Geologic history of water on mars

Geologic history of water on mars

J.W. RICE JR.

Mars Spaceflight Facility and Geosciences Department, Arizona

State University, USA ([email protected])

New data from spacecraft currently operating both on the sur-face and in orbit are revealing a very rich and complex history ofwater on Mars. Morphologic and remote sensing evidence fromthese missions indicates that volcanic, fluvial, lacustrine, glacialand aeolian processes have operated throughout Martian geologictime. However, the intensity of these processes appears to havedecreased 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 orbitnumerous deltas and alluvial fan complexes have been locatedat the mouths of channels. Evidence of persistent flow, multipleflow events, and long term paleoenvironmental change are foundin 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 sinceJanuary 2004. The discovery and analysis of bedrock outcrops bythe Opportunity Rover in Meridiani Planum has revealed thatliquid water played a major role in the formation and alterationof this sedimentary rock unit. This sulphate-rich rock is composedof numerous thinly laminated layers (several mm thick) with smallhematitic spherules (up to 6 mm diameter) randomly scatteredthroughout 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 pastlacustrine history of Gusev crater. To date groundwater appearsto have been the dominant type of aqueous activity in Gusevcrater.

Current results from the Mars Exploration Rovers, MarsGlobal Surveyor, and Mars Odyssey missions will be reportedon in this presentation.

doi:10.1016/j.gca.2006.06.978

Antiquity of harzburgitic diamonds fromthe Venetia kimberlite, Limpopo Belt,

Kaapvaal craton

S.H. RICHARDSON1, J.W. HARRIS

2, P.F. POML3

1Geological Sciences, University of Cape Town, Rondebosch

7701, South Africa ([email protected])2Geographical and Earth Sciences, University of Glasgow,

Glasgow G12 8QQ, UK ([email protected])3Institut fur Mineralogie, Westfalische Wilhelms-Universitat,

48149 Munster, Germany ([email protected])

The Sm/Nd and Rb/Sr isotope systematics of harzburgitic gar-net macrocrysts and inclusions in diamonds from the Venetiakimberlite have been investigated to determine their age and ori-gin. The 520 Ma Venetia kimberlite is located in the central zoneof the Limpopo Belt, considered to have been amalgamated withthe 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 Crcontents and represent the disaggregated mantle host rocks ofthe diamonds. Eight macrocrysts were prepared as interior splitsof 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/blankratios P103. All the garnets have negligible Rb.

The inclusion garnets have high Sr and Nd concentrations andlow Sm/Nd inversely correlated with Ca. Three out of the foursamples give an isochron age of 2.30 ± 0.04 Ga and a well-definedunradiogenic initial (eNd = �8). However, caution is requiredsince initial 87Sr/86Sr 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 initialNd composition requires a >3 Ga precursor, as observed forlherzolitic diamonds elsewhere.

The macrocryst garnets also show low Sm/Nd but much moreradiogenic Sr (60.720). Again, their least radiogenic Nd requiresan 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 inmodified Archean lithosphere following stabilization of the Limp-opo Belt within the Kaapvaal-Zimbabwe craton. The Venetiamacrocryst/inclusion garnets are strikingly similar to counterpartsfrom the 370 Ma Udachnaya kimberlite, Siberian craton, whereboth Archean and Proterozoic generations of harzburgitic tolherzolitic diamonds have been identified.

ReferencesEglington, 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