Geocheminar - winter-2019
When Did Plate Tectonics Begin?
Jan. 10, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Mark Harrison - UCLA
Subscribe to Calendar
The balance of forces in modern plate tectonics is broadly understood and subduction initiation appears to be a robust manifestation of the system. However, when and how it began are not. The principal hurdle to attaining mobile lid tectonics is overcoming the strength of the lithosphere, but this raises a chicken-or-egg paradox: what caused this strong layer to generate weakened planar features on which subduction could initiate? Answering this question will require knowledge of the interrelationships among the full range of relevant lithospheric rheologies but at present few firm conclusions can be drawn from theory. What is clear is that such models will never achieve ab initio reconstruction of early Earth behavior. Mantle convection, a highly non-linear, dispersive, chaotic system, is uninvertible and the recent recognition that global tectonic mode may be pathway dependent only underscores this fact. If we’re ever to understand when plate tectonics got underway, we’re going to have to acquire that knowledge from the geologic record.
Tracing the history of sediment grains using MET-IRSL
Jan. 17, 2019
noon - 1 p.m.
Slichter 3583
Presented By:
- Ed Rhodes - University of Sheffield
Subscribe to Calendar
Optical dating, based on OSL (Optically Stimulated Luminescence) or IRSL (Infra-Red Stimulated Luminescence) signals of quartz or feldspar, allows us to determine how long sediment grains have been buried since they last exposed to light, or heated above around 300°C. The luminescence signal measured in the laboratory results from the recombination and optical emission from electrons that became trapped at metastable sites in the crystal lattice as a result of environmental ionizing radiation. The dating technique works well on timescales of years to hundreds of thousands of years; an upper time limit is reached when all available electron trapping sites are filled. Technical improvements allow us to measure age estimates for individual sand-sized grains from sediment bodies. A new measurement approach termed MET (Multiple Elevated Temperature) IRSL, developed over the last five years or so, allows us to determine the duration of light exposure before burial commenced. This is clearly useful as a way to quantify whether sufficient daylight exposure was received by each grain to reset the IRSL signal fully (one of the remaining limitations of conventional IRSL and OSL approaches), but also has the potential for tracking the pathways for grains through the environment indifferent sediment transport systems. Combined with techniques to assess light exposure durations for rock surfaces using IRSL developed by Sohbati and colleagues at the Risø laboratory, Denmark, these methods have great potential for understanding many different aspects of sediment transport, including human modification of processes and source to sink storage.
Isotopic Fractionation of Moderately Volatile Elements During Moon Formation
Jan. 24, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3583
Presented By:
- Haolan Tang - UCLA
Subscribe to Calendar
Moderately volatile elements (MVE) are significant tracers of volatile depletion in planetary bodies. Investigating the mechanisms of MVE isotopic fractionation in the bulk silicate Moon is the key to our understanding of the thorough scenarios of Moon formation. Here we take the example of potassium to examine MVE isotopic fractionation during rapid accretion, lunar magma ocean evaporation, and vapor-melt exchange on the Moon. Accordingly, neither condensation inside of magma-atmosphere disk or evaporation in lunar magma ocean can reproduce the present distinguishing potassium isotopic composition in the lunar melt relative to terrestrial composition. Instead, both scenarios would generate the lunar melt with potassium isotopic composition either identical to or lighter than terrestrial composition. On the other hand, a silicate steady-state atmosphere can be rapidly formed above the lunar magma ocean. The isotopic fractionated vapor interacts with the lunar surface melt, which can efficiently shift K isotope composition toward positive value relative to bulk silicate Earth. To yield the present K isotopic composition in the bulk silicate Moon only takes 100 ~1000 years. This timescale coincides with previous estimate of duration of lunar magma ocean crystallization. In addition, given the evaporation properties of Mg and Si, the lunar isotopic composition of Mg and Si cannot be varied in such a short timescale. Our model interprets both isotopic difference of MVE and identical isotopic composition of lithospheric elements (e.g., Mg and Si) on the Moon relative to Earth and provides a new perspective on chemical and isotopic evolution of the Moon.
Geophysics of Chemical Heterogeneity in the Mantle
Jan. 31, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Lars Stixrude - UCLA
Subscribe to Calendar
Chemical heterogeneity, produced by the near-surface rock cycle and dominated volumetrically by subducted oceanic crust and its depleted residue, is continuously returned into the mantle. This lithologic-scale chemical heterogeneity may survive for as long as the age of Earth because chemical diffusion is inefficient. Estimates of rates of subduction and mantle processing over geologic history indicate that most or all of the mantle may be composed of lithologically heterogeneous material. Mineralogical models of the mantle show that chemical heterogeneity over many decades in length scale may be detectable by geophysical probes via its influence on seismic-wave propagation. Grain-scale heterogeneity influences the aggregate absolute seismic velocity and its lateral variation with temperature. The multi-phase nature of the mantle leads to many regions of reduced or negative thermal expansivity, which may influence mantle dynamics and thermal evolution. The elastic-wave velocity contrast associated with lithologic-scale heterogeneity may be sufficient to produce observable scattering of short- period seismic waves.
Methane, Mud Volcanos, and Microbes
Feb. 7, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Doug Rumble - Carnegie Institution for Science
Subscribe to Calendar
The two themes of the talk are: (1) How can measurements of isotopically-substitited methane be used to address geologic questions? Analysis of doubly-substituted, equilibrated methane molecules containing 12C-13C-1H-2H gives a robust estimate of formation temperature. This data can be used in the context of published earthquake hypocenter locations, seismic imaging of faults, and heat flow measurements to interpret a minimum depth of fluid flow and movement along active faults. (2) How can laboratory experiments on methane production both abiotically and by organisms be used to infer the origin of naturally occurring methane? Laboratory experiments on growing mono-cultures of methanogenic microbes, both anaerobic and aerobic, produce isotopically-substituted methane startlingly similar in its isotopic distributions to methane found in natural environments as different as Paleozoic hydrocarbon reservoirs and boreal lakes of the permafrost regions of the far North. Abiotic synthesis of methane under controlled hydrothermal conditions gives an experimental calibration of calculated isotope substitutions in methane.
Compositional Heterogeneity of Impact Melt Rocks at the Haughton Structure, Canada: Implications for Planetary Processes and Remote Sensing
Feb. 14, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3583
Presented By:
- Rebecca Greenberger - California Institute of Technology
Subscribe to Calendar
Meteorite impacts are the most common geological process in the solar system. Impacts modify and bring subsurface lithologies to the surface, where they can be studied through remote sensing and in situ measurements. However, questions remain as to how the compositions observed within impact structures represent subsurface crustal materials. Impact melt is an important product of meteorite impacts and forms through melting of target rocks during hypervelocity impacts, often containing blocks and clasts of variably shocked country rock. The melt rocks contain heterogeneities at a scale that cannot be sampled through standard geological field and laboratory methods. We use a novel technique, imaging spectroscopy in the field, to measure and sample compositions of entire outcrops of impact melt at the Haughton impact structure in the Canadian High Arctic. By comparing compositions from outcrop to outcrop, we assess the heterogeneity of impact melt in relation to the original target stratigraphy. I will discuss the implications for our understanding of how impact melt forms and for remote compositional analyses of other bodies in the solar system.
A melt inclusion protocol to study magmatic processes: The case study of Campi Flegrei (Southern Italy)
Feb. 21, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Rosario Esposito - UCLA
Subscribe to Calendar
One of the main goals of studying melt inclusions (MI) is to constrain the pre-eruptive physical and chemical processes that have occurred in a magma reservoir at the micro-scale. Recently, several studies that focused on magmatic differentiation of volcanic systems produced detailed interpretations based on data from MI trapped at different times and locations in the plumbing system. In this seminar, I present a protocol to select the most reliable MI from a dataset associated with a single magmatic system: the active volcanic system of Campi Flegrei (Southern Italy). Comparison of data obtained from reliable basaltic-trachybasaltic MI with rhyolite-MELTS predictions indicates that one group of MI records the geochemical evolution of a volatile-saturated magma differentiating by polybaric fractional crystallization from ≥200 MPa (≥7.5 km) to 30 MPa (~1 km). Another group of MI records recharge of the magma chamber by a primitive basaltic magma that mixes with the preexisting primitive trachybasaltic magma before eruption.
A 2D Perspective on CH4 Isotopologues
Feb. 28, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Ed Young - UCLA
Subscribe to Calendar
Isotopic bond ordering in methane gas molecules traces process, while bulk isotope ratios trace both process and source isotopic compositions. By measuring the isotopic bond ordering in CH4 gas it is possible to isolate processes from sources. The 13C/12C ratios of microbialgenic methane, for example, are controlled by the source of carbon as well as by the processes of methane formation. The D/H ratios of methane are controlled by the source of hydrogen (often water) as well as the reaction path to formation. The combination of 13CH3D and 12CH2D2 yields a measure of the process of formation irrespective of the 13C/12C or the D/H ratios of the source carbon and hydrogen. While more experimental work is required to investigate all possible reaction pathways, thus far it appears that the position of a methane datum in 13CH3D vs. 12CH2D2 space can be used to identify processes of formation independent of the uncertainties in source material. One can predict that one day the ability to trace the origins of methane independent of bulk carbon and hydrogen isotope ratios could prove invaluable for assessing the origins of CH4 gas on other solar system bodies where the meaning of bulk isotope ratios would be poorly known.
The thermodynamics of highly alkaline metamorphic fluids produced from dissolved mineral components: an experimental study
March 7, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Adam Makhluf - UCLA
Subscribe to Calendar
This talk will review recent high P-T experiments conducted in the system (Ab) NaAlSi3O8-(Qz) SiO2-H2O in order to understand potential sources, chemical pathways, and thermodynamic solution properties of highly alkaline metamorphic fluids produced from dissolved minerals such as albite. Constraints on the phase boundaries in the ternary system are also an outcome of this study. Our results show that highly alkaline solutions can be produced when albite dissolves in H2O at high-grade metamorphic conditions in the absence of moderating salty solutions such as NaCl, which serves to greatly neutralize the solutions by inhibiting the dissolution of albite. The alkalinity of albite solutions in pure H2O was tested by using the solubility of quartz as a proxy. A control group of quartz solubility experiments in KOH-H2O solutions was conducted to determine the relative alkalinity of the two solutions. Surprisingly, NaAlSi3O8-H2O solutions produce exponentially more hydroxide ions when compared to equimolar KOH solutions. The thermodynamic properties of the albite dissolution reaction can be extracted when combined with other aqueous equilibria data from the Deep Earth Water Model (Sverjensky et al. 2014). Extreme solubility of quartz (> 20 molal Qz for 1 molal Ab at 1000 ºC and 1GPa) in albite solutions naturally leads to supercritical behavior of the solutions at some temperature between 900-1000 ºC at 1 GPa. Our results also place tight constraints on the liquid-vapor miscibility gap at lower temperatures. In light of these results, mechanisms for ubiquitous features in metamorphic and igneous terrains such as quartz veining need to be reexamined. A location to test where a highly-charged, silica-rich fluid such as the one described in this work may have left an imprint on the geologic record is in the Catalina Schist where copious amounts of Si from metamorphic fluids infiltrated the ultramafic mélange from the subducted slab (Bebout and Barton 1989).
Evidence against ancient perchlorate in Gale Crater
March 14, 2019
3:30 p.m. - 4:30 p.m.
Slichter 3853
Presented By:
- Peter Martin - Caltech
Subscribe to Calendar
Perchlorate anion (ClO4-) was discovered in Martian soil by the Phoenix lander, with important implications for potential Martian biology, photochemistry, aqueous chemistry, and the chlorine cycle on Mars. Perchlorate has subsequently been reported in solid samples (both modern scooped sediments and ancient drilled bedrock) analyzed by the SAM instrument onboard the Curiosity rover, based on the release of O2 from these samples at temperatures of ~200-500°C. While the presence of perchlorate in modern sediments is consistent with the detection of perchlorate in soil by Phoenix, the suggestion of inclusion of highly soluble perchlorate in the Gale bedrock is unexpected based on the evidence for a continually wet lacustrine environment. Perchlorate’s preservation to modern day from ~3.5 Ga is also unlikely as a result of its low thermodynamic stability given the background energy source of radioactive decay. As perchlorate’s presence appears suspect, we attempt to present a comprehensive list of alternative O2 sources that could equally explain the data: sulfates, peroxides, nitrate, and metal oxides. Disqualifying issues with each alternative source are apparent. We therefore conclude that perchlorate is most likely present in Gale Crater. However, based on geologic and stability constraints, it must be Amazonian in age and have been introduced to the samples post-depositionally. The observation that perchlorate in Gale Crater (and likely Mars generally) is exclusively modern means that its impact on the processes noted above is also modern and that interpretations invoking perchlorate in an ancient setting must be reevaluated.