A study underway at Marshall Space Flight Center is further characterizing the effects of shock on isotopic ages. The study was inspired by the work of L. Nyquist et al. [1, 2], but goes beyond their work by investigating the spatial distribution of elements in lunar ferroan anorthosites (FANs) and magnesium-suite (Mg-suite) rocks in order to understand the processes that may influence the radioisotope ages obtained on early lunar samples. This paper discusses the first data set (major elements) obtained on FANs 62236 and 67075

Cohen, B. A.

Edmunson, J.

Spilde, M. N.

NASA Technical Reports Server

40th Lunar and Planetary Science Conference (2009) 	 2094.pdf
CHARACTERIZING THE EFFECT OF SHOCK ON ISOTOPIC AGES I: FERROAN ANORTHOSITE
MAJOR ELEMENTS. J. Edmunson 1 , B. A. Cohen2, and M. N. Spilde3, 1NASA Postdoctoral Program, Marshall
Space Flight Center (National Space Science and Technology Center, Huntsville AL 35805, Jenni-
fer.E.Edmunson@nasa.gov), 2Marshall Space Flight Center (Huntsville AL 35812, Barbara.A.Cohen@nasa.gov),
3Institute of Meteoritics (University of New Mexico, Albuquerque NM 87131, mspilde@unm.edu ).
Introduction: A study underway at Marshall
Space Flight Center is further characterizing the effects
of shock on isotopic ages. The study was inspired by
the work of L. Nyquist et al. [1, 2], but goes beyond
their work by investigating the spatial distribution of
elements in lunar ferroan anorthosites (FANs) and
magnesium-suite (Mg-suite) rocks in order to under-
stand the processes that may influence the radioisotope
ages obtained on early lunar samples. This abstract
discusses the first data set (major elements) obtained
on FANs 62236 and 67075.
Background: This study was designed to investi-
gate peculiarities of a selection of lunar highland
rocks: 62236 and its FAN geochemistry would imply
origin during primary lunar crust formation, but its
147 Sm- 143Nd
 isotopic age is relatively young (4.29
0.06 Ga [3]); isotopic ages for 67075 range from 3.66
0.63 Ga [4] to 4.47 Ga [5], and the calculated initial
ENd of approximately +8 [6] indicates derivation of
this FAN from a LREE-depleted source (unlike ex-
pected primary FAN compositions [7]); Mg-suite troc-
tolite 76535 has contradictory 87Rb-87Sr (4.57 0.07
Ga, X87 = 1.402 x10-11y-1 [8]) and 147 Sm- 143Nd (4.26
0.06 Ga [9]) ages of formation; and no 87Rb- 87 Sr iso-
chron could be discerned for Mg-suite troctolite 76335
[10].
Apollo 16 sample 62236 is classified as a ferroan
noritic anorthosite, despite the fact that it has both
noritic and troctolitic biases depending on the thin sec-
tion viewed [11, 12]. Warren and Wasson [11] con-
cluded that 62236 is pristine, and Nord and Wandless
[12] argue that 62236 is a monomict breccia. How-
ever, Shearer et al. [13] found two distinct populations
of olivine characterized by differences in their Ni and
Co abundances, indicating a polymict nature. They
further state that the relatively young age of 62236 [3]
may suggest metamorphic re-equilibration of the sam-
ple, and that “the extent and scale of re-equilibration
determines the significance of the Sm-Nd data”. Thus,
any re-equilibration of the sample must be quantified –
one of the goals of this study.
Sample 67075 is a friable brecciated anorthosite,
containing minor amounts of olivine, pyroxene, and
chromite [14, 15]. Like 62236, variations in the distri-
bution of mafic minerals produced different estimated
modal mineralogies for 67075. In the case of 67075,
the modal mineralogy of the thin section viewed likely
reflects the proportions of three components, with
slightly different chemical compositions, that make up
this sample [14]. The three components are a polygo-
nal microanorthosite, a coarse-grained cataclastic anor-
thosite, and a pyroxene-dominated constituent [14].
This sample is thought to be the result of brecciation
and mixing of layers from a single igneous pluton [15,
16], which should be ideal in the case of an isotopic
analysis. However, isotopic studies have produced a
range in ages for 67075 [4, 5], and a significantly posi-
tive calculated initial [6]. Further study of 67075
has indicated regions of annealing [15, 17], which
could influence the age of the sample. Investigating
the geochemistry of this sample will help establish the
extent of shock and annealing, as well as the validity of
the ages and the calculated LREE-depleted source for
this FAN.
Methodology: The methods employed in this
study mirror that of [18]; imaging and initial identifica-
tion of phases through scanning electron microscopy
(SEM), major element analysis by electron micro-
probe, and trace element abundance determination with
secondary ion mass spectrometry (SIMS). Ultimately,
153 quantitative major element microprobe analyses
were collected on 5 phases in both thin sections. These
points were selected for the following reasons: (1)
large mineral grains, core to rim, to look for diffusion
or igneous crystallization trends; (2) small mineral
grains, grain center analysis, to compare to larger
grains in major and trace element abundances; (3) in-
clusions within the large mineral grains to show high
silica phase production and equilibration or diffusion
within adjacent related minerals; and (4) inclusion host
grains, core to inclusion, to note any changes in chem-
istry associated with proximity to the inclusion. These
points were also selected to be suitable for subsequent
analysis by SIMS.
Expectations: Given the relatively low shock
pressures for 62236 (estimated at 200-300 kilobars
[12]) and 67075 (see below), any changes in major
element chemistry due to shock are expected to be
below detection limits of the microprobe, but ppm-
level changes should be observed with SIMS. In situ
melting of anorthite and olivine at grain boundaries has
been observed in 62236, but maskelynite formation has
https://ntrs.nasa.gov/search.jsp?R=20090025878 2019-08-30T07:25:27+00:00Z
40th Lunar and Planetary Science Conference (2009) 	 2094.pdf
not [12]. Maskelynite is thought to be a diaplectic
glass implying that the structure, but not the chemistry,
of the feldspar has been altered. Thus, samples such as
62236 and 67075 likely would not show significant
changes to their major element chemistry.
Observations: Thin section observations of 62236
are fairly consistent with those of Nord and Wandless
[12], who estimated that three shock events were nec-
essary to create this breccia. These observations in-
clude multiple episodes of mechanical faulting and
undulatory extinction in plagioclase (found by [12] to
be deformation in 62236 anorthite on a <1 μm scale).
However, no melting of “particulate olivine” was ob-
served in thin sections ,40 and ,58 (exceedingly fine-
grained olivine was observed, but did not appear under
SEM to be melted).
A study of thin section 67075 ,51 revealed planar
deformation features in a few plagioclase grains (after
[18]), as well as annealed clasts (e.g., Figure 1). Nei-
ther maskelynite, nor melting of “particulate olivine”,
was present in the thin section. Thus, the constraints
of 300 or 200 kilobars shock pressure, respectively,
that were applied to 62236 by [12] cannot be applied to
this portion of 67075. Also, no evidence of (001)
augite shock-induced twinning was found in section
,51, and without contradictory evidence, an upper
bound of 50 kilobars [19] can be established.
Figure 1: Annealed clast in 67075 ,51. Note polygo-
nal grains and abundance of 120° grain junctions.
Electron microprobe results were studied with re-
spect to spatial location and grain size to determine
whether diffusion of highly volatile elements occurred
readily in relatively small grains when compared to
larger grains. No relationship between the size of a
grain and its volatile major element content could be
established.
Low-resolution microprobe element maps for
67075 indicated a difference in the Na content of dif-
ferent plagioclase clasts. In fact, a plot of CaO versus
Na2O abundances in individual microprobe points
show scatter in Na2O outside that explainable by mi-
croprobe error. However this scatter is well within the
natural variations in terrestrial anorthosite complexes
such as the Bushveld Complex [19] or the Laramie
Complex [20], calculations indicate that the plagio-
clase grains are stoichiometric, and the An content of
both 62236 and 67075 only varies from An98-99 which
is similar to other FANs. Therefore, it will be impor-
tant in the next phase of this study to further investi-
gate Na, K, and Rb abundances in the individual grains
to verify whether or not shock has influenced the vola-
tile content of the mineral grains.
Study Status: There is currently no evidence to
suggest that the major elements of 62236 and 67075
have been redistributed due to shock. Scatter shown in
the Na2O content of plagioclase does not exceed scat-
ter in natural terrestrial anorthosites, but does suggest
that further study with more precise techniques of
similar volatile trace elements in these lunar samples is
necessary. An estimated shock pressure of <50 kilo-
bars is implied by the petrography of 67075 ,51.
Acknowledgements: This research was supported
by an appointment to the NASA Postdoctoral Program
at the Marshall Space Flight Center, administered by
Oak Ridge Associated Universities through a contract
with NASA.
References: [1] Nyquist L. W. et al. (1991) LPS
XXII, 985-986. [2] Nyquist L. E. et al. (1991) LPS
XXII, 987-988. [3] Borg L. E. et al. (1999) Geochim.
Cosmochim. Acta, 63, 2679-2691. [4] Nyquist L. E. et
al. (1976) Proc. LPS VII, 1507-1528. [5] Oberli F. et
al. (1979) LPS X, 940-942. [6] Shih C. –Y. et al.
(2005) LPS XXXVI, Abstract #1433. [7] Korotev R. L.
et al. (1980) Proc. LPS XI, 395-429. [8] Papanastas-
siou D. A. and Wasserburg G. J. (1976) Proc. LSC VII,
2035-2054. [9] Lugmair G. W. et al. (1976) Proc. LSC
VII, 2009-2033. [10] Edmunson J. (2007) Chron. Me-
teorites & Early Sol. Syst., Abstract #4069. [11] War-
ren P. H. and Wasson J. T. (1978) Proc. LPS IX, 185-
217. [12] Nord G. L., Jr. and Wandless M. –V. (1983)
Proc. LPS XIII, JGR, 88, A645-A657. [13] Shearer C.
K. et al. (2002) LPS XXXIII, Abstract #1517. [14]
Steele I. M. and Smith J. V. (1973) Proc. LSC IV, 519-
536. [15] McCallum I. S. et al. (1975) EPSL, 26, 36-
53. [16] Peckett A. and Brown G. M. (1973) Nature,
242, 252-255. [17] LSPET (1973) Science, 179, 23-34.
[18] Edmunson J. et al. (2005) Meteoritics & Planet.
Sci., 40, 1159-1174. [18] Hornemann U. and Müller
W. F. (1971) Neues Jahrb. Mineral. Monatsh., 247-
255. [19] Maier W. D. (1995) Canadian Mineralogist,
33, 1011-1022. [20] Mitchell J. N. et al. (1996) J. Pet.,
37, 637-660.


Characterizing the Effect of Shock on Isotopic Ages I: Ferroan Anorthosite Major Elements

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server