Influence of Deformation and Fluids on Ti Exchange in Natural Quartz

Abstract

Using a combination of microstructural, spectroscopic, and geochemical analyses, we investigate how subgrain rotation recrystallization and fluid migration affect Ti concentration [Ti] in naturally deformed quartz veins from the Prijakt Nappe (Austroalpine Unit, Eastern Alps). These coarse‐grained quartz veins, that formed at amphibolite facies conditions, were overprinted by lower greenschist facies deformation to different degrees. During the overprint, subgrain rotation recrystallization was dominant during progressive deformation to ultramylonitic stages. The initial [Ti] (3.0–4.7 ppm) and cathodoluminescence (CL) signature of the vein crystals decrease during deformation mainly depending on the availability of fluids across the microstructure. The amount of strain played a subordinate role in resetting to lower [Ti] and corresponding darker CL shades. Using a microstructurally controlled analysis we find that the most complete re‐equilibration in recrystallized aggregates ([Ti] of 0.2–0.6 p.m.) occurred (a) in strain shadows around quartz porphyroclasts, acting as fluid sinks, and (b) in localized microshear zones that channelized fluid percolation. [Ti] resetting is mainly observed along wetted high angle boundaries (misorientation angle >10–15°), with partial [Ti] resetting observed along dry low angle boundaries (<10–15°). This study shows for the first time that pure subgrain rotation recrystallization in combination with dissolution‐precipitation under retrograde condition provide microstructural domains suitable for the application of titanium‐in‐quartz geothermobarometry at deformation temperatures down to 300–350°C.

Quartz Vein Formation and Deformation during Porphyry Cu Deposit Formation: A Microstructural and Geochemical Analysis of the Butte, Montana, Ore Deposit

“…Grain boundary migration reset quartz towards the equilibrium Ti concentration for the recrystallization temperature. Volume diffusion of Ti through quartz is very slow [66,67], but strain-induced grain boundary migration can reset Ti in quartz towards a new equilibrium value at a rate that is much faster [68][69][70] especially in fluid-rich systems such as Butte [71]. Lower temperature recrystallization mechanisms-subgrain rotation and bulging recrystallization-can reset Ti to lower equilibrium values in certain conditions [71] but are generally much less likely to do so [71,72].…”

“…Volume diffusion of Ti through quartz is very slow [66,67], but strain-induced grain boundary migration can reset Ti in quartz towards a new equilibrium value at a rate that is much faster [68][69][70] especially in fluid-rich systems such as Butte [71]. Lower temperature recrystallization mechanisms-subgrain rotation and bulging recrystallization-can reset Ti to lower equilibrium values in certain conditions [71] but are generally much less likely to do so [71,72]. Haertel et al [73] found that synkinematic quartz veins that underwent retrograde bulging and subgrain rotation recrystallization during exhumation at temperatures less than 350 °C preserved Ti concentrations and gave thermometric estimates that were a close approximation of the temperature of original vein formation.…”

“…Marmo and Wilson [80] showed that a vein made of oriented columnar ice crystals in hard glide orientations remained undeformed during glacial flow as strain was accommodated in the surrounding easy glideoriented ice. Bestmann et al [71] also showed that the crystallographic orientation of quartz in metamorphic veins determined whether a crystal persisted as a relatively unstrained porphyroblast or was deformed into ribbon or mylonitic quartz. These results highlight the role that growth fabrics can have on rock strength during ductile deformation.…”

Cathodoluminescence as a tracing technique for quartz precipitation in low velocity shear experiments

“…Total intensity CL maps show a lower intensity CL emission from the blue precipitated quartz. In recent studies on recrystallization of deformed quartz, decreased total CL intensity was linked to a reset of the Ti concentration in quartz 19 , 20 . Also in our samples we observe a decreased total CL intensity in the precipitated quartz with blue CL emission.…”

“…Williams et al 18 used a cold-cathode CL system on a petrographic microscope to trace quartz precipitation in healed fractures and overgrowths after long duration quartz growth experiments (at 300–450 °C and 150 MPa, with an Al-enriched pore fluid containing amorphous silica). Recently, various authors used CL to visualize recrystallized quartz in experimentally and naturally deformed rocks, linking CL to trace element re-equilibration (especially Ti) resulting from the presence of grain boundary fluids during deformation, both at low and high temperatures 19 – 22 .…”