Trace and minor elements in garnet and quartz: novel approaches to understanding crustal evolution

Abstract

PART I: Garnet is a common and important mineral in metamorphic systems yet the mechanisms by which it incorporates Ti— one of the major elements in the crust— are not well constrained. This study uses garnets synthesized at a range of temperatures and pressures to understand Ti solubility and the substitution mechanisms that govern its incorporation into garnet at eclogite and granulite facies conditions. Garnets from these experiments can incorporate up to several wt. % TiO2. Comparing Ti content with deficits in Al and Si in garnet indicate that Ti is incorporated by at least two substitution mechanisms (VITi4+ + VIM2+ ↔ 2VIAl3+, and VITi4+ + IVAl3+ ↔ VIAl3+ + IVSi4+). Increasing Ti solubility is correlated with increasing Ca and Fe/Mg ratios in garnet, clinopyroxene and melt. The complexity of the substitution mechanisms that account for Ti solubility in garnet make practical Ti in garnet thermobarometry infeasible. However, a model fit to Ti partitioning between garnet and the melt from which it crystallized could be used to predict melt compositions in high grade metamorphic systems. Additionally, the solubility and substitution mechanisms described here can help explain the presence of crystallographically-aligned rutile needles in high grade metamorphic systems.

PART II: Ti can either be incorporated tetrahedrally (IVTi) or octahedrally (VITi) in most silicate minerals, and decades of research have yet resolve which coordination dominates in garnet. Ti Kα X-ray Absorption Fine Structure (XAFS) spectroscopy enables observation of Ti coordination in minerals and melts. In this study, XAFS is used to determine the coordination of Ti in experimental and natural garnets. Garnets grown experimentally at eclogite- and granulite-facies conditions can contain several wt. % TiO2, most of which is incorporated as VITi. This observation aligns with major element trends in these garnets. In natural garnets grown at lower temperatures and pressures, Ti is observed to occupy both the octahedral and tetrahedral sites in garnet— in some cases Ti is almost entirely IV-fold coordinated. Combined with previous research (see Chapter 2) on substitution mechanisms for VITi, the results of this study demonstrate that Ti is incorporated on multiple crystallographic sites in garnet by at least three primary substitution mechanisms. There is a discrete increase in VITi content in garnet relative to temperature and pressure between natural and experimental garnets suggesting increases in these two parameters may increase Ti solubility. However, a continuous increase in VITi with increasing grossular content also suggests Ca content of the garnet plays a critical role in VITi solubility.

PART III: Quartz is one of the most common minerals on the surface of the earth, and is a primary rock-forming mineral across the rock cycle. These two factors make quartz an obvious target for sediment provenance studies. Observations from experimental and natural samples demonstrate that the trace-element content of quartz often reflects the conditions of quartz formation. When quartz is weathered from its primary crystallization setting (i.e., quartz from a granitoid) it can retain many chemical signatures of formation throughout the sedimentation processes. These geochemical signatures can be used to understand the primary source of individual quartz grains within a sediment. Here we present a case study from the Bega River catchment to demonstrate that quartz grains in sediments at the mouth of the Bega River are sourced from granitoids within the drainage basin. Data presented here also indicate that a portion of the beach sediment is also derived from either – (i) sedimentary rocks within the basin or; (ii) mixing with sediments at the mouth of the river. The Bega River catchment was selected for this study because it is both small and has a well-constrained bedrock lithology, making it an ideal location to test the utility of this provenance technique. However, quartz trace-element provenance has broad applications to modern and ancient sediments and can be used in lieu of, or in conjunction with, other provenance techniques to elucidate sediment transport through time.