Formation conditions of aluminum-rich chondrules

Abstract

We have studied the formation conditions of Al-rich chondrules by doing isothermal and dynamic crystallization experiments at one atmosphere on four different chondrule analogue compositions within the pure CaO-MgO-Al 2 O 3 -SiO 2 system. For the dynamic crystallization experiments, we cooled from both liquidus and subliquidus peak temperatures (T max ), at cooling rates from 5-1000°C/h. The starting compositions include two with anorthite and two with forsterite as the dominant liquidus phases, all at or near spinel-saturation. One of each pair evolves towards diopside crystallization, and the others cordierite or enstatite crystallization, giving a total of four completely different crystallization sequences analogous to the four basic varieties of Al-rich chondrule recently proposed. Bulk composition is the main controlling factor, both in terms of mineralogy and texture. The textures of the anorthite-rich compositions are more sensitive to T max than they are to cooling rate, whereas the textures of the forsterite-rich compositions are more sensitive to cooling rate. Comparisons of natural Al-rich chondrules having similar compositions to our synthetic analogues indicate that the natural objects reflect a range of peak heating temperatures, $1400-1500°C, and cooling rates of 10-500°C/h for porphyritic chondrules and possibly higher (1000°C/h) for barred chondrules. These conditions are consistent with the conditions inferred for ferromagnesian chondrules but differ from those inferred for some calcium-aluminum-rich inclusions.

Petrology and mineralogy of the Ningqiang carbonaceous chondrite

“…Therefore, the presence of Al-enstatite further supports the conclusion that ARCs were formed by rapid cooling processes. This has been proven by the recent study by Tronche et al (2007). They concluded from isothermal and dynamic crystallization experiments that the formation conditions of ARCs are consistent with those inferred for ferromagnesian chondrules.…”

Refractory inclusions and aluminum‐rich chondrules in Sayh al Uhaymir 290 CH chondrite: Petrography and mineralogy

“…Dynamic crystallization experiments by Tronche et al (2007) also indicate that bulk composition is a main controlling factor in terms of mineralogy and texture. In NWA 739, Krot et al (2006a) observed that Al-diopside-rich and plagioclase-rich ARCs plot on different sides of the anorthite-forsterite (Krot et al 2006a).…”

Oxygen isotopic compositions of Allende Type C CAIs: Evidence for isotopic exchange during nebular melting and asteroidal metamorphism

“…The key assumption in that interpretation was that oxygen diffusion in spinel is much slower than in pyroxene, and slower in pyroxene than in melilite or anorthite. However, the validity of that Type C CAIs provide a natural laboratory for testing the partial melting model, because the igneous crystallization sequence of a Type C CAI (see Tronche et al, 2007) is very different from the above: spinel, anorthite, and melilite + pyroxene (whose relative order in this case is a function of bulk composition). The Type C oxygen isotope data presented in this paper in fact refute the partial melting model altogether.…”

“…One way to approach the problem is to study objects that have transitional properties between chondrules and CAIs. Al-rich chondrules are one such kind of intermediate object, and these have received much attention over the past 10 years (e.g., Russell et al, 1996Russell et al, , 2000Huss et al, 2001;MacPherson and Huss, 2005;Tronche et al, 2007). Another group of "intermediate" objects are the Type C CAIs, which are rare relative to other CAI varieties and which have been little studied (Wark, 1987).…”