The role of undercooling in producing igneous zoning trends in pyroxenes and maskelynites among basaltic Martian meteorites

Pyroxene and maskelynite are major minerals in basaltic Martian meteorites (Shergotty, Zagami, EETA79001, and QUE94201). They are chemically zoned in both major and minor elements, offering useful information on their crystal lization history, Pigeonite and augite show two distinct textural occurrenc es of zoning patterns. In Shergotty and Zagami, pigeonite and augite are us ually present as separate grains that are zoned from Mg-rich core to Fe-ric h rim, respectively. Both pigeonite and augite usually have homogeneous cor es, considered to be cumulus phases. Zagami pyroxenes are not zoned as exte nsively as those in Shergotty, but their mineralogy is quite similar. On th e other hand, pigeonite and augite in EETA79001 and QUE94201 are both prese nt in individual composite grains. These pyroxenes are complexly zoned, and typically the cores are Mg-rich pigeonites, mantled by Mg-rich augite, and the rims are Fe-rich pigeonite. Pyroxenes in lithology A of EETA79001 are small and show irregular zoning patterns, but most of them show similar zon ing patterns to lithology B of EETA79001 and QUE94201. Maskelynite composit ions also correspond to differences in pyroxene zoning. Maskelynites in She rgotty and Zagami are more alkali-rich than those in EETA79001 and QUE94201 . Shergotty and Zagami maskelynites apparently nucleated on pyroxene crysta ls and grew outward (to lower An content) into interstitial melts, whereas EETA79001 and QUE94201 maskelynites have the most An-rich portions in the c enters of grains, showing normal core-to-rim zoning. FeO in the maskelynite cores is different between Shergotty and Zagami (0.5-0.6 wt%) and EETA7900 1 and QUE94201 (0.3-0.4 wt%). The lower Fe content of EETA79001 and QUE9420 1 maskelynite cores reflects earlier crystallization of plagioclase. Al zon ing in pyroxenes also marks the beginning of plagioclase crystallization. D ecrease of the Al/Ti ratios observed in Al-Ti plots of pyroxenes further su ggests plagioclase crystallization, and the Al-Ti distribution shows a clea r difference between Shergotty and Zagami on one hand and EETA79001 and QUE 94201 on the other Such mineralogical differences of EETA79001 and QUE94201 from Shergotty and Zagami can be understood by undercooling of the magmas from which they have crystallized. We believe that Shergotty and Zagami exp erienced only slight undercooling, resulting initially in cotectic growth o f pigeonite and augite, later joined by plagioclase. On the other hand, EET A79001 and QUE94201 experienced significant undercooling of their melts, an d their crystallization sequence was pigeonite, augite, plagioclase, and th en Fe-rich pigeonite, with each phase crystallizing metastably alone, becau se the melt compositions did not follow the equilibrium phase boundaries. L ithology A of EETA79001 also experienced undercooling effects, but they are not so extensive as in lithology B, inasmuch as lithology A shows some fea tures similar to Shergotty and Zagami. All of these inferences concerning s upercooling are consistent with field emission gun scanning electron micros copy (FEG-SEM) observation of pyroxene microstructures if we consider that magmatic cooling rates are related to subsolidus cooling rates. That is, th e pyroxene cores of Shergotty and Zagami have fine exsolution lamellae ja f ew tens to a few hundreds nm), whereas those of EETA79001 and QUE94201 do n ot contain such exsolution lamellae, indicating faster cooling rate even at subsolidus. (C) 1999 Elsevier Science B.V. All rights reserved.