ion irradiation experiments on the murchison cm2 - USRA

46th Lunar and Planetary Science Conference (2015)
A. Dukes3, R. A. Baragiola3, and Z. Rahman2. 1ARES, Code XI3, NASA/JSC, Houston, TX 77058
([email protected]). 2Jacobs, NASA/JSC, Code XI, Houston, TX, 77058. 3Laboratory for Atomic and
Surface Physics, University of Virginia, Charlottesville, VA 22904.
irradiated material (Fig. 1) however, no differences
were observed in the 10 m silicate feature. Many irradiation studies of silicates show the preferential loss
of oxygen from irradiated surfaces (e.g., [3]), but
based on the FTIR results, at least some of the oxygen
loss from irradiated Murchison matrix is in the form of
Remote sensing observations
show that space weathering processes affect all airless
bodies in the Solar System to some degree. Sample
analyses and lab experiments provide insights into the
chemical, spectroscopic and mineralogic effects of
space weathering and aid in the interpretation of remote-sensing data. For example, analyses of particles
returned from the S-type asteroid Itokawa by the
Hayabusa mission revealed that space-weathering on
that body was dominated by interactions with the solar
wind acting on LL ordinary chondrite-like materials
[1, 2]. Understanding and predicting how the surface
regoliths of primitive carbonaceous asteroids respond
to space weathering processes is important for future
sample return missions (Hayabusa 2 and OSIRIS-REx)
that are targeting objects of this type. Here, we report
the results of our preliminary ion irradiation experiments on a hydrated carbonaceous chondrite with emphasis on microstructural and infrared spectral changes.
Samples and Methods. A polished thin section
of the Murchison CM2 carbonaceous chondrite was
irradiated with 4 kV He+ (normal incidence) to a total
dose of 1x1018 He+/cm2 over an area of ~5x5 mm2.
The irradiated area included abundant matrix and
chondrules. We obtained ex situ Fourier-transform
infrared (FTIR) reflectance spectra from multiple areas
of matrix, ~150 m2 in size, using a Hyperion microscope on a Vertex Bruker FTIR bench. A JEOL 7600F
field emission scanning electron microscope (SEM)
was used to study the morphological effects of the
irradiation. Following the SEM analyses, we extracted
thin sections from both irradiated and unirradiated
regions in matrix using focused ion beam (FIB) techniques. We used electron beam deposition for the protective carbon strap to minimize surface damage artifacts from the FIB milling. The FIB sections were analyzed using the JEOL 2500SE scanning and transmission electon microscope (STEM).
Results and Discussion. Optical examination
showed that the irradiated area was visibly darker than
the un-irradiated parts of the thin section. FTIR reflectance spectra were collected from irradiated and unirradiated regions of fine-grained matrix. The irradiated matrix showed lower reflectance in the near-IR and
a red-sloped continuum compared to the un-irradiated
matrix spectra. The depth of the 3 m feature is decreased in the irradiated regions relative to un-
Wavelength (m)
Figure 1. FTIR spectra from irradiated (red) and
unirradiated matrix regions of matrix. The spectra
have been normalized to the 10 m silicate feature.
SEM imaging shows that the irradiated matrix regions have a “bubbly” or “frothy” texture, with numerous sub-m rounded holes and voids relative to the
un-irradiated material (Fig. 2). TEM analysis of the
FIB sections show that the frothy texture in the irradiated matrix results from the formation of irregularyshaped 50-100 nm voids at the sample surface. In addition, there are smaller (20-50 nm dia.) vesicles in
some of the surface exposed grains.
High-resolution imaging shows that the phyllosilicates (mainly serpentine group minerals) have been
rendered amorphous from the irradiation to a depth of
~150-200 nm. Assuming a target density of ~1.3 (the
density of serpentine with 50% porosity), and allowing
46th Lunar and Planetary Science Conference (2015)
for reasonable changes in target density during the
irradiation, there is excellent agreement between the
total thickness of the amorphized layer and the He+ ion
damage depth obtained from SRIM calculations [4].
appears to have been amorphized by the He+ irradiation.
Conclusions. Irradiation of Murchison matrix
with 4 keV He+ produced several results including: the
amorphization of the phyllosilicates to a depth of ~200
nm, blistering and void development, a loss of OH
from the hydrated silicates, and the formation of
nanophase Fe-rich inclusions within the amorphized
phyllosilicates. Follow on analyses will focus on the
spectral changes in the VIS-NIR spectral region.
References. [1] Noguchi, T. et al. (2014) MAPS,
49, 188-214. [2] Keller, L. P. and Berger, E. L. (2014)
EPS, 66, 71-80. [3] Loeffler, M. J. et al. (2009) JGR,
114, E03003. [4] Ziegler, J.F. et al. (2006) Stopping
and Range of Ions in Matter [5] Keller,
L. P. et al. (2013) LPSC XLIV, #2404.
Acknowledgements. This work was supported by the
NASA Cosmochemsitry Program. We thank the
USNM for the Murchison chip used in this study.
Figure 2. SEM images of typical fine-grained
matrix regions in the un-irradiated (top) and irradiated (bottom) areas of the Murchison sample.
Note the “frothy” texture in the irradiated region
from implantation effects.
Dark-field STEM imaging reveals abundant
nanophase (2-5 nm) inclusions in the amorphized
phyllosilicates (Fig. 3). The nanophase grains are Ferich, and analyses are underway to determine their
exact mineralogy and oxidation state. Minor Fe-Ni and
S-bearing nanophase inclusions occur throughout
Murchison matrix. Larger (m-sized) FeNi sulfides
exposed at the surface show a preferential loss of sulfur by sputtering and the development of a thin 5-10
nm rim of nanophase Fe metal, similar to experimentally irradiated FeS [5]. A sub-m CaCO3 grain also
Figure 3. Dark-field STEM image of the amorphized surface of Murchison matrix. The dark
regions are irregularly shaped voids. The bright
specks indicate individual 2-5 nm Fe-rich particles
in the amorphized phyllosilicate. The white
dashed line indicates the uppermost surface of the