HAMMADAH AL HAMRA 119


R4, rumurutiite
standby for hammadah al hamra 119 photo
Found April 1995
28° 30.87' N., 12° 53.81' E.

Five fragments of this meteorite having a total weight of 352 g were found in the Libyan Sahara Desert. It is unpaired to any other Saharan R chondrite. Hammadah al Hamra 119 is a rare unbrecciated R chondrite having a petrologic grade of 4, reflected by its equilibrated olivines. It has been weakly shocked to stage S3, which has produced shock veins, and has been severely weathered to grade W4 on the Wlotzka weathering scale (1993), which has produced abundant staining in the olivine.

This is a unique group of chondrites having a higher volume of olivine (FeO-rich), a lower volume of pyroxene, and essentially no FeNi-metal as compared to all other chondrite groups. This is thought to be the result of metasomatic oxidation in which FeNi-metal and pyroxene reacted with water to form olivine (Isa et al., 2010). While R chondrites share some characteristics with the various chondrite groups, they differ in several chemical and petrographic trends and define a unique parent body. Although most R chondrites contain highly altered CAIs second in abundance only to carbonaceous chondrites, none have been observed in HaH 119. A high content of noble metals (Pt, Os, Au, Ir) is a characteristic feature of R chondrites. The Pt, along with associated Sn and As, resides in the mineral sperrylite which formed during conditions of increased metamorphism.

The difference in the oxygen isotopic abundances is greater between the R chondrites and ordinary chondrites than it is among the H, L, and LL ordinary chondrites, further resolving this group from the ordinary chondrites (Weber et al., 1997). On an oxygen 3-isotope diagram, the R chondrite group does not plot along the ordinary chondrite trend line; in fact, it plots farther from the carbonaceous and enstatite chondrite groups than it does from the ordinary chondrite groups (Rubin and Kallemeyn, 1989). R chondrites are also highly oxidized, have a low chondrule/matrix ratio, and have the highest 17O value of any other Solar System material. Hammadah al Hamra 119 has a CRE age based on cosmogenic 21Ne of 2.5 m.y. (Schultz et al., 2005).

This chondrite group was originally named for the Carlisle Lakes, Australia (49.5 g) meteorite, but was subsequently renamed for the only fall of the group from Rumuruti, Kenya (67 g). In past years there have been many new R chondrite finds from the hot and cold deserts of Africa, Australia, and Antarctica. The above specimen is a 0.7 g complete slice with remnant fusion crust.