The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Abstract

Abstract-In a search for evidence of evaporation during chondrule formation, the mesostases of 11Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K-isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had 841K compositions that were normal within error (typically ?3%, 20). The six "anomalous" points are probably all artifacts. Experiments have shown that free evaporation of K leads to large 41K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3%0 enrichment in 841K is produced by -12% loss of K. The range of L-chondrite-normalized K/Al ratios (a measure of the K-elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L-chondrite-like K abundances and the K loss occurred via Rayleigh fractionation, the most K-depleted chondrules would have had compositions of up to 841K = 200%0. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of WA1 fractionation in chondrules does not explain the K-isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K-isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim-matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K-isotopic fractionation in them requires that the K-isotopic exchange took place during chondrule formation, which would probably require gas-chondrule exchange. Potassium evaporated from fine-grained dust and chondrules during chondrule formation may have produced sufficient K-vapor pressure for gas-chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.

The onset of metamorphism in ordinary and carbonaceous chondrites

“…Figure 8, Fig. 9b, and Table 6 show that Na enters type I chondrules during metamorphism, raising the Na 2 O content of mesostasis, a feature that is also well documented in the literature (McCoy et al 1991;Hewins 1991;Sears et al 1995a;Alexander et al 2000). This process apparently occurred without devitrification of the glass: chondrites like Chainpur contain many type I chondrules with clear, isotropic glass that is rich in alkalis, which is a metamorphic overprint.…”

Relationships between type I and type II chondrules: Implications on chondrule formation processes

“…In these circumstances and depending on the oxygen fugacity, relatively modest PFe in the chondrule forming region may explain the concomitant enrichment in iron as well as the lack of measurable iron mass fractionation in chondrules (Alexander and Wang, 2001). This is analogous to suggestions that high partial pressure of other volatile and moderately volatile elements (K, Si, O) in the chondrule forming region can explain the very limited range of isotopic mass fractionation (Alexander et al, 2000;Georges et al, 2000;Kita et al, 2010;Fedkin et al, 2012). Finally, it is worth noting that if Fe enters the molten mesostasis from the gas phase (i.e., a solubility process) or from ferroan rims, this will dilute the other siderophile elements (SE metal = Ni, Co, Cr, P) dissolved in the melt from the Fe-Ni metal blebs by disturbing the Fe/SE metal ratio.…”

“…Alexander et al, 2000;Alexander and Wang, 2001;Yu et al, 2003;Lauretta et al, 2006;Alexander et al, 2008;Fedkin et al, 2012), (ii) metal grains in dusty olivines (Nipoor) and metal in type I chondrules (Ni-rich) do not have the same origin and thus could not have been formed simultaneously by an evaporation/reduction process (e.g. Rambaldi and Wasson, 1981;Leroux et al, 2003;Hewins et al, 2005), (iii) chemical gradients of volatiles and moderately volatiles elements in chondrule mesostasis and occurrence of mineralogical zoning in some chondrules argue in favor of condensation rather than evaporation processes (Matsunami et al, 1993;Nagahara et al, 1999;Libourel et al, 2003, Libourel et al, 2006Krot et al, 2004;Alexander et al , 2008;.…”

Re-examining the role of chondrules in producing the elemental fractionations in chondrites

“…At this stage, there will be some evaporation of volatile elements. The lack of K isotopic fractionation in chondrules (Alexander et al 2000;Alexander and Grossman 2005) and bulk chondrites (Humayun and Clayton 1995) requires that the chondrules and gas approach isotopic equilibrium.…”