Publication Date:
2014-12-07
Description:
The asterism effect of star garnet has been attributed to the oriented distribution of needle-like rutile inclusions. Rutile needles occur in garnet from a wide range of metamorphic settings and rock bulk compositions, and their origin has been ascribed to different mechanisms, such as exsolution, and used to interpret petrological and tectonic processes. Results from an optical and transmission electron microscopy of Idaho star garnet indicates a co-precipitation origin. It was found that rutile needles are predominantly oriented along the 〈103〉 rt //〈111〉 grt and 〈001〉 rt //〈001〉 grt directions following multiple crystallographic orientation relationships (CORs); i.e. COR-1, 2, 2’, 3, 4 & 5, in 6-ray star garnet, and are oriented solely along the 〈103〉 rt //〈111〉 grt directions following exclusively COR-2 in 4-ray star garnet. The sole presence of COR-2 〈111〉 grt needles in the common 4-ray star garnet, in contrast to the presence of both 〈111〉 grt and 〈001〉 grt needles with multiple CORs in the rare 6-ray star garnet, suggests that the COR-2 〈111〉 grt needle probably is the energetically most favoured variant, as is also supported by the coincidence site lattice considerations. The unique crystallography-controlled microstructures of 4-ray star garnet, including the cloudy domains behind the {111} grt or {100} grt fronts with abundant inclusions of rutile needle, rutile compound needle and multiple-phase-inclusion, as well as the clear domains behind the {110} grt fronts with only a few above inclusions concentrated exclusively within the linear, 〈110〉 grt -oriented, continuous tube-like domains, further suggest that the COR-2 〈111〉 grt needles in 4-ray star garnet most likely have a growth-in origin, co-precipitating with garnet at its growth fronts close to thermodynamic equilibrium conditions. The 6-ray star garnet, on the other hand, most likely formed under far-from equilibrium conditions, thereby yielding a maximum of 99 crystallographic variants of rutile needles with multiple CORs in a single crystal. In light of these findings, along with the common occurrences of the sole COR in many inclusion-host systems owing to the requirement to minimize the energy barrier in an exsolution process, the presence of both 〈103〉 rt //〈111〉 grt and 〈001〉 rt //〈001〉 grt needles with multiple CORs in garnet of Sulu eclogite and Erzegebirge quartzofeldspathic rock, would therefore cast doubt on the assertion of an exsolution origin of rutile needles in garnet from these ultrahigh-pressure rocks. This article is protected by copyright. All rights reserved.
Print ISSN:
0263-4929
Electronic ISSN:
1525-1314
Topics:
Geosciences
Permalink
