KREEP basalt petrogenesis: Insights from 15434,181

Returned lunar KREEP basalts originated through impact processes or endogenous melting of the lunar interior. Various methods have been used to distinguish between these two origins, with varying degrees of success. Apollo 15 KREEP basalts are generally considered to be endogenous melts of the lunar interior. For example, sample 15434,181 is reported to have formed by a two‐stage cooling process, with large orthopyroxene (Opx) phenocrysts forming first and eventually cocrystalizing with smaller plagioclase crystals. However, major and trace element analyses of Opx and plagioclase coupled with calculated equilibrium liquids are inconsistent with the large orthopyroxenes being a phenocryst phase. Equilibrium liquid rare earth element (REE) profiles are enriched relative to the whole rock (WR) composition, inconsistent with Opx being an early crystallizing phase, and these are distinct from the plagioclase REE equilibrium liquids. Fractional crystallization modeling using the Opx equilibrium liquids as a parental composition cannot reproduce the WR values even with crystallization of late‐stage phosphates and zircon. This work concludes that instead of being a phenocryst phase, the large Opx crystals are actually xenocrysts that were subsequently affected by pyroxene overgrowths that formed intergrowths with cocrystallizing plagioclase.     

Early diagenetic remineralization of sedimentary organic C in the Gulf of Papua deltaic complex (Papua New Guinea): Net loss of terrestrial C and diagenetic fractionation of C isotopes

Oceania supplies ∼40% of the global riverine flux of organic carbon, approximately half of which is injected onto broad continental shelves and processed in shallow deltaic systems. The Gulf of Papua, on the south coast of the large island of New Guinea, is one such deltaic clinoform complex. It receives ∼4 Mt yr⁻¹ particulate terrestrial organic carbon with initial particle C_org loading ∼0.7 mg m⁻². C_org loading is reduced to ∼0.3 mg m⁻² in the topset-upper foreset zones of the delta despite additional inputs of mangrove and planktonic detritus, and high net sediment accumulation rates of 1-4 cm yr⁻¹. Carbon isotopic analyses (δ¹³C, Δ¹⁴C) of ΣCO₂ and C_org demonstrate rapid (<100 yr) remineralization of both terrestrial (δ¹³C <−28.6) and marine C_org (δ¹³C ∼−20.5) ranging in average age from modern (bomb) (Δ¹⁴C ∼60) to ∼1000 yr (Δ¹⁴C ∼−140). Efficient and rapid remineralization in the topset-upper foreset zone is promoted by frequent physical reworking, bioturbation, exposure, and reoxidation of deposits. The seafloor in these regions, particularly <20 m, apparently functions as a periodically mixed, suboxic batch reactor dominated by microbial biomass. Although terrestrial sources can be the primary metabolic substrates at inshore sites, relatively young marine C_org often preferentially dominates pore water ΣCO₂ relative to bulk C_org in the upper foreset. Thus a small quantity of young, rapidly recycled marine organic material is often superimposed on a generally older, less reactive terrestrial background. Whereas the pore water ΣCO₂ reflects both rapidly cycled marine and terrestrial sources, terrestrial material dominates the slower overall net loss of C_org from particles in the topset-upper foreset zone (i.e. recycled marine C_org leaves little residue). Preferential utilization of C_org subpools and diagenetic fractionation of C isotopes supports the reactive continuum model as a conceptual basis for net decomposition kinetics. Early diagenetic fractionation of C isotopes relative to the bulk sedimentary C_org composition can produce changes in ¹⁴C activity independent of radioactive decay. In the Gulf of Papua topset-upper foreset, Δ¹⁴C of pore water ΣCO₂ averaged ∼ 300‰ greater than C_org sediment between ∼1-3 m depth in deposits. Diagenetic fractionation and decomposition aging of sedimentary C_org compromises simple application of ¹⁴C for determination of sediment accumulation rates in diagenetically reactive deposits.     

A test of the possible influence of seabird activity on the <Superscript>210</Superscript>Pb flux in high Arctic ponds at Cape Vera,Devon Island,Nunavut: implications for radiochronology

A large seabird colony of northern fulmars (Fulmarus glacialis) at Cape Vera on Devon Island in the Canadian high Arctic has been shown to act as an effective biological pump, transporting large quantities of marine-derived nutrients and contaminants to the ponds beneath their nesting sites. Previous research has shown that seabirds also have the potential to elevate radionuclide concentrations in their immediate surroundings. Here, we assess the influence of the Cape Vera seabird colony on the ²¹⁰Pb budgets of six affected ponds and one control site, and discuss potential implications with respect to ²¹⁰Pb dating methodologies. In general, fluxes of ²¹⁰Pb at Cape Vera were similar to, or less than, those measured at other Arctic locations not influenced by seabirds. Plots of unsupported ²¹⁰Pb activity versus depth showed a number of non-monotonic features in all sediment cores, necessitating the use of the constant-rate-of-supply (CRS) model to derive age-depth profiles. We conclude that seabird activity has not dramatically increased the ²¹⁰Pb flux to the ponds at Cape Vera. Overall, the low flux of excess ²¹⁰Pb made accurate dating problematic for some of the study cores, necessitating the use of independent dating markers (e.g., ¹³⁷Cs and PCB concentrations) to verify the reliability of the chronologies.     

Probabilistic evaluation of flood hazard in urban areas using Monte Carlo simulation

The goal of the presented research was the derivation of flood hazard maps, using Monte Carlo simulation of flood propagation at an urban site in the UK, specifically an urban area of the city of Glasgow. A hydrodynamic model describing the propagation of flood waves, based on the De Saint Venant equations in two‐dimensional form capable of accounting for the topographic complexity of the area (preferential outflow paths, buildings, manholes, etc.) and for the characteristics of prevailing imperviousness typical of the urban areas, has been used to derive the hydrodynamic characteristics of flood events (i.e. water depths and flow velocities). The knowledge of the water depth distribution and of the current velocities derived from the propagation model along with the knowledge of the topographic characteristics of the urban area from digital map data allowed for the production of hazard maps based on properly defined hazard indexes. These indexes are evaluated in a probabilistic framework to overcome the classical problem of single deterministic prediction of flood extent for the design event and to introduce the concept of the likelihood of flooding at a given point as the sum of data uncertainty, model structural error and parameterization uncertainty. Copyright © 2011 John Wiley & Sons, Ltd.     

Volcano collapse promoted by progressive strength reduction: new data from Mount St. Helens

Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse, even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction could promote collapses at other volcanic edifices.     

Sediment entrainment by a soft‐bedded glacier: a model based on regelation into the bed

Experimental results indicate that wet‐based, soft‐bedded glaciers may penetrate their substrates by regelation (melting and refreezing) and thereby entrain sediment. In principle, there should be a steady depth of penetration at which the downward regelation speed, driven by the interfacial effective pressure, equals the basal melt rate. Herein, the magnitude and distribution of penetration are estimated for the case of a glacier resting on a deformable bed with channelized basal drainage. The distance between channels and the distribution of effective pressure across the bed are calculated, and special attention is paid to the interdependence of the basal melt rate and effective pressure. A major uncertainty is whether fine‐grained tills may impede or prevent regelation as a result of high surface tension at the ice/water interface. Predicted penetration depths range from millimetres to a few decimetres, and thus, dirty basal ice layers of such thicknesses might be expected. Predicted distances between channels agree well with those indicated by borehole measurements at Ice Stream B, West Antarctica. Effective pressures, and hence penetration depths, increase toward channels. Therefore, the edges of interfluves, bounded by anastomosing subglacial channels, should be eroded preferentially. This motivates the testable hypothesis that such erosion contributes to the formation of streamlined landforms, such as drumlins. Copyright © 2000 John Wiley & Sons, Ltd.     

Clay mineralogy of Gulf of Papua Shelf and Pandora Trough deposits constrains sediment routing during the last sea-level cycle

The Gulf of Papua inner mid-shelf clinothem and lowstand deposits in Pandora Trough record sediment source and routing through the last sea-level cycle on 20 kyr cycles. Clay mineralogy tracked dispersal of sediment from the two types of rivers (wide versus narrow floodplains) to constrain the contributions of river systems to the Gulf of Papua clinothem and Pandora Trough deposits. Fly River sediment has higher illite:smectite than clays from the small mountainous rivers (Bamu, Turama, Kikori and Purari rivers) that drain regions with more limestones. X-ray diffraction shows high illite:smectite proximal to the Fly River delta that decrease towards the north-east. Downcore mineralogy of inner mid-shelf cores reveals that the largest shifts in illite:smectite correspond to changes in sediment units. The relict clinothem emplaced on the Gulf of Papua shelf during Marine Isotope Stage 3 has lower illite:smectite than the Holocene clinothem that has been building since 2 ka and the Marine Isotope Stage 5a relict clinothem. In the inner mid-shelf, downcore decreases in illite:smectite during Marine Isotope Stage 3 suggest that this region received less clay from the Fly River and more contributions from small mountainous rivers. During Marine Isotope Stage 3, the exposed physiography and narrower shelf in this region may have deflected Fly River sediment more south-eastward, where it bypassed the inner shelf via the Kiwai, Purutu and Umuda valleys and was deposited in the Pandora Trough. The Fly River may have been more susceptible to valley incision because of its limited shelf accommodation and higher ratio of water to sediment discharge. Such bypass of the inner mid-shelf by Fly River sediment during the Marine Isotope Stage 2 sea-level lowstand is recorded in Pandora Trough deposits with high illite:smectite ratios. Inner mid-shelf clinothems with compositional shifts on the order of 20 kyr may be influenced by shelf physiography, accommodation and the variable incision by small and large rivers.     

大别山英山和熊店榴辉岩单颗粒锆石SHRIMP U-Pb年代学研究

报道了对大别山高压-超高压带河南罗山熊店和湖北英山蜜蜂尖两地榴辉岩单颗粒锆石SHRIMP U-Pb原位(in-situ)定年结果.英山超高压榴辉岩年龄有4组:(773±20) Ma,(730±16) Ma,(461±7) Ma和(262±6) Ma,其中以(461±7) Ma占绝对优势.熊店高压榴辉岩有3组年龄:(449±14) Ma,(307±14) Ma和(216±4) Ma.两地榴辉岩中均存在约450 Ma的早古生代奥陶纪年龄,它们均具有极低的Zr含量(9×10-6～19×10-6).结合熊店榴辉岩样品具有类似于MORB的微量元素特征和εNd值(450 Ma时为+6.56),说明该古生代年龄应代表古生代洋壳和岛弧物质俯冲经高压-超高压变质作用的产物.由于奥陶纪高压-超高压事件的年龄同时出现在英山和熊店榴辉岩中,且熊店榴辉岩中清楚地记录了三叠纪高压-超高压事件,表明大别高压-超高压带北界无论是在奥陶纪或三叠纪时均应在熊店榴辉岩以北的桐柏-商城断裂.     

Birthplace of the São Francisco Craton,Brazil: Evidence from 3.60 to 3.64 Ga Gneisses of the Mairi Gneiss Complex

Records of Earth's primitive crust are scarce. Eoarchean (older than 3.6 Ga) banded mafic to felsic gneisses have been discovered in the São Francisco Craton, Brazil, pushing back by over 100 million years the oldest gneisses known to date in South America (3.5 Ga). Zircon U‐Pb data yield rock ages from 3,598 to 3,642 Ma with a few ca. 3.65–3.69 Ga grains suggesting even older rocks in the area. Zircon grains show significantly negative to nearly chondritic initial εHf values and two‐stage model ages from 3.82 to 4.33 Ga, which may indicate the existence of a recycled Hadean to early Eoarchean crust in the region. The felsic gneisses are chemically similar to the low‐pressure Tonalite‐Trondhjemite‐Granodiorite association whereas the mafic gneisses have geochemical signatures that resemble within‐plate basaltic andesite to andesite of Iceland (icelandites). The results are relevant to constrain the composition of Earth's first continental crust.