1194.pdf
Lunar and Planetary Science XXXII (2001)
MASS-DEPENDENT FRACTIONATION OF MG, SI, AND FE ISOTOPES IN FIVE STONY COSMIC
SPHERULES. C. M. O'D. Alexander1, S. Taylor2, J. Delaney3,5, P. Ma4,5 and G. F. Herzog4,5, 1Dept. Terrestrial
Magnetism, 5241 Broad Branch Road NW, Washington, DC 20001, 2CRREL, 72 Lyme Road, Hanover, NH
03755, 3Dept. Geological Sciences, 4Dept. Chemistry, 5Rutgers Univ., Piscataway, NJ 08854, <her-
zog@rutchem.rutgers.edu>.
Introduction: Some micrometeorites have been
Results: Compositionally, MM11 is a fairly
heated more than others [1]. We focus on the most
typical glass spherule; MM9 and MM14 resemble
strongly heated stony micrometeorites, the cosmic
other rare Ca-, Al-, and Ti-rich CS from the SPWW;
spherules (CS), some of which may have had
low Al and Ca contents in MM8 suggest a forsterite-
cometary progenitors [2]. All CS melted (by defini-
rich parent; MM15 has an unusually high Al content,
tion) and their compositions changed to varying de-
high Al/Ca ratio, and low Si content (Table 1). All
grees during atmospheric entry. An understanding of
particles contain little Fe, about 4% of what CIs have
these changes helps in characterizing the original
and 10% of the values typical for other sCS.
materials and perhaps their sources. Some calcula-
Alexander [8] has modeled the compositional
tions suggest, for example, that CS lost 80% of their
evolution of a CI melt evaporating at high tempera-
mass by evaporation [3], a process expected to enrich
ture (Fig. 1). Other types of meteoritic material
the heavier isotopes of elements. Heavy-isotope en-
40
orimaemgsphig2
g
f
richments are, in fact, common for metallic (I-type)
35
MM8
spherules [see 4] but infrequently reported for stony
MM11
CS [5]. We decided to search for isotopic evidence
30
of evap oration in the most strongly heated stony CS
25
(sCS). To identify them we relied on a simple crite-
20
rion, color. Light color indicates low iron content [6]
ite
er
rst
15
Among major elements, relatively volatile Fe should
fo
To
be especially susceptible to evaporative loss and iso-
10
topic fractionation. We therefore set out to analyze
MM9
Cosmic spherules
5
MM14
the isotopic composition of Fe (and of Si and Mg) in
CI path
MM15
0
five sCS.
0
10
20
30
40
50
60
Experimental Methods: We selected five, light-
Mg/Al (atom/atom)
colored sCS, from the South Pole Water Well
Figure 1. Mg/Al and Si/Al ratios of five stony
(SPWW) colle ction, weighed them, and measured
cosmic spherules.
their diameters. Elemental compositions were ana-
lyzed at the Rutgers University Microanalysis Facil-
evolve from star ting points higher in the diagram
ity using the JEOL JXA-8600 microprobe. Isotopic
than CI, but quickly converge to the CI trajectory.
analyses were carried out with the Carnegie Cameca
Thus the data points suggest that MM9, MM14 and,
6f ion microprobe using a 12.5-kV O- primary beam
MM15 have undergone some evaporation; we cannot,
in the shaped or flat-bottomed illumination mode, a
however, identify the precursors unambiguously. To
10-kV secondary accelerating voltage, a 50-eV en-
go further we turn to the isotopic data (Table 1),
ergy window and 100-m field aperture. Primary
which provide quantitative measures of the extent of
beam currents ranged between 0.3 nA and 5 nA; spot
evaporation.
sizes were ~20 m across. Because of potential in-
The heavy isotopes of Fe, Si, and Mg are enriched
in 5, 3, and 1 of the cosmic spherules, respectively.
were measured. The principal remaining interference
In general, isotopic fractionation is largest for Fe,
was 56FeH at 57Fe. To resolve the hydride from 57Fe,
smaller for Si, and smallest for Mg, implying a pat-
analyses were performed at a mass resolution of
tern of evaporation similar to that observed in labo-
~8000. The Mg and Si isotopes measurements were
ratory heating experiments [9]. We have used the
conducted at a mass resolution of ~3500 to r esolve
Rayleigh equation to calculate from the experimental
δ values the respective fractions, f, of Fe, Si, and Mg
24MgH from 25Mg and 28SiH from 29Si. The standard
used to measure the instrumental mass fractionation
retained by each micrometeorite (Table 1). The mi-
factors for all three elements was a BHVO basaltic
crometeorite with the largest fractionation effects,
glass [7]. The influence of composition on instru-
MM14, retains little Fe and about 61% of its Mg.
mental fractionation factors for MgO and SiO2 is not
Based on the values of f, we corrected measured ele-
known.
mental compositions for evaporative loss and aver-
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