Category Archives: UMD

University of Maryland geology department

UMD: Cohen on early solar system events

2017 Geology Colloquium Series

Friday, September 1st 2017 at 3:00 pm
in PLS 1130, the University of Maryland at College Park

Barbara Cohen
NASA Goddard Space Flight Center

The Violent Early Solar System, as Told by Planetary Sample Geochronology

One of the legacies of the samples collected by the Apollo and Luna missions is the link forged between radiometric ages of rocks and the timing of major geologic events, such as impact cratering. Our current understanding of the chronology of the inner solar system is based on the Moon, where evidence suggests that intense bombardment happened during planet formation, followed by a relatively quiescent period, and increased again in an extraordinary bombardment event (“cataclysm”) in the Earth-Moon system at ~3.9 Ga. The importance of the age of lunar samples goes far beyond assigning stratigraphic markers to lunar chronology. The temporal evolution of lunar bombardment is tied to the evolution of a habitable Earth, while the flux of impactors at the Moon drives dynamical models of the formation of the inner solar system. Dr. Cohen will discuss how we use the geochronology of lunar and meteorite samples to understand the past history of impact events on the Moon, the Earth, and throughout the solar system.

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UMD Geology: Zhang on Fe-Ni-S-C Liquid in the Earth’s Mantle

2017 University of Maryland Geology Colloquium Series

Friday, April 7th 2017 at 3:00 pm
in PLS 1140 (College Park campus)

Johnny Zhang
Scripps Institution of Oceanography

Fe-Ni-S-C Liquid in the Earth’s Mantle

Fe-Ni-S-C phases are accessory phases in the Earth’s mantle, but carry important geochemical and geophysical implications due to the contrasting physical and chemical properties between metallic and silicate phases. In the shallow mantle (<200 km), the metallic phase occurs as monosulfide solid solution (mss) or melt with near-monosulfide stoichiometries. To constrain the sulfide melt stability field and its Fe-Ni exchange with mantle silicate minerals, we performed experiments at comparable conditions (P, T, fO2) to Earth’s shallow mantle. In the deeper part of the upper mantle (200-410 km), the mantle become reduced, corresponding to an increase of metal activities in sulfide melt. To contain the composition of Fe-Ni-S melt and its storage of deep carbon, we performed experiments and thermodynamic calculations to show the evolution of Fe-Ni-S-C compositions and mantle silicates at deep upper mantle conditions. Based on the experim! ental and modeling exercise, further discussion will be made on the recent Fe-Ni-S-C liquid from deep diamonds (Smith et al. 2016). In the deepest part of Earth’ mantle (<2900 km), we propose that small quantities of Fe-Ni-S-C liquid is the cause for the two large low shear velocity provinces (LLSVPs). These Fe-Ni-S-C liquid is likely trapped during the crystallization of a dense basal magma ocean and therefore a potential carrier of primordial geochemical signature.

UMD: Viete on metamorphism as earthquake record

2017 Geology Colloquium Series

Friday, March 31st 2017 at 3:00 pm
in PLS 1140, University of Maryland, College Park

Daniel Viete
Johns Hopkins University

Metamorphism can record individual earthquake events in the subduction setting: evidence from the Franciscan Complex, California

Rhythmic major-element zoning has been documented in garnets from high pressure/low temperature (HP/LT) lenses within a number of worldwide subduction mélanges (e.g. California, Chinese Tianshan, Cuba, Greek Cyclades, Guatemala, Japan, Venezuela). These features reflect some fundamental process(es) in the subduction setting. In this talk, conditions of rhythmic zoning acquirement in HP/LT garnets of the Franciscan Complex, California are investigated by use of thermodynamic modeling of phase equilibria, and Raman and synchrotron Fourier transform infrared (FTIR) microspectroscopy.

Hornblende, omphacite and zoisite in the Franciscan rocks are also complexly zoned in major elements. Modeling of phase equilibria shows that modal contours for garnet, amphibole and zoisite are gently dipping in the P–T region that corresponds to the peak-metamorphic mineral assemblage. Metamorphic assemblage diagrams suggest that hydration/dehy! dration reactions involving garnet <—> zoisite (which also involve amphibole exchange or omphacite for glaucophane) are incredibly sensitive to changes in P (e.g. 5–10 vol.% absolute gain/loss of garnet for ΔP = 250 MPa). Major-element zoning in the Franciscan minerals may record repeated growth–partial dissolution cycles in response to P fluctuations in the subduction setting.

Quartz-in-garnet Raman barometry reveals varying P—on the order of 100–350 MPa, over radial distances of 10s of µm—in association with the major-element zoning in the Franciscan garnets. Results from synchrotron FTIR microspectroscopy demonstrate association between zone overgrowths and OH in garnet (a proxy for crystallization pressure in pyrope garnet). The microspectroscopy results confirm changes in P attended development of the rhythmic garnet zoning.

Steep compositional gradients defining the rhythmic major-element zo! ning limit time scales at peak T (and garnet growth–dissolution) conditions to < 1 Myr, requiring that individual growth–partial dissolution cycles were extremely brief. Overpressure on the order of 100s of MPa can develop by tectonic loading of the crust and is relieved with earthquake rupture. Seismic cycles represent ephemeral phenomena capable of accounting for development of rhythmic major-element zoning in HP/LT garnet, during subduction, as a result of fluctuations in P (and garnet stability). Metamorphic rocks may carry detailed records of the catastrophism that punctuates longer-term tectonometamorphic processes.

UMD Geology: Lewis (JHU) on Mars rover geology

2017 Geology Colloquium Series

Friday, February 24th 2017 at 3:00 pm
in PLS 1140

Kevin Lewis
Johns Hopkins University

Exploration of Gale Crater Mars with the Curiosity Mars Rover

The Curiosity Mars rover has been exploring its landing site at Gale crater since 2012. Over this time it has begun to climb the lower slopes of Mount Sharp, a 5 kilometer high mound of sedimentary rock located within the crater. In this talk, we will combine orbital and rover-based geological and geophysical tools to understand the formation of Mount Sharp, with potential implications for other crater-hosted mounds found commonly in the Martian equatorial region. The ultimate goal of this work, and one of the key objectives of the Curiosity mission, is to understand the climate information recorded in the strata of Mount sharp exposed along the rover traverse.

UMD Geology: Pratt on Coastal Plain strata in earthquakes

2017 Geology Colloquium Series

Friday, February 3rd 2017 at 3:00 pm
in PLS 1140

Tom Pratt
USGS-Reston

The influence of eastern U.S. Atlantic Coastal Plain strata on earthquake ground motions, and damage in Washington, DC, during the 2011 Mineral, Virginia, Earthquake

During the 2011 Mw5.8 Mineral, VA earthquake, many buildings in Washington, DC, including national landmarks like the Washington National Cathedral, the Smithsonian “Castle,” and the Washington Monument, sustained damage despite being 130 km from the epicenter. The surprisingly large amount of damage from weak ground motions raises questions of how much the southeast-thickening sedimentary strata of the Atlantic Coastal Plain (ACP) strata beneath the city amplify and trap seismic energy. Partially consolidated ACP marine sedimentary strata overlie crystalline or indurated sedimentary rocks throughout coastal regions of the eastern U.S., extending more than 200 km inland from the coast. The strata taper landward from as much as 1 km near the coast to pinching out in the Washington, DC area. Shallow sedimentary strata are known to amplify earthquake ground motions due to low seismic impedance and strong reverberations. Between November 2! 014 and August 2015, we used 27 seismometers to measure ground motions across Washington, DC, using four sites on crystalline rocks as reference sites. We also used data from continental-scale seismic experiments that span the ACP to examine the influence of the broader ACP strata on earthquake ground motions. Recordings of teleseisms and regional earthquakes provided data with sufficiently high signal-to-noise for computing amplitude ratios relative to the bedrock sites. Amplifications of 10 or greater are found in the Washington, DC area due to the ACP strata, with the peak amplifications occurring near the estimated resonant frequencies of buildings throughout the city. Amplitudes decrease as the strata thicken, but even coastal sites on 600 m of ACP strata show amplification factors as great as 5. We use the frequency of the resonance peaks to invert for an average velocity function within the ACP strata. This work indicates that amplification of short-period ground mot! ions by thin ACP strata contributed to the damage in Washington, DC, d uring the 2011 earthquake, and documents longer-period amplifications that could affect larger structures beneath coastal regions of the eastern U.S. during earthquakes.

UMD: Arevalo on planetary mass spectrometry

University of Maryland 2016 Geology Colloquium Series

Friday, November 18th 2016 at 3:00 pm
in PLS 1140

Ricardo Arevalo
NASA-GSFC

Planetary Exploration and the role of in situ mass spectrometry

Top-priority science questions drive the course of NASA (and ESA) mission selection, and are defined openly by groups of scientists, engineers and planetary advocates. As the ambitions of the community evolve, so do the technologies required to address them. For decades, mass spectrometers have served as low-risk, cost-efficient means to explore the inner and outer reaches of the solar system. Legacy analyzers have characterized a range of planetary environments, including the lunar exosphere, the surface of Mars, and the atmospheres of Venus, Mars and outer planets. However, the collection of complicated mass spectra and detection of organic compounds on Mars and Titan, coupled with ground-based measurements of organics observed in meteorites and cometary materials, has underlined the importance of molecular disambiguation in next generation instruments. In response to these demands, next generation mass spectrometers promise: compatibility with ! chemical separation techniques, such as two-step ionization methods and liquid or gas chromatography; isolation/enrichment of targeted ion signals and intentional fragmentation of precursor (or “parent”) molecules; and, intrinsically higher mass resolving powers to distinguish compounds with nearly identical mass-to-charge ratios.

Here, a review is provided on the process by which missions concepts are formulated, and the evolution of mass spectrometry as a versatile analytical tool for probing the chemical compositions of high-priority planetary environments.

UMD Geology: van Keken on computational geodynamics

2016 Geology Colloquium Series

Friday, November 11th 2016 at 3:00 pm
in PLS 1140

Peter van Keken
Carnegie Institution for Science

A computational geodynamicist’s journey through the Earth in three acts: chemical geodynamics, mantle plumes and subduction zones.