AGOULT


Eucrite
Monomict, noncumulate
(Residual)

standby for agoult photo
Found March 2000
30° 33' N., 4° 54' W.

An 82 g stone with fresh fusion crust was found in Morocco. Many paired stones were subsequently recovered, bringing the total known weight to over 650 g. This eucrite has an unbrecciated, homogeneous, very fine-grained granulitic texture composed of pyroxene (~100 µm) and plagioclase (~50 × 300 µm). Opaques present include ilmenite, Ti-chromite, and FeS. Agoult was probably formed and experienced metamorphism at depth and at high temperatures.

In their studies of a small number of metamorphosed eucrites, Yamaguchi et al. (2009) determined that a few of them exhibited differences from Main Group–Nuevo Laredo trend eucrites in that they show varying degrees of LREE depletion with positive Eu anomalies. They believe these features are the result of rapid partial melting caused by magma plume intrusions and/or impact events, with subsequent melt extraction. These reheating events were superimposed over a protracted metamorphism that was already occurring within the deep crust. The team proposes to call these eucrites (e.g., Agoult, A-87272, DaG 945, and NWA 2362) "residual eucrites", which have lost a partial melt. This melt phase in turn contaminated the Main Group–Nuevo Laredo trend eucrites to produce the Stannern trend eucrites.

A study of the volatization of zinc isotopes in HEDs was conducted by Panielloa et al. (2012). Their data showed that unbrecciated eucrites like Agoult were isotopically heavier than brecciated eucrites with regards to Zn, and also contain lower Zn concentrations. They suggest that unbrecciated eucrites likely represent very early crustal material that was ejected from the surface of Vesta to form the Vestoids, while ensuing chondritic impacts produced the brecciated nature of the much larger parent asteroid resulting in its unique Zn isotopic systematics.

It is noteworthy that Agoult has been described by some investigators as having elemental, chemical, and textural characteristics similar to those of the ungrouped eucrite-like achondrite Ibitira. A Pb–Pb age based on plagioclase was calculated for Agoult by Iizuka et al. (2013) and determined to be 4.5322 (±0.001) b.y. Utilizing relatively large zircons (up to 80 µm) present in Agoult, Iizuka et al. (2014) determined a Pb–Pb age of 4.5545 (±0.002) b.y.; this age is thought to represent the time of high-temperature metamorphism in the basaltic crust. By contrast, the precisely calculated Pb–Pb age for Ibitira, 4.55675 (± 0.00057) b.y. as reported by Iizuka et al. (2014), is slightly older. Any difference in the ages of these two meteorites might be understood in consideration that the age of Ibitira is interpreted to represent the time of onset of thermal metamorphism. In addition, the crystalization temperature of zircon formation was calculated based on Ti-thermometry, with the Ti content of 37–55 ppm corresponding to ~905 (±32) °C. Utilizing this temperature combined with Ce-oxybarometry, they determined that the zircon was formed under reducing conditions in the absence of any oxidizing aqueous fluid. The specimen of Agoult pictured above is a 0.25 g partial slice. The photo below is an excellent petrographic thin section micrograph of Agoult, shown courtesy of Peter Marmet.

standby for lodran photo
click on photo for a magnified view
Photo courtesy of Peter Marmet