Hsin-Wei Chen

Typhoon Lee

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Neutron-rich iron group isotopic anomalies among meteorites

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Neutron-rich iron group isotopic anomalies among meteorites. H. W. Chen1, 2 and T. Lee2, 1School of Earth Sciences, University of Bristol, 2Institute of Earth Sciences, Academia Sinica.  The solar system is made of the remnants with vast different isotopic compositions ejected from many different stars [1], yet theoretical model suggested that these materials were effectively homogenized in some stage of formation epoch [2].  Extensive experimental works on various meteoritic samples suggest all isotopic variations in the solar system can be explained by later modification processes such as isotopic fractionation or radionuclides decay from a uniform composition [3].  Some observed isotopic anomalies, especially for neutron-rich nuclear statistical equilibrium (nNSE) isotopes, e.g. 48Ca, 50Ti, and 54Cr, are endemic existed in the earliest solids [4], e.g., Ca-Al rich inclusions (CAIs) and hibonites, from primitive chondrites that had experienced very little thermal alteration.  This heterogeneities are suggested to be preserved by either solids condensed before the solar system materials were completed homogeneous or rare nNSE nuclides carrier grains were random captured by various sizes of refractory inclusions to cause the variation of NSE isotopic anomalous effects (ex. 50Ti variation: -6.8%~+27.3% in hibonites and 0~1.5‰ in normal CAIs).  However, the solar system was still believed as isotopic homogeneity in planetary scale [5]. Recently, high precision Ca, Ti and Cr isotope works had distinguished correlated 48Ca, 50Ti and 54Cr anomalies in numerous bulk differentiated meteorites [6-8] and defined an apparent linear correlation of 48Ca: 50Ti: 54Cr~ 2.1:2.0:1 (Neutron-rich iron-group Isotopes Correlated Heterogeneities; NICH).  The most likely stellar sources responsible for 48Ca, 50Ti and 54Cr are from a rare kind of neutron-rich Type Ia supernovae (nSNe Ia) with igniting density about 5.8~7.4×109g cm-3 [9]. However, the 48Ca and 50Ti anoamlies in CAIs are plotted on the 50Ti rich (or 48Ca deplete) side of the NICH line.  Further compared with other reported isotopic anomalies in CAIs, we found that assuming 48Ca came from single type of source, and the trend represents the intake of component from n-rich SNe-Ia, the excess 50Ti by subtracting the part that correlates with 48Ca could surprisingly show a linear correlation trend with 138La anomaly [10], a rare isotope most likely synthesized by neutrino process [11].  Thus according to present stellar nucleosynthetic model, there were two types of source for 50Ti, the first one came from n-rich SNe-Ia, which produced all 48Ca and part of other neutron-rich iron group isotopes, 50Ti, while the other came perhaps from O/Ne layer of SNe-II where the neutrino process is thought to be producing 138La, and also contribute rest of 50Ti anomaly by s process.  References: [1] Zinner (2004) Treatise on Geochemistry, Vol. 1, Ch#1.02. [2] Boss (2004) Treatise on Geochemistry, Vol. 1, Ch#1.04. [3] Podosek (1978) ARAA, 16, 293. [4] Lee (1988) Meteorites and the early solar system Ch#14.3. [5] Palme and Jones (2004) Treatise on Geochemistry, Vol. 1, Ch#1.03. [6] Chen et al. (2011) ApJ, 745, L23. [7] Trinquier et al. (2009) Science, 324, 374. [8] Trinquier et al. (2007) ApJ, 655, 1179. [9] Woosley (1997) ApJ, 476, 801. [10] Shen and Lee (2003) ApJ 596, L109-L112. [11] Heger A. ert al. (2005) PhLB 606, 258-264. 

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Neutron-rich iron group isotopic anomalies among meteorites

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