Titanium isotopic anomalies

Figure 16 °Ti isotopic composition versus calculated initial Al/ Al for hibonite-bearing CAIs from CM chondrites. Titanium isotope anomalies larger than 10%o do not occur in CAIs with initial Al/ Al 5 X 10 , and vice versa. Diagram is updated from Clayton et aL (1988, figure 7). Only analyses having Al/Mg > 20 are shown (sources Fahey etaL, 1987a Hinton etaL, 1984 Hutcheon etaL, 1980 Ireland, 1988, 1990 Ireland and Compston, 1987 ...

There are other refractory inclusions, e.g., grossite-bearing CAIs from CH chondrites (Weber et aL, 1995), that do not fit this model since they lack calcium and titanium isotopic anomalies as well as Al. One interpretation of such objects could be that CAI formation lasted several million years, but this is not supported by any independent evidence and there could well be other reasons for the lack of both short-lived radioactivity and large isotopic anomalies (aside from excesses Sahijpal et al., 1999) in these inclusions. Circumstantial arguments against a long time period for CAI formation are that it... 

Ireland T. R., Compston W., and Heydegger H. R. (1985) Titanium isotopic anomalies in hibonites from the Murchison carbonaceous chondrite. Geochim. Cosmochim. Acta 49, 1989-1993. 

Titanium-calcium. The first evidence for isotopic anomalies in the iron-group was found in Ti showing up to 10% excesses of Ti in hibonites from the Murray CM2 meteorite (Hutcheon et al. 1983 Fahey et al. 1985 Ireland et al. 1985 Hinton et al. 1987). Further studies in Murchison showed that Ti extended from -7% to +27% associated with Ca variation from -6% to +10% (Ireland 1988 Ireland 1990). Except for the magnitude of the variations, this is similar to the results from Allende inclusions. Only a few samples display mass-dependent fractionation for which it ranges up to 1.3 %/amu. In the majority of the samples, it is absent or very low (less than 1 %o/amu) for Ca-Ti. There is no correlation between the presence of linear fractionation and the magnitude of Ti effects. Ti variations are also present, but about an order of magnitude smaller than Ti. Variations affecting these two isotopes are related but not strictly correlated (Ireland 1988). 

Hashimoto A, Hinton RW, Davis AM, Grossman L, Mayeda TK, Clayton RN (1986) A hibonite-rich Murchison inclusion with anomalous oxygen isotopic composition. Lunar Planet Sci XVII 317-318 Heydegger HR, Foster JJ, Compston W (1979) Evidence of a new isotopic anomaly from titanium isotopic ratios in meteoritic material. Nature 278 704-707... 

CAIs are composed of a variety of minerals, primarily hibonite, perovskite, melilite, spinel, aluminum- and titanium-rich diopside, anorthite, forsterite, and occasionally corundum or grossite. They also show significant enrichments in refractory trace elements. CAIs exhibit a host of isotopic anomalies inherited from incorporated presolar grains or from the early nebula itself. 

The discovery by Clayton et al. (1973) that Allende CAls contain non-mass-dependent enrichments in (relative to 0) was in part responsible for setting both the excitement and tone of CAl research for the ensuing 30 years. This isotopic signature, together with evidence for extinct A1 and nonradiogenic isotope anomalies in elements such as titanium and calcium, was attributed to the isotopic compositions of the presolar precursor dust from which the CAIs formed. Hence, CAIs have astrophysical significance. 

The former presence of Be was extended to another important class of refractory objects, hibonite from the CM2 Murchison meteorite (Marhas et al., 2002). Hibonite [CaAli2-2 c(Mg cTi c)Oi9] is one of the most refractory minerals calculated to condense from a gas of solar composition, and is known to host numerous isotopic anomalies, especially in the heavy isotopes of calcium and titanium (Ireland et al., 1985 Zinner et al., 1986 Fahey et al., 1987). Curiously, when these anomalies are of an exceptionally large magnitude (in the several to 10% range), the hibonite grains show a distinct lack of evidence for having formed with Al (e.g., Ireland, 1988,1990) or Ca (Sahijpal etal, 1998,... 

For most other elements there is no difference between the isotopic composition of carbonaceous chondrites and the Earth. As of early 2000s, only two exceptions, chromium and titanium, are known for these two elements very small differences in the isotopic composition between carbonaceous chondrites and the Earth were found. Bulk carbonaceous chondrites have isotope anomalies in chromium and titanium. Isotopically unusual material may have been mixed to the CC-source after proto-earth material has accumulated to larger objects. 

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