1. Geologic history of water on mars
J.W. RICE JR.
Mars Spaceflight Facility and Geosciences Department, Arizona
State University, USA (jrice@asu.edu)
New data from spacecraft currently operating both on the sur-
face and in orbit are revealing a very rich and complex history of
water on Mars. Morphologic and remote sensing evidence from
these missions indicates that volcanic, fluvial, lacustrine, glacial
and aeolian processes have operated throughout Martian geologic
time. However, the intensity of these processes appears to have
decreased with time. Nevertheless, the most recent lava flows,
channels and gullies are dated, by crater size-frequency relation-
ships, to be between 1 and 20 million years old. From orbit
numerous deltas and alluvial fan complexes have been located
at the mouths of channels. Evidence of persistent flow, multiple
flow events, and long term paleoenvironmental change are found
in the observations of terraces, meanders, braided channels,
degree of highlands dissection, and inverted channels.
The Mars Exploration Rover Missions (Spirit and Opportuni-
ty) have been analyzing Gusev crater and Meridiani Planum since
January 2004. The discovery and analysis of bedrock outcrops by
the Opportunity Rover in Meridiani Planum has revealed that
liquid water played a major role in the formation and alteration
of this sedimentary rock unit. This sulphate-rich rock is composed
of numerous thinly laminated layers (several mm thick) with small
hematitic spherules (up to 6 mm diameter) randomly scattered
throughout the rock. Numerous sedimentary bedforms and struc-
tures also reveal that water played a major role in its deposition.
To date no conclusive evidence has been found regarding a past
lacustrine history of Gusev crater. To date groundwater appears
to have been the dominant type of aqueous activity in Gusev
crater.
Current results from the Mars Exploration Rovers, Mars
Global Surveyor, and Mars Odyssey missions will be reported
on in this presentation.
doi:10.1016/j.gca.2006.06.978
Antiquity of harzburgitic diamonds from
the Venetia kimberlite, Limpopo Belt,
Kaapvaal craton
S.H. RICHARDSON
1
, J.W. HARRIS
2
, P.F. PO¨ ML
3
1
Geological Sciences, University of Cape Town, Rondebosch
7701, South Africa (shr@geology.uct.ac.za)
2
Geographical and Earth Sciences, University of Glasgow,
Glasgow G12 8QQ, UK (J.Harris@ges.gla.ac.uk)
3
Institut fu¨r Mineralogie, Westfa¨lische Wilhelms-Universita¨t,
48149 Mu¨nster, Germany (philipp@poeml.de)
The Sm/Nd and Rb/Sr isotope systematics of harzburgitic gar-
net macrocrysts and inclusions in diamonds from the Venetia
kimberlite have been investigated to determine their age and ori-
gin. The 520 Ma Venetia kimberlite is located in the central zone
of the Limpopo Belt, considered to have been amalgamated with
the Kaapvaal-Zimbabwe craton by subduction-accretion process-
es some 2.7–2.5 Ga ago (Phillips et al., 1999; Eglington and Arm-
strong, 2004).
The inclusion garnets have low Ca and high Cr contents span-
ning the entire G10 field of cpx-free harzburgitic garnet composi-
tions. The macrocryst garnets have less extreme Ca and Cr
contents and represent the disaggregated mantle host rocks of
the diamonds. Eight macrocrysts were prepared as interior splits
of single grains (1–5 mg) whereas 140 inclusions (av. wt. 40 lg)
were combined into four Ca/Cr groups to obtain enough Nd,
Sm and Sr for total spike ID-TIMS analysis with sample/blank
ratios P103
. All the garnets have negligible Rb.
The inclusion garnets have high Sr and Nd concentrations and
low Sm/Nd inversely correlated with Ca. Three out of the four
samples give an isochron age of 2.30 ± 0.04 Ga and a well-defined
unradiogenic initial (eNd = À8). However, caution is required
since initial 87
Sr/86
Sr is variable and inversely correlated with reci-
procal Sr, suggesting mixing between a low Ca, low Sm/Nd,
radiogenic Sr (60.707) endmember and a higher Ca, higher Sm/
Nd, less radiogenic Sr (<0.705) endmember. Even so, the initial
Nd composition requires a >3 Ga precursor, as observed for
lherzolitic diamonds elsewhere.
The macrocryst garnets also show low Sm/Nd but much more
radiogenic Sr (60.720). Again, their least radiogenic Nd requires
an Archean precursor while their most radiogenic Sr points to iso-
lation of the inclusions by diamond in high Rb/Sr host rocks dur-
ing mantle storage on a billion year timescale.
Hence, our preferred interpretation is that the Venetia harz-
burgitic diamonds crystallized or recrystallized at ca. 2.3 Ga in
modified Archean lithosphere following stabilization of the Limp-
opo Belt within the Kaapvaal-Zimbabwe craton. The Venetia
macrocryst/inclusion garnets are strikingly similar to counterparts
from the 370 Ma Udachnaya kimberlite, Siberian craton, where
both Archean and Proterozoic generations of harzburgitic to
lherzolitic diamonds have been identified.
References
Eglington, B.M., Armstrong, R.A., 2004. SAJG 107, 13–32.
Phillips, D., 1999. In: Proc. 7IKC, 2, pp. 677–688.
doi:10.1016/j.gca.2006.06.979
Goldschmidt Conference Abstracts 2006 A531