Presence Of 26Al Research Articles

Crystallization and cooling conditions for diogenite formation in the turbulent magma ocean of asteroid 4 Vesta

Despite its small size, the asteroid 4 Vesta has been completely differentiated to core and mantle. Its composition is similar to howardite-eucrite-diogenite (HED) meteorites of which the detailed petrology is known. Therefore, 4 Vesta is a good target for understanding the differentiation of terrestrial planets. A new differentiation model for crust formation has been developed by taking magma ocean fluid dynamics, chemical equilibrium, the presence of 26Al, and cooling into consideration with a special focus on crystal separation. The role of crystal size, thickness of the conductive lid, and fO2 are evaluated as parameters. The results show that large crystals of at least 1 cm settled and formed a kilometer-thick cumulate layer of orthopyroxene with Mg# of 0.70–0.90 in ∼20 thousand years, which almost agrees with the Mg # of diogenites. Smaller grain sizes formed thinner layers.

Revisiting26Al-26Mg systematics of plagioclase in H4 chondrites

Zinner and Göpel (1992, 2002) found clear evidence for the former presence of 26Al in the H4 chondrites Ste. Marguerite and Forest Vale. They assumed that the 26Al-26Mg systematics of these chondrites date “metamorphic cooling of the H4 parent body.” Plagioclase in these chondrites can have very high Al/Mg ratios and low Mg concentrations, making these ion probe analyses susceptible to ratio bias, which is inversely proportional to the number of counts of the denominator isotope (Ogliore et al. 2011). Zinner and Göpel (2002) used the mean of the ratios to calculate the isotope ratios, which exacerbates this problem. We analyzed the Al/Mg ratios and Mg isotopic compositions of plagioclase grains in thin sections of Ste. Marguerite, Forest Vale, Beaver Creek, and Sena to evaluate the possible influence of ratio bias on the published initial 26Al/27Al ratios for these meteorites. We calculated the isotope ratios using total counts, a less biased method of calculating isotope ratios. The results from our analyses are consistent with those from Zinner and Göpel (2002), indicating that ratio bias does not significantly affect 26Al-26Mg results for plagioclase in these chondrites. Ste. Marguerite has a clear isochron with an initial 26Al/27Al ratio indicating that it cooled to below 450 °C 5.2 ± 0.2 Myr after CAIs. The isochrons for Forest Vale and Beaver Creek also show clear evidence that 26Al was alive when they cooled, but the initial 26Al/27Al ratios are not well constrained. Sena does not show evidence that 26Al was alive when it cooled to below the Al-Mg closure temperature. Given that metallographic cooling rates for Ste. Marguerite, Forest Vale, and Beaver Creek are atypical (>5000 °C/Myr at 500 °C) compared with most H4s, including Sena, which have cooling rates of 10–50 °C/Myr at 500 °C (Scott et al. 2014), we conclude that the Al-Mg systematics for Ste. Marguerite, Forest Vale, and Beaver Creek are the result of impact excavation of these chondrites and cooling at the surface of the parent body, instead of undisturbed cooling at depth in the H chondrite parent body, like many have assumed.

ON THE INJECTION OF SHORT-LIVED RADIONUCLIDES FROM A SUPERNOVA INTO THE SOLAR NEBULA: CONSTRAINTS FROM THE OXYGEN ISOTOPES

Injection of short-lived radionuclides from a nearby core-collapse Type II supernova into the already-formed solar protoplanetary disk was proposed to account for the former presence of 26Al, 41Ca, and 60Fe in the early solar system inferred from isotopic analysis of meteoritic samples. One potential corollary of this late-injection scenario is that the disk's initial (pre-injection) oxygen isotopic composition could be significantly altered, as supernova material that carried the short-lived radionuclides would also deliver oxygen components synthesized in that given star. Therefore, the change in the oxygen isotopic composition of the disk caused by injection could in principle be used to constrain the supernova injection models. Previous studies showed that although supernova oxygen could result in a wide range of shifts in 17O/16O and 18O/16O of the disk, a couple of cases existed where the calculated oxygen changes in the disk would be compatible with the meteoritic and solar wind data. Recently, the initial abundances of 41Ca and 60Fe in the solar system were revised to lower values, and the feasibility of supernova injection as a source for the three radionuclides was called into question. In this study, supernova parameters needed for matching 26Al, 41Ca, and 60Fe to their early solar system abundances were reinvestigated and then were used to infer the pre-injection O-isotope composition of the disk. The result suggested that a supernova undergoing mixing fallback might be a viable source for the three radionuclides.

26Al as a heat source for early melting of planetesimals: Results from isotopic studies of meteorites

Ion microprobe studies of magnesium isotopic composition in igneous components from several chondritic meteorites have been carried out to look for26Mg excess that may be attributed to the presence of the now-extinct radionuclide26Al(τ ∼ 1 Ma) at the time of formation of these objects. A positive evidence for the presence of26Al in the analysed objects will strengthen its case as the primary heat source for the early thermal metamorphism/melting of meteorite parent bodies. Based on calculated temperature profiles inside chondritic objects of different sizes and initial26Al/27Al ratios, we have estimated the initial abundances of26Al needed to provide the heat necessary for the wide range of thermal processing seen in various types of meteorites. The magnesium isotopic data obtained by us do not provide definitive evidence for the presence of26Al at the time of formation of the analysed igneous phases in different chondritic meteorites. Experimental evidence for a planetary scale distribution of26Al in the early solar system to serve as a significant heat source for the thermal metamorphism and melting of meteorite parent bodies (planetesimals) remains elusive.

Isotopes in comets: Implications from Cr in carbonaceous chondrites

According to their chemical composition, rich in volatile compounds, comets are thought to be primitive materials. They may provide prime samples for the study of nucleosynthetic components of the solar system and of the processes occurring during the formation of the outer planets. Their origin is largely a matter of conjecture. Chromium isotopic measurements in carbonaceous chondrites illustrate how the non-volatile part of cometary material can be investigated both for isotopic heterogeneity and for the extinct nuclide 53Mn. Questions like the possible presence of 26Al as a heat source can also be addressed by these measurements.

The isotopic composition of Ag in meteorites and the presence of 107Pd in protoplanets

The isotopic composition of silver in the metal phase of several types of iron meteorites has been determined. The ratios of 107Ag 109Ag are found to range from the terrestrial value of 1.09 to 9.2. These results show the presence of excess 107Ag ( 107Ag ∗) in almost all iron meteorites studied with a ratio of 108Pd 109 Ag greater than ~ 400. 107Ag ∗ has been found in iron meteorites of types IVA-IVB, IIIA-IIIB, IIB, and some anomalous types. The typical ratio found is 107Ag ∗ 108Pd ≈ 2 × 10 −5 in general agreement with the original report of Kelly and Wasserburg (1978). An intensive study of Gibeon (IVA) shows that the 107Ag 109Ag correlates with 108Pd 109Ag over a wide range for bulk samples of the metal rich phase and yield 107Ag ∗ 108Pd = (2.40 ± 0.05) × 10 −5 and an extrapolated initial ( 107Ag 109Ag) 0 = 1.11 ± 0.01 . This is considered to represent an isochron resulting from the in situ decay of 107Pd ( τ 1 2 = 6.5 × 10 6y ) on the parent planet. The bulk sulfide shows 107Ag ∗ uncorrelated with 108Pd and a highly variable isotopic composition of Ag. This phenomena is not well understood. We interpret the sulfide data as partly due to diffusion transport of Ag and segregation into minor phases. Some of this complexity is due to recrystallization after shock melting of FeS and anthropogenic heat treatment. Several other IVA meteorites were analyzed with a wide range in Pd/Ag and all showed the presence of 107Ag ∗. A sample of Santa Clara (IVB) metal was analyzed, and it was confirmed that this does not lie on the IVA correlation line. Analysis of metal, schreibersite, and sulfide from the unshocked meteorite Derrick Peak (IIB) shows a correlation of 107Ag ∗ with 108Pd and 107Ag ∗ 108Pd = ~1.5 × 10 −5 and initial ( 107Ag 109Ag ) 0 = 1.092 ± 0.002 . These observations demonstrate a correlation of 107Ag* with 108Pd within samples of the metal phase of a single meteorite and for samples of different meteorites. These results are compatible with the model of decay of 107Pd in the parent planets which initially contained normal Ag. The 107Ag* in the sulfide is attributed to diffusion, although this is not proven. The cooling rates for the parent bodies must have been much faster than 150°K/ma thus implying a radius less than ~30 km. The presence of 107Pd in the parent bodies implies that the time scale between production of 107Pd and planet formation, melting, and cooling must be less than ~10 7a. A small but distinct variation in 107Ag* content exists among different meteorites. If this variation is interpreted to represent an age difference, then the present results indicate a maximum difference in age of ~4 × 10 7 a. The nuclide 107Pd is inferred to have been widespread throughout the terrestrial-type protoplanets. While the nucleosynthetic origin of 107Pd and 26Al is distinct, their presence in the early solar system appears to fix the time scales for small planet formation relative to the nucleosynthetic sources that produced them. The observations are not consistent with a model in which protosolar material is mixed with 107Ag ∗ from the previous decay of extinct 107Pd in old interstellar dust grains. However, the presence of 26Al in protoplanets is not yet established, nor is the heat source for the later melting and planetary differentiation established.

Isotope abundance anomalies and the early solar system (some) facts and (some) implications

Variations have been found in the isotopic composition of a number of chemical elements which appear to reflect primordial variations in space and/or time of the composition of the proto-solar nebula. The agreement with theoretical predictions is the first proof that (some of) the nucleosynthetic processes invoked by astrophysicists to explain the abundance pattern of nuclides in the solar system may indeed have happened. Data for xenon, oxygen, titanium and evidence for the former existence of now extinct 244Pu and 26Al are discussed in some detail as well as implications from the presence of 26Al as an effective heat source in the early history of the solar system.

26Al and the partial ionization of the solar nebula

Calculation of the ionization state and consequent magnetic Reynolds number for the solar nebula shows that the presence of26Al will result in strong coupling of the gas to magnetic fields. In the absence of26Al,40K will still result in substantial ionization, but the degree of magnetic coupling is much more model dependent.