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High-Precision Nd Isotopes in Picrites from Hawaii and Iceland - No Evidence for an Early-Formed Enriched ReservoirNew high-precision Nd isotope measurements have shown that the present-day Nd-142/Nd-144 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 Nd-142/Nd-144 resides in the mantle to balance the bulk Earth to chondritic. Data from Archean Greenland rocks show anomalies in Nd-142/Nd-144 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 coremantle 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 Os-186-Os-187 that has been proposed to be a signature of core-mantle interaction were measured on the JSC Triton for high-precision Nd-142/Nd-144. For comparison, picrites from Hawaii and Iceland that do not show coupled enrichments in Os-186-Os-187 were measured. We have established an external precision for Nd-142/Nd-144 of 3.6 ppm (2 sigma) during the analytical campaign. The Iceland (n=5) and Hawaiian data (n=9) have Nd-142 ranging from -0.01 plus or minus 0.03 to +0.03 plus or minus 0.03 (2 sigma) and there is no resolvable difference between samples with Os isotope enrichments versus those without. The average epsilon Nd-142 of +0.011 plus or minus 0.034 (2 sigma ) for all of the samples (n=14) is not resolvable from the Ames and La Jolla standards with epsilon Nd-142 of +0.000 plus or minus 0.036 (n=16, 2 sigma). These data confirm recent measurements on lavas for the absence of an ancient enriched Nd-142 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.
Document ID
20070031134
Acquisition Source
Johnson Space Center
Document Type
Conference Paper
Authors
Brandon, A. D.
(NASA Johnson Space Center Houston, TX, United States)
Norman, M. D.
(Australian National Univ. Canberra, Australia)
Debaille, V.
(Lunar and Planetary Inst. Houston, TX, United States)
Date Acquired
August 23, 2013
Publication Date
January 1, 2007
Subject Category
Lunar And Planetary Science And Exploration
Meeting Information
Meeting: American Geophysical Union Meeting
Location: San Francisco, CA
Country: United States
Start Date: December 10, 2007
End Date: December 14, 2007
Distribution Limits
Public
Copyright
Other

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