Monomict breccia, cumulate
standby for northwest africa 1836 photo
Purchased in 2003
coordinates not recorded

A single, fresh, partially fusion-crusted stone, composed of two matching fragments with weights of 505.10 g and 596.77 g, was acquired by an American dealer from Rissani, Morocco, in 2003. A later recovery of a 334 g paired stone has been reported. Analysis and classification was completed at Northern Arizona University (T. Bunch and J. Wittke) and NWA 1836 was determined to be an unusual eucrite classified as a cumulate, monomict breccia. The eucrite clasts, which occur throughout, have highly contorted shapes, and they are surrounded by cataclastic mantles and zones of partially melted to recrystallized material. The shock level ranges from S1 to S5. The textural appearance of this cumulate eucrite prompted one scientist to give it the nickname ‘Twisted Sister’.

The likely genesis of the HED clan began with the development of a global magma ocean on a chondritic body, the result of heating by short-lived radiogenic elements. This was soon followed by the segregation of an ~75 km radius metallic core, dated at 4,566.1 (±1.2) m.y. ago based on Hf–W, Al–Mg, and Mn–Cr systematics (Touboul et al., 2008). As the critical melt fraction reached ~15%, equilibrium crystallization of olivine was initiated within a vigorously convecting magma. During this period, gravitational segregation became the predominant force and this resulted in the earliest silicate layer to form—a deep olivine-rich dunite layer ~150 km thick. Following this high-temperature phase, crystallization of an orthopyroxene-rich, ~13-km-thick, cumulate diogenite layer ensued. Thereafter, residual liquids which were subjected to fractional crystallization (Holzheid and Palme, 2007) were extruded. This period of volcanism produced basalt flows that solidified to form a thin crust ~15 km thick (Mayne et al., 2008). This basaltic crustal rock was buried in turn by continual insulating flows of lava, resulting in its reheating and metamorphism and eventual formation of the Main Group–Nuevo Laredo trend eucrites.

The late-stage ascent of a portion of this Main Group magma was contaminated with a crustal partial melt to become the incompatible-element-rich Stannern trend eucrites. Some of the residual liquid, or more likely a separate REE-enriched liquid, was trapped at depths of up to ~10 km and underwent late fractionation and re-equilibration processes to produce the cumulate eucrites; this cumulate material has been dated at ~60–100 m.y. after CAI formation based on Hf–W, Sm–Nd, and Lu–Hf systematics (Touboul et al., 2008). Thereafter, surface eucritic material underwent impact gardening to form a regolith, during which time both diogenitic and xenolithic material was intermixed. Ultimately, lithification occurred forming the polymict howardite members of the HED clan. Complete crystallization was achieved within ~20 m.y.

Cumulate eucrites exist in a wide compositional range, and were formed as a result of fractional crystallization and gravitational settling within an evolved magma which had a composition similar to that of Nuevo Laredo. The resulting cumulate, composed of pyroxene and plagioclase in equilibrium with a trapped eucritic melt component, remained as a chemically closed system within a magma chamber. The incompatible element abundances present in cumulate eucrites were contributed by the trapped melt component. Studies of REE equilibrium partition coefficients and the chemical effects of interstitial melt lead scientists to conclude that parental melts of cumulate eucrites were the same as, or similar to, those of noncumulate eucrites (Barrat, 2004). Final cooling probably occurred at depths of 3–8 km, consistent with intrusion into crustal material.

The asteroid 4 Vesta has a diameter of ~525 km with an outer basaltic crust thin enough (~10–25 km) to have been completely excavated down to diogenitic material, or deeper still into harzburgitic material, by the impact of a 10–20 km-sized impactor. This event would have produced the 460-km-wide, ~13-km-deep crater observed near the south pole. Some of the thousands of identified fragments that were spalled into space by these impacts, appropriately named Vestoids, form a bridge between the orbit of Vesta and both the v6 secular resonance and the 3:1 Kirkwood gap. These fragments were eventually perturbed into an Earth-crossing orbit on a time scale of tens of m.y., from which pieces could easily find their way to Earth. One Vestoid, 1929 Kollaa, has a spectral signature consistent with a pyroxene composition similar to that of cumulate eucrites.

The Ar–Ar age of NWA 1836 and other eucrites were reset by large impact events at a time closely corresponding to the impact reset ages of lunar meteorites, which occurred during the Late Heavy Bombardment period ~3.8–4.1 b.y. ago. Age clusters for eucrites include some that are slightly younger than those representing the Moon; specifically, clusters at ~3.45, ~3.55, and ~3.78 are younger, while other clusters at ~3.90 and 3.98–4.07 b.y. parallel those of lunar samples (Bogard and Garrison, 2009). Both lunar and Vesta chronometer resetting events likely represent the same population of impactors, with impacts on Vesta continuing for a longer time. CRE age distributions of a statistical sampling of HEDs show that at least two major impact events occurred, ~22 and 39 m.y. ago, which delivered samples such as NWA 1836 to Earth.

Northwest Africa 1836 is one among a relatively small cumulate eucrite group represented in part by the monomict breccias Medanitos, Binda, Talampaya, and Dhofar 007, the unbrecciated eucrites Moama, Moore County, Nagaria, and Serra de Magé, and various other eucrites from the hot and cold deserts of Northwest Africa and Antarctica, respectively. The photo above shows an 8.83 g partial slice of NWA 1836.