Cooling rates of diogenites: A study of Fe²⁺‐Mg ordering in orthopyroxene by single‐crystal x‐ray diffraction

Abstract

Abstract-The iron-magnesium exchange between MI and M2 sites in orthopyroxene is a reversible reaction that records the latest event in the thermal evolution of the host rock. A kinetic analysis of this process has been applied to 16 orthopyroxene single crystals from 7 different diogenites to constrain the cooling history of their parent body. The Fe2+-Mg ordering degrees have been determined by single-crystal x-ray difiction. The Fe2+-Mg ordering closure temperatures were very homogeneous within each sample and ranged, for all diogenites studied, between 31 1 2 29 "C and 408 2 10 "C. Cooling rates at these closure temperatures were calculated using a numerical method developed by Ganguly (1982). These ranged between -5 "C/104 year in Johnstown and -0.8 "C/year in Roda.A comparison with other achondrites studied with the same method showed that increasing closure temperatures correspond to increasing cooling rates and that meteorites from a same parent body exhibit similar closure temperature and cooling rate values.The cooling rates obtained for these diogenites, at their low closure temperatures, should probably be ascribed to a complex thermal history of their parent body, thus confirming Miyamoto and Takeda's theory (1 994a) of excavation of deep crustal material due to impact events. The differences on cooling rate values for different diogenites could be due to different burial depths in the fragment ejected from the parent body.

Mineralogy of V-type asteroids as a constraining tool of their past history

“…On the other hand, we have not found the presence of wollastonite. The latter is in agreement with Bowman et al (1997), but opposing to the value reported by Zema et al (1997). Bowman et al (1997) found some traces of olivine on Johnstown, while we have found an olivine content of 10%.…”

“…We have found a lower enstatite content for Johnstown than Zema et al (1997) and Bowman et al (1997). On the other hand, we have not found the presence of wollastonite.…”

“…MB-TXH-095-B (Johnstown): Zema et al (1997) inferred an average enstatite, ferrosilite and wollastonite content of 73.7%, 23.5% and 2.8%, respectively. We have found a lower enstatite content for Johnstown than Zema et al (1997) and Bowman et al (1997).…”

Magnesium oxide‐iron oxide mass balance constraints and a more detailed model for the relationship between eucrites and diogenites

“…Presumably, by analogy with trends among lunar samples (McKay et al, 1991), the four gas-rich howardites are especially "mature" (well-mixed) samples of the HED asteroid's regolith. al., 1982;Mittlefehldt, 1994;Zema et al, 1996), diogenites formed deeper in the asteroid than most (i.e., at least the noncumulate) eucrites. Provided the parent asteroid is basically intact, its surface regolith composition must be biased toward materials of shallow original provenance, as impact ejection (excavation) is relatively inefficient for materials of deep provenance (Melosh, 1989).…”

Thermal history of ALH 84001 meteorite by Fe2+‐Mg ordering in orthopyroxene

“…This calculation has been performed by several researchers (Virgo and Hafner 1969;Besancon 1981;Anovitz et al 1988;Saxena et al 1987Saxena et al , 1989Skogby 1992;SykesNord and Molin 1993;Ganguly and Tazzoli 1994;Zema et al 1999;Heinemann et al 2000;Zema et al 2003;Stimpfl et al 2005) thus allowing the orthopyroxene to be used as a "geospeedometer" for retrieving the thermal history of its host rock. This method has already been applied to several terrestrial and extraterrestrial rocks (Ganguly 1982;Molin et al 1994;Ganguly and Domeneghetti 1996;Kroll et al 1997;Zema et al 1997;Domeneghetti et al 2000;Ganguly and Stimpfl 2000;Stimpfl et al 2005;McCallum et al 2006), but has never been used for the ALH 84001 meteorite. The aim of this work is to apply the orthopyroxene geospeedometer to ALH 84001 sample by measuring the Fe 2+ -Mg ordering state through single-crystal X-ray diffraction (XRD).…”