Origin of rutile needles in star garnet and implications for interpretation of inclusion textures in ultrahigh‐pressure metamorphic rocks

Abstract

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 indicate 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 0 , 3, 4 and 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 the 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.

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

“…The rutile needles have anomalous (non-parallel) extinction (Griffin et al 1971). This reflects single or multiple definite crystallographic orientation relationships (COR) between rutile and host garnet that are now well characterized by electron backscatter diffraction and transmission electron microscope studies (Proyer et al 2013;Hwang et al 2014).…”

“…For example, unusual Idaho "star" garnets (Guinel and Norton 2006), well known among mineral and gem enthusiasts for their four-star or six-star asterism, have rutile needle inclusions that may have formed by precipitation from garnet or co-precipitation of rutile and garnet (Hwang et al 2014). Star garnets are rare.…”

Rutile inclusions in garnet from a dissolution‐reprecipitation mechanism

“…Oriented rutile needles are known to occur in garnet of various rock types formed under a wide range of P–T conditions, for example, kimberlitic eclogite (Griffin, Jensen, & Misra, ; Hills & Haggerty, ), ultrahigh‐pressure (UHP) eclogite (Hwang, Yui, et al, ; Liati, Gebauer, & Wysoczanski, ; Ye, Cong, & Ye, ; Zhang & Liou, ; Zhang et al, ), garnet peridotite (van Roermund, Drury, Barnhoorn, & Ronde, ), UHP gneiss and metapelite (Hwang, Yui, et al, ; Mposkos & Kostopoulos, ; Proyer et al, ) and amphibolite to granulite facies rocks (Ague & Eckert, ; Hwang, Shen, Chu, Yui, & Iizuka, ; Kawasaki, Nakano, & Osani, ; O'Brien, ; Vrana, ; Whitney, ). These oriented rutile crystals have long been considered to have formed during P–T changes as exsolution/precipitation products of the precursor Ti‐bearing garnet, based primarily on the profound shape‐preferred orientations of rutile crystals in garnet host (e.g.…”

“…Ague & Eckert, ; Mposkos & Kostopoulos, ; van Roermund et al, ; Ye et al, ; Zhang & Liou, ). However, detailed transmission electron microscopic (TEM) study on the oriented needle‐like rutile crystals in UHP garnet from a Sulu (China) eclogite and an Erzgebirge (Germany) quartzofeldspathic rock showed that the rutile needles do not follow a specific crystallographic orientation relationship (COR; Hwang, Yui, et al, ), but instead follow multiple CORs with respect to the garnet host (Hwang, Shen, Chu, & Yui, ; Hwang et al, , see compiled CORs in Table ). The rutile needles oriented along both <111> grt and <001> grt directions in a six‐ray star garnet from amphibolite facies metapelite also follow similar crystallographic relations (Hwang et al, ; see Table ).…”

“…However, detailed transmission electron microscopic (TEM) study on the oriented needle‐like rutile crystals in UHP garnet from a Sulu (China) eclogite and an Erzgebirge (Germany) quartzofeldspathic rock showed that the rutile needles do not follow a specific crystallographic orientation relationship (COR; Hwang, Yui, et al, ), but instead follow multiple CORs with respect to the garnet host (Hwang, Shen, Chu, & Yui, ; Hwang et al, , see compiled CORs in Table ). The rutile needles oriented along both <111> grt and <001> grt directions in a six‐ray star garnet from amphibolite facies metapelite also follow similar crystallographic relations (Hwang et al, ; see Table ). Moreover, multiple CORs with appreciable misfits were noted in an electron backscattered diffraction (EBSD) study of rutile needles in garnet from UHP metapelite of the Kimi Complex, Greece (Proyer et al, , see analysis in Hwang et al, ).…”