Isotopic and Chemical Evidence Concerning the Genesis and Contamination of Basaltic and Rhyolitic Magma Beneath the Yellowstone Plateau Volcanic Field

Since 2.2 Ma, the Yellowstone Plateau volcanic field has produced6000 km³ of rhyolite tuffs and lavas in >60 separate eruptions,as well as 100 km³ of tholeiitic basalt from >50 vents peripheralto the silicic focus. Intermediate eruptive products are absent.Large calderas collapsed at 2?0, 1?3, and 0?6 Ma on eruptionof ash-flow sheets representing at least 2500, 280, and 1000km³ of zoned magma. Early postcollapse rhyolites show largeshifts in Nd, Sr, Pb, and O isotopic compositions caused byassimilation of roof rocks and hydrothermal brines during collapseand resurgence. Younger intracaldera rhyolite lavas record partialisotopic recovery toward precaldera ration. Thirteen extracalderarhyolites show none of these effects and have sources independentof the subcaldera magma system. Contributions from the Archaeancrust have extreme values and wide ranges of Nd-, Sr-, and Pb-isotoperatios, but Yellowstone rhyolites have moderate values and limitedranges. This requires their deep-crustal sources to have beenpervasively hybridized (and the Archaean components diluted)by distributed intrusion of Cenozoic basalt, most of which wasprobably contemporaneous with the Pliocene and Qualernary volcanism.In hybrid sources yielding magmas parental to the subcalderarhyolites, half or more of the Nd and Sr may have been contributedby such young basalt. Parents for the extracaldera rhyolites,generated beyond the leading edge of the northeast-propagatingfocus of basaltic intrusion and deep-crustal mobilization, containedsmaller fractions of mantle-derived components. Most Yellowstone basalts had undergone cryptic clinopyroxenefractionation in the lower crust or crust-mantle transitionzone and, having also ascended through or adjacent to crustalzones of silicic-magma generation, most underwent some crustalcontamination. A high fraction of the Pb in most basalts isof crustal derivation. Anomalously low seismic velocities toa depth of 250 km and a high flux of ³He at Yellowstone suggestsublithospheric magma contributions. Elevated baseline Nd- andSr-isotopc ratios suggest additional contributions from oldlithospheric mantle, but this is hard to quantify because ofthe crustal overprint. Foundering of crustally contaminatedmain-stage cumulates into the low-viscosity upper mantle beneaththe principal focus of basaltic intrusion may influence theisotopic compositions of low-K tholeiites and Snake River olivinetholeiites subsequently generated along the Snake River Plainaxis in the wake of the main migrating melting anomaly.     

Exhumed channel sandstone networks within fluvial fan deposits from the Oligo-Miocene Caspe Formation, South-east Ebro Basin (North-east Spain)

The Oligo‐Miocene Caspe Formation corresponds to the middle fluvial facies of the wider Guadalope‐Matarranya fluvial fan, located in the South‐east Ebro foreland basin (North‐east Spain). At the time of the Caspe Formation deposition, this sector of the Ebro basin underwent a very continuous, moderate sedimentation rate. Lithofacies comprise deposits from channellized and unchannellized flows. Channellized flow lithofacies form multi‐storey ribbon‐like sandstone bodies that crop out as extensive sandstone ridges belonging to exhumed channel networks. Width/thickness ratios of these channel‐fill bodies average close to six. Sinuosity is usually low (most common values around 1·1), although it can be high locally (up to 2). Thicknesses range from a few metres to 15 m. Unchannellized flow lithofacies form tabular bodies that can be ascribed to overbank deposits (levées, crevasse splays and fine‐grained floodplain deposits) and also to frontal lobes, although recognition of this last case requires exceptional outcrop conditions or geophysical subsurface studies. The unchannellized flow lithofacies proportion ranges from 75% to 97·8%. Methods applied to this study include detailed three‐dimensional architectural analysis in addition to sedimentological analysis. The architecture is characterized by an intricate network of highly interconnected ribbon‐like sandstone bodies. Such bodies are connected by three kinds of connections: convergences, divergences and cross‐cuttings. Although the Caspe Formation lithofacies and architecture resemble anastomosed channels (low topographic gradient, high preservation potential, moderate aggradation rate, high lateral stability of the channels, dominance of the ribbon‐like morphologies and high proportion of floodplain to channel‐fill sediments), an unambiguous interpretation of the channel networks as anastomosed or single threaded cannot be established. Instead, the observed architecture could be considered as the product of the complex evolution of a fluvial fan segment, where different network morphologies could develop. A facies model for aggrading ephemeral fluvial systems in tectonically active, endorheic basins is proposed.     

Large silica‐rich igneous‐textured inclusions in the Buzzard Coulee chondrite: Condensates,differentiates, or impact melts?

Abstract– We studied three large (2–4 mm diameter) igneous‐textured inclusions in the Buzzard Coulee (H4) chondrite using microanalytical techniques (OLM, SEM, EMPA, SIMS) to better elucidate the origins of igneous inclusions in ordinary chondrites. The inclusions are clasts that come in two varieties (1) white inclusions Bz‐1 and Bz‐2 represent a nearly holocrystalline assemblage of low‐Ca and high‐Ca pyroxene (63–66 area%) and cristobalite (33–36%) and (2) tan inclusion Bz‐3 is glass‐rich (approximately 60%) with low‐Ca and high‐Ca pyroxene phenocrysts. The bulk compositions of the inclusions determined by modal reconstruction are all SiO₂‐rich (approximately 67 wt% for Bz‐1 and Bz‐2, approximately 62% for Bz‐3), but Bz‐3 is enriched in incompatible elements (e.g., REE approximately 4–5 × CI abundances), whereas Bz‐2 and Bz‐1 are depleted in those elements that are most incompatible in pyroxene (e.g., La‐Ho approximately 0.15–0.4 × CI abundances). These bulk compositions do not resemble what one would expect for partial or complete shock melting of a chondritic precursor, and show no evidence for overall volatility control. We infer that the inclusions originated through igneous differentiation and FeO reduction, with Bz‐3 forming as an “andesitic” partial melt (approximately 30–40% partial melting of an H chondrite precursor), and Bz‐1 and Bz‐2 forming as pyroxene‐cristobalite cumulates from an Si‐rich melt. We suggest that both types of melts experienced a period of transit through a cold, low‐pressure space environment in which cooling, FeO reduction, and interaction with a vapor occurred. Melts may have been lofted into space by excavation or splashing during collisions, or by pyroclastic volcanism. Our results indicate intriguing similarities between the inclusions in Buzzard Coulee and the silicates in some iron (IIE‐type) and stony iron (IVA‐type) meteorites, suggesting a genetic relationship.     

Chondrule formation by repeated evaporative melting and condensation in collisional debris clouds around planetesimals

Abstract– A synthesis of previous work leads to a model of chondrule formation that involves periodic melting of dispersed dust in debris clouds that were generated by collisions between chondritic planetesimals. I suggest that chondrules formed by the passage of nebular shock waves through these dust clumps, which temporarily surrounded disrupted planetesimals. Type I chondrules formed by more intense evaporative heating of fewer particles in tenuous clumps, or at the edges of dense clumps, and type II chondrules formed by less intense evaporative heating of more particles deeper within dense clumps. Chondrules reaccreted by self‐gravity into the planetesimals, mixing with less heated dust and rock. This process of disruption, melting, and reaccretion could have repeated many times. In this way, chondrite components of various origins and thermal histories could remain preserved in planetesimals as a distinctive mix of materials for extended periods of time, while still allowing for a repetitive melting process that converted some of the planetesimal debris into chondrules. I also suggest that during chondrule formation, the inner solar nebula gas was evolving by the gradual incorporation and heating of icy bodies depleted in ¹⁶O, causing a general increase in gaseous Δ¹⁷O with time in most places, especially close to the “snow line.” In this model, early formed type I chondrules in C chondrites with lower Δ¹⁷O values were produced inside the snow line, and later formed type I and type II chondrules in C and O chondrites with higher Δ¹⁷O values were created nearer the snow line after it had moved closer to the young Sun.     

Vesta as the howardite,eucrite and diogenite parent body: Implications for the size of a core and for large-scale differentiation

Abstract— If Vesta is the parent body of the howardite, eucrite, and diogenite (HED) meteorites, then geo-chemical and petrologic constraints for the meteorites may be used in conjunction with astronomical constraints for the size and mass of Vesta to (1) determine the size of a possible metal core in Vesta and (2) model the igneous differentiation and internal structure of Vesta. The density of Vesta and petrologic models for HED meteorites together suggest that the amount of metal in the parent body is <25 mass%, with a best estimate of ～5%, assuming no porosity. For a porosity of up to 5% in the silicate fraction of the asteroid, the permissible metal content is <30%. These results suggest that any metal core in the HED parent body and Vesta is not unusually large. A variety of geochemical and other data for HED meteorites are consistent with the idea that they originated in a magma ocean. It appears that diogenites formed by crystal accumulation in a magma ocean cumulate pile and that most noncumulate eucrites (excepting such eucrites as Bouvante and Statinem) formed by subsequent crystallization of the residual melts. Modelling results suggest that the HED parent body is enriched in rare earth elements by a factor of ～2.5–3.5 relative to CI-chondrites and that it has approximately chondritic Mg/Si and Al/Sc ratios. Stokes settling calculations for a Vesta-wide, nonturbulent magma ocean suggest that early-crystallizing magnesian olivine, orthopyroxene, and pigeonite would have settled relatively quickly, permitting fractional crystallization to occur, but that later-crystallizing phases would have settled (or floated) an order of magnitude more slowly, allowing, instead, a closer approach to equilibrium crystallization for the more evolved (eucritic) melts. This would have inhibited the formation of a plagioclase-flotation crust on Vesta. Plausible models for the interior of Vesta, which are consistent with the data for HED meteorites and Vesta, include a metal core (<130 km radius), an olivine-rich mantle (～65–220 km thick), a lower crustal unit (～12–43 km thick) composed of pyroxenite, from which diogenites were derived, and an upper crustal unit (～23–42 km thick), from which eucrites originated. The present shape of Vesta (with ～60 km difference in the maximum and minimum radius) suggests that all of the crustal materials, and possibly some of the underlying olivine from the mantle, could have been locally excavated or exposed by impact cratering.     

Complex brecciation and shock effects in the Buck Mountain Wash (H3–5) chondrite

Abstract Buck Mountain Wash (BMW) is a new genomict breccia (H3‐5) found in the Franconia (H5) strewn field in Arizona that shows complex brecciation and shock effects. It contains three distinct chondritic lithologies in sharp contact: a) a main lithology that consists primarily of petrographic type 5 material but which has finely intermixed type 3 and 4 material, b) a shock‐blackened (shock stage S5) type 3 lithology (lithology A), and c) a shock‐blackened type 3/4 lithology (lithology B). Buck Mountain Wash was lithified after impact‐mixing and impact‐melting of weakly and strongly metamorphosed materials, possibly at depth in the regolith of the parent body. Shock effects included brecciation on a fine scale, localized impact‐melting of silicates, partial melting, and mobilization of metal‐sulfide, and chemical fractionations that produced non‐H‐group composition kamacite by two disequilibrium mechanisms. Shock heating did not cause significant thermal metamorphism in the shock‐blackened lithologies of BMW, except possibly in areas adjacent to whole‐rock shock melt. During lithification, cooling must have been rapid at high temperatures to preserve glass and inhomogeneous silicate compositions, but not so fast at lower temperatures as to produce dendritic metal‐sulfide globules or martensite.