"Rock of the Month # 93, posted March 2009" ---

The Campo del Cielo iron meteorite is a very large type IA iron meteorite, discovered in Argentina in 1576. The iron is a coarse octahedrite, bandwidth 3.0 mm. This 600-gram individual fragment displays concave thumb-print indentations ("regmaglypts") on its shiny surface, which has been treated to remove rust. With such a long residence in human history, it is not surprising that synonyms abound, such as Otumpa and El Taco, Meson de Fierro and Gran Chaco. Specimen from Grenville Minerals, Kingston, Ontario.

Campo del Cielo, approximate recovered mass (total known weight, TKW) now approximately 115 tonnes, is considered the largest-known meteorite (by TKW - Wasson, 2019), though individual, intact masses are heavier than any one "Campo" individual, notably the 60-tonne Hoba iron in Namibia. Campo is an iron, like all its other large company: Hoba (60 T), Cape York (circa 58 T), Canyon Diablo (circa 30 T), Sikhote-Alin (27 T), Bacubirito (22 T), Gibeon (21 T), Armanty (20 T), Mundrabilla (17 T) and Willamette (15 T). Some of these irons, such as Hoba, Bacubirito and Willamette, were found as single large masses, whereas others, such as Canyon Diablo and Sikhote-Alin, fragmented on impact into a myriad of smaller masses and metallic shards. Material surviving from the fall of stony meteorite classes tends to be relatively smaller: two large examples are Jilin (4 T) and Allende (2 T).

This grand iron is noted for its complex, abundant silicate inclusions. Minerals reported in this iron include: sulphides such as sphalerite; oxides such as chromite; native copper; carbides; phosphates; and such silicates as clinopyroxene and orthopyroxene, plagioclase feldspar and olivine. Minerals such as graphite, troilite and the phosphide schreibersite form rims mantling silicate inclusions, separating them from the host nickel-iron alloys.

Not surprisingly, Campo del Cielo has been extensively studied (61 MINLIB records as of 2009, and 99 on 13 January 2020, dated 1932-2019). A selection of 18 references is appended: the research includes aspects of mineralogy, bulk chemistry and structure (metallography), as well as radionuclide analysis, content of sundry rare elements (platinum group elements, mercury, etc), rare-gas, carbon and nitrogen (not to mention chlorine, copper and zinc) isotopes, as well as the size, structure and origin of the associated field of impact craters. The latter is also well-studied. At least 20 small craters are distributed across an area of 19x3 km. The largest has a diameter of about 100 m, and the impact is dated at roughly 4,000 B.P. The energy of the impact has been estimated in the range of tens of kilotons of high explosive.

Models of the impactor prior to its encounter with Earth are based on evidence such as the abundances of cosmic-ray -generated radionuclides within the iron. In the case of Campo del Cielo, the geometrically-simplified model is basically a spheroid of natural steel, 6 metres in diameter. Given a mean specific gravity of 7.25, this implies a pre-atmospheric mass of some 820 tonnes. Thus the 15 tonnes of one individual large mass is just 2 percent of the original projectile, and the estimated TKW represents 14% or one-seventh of the incoming meteoroid.

John Wasson's lab at University College- Los Angeles has analysed many meteorites by instrumental neutron activation analysis (INAA), including a great number of irons. The lab has tested many supposedly new iron meteorites and found them to have chemistry within the field of the Campo suite. A plot of Ir versus Au provides the best discrimination, relevant to provenance studies, authentication and so, ultimately, to the nomenclature of chemically -similar meteorites. As of early 2018, there are 36 other irons that have compositions that cannot be resolved from the Campo compositional field. Irons with Campo-like compositions and generally nebulous recovery data include NWA 11859, Elmore County and Soledade, as well as various irons christened in the "Nova" series such as Nova 015, Nova 022 and Nova 028 (Wasson, 2019).

Selected References

Park,FR, Bunch,TE and Massalski,TB (1966) A study of the silicate inclusions and other phases in the Campo del Cielo meteorite. Geochim.Cosmochim.Acta 30, 399-414.

Deines,P and Wickman,FE (1975) A contribution to the stable carbon isotope geochemistry of iron meteorites. Geochim.Cosmochim.Acta 39, 547-557.

Buchwald,VF (1977) The mineralogy of iron meteorites. Phil.Trans.Roy.Soc.London A 285, 453-491.

Kissin,SA, Schwarcz,HP and Scott,SD (1986) Application of the sphalerite cosmobarometer to group IAB meteorites. Geochim.Cosmochim.Acta 50, 371-378.

Norton,OR (1994) Rocks from Space: Meteorites and Meteorite Hunters. Mountain Press Publishing Co., 449pp.

Cassidy,WA and Renard,ML (1996) Discovering research value in the Campo del Cielo, Argentina, meteorite craters. Meteoritics & Planetary Science 31, 433-448.

Benedix,GK, McCoy,TJ and Keil,K (1997) A cautionary tale of heterogeneity in IAB iron meteorites. Meteoritics & Planetary Science 32 no.4, supplement (Abs. 60th annual meeting, Hawaii), 11-12.

Benedix,GK, McCoy,TJ, Keil,K and Love,SG (2000) A petrologic study of the IAB iron meteorites: constraints on the formation of the IAB-winonaite parent body. Meteoritics & Planetary Science 35, 1127-1141.

Marvin,UB (2000) Iron meteorites and controversies over the origin of erratic boulders. Eclogae Geologicae Helvetiae 93, 25-31.

Honda,M (2001) Preatmospheric size of large irons by cosmogenic nuclides. Antarctic Meteorites 26, Nat.Inst.Polar Research, Tokyo, 183pp., 38-40.

Liberman,RG, Niello,JOF, Di Tada,ML, Fifield,LK, Masarik,J and Reedy,RC (2002) Campo del Cielo iron meteorite: sample shielding and meteoroid's preatmospheric size. Meteoritics & Planetary Science 37, 295-300.

Wasson,JT and Kallemeyn,GW (2002) The IAB iron-meteorite complex: a group, five subgroups, numerous grouplets, closely related, mainly formed by crystal segregation in rapidly cooling melts. Geochim.Cosmochim.Acta 66, 2445-2473.

Rocca,MCL (2006) A catalog of large meteorite specimens from Campo del Cielo meteorite shower, Chaco province, Argentina. Meteoritics & Planetary Science 41, A150.

Echaurren,JC (2007) Numerical estimations for impact conditions on Campo del Cielo, crater field, South America. Meteoritics & Planetary Science 42, A39.

Honda,M, Nagai,H, Nagao,K, Bajo,K and Oura,Y (2008) Cosmogenic histories in Gibeon and Campo del Cielo iron meteorites. Meteoritics & Planetary Science 43, A59.

McKenzie,R (2014) Meteorites: a Southern African Perspective. Struik Nature, Cape Town, 120pp.

Casado,JV and Allepuz i Sunye,D (2016) Meteoritos: introduccion y guia de reconocimiento. Private publisher, 3rd edition, 318pp. (in Sp.).

Wasson,JT (2019) Campo del Cielo: a Campo by any other name. Meteoritics & Planetary Science 54, 280-289.