Info

Not Chondrites

Primitive +

Hewardites & Eucrites a. Diogenites &

Arterites & G rachm ile s t

Ureiiiles - AuS rites

S tony-irons x

Martian * Lunar *

■a.00 -6.00 -4.00 -2.00 0.00 2.00 4.00 6.00 8.00 tO.OO 12.00 14.00 16.00 16.00 20.0

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Figure 5. Oxygen isotopic composition of whole-rock meteorites. TFL - Terrestrial Fractionation Line: the array on which data from terrestrial and lunar materials fall. Data from Clayton et al. (1991); Clayton and Mayeda (1996, 1999), [2-4],

Subsequent to accretion, most meteorites have experienced varying extents of thermal metamorphism or aqueous alteration. These secondary processes occurred on the meteorites' parent bodies, and did not affect the overall composition of the chondrites. Mobilisation and re-distribution of elements such as Fe, Mg and Ca occurred as primary silicates either became homogeneous in composition through heating, or through formation of secondary mineral phases (e.g., clay minerals, carbonates) during aqueous alteration. Tertiary processes of brecciation and shock melting are also recognised, as are features resulting from weathering during the terrestrial lifetime of a meteorite. A thorough review of the properties and classification of chondritic meteorites is given by Brearley and Jones [5].

2.2. Achondrites

Broadly speaking, an achondrite is a stony meteorite that formed from a melt on its parent body. Thus achondrites have differentiated compositions, having lost a large fraction of their primordial metal content, and generally do not contain chondrules. There are many different groups of achondrites, some of which can be linked together to form associations allied with specific parents (Figure 1). The separate associations have little, if any, genetic relationship to each other and, like the chondrites, can be distinguished on the basis of oxygen isotope composition (Figure 5). One sub-set of achondrites, the primitive achondrites, comprises several groups of meteorites that have only experienced limited partial melting of their chondritic precursors. It has been suggested that the primitive achondrites might be a bridge between chondrites and achondrites. In contrast, the Howardite-Eucrite-Diogenite association (HED) is a suite of generally brecciated igneous rocks ranging from coarse-grained orthopyroxenites (diogenites) to cumulates and fine-grained basalts (eucrites). The howardites are regolith breccias, rich in both solar wind gases and clasts of carbonaceous material. The HEDs all have similar oxygen isotopic compositions; one candidate for the HED parent body is the asteroid 4 Vesta. For a much more thorough review of the mineralogy and classification of achondrites, see Mittlefehldt et al. [6].

2.3. Stony-irons

These are meteorites with very approximately equal proportions of silicate minerals and iron-nickel metal. Stony-irons are sub-divided into two big groups, mesosiderites and pallasites, which have very different origins and histories. Pallasites have a most unusual appearance, and are perhaps the most strikingly beautiful of all meteorites. They are an approximately equal mixture of iron-nickel metal and silicates, predominantly olivine, and are presumed to represent material from the core-mantle boundary of their parent bodies. The mesosiderites are a much more heterogeneous class of meteorites than the pallasites. They are a mixture of varying amounts of iron-nickel metal with differentiated silicates, the whole assemblage of which seems to have been brecciated.

2.4. Irons

Iron meteorites are composed of iron metal, generally with between 5-20 wt. % nickel, and account for approx. 5% of all observed meteorites falls. The mineralogy of iron meteorites is dominantly an intergrowth of two phases, the iron-nickel alloys kamacite and taenite. Kamacite, or a-Fe,Ni, has a body-centred cubic structure and a Ni content < 7wt.%, whilst taenite, or y-Fe,Ni, is face-centred cubic and approx. 20-50 wt.% Ni. Iron meteorites are highly differentiated materials, products of extensive melting processes on their parents. One major division of iron meteorites is into 'magmatic' irons (those that have solidified by fractional crystallisation from a melt) and 'non-magmatic' irons (those that seem not have completely melted, and possibly formed during impact processes). The irons are sub-divided into 13 different groups on the basis of nickel and trace element chemistries (Ga, Ge and Ir contents). The non-magmatic irons are groups IAB and IIICD, and show a wide range in nickel contents. Many irons defy chemical classification, and simply remain "Ungrouped". Prior to classification on the basis of trace element chemistry, iron meteorites were classified in terms of their metallographic structure. Laths of kamacite intergrown with Ni-rich phases form the 'Widmanstatten pattern' revealed in polished and etched iron meteorites. The width of the kamacite lamellae is related to the cooling history of the parent bodies. The structural classification of iron meteorites was redefined and made systematic by V.F. Buchwald [7].

2.5. Non-asteroidal meteorites

Lunar meteorites: there are currently 18 known lunar meteorites, 15 of which have been collected in Antarctica. Several are gabbroic or basaltic in nature, but the majority are anorthositic regolith breccias. Material returned by the Apollo and Luna missions sampled a fairly restricted area of the lunar surface. The discovery of meteorites from the Moon is a fortunate opportunity that has allowed study of a more diverse range of lunar materials. Papike et al. [8] reviewed the mineralogy of lunar samples, which allows the significance of lunar meteorites to be placed in context with Apollo and Luna rocks.

Martian meteorites: there are currently 16 meteorites (20 separate stones) that almost certainly originate on Mars. The martian origin rests on the age, composition and noble gas inventory of the meteorites (see http://www.nhm.ac.uk/mineralogy/staff/grady/mars.html for a brief review). The meteorites are all igneous in nature, and can be sub-divided into 4 groups: the Shergottites (after the type specimen Shergotty), further sub-divided into basaltic and lherzolitic types. Nakhlites (after Nakhla) are shallow cumulates that have been exposed to the martian hydrosphere, and thus contain rich assemblages of carbonates, sulphates and halite. Chassigny, the only member of its group, is an olivine-rich dunite. ALH 84001 is the sole orthopyroxenite, and is rich in carbonates. For a much more thorough review of the mineralogy and classification of martian meteorites, see McSween and Treiman [9].

3. WHAT CAN BE LEARNT FROM METEORITES?

Solar System history starts with the collapse of an interstellar molecular cloud to a protoplanetary disk (the solar nebula) and continues through a complex process of accretion, coagulation, agglomeration, melting, differentiation and solidification, followed by the secondary influences of bombardment, collision, break-up, brecciation and re-formation. The rocks which are accessible for study at the Earth's crust are not representative of the original material that accreted from the solar nebula. Study of meteorites allows a more complete understanding of the processes undergone by the material that resulted in the Earth of today. The last decade has seen a greater understanding of the processes that led to the formation of the Sun and Solar System. Advances have resulted from astronomical observations of star formation regions in molecular clouds, the recognition and observation of protoplanetary disks and planetary systems around other stars, and also from refinement of chronologies based on short-lived radionuclides. The main events that led to the formation of the Earth and the Solar System can be followed by several different short-lived radiometric age-dating chronometers, utilising components from meteorites that formed at various stages in Solar System evolution. The subsequent processes of differentiation, segregation and core-formation in planetary-sized bodies can also be traced by reference to more long-lived nuclear chronometers.

3.1. Timescales of accretion

Gravitational instability within an interstellar molecular cloud results in collapse of a fragment of the cloud to form a protoplanetary disk [10-12]; the mechanism that triggers cloud collapse is not clear: several possibilities have been suggested, e.g. a shock wave from a nearby supernova or ejection of a planetary nebula from an AGB star [13]. The oldest components in meteorites are the CAIs, with ages (determined by U-Pb dating) of 4566 ^ [14], This age can be taken as a baseline from which the date of formation of other meteoritic components can be measured, and has led to the establishment of a relative timescale for the production of CAIs and chondrules, based on the presence of the decay products from shortlived radionuclides. The 'canonical' model for accretion rationalised that the short-lived radionuclides were produced externally to the presolar nebula (probably in a supernova), injected into the collapsing dust cloud and incorporated into the CAIs, etc, on timescales shorter than that of the radionuclide half life. For example, the presence of 26Mg (from the decay of 26A1; T[/2 ~ 0.73 Myr) within CAIs shows that the CAIs formed whilst 26A1 was still "live" in the solar nebula, i.e., agglomeration took place over a very short timescale, < 1 Myr

[15]. The 41Ca-4IK chronometer, with T1/2 ~ 0.15 Myr, implies even more rapid formation of CAIs, with an interval between nucleosynthesis and agglomeration of < 0.3 Myr [16]. Measurements on aluminium-rich chondrules using the Al-Mg chronometer also imply that chondrules formed several million years after CAI production, and that chondrule-formation took place over an extended period of time, commencing very shortly (perhaps 1-5 Myr) after CAI formation [17, 18].

However, the use as a chronometer of isotope abundance differences between chondritic components is dependent on the models of short-lived isotope formation. Over the past few years, an alternative mechanism to production of the radionuclides in a supernova has been proposed, whereby the species are formed by nuclear reactions close to the energetic early sun, followed by expulsion to planet-forming regions by the X-wind [19]. In this scenario, no chronological significance can be attached to the lack of daughter products from short-lived radionuclides - any effects are spatial, rather than chronological. Currently, the debate between proponents of the exotic (i.e., supernova) versus local (i.e., young sun) origin of short-live radionuclides is vigorous. The detection, in meteoritic components, of the decay products of short-lived isotopes thought not to be produced in supernovae, (e.g., 10B, from the decay of I0Be, T./, ~ 1.5 Myr) were at first taken to support the local production of such species by spallation [20], however, refinement of models of supernovae have suggested that 1 Be might be formed by spallation in the outer envelope of a supernova [21], Most recently, evidence for 7Li (from the decay of 7Be, Ty, ~ 52 days) in CAIs has again turned the tide of the argument towards local production of short-lived isotopes [22]. Efforts are also being directed towards detection in meteoritic components of products from radionuclides that can only be formed by spallation, e.g. 50V [23],

No matter what the mechanism for production of short-lived radionuclides, it is probable that the Solar System formed by the heterogeneous accretion of components. The growth of planet-sized bodies from micron-sized dust grains is controlled by several factors, such as the nature of the initial grains (fluffy or compact) and the degree of turbulence within the nebula, and has been modelled by many authors [24], End-member models for planetesimal formation are coagulation of material by gravitational instability in a quiescent nebula [25] or by coagulation during descent to the midplane of a turbulent nebula [26-27]. The aggregation of interstellar dust (< 0.1 fim in diameter) into increasingly large bodies, eventually forming kilometre-sized planetesimals and culminating in the asteroids and planets, took place over a time interval of some 8 Myr following formation of the CAIs [14].

3.2. Timescales of differentiation

In addition to illuminating the earliest history of the Solar System, and providing examples of the types of material from which the Solar System formed, meteorites also enable studies of the planetary differentiation processes that occurred subsequent to aggregation. Once the proto-parents had aggregated, internal heat from radioactive decay, combined with gravitational energy and collisional energy from planetesimal bombardment kept the bodies molten. As cooling progressed, reduction reactions within the convecting system resulted in production of metal-rich cores and silicate-rich crust-mantle structures. Achondrites record episodes of incipient melting and differentiation, and iron and stony-iron meteorites represent the highly-differentiated end-member compositions that result from core formation and metal-silicate segregation on parent bodies. The timescale over which planetary melting and core formation occurred can be deduced using several radiometric decay schemes.

Igneous activity on differentiated stony asteroids is traced by data from 53Mn-53Cr chronometry (Tm ~ 3.7 Myr). For the HED assemblage, whose parent-body is believed to asteroid 4 Vesta, core formation apparently occurred < 4 Myr after CAI formation; subsequent volcanic activity on the asteroid continued for a further 4 Myr or so [28]. This compressed timescale indicates the relatively short period that elapsed between accretion of primordial dust into small proto-planets, and the onset of volcanism, melting, and differentiation.

The recently-established I82Hf-182W chronometer [29] has been used to infer the timescale over which core formation occurred on both asteroidal and planetary bodies. The strongly lithophile 182Hf is partitioned into silicates, relative to the more siderophile W during differentiation, and subsequent variations in HfAV are caused by decay of 18 Hf to 182W (Ti/2 ~ 9 Myr). Models based on the I82Hf-182W chronometer indicate that core formation on iron meteorite parent bodies occurred fairly rapidly, within 5-10 Myr of the onset of CAI formation, whilst formation of the Earth's core took place more gradually, some 50 Myr or so after the differentiation of iron meteorite parent-bodies [29], Based on measurements from martian meteorites, it appears that core formation on Mars also occurred across an extended timescale, within ~ 30 Myr of CAI formation [30],

3.3. Mars from martian meteorites

Satellite imagery has shown that Mars has had ice and liquid water flowing across or just under its surface during past epochs. The first detailed topographic maps of Mars were produced with data from NASA's Mariner 9 orbiter mission of 1971-1972, in which channel and valley networks, volcanoes, canyons and craters were imaged at reasonable resolution for the first time. Mariner 9 was also the mission that returned pictures of layered (possibly sedimentary) terrain in Mars' polar regions. In 1976, NASA's two Viking landers sent back many images of Mars' landscape, showing panoramic scenes of broken boulders distributed over flat dusty plains. Viking also measured the elemental composition of both Mars' atmosphere [31] and surface soils [32], In combination with the Mariner data, the Viking results have allowed a picture of Mars as a rocky planet with a significant geological history to be built up. The recent Pathfinder mission of 1997 recorded spectacular images of a rock-strewn plain, with tantalising glimpses of rounded pebbles and possible layered structures and hollows within some of the rocks. Inferences drawn from the images are that some of the rocks might be sedimentary, possibly even conglomerates, thus implying a significant fluvial history. Paler, almost cream-coloured patches in the soil might be areas leached by fluid action, hard-grounds or evaporite deposits. The poorly-sorted landscape of part-rounded pebbles and boulders has been interpreted as the type of landscape remaining after catastrophic flooding, further evidence of the stability of liquid water at some time in Mars' past. Recent data from the laser altimeter aboard the Mars Global Surveyor have been interpreted as evidence for a very large ocean across Mars' northern hemisphere [33]. For water to have been present at Mars' surface, the martian atmosphere must have been much thicker, and surface temperatures much warmer than they are today [34]. A thicker atmosphere engenders greater protection from solar radiation; in a previous epoch, Mars would have had a warmer and wetter climate and provided all the conditions suitable for the emergence of life [35]. Given this framework, it is not surprising that so much interest has been focussed on a potential martian biosphere. But many questions about Mars remained unanswered, particularly about the fluvial and seismic history of the planet. Vital to interpretation of much of the chemical data returned by spacecraft missions have been data obtained from martian meteorites [36].

Measurements made on martian meteorites complement data obtained from spacecraft exploration of Mars. So, for instance, the recognition of complex assemblages of salt (halite) with carbonates and clay minerals in nakhlites [37] has allowed interpretation of the scale and mode of fluid flow on the surface of Mars: results from the nakhlites imply that when water was present on the surface, it was warm and briny, and restricted in flow [38]. In other words, it might have been locked in enclosed basins that occasionally overflowed in episodes of flash flooding. Petrogenetic analysis of a second group of martian meteorites, the shergottites, has allowed development of theories of martian magma genesis [39, 40]. Analyses of martian meteorites have also added impetus to the resurgence of interest in exobiology. The observation that at least one of the SNCs, EETA79001 (a specimen found in the Elephant Moraine region of Antarctica in 1979) contained indigenous (i.e., martian) organic material associated with carbonate minerals [41] sparked a debate on the possibility of martian meteorites containing evidence for extraterrestrial life. This work was followed, in 1994, by the discovery of similarly enhanced levels of organic carbon in close association with carbonates in a second Antarctic martian meteorite, ALH 84001 [42, 43]. ALH 84001 became a further focus of interest in 1996 when a team of scientists led by David McKay of NASA's Johnson Space Centre in Houston reported the description of nanometre-sized features within carbonate patches in ALH 84001, and claimed to have found evidence for a primitive "fossilised martian biota" [44], Identification of the "nanofossils" remains controversial, since much of the evidence is circumstantial and relies on the coincidence between a number of otherwise unrelated characteristics of the meteorite (carbonate minerals in ALH 84001; organic compounds and magnetite associated with the carbonates, and the external morphology of the carbonates, see ref. 38, for a review of the main arguments for and against the interpretation of McKay et al.). Indeed, the formation conditions of both the carbonate and magnetite, the relevance of the organic compounds and interpretation of the morphology of the "nanofossils" have all been subject to detailed investigation by several groups of scientists. Although many members of the scientific community are sceptical about interpretation of the structures in ALH 84001 as martian nanofossils, discussion of the features has stimulated an enormous amount of interest in the possibilities of life on Mars.

3.4. Interstellar grains

A volumetrically insignificant, but scientifically critical component within chondrites is their complement of interstellar and circumstellar grains. The grains comprise several populations of nanometre-sized diamond and micron-sized silicon carbide, graphite and aluminium oxide [45]. The presence of the grains was first inferred in the late 1970s to early 1980s, on the basis of the isotopic composition of noble gases released by pyrolysis of density separates and size-fractions of residues produced from déminéralisation of bulk chondrites [46]. The unusual isotopic signatures of the noble gases implied the existence of several different hosts; analyses of acid-resistant residues suggested that the hosts might be carbon-rich. In 1983, combustion of a set of residues yielded the first carbon and nitrogen isotopic compositions of the grains [47, 48]. Together with the noble gas results, these measurements pointed to a variety of extra-solar sources for the grains including supernovae and red giant stars. Over the subsequent 20 or so years, increasingly sophisticated instrumentation, such as the ion microprobe and the analytical transmission electron microscope, have revealed different generations of interstellar grains, including grains within grains [49], Astrophysicists have been able to construct, then fine-tune, models of stellar evolution on the basis of the combined isotopic data resulting from analysis of the grains [50, 51]. Currently, grains from at least 15 different extra-solar sources have been isolated from chondritic meteorites. These grains were presumably introduced into the pre-solar nebula prior to its collapse and the onset of proto-planet formation. The presence of such a variety of grains indicates that the Sun did not form in isolation, but was part of a busy neighbourhood in which AGB stars, supernovae, novae and planetary nebula co-existed. When coupled with astronomical observations (both ground- and space-based) of interstellar and circumstellar dust clouds, the meteorite data are enabling astronomers to model star and protoplanetary disk formation processes [52]. It is hoped that continued study of pre-solar grains with a new generation of instrumentation will allow both relative and absolute chronologies to be constructed for the different grain populations, leading to models for the history and evolution of this region of the Galaxy.

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