HR: 0800h
AN: U21B-0408    [Abstracts]
TI: What Comes Around Goes Around: Mantle Convection and the Meaning of Mantle Isochrons
AU: * Hauri, E H
EM: hauri@dtm.ciw.edu
AF: Carnegie Institution, DTM, 5241 Broad Branch Rd NW, Washington, DC 20015, United States
AU: Brandenburg, J
EM: jpbrande@umich.edu
AF: Dept of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States
AU: VanKeken, P
EM: keken@umich.edu
AF: Dept of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, United States
AU: Ballentine, C
EM: chris.ballentine@man.ac.uk
AF: Dept of Earth Sciences, University of Manchester, Manchester, M13 9PL, United Kingdom
AB: The perceived significance of isotopic data arrays for oceanic basalts has long occupied a middle ground between the endmember interpretations of mixing and age. Brooks et al. [1] were the first to attach age significance to the correlations between parent-daughter ratios and daughter isotope ratios that are a regular feature of the geochemistry of OIB and MORB [e.g. 2,3]. Pseudo-isochrons derived from mixed mantle can still have age significance if the various packets of source material have been physically juxtaposed for long periods of time, yet the current paradigm has generally been to focus on daughter ratios alone, and to interpret their variations in terms of multi-component mixing. Numerous high-quality geochemical data sets now exist, and continute to be generated, for specific regions of OIB and MORB volcanism. In order to take the next step in a more accurate interpretation of this data, a forward model is needed that delimits the bounds of chemical variability and isotopic correlations expected to arise from the major processes operative during terrestrial mantle convection. In this talk, we will present the results of cylindrical 2D convection models with force-balanced plates [4] and examine specifically the roles of subduction and convective mixing of oceanic crust, with extraction of continental crust superimposed. In these models, melting occurs at divergent plate boundaries and geochemical evolution is recorded by millions of passive (harzburgite) and active (basalt) tracers that record the times, extents of melting, and extents of degassing and continent extraction at every melting event, allowing the geochemical evolution of any isotope system to be easily calculated (and recalculated) in a post-processing algorithm that operates on the tracer data independently from the dynamic calculations. We will explore the range of isotopic variability in these models as functions of partition coefficients, chemical density of basalt tracers, convective vigor, and history of continental crust extraction. In particular, these models reveal relationships between mantle isochron "ages" and true tracer ages that is not obtainable from statistical box- model calculations incorporating idealized mixing scenarios [e.g. 3-5]. References: [1] Brooks, C., Hart S.R., Hofmann A.W. & James D.E. (1976) EPSL 32, 51-61. [2] Albarede F. (2001) EPSL 189, 59-73. [3] Donnelly, K.E., Goldstein, S.L., Langmuir C.H. & Spiegelman, M. (2004) EPSL 226, 347-366. [4] Brandenburg, J.P. et al (2007) in review. [5] Rudge J.F. (2006) EPSL 249, 494-513.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
SC: Union [U]
MN: 2007 Fall Meeting