要由不同类型的麻粒岩相岩石组成 ,其化学成分以镁铁质为主 ,深度范围为 2 5～ 4 2km。上地幔由超镁铁质的二辉橄榄岩组成 ,在尖晶石二辉橄榄岩和石榴石二辉橄榄岩之间有一过渡层。由地温线确定的壳幔边界位于 4 2km附近 ,与地震资料确定的莫霍面一致 ,但在壳幔边界之上的下地壳底部有下地壳麻粒岩和超镁铁质岩的互层。这一现象可以解释在下地壳底部常见的层状反射层。该区岩石圈底界大约在 95km ,其下的软流层仍由石榴石二辉橄榄岩组成     

Occurrence of chromian,hercynitic spinel (“pleonaste”) in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An₈₀-An₉₄), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide.