Siderophile element patterns,PGE nuggets and vapour condensation effects in Ni-rich quench chromite-bearing microkrystite spherules, ∼ 3.24 Ga S3 impact unit,Barberton greenstone belt,Kaapvaal Craton,South Africa

Energy dispersive spectrometry (EDS), laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) track analyses of chlorite-dominated quench-textured microkrystite spherules and LA-ICPMS spot analyses of intra-spherule Ni-rich skeletal quench chromites from the 3243 ± 4 Ma Barberton S3 impact fallout unit (lower part of the Mapepe Formation, Fig Tree Group, Barberton greenstone belt, Kaapvaal Craton, South Africa) reveal fractionated siderophile and PGE trace element patterns corresponding to chondrite-contaminated komatiite/basalt compositions. The chlorites, interpreted as altered glass, contain sharp siderophile elements and PGE spikes inherited from decomposed metal and Ni-rich chromite particles. LA-ICPMS spot analysis identifies PGE-rich micronuggets in Ni–chromites (Ir ∼ 12–100 ppm, Os ∼ 9–86 ppm, Ru ∼ 5–43 ppm) and lower levels of the volatile PGEs (Rh ∼ 1–11 ppm, Pd ∼ 0.68–0.96 ppm). Previously reported PGE anomalies in the order of hundreds of ppb in some Barberton microkrystite spherules are accounted for in terms of disintegration of PGE-rich micronuggets. Replacement of the Ni-chromites by sulphide masks primary chondritic patterns and condensation element distribution effects. High refractory/volatile PGE ratios pertain to both the chlorites and the Ni-rich chromites, consistent with similar compositional relations in microkrystite spherules from other impact fallout units in the Barberton greenstone belt and the Pilbara Craton, Western Australia. The near-consistent low Pt/Re and high V/Cr and V/Sc ratios in chlorite of the spherules, relative to komatiites, are suggestive of selective atmospheric condensation of the spherules which favored the relatively more refractory Re and V. Selective condensation may also be supported by depletion in the volatile Yb relative to Sm. Ni–Cr relationships allow estimates of the proportion of precursor crustal and meteoritic components of the spherules. Mass balance calculations based on the iridium flux allow estimates of the order of magnitude of the diameter of the chondritic projectile.     

Platinum-group element geochemistry of Cenozoic basalts from the North China Craton: Implications for mantle heterogeneity

Forty-two Cenozoic(mostly Miocene) basalt samples from Jining, Chifeng, Fansi, Xiyang, and Zuoquan areas of the North China Craton(the NCC basalts hereafter) were analyzed for platinum-group elements(PGE, including Os, Ir, Ru, Rh, Pt, and Pd). Most of them are alkaline basalts and tholeiites and all of them display little crustal contamination. The total PGE contents of the NCC basalts vary from 0.1 to 0.9 ppb, much lower than those of the primitive mantle values of 23.5 ppb. Primitive mantle-normalized PGE patterns of these basalts define positive slopes and Pd/Ir ratios vary from 1.2 to 25. In terms of both PGE contents and Pd/Ir ratios, they are quite similar to the mid-ocean ridge basalts. There are no obvious negative correlations between PGE vs. Mg O, Ni, and Cu in the NCC basalts, indicating that fractional crystallization of olivine, pyroxene, and/or sulfides during magmatic process cannot be the controlling factor for the observed PGE variation. The observed Pd/Ir variations of the NCC basalts require involvement of non-chondritic heterogeneous mantle sources. Based on Sr-Nd-Pb-Hf isotopic systematics and incompatible-element signatures, a mixing of partial melts from both asthenospheric peridotites and enclosed mantle eclogites at the top of asthenosphere was proposed for the origin of these NCC basalts. The lenses of eclogites are derived from upwelling of recycled continental crust during the westward subduction of the Pacific plate from the ~600 km discontinuity zone. The PGE geochemistry of these basalts provides independent evidence to support this conclusion and the observed Pd/Ir variations may reflect variations in proportions of tapped peridotitic and eclogitic melts.     

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.     

Rb-Sr,Sm-Nd,K-Ca,O, and H isotopic study of Cretaceous-Tertiary boundary sediments,Caravaca, Spain: evidence for an oceanic impact site

Isotopic ratios and trace element abundances were measured on samples of Ir-enriched clay at the Cretaceous-Tertiary boundary, and in carbonate and marl from 5 cm below and 3 cm above the boundary. Samples were leached with acetic acid to remove carbonate, and with hydrochloric acid. Leachates and residues were measured. The Sr, Nd, O and H isotopic compositions of the boundary clay residues are distinct from those of the stratigraphically neighboring materials. The data indicate that most of the clay material was derived from a terrestrial source with relatively low⁸⁷Sr/⁸⁶Sr and high¹⁴³Nd/¹⁴⁴Nd ratios. The δ¹⁸O data suggest that the detritus has been modified by submarine weathering. K-Ca and Rb-Sr systematics, as well as O isotope ratios of K-feldspar spherules within the boundary clay, suggest that they are predominantly authigenic and may have formed after the time of deposition. However, Sm-Nd and Rb-Sr isotopic data indicate that the spherules contain relict material that provides information on the nature of the original detritus. The isotopic evidence for foreign terrestrial detritus in the boundary clay, the low rare earth element concentrations and high Ni concentration, support the hypothesis of a terminal Cretaceous asteroidal impact that produced a global layer of fallout. The data are most easily explained if the impact site was on oceanic crust rather than continental crust, and if a substantial fraction of the fallout was derived from relatively deep within the lithosphere (>3 km). This would probably require a single large impactor.     

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6-15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.     

Hematite spherules in basaltic tephra altered under aqueous, acid-sulfate conditions on Mauna Kea volcano, Hawaii: Possible clues for the occurrence of hematite-rich spherules in the Burns formation at Meridiani Planum, Mars

Iron-rich spherules (> 90% Fe₂O₃ from electron microprobe analyses) ∼10-100 μm in diameter are found within sulfate-rich rocks formed by aqueous, acid-sulfate alteration of basaltic tephra on Mauna Kea volcano, Hawaii. Although some spherules are nearly pure Fe, most have two concentric compositional zones, with the core having a higher Fe / Al ratio than the rim. Oxide totals less than 100% (93-99%) suggest structural H₂O and/or OH^− 1. The transmission Mössbauer spectrum of a spherule-rich separate is dominated by a hematite (α-Fe₂O₃) sextet whose peaks are skewed toward zero velocity. Skewing is consistent with Al³⁺ for Fe³⁺ substitution and structural H₂O and/or OH^− 1. The grey color of the spherules implies specular hematite. Whole-rock powder X-ray diffraction spectra are dominated by peaks from smectite and the hydroxy sulfate mineral natroalunite as alteration products and plagioclase feldspar that was present in the precursor basaltic tephra. Whether spherule formation proceeded directly from basaltic material in one event (dissolution of basaltic material and precipitation of hematite spherules) or whether spherule formation required more than one event (formation of Fe-bearing sulfate rock and subsequent hydrolysis to hematite) is not currently constrained. By analogy, a formation pathway for the hematite spherules in sulfate-rich outcrops at Meridiani Planum on Mars (the Burns formation) is aqueous alteration of basaltic precursor material under acid-sulfate conditions. Although hydrothermal conditions are present on Mauna Kea, such conditions may not be required for spherule formation on Mars if the time interval for hydrolysis at lower temperatures is sufficiently long.     

Platinum-group element micronuggets and refertilization process in Lherz orogenic peridotite (northeastern Pyrenees,France)

Highly siderophile elements (Platinum-group elements, Au and Re) are currently assumed to reside inside base metal sulfides (BMS) in the convecting upper mantle. However, fertile lherzolites sampled by Pyrenean orogenic peridotite massifs are unexpectedly rich in 0.5–3 µm large micronuggets of platinum-group minerals (PGM). Among those, sulfides from the laurite-erlichmanite series (Ru, Os(Ir)S(As)₂), Pt–Ir–Os alloys and Pt–Pd–Te–Bi phases (moncheite–merenskyite) are predominant. Not only the BMS phases but also the PGM micronuggets must be taken into account in calculation of the PGE budget of orogenic fertile lherzolites. Laurite is a good candidate for equilibrating the whole-rock budget of Os, Ir and Ru while accounting for supra-chondritic Ru/Ir_N. Textural relationships between PGMs and BMS highlight heterogeneous mixing between refractory PGMs (laurite/Pt–Ir–Os alloys) inherited from ancient refractory lithospheric mantle and late-magmatic metasomatic sulfides precipitated from tholeiitic melts. “Low-temperature” PGMs, especially Pt–Pd bismuthotellurides should be added to the list of mineral indicators of lithosphere refertilization process. Now disseminated within fertile lherzolites, “lithospheric“ PGMs likely account for local preservation of ancient Os model ages (up to 2 Ga) detected in BMS by in-situ isotopic analyses. These PGMs also question the reliability of orogenic lherzolites for estimating the PGE signature of the Primitive Silicate Earth.     

Geochemical evidence for the characteristic of the 1908 Tunguska explosion body in Siberia, Russia

Twenty-two peat samples collected at different depths of a core including the layer affected by the 1908 explosion in Tunguska area of Central Siberia, Russia, and three basalt samples collected near the site, are analyzed by ICP-MS. The concentrations of Pd, Ni, Co, ΣREE, Ti and Sr in the event layers are 4–35 times higher than the background values in the normal layers. The variation of Pd is closely related to Ni, Co and ΣREE in the event layers, but not to these elements in the normal layers. It indicates that these excess elements came from the same source, i.e. the Tunguska explosion body. In addition, the patterns of Cl-chondrite-normalized REE in the event layers ((La/Yb)_N ≈2–3) are much flatter than those in the normal layers ((La/Yb)_N ≈7–143), and differ from those in the three basalt samples. The concentrations of REE in the three basalt samples are tens times higher than those in the event layers. It may be inferred that these excess elements could not be produced by the contamination of the terrestrial material, but probably by the Tunguska explosion body. Additionally, the ratios of Ti/Ni and Sr/Co in the event layers are close to those in comet. It implies that the solid part of the explosion body was compositionally similar to carbonaceous chondrites (Cl) and more probably a small comet. In terms of the Pd excess fluxes in the explosion area, it can be estimated that the celestial body that exploded over Tunguska in 1908 weighed more than 10⁷ tons, corresponding to a radius of > 126 m.     

A new approach to the Nd residence time in the ocean: the role of atmospheric inputs

Concentrations of rare earth elements (REE) and Nd isotopic ratios were analyzed for seawater, filtered suspension and sediment trap samples collected in the tropical Atlantic Ocean (EUMELI program, EUtrophic, MEsotrophic and oLIgotrophic sites, 20°N, 18°–21°W). This is the first REE/Nd dataset on solution and different-sized particles collected at the same site. We present direct evidence of the Nd isotopic exchange between particulate lithogenic fraction and seawater without significant mass transfer. This exchange is probably one of the main factors that simultaneously constrains the Nd concentration and isotopic ratio budget. We propose a new approach to estimate the residence time of Nd in the ocean (τ_Nd) based on isotopic exchange: 200 yr < τ_Nd< 1000 yr. The exchange requires a partial dissolution of lithogenic Nd. We estimate that the fraction of soluble Nd proportion in atmospheric dust is of the order of 20% based on the isotopic ratios. We suggest that the partial dissolution of atmospheric fallout is probably one of the main REE sources of the ocean.     

Siderophile element fractionation in enstatite chondrites

The concentration of 10 to 15 siderophile elements was determined in the magnetic and non-magnetic portions of Abee (E4) and Hvittis (E6). The results indicate that, with the exception of Cu, W and Fe, all elements are strongly concentrated in the metal phase. Unlike ordinary chondrites, the metal phase of Abee and Hvittis consists exclusively of kamacite, which is very homogeneous and shows no systematic variation in composition with grain size.Differences in siderophile element content between Abee and Hvittis can be accounted for exclusively by differences in metal content and composition. These differences reflect different degrees of refractory siderophile loss, metal-silicate fractionation and loss of moderately volatile elements. The Ir/Ni ratio is 25% lower in Abee than in Hvittis, indicating that more Ir (Os, Pt, etc.) was lost from Abee during the refractory element fractionation. Abee and the other E4–5 members have also lost no metal and are not depleted in moderately volatile elements. In Abee the non-refractory elements Fe to Ge are present in CI ratios, and this meteorite has also Ir/Re ratios ?CI.These differences, which are recorded in the composition of the metal phase, make a straightforward genetic relationship between the two enstatite chondrite groups difficult to accept. In particular, the different Ir/Ni ratios, which were established very early in the chemical history of these chondrites, at the time of the refractory element fractionation, force us to conclude that E4–5 and E6 chondrites evolved from two different reservoirs, and that exchange of material among them never occurred. However, members of both groups have similar cosmic ray exposure ages suggesting derivation from the same parent body, which poses some interesting problems.     

Terrestrial chondrules,glass spherules and accretionary lapilli from the suevite,Ries Crater,Germany

Until recently, no terrestrial analogues of meteoritic and lunar chondrules were known. Only rare glass spherules from the Lonar Crater, India, and black magnetic spherules from various localities have been recorded. The impact breccia suevite of the No¨rdlinger Ries Crater, Germany, contains both chondrules and glass spherules, and in addition, accretionary lapilli, all of which are found imbedded within the fine-grained matrix of the suevite. The chondrules display many of the textural features characteristic of meteoritic and lunar chondrules. Lithic chondrules and fluid drop chondrules are present, the latter having a composition quite similar to that of glass bombs and glass fragments in the suevite. Fluid drop chondrules developed from glass spherules by slow devitrification in the hot suevite ejecta masses after deposition. On the whole, fluid drop chondrules, lithic chondrules and glass spherules are rare in the suevite, with fluid drop chondrules prevailing. Detection of chondrules from a terrestrial impact crater supports theories of an impact origin for meteoritic and lunar chondrules. Accretionary lapilli also represent material formed as a result of impact.     

PGE geochemistry of Jiding ophiolite in Tibet and its constraint on mantle processes

The total PGE amount (σPGE) of mantle peridotite in the Jiding ophiolite is slightly higher than that of the primitive mantle, but the PGE contents of basalt are higher than those of the mid-ocean ridge basalt (MORB), with obviously lower Pd/Ir ratios. The accumulates, dyke swarm and basalts show remarkable negative Pt and positive Rh anomalies, resulting in the special N-type PGE patterns. Mantle peridotite and crustal rocks have similar distribution patterns. It is proposed that the PGE distribution patterns in the Jiding ophiolite are closely related with a higher degree of partial melting of the mantle in this region. Magmatic crystallization-differentiation led to PGE fractionation, thus making the contents of PGE in the accumulates decrease in the ascending direction. The higher content of Au in the Jiding ophiolite is the result of metasomatic alteration at later stages. Pt-Pd fractionation indicates that both the PGEs are controlled by their alloy and sulfide phases. Positive Rh anomalies seem to be related with higher oxygen fugacity in the melts.     

Measurement of53Mn in deep-sea iron and stony spherules

Cosmic-ray-produced⁵³Mn (t_1/2 = 3.7 × 10⁶years) was measured in individual and groups of deep-sea iron and stony spherules by highly sensitive neutron activation analysis. The activities found were less than 20 dpm⁵³Mn/kg Fe (10^?5?10^?6 dpm⁵³Mn/sample) in iron spherules except one iron spherule whose activity was 241 ± 73 dpm⁵³Mn/kg Fe. These low activities may indicate evaporative loss of⁵³Mn due to heating in the earth's atmosphere. On the other hand, all stony spherules contained 200–260 dpm⁵³Mn/kg Fe which is similar to chondritic values. These spherules may be ablation debris from large objects.     

Isotopic compositions of potassium and calcium in magnetic spherules from marine sediments

Isotopic compositions of potassium and calcium in individual magnetic spherules were determined. No significant anomaly was observed for potassium within twice the statistical error (2σ), although for calcium isotopes enrichments of⁴⁶Ca,⁴⁴Ca and⁴²Ca were observed in one spherule. The relative excess of⁴⁶Ca,⁴⁴Ca and⁴²Ca in the spherule agrees with the relative yield of spallogenic calcium isotopes observed in iron meteorites. This fact indicates that the enrichment in the calcium isotopes was caused by cosmic ray irradiation of the spherule in outer space.     

Comparative study of platinum-group elements in the Coto and Acoje blocks of the Zambales Ophiolite Complex, Philippines

Abstract The Zambales Ophiolite Complex (ZOC), Philippines, includes two geochemically distinct, ophiolitic assemblages: the high-Al chromitite-bearing Coto Block and high-Cr chromitite-bearing Acoje Block. This paper reports a comparative platinum-group element (PGE) study of these two blocks. The PGE data were obtained using Ni-sulfide fire assay preconcentration combined with inductively coupled plasma mass spectrometry (ICP-MS) measurement. Podiform chromitites in the Acoje Block have higher Cu, Ir, Ru and Rh contents than their equivalents in the Coto Block, although chromitites from both have similar Pt and Pd contents. The PGE mantle-normalized patterns of dunites from the two blocks are also different: dunites from the Coto Block are depleted in Pt, whereas those from the Acoje Block have a relatively flat pattern. The data demonstrate that Coto and Acoje Blocks have different origins in terms of their source region and partial melting processes. This study implies that the ZOC is a paired ophiolite belt formed in an island arc and back-arc basin environment.     

Chemical record of the projectile in the graded fall-back sedimentary unit from the Ries Crater, Germany

The so-called “Graded Unit” comprises 17.2 m of the core from the research borehole Nördlingen 1973 and is thought to represent fall-back material, which has been sorted similar to tephra. Samples from six levels of these air-borne impact debris sediments in the Ries Crater have been analyzed for Ir, Ni, Co, Cr and Fe. The concentrations of these elements are negatively correlated with average grain size and accordingly the depth of the samples indicating the presence of a surface correlated component and are hence consistent with vapour deposition of meteoritic material on small dust grains. The largest Ir concentration found is 230 pg/g, a factor of 28 higher than the indigenous concentration. Enrichment factors in the uppermost layer of the “Graded Unit” are about 28 for Ir, approximately 2 for Fe, Co, Ni and 1.5 for Cr. If the enrichment of all elements determined were of meteoritic origin, then the “net meteoritic composition” is most closely matched by aubrites as suggested previously. However, it seems more likely that the observed concentration trends of Cr, Fe, Co and Ni are due to mineral sorting. Therefore the only as yet certain indication of meteoritic material is the presence of Ir, which is not sufficient to further constrain the nature of the projectile.     

Chlorinated hydrocarbons in marine fish and shellfish of Nigeria

The concentration ranges of chlorinated hydrocarbons in marine fish were HCB (0.03–9.5 ppb), Lindane (0.02–5.3 ppb), Endosulphan (0.21–4.9 ppb), DDT (0.50–18.6 ppb), Aldrin (0.05–54.6 ppb) and PCB (4.78–225 ppb). Fish contained higher concentrations of Aldrin, Heptachlor, HCB and Lindane than shellfish, while a reverse trend was observed for DDT and PCBs. The concentrations of residues obtained were found to be lower than those reported in literature for industrialized countries. Predator fish species were found to concentrate more residues in muscle tissue than plankton feeders. The DDT/PCB values were less than 1 indicating a predominance of industrial activities over agricultural activities as the source of contamination of the marine environment. The fish Galeoides decadactylus is a potential bio-indicator for chlorinated hydrocarbons pollution monitoring in the study area.     

Mathematical modeling of PCB bioaccumulation in Perna viridis

In the present work, we built a mathematical model of polychlorinated biphenyl (PCB) bioaccumulation in Perna viridis, namely, a one-compartment model with a time dependent incorporation rate R (μg g⁻¹ lipid per ppb water per day), with positive substrate cooperativity as the underlying physical mechanism. The temporal change of the PCB concentration Q (μg g⁻¹ lipid) in the soft tissues of the mussel depends on the competition of the input rate RW and the output rate kQ, where W is the concentration of PCB in water (ppb water) and k is the elimination rate (per day). From our experimental data, k=0.181±0.017 d⁻¹. The critical concentration in water W_c for positive substrate cooperativity was found to be 2.4 ppb. Below W_c, R is a constant. For a water concentration of 0.5 ppb Aroclor 1254, R=24.0±2.4 μg g⁻¹ lipid ppb⁻¹ d⁻¹. Above W_c, positive substrate cooperativity comes into effect and R becomes a function of time and dependent on the concentration Q in a form R=γQ/(Q+δ). This is the case for a water concentration of 5 ppb Aroclor 1254, where γ=15.1 μg g⁻¹ lipid ppb⁻¹ d¹ and δ≈200 μg g⁻¹ lipid. From this model, the uptake is exponentially increasing when the PCB concentration in the mussel is small compared to 200 μg g⁻¹ lipid, and hyperbolically increasing when the concentration is large compared to 200 μg g⁻¹ lipid, which are consistent with the experimental data. The model is useful for understanding the true processes taking place during the bioaccumulation and for risk assessment with higher confidence. Future experimental data which challenge the present model are anticipated and in fact desirable for improvement and perfection of the model.     

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.     

Eruption of the dacite to andesite zoned Mateare Tephra,and associated tsunamis in Lake Managua,Nicaragua

The dacite to andesite zoned Mateare Tephra is the fallout of a predominantly plinian eruption from Chiltepe peninsula at the western shore of Lake Managua that occurred 3000–6000 years ago. It comprises four units: Unit A of high-silica dacite is stratified, ash-rich lapilli fallout generated by unsteady subplinian eruption pulses affected by minor water access to the conduit and conduit blocking by degassed magma. Unit B of less silicic dacite is well sorted, massive pumice lapilli fallout from the main, steady plinian phase of the eruption. Unit C is andesitic fallout that is continuous from unit B except for the rapid change in chemical composition, which had little influence on the ongoing eruption except for a minor transient reduction of the discharge rate and access of water to the conduit. After this, discharge rate re-established to a strong plinian eruption that emplaced the main part of unit C. This was again followed by water access to the conduit which increased through upper unit C. The lithic-rich lapilli to wet ash fallout of unit D is the product of the fully phreatomagmatic terminal phase of the eruption. A massive well-sorted sand layer, the Mateare Sand, replaces laterally variable parts of unit A and lowermost part of unit B in outcrops up to 32 m above present lake level. The corresponding interval missing in the primary fallout can be identified by comparing the composition of pumice entrained in the sand, and pumice from the local base of unit B on top of the sand, with the compositional gradient in undisturbed fallout. The amount of fallout entrained in the sand decreases with distance to the lake. The Mateare Sand occurs at elevations well above beach levels and its widespread continuous distribution defies a fluviatile origin. Instead, it was produced by lake tsunamis triggered by eruption pulses during the initial unsteady phase of activity. Such tsunamis could threaten areas not affected by fallout, and represent a hazard of particular importance in Nicaragua where two large lakes host several explosive volcanoes.