Jessica M. Warren

Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere

Significance Although plate tectonics has seen broad acceptance for Earth, the manner in which lithospheric plates are coupled to Earth’s deeper interior is still heavily debated. In particular, recent seismological observations suggest a sharp, flat base of the lithosphere, whereas thermal models suggest a gradational boundary that deepens with age. Based on laboratory experiments, we suggest that thermal models are most appropriate and that seismic studies are detecting features frozen into the lithosphere after melting at midocean ridges. Experiments on olivine aggregates demonstrate that the seismic characteristics of deforming upper mantle are dramatically different between melt-free and low-melt-fraction aggregates. A model of upper-mantle flow incorporating these results predicts seismological features in excellent agreement with observations beneath the Pacific Ocean basin. Tectonic plates are a key feature of Earth’s structure, and their behavior and dynamics are fundamental drivers in a wide range of large-scale processes. The operation of plate tectonics, in general, depends intimately on the manner in which lithospheric plates couple to the convecting interior. Current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth. Thus, the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear. Here we present results from laboratory-based torsion experiments on olivine aggregates with and without melt, yielding an improved database describing the crystallographic alignment of olivine grains. We combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins. Agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled. This model additionally supports the idea that discontinuities in velocity and anisotropy, often assumed to be the base of the lithosphere, are, instead, intralithospheric features reflecting a compositional boundary established at midocean ridges, not a rheological boundary.

Revisiting the electron microprobe method of spinel-olivine-orthopyroxene oxybarometry applied to spinel peridotitesk

Abstract 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.

Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry

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 Fe 3+ /ΣFe 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.

Size effects resolve debate in 40 years of work on low-temperature plasticity in olivine

When deforming by low-temperature plasticity, the strength of the mantle mineral olivine is controlled by its grain size. The strength of olivine at low temperatures and high stresses in Earth’s lithospheric mantle exerts a critical control on many geodynamic processes, including lithospheric flexure and the formation of plate boundaries. Unfortunately, laboratory-derived values of the strength of olivine at lithospheric conditions are highly variable and significantly disagree with those inferred from geophysical observations. We demonstrate via nanoindentation that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stresses. This “size effect” resolves discrepancies among previous measurements of olivine strength using other techniques. It also corroborates the most recent flow law for olivine, which proposes a much weaker lithospheric mantle than previously estimated, thus bringing experimental measurements into closer alignment with geophysical constraints. Further implications include an increased difficulty of activating plasticity in cold, fine-grained shear zones and an impact on the evolution of fault surface roughness due to the size-dependent deformation of nanometer- to micrometer-sized asperities.

Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model

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. n nHere 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.

New SIMS reference materials for measuring water in upper mantle minerals

Abstract Trace amounts of water in the nominally anhydrous minerals of the upper mantle can dramatically affect their thermodynamic and rheological properties. Secondary ion mass spectrometry (SIMS) has become a mainstream technique for quantifying small amounts of water in these minerals, but depends on standards with known concentrations of water. The current standards in use for mantle minerals are well-characterized (Hauri et al. 2002; Koga et al. 2003; Aubaud et al. 2007; Mosenfelder and Rossman 2013a, 2013b), but a lack of extra material has limited the spread of this technique to other laboratories. We present new SIMS measurements on natural mantle xenolith pyroxenes that are suitable for use as calibration reference materials. They are calibrated off of the pyroxene standards currently in use at the Department of Terrestrial Magnetism of the Carnegie Institution of Washington (Koga et al. 2003; Aubaud et al. 2007). They have homogeneous water contents, defined as a standard deviation of <10% for analyses across multiple grains. Reference materials for H2O cover ranges from 52 to 328 ppm and from 9 to 559 ppm in orthopyroxene and clinopyroxene, respectively, covering most of the observed range of mantle water contents. The samples are evenly distributed over those ranges. The orthopyroxene reference materials can also be used to measure water in olivine based on previous observations that these two minerals have similar calibration slopes. The new pyroxene reference materials can also be used to calibrate fluorine and phosphorus at low concentrations. We found that fluorine in particular was homogeneous in both orthopyroxene and clinopyroxene, with concentrations of 3 to 50 ppm in orthopyroxene and 0.5 to 118 ppm in clinopyroxene. Phosphorus ranges from below detection up to 19 ppm in orthopyroxene and up to 73 ppm in clinopyroxene, but was more heterogeneous within some samples. Most of the reference materials have concentrations at the lower end of the ranges for fluorine and phosphorus in this study, with only a few samples showing higher concentrations.