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Project Progress

We have made substantial progress on each of the subprojects in the past year:

(1) Our work in collaboration with the Dietrich GROUP was completed and published in Science. He-3 exposure ages were determined for the Box Canyon basalts of south central Idaho. A paper on the geochronology has been published EPSL. This is the first demonstration of the combined U-Th-4He-3He approach to simultaneously dating the eruption age and exposure age of lava flows. The data place strong constraints on the possible origins of the Canyon by sapping processes.

Lamb, M.P, Dietrich, W.E. Aciego, S.M. DePaolo, D.J. and Manga, M., Formation of Box Canyon, Idaho, by Megaflood: Implications for Seepage Erosion on Earth and Mars, Science v. 320, 1067-1070 (2008)

(2) Ca isotopic analyses on a sediment core from Badwater dry lake in Death Valley are now complete. This core we are using as an analogue for early Mars surface conditions to assess the role of non-biological processes in fractionating Ca isotopes. New samples of stream waters, lake waters, and bedrock have been collected in Death Valley, and these have now been analyzed for Ca, O and H isotopes, as well as chemical components. The final few data are just now being collected, and we are beginning a manuscript. Report on this work was made at Fall AGU 2006 and also at Fall AGU 2007. The manuscript is now in final revision prior to submission.

(3) We have also completed analyses of Ca in a soil section from the Atacama desert, another Mars analogue site. In this case we have observed considerable Ca isotope fractionation, unlike any we have found elsewhere in terrestrial materials, which we believe is due to kinetic effects in the dissolution and reprecipitation of calcium carbonate and sulfate in the soil over millions of years. The manuscript describing the results is now published. Further work on this problem was conducted. Additional analyses of other Atacama sites with differing rainfall were made to assess the effects of small differences in rainfall above the very minimal rainfall characterizing the first site. The second paper is currently in preparation.

Ewing, S.A. Yang, W., DePaolo, D.J., Michalski, G., Kendall, C., Stewart, B.W., Thiemens, M., and Amundson, R., Non-biological Fractionation of Stable Ca Isotopes in Soils of the Atacama Desert, Chile, Geochimica et Cosmochimica Acta, Vol 72/4 pp 1096-1110 (2008)

(4) Measurements and modeling of an 800 meter-long deep-sea carbonate core, and the associated pore fluids, were done as a means of further evaluating the equilibrium fractionation factor for 44Ca/40Ca between calcite and aqueous solution. The results show that slow inorganic precipitation of calcite results in no fractionation of Ca isotopes. This paper also proposes a mechanism by which the results of our study can be reconciled with those of laboratory precipitation experiments. This study opens up new opportunities for using stable isotope fractionation of Ca and other mid-mass isotope systems to study the molecular mechanisms of mineral precipitation from aqueous solutions. The results were reported on at the Fall 2007 AGU meeting, as well at the 2007 and 2008 Goldschmidt conferences.

Fantle, M.S. and DePaolo, D.J., Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca²⁺(aq)-calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments. Geochim. Cosmochim. Acta, v. 71, 2524-2546 (2007)

In a follow-up study we also studied the Ca isotope systematics of deep sea sediments that have a larger fraction of silicate (clay) component and in which there is microbial reduction of sulfate. This situation could be analogous to sediment deposition on early Mars if there was sufficient biological activity. It was found that the behavior of calcite in these sediments is much different from that in more clay-poor and organic carbon-poor sediments. In particular, the presence of organic material has a major effect on stabilizing calcite, such that calcite dissolution rates are reduced to effectively zero.

(5) We have begun analytical work on a larger sample of the Martian meteorite ALH84001 to better constrain the Ca and Fe isotopic composition of the igneous and secondary minerals, and thereby assess whether there is evidence that the precipitation of carbonate was biologically mediated. We have continued to spend substantial effort in refining the analytical techniques for Ca isotopic measurements, and we have established methods for investigating variations of radiogenic 40Ca in meteorites, which could provide insights into the early formation and bulk composition of Mars.

(6) In an unexpected development, our analyses of meteorite samples resulted in the discovery that there are non mass-dependent Ca isotope effects in some meteorites. Postdoc Justin Simon has now made measurements of several meteorites and Martian samples. The results suggest that there was a small amount of heterogeneity in the early solar system in Ca isotopes, and the patterns we are observing suggest that there may have been zonation in the nebula as the planets were accreting.

Interestingly, there appears to be a small amount of isotopic heterogeneity in the Mars samples that is similar in character to that seen in some primitive meteorites, and absent in terrestrial samples. The origin of these effects needs further investigation, but it could be an indication that Mars accreted considerably earlier than Earth as suggested by some models.

This work was reported at the LPSC in March 2006 and there was a follow-up talk at the 2008 meeting. A manuscript has been submitted to Astrophysical Journal Letters.