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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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22 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Concordant Rb-Sr and Sm-Nd Ages for NWA 1460: A 340 Ma Old Basaltic Shergottite Related to Lherzolitic Shergottites

Preliminary Rb-Sr and Sm-Nd ages reported by [1] for the NWA 1460 basaltic shergottite are refined to 336+/-14 Ma and 345+/-21 Ma, respectively. These concordant ages are interpreted as dating a lava flow on the Martian surface. The initial Sr and Nd isotopic compositions of NWA 1460 suggest it is an earlier melting product of a Martian mantle source region similar to those of the lherzolitic shergottites and basaltic shergottite EETA79001, lithology B. We also examine the suggestion that generally "young" ages for other Martian meteorites should be reinterpreted in light of Pb-207/Pb-206 - Pb-204/Pb-206 isotopic systematics [2]. Published U-Pb isotopic data for nakhlites are consistent with ages of approx.1.36 Ga. The UPb isotopic systematics of some Martian shergottites and lherzolites that have been suggested to be approx.4 Ga old [2] are complex. We nevertheless suggest the data are consistent with crystallization ages of approx.173 Ma when variations in the composition of in situ initial Pb as well as extraneous Pb components are considered.

Nyquist, L. E.↗
NASA NTRS2011

Rb-Sr and Sm-Nd Study of Asuka 881394: Evidence of "Late" Metamorphism

The Asuka 881394 achondrite contains fossil Al-26 and Mn-53 [1,2,3] and has a Pb-207/Pb-206 age of 4566.5 plus or minus 0.2 Ma [3], the oldest for an achondrite. Preliminary results showed initial Sm-146/Sm-144 = (7.4 plus or minus 1.2) x 10(exp -3), indicative of an ancient age, but Rb-87 - Sr-87 and Sm-147 - Nd-143 ages of 4370 plus or minus 60 and 4490 plus or minus 20 Ma, resp. [1], were younger than expected from the presence of short-lived nuclides. We revisit the Rb-Sr and Sm-Nd chronology of A881394 in an attempt to establish whether late metamorphism led to inconsistency in its apparent ages.

Nyquist, L. E.↗
NASA NTRS2012

Redetermination of the Sm-Nd Age and Initial (Epsilon)Nd of Lunar Troctolite 76535: Implications for Lunar Crustal Development

Lunar troctolite 76535 is an old lunar rock predating the era of the lunar cataclysmic bombardment, but its radiometrially determined ages have been discordant [1-3]. The most recent multi-chronometer study [4] gave preferred ages of 4226+/-35 Ma and 4236+/-15 Ma from a Pb-207/Pb-206 isochron and an U-Pb upper concordia intercept, resp. We derive an age of 4323+/-64 Ma from Sm-Nd data reported by [4] for the bulk rock and three mineral separates. They derived an age of approx.4.38 Ga from combined Rb-Sr data [3,4] by omitting data for olivine separates. Ar-39-Ar-40 ages of approx.4.2 Ga are summarized by [5]. New Sm-147-Nd-143 data presented here give an age of 4335+/-71 Ma in agreement with the Sm-Nd age from [4], whereas Sm-146-Nd-142 data give a model age T(sub LEW) = 4439+/-22 Ma. Further, initial (Epsilon)Nd-143 for 76535 conforms to the Nd-143 evolution expected in an urKREEP [6] reservoir, consistent with inheritance of urKREEP Sm-Nd systematics via assimilation. We show that urKREEP Sm-Nd systematics require the lunar initial (Epsilon)Nd-143 to exceed the Chondritic Uniform Reservoir (CHUR) value [7], but are consistent with evolution from initial (Epsilon)Nd-143 like that of the HED meteorite parent body as defined by a 4557+/-20 Ma internal isochron for the cumulate eucrites Y-980433 and Y- 980318 [8].

Nyquist, Laurence E.↗
NASA NTRS2019

Mechanisms and Consequences of Intra-Crystalline Enrichment of Ancient Radiogenic Pb in Detrital Hadean Zircons from the Jack Hills, Western Australia

The recent discovery of Pb* (radiogenic lead) -enriched domains (PEDs) in zircon using high-resolution scanning ion imaging, atom probe tomography and transmission electron microscopy indicates that the U-Pb isotopic system in zircon can be affected not only by Pb* loss but also by intra-crystalline Pb* enrichment. However, the formation mechanism of PEDs and the consequences for in-situ U-Pb dating remain elusive. To further understand these issues, scanning ion imaging and U-Pb dating were carried out on 51 Hadean detrital zircon grains from the Jack Hills, Western Australia. Of these, 8 grains were found to contain micrometer-scale PEDs with elevated Pb-207 and Pb-206 in the ion images. Isotope profiles across these PEDs confirm that they represent unsupported Pb* resulting from intra-grain mobilization and local enrichment of Pb* during ancient overprinting events. The PEDs are rare in most grains, with only 1 – 3 Pb* hotspots present in any ~70 μm × 70 μm imaged area, and are located either in dark-CL, high-U growth zones, or associated with bright-CL, low-Th recrystallized domains, suggesting that they were likely formed by local-scale processes. The PEDs located in dark-CL, high-U growth zones likely resulted from Pb* trapped in local, interconnected pathways produced by radiation damage during thermal overprinting events. Those associated with bright-CL, low-Th domains are interpreted as Pb* concentrated in nanoscale pores formed during fluid-present recrystallization of non-metamict zircon. Our interpretations therefore imply that intra-crystalline enrichment of Pb* in zircon can occur at various scales and through diverse mechanisms. Age calculations based on the ion images demonstrate that incorporation of PEDs in conventional in-situ SIMS spots produces spuriously old ages that are either reversely discordant or concordant, depending on the time of Pb* mobilization and the analytical precision. The implication is that careful investigation of intra-grain distribution of U-Th-Pb isotopes is required in order to better understand the U-Pb isotopic system and to obtain reliable crystallization ages for ancient zircons affected by complex Pb* mobilization and redistribution.

Unsupported radiogenic lead↗