TWO GENERATIONS OF SPHENE IN ONE GARNET FROM A FRANCISCAN ECLOGITE, HEALDSBURG, CALIFORNIA
Two different generations of sphene are found as inclusions in garnets in an eclogite from Healdsburg, CA. The eclogite is part of an exotic block from the Franciscan Complex and is overprinted by blueschist facies retrograde metamorphism. The matrix assemblage is garnet-chlorite-sphene-phengite-clinozoisite-glaucophane-omphacite with minor apatite and aragonite. Matrix sphenes have a maximum length of 2 mm and contain inclusions of clinozoisite and rutile. Garnets range in diameter from 0.5-4 mm and are highly chloritized and fractured. The garnets are inclusion rich, containing sphene, clinozoisite, rutile, quartz, phengite, chlorite, aragonite, galucophane, apatite, omphacite, zircon, and ilmenite. Inclusions range in size from several microns to several hundred microns in length, and in many cases show a foliation. Prograde zoning is retained in garnet, with Alm56Pyp11Grs30Sps3 cores and Alm55Pyp14Grs30Sps1 rims.
The two generations of sphene are compositionally different. The first, low-aluminum sphene (LAS), Ca.98Ti.90Al.07Fe.03O4.96OH.10F.002, is similar in composition to matrix sphene, Ca1.01Ti.94Al.06Fe.01O4.99OH.06F.006. The second, high-aluminum sphene (HAS), has an average composition of Ca.98Ti.76Al.21Fe.02O4.85OH.23F.002. OH substitution increases with Al content, unlike in other high-P sphenes which generally contain more F than OH. LAS is larger in diameter and more common as an inclusion than HAS, which is scarce in the garnet. No chemical zoning in garnet is detected around either type of inclusion. The two sphenes occur as close as 50 mm apart from one another. With additional thermodynamic data, the reaction 5 HAS + quartz=grossular + clinozoisite + 2 H2O can be applied to these rocks. Using the barometer clinozoisite + sphene=grossular + rutile + quartz + H2O, HAS yields higher maximum pressures than LAS.