Week in Review:

 

            

The 2014 Lunar and Planetary Science Conference (LPSC) took place in Houston, TX from March 17-21.

  Many staff members and postdoctoral associates from the Geophysical Laboratory attended this year.  Check here daily for live updates on each day's science presentations.

Monday, March 17

Valerie Hillgren presented during the How to Make a Planet session and discussed "Metal-silicate partitioning of Si and S under highly reducing conditions: implications for the evolution of Mercury.

Hillgren explained her experimental process based on measurements from the MESSENGER spacecraft of Mercury’s surface composition indicate that the FeO content is very low, and the S content is strikingly high. Recent gravity models suggests that in addition to an anomalously large core there may be a dense FeS layer, termed the anticrust, at the base of the mantle. These unique characteristics of Mercury can all potentially be tied to core formation in an extremely reducing environment where Si as well as S may be incorporated into the core. In highly reduced systems almost all the FeO is reduced to Fe and partitions into the core, and S is more soluble in silicate melts. In addition, the miscibility gap in the Fe-S-Si system could provide an origin for the FeS “anticrust.” In order to understand how reducing condtions may have shaped the differentiation and subsequent evolution of Mercury, Hillgren et al. have begun a study of the mutual partitioning of Si and S between metal and silicate under reducing condiditons at pressures relvant to core formation in Mercury. For the complete abstract, click here.

          

Tuesday, March 18

Neil Bennett and Yingwei Fei presented at the Emerging Worlds: Planetary Differentiation poster session and disccused their poster "The effect of thermal gradients on the major and trace element distribution in Fe-Ni-O melts: the implications for chemical redistribution during planetary accretion."

Bennett explained that it is well documented that when subject to a thermal gradient, multi-component melts may develop compositional gradients. The sense of major element redistribution is oriented to minimize the kinetic-energy imbalance between hot and cold portions of the system. The magnitude of compositional separation and exact arrangement of melt components depend upon activity-composition relationships; with strongly non-ideal systems promoting the formation of larger compositional gradients. Trace elements are controlled by the major element composition and are distributed in a manner that maintains their activity at an approximately constant value throughout the system. Bennett et al. have conducted experiments at high pressure and temperature to investigate the distribution of highly siderophile elements (HSE), W and Mn in metallic Fe-Ni-O melts subject to a thermal gradient. The results of these experiments can be used to determine the HSE distribution during equilibrium partitioning that involves an oxygen bearing metal phase.

                

Wednesday, March 19

Former GL postdoctoral associate Yoko Kebukawa presented during the Chondrites: Matrix, Water, and Accreting Parent Bodies session on "Isotope imaging and the kinetics of deuterium-hydrogen exchange between insoluble organic matter and water."

She explained how the high deuterium enrichment in insoluble organic matter (IOM) in chondrites has largely been attributed to low temperature chemistry in the interstellar medium (ISM) or the early outer Solar System. The IOM in carbonaceous chondrites has various D/H ratios that generally decrease with increasing alteration, while the IOM in ordinary chondrites has large D enrichments that increase with increasing metamorphism. In either case, the differences in H isotopic compositions among chondrites are likely the result of kinetic isotopic exchange between IOM and water. Kebukawa et al. have studied kinetics of D-H exchange between organic matter and water using laboratory analog of IOM derived from the polymerization of formaldehyde with incorporation of ammonia. The team previously proposed the three-dimensional diffusion model to fit the experimental curves, since this model gave the best fit. If the D-H exchange rates are controlled by diffusion, one expects to observe a classical diffusion profile into the organic grains. For the full report on the results of isotope microscope analyses of the D-H exchanged IOM analog, click here.

                        


Former GL postdoctoral associate and Carnegie fellow Francis McCubbin presented during the Lunar Volatiles session on "Apatite-melt partitioning in basaltic magmas: the importance of exchange equilibria and the incompatibility of the OH component in Halogen-rich apatite."

He explained that the mineral apatite [Ca5(PO4)3-(F,Cl,OH)] is present in a wide range of planetary materials. Due to the presence of volatiles within its crystal structure, many recent studies have attempted to use apatite to constrain the volatile contents of planetary magmas and mantle source. In order to use the volatile contents of apatite to precisely determine the abundances of volatiles in coexisting silicate melt or fluids, thermodynamic models for the apatite solid solution and for the apatite components in multicomponent silicate melts and fluids are required. Although some thermodynamic models for apatite have been developed, they are incomplete. Furthermore, no mixing model is available for all of the apatite components in silicate melts or fluids, especially for F and Cl components. Several experimental studies have investigated the apatite-melt and apatite-fluid partitioning behavior of F and Cl in terrestrial systems; however, the partitioning has proved to be compositionally dependent, and experiments on magmatic systems relevant to extraterrestrial magmas have not been conducted. In the present study, McCubbing et al. conducted apatite-melt partitioning experiments in a piston cylinder press at 1.0 GPa and 950-1000°C on a synthetic martian basalt composition equivalent to the basaltic shergottite Queen Alexandria Range (QUE) 94201. These experiments were conducted to assess the effects of apatite composition on the partitioning behavior of F, OH, and Cl between apatite and basaltic melt. For the results,click here.

 

Thursday, March 20

Former summer intern Mike Krawczynski presented "Iron isotope fractionation between metal and troilite: a new cooling speedometer for iron meteorites" during the Planetary Coressession.

Krawczynski explained a new method for estimating the cooling rate of iron meteorites based on metal-troilite partitioning of Fe isotopes. The equilibrium partitioning of Fe isotopes is strongly dependent on temperature, with larger fractionation at lower temperatures. Slowly cooled iron-troilite pairs will preserve larger Fe isotope fractionations than rapidly cooled pairs because they are able to equilibrate to lower temperatures. Troilite is a ubiquitous phase in all Fe-meteorite groups, and Fe self-diffusion data have been determined experimentally for metal and sulfides. Krawczynski et al. presented new data on the equilibrium fractionation of Fe isotopes between metal and troilite, and a numerical model for determining closure temperatures. To find out more, click here.

 


Staff scientist Yingwei Fei joined the Planetary Cores session by discussing "Percolative behavior of immiscbible liquids at high pressure and temperature: implications for composition of planetary cores."

Fei detailed that the percolation of liquid metal in solid silicate matrix is likely a dominant process in the initial differentiation when the temperature is not high enough to melt the entire planetary body. During the core formation, different amounts of multiple light elements enter the iron dominant core, depended on the initial composition and physical conditions of the core-forming event, such as pressure, temperature, and oxygen fugacity. Therefore, it is critical to understand percolative behavior of metallic liquid with multiple light elements in the solid matrix. The Fe-S-Si, Fe-S-O, and Fe-S-C systems form immiscible liquids at low pressure. Because the two immiscible liquids have different wetting properties, the immiscible liquids could be physically separated through percolation if one liquid forms smaller dihedral angle than 60° whereas the wetting angle of the second liquid is larger than the critical angle. This potentially is a novel mechanism for compositional separation during core formation for small planetary bodies, which has not been considered before. Read more here.

                                        

Michelle Scholtes, 17 March 2014

Scientific Area: