Martian Meteorites

Meteorites that are now generally accepted as being samples of Mars were for many years known as SNC meteorites after three individuals were originally identified as being similar to one another yet distinct from other groups [12]. These were Shergotty, Nakhla and Chassigny, each conventionally named after the place nearest to where they were recovered. These and other, similar meteorites are a petrologically diverse group of basalts and ultramafic igneous rocks [13]. Since they lack the chondrules present in many other groups of silicate-rich meteorites they are known as achondrites. There are, however, many different types of achondritic meteorites and it was not until the oxygen isotopic composition of SNC meteorites was measured that their common ancestry was confirmed [14]. A list of presently known Martian meteorites is given in Table 1.1.

Plate 1.1 One of the specimens comprising the Martian meteorite Los Angeles 001, a basaltic Shergottite. The meteorite comprised of two stones, each displaying a well defined fusion crust. They were originally found in the Mojave Desert but only identified in 1999 in Los Angeles. The stone shown has a mass of 245 g and is shown next to a 1 cm square cube for scale. Photograph reproduced courtesy of Ron Baalke 2000.

Plate 1.1 One of the specimens comprising the Martian meteorite Los Angeles 001, a basaltic Shergottite. The meteorite comprised of two stones, each displaying a well defined fusion crust. They were originally found in the Mojave Desert but only identified in 1999 in Los Angeles. The stone shown has a mass of 245 g and is shown next to a 1 cm square cube for scale. Photograph reproduced courtesy of Ron Baalke 2000.

Measurement of the oxygen isotopic composition of silicate minerals in meteorites has played a crucial role in identifying and characterising those belonging to discrete groups. So, for instance, it is possible to tell that one group of meteorites originated on a different parent body to any other (in most cases the parent bodies are different asteroids, but there are also lunar meteorites and, of course, the SNCs).

Table 1.1. Martian meteorites as at 1st June 2003. Samples include 35 specimens of 23 separate meteorites.

Meteorite

Paired

Wt.(g)

Ty pe

Where

Date

ALHA77005

482

LS

Antarctica

1977 (Find)

ALH 84001

1,931

O

Antarctica

1984 (Find)

Chassigny

4,000

C

France

1815 (Fall)

Dar al Gani 476

2,015

BS

Sahara

1998 (Find)

Dar al Gani 489

DaG 476

2,146

BS

Sahara

1997 (Find)

Dar al Gani 670

DaG 476

1,619

BS

Sahara

1999 (Find)

Dar al Gani 735

DaG 476

588

BS

Sahara

1996 (Find)

Dar al Gani 876

DaG 476

6

BS

Sahara

1998 (Find)

Dhofar 019

1,056

BS

Oman

2000 (Find)

Dhofar 378

15

BS

Oman

2000 (Find)

EETA 79001

7,982

BS

Antarctica

1979 (Find)

Governador Valadares

158

N

Brazil

1958 (Find)

GRV9927

9.97

LS

Antarctica

1999 (Find)

Lafayette

800

N

USA (Indiana)

1931 (Find)

LEW 88516

13

LS

Antarctica

1988 (Find)

Los Angeles (LA 001)

698

BS

USA (Mojave Desert)

1999 (Find)

Nakhla

10,000

N

Egypt:

1911 (Fall)

NWA 480

NWA 1110

28

BS

NW Africa

2000 (Find)

NWA 817

104

N

NW Africa

2000 (Find)

NWA 856

NWA 1110

320

BS

NW Africa

2001 (Find)

NWA 1068

654

BS

NW Africa

2001 (Find)

NWA 1110

118

PS

NW Africa

2001 (Find)

QUE 94201

12

BS

Antarctica

1994 (Find)

Sayh al Uhaymir 005

1,344

BS

Oman

1999 (Find)

Sayh al Uhaymir 008

S. al U. 005

8,579

BS

Oman

1999 (Find)

Sayh al Uhaymir 051

S. al U. 005

436

BS

Oman

2000 (Find)

Sayh al Uhaymir 060

S. al U. 005

42

BS

Oman

2001 (Find)

Sayh al Uhaymir 090

S. al U. 005

95

BS

Oman

2002 (Find)

Sayh al Uhaymir 094

S. al U. 005

223

BS

Oman

2000 (Find)

Shergotty

5,000

BS

India

1865 (Fall)

Y-793605

16

LS

Antarctica

1979 (Find)

Y-000593

13,700

N

Antarctica

2000 (Find)

Y-000749

Y-000593

1,300

N

Antarctica

2000 (Find)

Y-A1075

55

LS

Antarctica

2000 (Find)

Zagami

18,100

BS

Nigeria

1962 (Fall)

BS: Shergottite, LS: Lherzolitic Shergottite, PS: Picritic Shergottite C: Chassignite, N: Nakhlite, O: Orthopyroxenite.

BS: Shergottite, LS: Lherzolitic Shergottite, PS: Picritic Shergottite C: Chassignite, N: Nakhlite, O: Orthopyroxenite.

Three stable isotopes of oxygen exist with atomic masses of 16, 17 and 18. Originally only the two most abundant isotopes were measured, i.e. molecules having masses of 32 and 34 (16O +16O and 16O +18O respectively). The relative abundance of the two isotopes in terrestrial samples, typically 16O = 99.763 %, 18O = 0.1995 % [15], varies slightly according to the mineral in question and the temperature at which it formed, a property that has been widely used as a means of assessing alteration temperatures in metamorphic mineral assemblages. As analytical methods and mass spectrometers improved during the 1960s it became possible to measure accurately the third and least abundant isotope of oxygen, 17O (typical abundance 0.0375 %), as mass 33 (16O+17O). It was this advance that became of crucial importance in studies of extraterrestrial materials [16]. The abundance of the isotopes is usually expressed as a ratio of the minor to major isotopes relative to a standard (Standard Mean Ocean Water, SMOW is used for oxygen) and is quoted as a 5 (delta) value (517O, 518O for 17/16 and 18/16 ratios, 5D for D/H ratios). The details of the notation need not concern us here, suffice it to say that the resulting values are expressed in parts per thousand or per mil (%o). (For a full explanation see, for instance [17].) Efficient mixing of all terrestrial material during the period of global melting, which accompanied formation of the Earth, means that with the exception of a few sulphate rocks that derive an isotopic anomaly ultimately from stratospheric processes [18], there remains a constant relationship between 17O/16O and 18O/16O ratios of all oxygen-bearing materials (i.e. mantle rocks, sedimentary rocks, the oceans, the atmosphere, etc.). In other words the planet-forming event served to homogenise any isotopic variability present in the constituent planetesimals, dust and gas. Subsequent geological activity has caused changes in the initial oxygen isotopic ratios but in a way that is understood from normal chemical and physical principles. During most chemical and physical reaction processes, the behaviour of the two less abundant isotopes means that they fractionate between reactant and product in proportions that are approximately equal to the difference in mass between the isotopes. A rule of thumb is that the effects registered in 18O are twice as large as those in 17O, relative in both cases to 16O (i.e. 18 - 16 = 2 and 17 - 16 = 1). In fact the details of this phenomenon are quite complicated and there are all kinds of subtle variations inherent to different systems [19]. For our purposes it is sufficient to note that on a plot of 517O against 518O (known as a three isotope plot, [20]) all terrestrial samples fall on a straight line referred to as the terrestrial fractionation line (TF line; see Fig. 1.1). It transpires that the slope of this line is about 0.52. In contrast, extraterrestrial materials possess a wide range of abundances of the minor isotopes that reflect the relative contributions by material accreting to form their respective parent bodies and any processes of exchange between materials that may have taken place [16]. As such meteorites plot in different regions of a three isotope plot.

The abundance of the minor isotopes, 17O and 18O, in Martian meteorites is internally consistent yet distinctly different to that measured in terrestrial samples [21, 22]. In every case they possess a small excess of 17O, which means that on a three isotope plot they lie on a line parallel to, but displaced from the TF line

(Fig. 1.1). The magnitude of deviation from the TF line is defined as the A17O value. The consistent magnitude of the A17O offset found in these samples is powerful evidence that they originated on a single parent body elsewhere in the Solar System. The spread of points along the line reflects the operation of geological processes that post-date planetary formation. As only relatively large bodies undergo global scale melting the possible source of these meteorites must be restricted to larger planetary bodies. Furthermore the relatively young age at which the rocks were still molten, ~1.3 Ga (billion years) ago for Nakhla and Chassigny, as deduced from analysis of radiogenic isotopes [23, 24] and as low as a few tens or hundreds of Ma (million years) for some of the other members of the group [13], meant that the source has remained volcanically active until the relatively recent past.

Martian Fractionation Line

Terrestrial Fractionation Line

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