26 research outputs found
Evaluating mantle and crustal processes using isotope geochemistry
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999Geochemical studies are fundamental for understanding how the dynamic Earth
works and evolves. These studies place constraints on the composition, formation, age,
distribution, evolution and scales of geochemically distinct reservoirs such as the Earth's
crust, mantle and core. In this dissertation the strategy has been to work on a broad range
of topics to evaluate crustal and mantle processes. This study presents Re-Os systematics to
constrain the composition, formation and age of the lower continental crust and the mantle
lithosphere, examines melt inclusion from oceanic island basalts to evaluate the scale of the
mantle heterogeneities, and uses U-series isotope to constrain geodynamic parameters,
such as the upwelling velocities and porosities of mantle plumes.
The lower continental crust plays a pivotal role in understanding the composition
and evolution of the continental crust and the petrogenesis of continental basalts. This
chapter presents Re/Os isotope measurements which allow us to further our understanding
of these problems. Two well-characterized suites of lower crustal xenoliths from Northern
Queensland, Australia, which have average major and trace element compositions similar to
bulk lower crust, were analyzed for Re/Os isotope systematics. From this data, we infer
that the lower crust has 1 to 2 times as much as, about half of the Re and is less radiogenic
in 187OS/88OS than the upper continental crust. Our data show that assimilation and
fractional crystallization (AFC) are important processes in the formation of the lower crust
and lead to dramatic changes in the Os isotopic composition of basalts that pond andfractionate there. Because of this, the Re-Os system cannot be relied upon to yield accurate
mantle extraction ages for continental rocks.
Chapter 2 examines the Re-Os isotopic composition of the Horoman massif, Japan.
These data indicate that the Os isotope composition is controlled by the Re content, through
radiogenic ingrowth, while the Re content is governed by the extent of depletion in
"basaltic component" of the ultramafic rocks. Re-Os systematics suggest that depletion
model ages of â 1.8 Ga represent the age of the melting event. The colinearity between
mafic and ultramafic rocks in the Re-Os isochron diagram defines an apparent age of â
1Ga.. The similar "ages" determined by Re-Os and Sm-Nd isotopes and the high Re/Os
ratios in the most fertile peridotites plotting to the right of the geochron, indicate that the
mafic layers and the ultramafic rocks are genetically related by a refertilization process
which took place â 1 Ga ago. The Re-Os systematics for' other ophiolitic massifs indicate
that refertilization of the lithospheric mantle seems to be a more Widespread process than
previously thought.
Previous studies have suggested that melting processes are responsible for the trace
element variability observed in olivine-hosted basaltic melt inclusions. Melt inclusions from
four individual lava samples representing three mantle end-members HIMU, EMl and EMIl
(two from Mangaia, Cook Islands, one from Pitcairn, Gambier chain, and one from Tahaa,
Society chain), have heterogeneous Pb isotopic compositions, even though the erupted
lavas are isotopically homogeneous. The range of Pb isotopic compositions from individual
melt inclusions in a single lava flow spans 50% of the world-wide range observed for
ocean island basalts (OlB). The melt inclusion data can be explained by two-component
mixing for each island. Our data imply that magmas with different isotopic compositions
existed in the volcanic plumbing system prior to or during melt aggregation.
Evaluation of U-series disequilibrium, trace element composition and He, Sr, Nd
and Pb isotopes of Galapagos lavas indicates that magma mixing between plume and
asthenospheric melts has been the main process responsible for the geochemical variation
observed in the archipelago. Correlations between He isotopes and TilTi*, K/Rb and
Nb/La ratios suggest that the mantle plume has positive anomalies of Nb and Ti and
negative anomalies of K. 230Th excesses measured in the lavas indicate that the basalts from
Galapagos originated completely or partially in the garnet stability field. Mantle upwelling
velocity for the Galapagos plume (Fernandina) ranges from â 1 to 3 cm/y with a maximum
porosity of 0.3%, indicating that Galapagos is a mildly buoyant plume. Very slow mantle
upwelling rates and very low porosity for Pinta (0.5 to 1 crnIy and 0.1%) and Floreana
(0.1 em/year and <0.1%) islands, support the hypothesis that the movement of the plume
across the 91°50' transform fault into a younger and thinner lithosphere produced slow
upwelling and small extents of melting
Microstructural and geochemical constraints on the evolution of deep arc lithosphere
Mantle xenoliths from the Sierra Nevada, California, USA, sampled a vertical column (60â120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anticlockwise P-T-t path resulting in rapid cooling that effectively âquenched inâ features inherited from earlier high-temperature conditions. Here we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPOs) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse-protogranular, less-melt-infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine-porphyroclastic, highly melt-infiltrated garnet peridotite with weak, axial-[010] olivine CPO. In contrast to the observed axial-[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the (001)[100] slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5â10 and 30â500 wt ppm, respectively) are lower than that required to promote dominant (001)[100] slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P-T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the late Cenozoic
Globally elevated titanium, tantalum, and niobium (TITAN) in ocean island basalts with high 3He/4He
Author Posting. Š American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q04027, doi:10.1029/2007GC001876.We report evidence for a global Ti, Ta, and Nb (TITAN) enriched reservoir sampled by ocean island basalts (OIBs) with high 3He/4He ratios, an isotopic signature associated with the deep mantle. Excesses of Ti (and to a lesser degree Nb and Ta) correlate remarkably well with 3He/4He in a data set of global OIBs, demonstrating that a major element signature is associated with the high 3He/4He mantle. Additionally, we find that OIBs with high 3He/4He ratios have moderately radiogenic 187Os/188Os (>0.135). The TITAN enrichment and radiogenic 187Os/188Os in high 3He/4He OIBs indicate that they are melts of a mantle domain that hosts a nonprimitive (nonchondritic) component. The observation of TITAN enrichment in the high 3He/4He mantle may be important in balancing the Earth's budget for the TITAN elements. Understanding the origin of the TITAN enrichment is important for constraining the evolution of the enigmatic high 3He/4He mantle domain.Funds for helium measurements were provided by NSF-OCE
to M.D.K. Funds for major and trace element analyses were
provided by NSF-EAR 0509891 to S.R.H
Microstructural and geochemical constraints on the evolution of deep arc lithosphere
Mantle xenoliths from the Sierra Nevada, California, USA, sampled a vertical column (60â120 km) of lithosphere that formed during Mesozoic continental arc magmatism. This lithosphere experienced an anticlockwise P-T-t path resulting in rapid cooling that effectively âquenched inâ features inherited from earlier high-temperature conditions. Here we combine new mineral chemistry data (water, trace element, and major element concentrations) with mineral crystallographic preferred orientations (CPOs) to investigate the relationship between melt infiltration and deformation. The peridotites record a refertilization trend with increasing depth, starting from shallow, coarse-protogranular, less-melt-infiltrated spinel peridotite with strong, orthorhombic olivine CPO to deep, fine-porphyroclastic, highly melt-infiltrated garnet peridotite with weak, axial-[010] olivine CPO. In contrast to the observed axial-[010] CPOs, subgrain boundary orientations and misorientation axes suggest the dominant activation of the (001)[100] slip system, suggesting deformation under moderately hydrous conditions. After accounting for effects of subsolidus cooling, we see coherent trends between mineral trace element abundance and water content, indicating that melt infiltration led to an increase in water content of the peridotites. However, measured olivine and pyroxene water contents in all peridotites (5â10 and 30â500 wt ppm, respectively) are lower than that required to promote dominant (001)[100] slip system observed in both natural and experimental samples. These results suggest that deformation occurred earlier along the P-T path, probably during or shortly after hydrous melt infiltration. Subsequent rapid cooling at 90 Ma led to water loss from olivine (owing to decreased solubility at low temperature), leaving behind a deep arc lithosphere that remained viscously coupled to the Farallon slab until the opening of the slab window in the late Cenozoic
Parma consensus statement on metabolic disruptors
A multidisciplinary group of experts gathered in Parma Italy for a workshop hosted by the University of Parma, May 16â18, 2014 to address concerns about the potential relationship between environmental metabolic disrupting chemicals, obesity and related metabolic disorders. The objectives of the workshop were to: 1. Review findings related to the role of environmental chemicals, referred to as âmetabolic disruptorsâ, in obesity and metabolic syndrome with special attention to recent discoveries from animal model and epidemiology studies; 2. Identify conclusions that could be drawn with confidence from existing animal and human data; 3. Develop predictions based on current data; and 4. Identify critical knowledge gaps and areas of uncertainty. The consensus statements
are intended to aid in expanding understanding of the role of metabolic disruptors in the obesity and metabolic disease epidemics, to move the field forward by assessing the current state of the science and to identify research needs on the role of environmental chemical exposures in these diseases. We propose broadening the definition of obesogens to that of metabolic disruptors, to encompass chemicals that play a role in altered susceptibility to obesity, diabetes and related metabolic disorders including metabolic syndrome
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Role of f0(2) on Fluid Saturation in Oceanic Basalt - Reply
Earth and Planetary Science
Insights into the dynamics of mantle plumes from uranium-series geochemistry
The long-standing paradigm that hotspot volcanoes such as Hawaii or Iceland represent the surface expression of mantle plumesâhot, buoyant upwelling regions beneath the Earth's lithosphereâhas recently been the focus of controversy. Whether mantle plumes exist or not is pivotal for our understanding of the thermal, dynamic and compositional evolution of the Earth's mantle. Here we show that uranium-series disequilibria measured in hotspot lavas indicate that hotspots are indeed associated with hot and buoyant upwellings and that weaker (low buoyancy flux) hotspots such as Iceland and the Azores are characterized by lower excess temperatures than stronger hotspots such as Hawaii. This direct link between buoyancy flux and mantle temperature is evidence for the existence of mantle plumes.5 page(s
The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina
Abstract The Payenia region of Argentina (34.5â38°S) is a large PlioceneâQuaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of backâarc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new wholeârock major and trace element concentrations, Srâ, Ndâ, Hfâ, and Pbâisotope ratios and highâprecision olivine oxygenâisotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arcâinfluenced to intraplate signature. Variable crustal contamination and/or recent slabâderived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50â150 Ma, consistent with the timing of the breakup of Gondwana and the development of the protoâPacific Andean arc. Variations in δ18Oolivine values from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORBâlike δ18O values, providing a different explanation for the lowâδ18O signatures observed in continental arc settings