Ages and Mineralogical Variations of Lunar Mare Basalts

Details Ages and Mineralogical Variations of Lunar Mare Basalts Ages and Mineralogical Variations of Lunar Mare Basalts

May 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p21a04h&link_type=abstract

American Geophysical Union, Spring Meeting 2001, abstract #P21A-04 INVITED

Mathematics

Logic

5410 Composition, 5464 Remote Sensing, 5480 Volcanism (8450)

Scientific paper

In the lunar sample collection, three major groups of basalts have been identified: high-Ti (9-14 wt% TiO2), low-Ti (1-5 wt% TiO2)), and very-low-Ti (< 1 wt% TiO2)) basalts. Laboratory data of lunar basalt samples show a distinctive bimodal distribution of titanium concentrations with peaks at ~2.5-3 and 12-13 wt% TiO2, but remote sensing data suggest a continuous gradation from very-low-Ti to high-Ti mare basalts [1]. Lunar mare basalts not only exhibit a broad variety of mineralogical compositions, but also of ages. Early Ti-rich basalts flooded large regions in the eastern lunar hemisphere (Ap11, Ap17) in the early Imbrian Period (3.3-3.8 b.y.) [2]. These basalts were followed by widespread eruptions of less Ti-rich basalts of middle to late Imbrian age (Ap12, Ap15). Finally Ti-rich basalts, which have not been sampled so far, flooded parts of Mare Imbrium and Oceanus Procellarum in the early Eratosthenian Period (2.5-3.0 b.y.). We performed new crater counts for about 220 basalt units exposed in the large mare region of the lunar nearside [3]. Combined with new mineralogical data from Clementine [4] and Lunar Prospector [5, 6], these new crater counts allow a detailed investigation of the relationship between mineralogy (Ti, Fe, Th, Sm) and age of a basalt. Our investigation does not show any systematic relationship between the age and the titanium abundance of a lunar basalt. We see that titanium-rich basalts generally erupt simultaneously with titanium-poor basalts. We do not find any evidence for a trend with older basalts being systematically more titanium-rich. The titanium abundance varies between less than 1 and ~14 wt% and most basalts erupted between 3.3 and 3.8 b.y. The iron content of all investigated basalts varies between ~3 and ~19 wt% with most basalts showing iron concentrations of 13-19 wt%. We do not see a correlation between the iron abundance and age, but our data indicate that basalts older than ~3.3 b.y. have a wider variety in iron abundances than younger basalts. Older basalts show the full range of iron concentrations (i.e., 3-19 wt%); younger basalts are generally very similar in iron content, i.e. 13-19 wt%. Based on our study, we conclude that (1) the new data confirm our previous work in that they do not show systematic variations of the titanium abundances of the investigated basalts with time; (2) titanium-poor basalts occur simultaneously with titanium-rich basalts; (3) the range in titanium concentrations remains more or less constant over ~3 b.y.; (4) the range in iron content is significantly larger for basalts that are older than 3.3 b.y compared to younger basalts; (5) for basalts that erupted after 3.3 b.y. the iron abundances remains more or less constant; (6) variation of iron and titanium concentrations do not show identical/similar characteristics; (7) most basalts erupted prior to ~3.3 b.y. (8) the new data have significant influence on petrologic models. [1] Giguere et al., 2000, Meteoritics & Planetary Sciences 35, 193-200; [2] Head, 1976, Rev. of Geophys. Space Phys. 14, 265-300; [3] Hiesinger et al., 2000, J. Geophys. Res. 105, 29239-29275; [4] Lucey et al., 2000, J. Geophys. Res. 105, 20297-20305; [5] Lawrence et al., 2000, J. Geophys. Res. 105, 20307-20331; [6] Elphic et al., 2000, J. Geophys. Res. 105, 20333-20345.

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