Meteorite classification for building the Chinese Antarctic Meteorite Depository Introduction of the classification of 500 Grove Mountains meteorites

Meteorites provide an important window into the origins and evolution of the solar system. Since the first four meteorites were recovered in Grove Mountains, Antarctica, in 1998, a further total of 12665 meteorites have been collected over seven polar seasons in the Grove Mountains. All of these meteorites are owned and managed by the Chinese Antarctic Meteorite Depository （CAMD） at the Polar Research Institute of China （PRIC）. In recent years, another 500 Antarctic meteorites have been classified and characterized based on mineralogy and petrology. In this work we examine four samples that have been identified as terrestrial, and a further 496 samples that have been confirmed as meteorites. These meteorites are further divided into different types：488 ordinary chondrites, one eucrite, one ureilite, one CM2 carbonaceous chondrite, one EH4 enstatite chondrite, one mesosiderite and three iron meteorites. The classification of meteorites not only provides an abundance of fundamental scientific data, but is also significant for introducing meteorites and related scientific knowledge to the publics particularly via the website of Chinese Resource-sharing Platform of Polar Samples for scientific research and education.

Progress in study of Chinese Antarctic Meteorites

This paper reviews and summarizes the Chinese Antarctica meteorite search, classification and research. During the past four antarctic explorations, a total of 9834 meteorites were collected in the Grove Mountains region. Among them, 2431 meteorites were classified by the end of 2008. So far, 684 meteorites have been officially published in the Meteoritical Bulletin, Meteoritical Society, including 2 martian meteorites, 2 eucrites, 6 ureilites, 5 mesosiderites, 1 pallasite, 1 iron and 10 carbonaceous chondrites. Comprehensive studies were carried out on a number of these rare type meteorites. In addition, we propose to continue the meteorite searching project in Grove Mountains and other regions in Antarctica. We also suggest several key topics of the future researches on the Chinese Antarctic meteorites.

The chemical compositions of Antarctic iron meteorites and their classification

The concentrations of 13 elements in the metal of 52 Antarctic irons have been reported and these irons have been classified based on the structures and their Ga,Ge, Ni, Ir and other trace elemental contents. The 52 iron meteorites assigned to chemical group consist of 16 of ⅠAB,12 of ⅡAB, 1 of ⅡE,3 of ⅢAB, 1 of ⅢCD, 1Of ⅣA and 18 of ungrouped irons. The ⅠAB, ⅢCD and ⅡE iron meteorite groups areconsidered to be of nonmagmatic origin. Nonmagmatic ⅠAB, ⅢCD and ⅡE ironsformed as individual pools of a chondritic body. The other groups, ⅡAB, ⅢAB andⅣA show the evidences of having originated by fractional crystallization of a metallicmagma.The slopes on element-Ni trends in the magmatic are generally higher than inthe nonmagmatic groups. Most interestings are the high abundance Of ungrouped andⅠAB (47.2% and 27.8%,respectively) and low abundance of ⅢAB (5.8%).Antarctic irons of the group abundances can be distinguished from non-Antarctic irons,which provide the information about previously unsampled parent planets, mass,shockand collision,as well as nebula regions.

Grove Mountains (GRV) 024237: A new ureilite from Antarctica

Ureilites share the characteristics of differentiated meteorites and of primary chondrites. GRV 024237 is a ureilite, which was found in the 19th Chinese National Antarctic Research Expedition （CHINARE）, at the No. 4 moraine, Grove Moun- tains, Antarctica. GRV 024237 consists mainly of coarse-grained olivine （60 vol%）, pigeonite （30 vol%） and opaque minerals （10 vol% ）. Tri-junction texture between olivine and pigeonite is common. Carbonaceous materials with minor amounts of troilite and nickel-iron metal were observed as interstitial phases. The Fa value of olivine composition varies from 6.2 to 16.8 from rim to core, but pyroxene is homogeneous in composition, with Fs 14.0 to 15.5. Both olivine and pyroxene have normal extinctions. Net-like iron or limonite veins filled in the fractures of olivine and pyroxene, and no diamond was observed. Based on petrographic and mineralogical features, GRV 024237 is a Type I and Group 2 monomict ureilite.

GRV 051523:A new eucrite

Grove Mountains （GRV） 051523 is a newly identified eucrite, consisting mainly of coarse-grained pyroxene （62.9 vol% ） and plagioclase （34.2 vol% ）, with less abundant opaque minerals （2.7 vol% ）, minor silica and tiny FeO-rich olivine （FaTs）. Coarse-grained pyroxenes show exsolution of augite lamellae in pigeonite or vice versa. Width of most exsolution lamellae in pyroxenes is 1-3 μm. Opaque minerals are mainly chromite, ilmenite and sulfides. The meteorite was heavily shocked, as indicated by breccias and melt veins. Coarse-grained pyroxenes commonly contain abundant tiny or needle-like chromite inclusions with orientation, probably due to heavy shock events. Pyroxenes in various petrographic occurrences exhibit highly homogeneous compositions, indicating that GRV 051523 experienced intense thermal metamorphism in the parent asteroid 4 Vesta. GRV 051523 is classified as Type 5-6. This new eucrite will have additional constraints on chemical composition, magmatic differentiation, multi-stage shock and thermal history of Vesta.

In situ SIMS oxygen isotope analyses: Evidence for the formation of aluminum-rich chondrules from ordinary chondrites

Aluminum-rich chondrules （ARCs）, which share mineralogic and chemical properties with both Ca, Al-rich inclusions （CAIs） and ferromagnesian chondrules, play an important role in revealing their temporal and petrogenetic relationships. In this work, seven ARCs were found in three ordinary chondrites GRV 022410 （H4）, GRV 052722 （H3.7） and Julesburg （L3.6）. They contain bulk Al2O3 - 17%-33% and exhibit igneous textures composed of olivine, high- and low-Ca pyroxene, plagioclase, spinel and glass. In situ SIMS analyses show that ARCs have oxygen isotopic compositions （δ18O=-6.1‰-7.1‰; δ17O= -4.5‰-5.1‰） close to ferromagnesian chondrules but far more depleted in 160 than CAIs （δ18O=-40‰; δ17O=-40‰）. Most ARCs plot close to the terrestrial mass fractionation （TF） line, and a few between the TF and carbonaceous chondrite anhydrous mixing （CCAM） lines. Plagioclase, nepheline and glass suffered O-isotopic exchanges during the metamorphism processes in the parent body. Spinel, olivine and pyroxene represent the primary O-isotopic compositions of ARCs, and define a fitted line with a slope of- 0.7±0.1. Compared with the results of previous studies, shallower slope as well as more depleted 160 compositions further demonstrates that ARCs in ordinary chondrites are not a simple mixing product of ferromagnesian chondrules and CAIs. Instead, they probably experienced higher-degree oxygen isotope exchange with a δ6O-poor nebular gas reservoir during multiple melting episodes.