[meteorite-list] Evidence of sedimentary activity in SNCs - Part 1 of 2

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:51 2004
Message-ID: <3C50666D.F56B717C_at_lehrer1.rz.uni-karlsruhe.de>

"E.P. Grondine" wrote:

> is there any sign at all of any sustained hydrological
> action? Any banding at all? Any other sign?

Hello EP and List,

Here is something that may pique your interest:

Best regs,

Bernd

Meteoritics 33-4, 1998, A023-A024:

Traces Of Martian Sediment In Nakhla And Other SNC Meteorites

J.C. Bridges and M.M. Grady, Department of Mineralogy, Natural History
Museum, London SW7 5BD, UK

Introduction:

We report some results of a search in Nakhla, Lafayette, and EETA 79001
for mineralogical and trace-element evidence of trapped martian
sediment. The origin of these meteorites' parent rocks on or near the
martian surface means it is likely that traces of sediments were
incorporated into them. Salt pan deposits formed by evaporative
concentration of ground or standing water are thought to exist on Mars
[1]. Such evaporates would contain halite, anhydrite, and carbonate.
These could be incorporated with igneous rocks by three mechanisms:

(1) melting and incomplete mixing with crystallizing lava [2];

(2) hydrothermal remobilization [3]; and

(3) entrainment during shock melting [4].

The addition of sedimentary components, which had been in exchange with
the martian atmosphere, can help explain isotopic signatures such as the
high d13C values in carbonate from SNCs [5]. The first mechanism is seen
in Nakhla, the second is most clearly displayed in nakhlites [3], and
the third is seen in EETA 79001 [4].

Nakhla:

A halite-siderite-anhydrite assemblage has textures which suggest it
crystallized at 800°-1000°C from an ionic melt [2]. It is associated
with interstitial silicate rather than being spread throughout the rock
in cracks or voids. In some areas, halite has crystallized around grains
of plagioclase and silica [2], whereas in others, the salt melt has
forced open cracks leading from interstitial silicate into the margins
of surrounding olivine and augite grains. Neither texture is consistent
with deposition from low-temperature hydrous fluids such as those
associated with the clay veins. Siderite can be stabilized at high
temperatures by Na and Cl. The trapped salt melt component contains 97%
halite, 2% siderite, 1% anhydrite, and traces of chlorapatite.
Contrasting trace-element abundances between siderite in Nakhla and
terrestrial hydrothermal siderite (Fig. 1) provide further evidence that
the former crystallized at high temperature. Variation in REE abundances
between Nakhla siderite grains is due to extraction of REE-free halite
from ionic-melt fractions.

Clay in Nakhla and Lafayette:

EDS analyses of smectite/illite veins show they contain ? 3.7 wt% Cl in
Nakhla and ? 0.2 wt% in Lafayette. No halite has been found associated
with Nakhla clay in this or other studies [3]. A late generation of
sulphate and calcite is associated with it [3].
Halite has not yet been found in Lafayette. This, together with
relatively low Cl abundances in the clay, suggests that the Lafayette
parent rock originated in a less halite-rich area of the martian surface
than Nakhla.

Elephant Moraine 79001:

Some sulfates and Ca carbonate were derived by assimiliation into the
parent during shock-induced melting at ? 1100°C [4]. We are studying new
sections in an attempt to provide information on the relationship
between martian sediments and this assemblage.

References:

[1] Forsythe R.D., Zimbelman J.R. (1995) JGR 100, pp. 5553-5563.
[2] Bridges J.C. and Grady M.M. (1998) LPS XXIX pp. 1399-1400.
[3] Gooding J.L. et al. (1991) Meteoritics 26, pp. 135-144.
[4] Gooding J.L. et al. (1997) GCA 52, pp. 909-916.
[5] Wright I.P. et al. (1992) GCA 56, pp. 817-826.
Received on Thu 24 Jan 2002 02:54:21 PM PST