NSF Forms - Ridge 2000 Program

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1. Results from Prior NSF Support: Results from Prior NSF Support: Charles Langmuir OCE-0242618 “Collaborative Research: Integrated Hydrothermal and Petrological Studies of the Eastern Lau Spreading Center”: 08-01-2004 to 07-31-2007, $447,553.00 This proposal funded a sea-going program that took place in October 2004. Three new vent fields on the Eastern Lau Spreading center (ELSC) were discovered using a phased exploration approach with the Automated Benthic Explorer (ABE). One of these sites (the ABE site) has since been designated as the “bulls-eye” for the Ridge2000 Integrated Study Site. We also occupied 203 sampling sites along the ELSC and Valu Fa Ridge, and were able to map in detail the petrological and geochemical variations that take place along the spreading center at varying distances from the volcanic front. More than 600 samples have been analyzed for major elements by electron microprobe. High precision trace elements by ICP-MS have been determined on more than 150 samples, and 200 samples have been analyzed for major and trace elements by DCP. Isotope data collection on more than 50 samples is underway, and the same samples are being analyzed for water, CO 2 and Cl by Peter Michael, and for He isotopes by David Graham. These data are discussed below. Results from this cruise were supplied to subsequent cruises exploring the ELSC hydrothermal systems in more detail, have been imparted to the data management site, and have been presented in more than a dozen abstracts at the AGU meetings 2004 and 2005, and the Goldschmidt Conference in 2006. Publications supported by this grant are indicated by the * in the reference list. Results from Prior NSF-OCE Support: K.W.W. Sims OCE0137325 “Spatial and Temporal Variations in Mid-Ocean Ridge Basalt Geochemistry Along and Across the East Pacific Rise Crest, 9-10°N”, $179,630. OCE-9730967 “U-Series Analyses of Young Lavas from the East Pacific Rise 9°48'N to 9°52'N: Using Lavas from the 1991 Eruption as a Baseline to Constrain the Nature and Timing of MORB Petrogenesis”, $180,890. With these two grants we have measured U-Th-Ra and U-Pa disequilibria, Hf, Nd, Sr and Pb isotopic and major- and trace-element compositions in 54 samples, selected from a suite of MORB that spans the ridge crest from 9° 28'–52'N and across it for ~4 km to either side. Sr, Nd, Hf and 208Pb/206Pb isotopic compositions of the off-axis N-MORB are identical to the axial lavas, but have larger U-Th and Th-Ra disequilibria and younger model ages than would be predicted from their off-axis distance. The anomalously young ages of most off-axis lavas suggest that volcanic construction along this region occurs over a zone that is wider than previously thought. In addition to the nine published manuscripts from this work (indicated by ** in reference list), there are fifteen published abstracts resulting from the presentation of this work at professional meetings, including two invited talks for AGU (Spring, 1999, Boston MA; Spring, 2003, Nice France), and four invited keynote (Goldschmidt Conferences: 2000, Oxford, England; 2002 Davos, Switzerland; and 2005, Moscow Idaho, US; and IAVCEI, China 2006). J. Standish: No prior NSF support. 2. INTRODUCTION This proposal has two aims that can be elucidated through U-series analyses of newly collected samples from the Lau back-arc basin. The first aim is to carry out the most complete U-series study of a back-arc basin ever undertaken in order to constrain the sources of subduction components in back-arcs and compare these results to published data from the neighboring Tonga arc. The second is to develop the age constraints that are essential to relate volcanism, petrology, hydrothermal D-1 0649641
activity, tectonics, seismic axial magma chamber reflectors and the biology of hydrothermal vents. Developing these relationships is the essence of the Integrated Study Site (ISS) theme of the RIDGE 2000 program. The recent intensive sampling of the ELSC provides the best back-arc sample set in the world to address these problems. Comments on prior submission This is the second submission of this proposal. The panel summary stated: “without exception, the readers and the panel felt the PI team was talented and productive and that a U-series study is needed for Lau Basin lavas.” Nonetheless, half the reviewers raised specific concerns about the proposal, and we have revised the proposal accordingly. The “issues that loomed large in the Program's decision not to fund” include: 1) skepticism concerning the use of U-series “model ages” in back arc settings; 2) insufficient statement of testable hypotheses; 3) uncertainty about work on rocks from the upcoming Fisher cruise (Sept 2006); 4) concerns regarding the time allotted for the study given the fact that the U-series lab at Harvard is not yet set up; and 5) inadequate detail on the number of analyses. To address these concerns: 1) We have recast the dating component of this study to focus on the age constraints obtainable by U-series limits rather than “model ages”, and explained why age constraints are essential for development and testing of hypotheses. 2) We provide explicit examples of how the U-series measurements will be combined with other geochemical and isotopic data to test hypotheses, and how our predicted findings test published interpretations of Tonga Arc petrology. 3) We now propose to study a sample suite that comes entirely from samples currently in our collection. (In addition, should any important samples come back from the upcoming Fisher cruise these will also be incorporated into this study.) 4) We more clearly outline the relationship between Harvard and WHOI and point out that Jeff Standish will not be setting up the Harvard U- series lab from scratch, but will instead be transferring the techniques he learned at WHOI to the existing and operational Harvard clean lab using the techniques he learned as a WHOI student and Sims’s well-calibrated U, Th, Ra spikes. Should unforeseen problems arise, the chemistry can always be done at WHOI, which is only 90 minutes from Harvard. The mass spectrometry will also be performed at WHOI, which has a long record of published results on U-series disequilibria. Therefore, there are no technical impediments to the proposed work. 5) We provide a detailed work plan that identifies samples and the time line for task completion. We also note that the proposed study relies on the particular skills of Jeff Standish, who is experienced with ocean ridge studies and with U-series analysis [Standish & Sims, submitted; Standish 2005]. Standish is currently a post-doc at Harvard, and is not listed as co-PI only because of Harvard policies with respect to post-docs. Should the proposal not be funded, Standish’s availability will end, and the necessary personnel will no longer be in place. 3. BACKGROUND The Lau Integrated Study Site and Bulls-Eye Selection Subduction of the Pacific plate at the Tonga trench has led to the formation of the NE-SW trending Tonga volcanic arc and the associated Lau Basin (Fig. 1). Spreading in the southern portion of the basin is accommodated along two main rifts, the Central Lau Spreading Center (CLSC) and the Eastern Lau Spreading Center (ELSC), separated by an offset that contains the Intermediate Lau Spreading Center (ILSC). The spreading rate increases southward from 65 mm/yr at 21ºS to 90 mm/yr at 18ºS [Taylor et al., 1996]. Based on parallels with open ocean ridges, the ridge morphology would be expected to vary from crestal plateau in the north to rift valley morphology and greater tectonism in the south. Instead, at the southern end of the rift, where the D-2 0649641
Page 1 and 2: COVER SHEET FOR PROPOSAL TO THE NAT
Page 3 and 4: COVER SHEET FOR PROPOSAL TO THE NAT
Page 5 and 6: PROJECT SUMMARY Intellectual Merit:
Page 7: TABLE OF CONTENTS For font size and
Page 11 and 12: eflector disappears. These changes
Page 13 and 14: the north and south, perhaps relate
Page 15 and 16: Figure 6: Left panel - U excesses a
Page 17 and 18: There is thus a rich array of probl
Page 19 and 20: 226 Ra - 230 Th disequilibria it is
Page 21 and 22: and trace element analysis and inte
Page 23 and 24: References Asmerom, Y., S.A. DuFran
Page 25 and 26: Hoogewerff, J.A. et al., U-series,
Page 27 and 28: Spiegelman M. and Elliott T. (1992)
Page 29 and 30: CHARLES H. LANGMUIR Harvard Univers
Page 31 and 32: Jared Jeffrey Standish Harvard Univ
Page 33 and 34: Kenneth W. W. Sims Associate Scient

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