Dissolution possibly assisted by the presence of organic material is evident at the glass-gel palagonite interfaces. In the pore layers, varying cation
concentrations seem to relate to ion mobility, with Ti cations precipitating in the bottom gel layers, followed by Fe cations and finally Mg2+ in the top
smectite layer. In the nakhlites, the composition of the gel layers may have had some dependence on local transport of cations from surrounding
host grains which may explain the Fe enrichment at the gel-phyllosilicate Lafayette interface.
Some putative organic material has been identified in the nakhlites [8]. cavities at the
silicate-gel interfaces where aqueous and OM based fluid may have filled in these
locations could also be localities in the meteorites to find organic material.
H. G. Changela1, C. C. Cockell2, C. Bryce2 and A. J. Brearley1 1 University of New Mexico, Dept. Earth & Planetary Sciences,
Albuquerque, NM87131, USA. 2 UK Centre for Astrobiology, University of Edinburgh, Edinburgh, EH9 3JZ UK. (Email:[email protected])
SEM-TEM Study of Icelandic Palagonite: Application to Hydrated Silicate Gel
Interfaces in the Nakhlite Meteorites and Secondary Processes on Mars
References:[1] Stronik N. A. & Schmincke H. (2002) Int J. Earth Sci (Geol
Rundsch) 91,680–697 [2] Changela and Bridges (2010) MaPs. 45(12), 1847-
1867. [3] Trieman (2005) Chemie der Erde 65 203–270. [4] Ming-Shan Sun
(1957) American Mineralogist, 42, 525-533. [5] Cockell et al. (2009)
Geomicrobiology J. 26(7) 491-507. [6] Benzerara K. et al. (2007) EPSL 260(1-
2): 187-200. [7] Changela H.G. & Bridges J. C. (2010) 73rd MetSoc #5300.
[8] White L. M et al. (2014) Astrobiology 14(2):170-181.
300 µm
Palagonite is a collective term for altered volcanic glass. Rapidly quenched
volcanic glasses in a variety of environments ranging from hydrothermal vents,
sub marine and sub glacial environments can form palagonite [1]. Palagonite
typically contains an assemblage of primary basaltic glass that has been altered
into amorphous hydrated silicate ‘gel-palagonite’ and phyllosilicate (usually
smectite) [1]. Although the nakhlite martian meteorites and their secondary
alteration assemblages may have very different petrogeneses to palagonite, the
types of alteration within them, namely the hydrated silicate assemblages (the
most abundant form of alteration in the nakhlites), are also found in palagonite.
Of particular interest in the nakhlites is the zoned olivine fracture filling material
which dominates them - an Fe-Mg-Al hydrated silicate lacking any detectable
crystalline structure by electron diffraction and deemed as a gel [2]. A collective
term, iddingsite, has been used to describe these olivine based alteration
assemblages [3]. As well as ‘iddingsite’ based terrestrial rocks [4], palagonite
may also provide insights into the formation mechanisms of the structurally
complex, hydrated and perhaps metastable hydrated silicate assemblages in
the nakhlites. The interaction of organic material (OM) with these hydrated
assemblages by possible biotic and abiotic processes could also be
investigated, with palagonite shown to have strong affinities with them on Earth
[5,6]. We have been using FIB/SEM-TEM analysis to explore distinctive features
in Icelandic palagonite and at silicate-gel interfaces.
Pores
Basaltic Glass
Gel-palagonite
500 µm
Alteration Layers
50 µm
Glass
0.5 µm
Gel
Phyl
Phyl
0.9 nm
Phyl
1 µm
Phyl Gel
Gel
Fe Kα Mg Kα Ti Kα
Ti-Rich
Fe-Rich
Mg-Rich
Mg + Fe-Rich
Oli
Glass Interface
Left: TEM of extracted altered pore marked in the BSE image of
the pore above (marked a.). DF STEM montage. A 1-2 µm top
phyllosilicate (smectite) layer in pore transitions to the gel layers
which are Fe rich-Ti rich. Bright bands are rich in the high Z (Fe and
Ti) cations. Below the montage is A STEM-EDX map of the region
in red dashed rectangle in the montage. SAD of gel and
phyllosilicate regions show the difference in structure between
them. Dark blebs of carbonaceous material under are arrowed. Right: A phosphate inclusion in the gel-phyllosilicate region (blue
rectangle from montage) is margined with carbonaceous material. left: SEM-EDX of the primary
glass (blue spectra). STEM-
EDX of the layer closest to
the glass interface in the pore
(region 1) the smectite top
layer (region 2) from the
STEM DF montage top left.
Note the depleted Mg at the
interface and the presence of
Ti. Also note the higher
carbon peak in this texturally
more heterogeneous layer
than the smooth gel layers.
BSE Images of Icelandic Palagonite.
Above: Low Mag image showing the
generic phases. Bright region is
sideromelane - primary glass. Darker
material is gel-palagonite. Left: An altered
pore lined with the alteration zones in gel-
palagonite. Note the inset which shows the
layers of alteration that were sectioned
through with the FIB in the pores.
Left & Below: Extraction across
the gel--glass interface attached to
the altered pore (marked b.) The
interface as shown by TEM below
is filled with carbonaceous
material which feeds into the gel
layer. Note the textural similarity
between the heterogeneous,
broken down gel layer and the
bottom layer of the extracted
pores at their glass interface.
Gel-glass interface
Extracted pore
Nakhlites: BF TEM
image of olivine
fracturevphyllosilica
te-gel assemblage
in the Lafayette
nakhlite [7]. Dark
regions in the gel
are high Z, Fe rich
layers. Similar 2:1
phyllosilicatee(sme
ctite) fringes were
found in palagonite.
The phyllosilicate-gel assemblages differ by containing Fe enriched bands between
the gel-phyllosilicate interface rather than a gradual Mg enrichment in the palagonite.
1
2
1 2 3 4 5 6 7 80
2
4
6
8
10
12
14
16
Energy (eV)
STEM-EDX GEL SEM-EDX GLASS
Bright Field TEM
100 µm 1 µm
15 µm
Pore
Gel-Palagonite Glass
Gel
Gel
Phyl.
Glass Interface
0 1 2 3 4 5 6 70
100
200
300
400
Energy (eV)
1
2
Counts
(10
3)
Counts
Fe Kα
Ti Kα Ca Kα
Si Kα
Al Kα
Mg Kα
Na
O Kα
Fe Kα
Ca Kα
Ti Kα
Si Kα
Al
Mg C
TEM-EDX Gel-Palagonite Layers
a.
a.
b.
Glass Gel
Pore
Na
500 nm
b.
Gel
Glass
Car
bo
nac
eo
us
Mat
eria
l
1 µm
Carbonaceous
b. Pt Cap
Pt Cap
BSE STEM Dark Field
Gel
Carbon
