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Fibrous amphibole and clay mineral inclusions that form striking trapiche-like star patterns within quartz crystals from Inner Mongolia, China, present a challenge to uncover how these crystals grow and incorporate inclusions in a geological context. We propose that the patterns formed as a result of protogenic clay (ferrosaponite or nontronite) inclusions that were preferentially trapped on rough surfaces during quartz crystal growth. The rough surface texture of these crystals is the result of multiple growth centers during 2D nucleation and spread and split crystal formation. Observations via optical microscopy, cathodoluminescence, and three-dimensional micro-CT scanning highlight how the exterior surface textures on the termination of a complete quartz crystal mimic its interior inclusion patterns. Cathodoluminescence images, as well as varying aluminum concentrations along a core-to-exterior transect in a quartz crystal slice, suggest that the formation fluid underwent a heterogeneous chemical history. Measurements of Ti and observations of fluid inclusions suggest the quartz formed at a temperature of under 348 °C. This study presents the details surrounding split crystal growth in quartz in a natural geological setting, which has implications for inspiring new materials and may serve as an indicator for turbid and highly supersaturated formation fluid conditions in geological formations.

An estimate of TiO2activity (aTiO2melt−sat)$\left( a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}} \right)$is necessary for the application of trace-element thermobarometry of magmatic systems where melts are typically undersaturated with respect to rutile/anatase. Experiments were performed in the system SiO2-Na2O-TiO2to develop two independent methods of estimating aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$—one based on the commonly applied rutile-saturation technique and another utilizing a novel Ti-in-tridymite thermometer. It is demonstrated that the rutile-saturation model can lead to an overestimate of aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$relative to TiO2activity calculated using the solubility of Ti in tridymite (SiO2) coexisting with rutile. Overestimation via the rutile-saturation technique is due to variations in the solubility mechanisms of Ti in the melt phase as a function of Ti content. In natural systems, overestimates of aTiO2melt−sat$a_{\text{Ti}{{\text{O}}_{2}}}^{\text{melt}-\text{sat}}$will lead to an underestimation of crystallization temperatures by Ti-based trace-element thermobarometers. Although this study is not directly applicable to natural systems, it lays the groundwork for future research on natural composition magmas to constrain TiO2activity in melts.

The structure and composition of granites provide clues to the nature of silicic volcanism, the formation of continents, and the rheological and thermal properties of the Earth's upper crust as far back as the Hadean eon during the nascent stages of the planet’s formation1–4. The temperature of granite crystallization underpins our thinking about many of these phenomena, but evidence is emerging that this temperature may not be well constrained. The prevailing paradigm holds that granitic mineral assemblages crystallize entirely at or above about 650–700 degrees Celsius5–7. The granitoids of the Tuolumne Intrusive Suite in California tell a different story. Here we show that quartz crystals in Tuolumne samples record crystallization temperatures of 474–561 degrees Celsius. Titanium-in-quartz thermobarometry and diffusion modelling of titanium concentrations in quartz indicate that a sizeable proportion of the mineral assemblage of granitic rocks (for example, more than 80 per cent of the quartz) crystallizes about 100–200 degrees Celsius below the accepted solidus. This has widespread implications. Traditional models of magma formation require high-temperature magma bodies, but new data8,9suggest that volcanic rocks spend most of their existence at low temperatures; because granites are the intrusive complements of volcanic rocks, our downward revision of granite crystallization temperatures supports the observations of cold magma storage. It also affects the link between volcanoes, ore deposits and granites: ore bodies are fed by the release of fluids from granites below them in the crustal column; thus, if granitic fluids are hundreds of degrees cooler than previously thought, this has implications for research on porphyry ore deposits. Geophysical interpretations of the thermal structure of the crust and the temperature of active magmatic systems will also be affected. Thermobarometry and diffusion modelling in quartz crystals show that some granites may crystallize at much lower temperatures than we had thought, possibly explaining observations of cold magma storage.

Titanium can be incorporated either tetrahedrally (IVTi) or octahedrally (VITi) in most silicate minerals. Ti K-edge 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 synthetic and natural garnets. Garnets grown synthetically 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, on the other hand, Ti is observed to occupy both the octahedral and tetrahedral sites in garnet—in some cases Ti is almost entirely fourfold coordinated. Combined with previous research (see Ackerson et al. 2017, this issue) on substitution mechanisms for VITi, the results of this study demonstrate that Ti is incorporated on two crystallographic sites in garnet by at least three primary substitution mechanisms. In both natural and synthetic garnets, there is a discernible increase in VITi content in garnet with increasing temperature and pressure, suggesting a significant role for these two parameters in determining Ti solubility. However, a continuous increase in VITi with increasing grossular content also suggests that the Ca content of the garnet plays a critical role.

Garnet is a common and important mineral in metamorphic systems, but the mechanisms by which it incorporates Ti—one of the major elements in the crust—are not well constrained. This study draws upon 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 synthesis experiments can incorporate up to several wt% TiO2. Comparison of Ti content with deficits in Al and Si in garnet indicates 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 involved in Ti solubility in garnet makes practical Ti-in- garnet thermobarometry infeasible at present. However, a model fit to Ti partitioning between garnet and melt can 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.