Zircon inclusions in garnet (ZiG) can be used as an elastic thermobarometer to estimate the pressure-temperature (P-T) conditions of geologic processes. Although this method is being increasingly applied as a thermometer to natural samples, experimental evaluation of ZiG is limited and has not been investigated at geologically relevant pressures. We present the first high-pressure assessment of the reliability of ZiG thermobarometry by crystallizing almandine with zircon inclusions in piston-cylinder experiments between 700-900 degrees C and 2.0-3.0 GPa. Zircons entrapped in experiments at >2.0 GPa yield residual inclusion pressures within 1 sigma of predicted values and yield entrapment temperatures within 17-30 degrees C of experimental T. Zircon inclusions in experiments performed at 2.0 GPa yield average inclusion pressures that are similar to 0.09 GPa higher than predicted values and therefore yield entrapment temperatures 65-73 degrees C higher than the experimental T. Evaluation of inclusion pressure trends shows that ZiG host-inclusion systems did not undergo non-elastic deformation during experimental exhumation, confirming that ZiG elastic thermobarometry can accurately record P-T conditions of garnet crystallization so long as zircon inclusions do not go into tension post-entrapment. Our high-pressure experiments suggest that the ZiG host-inclusion system may be applicable to a variety of geologic settings and processes, such as subduction zone metamorphism, partial melting, and contact metamorphism.

The Manhattan Prong (a belt of ancient rock in southern New York, USA) underlies one of the largest population bases in North America yet its tectonometamorphic history remains poorly understood. We performed monazite U-Pb and garnet Sm-Nd petrochronology in association with garnet diffusion chronometry on a sample of the Manhattan Schist from Central Park, New York City. Monazite included in both garnet and matrix yielded a 206Pb/238U date of ca. 438 Ma, though one matrix grain yielded a distinctly younger date of ca. 391 Ma; all monazite analyzed showed identical rare earth element compositions. Garnet yielded a robust isochron date of ca. 386 Ma, interpreted to constrain the timing of garnet growth at near-peak metamorphic conditions. Retention of manganese zoning in 1-mm-scale garnet requires that near-peak metamorphic conditions at ca. 386 Ma persisted for <5 m.y. The Manhattan Schist records a polymetamorphic history involving Taconic greenschist-facies metamorphism at ca. 438 Ma, recorded by relict monazite, then short-lived Acadian kyanite-grade metamorphism at ca. 386 Ma, recorded by garnet and the major rock-forming assemblage. These results are consistent with tectonic models that involve localized, episodic heating of the middle crust, rather than gradual and more pervasive processes of heating and cooling. Our study is an example of the nuanced tectonometamorphic understanding that can be gained by combining accessory and major phase petrochronology.

Elastic thermobarometry has been rarely applied to quartz inclusions entrapped in garnet (QuiG) in granulite and igneous terranes, in part, because there is uncertainty about the reliability of the thermobarometric results arising from the quartz inclusions being subject to tensile strain and stress when examined at room conditions. Here, we present QuiG results from high-temperature metapelites from the Adirondacks, NY, USA and piston-cylinder experiments that give insight into the deformation behavior of quartz inclusions under tension. Measured remnant pressures (Pinc) of experimental and natural samples calculated using the quartz phonon mode Grüneisen tensor are too tensile with respect to the expected Pinc values based on experimental and petrologic constraints. We show that these discrepancies are not related to non-elastic deformation nor inaccuracies in the quartz equation of state. Evaluation of previous density functional theory (DFT) results shows that the structural response of quartz is non-linear with increasing tensile strain. Therefore, because the available quartz phonon mode Grüneisen tensor was determined with a linear fit optimized for compressive strains, obtained tensile strains using this tensor are too large in magnitude. Pinc values obtained using the hydrostatic calibrations of the 128 and 464 cm−1 peaks have better agreement with the expected values and return entrapment conditions that are consistent with petrologically constrained or known experimental pressures. Pinc values obtained through hydrostatic calibrations must nonetheless be treated with caution because the behavior of Raman phonon modes under tension has not been calibrated experimentally.

Accurately constraining the timing and tempo of specific metamorphic events is necessary to constrain the rates of important geodynamic processes, such as burial and exhumation in orogens and devolatilization in subduction zones. Accessory phase petrochronology is a popular tool employed to constrain the age and duration of metamorphic processes, including mineral nucleation and deformation fabric development. While accessory phases can produce robust dates, their thermodynamic data is often poorly constrained, requiring careful petrographic analysis to link dates to the pressure-temperature-deformation histories of metamorphic rocks. The dating of rock-forming minerals, like garnet and plagioclase, is typically more resource intensive than accessory phase petrochronology, but allows for directly linking dates to specific metamorphic events. Here, we consider the results of monazite accessory phase U-Th-Pb chemical dating with that of Sm-Nd dating of garnet from the Manhattan Schist, New York City, to explore how the synthesis of these two methods can be used to constrain robust tectonic histories. The Manhattan Schist is a Laurentia-derived aluminous pelite historically thought to record polymetamorphism characterized by: 1) upper amphibolite facies metamorphism and anatexis associated with the Taconic Orogeny (~520-470 Ma) and 2) lower grade overprinting during the Acadian orogeny (~416-360 Ma). This interpretation, however, is based on correlation with seemingly similar units elsewhere in New England rather than direct study of the Manhattan Schist, and is called to question by the data presented here. This work explores three samples, MAT-2017-01a, GWB-03, and CRT-06. All samples are grt-ky-bt-ms migmatites, that record a four-stage metamorphic history: 1) garnet growth starting at ~550°C and 4-6 kbar, 2) burial and heating to kyanite-grade anatexis at 700-750°C and 7-11 kbar, 3) ~1-2 kbar of isothermal exhumation to sillimanite (fibrolite) stability, and 4) continued exhumation to ~700°C and ~6 kbar. Monazite grains in all samples occur both in the matrix and as inclusions in garnet. Metamorphic monazite dates are dominated by a ~465 ±30 Ma (n=15) age consistent with growth during the Taconic Orogeny, with only a single date at ~383 ±25 Ma found in CRT-06. Bulk garnet Sm-Nd dating via TIMS in MAT-2017-01a, however, yields a late Acadian age of ~386 ±3.68 Ma (n=5, MSWD=0.94). The low MSWD coupled with the preservation of major element growth zoning in garnet, suggests that this value represents a single garnet age population reflective of growth at peak or near-peak conditions, entirely during the Acadian orogeny. Taken as a whole, our results suggest that the Manhattan schist experienced: 1) early greenschist facies metamorphism during the Taconic orogeny recorded in metamorphic monazite, and 2) peak metamorphism associated with collision and burial in the Acadian orogeny recorded in garnet. Additionally, the sample-to-sample heterogeneity in monazite ages within the same lithology suggests that monazite growth is controlled by (sub)cm-scale processes. Therefore, future studies should investigate multiple samples of the same formation, if not the same outcrop, before making tectonic interpretations based on accessory phase petrochronology.

Changes in δ13C value of bulk sedimentary organic matter (OM) throughout Earth's history are thought to reflect carbon cycle perturbations, but as sedimentary OM may derive from multiple sources, it could also record other processes. We measured δ13C of microscale components of shale OM using nano-EA-IRMS to investigate drivers of large-magnitude carbon isotope excursions (CIE) in the late Tonian Chuar Group, USA. Components included organic-walled microfossils, kerogen, graphite, and macerate size-fractions. Microfossils δ13C has a broad range within samples, but average values vary little throughout stratigraphy and are decoupled from bulk δ13Corg, showing that these positive CIEs are not driven by secular changes in the carbon cycle. Instead, our fine-scale approach identified enriched components that can account for the CIE: exogenous clasts of kerogen and graphite, a finer macerate fraction, and abundant Eosynechococcus-a bloom-forming phytoplankter. The presence of these 13C-enriched particles indicates that the positive CIE signals were driven by a combination of allochthonous input/enhanced productivity, as well as thermal alteration. Fine-scale measurements can tease apart contributors to bulk δ13Corg records and offer insights into the Proterozoic carbon cycle.

Abstract New results that employ Zr‐in‐rutile thermometry (ZiR) and quartz‐inclusion‐in‐garnet (QuiG) barometry constrain the P–T conditions of garnet formation in blueschists and eclogites from the island of Syros, Greece. QuiG barometry reveals that garnet from different regions across the island formed at pressures ranging from 1.1 to 1.8 GPa and ZiR thermometry on rutile inclusions in garnet constrains the minimum temperature of garnet formation to have been 475–550°C. Most importantly, there is no systematic difference in the conditions of garnet formation from different regions across the island and these results are nearly identical to those obtained from the islands of Sifnos and Ios, Greece. A model is proposed whereby the rocks from all three islands were initially metamorphosed along a relatively shallow geotherm of around 11°C/km to a depth of around 45 km and were then subjected to metamorphism along a geotherm of around 7–8°C/km, which could have been caused by either an increase in the dip of the subduction zone or an increase in the rate of subduction. Garnet formed along this steeper geotherm was accompanied by the release of significant H2O from the breakdown of chlorite over a duration of 1 Ma or less based on thermal and diffusion modeling. It is concluded that rocks from Syros, Sifnos and Ios all followed a similar, roughly counter‐clockwise prograde P–T path and that the present outcrop configuration is largely due to a complex exhumation history.

Inclusion–host elastic thermobarometers are widely used to determine the pressure and temperature (P–T) histories of metamorphic rocks. Complex metamorphic P–T paths can affect the pressures that develop in host–inclusion systems. There are limited experimental studies that investigate how changing P–T conditions may re-equilibrate or “reset” residual pressures of inclusions. To evaluate re-equilibration of the quartz-in-garnet (QuiG) elastic thermobarometer, we performed single-, two-, and three-stage isothermal experiments. In the first stage of the experiments, oxide starting materials hydrothermally crystallised to grow garnet crystals with quartz inclusions between 700 and 800 °C and 1.0 and 3.2 GPa with constant P–T conditions for 48 h. In the second and third stage of the experiments, we isothermally changed pressure by 1.0 to 1.2 GPa for durations up to 38 d. We used Raman spectroscopy to measure strain-induced changes to the 128, 207, and 465 cm−1 Raman bands of quartz inclusions to determine the inclusion pressures (Pinc) and entrapment pressures (Ptrap) at the experimental temperature. The multi-stage experiments show that elasticity primarily controlled changes to Pinc values that occur from Ptrap through quenching to room conditions and that Pinc values measured at room conditions along with elastic modelling can be used to accurately calculate Ptrap. Quartz Pinc values in two-stage experiments re-equilibrated to give Pinc values between P1 and P2. The three-stage isothermal experiments show that the observed changes to inclusion pressures are reversible along different P–T paths to restore the re-equilibrated Pinc values back to their original entrapment isomeke at Ptrap. For rocks that underwent protracted metamorphism along complicated P–T paths, the re-equilibration experiments and viscoelastic calculations show that QuiG may underestimate maximum Ptrap conditions.

Quartz crystals with zircon inclusions were synthesized using a piston-cylinder apparatus to experimentally evaluate the use of inclusions in “soft” host minerals for elastic thermobarometry. Synthesized zircon inclusion strains and, therefore, pressures (Pinc) were measured using Raman spectroscopy and then compared with the expected inclusion strains and pressures calculated from elastic models. Measured inclusion strains and inclusion pressures are systematically more tensile than the expected values and, thus, re-calculated entrapment pressures are overestimated. These discrepancies are not caused by analytical biases or assumptions in the elastic models and strain calculations. Analysis shows that inclusion strain discrepancies progressively decrease with decreasing experimental temperature in the α-quartz field. This behavior is consistent with inelastic deformation of the host–inclusion pairs induced by the development of large differential stresses during experimental cooling. Therefore, inclusion strains are more reliable for inclusions trapped at lower temperature conditions in the α-quartz field where there is less inelastic deformation of the host–inclusion systems. On the other hand, entrapment isomekes of zircon inclusions entrapped in the β-quartz stability field plot along the α–β quartz phase boundary, suggesting that the inclusion strains were mechanically reset at the phase boundary during experimental cooling and decompression. Therefore, inclusions contained in soft host minerals can be used for elastic thermobarometry and inclusions contained in β-quartz may provide constraints on the P–T at which the host–inclusion system crossed the phase boundary during exhumation.

High-pressure low-temperature rocks from Svalbard are an excellent target for studying metamorphic reactions in Phanerozoic subduction zones. This study reveals the presence of monazite in an eclogite and a blueschist from the Vestgötabreen Complex, southwestern Svalbard. In order to investigate the monazite-forming reaction, we obtained pressure–temperature estimates coupled with U–Pb and Lu–Hf dating. Combined geothermobarometry allows to constrain three evolutionary stages of garnet growth in the eclogite: nucleation (1.6 ± 0.3 GPa at 460 ± 60 °C), peak-pressure (2.3 ± 0.3 GPa at 507 ± 60 °C), and peak-temperature (2.1 ± 0.3 GPa at 553 ± 60 °C). A zircon age of 482 ± 10 Ma is interpreted to belong to the prograde part of the pressure–temperature path. Monazite forms inclusions within garnet rims, or it is surrounded by allanite and apatite, altogether forming pseudomorphs of a tabular shape in the matrix. Textures, geothermobarometry and geochronology support the conclusion the monazite formed under high-pressure conditions at 471 ± 6 Ma. We propose that the monazite crystallization in the eclogite happened due to a decomposition of accessory phases during the decompression after peak-pressure of the metamorphic cycle. Monazite in the blueschist occurs as inclusions in garnet cores and gives an indicative age of 486 ± 6 Ma, which is interpreted to reflect the prograde growth of the garnet. Lu–Hf garnet dating resolves an age of peak-pressure metamorphism in the blueschist at 471.1 ± 4 Ma under conditions of 2.0 ± 0.03 GPa and 500 ± 30 °C. The Vestgötabreen Complex provides evidence for an early Ordovician modern-style subduction system in the proximity of the Baltica margin. Hence, this study also supports the tectonic models that favour a mixed Baltican and Laurentian provenance of south-western Svalbard.

Host-inclusion elastic thermobarometers are widely used to determine the pressure and temperature (P-T) conditions of formation and deformation, and histories of metamorphic rocks.Metamorphic P-T paths may be complex, and this complexity can affect the stress state of mineral inclusions.Most previous experimental studies for host-inclusion thermobarometry have included only static P-T conditions.Further refinement and experimentation are necessary to better understand how hostinclusion systems behave during progressive metamorphism and exhumation.This study is the first to apply dynamic experimental pressure-temperature-time (P-T-t) conditions for host-inclusion thermobarometers.Quartz inclusions in garnet hosts were grown hydrothermally from powdered oxide materials in a piston-cylinder device in two-stage isothermal or isobaric experiments.Isothermal experiments were conducted as both pressurization and depressurization experiments ranging from 1.0-3.2GPa.Isobaric experiments were conducted as both heating and cooling experiments from 300-800°C.First-stage conditions were held constant for 48 hours.Second-stage experimental conditions were held constant for 0-912 hours.Changes in experimental pressures ranged from 0.5-1.2GPa.Changes in temperature ranged from 100-500°C.Individual quartz inclusions were measured for changes to the 128, 206, and 464 cm -1 Raman bands to determine the entrapment pressures (P trap ).Over 200 individual inclusion measurements on each experiment indicate that the majority of inclusions reequilibrated towards the new experimental P-T conditions.Average P trap values for all experiments are between the two pressure stages.There is no evidence suggesting that significant numbers of inclusions were entrapped after adjusting P-T conditions to the second-stage condition.This study demonstrates that the stress state of quartz inclusions in garnet have an immediate elastic response to changes in P-T conditions followed by a protracted viscoelastic response.Experimental results indicate that changes to P-T conditions during metamorphism will modify the stress state of quartz inclusions, and the final stress state of quartz inclusions provides information on the final metamorphic conditions.

High-pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P-T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)-the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr-in-rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65-2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be similar to 480-500 degrees C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P-T trajectory of similar to 11 degrees C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of similar to 45 km reaching similar peak metamorphic conditions of similar to 500-525 degrees C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high-pressure prograde mineral assemblages during exhumation. The existence of similar P-T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.

Garnet-kyanite-staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P-T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet-mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet-biotite thermometry, delineate a prograde path from 480 to 600 degrees C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665 degrees C at <= 0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U-Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P-T conditions at 397 +/- 2 Ma (2 sigma) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 +/- 2 Ma. Monazite III, found only in the garbenschist, is 374 +/- 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P-T path that remained in the kyanite stability field. These results point to a relatively short (approximate to 12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons-similar to the 390 +/- 10 Ma Cape Woods granite located 40 km across strike from the fault-that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.

The measurement of residual stresses in exhumed rocks yields valuable information about metamorphic temperature and pressure, deformation and rheology, and stress state. However, the state of elastic strain and stress at the surface of a sample does not necessarily correspond to the state well below the surface. When a sample under elastic strain is cut, polished, or otherwise prepared for analysis, a part of the constraining rock is removed, allowing for the partial relaxation of the elastic strain. To be able to work with residual elastic strain and stress with analytical methods that probe the upper few microns of a sample, the process of strain relaxation must be well understood. For this work we used high-angular resolution EBSD to analyse stressed quartz inclusions in natural garnet from a range of settings, and in several samples grown in piston-cylinder experiments that were previously analysed with Raman spectroscopy for inclusion pressures. The experimental samples are not expected to have undergone plastic deformation in the garnet during cooling, as the majority of the pressure within the inclusion built up during decompression at room temperature. Additionally, the inclusion pressures in buried inclusions matches what is expected for the experimental conditions, suggesting no plastic yielding. Thus, in these samples we can isolate elastic strain from potential plastic deformation. One of the experimental samples was analysed with TEM to test this expectation. Forescatter images reveal topographical effects resembling quartz and adjacent garnet “extruding” out of the sample. Furthermore, rotations of the quartz lattice and the garnet lattice immediately around the quartz inclusion are observed. The rotation axis of the misorientation generally lies in the plane of the sample surface. TEM analysis revealed a number of dislocations in experimental garnet where these were not expected. However, a significant degree of bending of a wedge of garnet between the original sample surface and a quartz inclusion is also observed. The dislocations observed with TEM do not fit with the model of the experiments. Also, the formation of dislocations before sample preparation does not explain the dependence of the rotation axis on the surface orientation. A likely scenario for the deformation measured with EBSD is that the partial relaxation of elastic strains in stressed quartz inclusions in garnet as result of sample preparation induced local distortion of the inclusion and host. Additionally, the persistence of topographical features related to this relaxation despite several steps of polishing suggests that relaxation is not instantaneous but occurs over time.

New experiments to study titanium solubility in quartz were conducted at conditions not previously explored to extend and improve existing Ti-in-quartz solubility models for thermobarometric applications. Starting materials for experiments included silica glass, anatase, synthetic and natural rutile, Ti-enriched silica gel, Ti-enriched melts, zirconia, and HF and H2O fluids. Additional experimental data enabled us to characterize Ti-in-quartz solubility across much of the α- and β-quartz stability fields from 2 to 30 kbar and 550 to 1050 °C. Mutual occurrences of mineral inclusions in one another and Raman spectroscopy of mineral phases confirmed co-crystallization of quartz, rutile, and zircon. Electron microprobe measurements and cathodoluminescence images show that Ti concentrations in quartz crystals from all experiments are relatively uniform, and Ti concentrations of quartz crystals grown at the same experimental conditions using several Ti–rich starting materials and several different growth media are the same within experimental and analytical uncertainties. There are no significant differences in Ti concentrations of quartz across the α–β quartz transition. The Ti concentration in quartz crystals, $${X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ , systematically increases with temperature, but the quantity $$RT \mathrm{ln}{X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ is a constant at fixed pressure. The Ti concentration in quartz decreases non-linearly with pressure. To account for the observed P–T dependent changes to Ti in quartz, we developed the Ti-in-quartz solubility model: $$RT \mathrm{ln}{X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}=- 55.287-[P\left(\mathrm{kbar}\right)\bullet (-2.625+0.0403 P\left(\mathrm{kbar}\right))]+RT \mathrm{ln}{a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ where R is the gas constant 0.0083145 kJ/K, T is temperature in Kelvin, P is the pressure in kbar, $${X}_{{\mathrm{TiO}}_{2}}^{\mathrm{quartz}}$$ is the mole fraction of TiO2 in quartz, and $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ is the activity of TiO2 in the growth media (e.g., fluid, melt) referenced to rutile at standard state conditions of 1 bar and 25 °C. Experiments that co-crystallized quartz, rutile, and zircon permitted us to cross-check thermobarometric results from our Ti-in-quartz solubility models against the widely accepted Zr-in-rutile solubility models. We further tested our Ti-in-quartz solubility models using experiments that co-crystallized quartz, wollastonite, and titanite to fix $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ < 1. Concentrations of Ti in quartz crystallized from the sub-unity $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ experiments in the α- and β-quartz fields predict activities that match those calculated using the mineral reaction equilibrium and available thermodynamic data. Demonstrated agreement between calculated and measured experimental P–T conditions using the Zr-in-rutile and Ti-in-quartz solubility models and the consistent reduction of Ti concentrations in systems with $${a}_{{\mathrm{TiO}}_{2}}^{\mathrm{rutile}}$$ < 1 provide evidence that our experimental results accurately describe the equilibrium solubility of Ti in quartz.