Abstract-A new grouplet of primitive, metal-rich chondrites, here called the CB (C, carbonaceous; B, bencubbinite) chondrites, has been recognized. It includes Bencubbin, Weatherford, Hammadah a1 Hamra (HH) 237 and Queen Alexandra Range (QUE) 94411, paired with QUE 94627. Their mineral compositions, as well as their oxygen and nitrogen isotopic compositions, indicate that they are closely related to the CR and CH chondrites, all of which are members of the more inclusive CR clan. CB chondrites have much greater metal/silicate ratios than any other chondrite group, widely increasing the range of metal/silicate fractionation recorded in solar nebular processes. They also have the greatest moderately volatile lithophile element depletions of any chondritic materials. Metal has compositional trends and zoning patterns that suggest a primitive condensation origin, in contrast with metal from other chondrite groups. CB chondrites, as well as other CR clan chondrites, have much heavier nitrogen (higher 15N/14N) than that in other chondrite groups. The primitive characteristics of the CB chondrites suggest that they contain one of the best records of early nebular processes.Another chondrite, Grosvenor Mountains 9555 1, is petrographically similar to the CB chondrites, but its mineral and oxygen and nitrogen isotope compositions indicate that it formed from a different nebular reservoir.

The accretion of small bodies in the solar system is a fundamental process that was followed by planet formation. Chronological information of meteorites can constrain when asteroids formed. secondary carbonates show extremely old 53 mn-53 Cr radiometric ages, indicating that some hydrous asteroids accreted rapidly. However, previous studies have failed to define accurate mn/Cr ratios; hence, these old ages could be artefacts. Here we develop a new method for accurate mn/Cr determination, and report a reliable age of 4,563.4+0.4/-0.5 million years ago for carbonates in carbonaceous chondrites. We find that these carbonates have identical ages, which are younger than those previously estimated. This result suggests the late onset of aqueous activities in the solar system. The young carbonate age cannot be explained if the parent asteroid accreted within 3 million years after the birth of the solar system. Thus, we conclude that hydrous asteroids accreted later than differentiated and metamorphosed asteroids.

Abstract-We look at the relationship between the value of e 54 Cr in bulk meteorites and the time (after calcium-aluminum-rich inclusion, CAI) when their parent bodies accreted. To obtain accretion ages of chondrite parent bodies, we estimated the maximum temperature reached in the insulated interior of each parent body, and estimated the initial 26 Al/ 27 Al for this temperature to be achieved. This initial 26 Al/ 27 Al corresponds to the time (after CAI formation) when cold accretion of the parent body would have occurred, assuming 26 Al/ 27 Al throughout the solar system began with the canonical value of 5.2 9 10 À5 . In cases of iron meteorite parent bodies, achondrite parent bodies, and carbonaceous chondrite parent bodies, we use published isotopic ages of events (such as core formation, magma crystallization, and growth of secondary minerals) in each body's history to obtain the probable time of accretion. We find that e 54 Cr correlates with accretion age: the oldest accretion ages (1 AE 0.5 Ma) are for iron and certain other differentiated meteorites with e 54 Cr of À0.75 AE 0.5, and the youngest ages (3.5 AE 0.5 Ma) are for hydrated carbonaceous chondrites with e 54 Cr values of 1.5 AE 0.5. Despite some outliers (notably Northwest Africa [NWA] 011 and Tafassasset), we feel that the correlation is significant and we suggest that it resulted from late, localized injection of dust with extremely high e 54 Cr.

The discrepancy between the magnetic hysteresis properties of magnetite crystals that are precipitated from solution (<0.3 micrometer) and of crushed sifted grains (>0.3 micrometer) is not an inherent property of magnetite but is caused by the highly stressed state of crushed material and by adhering finer fragments. The size trends of magnetic properties exhibited by submicrometer-size precipitated grains continue in the size range from 1 micrometer to 1 millimeter in a set of hydrothermally recrystallized magnetite crystals. Coercive forces of these narrowly sized crystals follow a power law over a wide size range (0.1 micrometer to 1 millimeter) as predicted by theory. Dislocation etch pits show similar dislocation densities for hydrothermally grown (3 x 10(10) meter (-2)) and natural (1 x 10(10) meter(-2)) magnetite crystals. Hysteresis parameters of hydrothermally grown crystals are similar to those of natural crystals but are about one-fifth of those for crushed grains.