Recent detailed mapping has revealed numerous examples of Martian deltas. The location and morphology of these deltas are described. Factors that contribute to delta morphology are river regime, coastal processes, structural stability, and climate. The largest delta systems on Mars are located near the mouths of Maja, Maumee, Vedra, Ma'adim, Kasei, and Brazos Valles. There are also several smaller-scale deltas emplaced near channel mouths situated in Ismenius Lacus, Memnonia, and Arabia. Delta morphology was used to reconstruct type, quantity, and sediment load size transported by the debouching channel systems. Methods initially developed for terrestrial systems were used to gain information on the relationships between Martian delta morphology, river regime, and coastal processes

Rice, J. W., Jr.

Scott, D. H.

English

NASA Technical Reports Server

LPI TechnicaI Report 93-06. Part I 45
Coexisting pigeonite and augite are also found in ~ 12 categories
of lunar mare basalts and well-developed skeletal pigeonite mantled
by augite is found in Apollo 15 and Apollo 12 pigeonite basalts [6].
All 12 of these basalt groups are magnesium rich, ranging in weight
percent MgO from 7.03% to 19.97%, and averaging 10.7% [6]. Like
komatiites, the coexisting pyroxenes in the Apollo 12 pigeonite
basalts are thought to form by rapid metastable crystallization of a
supercooled liquid [7]. Regardless of the specific mechanism that
forms skeletal textures and coexisting pigeonite and augite, these
features are characteristic of highly magnesian lavas like komatiites.
Discussion: Using ISM data, Mustard et al. [I] estimated the
composition of Syrtis Major pyroxenes. According to Mustard et al.,
the pyroxene compositions represented by their ISM analysis falls in
an area of"unusual" composition on the pyroxene quadrilateral, and
therefore the data may represent an average composition of augite
and pigeonite as intimate exsolution lamella. Telescopic reflectance
measurements of several dark regions on Mars are indicative of-
coexisting pyroxenes, which correlates with the modal mineralogy of
Shergotite meteorites, which may have originated from Mars [8].
Comparison with Shergotite meteorites and terrestrial komatiites
suggests that exsolution lamella are not required by the results of[ 1].
If the ISM data represent an average of two pyroxene compositions,
then discrete pyroxenes may be implied. Furthermore, if coexisting
pyroxenes were detected by the ISM on Syrtis Major, then they may
have formed by a mechanism like that previously described for the
crystallization ofpyroxenes in mafic komatiite and Apollo 12 pigeo-
nite basalts.
Figure 1 shows a pyroxene quadrilateral with the Syrtis Major
ISM pyroxene field of [11 superposed over the compositions of
pyroxenes found in a pyroxenitic komatiite flow, the Shergotile
meteorite, and an Apollo 12 pigeonite basalt. The majority of
pyroxenitic komatiite pyroxene analyses do not fall within the ISM
field because their source liquid contained relatively little Fe com-
pared to potential martian lavas [e.g., 3,9]. Many individual pyrox-
erie analyses from the Shergotite and the Apollo 12 pigeonite basalt
do fall within the ISM field. More importantly, the averaged compo-
sition of coexisting pyroxenes in the plotted samples will also fall
within the ISM field. These samples perform better than the komatiite
because the higher Fe content in these rocks allows greater substi-
tution of Fe in the pyroxene structure as Mg is consumed initially.
A crystallization trend is defined that places these pyroxene analy-
(Ng}
En
(Ca)
D_. I lid
o .. , .... %,
___.a_m. __ .............
........... ::: :\
50 i'm
(Fe]
Fig. 1. Pyroxene compositions from Syrtis Major ISMand analogs. ISM field
(rhomboid) from [1 ]: pyroxenitic komatiite data (squares) provided by D. P.
Reyes. [sample DPY- 10,unpublisheddata, Reyes. 1993]:andApollo 12pigeonite
basalt (circles) and Shergotite meteorite (stars) data from [6].
ses within and below the ISM field, as seen in Fig. I. This analysis
supports the possibility of komatiite-type lava on the Syrtis Major
plateau because these coexisting pyroxenes are indicative of magne-
sian komatiite-type lava.
An important caveat to the analysis presented here and to that of
[I ] is that Syrtis Major plateau is composed of a large sand sheet and
dune field, with varying degrees of variable fine-grained dust coat-
ings [1]. The question arises whether the materials observed repre-
sent locally derived or transported materials. Additionally, if local or
distal in origin, certain phases in the Syrtis Major material may have
been preferentially concentrated by eolian activity or chemical weath-
ering. The relatively low spatial resolution of ISM (24 km) makes
these questions difficult to answer.
A thermal emission spectrometer (TES) with a spectral resolution
of 5-10 cm -l, from 5 to 60 pm, and a spatial resohition of 3 km (e.g.
the Mars Observer Thermal Emission Spectrometer [I0]), would
easily reveal the presence and nature of komatiitic lavas on Syrtis
Major as indicated by the evidence discussed. In addition, TES-type
data could be used to study weathering products and to trace sedi-
ment transport paths in an attempt to distinguish primary igneous
materials from reworked sediments. Finally, a TES would reveal if
Syrtis Major lava composition evolves from peridotitic to basaltic
compositions over time as seen in terrestrial komatiites [3].
References: [I] Mustard J. F. et al., (1993) JGR, 98, 3387-
3400. [2] Takahashi E. (1991) JGR, 95, 15941-15954. [3] Amdt
N.T. et al. (1977) J. Pet., 18, 319-369. [4] Cameron W.E. and
Nisbet E. G. (1982) In Komatiites (Arndt N. T. and Nisbet E G.,
eds.), George Allen and Unwin, Boston, 29-50. [5] Campbell I. H.
and Arndt N. T. (1982) Geol. Mag., 119, 605-610. [6] BVSP,
Basaltic Volcanism Study Project (1981) Pergamon, New York,
1286 pp. [7] Klein C. et al. (1971) Proc. LSC 2,d, 265-284.
[8] Straub D. W. et al. ( 1991 )JGR, 96, 18819-18830.19] McGetchin
T. R. and Smyth J. R. (1978) Icarus, 34, 512-536. [ 10] Christensen
P. R. et al. (1992) JGR, 97, 7719- 772'("___N94- 33230
MARTIAN DELTAS: MORPHOLOGY AND DISTRIBU-[ 1 f
TION. J.W.RiceJr) andD.H.Scott-',lDepartmentofGeography,
Arizona State University, Tempe AZ 85283, USA, -_Astrogeology
Branch, U.S. Geological Survey, 2255 N. Gemini Drive, Flagstaff
AZ 86001, USA.
The identification ofdeltas on Mars has been an enigma over the
years for planetary geologists. However, recent detailed mapping
(I :500,000 scale) has revealed numerous examples of martian del-
tas. We will document and describe the location and morphology of
these deltas.
Deltas are alluvial regions composed of sediment deposited in
relatively still water (lakes, bays, seas) at river mouths. Deposition on
deltas occurs when river velocity is reduced upon entering standing
bodies of water. Factors that contribute to delta morphology are river
regime, coastal processes, structural stability, and climate [I].
The largest delta systems on Mars are located near the mouths
of Maja, Maumee, Vedra, Ma'adim, Kasei, and Brazos Valles.
There are also several smaller-scale deltas emplaced near channel
mouths situated in lsmenius Lacus, Memnonia, and Arabia.
Delta morphology will be used to reconstruct type, quantity, and
sediment load size transported by the debouching channel systems
https://ntrs.nasa.gov/search.jsp?R=19940028724 2020-06-16T12:14:22+00:00Z
46 Resu#s from the MSA77 Program
[2,3]. For example, arcuate deltas are deposited by bed load or
mixed load systems that bring large quantities of coarse material to
the coastline as opposed to the elongate (birdfoot) deltas composed
of large quantities of fine sediment deposited by a combination of
suspended load and mixed load rivers. Methods initially developed
for terrestrial systems will be used to gain information on the
relationships between martian delta morphology, river regime, and
coastal processes.
References: [1] Morgan J. P. (1970) SEPM, 15, 31-47.
[2] Axelsson V. (1967) Geogr. Ann., 49, 2-127. [3] Galloway W. E.
(1975) Houston Geol. Soc., 87-98. N94- 33231
CARBONATE FORMJ_TION ON MARS: LATEST EXPER-
r, IMENTS. S.K. Stephens I, D. J. Stevenson 1,G. R. Rossman t , and
L. F. Keyser 2, t Division of Geological and Planetary Sciences, 170-
25, California Institute of Technology, Pasadena CA 91125, USA,
2Earth and Space Sciences Division, 183-901, Jet Propulsion
Laboratory, California Institute of Technology, Pasadena CA 91109,
USA.
Introduction: Laboratory simulations of martian CO 2 storage
address a fundamental question about martian climate history:
Could carbonate formation have reduced CO: pressure from a hypo-
thetical >1 bar to the present 7 mbar in <3-4 b.y.? We address this
problem with experiments and analysis [ 1,2] designed to verify and
improve previous [3,4] kinetic measurements, reaction mechanisms,
and product characterizations. Our theoretical modeling [5,6] has
sought to improve existing models of martian CO 2 history [7-I0],
which generally assume an early CO 2greenhouse atmosphere (ques-
tioned by Kasting [I l l).
Experimental Results: A sensitive manometer monitored the
pressure drop (laD) of CO 2 due to uptake by powdered silicate for
periods of 3 to 100+ days. Initially we focused on monomineralic,
crystalline samples, but more recently we ran basaltic glass (see
Table I). Grinding was done in a tungsten-carbide shattcrbox. Di I
was not treated further, and all other monomineralic samples were
heated at 120°C for ~1 day after sitting in weak acetic acid for 1+
days. Bas was simply heated. Runs were performed at warm (25°C)
and cold (-25°C) temperatures, about 1 bar CO-, pressure, and
specific surface areas of-I m'-/g (-I pm particles). Experiments
included "vapor" (H20 premixed with CO._, for ~1-10 monolayer
coverage), "damp" (H20 pipetted onto powder before soaking in),
and "wet" (ma, o ~ m_mpl e) runs.
BET specific surface areas were determined for Di I and Bas, and
quantitative results are given for experiments using these starting
compositions (Table 2). Pressure drops (Ap) for Di I and Bas show
-apid short-term (~ 1 day) CO, uptake and considerably slower Iong-
:rm AP (Figs. 1 and 2). We know the changes are not due to H20
dsorption since there is always sufficient water to maintain
P,,ap(H20). However, we assume the short-term signal reflects CO 2
adsorption (consistent with previous measurements [7,12] and con-
firmed by desorptlon experiments). Curves for Di2, Ol I, and OI 2
are qualitatively similar to those for Dil, whereas Qtz and Plag
show near-zero short-term AP and very slow long-term signal--
indistinguishable from a leak (<10 tt mol/m:/s).
Discussion: Thermodynamic calculations [131 suggest that
reactions should form carbonates from diopside and olivine, and
possibly plagioclase, at martian Pand T. PD results can be classified
into three groups: diopsides and olivines, quartz and plagioclase,
and basalt. Bas takes up more CO 2 than monominerallic, crystalline
diopside, and olivine, despite the fact that it is probably >50%
feldspar and quartz (i.e., relatively unreactive), suggesting a role for
glass (vs. crystalline mineral).
No long-term AP was expected for Qtz (no cations), although
adsorption was anticipated, while with Plag we expected a possible
long-term signal, and again the absence of an adsorption effect was
puzzling. One explanation involves the progressive polymerization
of silicate tetrahedra: olivine (isolated tetrahedra) and pyroxene
(chains) vs. feldspar and quartz (three-dimensional frameworks).
The structure of olivine and diopside may make it easier for cations
to physically adsorb (and/or chemically bond with) anions.
Also puzzling is the result of an X-ray photoelectron spectros-
copy (XPS) analysis performed on Di2 exposed to similar P(CO 2)
and H20 vapor conditions as the PD experiment with Dil (vapor)
TABLE I. Minerals and rocks used for experimental samples.
Sample Approximate Composition Sample Description Locale
Diopside I CaMgSi20 s Bulk. broken crystals,
[Di I] -5% impurities
Diopside 2 CaM gSi._O,, Green.euhedral crystals.
[Di2] <2% impurities
Olivine 1 Mg _Fe,,SiO_ Bulk,broken crystals.
[OI 1] [Forsterite 98%] -5% impurities
Olivine 2 Mg,_Fe.j:SiO, Translucent green pebbles (crystals).
[OI2] <2% impurities
Quartz SiO 2 Clear, euhedral crystal.
[Qtz] < 1% impurities
Plagioclase Ab__An.7 Clear, euhedral crystal
[Plag] [Ab - NaAISi._Os, < 1% impurities
An - CaAl:Si__Os]
Basalt Tholeiite -98 wl% black glass, -..0.3 wt% dissolved H20
[Bas] [50 wt%, SiO 2. 13% AI20._. -I wt% crystals, 1991 lava flow,
12% FeO. 9% MgO. 11% CaO] quenched in air
Dog Lake. Quebec
Rajastan, India
Gabbs, Nevada
San Carlos, Arizona
Mr. Ida, Arkansas
Ponderosa Mine. Oregon
Kilauea, Hawaii


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