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].     

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.     

Palaeomagnetism of the dyke swarms of the Gardar Igneous Province,south Greenland

In the western part of the Gardar Igneous Province of southern Greenland, lamprophyre dykes intruded at ca. 1276-1254 m.y. RbSr biotite ages yield a palaeomagnetic pole at 206.5°E,3°N (nine sites, dψ = 5.1°, d_χ = 10.1°) Slightly younger dolerite dykes with RbSr biotite ages in the range 1278-1263 m.y. give a pole at 201.5°E,8.5°N (24 sites, dψ = 4.7°, d_χ = 9.4°), and the syeno-gabbro ring dyke of the Kûngnât complex (RbSr isochron age 1245 ± 17 m.y.) cutting both of these dykes swarms, gives a pole at 198.5°E, 3.5°N (four sites, dψ = 2.3°,d_χ = 4.4°). All these rock units have the same polarity and the poles are identical to those from Mackenzie and related igneous rocks of North America (1280-1220 m.y.) after closure of the Davis Strait; they confirm that this part of the Gardar Province is a lateral extension of the Mackenzie igneous episode within the Laurentian craton.In the Tugtutôq region of the eastern part of the Gardar Province 47 NNE-trending dykes of various petrologic types, and intruded between 1175 ± 9 and 1168 ± 37 m.y. (RbSr isochron ages) yield a palaeomagnetic pole at 223.9° E, 36.4°N (dψ = 4.1°, d_χ = 6.1°). Fifteen other dykes in this swarm were intruded during a transitional phase of the magnetic field which, however, does not appear to have achieved a complete reversal over a period of several millions of years. The majority of dykes studied are highly stable to AF and thermal demagnetisation and contain single high blocking temperature components with single Curie points in the range 380–560°C.Palaeomagnetic poles from the Gardar Province between ca. 1330 and 1160 m.y. in age define the earlier part of the Great Logan apparent polar-wander loop; they correlate closely with contemporaneous North American results and confirm the coherence of the Laurentian craton in Upper Proterozoic times.     

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.     

Magnetic properties of high-Ti basaltic rocks from the Krušné hory/Erzgebirge MTS. (Bohemia/Saxony), and their relation to mineral chemistry

This study provides new thermomagnetic and petrographic data on specific basaltic rock association from the broader vicinity of the Lou?ná-Oberwiesenthal volcanic centre, western Bohemia/Saxony. Two types of volcanic rocks were recognized there: (i) high-Ti types (3.5–5.2 wt% TiO₂) represented by (mela)nephelinite s.s., and sporadically present (ii) medium-Ti types (2.5–3.5 wt% TiO₂) of olivine nephelinite, nepheline basanite and phonotephrite compositions. In order to examine the rock-magnetic behaviour, they were studied for their variations in the Curie temperature (T_C) and field-dependent susceptibility, spinel group minerals, chemistry and petrology. Magnetic susceptibility of ulvöspinel-rich titanomagnetite, as a dominant magnetic carrier, depends on the amplitude of measured magnetic field, whereas pure magnetite is field-independent. Field dependence parameter ^kHD of the studied basaltic rocks ranges from 0.8 to 18.7%, TiO₂ contents in titanomagnetite range from 12.7 to 20.1 wt.%. TiO₂ content in titanomagnetite does not correlate with whole-rock TiO₂ content (2.8 to 5.6 wt.%). The content of substituted titanium in the sublattice of magnetite is also sensitively reflected in the Curie temperature, ranging from 200 to 580°C. The spinel group minerals are designated as titanomagnetite with the dominance of ulvöspinel, magnetite and magnesioferrite components, or titanomagnetite with the magnetite, ulvöspinel and magnesioferrite components. Only two samples are characterized by a significant presence of Cr-spinel and magnesiochromite components forming cores of titanomagnetites. The titanomagnetite-bearing rocks in the studied area, likewise the low- to medium-Ti basaltic rocks from the ?eské st?edoho?í Mts., provide similar thermomagnetic curves.     

Zircon U-Pb and geochemical analyses for leucocratic intrusive rocks in pillow lavas in the Danfeng Group,north Qinling Mountains,China

Field observation showed that there are many irregular leucocratic intrusive rocks in pillow lavas in the Danfeng Group in the Xiaowangjian area, north Qinling orogenic belt. Photomicrographs indicated that the protoliths of those altered leucocratic intrusive rocks are dioritic rocks. Geochemical analyses showed that pillow lavas have a range of SiO2 from 47.35% to 51.20%, low abundance of TiO2 from 0.97% to 1.72%, and percentages of MgO (MgO#=41―49). Chondrite-normalized REE patterns of pillow lavas are even, indicative of a weak differentiation between LREE and HREE (La/YbN=1.52―0.99). N-MORB-normalized trace element abundances showed that pillow lavas are enriched in incompatible elements (e.g., K, Rb, and Ba). Leucocratic intrusive rocks in pillow lavas have a wide range of SiO2 from 53.85%―67.20%, low abundances of TiO2 from 0.51%―1.10%, and MgO (MgO#=40―51), and higher percentages of Al2O3 (13.32%―16.62%) and concentration of Sr (342-539 μg/g), ratios of Na2O/K2O (2―7) and Sr/Y (17―28). Chondrite-normalized REE patterns of leucocratic intrusive rocks showed highly differentiation between LREE and HREE (La/YbN=12.26―19.41). N-MORB-normalized trace element abundances showed that leucocratic intrusive rocks are enriched in incompatible elements (e.g., K, Rb, and Ba), and significantly depleted in HFSE (e.g., Nb, Ta, Zr and Ti), indicative of a relationship to subduction. Isotopically, leucocratic intrusive rocks have a similar εNd(t) ( 7.45― 13.14) to that of MORB ( 8.8― 9.7), which indicates that those leucocratic intrusive rocks sourced from depleted mantle most likely. SHRIMP U-Pb analyses for zircon showed that those leucocratic intrusive rocks were formed at 442±7 Ma, yielding an age of subduction in the early Paleozoic in the north Qinling orogenic belt.     

Tectonic evolution of lower crustal rocks in an exposed magmatic arc section in the Hidaka metamorphic belt, Hokkaido, northern Japan

Abstract : The Hidaka metamorphic belt consists of an island-arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D₀rs, D₁, D₂rs, and D₃, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D₀) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D₁ is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S-type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D₁ stage, mafic and intermediate magmas supplied and transported heat to form the arc-type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D₂ stage is characterized by subhorizontal top-to-the-south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S-type tonalite. Deformation structures and textures of high-temperature mylonites formed along the décollement, as well as the duplex structures, show that the D₂ stage movement occurred under a N-S trending compressional tectonic regime. The depth of intra-crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust-mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral-reverse-slip (dextral transpression) movement accompanied by low-temperature mylonitization with retrograde metamorphism, the stage defined as D₃. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D₃ stage. During D₂ and D₃ stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.     

Modeling of mineralogical composition,density and elastic wave velocities in anhydrous magmatic rocks

We use the technique of direct minimization of the Gibbs free energy of the 8-component (K₂O-Na₂O-Fe₂O₃-FeO-CaO-MgO-Al₂O₃-SiO₂) multiphase system in order to determine the equilibrium mineral assemblages of rocks of different bulk chemical compositions equilibrated at various P-T conditions. The calculated modal compositions of rocks and experimental data on elastic moduli of single crystals are then used to calculate densities and isotropic elastic wave velocities of rocks together with their pressure and temperature derivatives. Sufficient accuracy of the calculations is confirmed by comparison with experimental data on the gabbro-eclogite transformation and precise ultrasonic measurements of elastic wave velocities in a number of magmatic and metamorphic rocks.We present calculated phase diagrams with isolines of density, elastic wave velocities, and their pressure and temperature derivatives for several anhydrous magmatic rocks, from granite to lherzolite. Density and elastic properties of rocks are controlled by their chemical compositions, especially the SiO₂ content, and by P-T of equilibration, and they increase with pressure due to mineral reactions changing mineral assemblages from plagioclase-bearing and garnet-free to garnetbearing and plagioclase-free. TheV _p -density correlation is high, and shows two clear trends: one for iron-poor ultramafic rocks and another for all the other rocks considered. Mineral reactions, which occur at high pressures, changeV _p and density of anhydrous magmatic rocks following the well-known Birch (or a similar) law.Felsic, intermediate and mafic rocks can be well distinguished in theV _p -V _p /V _s - diagram, although their values ofV _p can be close to one another. TheV _p -V _p /V _s -density diagrams together with calculated phase diagrams can serve as efficient instruments for petrologic interpretation of seismic velocities.     

Relationship between statistical thermal alteration index and vitrinite reflectance for sedimentary rocks in northern Japan with reference to effects for unconformity,faulting, and contact metamorphism

Two organic maturity indices, the statistical thermal alteration index (stTAI) and vitrinite reflectance (R_O), are used to gain insight into the geological histories of sequences of Tertiary and Upper Cretaceous sediments in northern Japan that contain an unconformity and which are affected by faulting and contact metamorphism. The stTAI is based on the brightness, or gray level, of fossil pollen of Pinus, Podocarpus, Abies, and Picea species. Pollen brightness is measured using a transmitted‐light microscope equipped with a computer‐driven digital image processor. The stTAI represents the mean value of the modes for the complete array of indigenous pollen in rock samples. The stTAI indicates the level of organic maturation for Neogene sedimentary rocks of Japan, from incipient diagenesis to early catagenesis (R_O ≤1.0%). With the progressive diagenesis and catagenesis of sedimentary rocks, stTAI values generally show a progressive decrease, whereas R_O values increase. The effects of an unconformity and faulting are more clearly recorded in stTAI trends than in R_O trends. During early stages of organic maturation (R_O ≤0.7%), stTAI shows a rapid decrease, while R_O shows a rapid increase during the mature and post‐mature stages (R_O ≥0.8%). The occurrence of a range in R_O values for a given level of organic maturity makes it difficult to determine the influence of the unconformity on the increasing R_O trend. R_O values show a progressive increase toward an igneous dyke, but this trend is not apparent in stTAI values. Measurements of Tertiary and Upper Cretaceous rocks in Japan reveal that stTAI is more sensitive to heating duration than R_O, although R_O is more sensitive to heating temperature.     

A new high-pressure phase of spinel

A new phase which is much denser than the component oxides of spinel (MgAl₂O₄) was synthesised at loading pressures greater than 250 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. The new phase (ε-MgAl₂O₄) was indexed on the basis of an orthorhombic cell with a = 8.507 ± 0.004, b = 2.740 ± 0.003, c = 9.407 ± 0.005Å, and Z = 4 at room temperature and 1 bar pressure. Thus the molar volume for ε-MgAl₂O₄ at the above conditions was calculated to be 33.01 ± 0.07 cm³, which is 10.3% less than that of the mixture corundum plus periclase. The dense phase of spinel found in shock-wave experiments can be reasonably interpreted as ε-MgAl₂O₄, and this may be a potentially important mineral component of the earth's lower mantle. The new structure may also provide a possible candidate for the dense phases of Fe₃O₄ and Mg₂SiO₄ which were found by shock experiments.     

Evidence for244Pu fission tracks in hibonites from Murchison carbonaceous chondrite

We have developed a technique for revealing nuclear tracks in the mineral hibonite (CaAl₁₂O₁₉), found in the refractory inclusions from carbonaceous chondrites. The tracks in hibonitesfrom Murchison carbonaous chondrite are dominated by fission tracks from²⁴⁴Pu (constituting more than 90% of the total). The measured uranium contents in these crystals range from 1.2 to 62 ppb. We deduce that the average value for the²⁴⁴Pu/²³⁸U ratio in most of the Murchison hibonites at the time of track retention is0.022 ± 0.011.     

The stratigraphic distribution of Ni-rich spinels in Cretaceous-Tertiary boundary rocks at El Kef (Tunisia), Caravaca (Spain) and Hole 761C (Leg 122)

Ni-rich spinels have been found throughhout the world at the Cretaceous-Tertiary boundary. These minerals have no counterparts in terrestrial rocks but have been observed in meteoroid ablation material and impact debris, suggesting that the Cretaceous-Tertiary boundary spinels result from an extraterrestrial event. Their stratigraphic distribution, which has been observed at the Cretaceous-Tertiary boundary sites of El Kef (Tunisia), Caravaca (Spain) and Hole 761C (ODP Leg 122, Indian Ocean), are markedly different from site to site and are all much less extended than the Ir anomalies. Regional differences in Ni-rich spinel distibution are entirely explained by a brief event recorded under different bioturbation conditions. The wider stratigraphic dispersion of Ir is attributed to postdepositional chemical processes. At El Kef, the site which appears to be the least affected by bioturbation and which has the highest sedimentation rate, the 1–2 mm thick Ni-rich spinel spike indicates that the deposition time did not exceed an upper limit of 100 years. This upper limit is consistent only with the collision of an asteroid or a comet at the close of the Cretaceous.     

Supra-subduction and mid-ocean ridge peridotites from the Piranshahr area,NW Iran

The Piranshahr metaperidotites in the northwestern end of the Zagros orogen were emplaced following the closure of the Neotethys ocean. The ophiolitic rocks were emplaced onto the passive margin of the northern edge of the Arabian plate as a result of northeastward subduction and subsequent accretion of the continental fragments. The metaperidotites have compositions ranging from low-clinopyroxene lherzolite to harzburgite and dunite. They are mantle residues with distinct geochemical signatures of both mid-ocean ridge and supra subduction zone (SSZ) affinities. The abyssal peridotites are characterized by high Al₂O₃ and Cr₂O₃ contents and low Mg-number in pyroxenes. The Cr-number in the coexisting spinel is also low. The SSZ mantle peridotites are characterized by low Al₂O₃ contents in pyroxenes as well as low Al₂O₃ and high Cr-number in spinel. Mineral chemical data indicate that the MOR- and SSZ-type peridotites are the residues from ∼15–20% and ∼30–35% of mantle melting, respectively. Considering petrography, mineralogy and textural evidence, the petrological history of the Piranshahr metaperidotites can be interpreted in three stages: mantle stable stage, serpentinization and metamorphism. The temperature conditions in the mantle are estimated using the Ca-in-orthopyroxene thermometer as 1210 ± 26 °C. The rocks have experienced serpentinization. Based on the textural observations, olivine and pyroxene transformed into lizardite and/or chrysotile with pseudomorphic textures at temperatures below 300 °C during the initial stage of serpentinization. Subsequent orogenic metamorphism affected the rocks at temperatures lower than 600 °C under lower-amphibolite facies metamorphism.     

Evaluation of volcanic gas analysis from surtsey volcano, iceland 1964–1967

Methods used previously to remove compositional modifications from volcanic gas analyses for Mount Etna and Erta'Ale lava lake have bean employed to estimate the gas phase composition at Nyiragongo lava lake, based on samples obtained in 1959. H₂O data were not reported in 11 of the 13 original analyses. The restoration methods have been used to estimate the H₂O contents of the samples and to correct the analyses for atmospheric contamination, loss of sulfur and for pre- and pest-collection oxidation of H₂S, S₂, and H₂. The estimated gas compositions are relatively CO₂-rich, low in total sulfur and reduced. They contain approximately 35–50% CO₂ 45–55% H₂O, 1–2% SO₂, 1–2% H₂., 2–3% CO, 1.5–2.5% H₂S, 0.5% S₂ and 0.1% COS over,he collection temperature range 102° to 960° C. The oxygen fugacities of the gases are consistently about half an order of magnitude below quartz-magnetite-fayalite. The low total sulfur content and resulting low atomic S/C of the Nyiragongo gases appear to be related to the relatively low f_O2 of the crystallizing lava. At temperatures above 800°C and pressures of 1–1.5 k bar, the Nyiragongo gas compositions resemble those observed in primary fluid inclusions believed to have formed at similar temperatures and pressures in nephelines of intrusive alkaline rocks. Cooling to 300°C, with f_O2 buffered by the rock, results in gas compositions very rich in CH₄ (50–70%) and resembling secondary fluid inclusions formed at 200–500°C in alkaline rocks. Below 600°C the gases become supersaturated in carbon as graphite. These inferences are corroborated by several reports of hydrocarbons in plutonic alkaline rocks, and by the presence of CH₄-rich waters in Lake Kivu — a lake on the flanks of Nyiragongo volcano.     

Iridium anomalies and fractionated siderophile element patterns in impact ejecta, Brockman Iron Formation, Hamersley Basin, Western Australia: evidence for a major asteroid impact in simatic crustal regions of the early Proterozoic earth

A stratigraphically consistent <20-cm-thick unit of microkrystite spherule and microtektite-bearing impact fallout ejecta overlying volcanic tuff of the 4th Shale Macroband (DGS4) of the Dales Gorge Member (2.47–2.50 Ga), Brockman Iron Formation, Hamersley Group, Western Australia, displays anomalous platinum group element (PGE) and other trace metal patterns. The unit has high Ir (13 ppb) and Pt (35 ppb), and low Pd (2.7 ppb) and Au (1.55–1.88 ppb). The low Pd/Ir ratios and low Cr/V suggest depletion in volatile PGE and metals relative to refractory PGE and V, contrasted to the ubiquitous high Pd/Ir of most terrestrial rocks. Marked depletion in the volatile Rare Earth Element (REE) abundances in stilpnomelane spherule cores is consistent with this model. The loss of volatile PGE, analogous to relations in 3.24 Ga impact fallout units of the Barberton greenstone belt (S3 and S4), suggests fractionation related to atmospheric spherule condensation. The microkrystite spherule unit locally incorporate fragments and up to meter-scale boulders of banded chert and stromatolite carbonate, suggesting tsunami transport postdating spherule deposition. DGS4 microkrystite spherules are dominated by stilpnomelane mantled by K-feldspar shells, which consist of inward-radiating fibrous feldspar aggregates suggestive of devitrification. The K and REE enrichment of spherule margins are contrasted to flat REE patterns of the stilpnomelane cores, suggesting adsorption of lithophile elements during settling of the spherules through the hydrosphere. K-feldspar shells contain submicron-scale Ni metal, oxide, sulfide and arsenide grains and euhedral needles of feldspar-exsolved ilmenite. Associated magnetite may have high nickel (<1.25% NiO). The generally mafic composition of the spherules and high Ni/Cr and Ni/Co are consistent with a target mafic-ultramafic crust, consistent with the lack of shock-metamorphosed quartz. Mixing calculations suggest a contribution of 2.5–3% projectile component to the impact-generated volatile cloud. Conservative mass balance estimates derived from the Ir and Pt flux, assuming global extent of a 10-cm-thick spherule unit and chondritic projectile composition, suggest an asteroid diameter on the scale of 30 km. Similar estimates are obtained from spherule sizes, which in DGS4 reach a mean diameter of 2.0 mm in aerodynamically elongate spherules. The evidence implies formation of an impact basin on the scale of 400 km in simatic/oceanic regions of the early Proterozoic crust.     

Carbon isotopic compositions of organic matter across continental Cretaceous–Tertiary (K–T) boundary sections: Implications for paleoenvironment after the K–T impact event

To assess the environmental perturbation induced by the impact event that marks the Cretaceous–Tertiary (K–T) boundary, concentrations and isotopic compositions of bulk organic carbon were determined in sedimentary rocks that span the terrestrial K–T boundary at Dogie Creek, Montana, and Brownie Butte, Wyoming in the Western Interior of the United States. The boundary clays at both sites are not bounded by coals. Although coals consist mainly of organic matter derived from plant tissue, siliceous sedimentary rocks, such as shale and clay, may contain organic matter derived from microbiota as well as plants. Coals record δ¹³C values of plant-derived organic matter, reflecting the δ¹³C value of atmospheric CO₂, whereas siliceous sedimentary rocks record the δ¹³C values of organic matter derived from plants and microbiota. The microbiota δ¹³C value reflects not only the δ¹³C value of atmospheric CO₂, but also biological productivity. Therefore, the siliceous rocks from these sites yields information that differs from that obtained previously from coal beds.Across the freshwater K–T boundary at Brownie Butte, the δ¹³C values decrease by 2.6‰ (from − 26.15‰ below the boundary clay to − 28.78‰ above the boundary clay), similar to the trend in carbonate at marine K–T sites. This means that the organic δ¹³C values reflect the variation of δ¹³C of atmospheric CO₂, which is in equilibrium with carbon isotopes at the ocean surface. Although a decrease in δ¹³C values is observed across the K–T boundary at Dogie Creek (from − 25.32‰ below the boundary clay to − 26.11‰ above the boundary clay), the degree of δ¹³C-decrease at Dogie Creek is smaller than that at Brownie Butte and that for marine carbonate.About 2‰ decrease in δ¹³C of atmospheric CO₂ was expected from the δ¹³C variation of marine carbonate at the K–T boundary. This δ¹³C-decrease of atmospheric CO₂ should affect the δ¹³C values of organic matter derived from plant tissue. As such a decrease in δ¹³C value was not observed at Dogie Creek, a process that compensates the δ¹³C-decrease of atmospheric CO₂ should be involved. For example, the enhanced contribution of ¹³C-enriched organic matter derived from algae in a high-productivity environment could be responsible. The δ¹³C values of algal organic matter become higher than, and thus distinguishable from, those of plant organic matter in situations with high productivity, where dissolved HCO₃⁻ becomes an important carbon source, as well as dissolved CO₂. As the δ¹³C-decrease of atmospheric CO₂ reflected a reduction of marine productivity, the compensation of the δ¹³C decrease by the enhanced activity of the terrestrial microbiota means that the microbiota at freshwater environment recovered more rapidly than those in the marine environment.A distinct positive δ¹³C excursion of 2‰ in the K–T boundary clays is superimposed on the overall decreasing trend at Dogie Creek; this coincides with an increase in the content of organic carbon. We conclude that the K–T boundary clays include ¹³C-enriched organic matter derived from highly productive algae. Such a high biological productivity was induced by phenomena resulting from the K–T impact, such as nitrogen fertilization and/or eutrophication induced by enhanced sulfide formation. The high productivity recorded in the K–T boundary clays means that the freshwater environments (in contrast to marine environments) recovered rapidly enough to almost immediately (within 10 yr) respond to the impact-related environmental perturbations.     

MnO/TiO2/P2O5: a minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis

A ternary diagram using MnO, TiO₂, P₂O₅ can discriminate between five petrotectonic environments of basaltic rocks (45–54% SiO₂). Fields for mid-ocean ridge, island arc tholeiite, island arc calc-alkaline, ocean island tholeiite, and ocean island alkalic rocks were distinguished on the basis of 507 analyses from well-defined environments. Boninites plot within island arc fields. Continental tholeiites, such as the Columbia River basalts, are high in P₂O₅ relative to MnO and TiO₂, and overlap portions of all five oceanic fields.MnO is depleted relative to TiO₂ in mid-ocean ridge analyses and may be controlled by early fractionation of olivine and/or clinopyroxene under conditions of lowf_O2. In island arc rocks, MnO is enriched relative to TiO₂ due to early crystallization of titanomagnetite in a high-f_O2 environment. Primitive mid-ocean ridge and arc tholeiites have similar MnO/TiO₂/P₂O₅ ratios which indicate a grossly similar parent magma. Increasingly differentiated basaltic rocks are more easily classified by the diagram. High relative abundances of TiO₂ and P₂O₅ in ocean island rocks are consistent with their derivation from a separate source.Despite the purported high mobility of MnO, the MnO/TiO₂/P₂O₅ discriminant diagram may be applied to unspilitized and moderately spilitized zeolite to greenschist facies greenstones with good agreement between the environment determined by MnO/TiO₂/P₂O₅ and by other means such as trace elements, REE, or field relations.     

Palaeomagnetism and K-Ar ages of triassic igneous rocks from the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation,Argentina

The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α₉₅ = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α₉₅ = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α₉₅ = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α₉₅ = 4°, k = 131).     

Petrology and phase equilibrium of newly found eclogites from Kekesu Valley in eastern part of southwest Tianshan HP-UHP metamorphic belt, China, and its tectonic significance

Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt,Xinjiang,northwest China.After detailed petrological study,three samples including one fresh eclogite TK003,one blueschist sample TK026-8 and one retrograded eclogite TK027,were selected for phase equilibrium modeling under NC(K)MnFMASHO(N2O-CaO-K2O-MnO-FeO-MgO-Al2O3-SiO2-H2O-O)system,by thermocalc 3.33 software.Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing,Xspss decreasing from core to rim.Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evolution trajectory,with a near iso-baric heating in the early stage,and reached eclogite facies metamorphic field with peak P-T regime of 480–515°C,2.00–2.30 GPa.Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515–519°C,1.78–1.93 GPa.Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition,as well as a variation in fluid activity during exhumation.P-T calculation and a peak geothermal gradient of 6–7°C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism.Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized,extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly200 km.However,UHP evidence has not been found in the Kekesu terrane,perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.     

Disproportionation of spinels to mixed oxides: significance of cation configuration and implications for the mantle

Experimental study of the phase boundary for the disproportionation of the inverse spinel Mg₂SnO₄ into its isochemical mixed oxides indicates a slope dP/dT = 40 ± 10bars/°K. This positive slope is consistent with the large entropies of inverse (relative to normal) spinels predicted from high-temperature entropy-molar volume systematics. Thermodynamic data do not support the existence of a distinctly negative slope for the proposed disproportionation of Mg₂SiO₄ normal spinel. Evidence from X-ray and phase equilibria studies suggests the possibility that Si⁴⁺, Mg²⁺, and Fe²⁺ share the octahedral sites in silicate spinels under mantle conditions. The consequences of this partial inverse character are a positive slope for the postulated spinel-mixed oxide phase boundary near 650 km depth, removal of a widely accepted constraint on mantle-wide convection, and anomalous elasticity-density behaviour within the transition zone.