Redox equilibria and Martian Redox equilibria and Martian basaltic meteoritesbasaltic meteorites

Martian basaltic meteoritesMartian basaltic meteorites

• ≥≥31 pieces of igneous rocks with 31 pieces of igneous rocks with crystallization ages of 150-180 Ma,crystallization ages of 150-180 Ma, 1.3 Ga, and 1.3 Ga, and >>4.3 Ga4.3 Ga

• “ “Left” Mars 0.5Left” Mars 0.5 -- 20 Ma ago20 Ma ago

Martian meteoriteMartian meteorite

• Shergottites:Shergottites: microgabbros microgabbros with and with and without Ol phenocrysts or xenocrysts without Ol phenocrysts or xenocrysts (bulk mg# = 0.24-0.60) and lherzolites (bulk mg# = 0.24-0.60) and lherzolites (bulk mg# = 0.68-0.75).(bulk mg# = 0.68-0.75).

• Nakhlites:Nakhlites: Pyroxenites (bulk mg# = Pyroxenites (bulk mg# = 0.50-0.72).0.50-0.72).

• Chassignites:Chassignites: Dunite Dunite (bulk mg# = (bulk mg# = 0.68).0.68).

In situ observations & Martian In situ observations & Martian meteoritesmeteorites

From McSween et al (Science) 2004From McSween et al (Science) 2004

MgO (wt% )

0 20 40 60 80 100

FeOT (wt% )

0

20

40

60

80

100

Na2O + K2O (wt% )

0

20

40

60

80

100

Pyroxenite (4.5 Ma)Pyroxenites & Dunite (1.3 Ga)Microgabbros (330-575 Ma)Microgabbros (age unknown)Lherzolites (180 Ma)Microgabbros (175 Ma)Model mantle compositions

Mars Pathfinder rocksThingmuli (Iceland)Santorini (Aegean Arc)

Fe

Al

Ti

Ca

Martian meteorites:Martian meteorites: Oxides & Oxides & silicatessilicates

from Wittke et al (2006), LPSC 37th, NWA 2975

Martian meteorites: Oxides & silicatesMartian meteorites: Oxides & silicates

Martian meteorites and oxygen Martian meteorites and oxygen fugacityfugacity

• Wadhwa (Science, 2001): basalts oxidize through assimilation of an altered by aqueous fluids and incompatible-element enriched crust

• Herd et al. (GCA 2002) modeled the variation in f O2 as mixing between mantle-like and crust-like reservoirs

• Herd (M&PS 2003) argued that fractional crystallization of a magma ocean would result in a mantle comprised of a reduced, cumulate portion, and an oxidized, trapped-liquid portion – the long-term incompatible-element depleted and enriched components, respectively. In this model, the f O2 of a martian basalt is a function of the relative involvement of the two components during partial melting of mantle sources.

Martian meteorites and oxygen Martian meteorites and oxygen fugacityfugacity

• Phase equilibria & Phase equilibria & f f OO22 variations variations (internal buffering): (internal buffering):

– Oxide-silicate reactions may result in Oxide-silicate reactions may result in relative oxidation during coolingrelative oxidation during cooling

– Fe metal-bearing rocks may exhibit Fe metal-bearing rocks may exhibit relative reduction during coolingrelative reduction during cooling

T (oC)

600 700 800 900 1000 1100

logfO

2FM

Q

-4

-2

0

2

2OxidesOl-Px-Spl2Px + 2OxidesFa-2Ox-Silica-Fe metal

Zagami

ShergottyQUE94201

NWA 1110

Los Angeles

ObservationsObservations

• Ti oxide pairs record “snapshots” of the evolution Ti oxide pairs record “snapshots” of the evolution of of f f OO22 with respect to T with respect to T

• If modeled properly, oxide-silicate equilibria will If modeled properly, oxide-silicate equilibria will give a more dynamic view of the evolution of give a more dynamic view of the evolution of f f OO22 with respect to T, and in this case they appear to with respect to T, and in this case they appear to record relative oxidation with coolingrecord relative oxidation with cooling

• In contrast, in Fe metal-bearing lunar basalts the In contrast, in Fe metal-bearing lunar basalts the equilibria among Fe metal, oxides, and silicates equilibria among Fe metal, oxides, and silicates suggest relative reduction with coolingsuggest relative reduction with cooling

ConclusionsConclusions

• Oxygen fugacity in Martian basaltic Oxygen fugacity in Martian basaltic meteorites is buffered internally meteorites is buffered internally

• The observed range and variations The observed range and variations likely result from phase equilibria likely result from phase equilibria control and not mixing of reduced control and not mixing of reduced and oxidized components and oxidized components