SYNERGISTIC COUPLING OF QUARTZ TITANIUM ZONING AND MELT INCLUSION COMPOSITIONS: UTILIZING THE QUARTZ GROWTH RECORD TO ELUCIDATE MAGMATIC ASSEMBLY AND STORAGE OF THE YOUNGEST TOBA TUFF, INDONESIA

Use of quartz in assessing the conditions, origin, and persistence of zoning within the magma reservoir responsible for the enormous (2800 km³) and compositionally zoned (68-77 wt. % SiO₂) 74 ka Youngest Toba Tuff (YTT) is limited by debate over the role of crystal settling and poor constraints on the factors that control Ti partitioning. Zoning in those quartz records a dynamic growth history, wherein high silica rhyolite (HSR) quartz have lower Ti and minor variations relative to low silica rhyolite (LSR) quartz. Here we provide a critical context for interpreting these conditions from coupling major and trace element compositions of 120 quartz-hosted melt inclusion (MI) with Ti-calibrated cathodoluminescence maps in crystals from five diverse YTT pumices.

We find that SiO₂ is highly variable between MI. In contrast, key trace elements have affinities to host pumice and show strong correlations between each other and with Ti in adjacent quartz. MI populations in HSR are evolved, overlap in composition, and exhibit minor inter-crystal variations. LSR MIs are bimodal, with some MI in crystal cores partially overlapping HSR compositions, and a dominant population of strikingly less evolved MI occurring in crystal interiors and rims.

The affinity between quartz MI compositions and host pumice indicates that quartz growth occurred virtually in-situ in a zoned magma body. Accordingly, the greater scatter of the MI SiO₂ record may be due to post-entrapment crystallization of quartz that modified SiO₂ but had little impact on trace elements. The limited heterogeneity in feldspar-controlled elements within HSR MI, coupled with little variation in quartz Ti zoning, is consistent with protracted crystallization under melt composition- and temperature-buffered granite minimum (eutectoid) conditions. The evolved MI in some LSR quartz cores could support minor crystal settling from HSR zones, albeit from slightly deeper eutectoid conditions. The excursions to much less evolved compositions between these and rim-ward inclusions implicates sanidine dissolution and subsequent quartz crystallization under non-eutectoid conditions prior to growth of higher Ti rims. Correlations between Ti in MI and adjacent quartz suggest that increased aTiO₂ accompanied growth of distinct, high Ti rims, possibly at higher temperatures.