Welcome to GeoUnion, the graduate student body of the Department of Earth, Environmental and Planetary Sciences. GeoUnion strives to supplement the overall graduate student experience at Rice and DEEPS. GeoUnion represents DEEPS in the overall Rice grad student community, acts as a liaison between students and faculty and organizes a number of intra- and inter-departmental events throughout the academic year.
Mineralogical Society of America has selected EEPS postdoctoral researcher Dr. Chenguang Sun for the society’s 2021 MSA Award. The Mineralogical Society of America Award is intended to recognize outstanding published contributions to the science of mineralogy by relatively young individuals or individuals near the beginning of their professional careers. The work must have been accomplished either before reaching the age of 35 or within 7 years of the awarding of the terminal degree. Dr. Sun is recognized for his work on interpretation of trace elements in minerals and rocks and interactions between carbonatite and peridotite and genesis of kimberlitic magmas.
Dr. Sun received his Ph.D. from Brown University in 2014 and held a post-doctoral fellowship at Woods Hole Oceanographic Institution from 2014 to 2016. Since summer of 2016, Dr. Sun has been a post-doctoral scholar at the EEPS department of Rice University. Starting this month, Dr. Sun will join University of Texas at Austin as a tenure-track assistant professor.
Martian geologist Kirsten Siebach among 13 chosen by NASA for rover mission
Kirsten Siebach has to persevere a little longer, waiting for her ship to come in.
That ship is in space, carrying a rover called Perseverance to Mars. And Siebach, a Martian geologist at Rice University, is now one of 13 scientists recently selected by NASA to help operate the rover and scout for samples that will eventually be returned to Earth.
The rover, launched in July and landing next February, is the first of three missions that will relay pieces of the Red Planet back home. Perseverance will identify, analyze and then collect samples that scientists hope contain signs of ancient microbial life.
A second mission led by the European Space Agency will pick up the collected promising samples and launch them to Mars orbit. A third mission will dock with the orbiter, take the samples and bring them to Earth, likely in the early 2030s.
Siebach, an assistant professor of Earth, Environmental and Planetary Sciences, will be a main player in the first mission, helping direct Perseverance as it stops along an appointed path to look for interesting features and select the precious samples. Her proposal was one of 119 submitted to NASA for funding.
“Everybody selected to be on the team is expected to put some time into general operations as well as accomplishing their own research,” she said. “My co-investigators here at Rice and I will do research to understand the origin of the rocks Perseverance observes, and I will also participate in operating the rover.”
That duty, not unfamiliar to her as a member of the Curiosity rover team, will help her choose Mars rock and sand targets for analysis by Rice data scientist Yueyang Jiang, an expert in machine-learning algorithms, and research scientist Gelu Costin, a mineralogist.
“Because there is only one rover, the whole team at NASA has to agree about what to look at, or analyze, or where to drive on any given day,” Siebach said. “None of the rovers’ actions are unilateral decisions. But it is a privilege to be part of the discussion and to get to argue for observations of rocks that will be important to our understanding of Mars for decades.”
The landing site, the 28-mile-wide Jezero Crater, was selected for its history; it once hosted a lake and river delta where microbial life may have thrived over 3 billion years ago. Siebach is particularly excited to investigate carbonates, the products of atmospheric carbon dioxide dissolved in water that on Earth usually settle into the landscape as limestone. They often contain fossils.
“There are huge packages of limestone all over Earth, but for some reason it’s extremely rare on Mars,” she said. “This particular landing site includes one of the few orbital detections of carbonate and it appears to have a couple of different units including carbonates within this lake deposit. The carbonates will be a highlight of we’re looking for, but we’re interested in basically all types of minerals.”
The primary instrument the Rice team will be using on Perseverance is PIXL, short for Planetary Instrument for X-ray Lithochemistry, which is designed to identify chemical elements while also providing closeups of soil and rocks with a resolution about the size of a grain of salt.
Siebach, Liang and Costin plan to develop computational and machine-learning methods that produce mineral maps of samples based on their high-resolution chemistry. They also aim to establish a context for samples that will eventually come back to Earth and could reveal the signatures of historic life on Mars.
It will take a couple months after landing to validate Perseverance before Siebach and the others get to start their scientific inquiry. Then the long game begins.
“Occasionally, something hits Mars hard enough to knock a meteorite out, and it lands on Earth,” she said. “We have a few of those. But we’ve never been able to select where a sample came from and to understand its geologic context. So these samples will be revolutionary.”
Authors: Sriparna Saha, Ye Peng, Rajdeep Dasgupta, Mainak Mookherjee, Karen M. Fischer
Abstract: A number of possible hypotheses have been proposed to explain the origin of mid-lithospheric discontinuities (MLDs), typically characterized by ∼2-6% reductions in seismic shear wave velocity (Vs) at depths of 60 km to ∼150 km in the cratonic sub-continental lithospheric mantle (SCLM). One such hypothesis is the presence of low-shear wave velocity, hydrous and carbonate mineral phases. Although, the presence of hydrous silicates and carbonates can cause a reduction in the shear wave velocity of mantle domains, the contribution of volatile metasomatism to the origins of MLDs has remained incompletely evaluated. To assess the metasomatic origin of MLDs, we compiled experimental phase assemblages, phase proportions, and phase compositions from the literature in peridotite +H2O, peridotite +CO2, and peridotite +H2O +CO2 systems at P-T conditions where hydrous silicate and/or carbonate minerals are stable. By comparing the experimental assemblages with the compiled bulk peridotite compositions for cratons, we bracket plausible proportions and compositions of hydrous silicate and carbonate mineral phases that can be expected in cratonic SCLMs. Based on the CaO and K2O contents of cratonic peridotite xenoliths and the estimated upper limit of CO2content in SCLM, ≤∼10 vol.% pargasitic amphibole, ≤∼2.1 vol.% phlogopite and ≤∼0.2 vol.% magnesite solid solution can be stable in the SCLM. We also present new elasticity data for the pargasite end member of amphibole based on first principles simulations for more accurate estimates of aggregate Vs for metasomatized domains in cratonic mantle. Using the bracketed phase compositions, phase proportions, and updated values of elastic constants for relevant mineral end members, we further calculate aggregate Vs at MLD depths for three seismic stations in the northern continental U.S. Depending on the choice of background wave speeds of unmetasomatized peridotite and the cratonic geotherm, the composition and abundance of volatile-bearing mineral phases bracketed here can explain as much as 2.01 to 3.01% reduction in Vs. While various craton formation scenarios allow formation of the amphibole and phlogopite abundances bracketed here, presence of volatile-bearing phases in an average cratonic SCLM composition cannot explain the entire range of velocity reductions observed at MLDs. Other possible velocity reduction mechanisms thus must be considered to explain the full estimated range of shear wave speed reduction at MLD depths globally.
Saha, S., Peng, Y., Dasgupta, R., Mookherjee, M. & Fischer, K. M. (2021). Assessing the presence of volatile-bearing mineral phases in the cratonic mantle as a possible cause of mid-lithospheric discontinuities. Earth and Planetary Science Letters 553, 116602. doi:10.1016/j.epsl.2020.116602
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