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C. M. O’D. Alexander

Publications and source records attributed to C. M. O’D. Alexander.

5 recordsLinked to original sources
USGS2010

Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel-Lévy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These “bleached” chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4–6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x-ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low-temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium-rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid-chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.

Meteoritics and Planetary Science↗
USGS2010

The lack of potassium-isotopic fractionation in Bishunpur chondrules

In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur 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 δ 41 K compositions that were normal within error (typically ±3%, 2s̀). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large 41 K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ 41 K 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 δ 41 K ≅ 200%. 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 K/Al 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.

Meteoritics and Planetary Science↗
USGS2010

Zoned chondrules in Semarkona: Evidence for high-and low-temperature processing

At least 15% of the low-FeO chondrules in Semarkona (LL3.0) have mesostases that are concentrically zoned in Na, with enrichments near the outer margins. We have studied zoned chondrules using electron microprobe methods (x-ray mapping plus quantitative analysis), ion microprobe analysis for trace elements and hydrogen isotopes, cathodoluminescence imaging, and transmission electron microscopy in order to determine what these objects can tell us about the environment in which chondrules formed and evolved. Mesostases in these chondrules are strongly zoned in all moderately volatile elements and H (interpreted as water). Calcium is depleted in areas of volatile enrichment. Titanium and Cr generally decrease toward the chondrule surfaces, whereas Al and Si may either increase or decrease, generally in opposite directions to one another; Mn follows Na in some chondrules but not in others; Fe and Mg are unzoned. D/H ratios increase in the water-rich areas of zoned chondrules. Mesostasis shows cathodoluminescence zoning in most zoned chondrules, with the brightest yellow color near the outside. Mesostasis in zoned chondrules appears to be glassy, with no evidence for devitrification. Systematic variations in zoning patterns among pyroxene- and olivine-rich chondrules may indicate that fractionation of low- and high-Ca pyroxene played some role in Ti, Cr, Mn, Si, Al, and some Ca zoning. But direct condensation of elements into hot chondrules, secondary melting of late condensates into the outer portions of chondrules, and subsolidus diffusion of elements into warm chondrules cannot account for the sub-parallel zoning profiles of many elements, the presence of H 2 O, or elemental abundance patterns. Zoning of moderately volatile elements and Ca may have been produced by hydration of chondrule glass without devitrification during aqueous alteration on the parent asteroid. This could have induced structural changes in the glass allowing rapid diffusion and exchange of elements between altered glass and surrounding matrix and rim material. Calcium was mainly lost during this process, and other nonvolatile elements may have been mobile as well. Some unzoned, low-FeO chondrules appear to have fully altered mesostasis.

Meteoritics and Planetary Science↗
USGS2008

The formation conditions of chondrules and chondrites

Chondrules, which are roughly millimeter-sized silicate-rich spherules, dominate the most primitive meteorites, the chondrites. They formed as molten droplets and, judging from their abundances in chondrites, are the products of one of the most energetic processes that operated in the early inner solar system. The conditions and mechanism of chondrule formation remain poorly understood. Here we show that the abundance of the volatile element sodium remained relatively constant during chondrule formation. Prevention of the evaporation of sodium requires that chondrules formed in regions with much higher solid densities than predicted by known nebular concentration mechanisms. These regions would probably have been self-gravitating. Our model explains many other chemical characteristics of chondrules and also implies that chondrule and planetesimal formation were linked.

Science↗