LIGHT LITHOPHILE ELEMENT MICRODISTRIBUTIONS in PYROXENES of the MARTIAN METEORITES. C. D. Williams1, M. Wadhwa1, D. R. Bell1, and R

41st Lunar and Planetary Science Conference (2010) 2641.pdf

LIGHT LITHOPHILE ELEMENT MICRODISTRIBUTIONS IN PYROXENES OF THE MARTIAN METEORITES. C. D. Williams1, M. Wadhwa1, D. R. Bell1, and R. Hervig1. 1School of Earth and Space Explora- tion, Arizona State University, Tempe, AZ 85287-1404, USA

Introduction: The light lithophile elements (LLE) methods similar to those described by [8-10]. Ion mi- such as Li, B, and Be, along with F and H, can be use- croprobe analyses were conducted with a 3 nA O- ful as tracers of magmatic-hydrothermal processes. beam with a spot diameter of 20 to 30 µm. The counts 7 + 9 + 11 + 47 + The abundances and zonation patterns of these ele- at the masses of interest, i.e., Li , Be , B , Ti , 30 + ments in primary igneous minerals in the martian me- Si , were integrated for 2, 2, 5, 1, and 5 seconds, re- teorites may shed light on the volatile contents of the spectively, for each cycle and each measurement con- Mars mantle and the magmas derived from it. In recent sisted of 50 cycles. years, several studies have attempted to address the Results and Discussion: Figures 1-3 show the question of the degassing of water from martian mag- measured concentrations of Li, B and Be plotted versus mas using the distributions of light elements with vola- the TiO2 contents of pyroxenes from the three shergot- tile affinities (Li and B, in particular) in the pyroxenes tites (Shergotty, Zagami and SaU 005) and the nakhlite of some of the martian meteorites [1-7]. However, the Yamato 000593. The abundances of these elements results from these studies have been somewhat am- have been reported previously for other shergottites biguous and controversial, and have raised additional (including Shergotty, Zagami and NWA 480 [1-5]) and questions regarding the role of secondary processes nakhlites (including Nakhla, Lafayette, MIL 03346 and such as diffusional equilibration in producing the ob- NWA 817 [6,7]) and these are shown for comparison served distributions of light lithophile elements in in these figures as the light and dark gray shaded areas, these pyroxenes. It has been shown that combining the respectively. information from abundances and isotopic composi- As can be seen in Fig. 1, pigeonite crystals of the tions of LLE in martian pyroxenes may help to better basaltic shergottites Shergotty and Zagami (for which address these questions (e.g., [6]. volatile degassing of their parent magmas has been In an attempt to address the questions related to the suggested [1,2]) show a gradual decrease in Li abun- degassing of martian magmas during their emplace- dances as TiO2 contents increase from cores to rims, ment and crystallization and the effects of igneous consistent with previous studies [1,2,5]. Note that al- fractionation and subsolidus equilibration, we have though the Li abundances reported here for Shergotty initiated an investigation of the abundances and isotope and Zagami pyroxenes fall within the range reported systematics of light lithophile elements in pyroxenes of previously, they do fall towards the low end of this several shergottites and nakhlites, and we plan to ex- range. Abundances of Li in pigeonites of the olivine- tend our analyses to the abundances of F (which ap- phyric shergottite SaU 005 are the lowest observed for pears to act crystal-chemically in a manner similar to any martian meteorite, and appear to be uniform (~1 OH). Here we report some preliminary results from ppm) over the range of TiO2 contents. Augite crystals this investigation that include LLE abundances in py- of Yamato 000593 define a much narrower range in roxenes of three shergottites( basaltic shergottites TiO2 contents, and may show a slight decrease of Li Shergotty and Zagami as well as the olivine-phyric abundances with TiO2 contents. shergottite SaU 005) and one nakhlite (Yamato The abundances of B show a substantial scatter in 000593). Shergotty and SaU 005 (with concentrations tending to Analytical Procedures: All measurements were be higher in the cores), although they are fairly con- made on polished thin sections of Shergotty, Zagami, stant in Zagami, with increasing TiO2 contents (Fig. 2). SaU 005 and Yamato 000593. Major element zona- The B data reported here for Zagami are consistent tions in pyroxene grains were mapped by backscattered with those reported previously by [5]. Given that Sher- electron imaging on a JEOL 845 scanning electron gotty and Zagami have very similar petrogenetic histo- microscope at Arizona State University (ASU) and ries, this suggests that the higher and more variable B were used to locate suitable areas for the ion micro- concentrations in Shergotty (and SaU 005) may be due probe analyses. To eliminate B contamination prior to to incomplete removal of surface B contamination. the ion microprobe analyses, each thin section was Yamato 000593 augites have uniformly low B concen- washed in a 1.82% manitol solution and rinsed in ultra trations that increase very slightly with increasing TiO2 pure water, and then was immediately gold coated. The contents (Fig. 2). abundances of Li, Be, B, and Ti were measured using The abundances of Be in pyroxenes of all martian the Cameca ims-6f ion microprobe at ASU using meteorites analyzed here show increases as their TiO2 41st Lunar and Planetary Science Conference (2010) 2641.pdf

contents increase (Fig. 3). This is consistent with the incompatible behavior of Be during igneous fractionation processes.

Figure 3. Plot of Be concentrations versus TiO2 contents in pigeonites of Shergotty (red squares), Zagami (yellow squares) and SaU 005 (blue squares), and in augites of Ya- Figure 1. Plot of Li concentrations versus TiO2 contents in mato 000593 (green circles). Low Ti contents correspond to pigeonites of Shergotty (red squares), Zagami (yellow core compositions and high Ti contents correspond to rim squares) and SaU 005 (blue squares), and in augites of Ya- compositions of these pyroxenes. The light and dark gray mato 000593 (green circles). Low Ti contents correspond to core compositions and high Ti contents correspond to rim shaded areas show the ranges of pyroxene compositions in compositions of these pyroxenes. The light and dark gray shergottites and nakhlites, respectively, from previous studies shaded areas show the ranges of pyroxene compositions in [1-7]. shergottites and nakhlites, respectively, from previous studies The distributions of the Li and B reported here in [1-7]. pyroxenes of the three shergottites (Shergotty, Zagami and SaU 005) and nakhlite Yamato 00059 are currently most consistent with subsolidus diffusional processes (as suggested by [7]), while Be distributions may be recording igneous fractionation processes. Further analyses of F abundances and Li isotopic compositions of these same pyroxenes will help to better assess the roles of processes such as magmatic degassing, igneous fractionation and subsolidus diffusion in the petro- genesis of the martian meteorites. Acknowledgements: We thank the Smithsonian Museum of Natural History for sections of Shergotty and Zagami, and the NIPR for the section of Yamato 000593. We are grateful for the assistance with ion microprobe analyses provided by L. Williams. Figure 2. Plot of B concentrations versus TiO2 contents in References: [1] McSween et al. (2001) Nature pigeonites of Shergotty (red squares), Zagami (yellow 409, 487-490. [2] Lentz R. C. F. et al. (2001) GCA, 65, squares) and SaU 005 (blue squares), and in augites of Ya- 4551-4565. [3] Beck et al. (2004) GCA, 70, 2925- mato 000593 (green circles). Low Ti contents correspond to 2933. [4] Herd et al. (2004) GCA, 68, 2925-2933. [5] core compositions and high Ti contents correspond to rim compositions of these pyroxenes. The light and dark gray Herd et al. (2005) GCA, 69, 2431-2445716.0. [6] Beck shaded areas show the ranges of pyroxene compositions in et al. (2006) GCA, 70, 4813–4825. [7] Treiman et al. shergottites and nakhlites, respectively, from previous studies (2006) GCA, 70, 2919-2934. [8] Hervig R. (1996) Rev. [1-7]. in Mineralogy, 33, 789-803. [9] Hervig R. (2002) In Beryllium: Mineralogy, petrology, and geochemistry in the Earth's crust, 50, 319-332. [10] Williams L. and Hervig R. (2005) GCA, 69, 5705-5716.