Constraints from mantle xenoliths on the geodynamic evolution of Earth’s upper mantle

Geophysical studies identify a region of slow seismic velocity mantle beneath the central Rio Grande rift that potentially represents a region of hot asthenospheric mantle that has replaced destabilized lithosphere. We determine that the majority of mantle xenoliths from Elephant Butte on the central Rio Grande rift axis are, based on their geochemical affinity to depleted mantle, derived from asthenospheric mantle that has accreted to the base of the Proterozoic lithosphere. Using mantle heat flow models, we estimate the boundary between residual lithosphere and accreted asthenosphere to be at ~45km depth. The amount of lithosphere thinning that has occurred cannot be accounted for by rift-related extension and we therefore suggest that convective removal of a large portion of Proterozoic lithosphere has occurred. Convecting upper mantle-derived peridotites display extreme isotopic depletions that are not observed in mid-ocean ridge basalts (MORB). Previous studies suggest that these isotopically ultradepleted domains represent rare refractory mantle domains that do not participate in MORB petrogenesis. We demonstrate the isotopically ultradepleted domains are not only a ubiquitous feature of convecting upper mantle, but are also capable of melting if advected beneath mid-ocean ridges. To explain the lack of MORB with ultradepleted isotopes, we suggest that MORB compositions are biased towards fertile enriched source components. Estimates of upper mantle composition based on MORB therefore overestimate the fertility of the upper mantle. The Lu-Hf system is commonly used to estimate the timing of melt depletion events in the lithospheric mantle. This is typically done with pseudoisochrons from genetically related mantle xenoliths. Most studies, however, misinterpret Lu/Hf – Hf isotopes correlations by using Cpx Lu/Hf ratios when whole rock Lu/Hf ratios are appropriate due to equilibration above the Lu-Hf closure temperature. Hafnium isotopes do not typically correlate with indicators of melt depletion, which suggests that Hf isotopes do not record ancient melting events. This is likely due to overprinting of Hf isotopes by later metasomatic events.Geological Science

The impact of pre-injury controlled substance use on clinical outcomes after trauma

A disproportionately high percentage of trauma patients use controlled substances, and they often co-ingest multiple drugs. Previous studies have evaluated the effect of individual drugs on clinical outcomes following trauma. However, the impact of all drugs included in a comprehensive screening panel has not yet been compared in a single cohort of patients

An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt

The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish “deep” S sources (e.g. magmas) from “surface” S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent fractionation of S isotopes (Δ33S = −0.6‰ to +1.0‰) and the distribution of the analyses in the Δ33S–δ34S space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O–CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmospheric signature suggests the hypothesis that the source of Au should also be identified in the supracrustal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean

Military Administration in Thirteenth Century England: A Study Emphasizing the Contributions of the Wardrobe in the Wars of Henry Iii and Edward I, 1216-1307

153 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1965.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Early Earth mantle heterogeneity revealed by light oxygen isotopes of Archaean komatiites

© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. Geodynamic processes on early Earth, especially the interaction between the crust and deep mantle, are poorly constrained and subject to much debate. The rarity of fresh igneous materials more than 3 billion years old accounts for much of this uncertainty. Here we examine 3.27-billion-year-old komatiite lavas from Weltevreden Formation in the Barberton greenstone belt, which is part of the Kaapvaal Craton in Southern Africa. We show that primary magmatic compositions of olivine are well preserved in these lavas based on major and trace element systematics. These komatiitic lavas represent products of deep mantle plumes. Oxygen isotope compositions (d18O) of the fresh olivine measured by laser fluorination are consistently lighter (about 2h) than those obtained from modern mantle-derived volcanic rocks. These results suggest a mantle source for the Weltevreden komatiites that is unlike the modern mantle and one that reflects mantle heterogeneity left over from a Hadean magma ocean. The anomalously light d18O may have resulted from fractionation of deep magma ocean phases, as has been proposed to explain lithophile and siderophile isotope compositions of Archaean komatiites

Carbonatite Versus Silicate Melt Metasomatism Impacts Grain Scale 87Sr/86Sr and 143Nd/144Nd Heterogeneity in Polynesian Mantle Peridotite Xenoliths

Abstract The Earth's upper mantle is isotopically heterogeneous over large lengthscales, but the lower limit of these heterogeneities is not well quantified. Grain scale trace elemental variability has been observed in mantle peridotites, which suggests that isotopic heterogeneity may be preserved as well. Recent advances in isotope ratio mass spectrometry enable isotopic analysis of very small samples (e.g., nanograms or less of analyte) while maintaining the precision necessary for meaningful interpretation. Here we examine four peridotite xenoliths—hosted in lavas from Savai'i (Samoa hotspot) and Tahiti (Societies hotspot) islands—that exhibit grain scale trace element heterogeneity likely related to trapped fluid and/or melt inclusions. To evaluate whether this heterogeneity is also reflected in grain scale isotopic heterogeneity, we separated clinopyroxene, orthopyroxene, and (in the most geochemically enriched xenolith) olivine for single‐grain 87Sr/86Sr and 143Nd/144Nd analyses. We find, in some xenoliths, extreme intra‐xenolith isotopic heterogeneity. For example, in one xenolith, different mineral grains range in 87Sr/86Sr from 0.70987 to 0.71321, with corresponding variability in 143Nd/144Nd from 0.512331 to 0.512462. However, not all peridotite xenoliths which display trace elemental heterogeneity exhibit isotopic heterogeneity. Based on coupled isotopic and trace element data (i.e., a negatively‐sloping trend in 87Sr/86Sr vs. Ti/Eu), we suggest that carbonatitic metasomatism is responsible for creating the intra‐xenolith isotopic heterogeneities which we observe. This carbonatitic component falls off the array defined in 87Sr/86Sr‐143Nd/144Nd space by Samoa hotspot basalts, which suggests a second, distinct EM2 (enriched mantle II) component is present in the Samoa hotspot that is not readily recognized in erupted products, but is instead seen only in mantle peridotite xenoliths

Heterogeneous Hadean crust with ambient mantle affinity recorded in detrital zircons of the Green Sandstone Bed, South Africa

The nature of Earth\u27s earliest crust and the processes by which it formed remain major issues in Precambrian geology. Due to the absence of a rock record older than ∼4.02 Ga, the only direct record of the Hadean is from rare detrital zircon and that largely from a single area: the Jack Hills and Mount Narryer region of Western Australia. Here, we report on the geochemistry of Hadean detrital zircons as old as 4.15 Ga from the newly discovered Green Sandstone Bed in the Barberton greenstone belt, South Africa. We demonstrate that the U-Nb-Sc-Yb systematics of the majority of these Hadean zircons show a mantle affinity as seen in zircon from modern plume-type mantle environments and do not resemble zircon from modern continental or oceanic arcs. The zircon trace element compositions furthermore suggest magma compositions ranging from higher temperature, primitive to lower temperature, and more evolved tonalite-trondhjemite-granodiorite (TTG)-like magmas that experienced some reworking of hydrated crust. We propose that the Hadean parental magmas of the Green Sandstone Bed zircons formed from remelting of mafic, mantle-derived crust that experienced some hydrous input during melting but not from the processes seen in modern arc magmatism

Elements of the integral calculus : with a key to the solution of differential equations, and A short table of integrals /

"A short table of integrals," compiled by B.O. Peirce, p. [1]-28 at end has separate t.-p.A sequel to the author's work on differential calculus, chapter v of which (Integration) is here reprinted as an appendix. cf. Pref.Mode of access: Internet