The last bourgogne grapes, plump with promise, ripened in the fading autumn sun near Langres, on the plateau above Dijon. France's famous white cows, les charolais, stood like sentries in the fields of clover, guarding nothing. The third dawn of October in 1815 revealed a high sky unblemished by clouds. A faint whisper of breeze encouraged quiet rÍverie.
Perhaps this pastoral postcard of a countryside embroidered with the well-tended vineyards of the Cote-D'Or predicated the return of peaceful times. Perhaps the horrible, if necessary, French days of Enlightenment, Revolution and Terror, the years of Napoleonic expansion, contraction and catharsis were finally over. When the artillery has roared incessantly, a stillness this deep is consciously embraced.
But like all stillness, this one was also broken. Not from an echo of Waterloo, but by a volley from the God of War.
"...at about 8:30 in the morning a loud report which could be likened to the discharge of numerous muskets and into which the thunderous noise of cannon ball discharges seemed to be intermingled, was heard in the community of Chassigny and also in the surrounding villages at a distance of three or four leagues. This noise, which seemed to come from a cloudlet above the horizon in the northeast which had a vague form and was of a gray color, had already persisted for a few minutes, when a man, who was working in a vineyard some distance from the village and had his eyes fixed on the selfsame cloud, saw an opaque body from which thick smoke emanated fall to the ground with a hissing sound like that of a cannonball about 400 meters from where he was standing. He instantly ran to that spot and noticed a hole in the ground about 0.27m deep and 0.50 or 0.60 meters in diameter in freshly plowed soil. All around the impact hole lay bits and pieces of a stone which looked unusual to him. When he picked up one of these fragments he found it to be hot (to the touch) as if it had been exposed to intense sunlight."
The description of the fall of Chassigny, our first Mars' meteorite, "as told to M. Virey by M. Pistollet, physician of the same town", appeared in the Paris science journal Annales de Chimie et de Physique in 1816.
I believe this English translation of the account by Bernd Pauley to be the most complete ever published.
"He took it home to the village, and when the news of the event had spread there, several other inhabitants went to the place of the fall and picked up some of these stones. I arrived in this village two days later and soon realized that it was an aerolite, because I was already in possession of a similar stone that had been sent to me from Germany and which differed from this one only insofar as it was finer grained and had a more compact texture. After I had been shown to the place of the fall by the farmer...I found about 60 more small fragments, some of which, covered with soil and penetrated by humidity, easily crumbled between my fingers."
"A fireball, which is usually seen to accompany the fall of aerolites, was not observed in this case. No vapor was seen to come from the cloud when these successive detonations occurred. Neither its altitude nor its form could be estimated and described because there was nothing extraordinary about it apart from the color that different persons who had seen it compared to the dark gray smoke of burning straw. What can be certified is that the noise ceased after the fall of these stones."
"According to different eyewitness reports...at the same moment, other stones were thrown in different directions, but as none has been found (so far) this couldn't be sufficiently substantiated. One exception - a piece of considerable size was found seven or eight days later in a vineyard about 160 meters from the place the other stones had fallen. The total mass of all the pieces collected so far amounts to about four kilograms. Undoubtedly, all these fragments are part of the same stone...I am also inclined to firmly believe that what we have collected so far is only a fragment of a stone of more considerable size that broke up in the air. A piece in my possession, weighing almost one kilogram shows two broken surfaces which leads to the conclusion that the stone had a mass of at least eight kilograms."
"Although it has a considerable specific weight...this value is somewhat different in each of the fragments, some of which display higher densities."
"I almost forgot to tell you that several persons, both from the village of Chassigny and the neighboring communities, who had been sitting on the ground...said they felt a shaking of the ground during the detonations. The farmer who eyewitnessed the fall said he didn't experience anything like this. I should also tell you that there was a piece of local lava at the bottom of the impact hole, which might lead you to the conclusion that the aerolite only broke when it hit this hard object on impact; but on the other hand, not a single piece of the aerolite was found inside this impact hole, the fragments were found scattered all around the hole within a radius of 80-100 centimeters in such small pieces that this was probably the result of an explosion and that it did not fragment as a consequence of the fall itself."
THE ACID TEST
On the pages following Pistollet's report in the Annales begins M. Vauquelin's analysis of the "aerolite".
Vauquelin "powderized" ten grams of Chassigny and dissolved them in sulfuric acid. He looked for effervescence - "movement" - and for changes in the color of the acid. The solution was "decanted", the residue dried, heated and weighed. The sulfuric acid treatment yielded magnesium, silica, iron and metallic chrome. Vauquelin was puzzled because "the meteoric stone from Langres does neither contain sulfur nor nickel and the iron it contains is totally oxidized, whereas these two substances can be found in all the other aerolites...iron always occurs in its metallic state."
Researchers later praised this primitive alchemy of Vauquelin which resolved that "the meteoric stone that fell in the vicinity of Langres consists of 33.9% Silicon, 31% Iron oxide, 32% Magnesium and 2% metallic chrome."
Of course, his numbers didn't add up. Vauquelin couldn't isolate and identify that last percentage point of mass, an omission of little consequence at the time. We'll learn later that for this presumed piece of Mars, the bell tolls loudest for matter measured in microns.
AN HISTORICAL PERSPECTIVE
Chassigny is the rarest of meteorites, an olivine achondrite having no common petrology with any other achondrite. Not just another orphaned, "ungrouped" meteorite, Chassigny is a chassignite. In fact, as no others exist, Chassigny is the only chassignite.
Let me digress for a moment of meteorite history. G. Rose proposed the first classification system for meteorites in 1864. Beginning with Irons and ending with silicates, the system was based on a meteorite's principle minerals in order of decreasing specific gravity. Rose is credited with authoring the term "chassignites." He included it within the category of stone meteorites "where pyroxenes and olivine form the main constituents". He described Chassigny as "consisting of a small-grained, almost homogenous, rather friable mass, greenish-yellow to gray in color."
In 1885, Gustav Tschermak, the Director of the Mineralogical and Petrographical Institute of the University of Vienna, wrote the transcendental treatise The Microscopic Properties of Meteorites. John and Mathilde Wood of the Smithsonian translated the book into English in 1963. Seventy-eight years after its first printing they called it "the largest and most comprehensive collection of photographs of the microstructures of stony meteorites in the literature" adding that "Tschermak's...identifications were rarely wrong. They were made on the basis of crystal forms, cleavage, twinning, optical properties, and the chemical analysis of small samples painstakingly hand-picked from the stone."
Studying one of only three polished thin sections of Chassigny prepared until recently, Tschermak observed, "In thin section, Chassigny is seen to consist of equal-sized pale yellow-green grains, which fit tightly against one another and are cut by the coarse and fine cracks characteristic of meteoric olivine. They contain only a few brownish glass inclusions. Here and there between the olivine grains small interstitial areas (often triangular in shape) filled with colorless or brown glass can be seen. Frequently, systems of cracks in the olivine grains radiate away from the glass areas. When highly magnified, the glass is often seen to contain numerous colorless birefringent grains, fine birefringent needle, or brown crystallites. Thus some of the glass has become partly devitrified."
It is those little glass inclusions first described by Gustav Tschermak in 1885 that have become the object of a 21st century meteoritical treasure hunt.
Other than the groundbreaking work of Tschermak, an inconsequential study by the French scientist Damour in 1862 ("it resembles terrestrial peridot") and a brief mention by A. Lacroix in the Paris Muse'um Catalogue in 1927, ("this chassignite...is an achondrite of the magnesian type, peridotic, granular, with a black, dull fusion crust...") the Chassigny field of research lay fallow for decades. Surprisingly, M. Stanislas Meunier, an historically important meteoriticist who published the first Géologie Comparée of Nakhla in October, 1911, never felt compelled to do a study on Chassigny, even while mentioning it by name in his Nakhla paper.
A SHOCK FROM THE ROCK
Not until 1962, when E. Jérémine, J. Orcel and A. Sandréa, published the "Mineralogical and Structural Study of the Chassigny Meteorite" was a complete microprobe analysis performed. They noted that "it is holocrystalline and essentially composed of olivine and also of rare enstatite and clinohypersthene crystals; some feldspathic grains fill the interstices." They refined the meteorite's bulk chemistry first ascertained by Vauquelin.
Their work proceeded nominally until they saw something remarkable in a Chassigny thin section.
"We could observe a strange, suggestive phenomenon. It is found in the olivine and forms an enclave with rounded or ovoid contours which are clearly delineated and promote the development of radial fissures. This is suggestive of an increase of size by growth. This inclusion is dominantly composed of elongated hypersthene crystals which are slightly twisted and which follow the external, ovoid contours of the rim...There is also a thin ring showing a unique optical orientation, more birefringent than the hyperstene of the central part (probably diopside). This is a perfect preliminary form of a small (polysomatic) chondrule."
Chondrules in an achondrite? Mon dieu!
According to Jérémine et. al., chondrules were "silicate magma droplets which formed spherical blebs when they solidified from exploding planetary bodies" or "as a consequence of the collision of several asteroidal bodies". Cognizant of their observed meteoritical oxymoron, they added "the problem of chondrule genesis has not yet been resolved satisfactorily. Obviously, it will be necessary to look for another explanation for the presence of small (internal) chondrules in Chassigny."
Leaving you to ponder the imponderable, another observation of this trio of researchers bears mention. They noted that Chassigny contains minerals in relative proportions that are not duplicated in any other terrestrial or extraterrestrial material.
"Peridot (and this includes dunite) in no way resembles this chassignite material, neither macroscopically or microscopically. And no terrestrial rock of the same family is identical to chassignite. Such terrestrial rocks always contain Fe2O3 and there is much more MgO than FeO. In the Chassigny meteorite, these two oxides are present in almost equal amounts."
And for the record, definitely not present in equal amounts in other Mars' rocks.
Brian Mason et al., described their search for Jérémine's chondrules in a 1975 paper, "The Composition of the Chassigny Meteorite".
"In a thin section of Chassigny from the Smithsonian collection we have recognized structures similar to those described by Jérémine et al., but are undecided on their interpretation. Whether they are truly incipient chondrules, or chondrules which have been almost erased by recrystallization, or some unrelated structures is difficult to determine from the limited evidence."
This is the last dedicated search for the elusive Chassigny "chondrules".
Mason's group focused on determining the minor and trace elements of Chassigny. Because of the absence of a marked Eu (Europium) anomaly, they supposed that the meteorite could not be derived from a melt of chondritic composition. Additionally, "for Chassigny, the major mineralogical differences from the chondrites are the absence and near-absence of nickel-iron and troilite."
They agreed with the research of workers Schmitt and Smith (1963) who found an REE (Rare Earth Element) distribution similar to Nakhla, but at lower concentrations. They wondered if Chassigny was an early cumulate from the same parent liquid as Nakhla.
Brian Mason also contributed to the "Brachina Meteorite - A Chassignite from South Australia" (Johnson J.E., et al., 1977). Brachina lost this classification when Nehru, et al. (1983), used oxygen isotopic data derived by Clayton and Mayeda (1983) and declared a new type of meteorite, a brachinite. But the original arguments for its inclusion as the second chassignite were persuasive. According to Johnson et al., "the distribution patterns of REE abundance are quite similar, showing a rapid decline in relative abundances for the light REE (La-Sm) (Lanthanum-Samarium) followed by a slight positive Eu anomaly and practically uniform relative abundances for the heavy REE (Ga-Yb) (Gadolinium-Ytterbium). This distribution pattern may be unique to Brachina and Chassigny; the only comparable pattern among meteorites is that for Nakhla...the REE distribution pattern for Nakhla, however, shows a uniform decline in relative abundances from La to Yb, and no Eu anomaly. Nakhla is classified as a calcium-rich achondrite and Chassigny and Brachina are calcium-poor, but a genetic relationship may exist between them."
The incredible difficulty in pinning down exactly what Brachina and Chassigny were is evident in some of their other observations. "Although classified as achondrites, Chassigny and Brachina are chemically comparable to the chondrites, specifically the L and LL chondrites. Unlike the chondrites, Brachina contains no nickel-iron metal, and its SiO2/MgO ratio is lower than most chondrites. The classification of Brachina and Chassigny as achondrites is determined essentially by their granular non-chondritic textures."
In 1978, R.J. Floran, the late Martin Prinz, and four others cooperated on "The Chassigny meteorite: a cumulate dunite with hydrous amphibole-bearing inclusions". This work presented a detailed petrogenesis of Chassigny and it is here that the significance of Chassigny's glass melt inclusions was first noted.
Melt inclusions composed of glass are significant because when they are present with other silicates they can be used to calculate the equilibrium crystallization sequence, the relative and absolute cooling rates, and the composition of the magma when the inclusions were isolated from the melt. They are the key that unlocks many doors.
Although Floran, et al., found similarities in the bulk melt inclusion compositional data to major element abundances in the silicate portion of LL group chondrites, the oxidizing conditions that Chassigny formed under suggested to them that the parental melt could not be directly derived from a chondritic composition.
Using an ARL ion microprobe mass analyzer, they bombarded the Smithsonian's well-trafficked NMNH 624 thin section (provided by Dr. Roy S. Clarke) with a negatively charged 5 micrometer-sized beam of monatomic oxygen "to obtain 'semi quantitative' estimates of the hydrogen and fluorine contents of the Kaersutite amphiboles found (only) in the melt inclusions." They discovered that the amphiboles contained substantial hydrogen, and to them it seemed likely that "these crystals represent the first occurrence of extraterrestrial hydrous amphibole".
Surprisingly, they also concluded that although the inclusions were saturated in respect to olivine, the inclusions "could not have been the melt from which Chassigny crystallized."
So where did they come from?
Determining the origins of a cumulate rock is difficult. Chassigny is a cumulate, as suggested by its iron-rich olivine with anhedral to euhedral shaped crystals. But its bulk rock composition is not necessarily equivalent to a liquid and its parent magma composition can only be inferred indirectly.
Floran, et al., acknowledged this dilemma. "Because of inclusion-host interactions after trapping of the melt, indirect methods must be used to obtain estimates of magma composition", but "none of these methods can be used for the melt inclusions in Chassigny."
They decided to estimate the bulk chemical characteristics of the magma at the time the inclusions became isolated. Their result, a so-called "hypothetical magmatic composition table", was proffered with this caveat, "it should be emphasized that it is not possible to uniquely choose which, if any, of the compositions in Table 5 might be approximately representative of the melt from which Chassigny was derived."
Their "hypothetical magma v. LL silicates" graph of mineral percentages led them to the ambiguous conclusion that "Chassigny may have formed during a major melting event from a source region whose major element chemistry at some time earlier in its history was grossly similar to LL group chondritic silicates."
Vague as that was, their concurrent work on trace element relationships inferred the opposite, pounding a stake through their first theory's barely-beating
heart - "Chassigny is probably not directly related to LL group chondrites."
Continuing along this bumpy road, Floran et al., posit that olivine-rich rocks like Chassigny (and Brachina) "play a key role in models portraying the early thermal and chemical evolution of differentiated planetary bodies." They had no thoughts of a Martian origin for Chassigny, rather it was supposed that "olivine achondrites probably originated in the interior or near-surface of one or more now fragmented parent bodies which formed in the asteroid belt."
Florian et al. close out their paper with the remainder of Chassigny's petrogenesis. They invoke the work of Lancet and Lancet (1971) citing "shock deformation during one or more preterrestrial collisions; a K/Ar (Potassium/Argon) age of 1.39 Ga (billion years) may date the event" and a cosmic ray exposure age of 9 Ma (million years) after ejection "from the asteroid belt". Disregarding any historical viticultural reference, they dryly note its "entry into the Earth's atmosphere and recovery in 1815." C'est la vie.