skip to main content


Title: Locating the granitic composition water-saturated solidus
The accepted water-saturated solidus for granitic compositions (granitic water-saturated solidus, G-WSS) was largely determined >60 years ago. Significant experimental resources using modern approaches have been allocated to defining, refining, and parameterizing the solidus positions for other rock compositions, but limited work has been performed to accurately define G-WSS. Modern experimental and analytical techniques afford the opportunity to re-investigate the position of the G-WSS. Various thermobarometric applications to many granitic and rhyolitic composition rocks commonly return temperature estimates significantly lower than the widely accepted G-WSS determined largely by Tuttle and Bowen (1958). To evaluate the apparent discrepancies and help distinguish igneous from metamorphic processes recorded in granitic mineral assemblages, we performed experiments at temperatures ranging from 575 to 900°C and 0.5 to 10 kbar on 12 granitoid compositions composed of natural and synthetic starting materials. Most starting materials were melted in the presence of water at 10 kbar and 900°C in piston cylinders and quenched to room temperature in under one minute to produce water-saturated glasses for usage in subsequent crystallization experiments. The results of experiments on glass compositions were further validated in several runs using finely-ground crystalline starting materials. We ran crystallization experiments at P-T conditions spanning the accepted G-WSS. Decreasing experimental temperatures along each isobar caused systematic decreases in melt percentages until achieving complete crystallization at solidus conditions. A time-series of experiments at P-T conditions with ~20% melt did not reveal any kinetic effects on melt crystallization. All compositions investigated contained melt to temperatures ~75 to 100°C below the accepted G-WSS. Experimental results demonstrating that the G-WSS is significantly lower than unanimously accepted estimates will help us to better understand the storage conditions and evolution of silicic magmatic systems. Tuttle O, Bowen N (1958) Origin of Granite in the Light of Experimental Studies in the System NaAlSi3O8–KAlSi3O8–SiO2–H2O. Geological Society of America  more » « less
Award ID(s):
2120598
NSF-PAR ID:
10451542
Author(s) / Creator(s):
Date Published:
Journal Name:
AGU Fall Meeting
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. The granitic water-saturated solidus (G-WSS) is the lower temperature limit of magmatic mineral crystallization. The accepted water-saturated solidus for granitic compositions was largely determined >60 years ago1. More recent advances in experimental petrology, improved analytical techniques, and recent observations that granitic systems can remain active or spend a significant proportion of their lives at conditions below the traditional G-WSS2–5 necessitate a careful experimental investigation of the near-solidus regions of granitic systems. Natural and synthetic starting materials were melted at 10 kbar and 900°C with 48 wt% H2O to produce hydrous glasses for subsequent experiments at lower PT conditions used to locate the G-WSS. We performed crystallization experiments and melting experiments at temperatures ranging from 575 to 800°C and 1, 6, 8, and 10 kbar on 12 granitoid compositions. First, we ran a series of isothermal crystallization experiments along each isobar at progressively lower temperatures until runs completely crystallized to identify apparent solidus temperatures. Geochemical analyses of quenched glass compositions demonstrate that progressive crystallization drives all starting compositions towards silica-rich, water-saturated rhyolitic/granitic melts (e.g., ~7578 wt% SiO2). After identifying the apparent solidus temperatures at which the various compositions crystallized, we then ran series of reversal-type melting experiments. With the goal of producing rocks with hydrous equilibrium microstructures, we crystallized compositions at temperatures ~10°C below the apparent solidus identified in crystallization experiments, and then heated isobarically to conditions that produced ~20% melt during the crystallization experiments. Importantly, crystallization experiments and heating experiments at the same PT conditions produced similar proportions of melt, crystals, and vapor. A time-series of experiments 230 days at PT conditions previously identified to produce ~10% to 20% melt did not reveal any kinetic effects on melt crystallization. Experiments at 6 to 10 kbar crystallized/melted at temperatures close to the published G-WSS. However, at lower pressures where the published G-WSS is strongly curved in PT space, all compositions investigated contained melt to temperatures ~75 to 100°C below the accepted G-WSS. The similarity of crystallization temperatures for the higher-pressure experiments to previously published results, similar phase proportions in melting and crystallization experiments, and the lack of kinetic effects on crystallization collectively suggest that our lower pressure constraints on the G-WSS are accurate. The new experimental results demonstrating that the lower-pressure G-WSS is significantly lower than unanimously accepted estimates will help us to better understand the storage conditions, evolution, and potential for eruption in mid- to upper-crustal silicic magmatic systems. (1) Tuttle, O.; Bowen, N. Origin of Granite in the Light of Experimental Studies in the System NaAlSi3O8–KAlSi3O8–SiO2–H2O; Geological Society of America Memoirs; Geological Society of America, 1958; Vol. 74. https://doi.org/10.1130/MEM74. (2) Rubin, A. E.; Cooper, K. M.; Till, C. B.; Kent, A. J. R.; Costa, F.; Bose, M.; Gravley, D.; Deering, C.; Cole, J. Rapid Cooling and Cold Storage in a Silicic Magma Reservoir Recorded in Individual Crystals. Science 2017, 356 (6343), 1154–1156. https://doi.org/10.1126/science.aam8720. (3) Andersen, N. L.; Jicha, B. R.; Singer, B. S.; Hildreth, W. Incremental Heating of Bishop Tuff Sanidine Reveals Preeruptive Radiogenic Ar and Rapid Remobilization from Cold Storage. Proceedings of the National Academy of Sciences 2017, 114 (47), 12407–12412. https://doi.org/10.1073/pnas.1709581114. (4) Ackerson, M. R.; Mysen, B. O.; Tailby, N. D.; Watson, E. B. Low-Temperature Crystallization of Granites and the Implications for Crustal Magmatism. Nature 2018, 559 (7712), 94–97. https://doi.org/10.1038/s41586-018-0264-2. (5) Glazner, A. F.; Bartley, J. M.; Coleman, D. S.; Lindgren, K. Aplite Diking and Infiltration: A Differentiation Mechanism Restricted to Plutonic Rocks. Contributions to Mineralogy and Petrology 2020, 175 (4). https://doi.org/10.1007/s00410-020-01677-1. 
    more » « less
  2. Abstract

    For rocky exoplanets, knowledge of their geologic characteristics such as composition and mineralogy, surface recycling mechanisms, and volcanic behavior are key to determining their suitability to host life. Thus, determining exoplanet habitability requires an understanding of surface chemistry, and understanding the composition of exoplanet surfaces necessitates applying methods from the field of igneous petrology. Piston‐cylinder partial melting experiments were conducted on two hypothetical rocky exoplanet bulk silicate compositions. HEX1, a composition with molar Mg/Si = 1.42 (higher than bulk silicate Earth's Mg/Si = 1.23) yields a solidus similar to that of Earth's undepleted mantle. However, HEX2, a composition with molar Ca/Al = 1.07 (higher than Earth Ca/Al = 0.72) has a solidus with a slope of ∼10°C/kbar (vs. ∼15°C/kbar for Earth) and as result, has much lower melting temperatures than Earth. The majority of predicted adiabats point toward the likely formation of a silicate magma ocean for exoplanets with a mantle composition similar to HEX2. For adiabats that do intersect HEX2's solidus, decompression melting initiates at pressures more than 4x greater than in the modern Earth's undepleted mantle. The experimental partial melt compositions for these exoplanet mantle analogs are broadly similar to primitive terrestrial magmas but with higher CaO, and for the HEX2 composition, higher SiO2for a given degree of melting. This first of its kind exoplanetary experimental data can be used to calibrate future exoplanet petrologic models and predict volatile solubilities, volcanic degassing, and crust compositions for exoplanets with bulk compositions and ƒO2similar to those explored herein.

     
    more » « less
  3. The concept of long-wavelength ductile flow of lower crustal material, or channel flow, has emerged to explain the evolution of large hot orogens. In this model, growth of heat producing crust during collision leads to melt-weakening and flow of lower crust in response to tectonic forcing or long-wavelength gradients in gravitational potential energy. In the Himalayan-Tibetan (HT) orogen where the model was originally proposed, it has been hypothesized that a Miocene orogen-normal channel was active and that there was a more recent switch to orogen-parallel “escape” flow as the front of the orogen began to deform as a thrust wedge. However, because this hypothesized HT orogenic channel is largely subsurface it cannot be directly examined, making it difficult to test these hypotheses. The Inner Piedmont (IP), southern Appalachians has been proposed to be an exhumed orogenic channel based on inverted metamorphic isograds, extensive migmatization, and a large-scale curved mineral lineation pattern that is consistent with a shift from orogen-normal to orogen-parallel flow. To test the viability of the channel flow model in the IP, we construct pressure-temperature-time (P-T-t) paths and compare these to existing models which indicate that peak temperatures and residence times will differ between thrust wedge and channel flow models. The P-T-t paths are constructed using isochemical phase diagram sections (pseudosections), garnet compositions, monazite geochronology, and 40Ar/39Ar thermochronology to define prograde to retrograde conditions and residence times. The channel flow models require temperatures above 700-750°C to initiate and maintain flow. Preliminary pseudosections from the northern IP Brindle Creek fault zone indicate prograde to peak conditions of 815–820 °C and 7.9–9.3 kbar, and retrograde conditions of 720–730 °C and 5.3–5.4 kbar based on observed garnet compositions and sample mineralogy (Qtz + Pl + Bt + Sil + Grt ± Ms ± Ep ± Ilm ± Rt). Pseudosections are still being revised, however if confirmed, the P-T conditions are compatible with channel flow in the IP. Future model revisions and age data from samples forming a transect across the IP and into the adjacent Carolina superterrane and eastern Blue Ridge will be used to compare the P-T-t histories between the prdoposed channel and surrounding units. 
    more » « less
  4. Abstract

    The Quaternary Big Pine (BP) volcanic field in eastern California is notable for the occurrence of mantle xenoliths in several flows. This points to rapid ascent of basalt through the crust and precludes prolonged storage in a crustal reservoir. In this study, the hypothesis of phenocryst growth during ascent is tested for several basalts (13–7 wt% MgO) and shown to be viable. Phenocrysts of olivine and clinopyroxene frequently display diffusion‐limited growth textures, and clinopyroxene compositions are consistent with polybaric crystallization. When the most Mg‐rich olivine in each sample is paired with the whole‐rock composition, resulting(olivine‐melt) values (0.31–0.36) match those calculated from literature models (0.32–0.36). Application of a Mg‐ and a Ni‐based olivine‐melt thermometer from the literature, both calibrated on the same experimental data set, leads to two sets of temperatures that vary linearly with whole‐rock MgO wt%. Because the Ni thermometer is independent of water content, it provides the actual temperature at the onset of olivine crystallization (1247–1097°C), whereas the Mg thermometer gives the temperature under anhydrous conditions and thus allows ΔT(=TMg − TNi = depression of liquidus due to water) to be obtained. The average ΔTfor all samples is ~59°C, which is consistent with analyzed water contents of 1.5–3.0 wt% in olivine‐hosted melt inclusions from the literature. Because the application of olivine‐melt thermometry/hygrometry at the liquidus only requires microprobe analyses of olivine combined with whole‐rock compositions, it can be used to obtain large global data sets of the temperature and water contents of basalts from different tectonic settings.

     
    more » « less
  5. BSE mosaics of mushes and experimental products. Abstract: "We conducted experiments to study melt migration in crystal-rich mushes, with application to magma ascent within transcrustal magma reservoirs. Mushes with crystal volume fractions of 0.59 to 0.83 were prepared by hot-pressing crushed borosilicate glass mixed with different amounts quartz sand particles. Each experimental sample comprises stacked disks of mush and soda-lime glass, a proxy for crystal-free magma. Samples were subjected to confining pressures of 100 to 300 MPa and a temperature of 900°C (above the glass transition temperatures of the borosilicate and soda-lime glasses) for up to 6 h. The bottom and circumference of the mush and soda lime disks experience the confining pressure, but the top of the mush disks are at room pressure, resulting in a pore-pressure gradient across the mush layer. Following cooling and decompression, we determined the area fraction and morphology of soda-lime melt that migrated into the mush layer during experiments. Melt fraction is more strongly correlated to crystal fraction than pore-pressure gradient, increasing with crystal fraction before sharply decreasing as crystal fractions exceed 0.8. This change at 0.8 coincides with the transition from crystals in the mush moving during soda-lime migration to crystals forming a continuous rigid network. In our experiments, melt migration occurred by viscous fingering, but near the mobile-to-rigid transition, melt migration is enhanced by additional capillary action. Our results indicate that magma migration may peak when rigid mushes “unlock” to become mobile. This transition may mark an increase in magma migration, a potential precursor to volcanic unrest and eruption." Imaging: "Transverse sections cut from the top and/or bottom of the vacuum hot-pressed mushes were polished, carbon-coated, and imaged in BSE mode using the JEOL JXA-8530FPlus Electron Probe Microanalyzer (EPMA) at UMN (15 kV, 10 nA). About ten 50x magnification images were taken per sample and then compiled into BSE mosaics using Affinity Designer. The different compositions of the borosilicate glass and crystalline materials are distinguishable by greyscale in BSE images. [...] Following each experiment, sample assemblies were cut longitudinally along the cylindrical axis to produce sections for microstructural analysis. Scored samples (pHi-19s, Int-20s, Lo-21s) were cut again to produce sections tangential to the sample cylinder. Cut sections were vacuum impregnated with EpoFix resin and hand-polished on diamond lapping film from 30 to 0.5 μm grit. Polished and carbon-coated samples were imaged in BSE mode in the EPMA at UMN (15 kV, 10 nA). The different compositions of the soda-lime glass, borosilicate glass, and crystalline materials are distinguishable by greyscale in BSE images. Twenty to forty 50x magnification images were taken per sample and then compiled into sample-scale BSE mosaics using Affinity Designer." 
    more » « less