NORTHWEST AFRICA 1763


CV3.5

standby for nwa 1763 photo
click on photos for a magnified view

Found October 2001
no coordinates recorded

A single small stone weighing 57.1 g was probably found near Boudnib, Morocco, and was subsequently purchased in Erfoud. Analysis is ongoing at the Muséum National d'Histoire Naturelle in Paris. The analyses thus far have determined that NWA 1763 is a carbonaceous chondrite of the CV group with a petrologic type of 3.5 (Fa0.2734.91; Fs0.692.34). It is both unshocked (S0) and unweathered (W0), with an unoxdized matrix and an abundance of intact sulfide. The pristine condition of this CV3.5 chondrite should prove helpful in continued studies of this carbonaceous chondrite group.

Northwest Africa 1763, along with most carbonaceous chondrites, contains many refractory inclusions of calcium and aluminum (CAIs) that were probably formed by condensation at temperatures high enough to vaporize the Fe and Mg silicates. One theory places their formation early in nebular history when the heat source was the gravitational energy of the accreting stellar disk. Outward diffusion mechanisms allowed some CAIs to escape solar accretion and become stabilized in the outer, zero-drag, newly formed Jovian gap. It was further proposed that surface flares from Jupiter then formed chondrules, and both components were ultimately incorporated into a carbonaceous assemblage. More recently, theories have attributed the formation of chondrules to shock waves caused by gravitational instabilities.

Yet, another theory of CAI origin places the formation at a later period, when the accretion phase was over and the Sun was in its T Tauri phase. The solar wind swept the volatile-rich gas from the outer layers of the nebular disk leaving only refractory-rich dust behind. Shock wave heating then evaporated the Fe and Mg silicates, leaving the dust enriched in Al. This dust eventually coalesced, and was melted to form the CAIs that are present in most carbonaceous meteorites. More recently, studies have shown that CAIs were accreted rapidly into larger bodies, heated by the decay of 26Al, and thermally metamorphosed. Thereafter, these bodies were disrupted, and the CAIs were returned to the nebula to be remixed and recycled into later carbonaceous chondrite bodies. Layered rims surrounding some CAIs, referred to as Wark–Lovering rims or accretionary rims, were possibly formed by a flash heating event in a more oxidizing environment of the solar nebula. This heating event was measured in fractions of a second and resulted in a loss of volatiles with enrichment of the refractory component, along with the subsequent diffusion of O and Mg. During the flash heating event, temperatures at the rim are inferred to have approached 3000°C, steeply decreasing to temperatures of ~1700°C just 1 mm below the rim. Subsequent chemical and isotopic exchange, corresponding to the grain size and porosity of specific minerals, most likely occurred in situ on the parent body.

The CV3 group has been subdivided into four subgroups (McSween, 1977; Weisberg et al., 1997; Wasson et al., 2013):

  1. Reduced subgroup: e.g., Arch, Efremovka, Leoville, Vigarano, and QUE 93429
  2. Oxidized-Allende subgroup: e.g., Allende, Axtell, Tibooburra, and ALH 84028
  3. Oxidized-Bali subgroup: e.g., Bali, Grosnaja, Kaba, and Mokoia

The oxidized-Allende and reduced subgroups are separated on the basis of metal abundances and the Ni content of sulfide (Howard et al., 2010). The previously used discriminator, magnetite abundance, has been shown to overlap between oxidized and reduced subgroups. The oxidized-Bali subgroup has a higher degree of aqueous alteration than oxidized-Allende (for more mineralogical relationships, see Appendix I, Carbonaceous Chondrites). The subgroup to which NWA 1763 belongs has not yet been published, but it likely belongs to the oxidized-Allende subgroup.

Some investigators (e.g., Greenwood et al., 2003 and Wasson et al., 2013) have proposed that the CK chondrites could represent an extension of the CV group. This subgroup is considered to reflect varying degrees of metamorphism including impact-generated crushing, thermal alteration, and recrystallization processes (Wasson et al., 2013). In a subsequent study, Dunn et al. (2016) compared magnetite in a number of CK and CV chondrites, and presented geochemical, mineralogical, and petrographic evidence which is more consistent with separate CV and CK parent bodies; details of their study can also be found on the Dhofar 015 page.

The first naturally occuring quasicrystal, dubbed icosahedrite, was discovered by Princeton University scientists embedded in stishovite grains inside a rare mineral called khatyrkite. Oxygen isotopic values of the icosahedrite quasicrystals are similar to those of CV3 carbonaceous chondrites which suggests a common extraterrestrial origin in the solar nebula over 4.5 billion years ago (Bindi et al., 2012, PNAS). The specimen of NWA 1763 pictured above is a 3.35 g slice (some saw marks are evident in the photo). A photo of the complete, fresh stone as found is shown below.

standby for nwa1763 photo
Photo courtesy of Philippe Thomas—Meteoritica