Dark Micrometeorites in Blue Ices Relationships with Hydrous Carbonaceous Chondrites

During the Antarctic summer field seasons of 1987, 1990, and 1994, a team set off for the "blue ice" fields of Cap-Prudhomme in Antarctica, to recover the glacial sand trapped in the ice. This sand turned to be amazingly rich in large unmelted micrometeorites (Maurette et al., 1991). In the best 50-100-^m size fraction, a daily collect recovered from 10 to 15 m3 of melted ice, typically yields approximately 2000 unmelted micrometeorites, approximately 500 cosmic spherules (i.e., melted micrometeorites), and approximately 10,000 terrestrial particles - mostly morainic debris. In this sand, about 20% of the particles are unmelted micrometeorites (Fig. 3.1).

About 2,000 Antarctic micrometeorites (AMMs) have already been analyzed by a consortium mostly including M. Christophe, E. Delousle, J. Duprat, C. Engrand, M. Gounelle, C. Hammer, G. Kurat, G. Matrajt, M. Maurette, and C. Olinger (see Maurette et al., 1991; Kurat et al., 1994; Engrand and Maurette, 1998; Maurette et al., 2000; Matrajt et al., 2001; Maurette et al.,

Fig. 3.1. Glacial sand recovered from the blue ice fields of Cap-Prudhomme, at approximately 2 km from the margin of the Antarctica ice sheet. Steam generators, adjusted to deliver hot water at approximately 70°C, were used to make pockets of melt ice water at a few degree centigrades. Each daily collect was recovered from a total volume of water of about 10-15 m3, which was pumped and filtered on stainless steel sieves with openings ranging from about 25 ^m up to 400 ^m. This picture represents the best 50-100-^m size fraction of this sand, where one grain out of five is an unmelted micrometeorite. Unmelted and melted micrometeorites appear as "dark-irregular-charcoal" looking grains and glassy cosmic spherules, respectively.

Fig. 3.1. Glacial sand recovered from the blue ice fields of Cap-Prudhomme, at approximately 2 km from the margin of the Antarctica ice sheet. Steam generators, adjusted to deliver hot water at approximately 70°C, were used to make pockets of melt ice water at a few degree centigrades. Each daily collect was recovered from a total volume of water of about 10-15 m3, which was pumped and filtered on stainless steel sieves with openings ranging from about 25 ^m up to 400 ^m. This picture represents the best 50-100-^m size fraction of this sand, where one grain out of five is an unmelted micrometeorite. Unmelted and melted micrometeorites appear as "dark-irregular-charcoal" looking grains and glassy cosmic spherules, respectively.

2001; Duprat et al., 2003). Their mineralogical, chemical, and isotopic compositions first show that about 99% of them are related only to the group of the HCCs. These relatively rare meteorites (about 2.5% of the meteorite falls) include the three subgroups of the CI1, CM2, and CR2 type chondrites, containing about 100, 50, and 25% of hydrous minerals, respectively.

In the complex classification of meteorites, which already involved about 80 distinct groups in 1980 (Dodd, 1980) - 50% of them correspond to minor groups of iron meteorites - HCCs are considered as the most primitive meteorites, i.e., their constituent grains have never been exposed to temperatures higher than approximately 600 K since their formation in the early solar nebula. These "wet" carbonaceous objects are the most volatile-rich material delivered to the Earth. They can release neon, nitrogen, water, CO2, SO2, etc., during their volatilization and/or melting upon atmospheric entry, but also during the subduction of the oceanic crust on which they end up being deposited.

More surprisingly, about 95% of the AMMs are only related to the CM subgroup of HCCs, thought to originate from a single family of carbonaceous asteroids. The first hints about this "deceptively" simple micrometeorite classification led to disappointment. Indeed, the major initial objective of the "blue ice" team was to discover new solar system objects, not represented as yet in the complex classification of meteorites, and the hope to find new objects among AMMs was quickly vanishing.

However, this disappointment turned to excitement when the following deductions were made: (i) this simple classification implies that AMMs are not a mixture of microscopic fragments originating from the >80 distinct parent bodies of meteorites. Therefore, they likely originate from other types of parent bodies; (ii) the chemical and isotopic compositions of their volatile species are well mimicking those of the Earth's atmosphere; (iii) the proportion of large micrometeorites with sizes >100 |m which survive unmelted upon atmospheric entry is about 25%. This value, which is two times higher than previous estimates, was deduced for a new set of unweathered-highly-friable AMMs collected by Duprat and Engrand in central Antarctica (Duprat et al., 2003). Today, with this revised value, micrometeorites deliver to the Earth's surface a mass of unmelted hydrous-carbonaceous material at least ^20,000 times larger than that expected for the HCCs . If this enhancement factor was not decreased by a factor >1,000 in the distant past, the micrometeorite flux was the major extraterrestrial source of volatiles and organics on the early Earth's surface, after the giant impact by a Mars-sized body, which blew off the complex pre-lunar atmosphere of the young Earth (see first rubric in Sect. 3.10).

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