NORTHWEST AFRICA 753


R3.9, rumurutiite
standby for northwest africa 753 photo
Found January 2000
coordinates not recorded

Multiple fragments weighing together 12 kg were purchased in Rissani, Morocco, while the actual find location was probably the Kem Kem region of the Sahara. Classification was performed at the Institut für Planetologie in Münster, Germany. Northwest Africa 753 is a brecciated, very weakly shocked (S2) meteorite that is relatively fresh compared to most other R chondrite finds—designated W2 on the Wlotzka weathering scale (1993). Abundant sulfides, typically pyrrhotites, attest to a S-rich environment of formation (Jackson and Lauretta, 2010). It has been established through laboratory analyses that several pairings to NWA 753 exist, including NWA 1472, 1476, 1477, 1478, and 1566.

Results of Mn–Cr isotopic systematics have established an initial age for NWA 753 of 4.561 (±2) m.y., which is a few m.y. younger than the age established for carbonaceous chondrites (~4.566 m.y.) (Jogo et al., 2006). A possible correlation of Cr-isotopic compositions with O-isotopic variations for ordinary and R chondrites has been demonstrated.

In their search for CAIs and other Al-rich objects in R chondrites, Rout and Bischoff (2008) and Rout et al. (2009) discovered a high abundance of such objects in unequilibrated clasts from NWA 753. They determined that the abundance of CAIs in R chondrites is less than it is in carbonaceous chondrites, but greater than in ordinary and enstatite chondrites. These CAIs in R chondrites are significantly smaller than those in CM and CV chondrites, but similar in size to those in CH, O, and E chondrites. Based on Δ17O values, the CAIs in R chondrites were divided into 16O-rich (~ –23‰ to –26‰), 16O-depleted (~ –2‰), and heterogeneous (–25‰ to +5‰). As with CAIs of other chondrite groups, R chondrite CAIs were likely formed in an 16O-rich nebular region, with some sustaining subsequent isotopic exchange with an 16O-depleted nebular gas or through metasomatism on the parent asteroid. Taking into account the differences in mineralogy between the majority of R chondrite CAIs and those from other groups, R chondrites contain a unique subset of CAIs. That said, the investigators argue that certain types of CAIs present in R chondrites strongly resemble some of those found in the CO-chondrite group, especially those present in the more highly metamorphosed meteorites like Ornans, Moss, Isna, and Lancé. Other Al-rich objects in R chondrites share similarities to those from the O- and E-chondrite groups.

Kita et al. (2013) employed the same technique for NWA 753 that is applied to ordinary chondrite chondrules for determination of the petrologic subtype from 3.00 to 3.2, which is based on the alteration resistant Cr content in ferroan olivine (Grossman, 2004; Grossman and Brearley, 2005). They determined that the corresponding petrologic subtype for the least equilibrated material in NWA 753 is 3.15–3.20, the lowest subtype found among R chondrites. Crowther et al. (2015) utilized I–Xe chronometry in a preliminary study of four samples of NWA 753 having a petrologic type range of 3.15–3.9. They found that a correlation exists between the closure age and the petrologic type of a sample, with a younger age being correlated with a higher degree of metamorphism. This range of petrologic types reflects a difference in age of ~5 m.y. years and is consistent with cooling within an onion-shell structure on the R-chondrite parent body.

Consistent with the absence of solar gases, NWA 753 is not thought to have been part of a regolith for any significant length of time. Evidence gathered thus far indicates that NWA 753 experienced a relatively rapid transfer from the asteroid belt to the Earth, having a 21Ne-based CRE age of 12.8 m.y. (Schultz et al., 2005). Subsequent noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). Their ~16 groupings vary slightly from the previous pairings suggested by other investigators, and along with NWA 753, they propose an ~14 m.y. CRE age group representing possible common source craters and/or fall events which includes those R chondrites with the NWA series designations 1472, 1476, 1478, 1566, 4360, 4360, 4419, and 5606.

This unique chondrite group was originally named for the Carlisle Lakes, Australia (49.5 g) specimen, but has since been renamed for the only fall of the group from Rumuruti, Kenya (67 g) in 1934. Two views of a 0.54 g partial slice of NWA 753 are shown above: the right photo shows the melted fusion-crusted surface, while the left photo shows the abundant tiny chondrules characteristic of this chondrite group.