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:changela@unm.edu)

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