Crust/Mantle Recycling at convergence zones : Edited by S. R. Hart and L. Gulen. Kluwer Academic Publishers, 1989, xvii + 279 p., US $79.00 (ISBN 0-7923-0066-1)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28854/1/0000689.pd

Prograde and retrograde history of eclogites from the Eastern Blue Ridge, North Carolina, USA

The prograde metamorphism of eclogites is typically obscured by chemical equilibration at peak conditions and by partial requilibration during retrograde metamorphism. Eclogites from the Eastern Blue Ridge of North Carolina retain evidence of their prograde path in the form of inclusions preserved in garnet. These eclogites, from the vicinity of Bakersville, North Carolina, USA are primarily comprised of garnet–clinopyroxene–rutile–hornblende–plagioclase–quartz. Quartz, clinopyroxene, hornblende, rutile, epidote, titanite and biotite are found as inclusions in garnet cores. Included hornblende and clinopyroxene are chemically distinct from their matrix counterparts. Thermobarometry of inclusion sets from different garnets record different conditions. Inclusions of clinozoisite, titanite, rutile and quartz (clinozoisite + titanite = grossular + rutile + quartz + H 2 O) yield pressures (6–10 kbar, 400–600 °C and 8–12 kbar 450–680 °C) at or below the minimum peak conditions from matrix phases (10–13 kbar at 600–800 °C). Inclusions of hornblende, biotite and quartz give higher pressures (13–16 kbar and 630–660 °C). Early matrix pyroxene is partially or fully broken down to a diopside–plagioclase symplectite, and both garnet and pyroxene are rimmed with plagioclase and hornblende. Hypersthene is found as a minor phase in some diopside + plagioclase symplectites, which suggests retrogression through the granulite facies. Two-pyroxene thermometry of this assemblage gives a temperature of c. 750 °C. Pairing the most Mg-rich garnet composition with the assemblage plagioclase–diopside–hypersthene–quartz gives pressures of 14–16 kbar at this temperature. The hornblende–plagioclase–garnet rim–quartz assemblage yields 9–12 kbar and 500–550 °C. The combined P–T data show a clockwise loop from the amphibolite to eclogite to granulite facies, all of which are overprinted by a texturally late amphibolite facies assemblage. This loop provides an unusually complete P–T history of an eclogite, recording events during and following subduction and continental collision in the early Palaeozoic.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73522/1/j.1525-1314.2003.00479.x.pd

Diagenetic incorporation of Sr into aragonitic bivalve shells: implications for chronostratigraphic and palaeoenvironmental interpretations

Aragonite is easily altered during diagenesis, therefore presumed pristine when present. In effect, beyond polymorphic transformation to calcite, alteration paths of aragonite remain poorly understood despite heavy reliance on such material to produce palaeoenvironmental and chronostratigraphic interpretations. Previous work on core material from Southern McMurdo Sound, Antarctica, showed that unlike their calcitic counterparts, seemingly unaltered aragonite shell fragments invariably produced older than expected 87Sr/86Sr ages. In this study, we pursued additional analyses of these aragonite shells and of the porewater of the core to understand this discrepancy. Aragonite mineralogy was reconfirmed and elemental mapping of shell fragments revealed growth lines within the middle layer suggestive of good preservation. The outer layer, however, showed anomalously high Sr concentrations (average 4·5 ± 0·6 mole% SrCO3; ca 25 mmol mol−1 Sr/Ca) and was depleted in 18O and 13C compared to the middle layer, both features inconsistent with pristine material. The δ18O values and Sr concentrations of the porewater were used to model outer layer compositions reasonably well. Coincidentally, porewater Sr isotope composition was in general agreement with the age model of the core only at the aragonite‐bearing interval suggesting that Sr‐isotopic disequilibrium between porewater and the carbonates was the rule rather than the exception in the core. The Sr isotope compositions of the aragonite shells are most likely the result of early diagenesis as suggested by the inconsistent O and C isotope compositions between shell layers and the anomalously high Sr concentrations. We conclude that knowledge of Sr concentration and distribution in shells is critical to determine the viability of Sr stratigraphy and the scale at which it may be applied. Reliance on traditional indicators of lack of alteration, such as cathodoluminescence, Mn‐Fe concentration, and the presence of labile mineralogies to assert chronostratigraphic and palaeoenvironmental questions may produce erroneous conclusions due to obscurely altered material.Compositional map of Sr over BSE image and representative SEM detail overlay from a Miocene aragonitic Retrotape andrillorum bivalve, Southern McMurdo Sound, Antarctica. Signs of apparent good preservation, such as the presence of growth bands and the preservation of crystalline structure and mineralogy, appear in contrast to anomalously high Sr concentrations. Results suggest that relying solely on traditional methods to rule out diagenesis may in some cases lead to erroneous conclusions due to obscure alteration patterns.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113152/1/dep23.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/113152/2/dep23-sup-0003-AppendixS3.pd

Age and petrogenesis of the Sarmiento ophiolite complex of southern Chile

Zircon fractions separated from fine-grained plagiogranites, interpreted to be cogenetic with the mafic rocks of the Sarmiento ophiolite complex in southern Chile, yield slightly to grossly discordant age patterns for which the lower concordia intercept U-Pb ages of 140.7 +/- 0.7 Ma (Lolos Fjord) and 137.1 +/- 0.6 Ma (Encuentro Fjord) are well constrained. These dates are interpreted as formation ages for the northern portion of the igneous floor of the Rocas Verdes basin, and they are younger than the age of 150 Ma determined for a more southern portion of the floor of this basin on South Georgia Island. Coarse-grained trondjemites within the gabbro units of the Sarmiento complex yield a lower concordia intercept U-Pb age of 147 +/- 10 Ma and a poorly defined upper intercept reflecting an inherited zircon component, possibly of Proterozoic age. These rocks are interpreted as remobilized fragments of country rocks entrapped within the essentially mantle-derived rocks of the ophiolite complex.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29957/1/0000318.pd

The Nd-, Sr- and Pb-isotopic character of lavas from Taal, Laguna de Bay and Arayat volcanoes, southwestern Luzon, Philippines: Implications for arc magma petrogenesis

Following the amalgamation of a collage of pre-Neogene terranes largely by strike-slip and convergence mechanisms to form the Philippine islands, volcanic chains, related to oppositely dipping subduction zones, developed along the eastern and western margins of the archipelago. There is ample field evidence that this volcanic activity, predominantly calc-alkaline in chemical character, had commenced by the Oligocene.Volcanoes resulting from subduction along the Manila-Negros trench in the west (e.g. Taal, Laguna de Bay and Arayat) form a high-angle linear array, trending away from the MORE field on Pb-isotopic covariation diagrams; have the highest Sr- and lowest Nd-isotopic compositions, of the two chains (but nevertheless plotting above bulk earth on the 87Sr/86Sr versus 143Nd/144Nd covariation diagram); and exhibit Sm/Nd and Rb/Sr values that are lower and higher, respectively, than the estimated values for bulk earth. While the Sm/Nd and Rb/Sr characteristics are common to both chains, volcanoes associated with the Philippine-East Luzon trench have Pb-isotopic compositions that fall in the Indian Ocean MORB field and that require time-integrated evolution in a high Th/U environment. They also have higher Nd- and lower Sr-isotopic ratios.The source materials of Philippine volcanoes, therefore, have undergone varied recent enrichments in LILE, as indicated by the decoupling of isotopic and elemental ratios. These enrichments, particularly for the western volcanoes, cannot be entirely due to small degrees of partial melting in the mantle wedge, considering that they were accompanied by elevations in radiogenic Pb. Elevated Pb ratios are best explained by the introduction of subducted, continentally derived sediments. The sedimentary component in the western volcanoes is probably the South China Sea sediments derived largely from Eurasia. That this component is not available in the Philippine-East Luzon trench is reflected by the fact that the eastern volcanoes have higher Nd- and lower Sr-isotopic ratios as well as less radiogenic common Pb.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31432/1/0000350.pd

Hf- and O-isotope data from detrital and granitoid zircons reveal characteristics of the Permian–Triassic magmatic belt along the Antarctic sector of Gondwana

Permian–Triassic strata in the Transantarctic Mountains and West Antarctica carry a significant detrital component derived from a contemporaneous magmatic belt along the Gondwana margin. Hf- and O-isotope characteristics were determined for near-contemporaneous (as shown by U-Pb zircon geochronology) detrital igneous zircons in Upper Permian and Triassic sandstones. Zircons from six granitoids in the contemporaneous magmatic belt were also analyzed for Hf and O isotopes in order to gain insight into the potential detrital zircon sources. Although the ages of these granitoids only loosely correspond with the depositional ages of the sandstones, the initial εHf and δ18O isotope compositions for these igneous zircon grains, in general, overlap those recorded for the detrital igneous zircon grains. Results demonstrate a range of εHf and δ18O values. Features of particular interest are the very low δ18O values in two of the granitoids, and similar low values also recorded in the detrital igneous zircons in two sandstones. The distribution of Permian–Triassic granitoids must be much greater than is apparent from the existing outcrops in the extensively ice-covered region. The Permian and one of the Triassic granitoids have Hf-isotope characteristics similar to the Cretaceous granites and Devonian–Carboniferous plutons of West Antarctica, whereas the other Triassic granite differs from both. Importantly, the zircon isotopic data from the Permian–Triassic rocks suggest that an Hf-defined Upper Mesoproterozoic lithosphere underlies much of the magmatic belt

Patterns of nucleotide diversity in Meisa1 and G3pdh in wild and cultivated cassava

The distribution and frequency of single nucleotide polymorphisms (snps) is an excellent tool for discerning evolutionary relatedness between cultivated and wild plant genomes. This type of information is scanty for the genus Manihot, and thus limiting systematic approaches in the genetic improvement of cassava. Here, we present a detailed description of the comparative patterns of snps in Isoamylase1 (Meisa1) and Glyceraldehyde-3-phosphate dehydrogenase (G3pdh) in 10 accessions of wild (Manihot esculenta subsp. flabellifolia) and 12 accessions of cultivated cassava (M. esculenta). The results show that Meisa1 is more variable in cultivated cassava than that in subspecies flabellifolia, where the 954 bp sequence region differs at 1 in 111 and 250 nucleotides of cultivated and wild species, respectively. Frequency analysis shows that snp occurs once every 42 bp in cultivated and every 70 bp in wild. Tajima’s D test statistics showed that Meisa1 has been evolving under different selection pressures, diversifying in cultivated and purifying in wild. G3pdh is under diversifying selection in both populations. This may indicate the importance for isoamylase1 in starch quality traits in cassava, a trait that is likely to have been the target for artificial selection by farmers and breeders, in addition to natural selection. This study also suggests that G3pdh may be a good marker for phylogeny study while Meisa1 may be useful for intra and inter-cultivar diversity studies. The non-synonymous snps that changed the amino acid property were identified and the potential implication of the change in protein function was analyzed and discussed

Exhumation history along the eastern Amundsen Sea coast, West Antarctica, revealed by low-temperature thermochronology

West Antarctica experienced a complex tectonic history, which is still poorly documented, in part due to extensive ice cover. Here we reconstruct the Cretaceous to present thermotectonic history of Pine Island Bay area and its adjacent coasts, based on a combination of apatite and zircon fission track and apatite (U-Th-Sm)/He thermochronology. In addition, we report petrographic information for the catchments of Pine Island, Thurston Island, and Thwaites glaciers. Our data suggest that the underlying bedrock of the Pine Island and Thwaites Glacier catchments are very different and vary from granitoids to (Cenozoic?) volcanogenic sequences and low-grade metamorphics. Our thermochronology data show that the upper crustal rocks of Pine Island Bay experienced very rapid cooling during the late Cretaceous. We attribute this rapid cooling of basement rocks and associated reduction in mean elevation to tectonic denudation driven by gravitational collapse of the Cretaceous orogen along the proto-Pacific Gondwana margin. Rapid Cretaceous crustal cooling was followed by very slow cooling during the Cenozoic, with no erosional response—within the limits of thermochronological methods—to the onset of glaciation and subsequent climatic changes. Cenozoic rifting within the West Antarctic Rift appears to have had little effect on erosion processes around Pine Island Bay; instead, our data suggest Cenozoic crustal tilting toward Pine Island Trough, a major geomorphic feature previously suggested to be a branch of the rift system

Magmatic modification of the uppermost mantle beneath the Basin and Range to Colorado Plateau Transition Zone; Evidence from xenoliths, Wikieup, Arizona

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform 143 Nd/ 144 Nd for the mineral separates and whole rocks in spite of the significantly wide range in their 147 Sm/ 144 Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42222/1/410-128-1-52_71280052.pd