HR: 1340h
AN: V33A-1173    [Abstracts]
TI: High-Precision Nd isotopes in Picrites from Hawaii and Iceland - No Evidence for an Early- Formed Enriched Reservoir
AU: * Brandon, A D
EM: alan.d.brandon@nasa.gov
AF: NASA Johnson Space Center, Mail Code KR, 2101 NASA Parkway, Houston, TX 77058, United States
AU: Norman, M D
EM: Marc.Norman@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, 0200, Australia
AU: Debaille, V
EM: Debaille@lpi.usra.edu
AF: Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058, United States
AB: New high-precision Nd isotope measurements have shown that the present-day ¹⁴²Nd/¹⁴⁴Nd for average chondrites is approximately 20 ppm lower than that for the terrestrial standard and all recent mantle- derived samples measured to date. One explanation for these differences is that an enriched ‘missing' reservoir with lower ¹⁴²Nd/¹⁴⁴Nd resides in the mantle to balance the bulk Earth to chondritic. Data from Archean Greenland rocks show anomalies in ¹⁴²Nd/¹⁴⁴Nd of up to 40 ppm higher than the proposed average for chondrites. This difference between the Archean Greenland rocks and present-day mantle-derived samples has been interpreted to result from remixing of an early-formed enriched reservoir into the convecting mantle during the Archean. If so, the implication from this is that remixing of the enriched reservoir largely shut down some time in the past such that it must at present reside in a region in the mantle that infrequently participates dynamically and leading to volcanism at the surface. Several studies have suggested that the missing reservoir may be present just above the core-mantle boundary (CMB). Depending on the size of this reservoir it could potentially make up all of D" or exist as distinct domains within it. If such a reservoir does exist, then mantle-derived samples from plume systems that are thought to come from the CMB may be the best opportunity to identify this component using high-precision Nd isotope measurements. To test this, picrites from Hawaii with coupled enrichments in ¹⁸⁶Os-¹⁸⁷Os that has been proposed to be a signature of core-mantle interaction were measured on the JSC Triton for high-precision ¹⁴²Nd/¹⁴⁴Nd. For comparison, picrites from Hawaii and Iceland that do not show coupled enrichments in ¹⁸⁶Os-¹⁸⁷Os were measured. We have established an external precision for ¹⁴²Nd/¹⁴⁴Nd of ±3.6 ppm (2σ) during the analytical campaign. The Iceland (n=5) and Hawaiian data (n=9) have ε¹⁴²Nd ranging from -0.01±0.03 to +0.03±0.03 (2σ) and there is no resolvable difference between samples with Os isotope enrichments versus those without. The average ε¹⁴²Nd of +0.011±0.034 (2σ) for all of the samples (n=14) is not resolvable from the Ames and La Jolla standards with ε¹⁴²Nd of 0.000±0.036 (n=16, 2σ). These data confirm recent measurements on lavas for the absence of an ancient enriched ¹⁴²Nd isotopic signature in plume systems likely derived from D" and indicates that such a reservoir, if existing, must now reside in areas of the deep mantle that plumes do not sample.
DE: 1025 Composition of the mantle
DE: 1027 Composition of the planets
DE: 1040 Radiogenic isotope geochemistry
DE: 1115 Radioisotope geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting