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46th Lunar and Planetary Science Conference (2015)
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SPACE WEATHERING OF FE AND MG END-MEMBER PHYLLOSILICATES. H. M. Kaluna1 and J. J.
Gillis-Davis2, 1Institute for Astronomy, University of Hawaiʻi, Honolulu-HI-96822, 2Hawaiʻi Institute for Geophysics and Planetology, University of Hawaiʻi, Honolulu-HI-96822.
Introduction: Space weathering effects are difficult to identify on C-type asteroids due to their low
albedos and shallow absorption features [1,2]. Recent
observational studies indicate that C-type asteroids do
experience space weathering, but the results show contrasting trends. Two studies find the spectral slopes
decrease with exposure age [3,4], whereas another
finds the slopes increase (i.e. redden) [5]. Laboratory
space weathering of carbonaceous chondrites (CCs)
may offer clues towards solving the discrepancy found
in asteroid observations.
Experimental space weathering of CCs is still
largely unexplored given the significant heterogeneity
in CC compositions. Past ion bombardment and laser
irradiation experiments on CCs also show a variety of
spectral trends including darkening, brightening, reddening [5-8] and blueing (slope decrease) [9-11].
Hence, targeted experiments and observations are necessary to understand how individual components of
CCs contribute to the complicated response of C-type
space weathering.
The aim of this work is to investigate the discrepancy between the spectral trends seen in space weathered C-type asteroids and CC meteorites by performing
a targeted series of lab experiments. Our experimental
work simulates micrometeorite impacts on phyllosilicate minerals, one of the major constituents in CCs.
We compare the spectral response of two end-member
phyllosilicates, cronstedtite (Fe22+Fe3+(SiFe3+)O5OH4)
and serpentine (Mg3Si2O5)(OH4), with olivine, an anhydrous silicate.
Methods: Each mineral was crushed and drysieved to <75 µm. X-ray diffraction analysis showed
the lizardite to be 95% pure and the cronstedtite to be a
mixture of cronstedtite, pyrite (FeS2) and siderite
(FeCO3) (~73, 23 and 3 wt.% respectively).
Samples were irradiated with a Nd:YAG, 1064 nm
wavelength, pulsed laser. Laser spot size on the sample is 0.25 mm with an incident energy of 30mJ. A 20
Hz frequency and 6-8 nanosecond pulse duration was
chosen to simulate timescales of real micrometeorite
dust impacts [12]. The samples were irradiated at intervals of 2.5, 2.5, 5.0, 5.0, 5.0, 10.0, and 10.0 for a
total of 40 minutes. Three 0.5g samples of each mineral were irradiated. Samples were irradiated as uncompressed powers and under vacuum pressures of 105
to 10-6 mbar. The partial pressure of H2O along with
other volatiles was measured inline with a Stanford
Research Systems 100 amu residual gas analyzer
(RGA), which is a mass spectrometer consisting of a
quadrupole probe. The RGA measured the partial pressure of the gases with time throughout the duration of
the experiment.
Spectral data were taken using a Vis/NIR (3502500nm) Analytical Spectral Devices Inc. (ASD)
Fieldspec 4 spectroradiometer. Bidirectional reflectance spectra were acquired at a standard viewing geometry of i=30° and e=0°, and measured relative to a
99% reflectance LabSphere Spectralon Standard.
Results: Spectral changes as a function of laser
space weathering were measured for each mineral (Fig.
1). The most significant spectral changes were seen in
the olivine slopes, which redden ~10x more than the
phyllosilicates. The lizardite spectra also became redder with increased irradiation. The cronstedtite samples, however, initially redden then reverse and become bluer as irradiation time increased. Lizardite
shows little to no changes in reflectance (our proxy for
albedo) at all wavelengths, but olivine and cronstedtite
both experience darkening in the visible and NIR. Initially the cronstedtite darkens more quickly in the visible than the NIR, but after ~10 minutes only the NIR
continues to darken. Lizardite, which has a large suite
of absorption bands in the visible and NIR, shows a
smaller reduction in band depths at longer wavelengths.
The RGA data show water released during irradiation are comparable for lizardite and cronstedtite at all
stages of weathering. The water released by the phyllosilicates decreases slightly with increasing irradiation, but do not show evidence of dropping to olivine
levels. These data suggest that the sample does not
need to be fully dehydrated to have a spectral response
to space weathering [13].
Discussion: Phyllosilicates are an important tracer
of aqueous alteration. Modal mineralogy of CCs show
a progression from Fe-cronstedtite to Mg-serpentines
as aqueous alteration progresses from petrologic type 2
to 1 [14,15]. Furthermore, phyllosilicates can make up
a significant fraction of CC meteorties (~80 wt. % in
Murchison [16]), and have been detected in ~50% of
C-type asteroids [17]. Hence we chose to focus on the
response of phyllosilicates to simulated space weathering. Lizardite and cronstedtite are only two minerals
in the phyllosilicate class, but these two Fe-Mg endmember compositions exhibit a divergent set of spectral trends.
46th Lunar and Planetary Science Conference (2015)
Cronstedtite is the first mineral to show a decrease
in spectral slopes in response to laser space weathering. Previously only organics have shown spectral
blueing in response to ion bombardment [11]. The
spectral blueing in cronstedtite appears to result from
the continued darkening in the NIR while darkening
saturates in the visible. Space weathering models of
lunar materials attribute darkening and reddening to
the optical influence of fine-grained iron particles 10's
of nm in size [1]. Iron particles larger than 1µm will
cause material to darken and not produce reddening
[18]. The initial reddening in cronstedite is likely a
result of the rapid production of small nm-sized particles. We postulate two possibilities for the spectral
blueing that occurs after the first 10 minutes of laser
irradiation. The first possibility is the production of
iron particles greater than 1µm dominating the spectral
properties of the sample beyond 10 minutes. The second possibility is the creation of a spectrally neutral
phase (e.g. carbon from the breakdown of siderite).
Both of these possibilities need to be examined with
radiative transfer modeling and Transmission Electron
Microscopy (TEM).
Lizardite shows a small degree of darkening in the
visible and no darkening in the NIR. The resulting
spectra become redder as a function of laser space
weathering. We interpret these spectral trends to be
consistent with the production of fine grained iron particles like those seen in olivine and lunar soils.
Conclusions: Our cronstedtite and lizardite samples yield different spectral responses to space weathering. The correlation of these two phyllosilicates with
petrologic type suggests that both spectral reddening
and blueing should be expected in the C-type population, as also observed by [19]. Further, the spectral
response of C-type asteroid families to space weathering may be used to reveal the degree of aqueous alteration the parent body has experienced. Future work is
necessary to understand the relative contribution of
cronstedtite and serpentine to space weathering of carbonaceous materials (e.g., TEM and radiative transfer
modeling). Additional lab experiments are needed to
characterize the spectral behavior of other aqueous
alteration products (e.g. carbon, carbonates, organics
and oxides).
References: [1] Hapke B. (2001) JGR, 106, 1003910074. [2] Chapman C. R. (2004) Annual Review of
Earth and Planetary Science, 32, 539-567.
[3] Nesvorny D. et al. (2005) Icarus, 173, 132-152. [4]
Lantz C. et al. (2013) A&A, 554, A138. [5] Lazzarin
M. et al. (2006) ApJ, 647, L179-182. [6] Moroz L. V.
et al. (1996) Icarus, 122, 366-382. [7] Moroz L. V. et
al. (2004) LPS XXXV, Abstract #1279. [8] Shingareva
T. V. et al. (2004) LPS XXXV, Abstract #1137. [9] Hi-
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Hiroi T. et al. (2004) LPS XXXV, Abstract #1616. [11]
Moroz L. et al. (2004) Icarus, 170, 214-228. [12] Yamada M. et al. (1999) Earth, Planets and Space, 51,
1255. [13] Rivkin A. S. et al. (2002) Asteroids III, 235253. [14] Howard K. T. et al. (2009) Geochim. Cosmochim. Acta, 73, 4576-4589. [15] Howard K. T. et al.
(2011) Geochim. Cosmochim. Acta, 75, 2735-2751.
[16] Cloutis E. A. et al. (2011) Icarus, 216, 309-346.
[17] Fornasier S. et al. (2014) Icarus, 233, 163-178.
[18] Lucey P. G. and Riner M. A. (2011) Icarus, 212,
451-462. [19] Gillis-Davis, J. J. (2014) this LPSC.
Acknowledgements: We would like to say mahalo nui to Karen Meech, Bin Yang, Patrick Gasda,
Jeff Taylor, Eric Hellebrand, and Matt Markley for
their help and support of this work. This material is
based upon work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement No.
NNA04CC08A issued through the Office of Space
Science, by NASA Grant No. NNX07A044G.
a)
b)
Figure 1: a) Reflectance spectra of cronstedtite
(blue), lizardite (green) and olivine (red) b) Reflectance spectra normalized to 0.55µm. The data
follow the same color scheme as a).