ORIENTED RUTILE NEEDLES IN GARNET AND THEIR IMPLICATIONS FOR EXTREME METAMORPHIC CONDITIONS

Oriented inclusions in garnet are potential indicators of crystallization at high-pressure/ultrahigh-pressure (HP/UHP) or high-temperature/ultrahigh-temperature (HT/UHT) conditions. Needles, lamellae, and plates of rutile ± ilmenite are the most common inclusions, but other phases including apatite, pyroxenes, micas, and quartz (or coesite) are also reported. Rutile ± ilmenite (“oxide”) needles are usually described from UHP, granulite facies, and mantle rocks. They are typically oriented parallel to garnet <111> and have a definite crystallographic orientation relationship with host garnet (Proyer et al., 2013). The most widely cited mechanism for needle formation is precipitation from Ti-bearing garnet during retrogression, as the original Ti content of garnet can be considerable at elevated P-T conditions. Other mechanisms, including recrystallization or coprecipitation of oxide and garnet are also possible. We examine oxide inclusions from a range of natural garnets to assess inclusion morphology as a function of metamorphic grade. Tiny (<1 um) oxide particles precipitated on dislocations can be found in amphibolite facies garnets and are not diagnostic of extreme P-T conditions. Needles start to become noticeable under the microscope at high magnifications in ~800 °C granulite facies rocks, but are very fine (<~1 um wide) and are typically not obvious under normal illumination and magnification conditions. With increasing metamorphic intensity, needles tend to get more volumetrically abundant, thicker, and/or longer. In UHT garnet from Connecticut, USA, they can be nearly 1 mm long and 9 um wide (Ague & Eckert, 2012; Axler & Ague, 2015), and in UHP Erzgebirge garnet up to 200 um long and 3 um wide. These needles are “petrographically obvious”, but the microscope condenser may still be required to see them fully. Literature data from 30 localities shows that such needles are found in rocks formed at ultrahigh-T > ~900 °C and/or P in excess of ~1.8 GPa. However, it is also common for garnets that crystallized at these conditions to lack needles, suggesting that oxide precipitation is highly sensitive to multiple factors including bulk Ti content and cooling history. Oriented oxide needles in garnet can be important indicators of extreme metamorphism, but must be interpreted with care.