Stein B. Jacobsen - US grants

This award provides one-half the funding necessary to acquire an inductively-coupled plasma mass spectrometer (ICP-MS) system that will be installed in the Department of Earth and Planetary Sciences at Harvard University. Harvard University is committed to providing the remaining funds needed for the acquisition. The ICP-MS system will satisfy the analytical needs of a number of research groups at Harvard, mostly (an estimated 70% of machine time) in the departments of Earth and Planetary Sciences and Anthropology. The remaining machine time is expected to be devoted to the needs of researchers in the Division of Applied Sciences, and the departments of Chemistry and Biology. ICP-MS is the technique of choice for obtaining both rapid and highly sensitive analysis of a wide range of elements and concentrations. Research projects at Harvard in need of such analyses cover a range of subjects from the study of ore forming processes, to the chemical evolution of the oceans, to river water chemistry, anthropology and archaeology, and ice cores.

This research is on variations in the Nd and Sr isotopic composition and REE abundance patterns in Proterozoic and Archean chemical sediments. It's purpose is to trace the major mass fluxes in the exogenic geochemical cycle through the Proterozoic and to understand the relationship between the evolution of the sedimentary reservoir, the oceans and the atmosphere during this time period. We will concentrate on measurements of Nd and Sr isotope variations and REE abundance patterns on samples of Proterozoic marine sediments. The following specific projects are proposed: 1) Determine 143Nd/144Nd variations in Proterozoic carbonates and shales.; 2) Determine REE patterns of banded iron formations ranging in age from - 0.8 Ga to 3.5 Ga.; 3) Modeling of the Sr and Nd isotope records of the exogenic geochemical cycle through the past 3.5 Ga, emphasizing the Proterozoic.; and 4) Extend the record of high precision Sr isotopic data of carbonates from the mid-Cambrian to - 2.0 Ga ago, using well dated and documented samples from Victoria Island (N.W.T., Canada), and various sedimentary basins in Australia. We propose to establish a detailed, precise record of Sr and Nd isotopic variations of chemical sediments for the time between 0.5 and 2.0 Ga ago. During the Proterozoic, seawater Nd and Sr budgets appear to have changed from dominantly hydrothermal inputs (as in the Archean) to dominantly continental (river) inputs. Data obtained from this research will be used to constrain the changes in rates of hydrothermal circulation, continental erosion rates and variations in the mean age of continental erosion products through the Proterozoic.

This award provides support for an integrated, multi- disciplinary investigation of the earth's continental lithosphere where an abrupt boundary exist between highly extended crust and a tectonically stable block. The main research objective of this study is to use the "natural experiment" of a large strain gradient in the uppermost part of the lithosphere to understand how the Moho and other deep discontinuities respond to extension, thus quantifying how large horizontal strains of the lithosphere relate to vertical motions controlled by isostasy. A number of such regions have been identified by geological mapping in the Basin and Range province. Of these, the western margin of the Colorado Plateau is the best-suited example for accomplishing the research objectives. It has the virtues of (1) abruptness; (2) a geologic setting that indicates a simple pre-extensional crustal structure; and (3) a well understood large-scale extensional kinematic history. Achieving these research goals will require both focus and broad expertise. The principal investigators will integrate geologic, petrologic, paleomagnetic, and geochronologic studies and continuum modeling to unravel the geometry, kinematics and dynamics of the lithosphere's response to severe extensional strain.

The Black Mountains, Death Valley, CA, present an opportunity to characterize a magmatic system developed during large-scale intracontinental extension. The goals of this project are to: 1) determine the relationship of magma sources and magma evolution to the geometry and stages of extension, 2) determine the chemical and isotopic signature of new additions to the crust in extensional provinces, 3) determine the crust-mantle mass balance for the southern Basin and Range Province, 4) determine whether or not the geochemistry of the intrusive series is reflected in the eruptive sequence. The PI hopes to: 1) determine the ages of various plutonic bodies through U-Pb, Rb-Sr and fission track studies, 2) measure the Nd, Sr, and Pb isotope, and trace element composition, and 3) apply geobarometry studies. All this wil be done following careful field work.

Recent investigation point to the latest Proterozoic (0.5-0.9 Ga) as being a time of vast evolutionary change. Well-preserved sedimentary successions in Namibia, Spitsbergen, South Africa and China have been studied in detail as they contain important economic deposits and remnants of the first metazoan faunas. In addition, strong carbon- and strontium-isotopic variations are found in these sequences and may be useful for the correlation of coeval strata world-wide. We propose to continue research on the isotopic variations in C, Sr and Nd in well-characterized and well- preserved stratigraphic samples from the Nama, Polarisbreen and Kango groups as well as the Doushantuo and Dengying formations to better characterize the rate of isotopic change during the latter part of the Proterozoic Eon. Given a better knowledge of the rate of isotopic changes in latest Proterozoic seawater our goals are to: 1) trace the major mass fluxes in the exogenic geochemical cycle and understand the relationship between the evolution of the sedimentary reservoir, the oceans and the atmosphere during the Late Proterozoic, 2) determine rates of erosion, organic carbon burial and hydrothermal alteration and their relation to global tectonics, 3) investigate the relationship of strong isotopic variations to known events of glaciations, phosphorite and banded iron-formation deposition, and 4) strengthen the tie between chemo- stratigraphic and bio-stratigraphic correlations between these Late Proterozoic successions. During the Proterozoic, seawater Nd and Sr budgets appear to have changed from dominantly hydrothermal inputs (as in the Archean) to dominantly continental (river) inputs. Modeling of the data obtained from this research will be used to constrain the changes in rates of hydrothermal circulation, continental erosion rates, burial rates of organic carbon and variations in the mean age of continental erosion products through the late Proterozoic. Implications for the evolution of O2 and CO2 in the Earth's atmosphere will also be considered. We have demonstrated with our previous work that high resolution Sr and C isotopic curves may be obtained for late Proterozoic sedimentary successions and are confident that this research will lead to major breakthroughs in our understanding of Late Proterozoic evolution.

Recent advances in high precision mass spectrometry have made it possible to study the early history of the Earth (4- 4.6 Ga ago) using the decay signature of the extinct nuclide 146Sm. 146Sm was present at the beginning of solar system history with an abundance of only 0.25% of Sm. Observable variations in the abundance of its daughter nuclide 142Nd record differentiation processes active only during the first 300 Ma of Earth history. By linking the short-lived 146Sm-142Nd and long-lived 147Sm-143Nd chronometers into a "coupled" systematics, it is possible to precisely constrain the formation history of the Earth's primordial protocrust. The Hadean epoch (4.0-4.6 Ga) is the least well-known period in Earth history. This project will map out a basic chronological outline of global geochemical differentiation.

9405374 Jacobsen Project involves development of a high-precision 87Rb-86Sr dating technique applicable to both lavas and foraminifera. One objective is the evaluation of this method for leucite-bearing Italian volcanics by study of groundmass minerals and phenocrysts in historical lavas. A second is an evaluation of the disequilibrium dating method applied to both shallow and deep water samples containing forams from the Pacific, plus initial investigation using del 234U dating of forminiferal ooze in Toba Ash horizons. The ultimate goal is to provide precisely time-resolved climate proxy records to increase our ability to understand causes of cyclic variation in climate.

9506517 Jacobsen The compositional variation of the present-day mantle results from a number of crust-mantle and intra-mantle differentiation processes operating over geologic time. Some of these ancient processes many have been markedly different from those occurring today. The production of continental crust has had the greatest influence in establishing the composition of the present-day mantle, but exactly how and when this occurred is controversial. It is proposed to conduct a series of high quality geochemical measurements of modern and ancient mafic to ultramafic lavas and their melt inclusions that will yield new insights into the secular evolution in the composition of the mantle. Foremost among these studies will be the use of microbeam techniques on olivine-hosted melt inclusions in ancient lavas to determine the compositional characteristics of the primary lavas and their source regions. Komatiitic samples containing olivine-hosted melt inclusions from widely varying temporal and spatial associations will be investigated. These microbeam techniques will also be applied to determine the compositions of glassy portions of modern oceanic basalts in order to establish variations in "constant ratios" of incompatible elements in the lavas and their source regions. Characterizing the secular evolution of the composition of the mantle will lead to a enhanced understanding of crust-mantle and intra-mantle differentiation processes and the rate at which they occurred.

0093645
Jacobsen

This grant supports the acquisition of a frequency quintupled (193 nm wavelength) deep ultraviolet (DUV) laser to be interfaced with inductively coupled plasma mass spectrometers (both a quadrupole and a multicollector) in the Department of Earth and Planetary Sciences (EAPS) at Harvard University. The laser ablation system will allow continued studies requiring in situ and spatially resolved isotopic analysis of geological and extraterrestrial materials. The laser system will support research by several faculty members at Harvard including Stein Jacobsen (crust-mantle geochemistry and cosmochemistry), Dan Schrag (marine chemistry and paleoceanography), Paul Hoffman (Neoproterozoic tectonics and climate) and Heinrich Holland (evolution of Earth's early atmosphere) as well as their graduate students and postdocs.
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Jacobsen
0112563

The investigators will work on several closely related subjects that will address important problems with respect to the structure and chemical and dynamical evolution of the Earth's mantle. The work will entail a combination of approaches using geochemical reservoir models that take into account the dispersion in both radiogenic isotope ratios and parent/daughter ratios in the mantle to obtain a simple theory that can combine fluxes and residence time estimates from traditional geochemical reservoir models with mixing times from three dimensional mantle convection models. Test results for the Nd and Sr isotopic systems demonstrate the validity of this approach. It is also proposed to combine reservoir models for isotopic transport and evolution with a parameterized convection model that will integrate the chemical evolution with thermal evolution. The parameterized models will include important effects that have only recently been identified, such as the bending of the lithosphere.

0093645
Jacobsen

This grant supports the acquisition of a frequency quintupled (193 nm wavelength) deep ultraviolet (DUV) laser to be interfaced with inductively coupled plasma mass spectrometers (both a quadrupole and a multicollector) in the Department of Earth and Planetary Sciences (EAPS) at Harvard University. The laser ablation system will allow continued studies requiring in situ and spatially resolved isotopic analysis of geological and extraterrestrial materials. The laser system will support research by several faculty members at Harvard including Stein Jacobsen (crust-mantle geochemistry and cosmochemistry), Dan Schrag (marine chemistry and paleoceanography), Paul Hoffman (Neoproterozoic tectonics and climate) and Heinrich Holland (evolution of Earth's early atmosphere) as well as their graduate students and postdocs.
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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5)

The proposed EaGER (Early Grant for Exploratory Research) is designed to facilitate the application of a new geochemical tool (Ca isotopes) to the field of mantle geochemistry using the well-studied Hawaiian lavas as a case study. This is an exploratory study that is still in its early stages of development and, hopefully, will demonstrate through further work that Ca isotopes can be used as a reliable test of recycled sediments in mantle sources. This proposal involves two early career scientists (Huang and Farkas) at Harvard University who will combine their expertise in different fields, and will educate and train graduate and undergraduate students in state-of-the-art analytical techniques and their applications to geosciences. The proposed research will also leverage previous NSF investments by using samples recovered from Hawaii Scientific Drilling Project and Koolau Scientific Drilling Project that were funded via NSF programs.

As the fifth most abundant element on the Earth, Ca is a major constituent of three out of the four major minerals present in the Earth's mantle, and it plays an important role in our understanding of mantle melting and mantle convection. Recent development in Ca isotopic analysis revealed large isotopic variations in modern and ancient carbonates, with ancient carbonates having significantly lighter Ca isotopic ratios (lower 44Ca/40Ca) than modern mantle values. Recycling of ancient carbonates into the mantle can, therefore, introduce Ca isotopic variations in mantle-derived lavas. Hawaiian volcanoes are one of the best examples of mantle plume-derived volcanism, and extensive geochemical and isotopic studies on Hawaiian lavas indicate that recycled ancient oceanic crust and sediments are important in the petrogenesis of Hawaiian lavas. The ultimate purpose of this EaGER is to expand preliminary analyses that have shown a slightly lower 44Ca/40Ca than typical mantle 44Ca/40Ca value for some Hawaiian lavas that could be due to the incorporation of recycled carbonate sediments. If fully successful, the results will have great impact on our current understanding of mantle convection and global carbonate cycles, as well as the behavior of Ca isotopes during magmatic processes.

This award provides funds, under the auspices of the EArly-concept Grants for Exploratory Research (EAGER) program, to investigate whether a bolide impact occurred at the beginning of the Younger Dryas (approximately 12,900-11,500 years ago) thereby bringing about dramatic climatic change. The research team plans to accomplish this science goal by measuring iridium concentrations in the Greenland Ice Sheet Project Two (GISP2) ice core across the Allerød-Younger Dryas boundary.

The researchers argue that, as with other impact studies, anomalously high iridium concentrations are an indicator of a major bolide impact and that the ice core analyses should provides a continuous, well-documented, and well-preserved stratigraphic sequence in which to search for an anomaly. They estimate that an impact of the currently hypothesized magnitude would have produced an iridium signal between 200 and 130,000 times the background iridium level due to the continuous accumulation of terrestrial and cosmic dust. As such, the researchers postulate that their analyses will be a definitive test of the impact.

The research could have broad impact on the wider science community by catalyzing new scientific thinking about the existing scientific theory that a bolide impact was a contributory cause of abrupt climatic change at the Younger Dryas boundary.

The planned activities fit well into the potentially transformative, high risk, and exploratory nature of the EAGER programs.

Ca Isotope Variations in Oceanic Basalts

Intellectual Merit. Preliminary work demonstrates that measurable mass dependent 44Ca/40Ca isotopic variations Hawaiian basaltic lavas can be attributed to the presence of recycled ancient carbonates in the underlying mantle. It is proposed to extend this work to other oceanic islands and other tectonic settings to determine if Hawaii is a unique case or if recycling of ancient carbonates is a more general process in generating mantle heterogeneity? It is predicted that such a process would significantly affect the Ca and Sr, as well as C, budgets of the Earth?s mantle. Radiogenic isotope geochemistry (Rb­Sr, Sm-Nd, U-Th­Pb, Lu­Hf, and noble gas systems) provides compelling evidence that widespread mantle heterogeneity exists, and can be described in terms of apparent isotopic end­member compositions (e.g., EM1, EM2, HIMU and DMM). It is proposed to determine Ca isotope compositions of representative oceanic basalts to see how this parameter varies with the other isotopically determined end­member variants, and to further constrain the importance of carbonate recycling. Specifically, the new measurements will be used to [1] test current hypothesis that mass dependent 44Ca/40Ca variations are due to carbonate recycling, [2] investigate if non-mass dependent 44Ca/40Ca variations, due to 40K decay, are present in samples with very high 87Sr/86Sr ratios and [3] quantify fluxes of recycled materials that may have caused the variations. Processes that could account for mass dependent 44Ca/40Ca variations incluse: [1] carbonate recycling, [2] altered oceanic crust subduction, [3] recycling of plagioclase­rich gabbroic crust, and [4] igneous fractionation (diffusion, partial meting and fractional crystallization). A research plan is designed to help resolve the following questions: [1] Is carbonate recycling a common feature in oceanic basalts with high 87Sr/86Sr or is it limited to rare and unique sites like Hawaii? [2] Are Ca isotope data consistent with upper crustal recycling proposed to explain very high 87Sr/86Sr ratios in Samoan lavas? This research aims to establish a Ca isotope constraint on the magnitude of upper crustal recycling in mantle sources of basalts and also to test the influence of garnet and plagioclase fractionation on isotopic variations in oceanic basalts.

Broader Impacts. T the proposed research facilitates for the first time extensive application of Ca isotopes to the field of mantle geochemistry. This proposal involves an early career research scientist (Huang) at Harvard University, and will educate and train graduate and undergraduate students in application of state­of­the-art analytical techniques to geosciences. Part of the proposed study will evolve into the senior thesis of an undergraduate student. The proposed research will leverage previous NSF investments by using samples recovered from previous NSF funded programs, such as ODP. The scientific results from this research will be presented in an educational website, which will be used in undergraduate and graduate education. As part of the proposed study, a new silicate standard will be prepared for intercalibration of future Ca isotopic work. Finally, results of the proposed study will be made available for outreach education via a project web page and an exhibition setup.