A SYSTEMATIC CLASSIFICATION OF METEORITES


MARTIAN METEORITES

A Geochemical Classification
(based in part on Mg# as an indicator for extent of evolution, and initial ε143Nd value as a measure of enrichment/depletion)

SHERGOTTITES (3 subclasses based on REE patterns and Sr–Nd systematics)
1. ENRICHED SUITE–initial ε143Nd value of ~–7; shallowest source region (variable compositions)
   a. more evolved (e.g. Los Angeles [Mg# 23, εNd ~–7], NWA 856 [Mg# 42], NWA 2800, Dhofar 378, NWA 2975/2986)
   b. moderately evolved (e.g. Zagami [Mg# 52, εNd ~–6], Shergotty [Mg# 46, εNd ~–8], NWA 3171, NWA 5298)
   c. less evolved (or olivine accumulation) (e.g. NWA 1068 and pairings [Mg# 59, εNd ~–7], NWA 4468 [Mg# 64, εNd ~–6.9], RBT 04261/2 [Mg# 65, εNd ~–7], LAR 06319 [εNd ~–7], NWA 5029)
2. INTERMEDIATE SUITE (Bulk Mars)–initial ε143Nd value of +8 to +17; (variable compositions)
   a. more evolved (e.g. EETA79001B [Mg# 43, εNd ~+16], NWA 2646, NWA 4480)
   b. moderately evolved (e.g. NWA 480/1460 [Mg# 48, εNd ~+9], EETA79001A [Mg# 58, εNd ~+17], NWA 6234/2990/5960/6710 [Mg# 59 near primary mantle melt], NWA 4797)
   c. less evolved (e.g. ALHA77005 [Mg# 71 near primary mantle melt, εNd ~+10], LEW 88516 [Mg# 72, εNd ~+10], Y-793605 [Mg# 71, εNd ~+10])
3. DEPLETED SUITE–initial ε143Nd value of +36 to +40; deepest source region (variable compositions)
   a. more evolved (e.g. QUE 94201 [Mg# 38 after 43% fractional crystallization of Y98-like magma, εNd ~+50], NWA 5990 [εNd ~+55.13])
   b. moderately evolved (e.g. Dho 019 [Mg# 58, εNd ~+40], NWA 4925)
   c. less evolved (e.g. Y-980459 (ol-websterite) [Mg# 67 near primary mantle melt], NWA 5789 [Mg# 67 near primary mantle melt; εNd +51.89], SaU 005/008/094 [εNd ~+40], DaG 476 and pairings [Mg# 68, εNd ~+40], NWA 1195, NWA 2046, NWA 2626)

According to Papike et al. (2009), both depleted and enriched material were formed early in the planet's history as a result of magma ocean crystallization. Although complex two- and three-stage petrogenetic models have been proposed, martian lithologies can be resolved into four basic reservoirs based on geochemical, isotopic, and mineralogical characteristics. These reservoirs represent the shergottites and lherzolites through contrasting models: one involving mantle magma assimilation of crustal material, or perhaps more plausible, one involving the mixing of multiple mantle magma reservoirs linked by fractional crystallization. These four basic reservoirs also represent the nakhlites, conceivably originating from a deep reservoir, and the single known orthopyroxenite ALH 84001, conceivably originating from a shallow reservoir. These four basic reservoirs are distinguished in the following table:

Comparison of Four Silicate Reservoirs on Mars (Wadhwa and Borg, 2006; Papike et al., 2009)
  Trace Elements Redox 147Sm/144Nd ε182W ε142Nd 180Hf/183W
Depleted Mantle
Reservoir 1		 LREE Depleted
(shergottites Y-980459,
QUE 94201, DaG 476, SaU 005)		 reduced
(IW–IW+1)		 ≥0.285 ≥0.6 ≥0.9 ≥18
Depleted Mantle
Reservoir 2		 LREE Depleted
(nakhlites)		 oxidized
(≥IW+3.5)		 ~0.255–0.266 ~2.95 ~0.74 ~22–43
Depleted Mantle
Reservoir 3		 LREE Undepleted
(orthopyroxenite)		 reduced
(IW–IW+1)		 ~0.214 ~0.49 ~0.19 ~19
Enriched Mantle
Reservoir		 LREE Enriched
(shergottites Shergotty,
Zagami, LA, NWA 1068)		 oxidized
(>IW+2)		 <0.182 ≤0.3 ≤–0.2 ≤11

A new hybridized model developed by Borg and Draper (2003) suggests that the martian magma ocean crystallized and produced a depleted mantle (45% opx, 38% ol, 14% cpx, and 3% majoritic garnet) plus an enriched, trapped, late-stage liquid (after 99.5% crystallization of magma ocean). This stage was followed by cumulate melting which generated partial melts compositionally similar to the most primitive martian meteorites. The other meteorites were generated by fractionation of olivine and orthopyroxene to form parental melts in initial conditions of high pressure (≥12 GPa), superchondritic CaO/AlO ratio, high Mg# (~80), and an FeO component of ~13.5 wt%. The late-stage liquid was trapped in the cumulate pile after 98–99.5% crystallization, representing a component analogous to lunar KREEP. Basaltic martian meteorites were derived from the melting of mixtures of cumulates and late-stage liquids that crystallized ~4.5 b.y. ago.

The above hybridized model was refined by Lapen et al. (2010) to describe the mantle source of shergottites as well as the orthopyroxenite ALH 84001. Although their model generally agrees with that of Borg and Draper (2003) in which the various martian lithologies were produced from variable mixtures of depleted cumulate material and enriched, trapped, residual liquids, improved age and isotopic data indicate that residual liquid remaining after ~93–98% crystallization of the magma ocean was not part of the mixing array that produced enriched shergottites. The average mixture of shergottites was determined to be 94% cumulates to 6% trapped residual liquids. The depth of the mantle reservoir consistent with partial melting characteristics and observed incompatible element abundances was calculated to be 250–400 km. Thus far the team recognizes two distinct mantle reservoirs: that in which the shergottites and ALH 84001 formed, and that in which the nakhlites formed.

Classification schemes and data adapted from the following sources:

Borg and Draper, 2003
Warren and Bridges, 2005
Irving et al., 2007
Symes et al., 2008
Bunch et al., 2008
Irving and Kuehner, 2008
Shih et al., 2009
Rumble III and Irving, 2009
Papike et al., 2009
Irving et al., 2010
Lapen et al., 2010

Further information on this shergottite classification scheme can be found on the NWA 1195 page.

A more in-depth treatment of the classification of martian meteorites can be found on Dr. Tony Irving's webpage, University of Washington at Seattle.



© 1997–2012 by David Weir