TY - JOUR
T1 - A new workflow to assess emplacement duration and melt residence time of compositionally diverse magmas emplaced in a sub-volcanic reservoir
AU - Ratschbacher, Barbara C.
AU - Brenhin Keller, C.
AU - Schoene, Blair
AU - Paterson, Scott R.
AU - Lawford Anderson, J.
AU - Okaya, David
AU - Putirka, Keith
AU - Lippoldt, Rachel
N1 - Funding Information:
Katie Ardill is thanked for help with microprobe work at the USGS facility at Menlo Park. The authors thank Peter Nabelek for sharing the SILLS code for conductive cooling calculations. Rosario Esposito at UCLA and Leslie Hayden at the USGS are thanked for helping with microprobe work, and Andrew Kylander-Clark for work on LA-ICP-MS at UCSB. We further thank Cal Barnes for comments on an earlier version of this paper, which helped to improve it. The authors further thank Jonathan Miller and two anonymous reviewers for their comments and feedback, which helped to improve the paper. We also gratefully mention Jim Beard for editorial handling and comments on the paper. This work was funded by NSF grant EAR 1250219 to K.P. and S.R.P. Geochronology work was made possible by an NSF EarthScope Geochronology Graduate Student Award to B.C.R. C.B.K. was supported in part by a US Department of Energy Computational Science Graduate Fellowship under contract DE-FG02-97ER25308. Computational resources were provided by the Princeton Institute for Computational Science and Engineering
Funding Information:
This work was funded by NSF grant EAR 1250219 to K.P. and S.R.P. Geochronology work was made possible by an NSF EarthScope Geochronology Graduate Student Award to B.C.R. C.B.K. was supported in part by a US Department of Energy Computational Science Graduate Fellowship under contract DE-FG02-97ER25308. Computational resources were provided by the Princeton Institute for Computational Science and Engineering.
Publisher Copyright:
© The Author(s) 2018. Published by Oxford University Press. All rights reserved.
PY - 2018/9/1
Y1 - 2018/9/1
N2 - Construction durations of magma reservoirs are commonly inferred from U-Pb zircon geochronology using various statistical methods to interpret zircon U-Pb age spectra (e.g. weighted mean ages of concordant zircon populations). However, in compositionally different magmas, zircon saturation and crystallization are predicted to occur at different times relative to other mineral phases and the geological event of interest; for instance, magma emplacement. The timescales of these processes can be predicted by numerical modeling and measured using U-Pb zircon thermal ionization mass spectrometry (TIMS) geochronology, therefore creating an opportunity to quantify magma emplacement in space and time to constrain the size and longevity of magma reservoirs during pluton construction. The Jurassic tilted, bimodal (gabbroic and granitic) Guadalupe igneous complex (GIC) in the Sierra Nevada arc presents an exceptional opportunity to study the construction duration of a shallow (1-10 km) magma reservoir comprising multiple magma batches. We present a new workflow to constrain emplacement ages from zircon geochronology of compositionally different magma batches and evaluate melt-present timescales. High-precision U-Pb chemical ablation isotope dilution (CA-ID)-TIMS zircon ages are combined with MELTS modeling to calculate zircon saturation ages for each dated sample. Bayesian statistics are then used to compare calculated zircon saturation distributions with zircon age distributions from TIMS data to predict time, temperature, and melt fraction at zircon saturation and solidus. In addition, we use mineral thermometry and cooling rate calculations to relate zircon saturation ages to emplacement ages for felsic and mafic rocks, resulting in a best estimate for the total construction duration of 295±110 kyr for the GIC. Rhyolites exposed at the top of the GIC are ~2-3 Myr older and thus not part of the same magmatic system. The good agreement between Ti-in-zircon crystallization temperatures and calculated zircon saturation temperatures by MELTS implies that bulk-rock compositions of both mafic and felsic rocks are close to liquid compositions. Mafic and felsic magmas experienced extensive mingling at the emplacement level in a magma chamber (which, as defined here, has temperatures above the solidus of the respective rock composition) encompassing ~60% of the exposed map area of the complex shortly after construction. Melt was present within the system for a total duration of ~550 kyr as constrained by two-dimensional thermal finite-difference modeling using an incremental growth and sill emplacement model. The construction and meltpresent timescales calculated in this study for the shallow GIC have implications for the potential of in situ differentiation, mixing and mingling timescales and eruption in shallow magmatic systems.
AB - Construction durations of magma reservoirs are commonly inferred from U-Pb zircon geochronology using various statistical methods to interpret zircon U-Pb age spectra (e.g. weighted mean ages of concordant zircon populations). However, in compositionally different magmas, zircon saturation and crystallization are predicted to occur at different times relative to other mineral phases and the geological event of interest; for instance, magma emplacement. The timescales of these processes can be predicted by numerical modeling and measured using U-Pb zircon thermal ionization mass spectrometry (TIMS) geochronology, therefore creating an opportunity to quantify magma emplacement in space and time to constrain the size and longevity of magma reservoirs during pluton construction. The Jurassic tilted, bimodal (gabbroic and granitic) Guadalupe igneous complex (GIC) in the Sierra Nevada arc presents an exceptional opportunity to study the construction duration of a shallow (1-10 km) magma reservoir comprising multiple magma batches. We present a new workflow to constrain emplacement ages from zircon geochronology of compositionally different magma batches and evaluate melt-present timescales. High-precision U-Pb chemical ablation isotope dilution (CA-ID)-TIMS zircon ages are combined with MELTS modeling to calculate zircon saturation ages for each dated sample. Bayesian statistics are then used to compare calculated zircon saturation distributions with zircon age distributions from TIMS data to predict time, temperature, and melt fraction at zircon saturation and solidus. In addition, we use mineral thermometry and cooling rate calculations to relate zircon saturation ages to emplacement ages for felsic and mafic rocks, resulting in a best estimate for the total construction duration of 295±110 kyr for the GIC. Rhyolites exposed at the top of the GIC are ~2-3 Myr older and thus not part of the same magmatic system. The good agreement between Ti-in-zircon crystallization temperatures and calculated zircon saturation temperatures by MELTS implies that bulk-rock compositions of both mafic and felsic rocks are close to liquid compositions. Mafic and felsic magmas experienced extensive mingling at the emplacement level in a magma chamber (which, as defined here, has temperatures above the solidus of the respective rock composition) encompassing ~60% of the exposed map area of the complex shortly after construction. Melt was present within the system for a total duration of ~550 kyr as constrained by two-dimensional thermal finite-difference modeling using an incremental growth and sill emplacement model. The construction and meltpresent timescales calculated in this study for the shallow GIC have implications for the potential of in situ differentiation, mixing and mingling timescales and eruption in shallow magmatic systems.
KW - Compositional bimodality
KW - Finite-difference thermal modeling
KW - Magma chamber
KW - U-Pb zircon ID-TIMS
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U2 - 10.1093/petrology/egy079
DO - 10.1093/petrology/egy079
M3 - Article
AN - SCOPUS:85057175972
SN - 0022-3530
VL - 59
SP - 1787
EP - 1810
JO - Journal of Petrology
JF - Journal of Petrology
IS - 9
ER -