Browsing by Author "Warren, Jessica M."
Now showing 1 - 11 of 11
Results Per Page
Sort Options
Item Comparison of thermal modeling, microstructural analysis, and Ti-in-quartz thermobarometry to constrain the thermal history of a cooling pluton during deformation in the Mount Abbot Quadrangle, CA(American Geophysical Union, 2017-03-30) Nevitt, Johanna M.; Warren, Jessica M.; Kidder, Steven; Pollard, David D.; Johanna M. Nevitt, Jessica M. Warren, Steven Kidder, and David D. Pollard; Warren, Jessica M.Granitic plutons commonly preserve evidence for jointing, faulting, and ductile fabric development during cooling. Constraining the spatial variation and temporal evolution of temperature during this deformation could facilitate an integrated analysis of heterogeneous deformation over multiple length-scales through time. Here, we constrain the evolving temperature of the Lake Edison granodiorite within the Mount Abbot Quadrangle (central Sierra Nevada, CA) during late Cretaceous deformation by combining microstructural analysis, titanium-in-quartz thermobarometry (TitaniQ), and thermal modeling. Microstructural and TitaniQ analyses were applied to 12 samples collected throughout the pluton, representative of either the penetrative ‘‘regional’’ fabric or the locally strong ‘‘fault-related’’ fabric. Overprinting textures and mineral assemblages indicate the temperature decreased from 400–5008C to <3508C during faulting. TitaniQ reveals consistently lower Ti concentrations for partially reset fault-related fabrics (average: 1264 ppm) than for regional fabrics (average: 31612 ppm), suggesting fault-related fabrics developed later, following a period of pluton cooling. Uncertainties, particularly in TiO2 activity, significantly limit further quantitative thermal estimates using TitaniQ. In addition, we present a 1-D heat conduction model that suggests average pluton temperature decreased from 5858C at 85 Ma to 3328C at 79 Ma, consistent with radiometric age data for the field. Integrated with the model results, microstructural temperature constraints suggest faulting initiated by 83 Ma, when the temperature was nearly uniform across the pluton. Thus, spatially heterogeneous deformation cannot be attributed to a persistent temperature gradient, but may be related to regional structures that develop in cooling plutons.Item Evidence for chemically heterogeneous Arctic mantle beneath the Gakkel Ridge(Elsevier, 2015-12-07) D’Errico, Megan E.; Warren, Jessica M.; Godard, Marguerite; Megan E. D’Errico, Jessica M. Warren, Marguerite Godard; Warren, Jessica M.Ultraslow spreading at mid-ocean ridges limits melting due to on-axis conductive cooling, leading to the prediction that peridotites from these ridges are relatively fertile. To test this, we examined abyssal peridotites from the Gakkel Ridge, the slowest spreading ridge in the global ocean ridge system. Major and trace element concentrations in pyroxene and olivine minerals are reported for 14 dredged abyssal peridotite samples from the Sparsely Magmatic (SMZ) and Eastern Volcanic (EVZ) Zones. We observe large compositional variations among peridotites from the same dredge and among dredges in close proximity to each other. Modeling of lherzolite trace element compositions indicates varying degrees of non-modal fractional mantle melting, whereas most harzburgite samples require open-system melting involving interaction with a percolating melt. All peridotite chemistry suggests significant melting that would generate a thick crust, which is inconsistent with geophysical observations at Gakkel Ridge. The refractory harzburgites and thin overlying oceanic crust are best explained by low present-day melting of a previously melted heterogeneous mantle. Observed peridotite compositional variations and evidence for melt infiltration demonstrates that fertile mantle components are present and co-existing with infertile mantle components. Melt generated in the Gakkel mantle becomes trapped on short length-scales, which produces selective enrichments in very incompatible rare earth elements. Melt migration and extraction may be significantly controlled by the thick lithosphere induced by cooling at such slow spreading rates. We propose the heterogeneous mantle that exists beneath Gakkel Ridge is the consequence of ancient melting, combined with subsequent melt percolation and entrapment.Item Global variations in abyssal peridotite compositions(Elsevier, 2016-01-09) Warren, Jessica M.; Jessica M.Warren; Warren, Jessica M.Abyssal peridotites are ultramafic rocks collected frommid-ocean ridges that are the residues of adiabatic decompression melting. Their compositions provide information on the degree of melting and melt–rock interaction involved in the formation of oceanic lithosphere, as well as providing constraints on pre-existing mantle heterogeneities. This review presents a compilation of abyssal peridotite geochemical data (modes, mineral major elements, and clinopyroxene trace elements) for N1200 samples from 53 localities on 6 major ridge systems. On the basis of composition and petrography, peridotites are classified into one of five lithological groups: (1) residual peridotite, (2) dunite, (3) gabbro-veined and/or plagioclase-bearing peridotite, (4) pyroxenite-veined peridotite, and (5) other types of melt-added peridotite. Almost a third of abyssal peridotites are veined, indicating that the oceanic lithospheric mantle is more fertile, on average, than estimates based on residual peridotites alone imply. All veins appear to have formed recently during melt transport beneath the ridge, though some pyroxenites may be derived from melting of recycled oceanic crust. A limited number of samples are available at intermediate and fast spreading rates, with samples from the East Pacific Rise indicating high degrees of melting. At slow and ultra-slow spreading rates, residual abyssal peridotites define a large (0–15% modal clinopyroxene and spinel Cr#=0.1–0.6) compositional range. These variations do not match the prediction for how degree of melting should vary as a function of spreading rate. Instead, the compositional ranges of residual peridotites are derived from a combination of melting, melt–rock interaction and pre-existing compositional variability, where melt–rock interaction is used here as a general term to refer to the wide range of processes that can occur during melt transport in the mantle. Globally, ~10% of abyssal peridotites are refractory (0% clinopyroxene, spinel Cr# N 0.5, bulk Al2O3 b 1wt.%) and someridge sections are dominated by harzburgiteswhile lacking a significant basaltic crust. Abyssal ultramafic samples thus indicate that the mantle ismulti-component, probably consisting of at least three components (lherzolite, harzburgite, and pyroxenite). Overall, the large compositional rangeamong residual andmelt-added peridotites implies that the oceanic lithospheric mantle is heterogeneous, which will lead to the generation of further heterogeneities upon subduction back into the mantle.Item Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry(Geological Society of America, 2016-06-01) Birner, Suzanne K.; Warren, Jessica M.; Cottrell, Elizabeth; Davis, Fred A.; Suzanne K. Birner, Jessica M. Warren, Elizabeth Cottrell, and Fred A. Davis; Warren, Jessica M.Olivine, orthopyroxene, and spinel compositions within seafloor peridotites yield important information about the nature of Earth’s mantle. Major element compositions of these minerals can be used to calculate oxygen fugacity, a thermodynamic property critical to understanding phase equilibria in the upper mantle. This study examines how hydrothermal alteration at the seafloor influences peridotite chemistry. The Tonga Trench (South Pacific Ocean) exposes lithospheric forearc peridotites that range from highly altered to completely unaltered and provides an ideal sample suite for investigating the effect of alteration on spinel peridotite major element chemistry and calculated oxygen fugacity. Using the Tonga peridotites, we develop a qualitative alteration scale rooted in traditional point-counting methodology. We show that high degrees of serpentinization do not affect mineral parameters such as forsterite number in olivine, iron site occupancy in orthopyroxene, and Fe3+/SFe ratio in spinel. Additionally, while serpentinization is a redox reaction that leaves behind an oxidized residue, the oxygen fugacity recorded by mantle minerals is unaffected by nearby low-temperature serpentinization. As a result, oxygen fugacity measured by spinel oxybarometry in seafloor peridotites is representative of mantle processes, rather than an artifact of late-stage seafloor alteration.Item Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology(Mineralogical Society of America, 2015-12-14) Harvey, Jason; Warren, Jessica M.; Shirey, Steven B.; Jason Harvey, Jessica M. Warren, Steven B. Shirey; Warren, Jessica M.Item Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotites(Mineralogical Society of America) Davis, Fred A.; Cottrell, Elizabeth; Birner, Suzanne K.; Warren, Jessica M.; Lopez, Oscar G.; Fred A. Davis, Elizabeth Cottrell, Suzanne K. Birner, Jessica M. Warren, and Oscar G. Lopez; Warren, Jessica M.Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity fO2 of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe3+/∑Fe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe3+/∑Fe ratios that relies on a reported correlation between the difference in Fe3+/∑Fe ratio by Mössbauer spectroscopy and by electron microprobe (ΔFe3+/∑FeMöss-EPMA) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mössbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe3+/∑FeMöss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe3+/∑Fe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = 0.031 and corrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe3+/∑Fe ratios despite the absence of a correlation between ΔFe3+/∑FeMöss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe3+/∑Fe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive fO2 . Because there is a logarithmic relationship between the activity of magnetite and logfO2, the uncertainty in fO2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe3+/∑Fe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe3+/∑Fe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative fO2. We also document and correct errors that appear in the literature when formulating fO2 that, combined, could yield errors in absolute fO2 of greater than 0.75 log units—even with perfectly accurate Fe3+/∑Fe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe3+/∑Fe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of fO2 from spinel peridotite mineral compositions.Item Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotites(Mineralogical Society of America, 2017-02-09) Davis, Fred A.; Cottrell, Elizabeth; Birner, Suzanne K.; Warren, Jessica M.; Lopez, Oscar G.; Fred A. Davis, Elizabeth Cottrell, Suzanne K. Birner, Jessica M. Warren, and Oscar G. Lopez; Warren, Jessica M.Natural peridotite samples containing olivine, orthopyroxene, and spinel can be used to assess the oxygen fugacity (fO2) of the upper mantle. The calculation requires accurate and precise quantification of spinel Fe3+/∑Fe ratios. Wood and Virgo (1989) presented a correction procedure for electron microprobe (EPMA) measurements of spinel Fe3+/∑Fe ratios that relies on a reported correlation between the difference in Fe3+/∑Fe ratio by Mössbauer spectroscopy and by electron microprobe (ΔFe3+/∑FeMöss-EPMA) and the Cr# [Cr/(Al+Cr)] of spinel. This procedure has not been universally adopted, in part, because of debate as to the necessity and effectiveness of the correction. We have performed a series of replicate EPMA analyses of several spinels, previously characterized by Mössbauer spectroscopy, to test the accuracy and precision of the Wood and Virgo correction. While we do not consistently observe a correlation between Cr# and ΔFe3+/∑FeMöss-EPMA in measurements of the correction standards, we nonetheless find that accuracy of Fe3+/ZFe ratios determined for spinel samples treated as unknowns improves when the correction is applied. Uncorrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = 0.031 and corrected measurements have a mean ΔFe3+/∑FeMöss-EPMA = −0.004. We explain how the reliance of the correction on a global correlation between Cr# and MgO concentration in peridotitic spinels improves the accuracy of Fe3+/ZFe ratios despite the absence of a correlation between ΔFe3+/∑FeMöss-EPMA and Cr# in some analytical sessions. Precision of corrected Fe3+/∑Fe ratios depends on the total concentration of Fe, and varies from ±0.012 to ±0.032 (1σ) in the samples analyzed; precision of uncorrected analyses is poorer by approximately a factor of two. We also present an examination of the uncertainties in the calculation contributed by the other variables used to derive fO2. Because there is a logarithmic relationship between the activity of magnetite and logfO2, the uncertainty in fO2 relative to the QFM buffer contributed by the electron microprobe analysis of spinel is asymmetrical and larger at low ferric Fe concentrations (+0.3/−0.4 log units, 1σ, at Fe3+/∑Fe = 0.10) than at higher ferric Fe concentrations (±0.1 log units, 1σ, at Fe3+/EFe = 0.40). Electron microprobe analysis of olivine and orthopyroxene together contribute another ±0.1 to ±0.2 log units of uncertainty (1σ). Uncertainty in the temperature and pressure of equilibration introduce additional errors on the order of tenths of log units to the calculation of relative fO2. We also document and correct errors that appear in the literature when formulating fO2 that, combined, could yield errors in absolute fO2 of greater than 0.75 log units—even with perfectly accurate Fe3+/∑Fe ratios. Finally, we propose a strategy for calculating the activity of magnetite in spinel that preserves information gained during analysis about the ferric iron content of the spinel. This study demonstrates the superior accuracy and precision of corrected EPMA measurements of spinel Fe3+/∑Fe ratios compared to uncorrected measurements. It also provides an objective method for quantifying uncertainties in the calculation of fO2 from spinel peridotite mineral compositions.Item Trace elements in abyssal peridotite olivine record melting, thermal evolution, and melt refertilization in the oceanic upper mantle(Contributions to Mineralogy and Petrology, 2023-09-06) Lin, Kuan-Yu; Warren, Jessica M.; Davis, Fred A.Trace element concentrations in abyssal peridotite olivine provide insights into the formation and evolution of the oceanic lithosphere. We present olivine trace element compositions (Al, Ca, Ti, V, Cr, Mn, Co, Ni, Zn, Y, Yb) from abyssal peridotites to investigate partial melting, melt–rock interaction, and subsolidus cooling at mid-ocean ridges and intra-oceanic forearcs. We targeted 44 peridotites from fast (Hess Deep, East Pacific Rise) and ultraslow (Gakkel and Southwest Indian Ridges) spreading ridges and the Tonga trench, including 5 peridotites that contain melt veins. We found that the abundances of Ti, Mn, Co, and Zn increase, while Ni decreases in melt-veined samples relative to unveined samples, suggesting that these elements are useful tracers of melt infiltration. The abundances of Al, Ca, Cr, and V in olivine are temperature sensitive. Thermometers utilizing Al and Ca in olivine indicate temperatures of 650–1000 °C, with variations corresponding to the contrasting cooling rates the peridotites experienced in different tectonic environments. Finally, we demonstrate with a two-stage model that olivine Y and Yb abundances reflect both partial melting and subsolidus re-equilibration. Samples that record lower Al- and Ca-in-olivine temperatures experienced higher extents of diffusive Y and Yb loss during cooling. Altogether, we demonstrate that olivine trace elements document both high-temperature melting and melt–rock interaction events, as well as subsolidus cooling related to their exhumation and emplacement onto the seafloor. This makes them useful tools to study processes associated with seafloor spreading and mid-ocean ridge tectonics.Item Viscous anisotropy of textured olivine aggregates, Part 1: Measurement of the magnitude and evolution of anisotropy(AGU Publications, 2016-04-22) Hansen, Lars N.; Warren, Jessica M.; Zimmerman, Mark E.; Kohlstedt, David L.; Lars N.Hansen, Jessica M.Warren, Mark E.Zimmerman, David L.Kohlstedt; Warren, Jessica M.The development of crystallographic textures in olivine-rich rocks leads to a marked anisotropy in viscosity of the upper mantle, strongly influencing a variety of large-scale geodynamic processes. Most estimates of the magnitude of viscous anisotropy in the upper mantle are derived from micromechanical models that predict textural and mechanical evolution numerically. Unfortunately, relatively few data exist with which to benchmark these models, and therefore their applicability to geodynamic processes remains in question. Here we present the results from a series of laboratory deformation experiments that yield insight into the magnitude and evolution of the anisotropy of olivine aggregates during deformation along complex loading paths. Aggregates of Fo50olivine were first deformed in extension in a gas-medium apparatus at a temperature of 1473K, confining pressure of 300MPa, and a variety of stresses and strain rates. Early in the extension experiments, samples exhibited viscosities similar to those previously determined for isotropic aggregates. Extensional deformation was accompanied by formation of crystallographic textures with [100] axes dominantly aligned with the extension axis. Samples were subsequently deformed in torsion under similar conditions to shear strains of up to 15.5. Early in the torsion experiments, samples supported stresses a factor of ∼2 larger than measured at the end of extension experiments, demonstrating a marked anisotropy in viscosity. Textures at the end of torsion experiments exhibited [100] axes dominantly aligned with the shear direction, comparable to previous experimental observations. Evolution of the textures resulting from extension to those resulting from torsion was analyzed through examination of radial sections of torsion samples. Our results confirm that texture produces viscous anisotropy in olivine aggregates, and we provide a simple, calibrated parameterization of viscous anisotropy for use in geodynamic models. Our results also provide an extensive dataset for future calibration of micromechanical models that track the evolution of anisotropy in upper mantle rocks.Item Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model(AGU Publications, 2016-10-25) Hansen, Lars N.; Conrad, Clinton P.; Boneh, Yuval; Skemer, Philip; Warren, Jessica M.; Kohlstedt, David L.; Lars N. Hansen, Clinton P. Conrad, Yuval Boneh, Philip Skemer, Jessica M. Warren, and David L. Kohlstedt; Warren, Jessica M.The significant viscous anisotropy that results from crystallographic alignment (texture) of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes. Our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution. Unfortunately, existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths, making them less general than desired for many large-scale geodynamic scenarios. Here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories. For the mechanical behavior, we explore two extreme scenarios, one in which each grain experiences the same stress tensor (Sachs model) and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate (pseudo-Taylor model). For the textural evolution, we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation. Only recently has enough laboratory data on the deformation of olivine become available to calibrate these models. We use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models. These inversions demonstrate that the calibrated pseudo-Taylor model best reproduces the mechanical observations. Additionally, the pseudo-Taylor textural evolution model can reasonably reproduce the observed texture strength, shape, and orientation after large and complex deformations. A quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution. In addition, we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large-strain torsion tests. This set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large-scale numerical simulations.Item Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model(American Geophysical Union, 2016-09-27) Hansen, Lars N.; Conrad, Clinton P.; Boneh, Yuval; Skemer, Philip; Warren, Jessica M.; Kohlstedt, David L.; Lars N. Hansen, Clinton P. Conrad, Yuval Boneh, Philip Skemer, Jessica M. Warren, and David L. Kohlstedt; Warren, Jessica M.The significant viscous anisotropy that results from crystallographic alignment (texture) of olivine grains in deformed upper mantle rocks strongly influences a large variety of geodynamic processes. Our ability to explore the effects of anisotropic viscosity in simulations of these processes requires a mechanical model that can predict the magnitude of anisotropy and its evolution. Unfortunately, existing models of olivine textural evolution and viscous anisotropy are calibrated for relatively small deformations and simple strain paths, making them less general than desired for many large-scale geodynamic scenarios. Here we develop a new set of micromechanical models to describe the mechanical behavior and textural evolution of olivine through a large range of strains and complex strain histories. For the mechanical behavior, we explore two extreme scenarios, one in which each grain experiences the same stress tensor (Sachs model) and one in which each grain undergoes a strain rate as close as possible to the macroscopic strain rate (pseudo-Taylor model). For the textural evolution, we develop a new model in which the director method is used to control the rate of grain rotation and the available slip systems in olivine are used to control the axis of rotation. Only recently has enough laboratory data on the deformation of olivine become available to calibrate these models. We use these new data to conduct inversions for the best parameters to characterize both the mechanical and textural evolution models. These inversions demonstrate that the calibrated pseudo-Taylor model best reproduces the mechanical observations. Additionally, the pseudo-Taylor textural evolution model can reasonably reproduce the observed texture strength, shape, and orientation after large and complex deformations. A quantitative comparison between our calibrated models and previously published models reveals that our new models excel in predicting the magnitude of viscous anisotropy and the details of the textural evolution. In addition, we demonstrate that the mechanical and textural evolution models can be coupled and used to reproduce mechanical evolution during large-strain torsion tests. This set of models therefore provides a new geodynamic tool for incorporating viscous anisotropy into large-scale numerical simulations.