46th Lunar and Planetary Science Conference (2015) 2793.pdf SAMARIUM-NEODYMIUM CHRONOLOGY OF AN ALLENDE TYPE A CAI AlNO 1-16. N. E. Marks1, L. E. Borg1, I. D. Hutcheon1, B. Jacobsen1, R.N. Clayton2, T. K. Mayeda2, 1Lawrence Livermore National Laboratory, 7000 East Ave L-231, Livermore, CA 94550, USA ([email protected]), 2Department of Chemistry, The University of Chicago, 5640 S. Ellis Ave, Chicago, IL, 60637, USA. Sm-Nd Systematics of AlNO-1-16: Results of 147Sm143 Nd isotopic analyses for AlNO 1-16 are presented in an isochron plot in Figure 1. A linear regression fitted through five points yields an apparent age of 4.89 ± 2.5 Ga. This regression excludes the lightest density separate, which appears to be disturbed. The large uncertainty in the age reflects the very limited spread in the 147 Sm/144Nd ratios of the mineral fractions. In fact, the total measured variation in 147Sm/144Nd is less than 3%. In comparison, the spread we recently determined in the Type B CAI Al3S4 was over 25% . This sample yielded a 147Sm-143Nd age of 4569 ± 36 Ma and an initial 143Nd/144Nd ratio of 0.506646 ± 48. The Al3S4 isochron is in excellent agreement with Pb-Pb ages determined on CAIs [7,8]. A combined plot of the ALNO-1-16 and Al3S4 data demonstrate that the density separates from AlNO 1-16 fall along the Al3S4 isochron suggesting both samples have concordant crystallization ages. A linear regression fitted to all the points from both CAIs results in an age of 4562 ± 170 Ma. The greater uncertainty in the combined age most likely reflects the higher proportion of secondary phases in Type A CAI compared to Type B CAIs. 0.5145% 0.5140% Pyroxene% 143Nd/144Nd' Introduction: Calcium–aluminum-rich inclusions (CAIs) are primitive objects that formed within the protoplanetary disk surrounding the young Sun. Recent Pb–Pb chronologic studies have demonstrated that CAIs are the oldest solar system solids, crystallizing 4567 Ma ago. Two primary types of CAIs have been identified: Type A inclusions are composed almost entirely of melilite, with minor amounts of spinel, clinopyroxene, grossular, perovskite, and hibonite, while Type B inclusions contain subequal amounts of melilite and clinopryoxene with lesser amounts of spinel and anorthite (e.g. ). The isotope systematics of CAIs provide critical insight into the earliest history of the Solar System. Although the Sm–Nd geochronometers are highly effective tools for investigating cosmochemical evolution in the early Solar System, Type A CAIs have not historically lent themselves to this type of study. Because they are essentially monomineralic, mineral separates with variable 147Sm/144Nd are quite difficult to produce. Here we report new data from density separates of Allende CAI AlNO-1-16 from both the long-lived (147Sm–143Nd) and short-lived (146Sm–142Nd) isotopic systems derived from density separates of the Type A CAI AlNO-1-16. Petrography and Mineral Fractions: AlNO 1-16 is a coarse-grained compact Type A CAI from Allende that was first separated by heavy liquid and magnetic techniques at the University of Chicago . The inclusion is comprised primarily of melilite that poikilitically encloses spinel; the spinel is characterized by unusual epitaxial overgrowths of perovskite . Although the textures of the spinel-perovskite intergrowths imply condensation from the gas phase (i.e  Hutcheon, 1977) the mineralogy reflects crystallization from a liquid [4,5]. Five density separates plus a whole-rock (fines) fraction were analyzed for Sm-Nd isotopic composition. The density separates comprise primarily melilite, with variable amounts of spinel and grossular. 0.5135% 0.5130% ρ>3.26% Fines% 0.5125% 0.5120% 0.5115% 0.15% Px5Mel% ρ<3.26% Melilite% 0.20% 0.25% 147Sm/144Nd' Figure 1. A 147Sm-143Nd isochron for density separates from AlNO 1-16 (red diamonds). Most of the AlNO-1-16 fractions fall along the isochron defined by mineral separates from Type B CAI Al3S4 (yellow circles) that yielded an age of 4560 ± 34 . The melilite fraction from AlNO 1-16 appears to be disturbed. Results of 147Sm-142Nd isotopic analyses for AlNO 116 are presented in an isochron plot in Figure 2. A linear regression fitted through five AlNO 1-16 fractions yields an age of 4519 +55/-88 Ma (initial ε142Nd = 2.2 ± 1.7). The MSWD for this isochron is 2.2, indicating a good fit of the data. However, like the 147Sm143 Nd isochron, uncertainty in the age is strongly affected by the limited spread in 147Sm/144Nd ratio. An age of 4567 +11/-12 Ma is calculated from the data obtained from the mineral fractions from both AlNO 1-16 and Al3S4 using 146Sm/144Sm = 0.00828 and 146Sm t½= 103 Ma. This isochron has an initial of ε142Nd = 46th Lunar and Planetary Science Conference (2015) 3.0 ± 1.1 that is in good agreement with previous estimated of the Solar Systems initial Nd isotopic composition. 1.14190% Pyroxene% 142Nd/144Nd' 1.14188% 1.14186% ρ>3.26% 1.14184% 1.14182% 1.141850% Fines% Px5Mel% 1.141845% 1.141840% 1.14180% 1.141835% ρ<3.26% 1.141830% 1.14178% Melilite% 1.14176% 0.17% 0.18% 1.141825% 0.19% 0.19% 0.2% 0.21% 0.22% geochron plot, as it eliminates issues associated Sm fractionation from Nd through alteration and also through laboratory preparation. Discussion: Despite the very limited spread in Sm/Nd for the Type A CAI AlNO 1-16, the density separate data yield 146Sm-142Nd and Nd-Nd ages that are concordant with previous measurements of mineral separates from the Allende CAI Al3S4 , as well as PbPb ages determined on other CAIs [7,8]. This agreement confirms evidence from the 26Al-26Mg and 207Pb206 Pb isotope systems  that common types of CAIs crystallized contemporaneously and demonstrates the Sm-Nd systems of both CAIs were minimally affected by alteration on the parent body. 1.14192% Age(all%points)%=%4567%+11/512% IniGal%ε142Nd=3.0+1.1% MSWD%=%1.9% 2793.pdf 0.20% 0.23% 1.14192% 0.24% 1.14190% 147Sm/144Nd' Allende Type A CAIs are typically characterized by alteration, and AlNO 1-16 is no exception. The longlived and short-lived Sm-Nd systems are vulnerable to disturbance because this process can fractionate Sm from Nd. Calculating ages using the 143Nd-142Nd isotopic reduces the affects of disturbance by eliminating the uncertainties associated with the Sm/Nd ratio. This approach is illustrated in Fig. 3 where 142Nd/144Nd versus 143Nd/142Nd of mineral fractions from AlNO 1-16 yield an age of 4581 +44/-88 Ma (MSWD = 0.5) that is more precise than either the 147Sm-143Nd or the 146Sm142 Nd isochron ages determined from these fractions. If mineral fractions from the Type B CAI Al3S4  are included in the calculation, an age of 4570 +7/-8 Ma with an MSWD of 1.3 is determined. The excellent agreement between the Nd-Nd age and the Pb-Pb ages determined on CAIs by Amelin et al.  and Connelly et al.  indicate that this system most accurately records the formation age of CAIs. Like the Pb-Pb isotopic system, if alteration occurs when both Sm parent isotopes are extant, then fractionation of parent and daughter isotopes associated with the alteration event minimally effects the age. The lowest density separate for AlNO 1-16, ρ=2.883.20 g/cm3, was determined by  to contain primarily melilite with trace spinel and garnet. Data for this separate falls off the 147Sm-143Nd and 146Sm/142Nd isochrons, although it does fall on the Nd-Nd isochron. This is a good illustration of the utility of the Nd-Nd 142Nd/144Nd' Figure 2 A 146Sm-142Nd isochron for density separates from ALNO 1-16 (red diamonds). The AlNO 1-16 fractions lie on the isochron defined by Type B CAI Al3S4 (yellow points). A regression through all of the points yields an age of 4567 +11/-12 Ma with a MSWD of 1.9. Pyroxene% 1.14188% 1.14186% ρ>3.26% 1.14184% Fines% 1.14182% Px5Mel% 1.14180% 1.14178% ρ<3.26% Nd5Nd%Age(all%points)%=%4570%+7/58% Melilite% 1.14176% 0.5115% 0.5120% Slope%=0.0568±0.0029%(2σ)%% MSWD%=%1.3% 0.5125% 0.5130% 0.5135% 0.5140% 143Nd/144Nd' Figure 3 Nd-Nd geochron with CAI isochron defined by both Al3S4 and AlNO 1-16 data yielding an age of 4570 +7/-8 Ma. The isochron defined by data from the mineral separates from Al3S4 and the density separates from AlNO 1-16 provides additional confirmation that the 103 Ma half-life for 146Sm is the best value. The concordance of the two CAI sample sets indicates that the initial 146 Sm/144Sm of the solar system is well represented by 0.00828 ± 44 determined by Marks et al. . References:  Grossman, L (1980) Ann. Rev. Earth Planet. Sci., 8 559–608.  Clayton et al. (1977) Earth Planet. Sci. Lett., 34 209–224  Hutcheon, I.D. (1976) Metoritics, 11, 304.  Hutcheon, I.D. (1977) LPSC VII, 8 487-489.  Blander, M. & Fuchs, L.H (1975) Geochim. Cosmochim. Acta., 39(12) 1605-1619  Marks et al. (2014) Earth Planet Sci. Lett., 405 15-24  Amelin et al. (2002) Sci. 297, 1678.  Connelly et al. (2008) Sci. 651.  Jacobsen et al. (2008) Earth Planet Sci. Lett., 272, 353-364. This work was performed under the auspices of the U.S. DOE by LLNL under contract DEAC52-07NA27344.
© Copyright 2018 ExploreDoc