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Lackey J.S.,Pomona College | Romero G.A.,Pomona College | Bouvier A.-S.,University of Wisconsin - Madison | Bouvier A.-S.,Institute Of Mineralogie Et Geochimie | Valley J.W.,University of Wisconsin - Madison
Geology | Year: 2012

In order to improve understanding of how accessory garnet crystallizes in igneous rocks, and evaluate it as a mineral recorder of magma history, we analyzed δ18O of garnets from the Hallowell and Togus plutons in south-central Maine (United States) by laser fl uorination, and in situ by ion microprobe. Two types of garnet are recognized, magmatic and locally derived peritectic. Traverses of some single crystals show both gradual and abrupt changes of δ18O(garnet), commonly >1‰, while other garnet grains are isotopically homogeneous. Rimward increase of δ18O in many crystals indicates that garnet grew while high δ18O metamorphic wall rocks were assimilated. Peritectic grains have a complementary record of the transfer of high δ18O melts to the plutons. In some rocks, δ18O varies among neighboring grains, evidence that crystals grew episodically or were juxtaposed from different sources during magma mixing. Garnet faithfully records changing magmatic δ18O, and is a valuable tool to decipher magma petrogenesis. © 2012 Geological Society of America.

Trincal V.,University of Franche Comte | Charpentier D.,University of Franche Comte | Buatier M.D.,University of Franche Comte | Grobety B.,University of Fribourg | And 3 more authors.
Marine and Petroleum Geology | Year: 2014

In fold-and-thrust belts, shortening is mainly accommodated by thrust faults which are preferential zones for recrystallisation and mass transfer. This study focuses on a detachment fault related to the emplacement of the Monte Perdido thrust unit in the southern Pyrenees. The studied fault zone consists of a 10m thick intensively foliated phyllonite developed within the Millaris marls, of Eocene age. The lithological homogeneity of the hanging wall and footwall allows us to compare the Millaris marls outside the fault zone with the highly deformed marls located in the fault zone and to quantify the chemical, mineralogical and volumetric changes related to deformation processes along the fault. The Millaris marls are composed of detrital quartz, illite, chlorite, minor albite and pyrite, in a micritic calcite matrix. In the fault zone, the cleavage planes are marked by clay minerals and calcite±chlorite veins attest to fluid-mineral interactions during deformation. The mineral proportions in all samples from both the fault zone and Millaris marls have been quantified by two methods: (1) X-ray diffraction and Rietveld refinement, and (2) bulk chemical analyses as well as microprobe analyses to calculate modal composition. The excellent agreement between the results of these two methods allows us to estimate mineralogical variations using a modification of the Gresens' equation. During fault activation, up to 45wt% of calcite was lost while the amounts of quartz and chlorite remained unchanged. Illite content remained constant to slightly enriched. The mineralogical variations were coupled with a significant volume loss (up to 45%) mostly due to the dissolution of micritic calcite grains. Deformation was accompanied by pressure solution and phyllosilicates recrystallisation. These processes accommodated slip along the fault. They required fluids as catalyst, but they did not necessitate major chemical transfers. © 2013 Elsevier Ltd.

Tutken T.,University of Bonn | Vennemann T.W.,Institute Of Mineralogie Et Geochimie | Pfretzschner H.-U.,University of Tübingen
Geochimica et Cosmochimica Acta | Year: 2011

Rare earth elements (REE), while not essential for the physiologic functions of animals, are ingested and incorporated in ppb concentrations in bones and teeth. Nd isotope compositions of modern bones of animals from isotopically distinct habitats demonstrate that the 143Nd/144Nd of the apatite can be used as a fingerprint for bedrock geology or ambient water mass. This potentially allows the provenance and migration of extant vertebrates to be traced, similar to the use of Sr isotopes. Although REE may be enriched by up to 5 orders of magnitude during diagenesis and recrystallization of bone apatite, in vivo 143Nd/144Nd may be preserved in the inner cortex of fossil bones or enamel. However, tracking the provenance of ancient or extinct vertebrates is possible only for well-preserved archeological and paleontological skeletal remains with in vivo-like Nd contents at the ppb-level. Intra-bone and -tooth REE analysis can be used to screen for appropriate areas. Large intra-bone Nd concentration gradients of 101-103 are often measured. Nd concentrations in the inner bone cortex increase over timescales of millions of years, while bone rims may be enriched over millenial timescales. Nevertheless, εNd values are often similar within one εNd unit within a single bone. Larger intra-bone differences in specimens may either reflect a partial preservation of in vivo values or changing εNd values of the diagenetic fluid during fossilization. However, most fossil specimens and the outer rims of bones will record taphonomic 143Nd/144Nd incorporated post mortem during diagenesis. Unlike REE patterns, 143Nd/144Nd are not biased by fractionation processes during REE-uptake into the apatite crystal lattice, hence the εNd value is an important tracer for taphonomy and reworking. Bones and teeth from autochthonous fossil assemblages have small variations of ±1εNd unit only. In contrast, fossil bones and teeth from over 20 different marine and terrestrial fossil sites have a total range of εNd values from -13.0 to 4.9 (n=80), often matching the composition of the embedding sediment. This implies that the surrounding sediment is the source of Nd in the fossil bones and that the specimens of this study seem not to have been reworked. Differences in εNd values between skeletal remains and embedding sediment may either indicate reworking of fossils and/or a REE-uptake from a diagenetic fluid with non-sediment derived εNd values. The latter often applies to fossil shark teeth, which may preserve paleo-seawater values. Complementary to εNd values, 87Sr/86Sr can help to further constrain the fossil provenance and reworking. © 2011 Elsevier Ltd.

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