CHASSIGNY: The First Martian Harvest

By Kevin Kichinka
MARSROX@aol.com

PART TWO

AN EMERGING MARTIAN TRIUMVIRATE


Only an oenologist knows whether the burgundy grapes growing near Chassigny, France in 1815 fulfilled their promise and yielded a peppery Cote-D'Or, bold of body and nose.

But all readers of this quarterly know that for meteorites, 1815 was a vintage year, the year we reaped our first harvest from Mars.

We rejoin our story in 1979, when Hap McSween, et al., and Wasson and Wetherill published papers dating Chassigny, Shergotty and Nakhla as meteorites "too young" to be of an asteroidal origin. In 1981, C.A. Wood and L.D. Ashwal included Chassigny when posing the profound question, "SNC Meteorites - Igneous Rocks from Mars?"

Those masters of heavy element analysis, N. Nakamura, H. Komi and H. Kagami, found "evidence for a close genetic relationship of Chassigny and Nakhla" in their paper, "Rb-Sr (Rubidium-Strontium) Isotopic and REE Abundances in the Chassigny Meteorite" (1982a). Data indicated a planetary origin for Nakhla and "the lack of Rb-Sr isotopic disturbance and age agreement with that from K-Ar system...suggests that the age of 1.23 Ga obtained from Chassigny probably represents the time of igneous crystallization rather than (an) impact event."

Evidence for Chassigny's tie-in with the shergottites and Nakhla, and a planetary, rather than asteroidal origin accrued even further with R.N.Clayton and T.K.Mayeda's paper "Oxygen isotopes in eucrites, shergottites, nakhlites and chassignites" (1983).

"Oxygen isotopic analysis has been carried out on three shergottites, two nakhlites, nine eucrites and seventeen terrestrial samples. The eucrites define a fractionation line displaced from the terrestrial line...the shergottites, nakhlites and Chassigny define another fractionation line...within each of these two groups, the meteorites have been derived from a common oxygen reservoir, and perhaps a common parent body. The differences between the two groups require separate reservoirs. Surprisingly, Brachina, previously classified as a chassignite, has an oxygen isotopic composition that lies within the eucrite group."

Allan Treiman, et al., (1986) noted that although the calculated parent magmas of the SNC meteorite subgroups differed significantly from each other, they all had one common feature - relatively low Al2O3.

U. Ott sacrificed 98.7 mg. of powdered Chassigny for his study "Noble Gases in SNC meteorites: Shergotty, Nakhla, Chassigny" (1988). "Differences in Xe (Xenon) isotopic composition between EETA 79001 glass (putative Martian atmosphere) and Chassigny (which then must be representative of some interior portion of the planet) contain valuable information on the history of Mars, if a Martian origin of the SNC's is assured."

Although apparently related by their young crystallization ages, no defining link between the supposed Mars meteorites was uncovered from his study of their trapped gases. "It is obvious from the data...the trapped gases found in Shergotty, Nakhla and Chassigny are different in composition from each other...and by inference, from those in the Martian atmosphere."

In Shergotty, Ott found a mixture of two types of gases, one possibly introduced by shock, the other a pure sample of those found in Chassigny. How unusual were the gases in Chassigny? "No other known reservoir of gases shows a composition quite like Chassigny." He found Chassigny's Xe isotopic gas component to be so solar-like as to wonder how this could have found its way into the interior of the "supposed planet body" - Mars.

Ott's table of cosmic ray exposure ages, based on residual amounts of 3He (Helium), 21Ne (Neon) and 38Ar, gave exposure ages of 8.3-14.3 Ma (million years) for Chassigny and 9.0-11.3 Ma for Nakhla. Ott wondered "if the ages of Nakhla and Chassigny are in fact identical" and "whether such chemically disparate meteorites were ejected from the same parent body in the same event."

THERE'S WATER IN THE MELT

Taking Chassigny research to the next level, a trio from the Brown University geology department, Marie Johnson, Malcolm Rutherford and Paul Hess published "Chassigny petrogenesis: Melt compositions, intensive parameters, and water contents of Martian (?) magmas" (1991).

"Chassigny...is one of the most primitive SNC meteorites, and thus it is most likely to reveal information about the SNC basalt source region. This study presents a detailed examination of partially crystallized melt inclusions in cumulus olivine grains in Chassigny. These trapped melts are argued to be representative samples of the melt that existed when Chassigny crystallized."

They noted that "the melt in these inclusions did not quench to glass; instead the melt crystallized into a variety of phases...in a closed system."

Phases were identified from about twenty-five melt inclusions using the Smithsonian's Chassigny polished thin section USNM 624-1. Melts trapped by olivine crystals had crystallized to high-Ca (Calcium) pyroxene (determined to be augite), low-Ca pyroxene (assumed to be an extremely aluminous orthopyroxene), kaersutite, chromite, chlorapatite, troilite, pentlandite, and glass. The glass was considered to have formed as shock melt during ejection from the parent body. Plagioclase was not found. The compositions of these phases were important in calculating the pressure, temperature, water and oxygen fugacity conditions during the time the phases crystallized.

Johnson, et al., took a two-step approach in deciphering Chassigny's petrogenesis. First, all phases in the inclusions were microprobed to create a compositional database. Then kaersutite/melt equilibrium possibilities were tested experimentally to "determine the intensive parameters which stabilize igneous kaersutite and to study the melt compositions that coexist with kaersutite."

Since separating out any Chassigny kaersutite for experimentation was almost impossible, a terrestrial kaersutite was substituted. It was thought best to conduct experiments to equilibrate kaersutite with melt under water-saturated conditions. Using a 50/50 (wt%) mix of Fe-Ti (Iron-Titanium) basalt and natural kaersutite, equilibrium was achieved under measured constraints.

A surprising discovery was made while investigating the Chassigny kaersutite amphibole - one of the inclusions contained the first reported extraterrestrial biotite, another hydrous byproduct. (Watson, et al., measured the deuterium contents of the biotite in 1994. Two years later, Watson, divorced and using her maiden name, Leshin, (et al.), claimed the deuterium level was "indistinguishable from terrestrial values.")

Johnson's group gave estimates of the conditions present during Chassigny's crystallization for the first time.

Floran et al.,'s 1978 discovery of the first extraterrestrial hydrous amphibole-bearing inclusions was validated. The trapped liquid initially contained 1.5% dissolved water. The crystallization of anhydrous phases had caused water to build up in the trapped melt.

Prior to kaersutite crystallization, the melt must have contained at least 4 wt% dissolved water, stable at a minimum pressure of 1.5 kbar. One kilobar of pressure on Mars would represent a burial depth of 7.5 kilometers. Two-pyroxene geothermometry indicated that equilibrium temperatures were 1,000º ± 50ºC. These factors together support a mantle origin for Chassigny.

Jay Melosh predicted in 1988 a shock pressure of at least 700 kbars, a blow that's barely a fender-bender on the planetary level, would be needed to blast this material to escape velocity from the near surface of Mars.

By looking closer than anyone had before, Susan Wentworth and James Gooding (1994) found more evidence for the past presence of water. Using mm-sized grains of whole rock material obtained from I.P.Wright and Monica Grady (Open University), generous cm-sized chips donated by Paul Pellas of the Paris Museum and a freshly-shaved polished thin section borrowed from the Johnson Space Center, they were able to detect the fingerprints of a wet environment in discontinuous veins of Ca-carbonate, Mg- (Magnesium) carbonate and Ca-sulfate grains varying in size from 1-10 microns. Their search was aided by the use of a scanning microscope and energy-dispersive X-ray spectrometry. "Although a preterrestrial origin for these salts is not absolutely verifiable, similar studies by Gooding et al. (1988) on EETA 79001, Nakhla (Gooding et al., 1991) and Lafayette (Treiman, et al., 1993) respectively, showed preterrestrial origins for similar salts in those meteorites."

Wentworth and Gooding's study indicated that following crystallization, Nakhla, Lafayette, EETA 79001, and Chassigny "were all exposed to similar water-based oxidizing solutions." Additionally, a lack of associated silicates or oxides suggested to them that the Chassigny parent rocks "were exposed to aqueous fluids, i.e. saline solutions that were probably cold and short-lived."

A turn away from a Nakhla kinship came when Hap McSween and Ralph Harvey discovered cumulate orthopyroxene in Chassigny. Published in a 1994 issue of Meteoritics, this discovery gave Chassigny a push towards the family of shergottites, or more inclusively, the possibility for a planet-wide "generic brand Al-poor basaltic magma" on Mars.

Another nudge away from Nakhla came in 1998. Dario Terribilini, et al., writing about trapped argon-40/argon-36 in Martian meteorites determined that Chassigny's noble gases were a non-homogeneous mixture of Mars' atmospheric and mantle argon and not introduced by shock as in the case of Nakhla (and every other Mars' meteorite). This dissimilarity, and their far different petrologies, led these researchers to consider that Chassigny and Nakhla "originated from different regimes of the impact site or not from the same ejection event."

INCLUDING THE EXCLUDED INCLUSIONS

M.E. Varela, Gero Kurat of the Vienna Museum and others revisited Chassigny's glass-bearing inclusions in 1998. They wrote, "Glass bearing inclusions are generally believed to be residuals of the parental melt that were trapped during growth of the host from the melt. According to that model, subsequent "closed-system cooling" of these inclusions produced an assemblage of "daughter phases" and quenched glass. This model assumption unfortunately has led to the exclusion of inclusions smaller than 25-30 µm from the studies because they cannot be considered to be representative samples of the host melt. We believe...they should not be neglected."

Neglected no more, relentless and thorough efforts were made by these workers to quantify the origins of all the inclusions and divine the chemistry of Chassigny's parent melt. Every tool in the box was brought to bear for this task.

There were challenges. They wanted to work with glass inclusions having a depth of <40 µm and exposed on both sides of the thin section to avoid measurement interference with the host olivine. The scarcity of inclusions they found meeting these criteria limited their analysis. Heating experiments failed when fractures in the olivine caused samples to darken. Then there was the case for the missing bubbles.

Varela, et al., explained that during the natural cooling of melt inclusions the different contraction rates of the host and melt cause a "shrinkage bubble" to form. None of the inclusions in the Chassigny olivine has a bubble. "We cannot invoke a single cooling process of a closed system accompanied by post-entrapment crystallization because, if this is the case, the system would have to develop a bubble."

Could the impact that sent Chassigny from Mars to the former empire of Charlemagne have left the inclusions melted but so rapidly quenched that bubbles couldn't form? "Formation of bubble-free inclusions and their glass by a shock melting event seems to be highly unlikely." Other scenarios were suggested but discharged.

There could be no bubbles only if there was no differential shrinkage between the host olivine and the melt. Therefore, there could be no bubbles only if the inclusions were trapped at sub-igneous temperatures.

"A new mechanism for the formation of these glass-bearing inclusions is proposed that has a direct repercussion on the petrogenetic scenario of this rock."

And that repercussion has reverberated through every paper on Chassigny written since.

A NEW BUBBLE OF PERCEPTION

Varela, et al. dismissed without prejudice the past findings of all who came before them in describing the formation of Chassigny. "We see that many of (our) observations do not fit the classical scenario generally proposed...it is not our intention to ignore the detailed work performed on Chassigny for many years, but our new observations and data suggest a genetic model radically different from the standard SNC meteorite model."

"The standard SNC meteorite model has related the glass inclusions in minerals of Chassigny to parent magma trapping during the growth of the host from that melt. However, our experiments demonstrate that the inclusions in Chassigny olivine are not the product of a post-entrapment, closed-system evolution of an originally homogeneous melt. (Petrographic studies) point to an origin by heterogeneous trapping of crystals and solid glass precursors in the Chassigny olivine. (Other evidences) suggest that not only have olivines sampled heterogeneous phases in varying proportions but that this trapping could have happened at fairly low temperatures that prevented chemical homogenization of the glasses."

Elemental distributions and K/Rb and Rb/Sr ratios within the glass inclusions led them to suggest "a chondritic rather than a fractionated planetary source (for) the trapped phases and the fluids associated with them."

"These new data suggest low temperature prevailing during the heterogeneous trapping and thus, a likely non-igneous origin for the primary glass-bearing inclusions in Chassigny olivine...the direct implications suggest that Chassigny likely was formed at sub-igneous temperatures, probably by aggregation of precipitates from a fluid (gas) phase. There are indications for a primitive, chondrite-like source for that fluid."

Shall we blow the dust from that 1978 paper by Florian, et al. and their self-refuted LL chondritic silicate source?

SEEKING COMMON GROUND

An analysis of newly found desert Mars rock Dar al Gani 476 complicated the issue further. E. Jagoutz and R. Jotter of the Max Planck Institute reported at the 62nd Annual Meteoritical Society Meeting that Dar al Gani 476 fell between Nakhla and Antarctic-find QUE94201 on a Sm-Nd (Samarium-Neodymium) isochronal plot. They proposed an 800 Ma event where "Nakhla, Chassigny, and QUE94201, as well as DaG 476, were formed from a L(ight)REE depleted reservoir (NCQD-reservoir)...The NCQD-reservoir was formed presumably from a chondritic reservoir at 4.5 Ga...There is good reason to believe that the NCQD-reservoir had a residual mineralogy consisting essentially of olivine and pyroxenes."

"At 800 Ma, melt was generated from the NCQD reservoir, represented by QUE94201 and the matrix of DaG 476. In addition, residues and cumulates were formed represented by Chassigny and Nakhlites. The melt was depleted in LREE and the residues or cumulates were enriched in LREE."

Jagoutz and Jotter's follow-up work, "New Sm-Nd isotope data on Nakhla minerals" supported this theory. They drew on additional Sm-Nd data gathered by Shih et al. (1999) on Governador Valadares and their own, yet unpublished research on Lafayette and Chassigny.

They offered three explanative scenarios from which they readily discarded two. Their favored theory "attributes the observed isotopic patterns to mixing processes accompanying the impacting of a heterogeneous regolith."

In other words, let's add the necessary flavor by stirring the cake with an asteroidal/cometary cinnamon stick!

"Using a mixing calculation, the observed major and trace element contents of the SNC meteorites can be explained in terms of three end-member (mafic, felsic and trace-element-enriched) components. Thus, olivine and pyroxene are contributed by the mafic component; elements having a constant ratio to Al, such as P, Ti, H(igh)REE, etc., are controlled by the felsic component; and Rb, Sr, LREE, K, etc. are derived dominantly from the trace element component. In this scenario the three isochrones cannot all have age significance, and at least two of these lines must be mixing lines."

"Regardless, the most important conclusion based on these new Sm-Nd data is that Chassigny, Nakhla, Governador Valadares, Lafayette, DaG 476 and QUE 94201 derive from the same source despite their very different exposure ages. This conclusion seems to be supported by existing Rb-Sr data, which show all these meteorites to have unradiogenic Sr isotopes. Furthermore, these meteorites all share a Chassigny-type rare gas composition and a similar 142Nd excess."

After years of research that alternately pulled together or pushed apart the members of the Martian meteorite family, could it be that Nakhla, Shergotty and Chassigny are kin to the same maternal Martian melt?

Wait a minute.

Shall we so easily dismiss the well-established 1.3 Ga crystallization ages of Nakhla and Chassigny or the 180 Ma age of the shergottites?

What of Jérémine's petits chondres naissants, mes amis?

And what's the buzz behind Chassigny's bulk melt inclusion compositional data that compares favorably with the major element abundances in the silicate portion of LL group chondrites? Where do you place Chassigny's solar-like Xe isotopic gas component in the evolution of a differentiated planet?

The petrogenesis of this rock remains as elusive as the "sub-igneous vapours" that born Varela et al.'s "aggregation of precipitants". The story continues.

KEEP ON KEEPIN' ON

A couple of items of practical interest remain.

The Name. Those challenged by the Roman languages call this meteorite "chas-sig'-knee". FYI - The French pronounce it "shah-see-nyee".

The Coincidence. Chassigny fell near some cows in France on October 3, 1815. Zagami fell near some cows in Nigeria on October 3, 1962. Mere coincidence? Allan Treiman wrote a paper published in Meteoritics in 1992 entitled, "Fall days of the SNC meteorites: Evidence for an SNC meteoroid stream, and a common site of origin."

He considered the probability of random coincidence and the precession of the orbit of a meteoroid stream. Treiman calculated the procession of the Earth's orbit. Different ejection scenarios were considered in light of Mars' meteorites' respective cosmic ray exposure ages. There were flies in all the ointments.

Should we stay up late every October 3rd to watch for a shower of Chassigny, Nakhla and Zagami meteorites? According to a personal communication Treiman received from respected meteoriticist George Wetherill in 1984, "the odds of obtaining two of the SNC meteorites from a single ejectum are calculated to be <2 x 10¯4."

The cows are safe. The sleeping dogs will lie.

The Weight. The Catalogue of Meteorites lists the TKW (total known weight) of Chassigny as four kilograms. This is the collected weight according to M. Pistollet, who arrived two days after the fall. There is no evidence that any pieces were weighed. Pistollet believed that eight kilos had fallen based on the missing pieces of a fractured, one-kilo specimen he obtained.

The actual weight of the Chassigny specimens listed in the Catalogue totals about 657 grams. The largest fragment is a 344 gram specimen in Paris.

Russ Kempton, Director of the New England Meteoritical Services (NEMS) has sent me a more-detailed tally that totals 829.3 grams. His list shows the largest Paris specimen weighing 376 grams.

Charles Meyer Jr.'s "Mars Compendium" web site (http://www-curator.jsc.nasa.gov/curator/antmet/mmc/mmc.htm) has a photo of a Parisian 215 gram specimen and mentions a 119 gram Paris Museum piece. Neither of these is documented on the first two lists.

If Pistollet's "guestimate" of the TKW was correct, more than three kilograms of Chassigny are gone.

Certainly other unlisted pieces exist in private collections. A spectacular 13.5 gram fragment with fusion crust, "the only specimen in private hands available in the world" (sic) was part of a failed Guernsey's auction on November 20, 1996 in New York City. No specimen of exactly 13.5 grams is listed in any record of repositories. The consignor is well known among meteorite cognoscenti, but preferred anonymity for this event, which included a 65 gram specimen of Nakhla and 420 grams of Zagami. Expectations for the lot pushed US$2 million.

So where did the missing material go? Unlike some meteorites that become underemployed doorstops and tire jacks, Chassigny is far too friable for such utilitarian labor.

Maybe it's just...gone. Losing little pieces of rock over two hundred years is not unusual. Reviewing the universe of old, historic meteorites we find that inventories often shrink with the passing years.

Barbotan fell in France in 1790. The Catalogue records "a shower of stones, the largest of nine kilos" but now lists only 3,678 grams extant.

Ensisheim is recorded to have weighed 127 kilos when it fell near the church in 1492. Sixty-nine kilos remain.

The Juvinas eucrite was a single 91 kilogram stone when it fell in 1821. Forty-nine and-a-half kilos are preserved.

Where are the missing three kilos of fragments? Some pieces were immediately shoved into the crowded pockets of excited little boys in 1815, an event horizon once breeched from which nothing returns. Other fragments were stored in a drawer in the kitchen of a farmer's thatched-roofed cottage, buried under a slow-motion avalanche of wine bottle corks, stick matches, and garlic presses. The rest were passed from father to son, and losing their emotional momentum, were forever forgotten.

But despair not, enough remains for serious science, mere milligrams matter. Great people, some yet unborn, will rise to the compelling challenge of finding the secrets still stashed within Chassigny's cosmic heart.

I would like to express my appreciation to Bernd Pauley for his advice, patience, good humor, and research assistance. His translations of the original French articles to English were critical to the success of this endeavor. Allan Treiman of the Lunar and Planetary Institute allowed the use and description of his spectacular photo of Chassigny polished thin section NMNH 624-1. I am incredibly fortunate to have academic access to one of this centuries' great meteoriticists. Kudos to Russ Kempton of NEMS for sharing with me the technical data he pried loose from the Ivory Towers of certain Eastern Establishments of Higher Learning. A special thanks to Alan Rubin of UCLA, a consulting scientist for Meteorite who performed the peer review of this feature and helped me locate the rocks I lost in the crater. A note of appreciation to Bill and the other librarians at the Rutenberg Branch of the Lee County, Florida system who never flinched when I casually requested "Interlibrary Loans" for articles published in obscure French periodicals in 1815.

Most importantly, I owe a thousand nights of sandman slumber to my fiancée, Kathy Morgan, who weathered my 3AM Martian brainstorms, seemingly ad infinitum.

Kevin Kichinka can be contacted at MARSROX@aol.com

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