While rolling over the Meridiani Planum sedimentary terrane, the rover Opportunity has occasionally discovered large, > 10 cm erratics. Most of these have proven to be meteorites [1], but one - Bounce Rock - is a martian basaltic rock similar in composition to the meteorite EETA79001 lithology B [2]. Presently, Opportunity is intensively investigating an --30 cm tall rock named Marquette Island that may be a distinct type of martian mafic lithology. We report the results of its continuing investigation using the Microscopic Imager (MI); Mossbauer Spectrometer (MB) and Alpha Particle X-ray Spectrometer (APXS). A companion abstract discusses the results of Panoramic Camera (Pancam) imaging of the rock [3]

Clark, B. C.

Cohen, B. A.

Fleischer, I.

Gellert, R.

Herkenhoff, K. E.

Jolliff, B. L.

Klingelhoefer, G.

Ming, D. W.

Mittlefehldt, David W.

Yingst, R. A.

NASA Technical Reports Server

MARQUETTE ISLAND: A DISTINCT MAFIC LITHOLOGY DISCOVERED BY OPPORTUNITY. D. W.
Mittlefehldt', R. Gellert` , K. E. Herkenhoff3 , R. V. Morris' ; B. C. Clark4, B. A. Cohen' ; I. Fleischer', B. L. Jolliff',
G. Klingelhofer', D. W. Ming', R. A. Yingst 8 and the Athena Science Team, 'NASA/JSC, Houston, TX USA
(day-•id.w.mittlefehldt(dnasa.gov ), 2Univ. Guelph, Guelph, ON CA, 3USGS, Flagstaff, AZ USA, 4SSI, Boulder, CO
USA,'NASA/MSFC, Huntsville, AL USA, 'Johannes Gutenberg-Univ., Mainz, DE,' Washington Univ., St. Louis,
MO USA, UUniv. Wisconsin Green Bay, Green Bay, WI USA.
Introduction: While rolling over the Meridians
Planum sedimentary terrane, the rover Opportunity has
occasionally discovered large, X 10 cm erratics. Most
of these have proven to be meteorites [1], but one -
Bounce Rock — is a martian basaltic rock similar in
composition to the meteorite EETA79001 lithology B
[2]. Presently, Opportunity is intensively investigating
an --30 cm tall rock named Marquette Island that may
be a distinct type of martian mafic lithology. We re-
port the results of its continuing investigation using the
Microscopic Imager (MI); Mossbauer Spectrometer
(MB) and Alpha Particle X-ray Spectrometer (APXS).
A companion abstract discusses the results of Pano-
ramic Camera (Pancam) imaging of the rock [3].
Microscopic Imager: The MI has imaged undis-
turbed and brushed surfaces, but the underlying texture
remains obscure. Some undisturbed regions appear to
show blocky, mm-sized grains. The brushed surface is
hackly and contains numerous glints from regions of a
highly reflective surface (Fig. 1). Regions of subdued
reflectivity show a dark-light structure with darker
material interstitial to brighter equant "grains" of mm
size. Thus, Marquette Island may be comprised of
what appear to be individual clastic, angular grains
embedded in a finer-grained matrix. The larger clast-
supported grains are poorly-sorted, with no preferred
orientation. There is no evidence for sedimentary
structures or layering. The texture is not highly diag-
nostic of a particular process or origin, but is grossly
similar to the target Seminole, imaged by Spirit in
Gusev Crater. There are a few instances of what ap-
pear to be semi-arcuate structures. This and the fact
that several facets are highly reflective, suggest that
the parent rock may be a breccia containing glassy
shards. However, the high reflectivity may simply be
a result of crystal facets sitting at advantageous angles
to reflect sunlight into the MI. The semi-arcuate struc-
tures are not common; and the rock appears very well-
lithifred, which would argue against brecciation. Some
regions show possible igneous texture, confounding
the textural interpretation of Marquette Island.
Figure 1. Part of radiometrically-calibrated MI image IM311956850
of target "Peck Bay", taken by Opportunity on Sol 2070. Subframe
is about 3 cm across, with direct stn illumination from upper left.
Dust has been removed by RAT brushing. except in circular area at
bottom center and upper right and left corners.
Mossbauer Spectrometer: We have acquired
over 150 hours of combined MB integration time on
two targets on Marquette Island. The major iron-
bearing minerals are olivine and pyroxene. Most of
the total rock Fe is contained in olivine with Fe-
ot/(Feoi+Fein) -- 0.8 (Fig. 2). A small amount of the
total rock Fe is contained in nanophase ferric oxide. A
Mossbauer sextet consistent with magnetite,
maghemite or hematite has not been detected.
	
Marquette Island	 Olivine o
	
Islington Bay	 Pyroxene
npOx
Data
Fit
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Figure 2. Mossbauer spectrum from the target Islington Bay, domi-
nated by olivine with contributions from pyroxene and nanophase
ferric oxide.
Alpha Particle X-ray Spectrometer: We have
done APXS measurements on two brushed spots on
Marquette Island. Some dust remained in the APXS
field of view (Fig. 1) and we do not have a contamina-
tion-free composition yet. Nevertheless, Marquette
Island is compositionally distinct from all other mate-
rials analyzed by Opportunity (Fig . 3). Among the
major elements, Marquette Island has hi gher Mg/Si,
Al/Si and lower Ca/Si than the basalt Bounce Rock.
The high-Ni cobbles; probable silicate-rich meteorites
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possibly similar to mesosiderites [1], have Si-
normalized ratios of Mg, Cr and Ni far above, and Al
and Ca lower than, those of Marquette Island. The
low-Ni cobbles are of uncertain ori gin, but their com-
positions are markedly different from that of Mar-
quette Island. However, these small cobbles do not fill
the APXS field of view and thus their compositions
are compromised by their soil substrate to varying de-
grees.
The closest compositional match to Marquette Is-
land among materials studied by the MER mission are
Adirondack Class basalts from Gusev Crater (Fig. 3),
but there are important differences — higher Mg/Si,
Al/Si and lower Ca/Si in the former. Martian meteor-
ites do not match Marquette Island in composition.
Martian meteorites with similar Mg/Si have Al/Si that
are much lower and Ca/Si that are much higher.
While the composition of Marquette Island is simi-
lar to Adirondack Class rocks (Fig. 3) and basaltic
soils at both landing sites, there is a clear distinction in
the Pu x-ray scatter peaks of Marquette Island com-
pared to other basaltic samples. These peaks are sensi-
tive to the overall composition of the sample at greater
depth, including x-ray-invisible light elements such as
H, O and C. Therefore it might be that either Mar-
quette Island includes significant amounts of "unseen"
light elements, or that the rock has a significantly dif-
ferent composition at greater depth with a higher light
element content.	 y
Discussion: Marquette Island is very distinct from
the high-Ni cobbles such as Barberton that are prob-
able silicate-rich meteorites, possibly similar to
mesosiderites [1]. Because of the low Ni/Si and lack
of kamacite or troilite, we do not consider a meteoritic
origin for Marquette Island to be likely.
Marquette Island is mafic in composition. Current
MI images do not allow for confident textural interpre-
tation: igneous and clastic textures remain possibilities.
Our working hypothesis is that Marquette Island is a
distinct martian mafic lithology with compositional
similarities to Adirondack Class basalts of Gusev Cra-
ter. The MB spectra show that the Fe mineralogy is
dominated by olivine with subordinate pyroxene. Pan-
cam spectra on some regions of the brushed surface
show no upturn in relative reflectance at the longest
wavelength, consistent with the presence of olivine
[3]. A normative mineralogy calculated for low Fe" is
consistent with the MB Fe mineralogy. Although gen-
erally similar to Adirondack Class basalts, there are
nonetheless important differences in composition. The
low Ca/Si in Marquette Island results in --2 wt% nor-
mative diopside, compared to 14-16 wt% for brushed
Adirondack Class basalts. The excess in light ele-
ments suggests that Marquette Island may be an altered
rock ; and thus not a true mafic igneous lithology.
Bounce Rock is interpreted as being a mafic igne-
ous ejecta block deposited on Meridians Planum [2].
Marquette Island probably is also crater ejecta, possi-
bly from the same crater. However, the compositions
of Bounce Rock and Marquette Island are not simply
related by igneous fractionation. Bounce Rock is an
olivine-poor, pyroxene-rich basalt with lower
Mg/(Mg+Fe), Mg/Si, Cr/Si and Ni/Si, and higher
Ca/Si suggesting that it could be a more fractionated
member of a conunon magma series. However ; the
hi gher Al/Si of Marquette Island is inconsistent with
this scenario. Thus, if these two mafic erratics were
ejecta from the same crater, they appear to represent
unrelated rock series.
Marquette Island —<>- Bounce Roc
	
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Figure 3. Element/Si ratios for Marquette Island compared to other
rocks front Planum (tipper) and Gusev Crater Adirondack
Class basalts (lower). Element ratios are normalized to an average
brushed Meridiani outcrop composition.
References: [1] Schroder C. et al. (2008) JGR,
113, doi:10.1029/2007JE002990. [2] Squyres S. W. et
al. (2004) Science, 306, 1698. [3] Farrand W. H. et al.
(2010) This conference.
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Marquette Island: A Distinct Mafic Lithology Discovered by Opportunity

Marquette Island: A Distinct Mafic Lithology Discovered by Opportunity

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