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VARIATION in Mn/Fe SYSTEMATICS in PYROXENE from the MOON: the EFFECTS of CONTRASTING Ti and Al ACTIVITIES

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VARIATION in Mn/Fe SYSTEMATICS in PYROXENE from the MOON: the EFFECTS of CONTRASTING Ti and Al ACTIVITIES 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1413.pdf VARIATION IN Mn/Fe SYSTEMATICS IN PYROXENE FROM THE MOON: THE EFFECTS OF CONTRASTING Ti AND Al ACTIVITIES. J. J. Papike, S. B. Simon, and C. K. Shearer. Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A. ([email protected]). Introduction: Papike et al. [1] introduced a tech- components are very small. Coupled substitutions are nique where the Mn/Fe ratios of olivine or pyroxene required to incorporate trivalent cations and Ti4+ into and the anorthite content of plagioclase provided a tool the pyroxene crystal structure because they replace to identify the planetary parentage of newly discovered divalent Mg and Fe in the M1 site [7], thus creating an meteorites. A recent paper [2] refined this technique, excess charge of +1 or +2, respectively. Crystal charge focusing on the angrite meteorites, and a new, robust balance must be maintained, and and balancing of determinative curve was obtained. On a plot of Mn vs. charge excesses can be accomplished by simultaneous- Fe cations per formula unit for pyroxene from basalts, ly substituting: 1) one Al cation into the tetrahedral site the slopes of the data trends decrease in the sequence for Si; or 2) one Na cation into the M2 site for Ca, per Vesta, Mars, Earth, and Moon. Analyses of angrite unit of excess charge. Although pyroxenes from the pyroxenes are not directly comparable to those from three mare basalt groups exhibit a range of Mn/Fe rati- the other bodies because in angrite pyroxenes the M2 os, the positioning of an individual point on a plot of sites are completely filled by Ca [e.g., 3] and therefore Mn vs. Fe cations does not follow a simple sweep of have different substitution systematics. Despite the fact Mn/Fe slope dependent only on Ti content. This is that with enough high-quality electron microprobe data because, although Ti4+(M1) – 2Al (tet.) exchange is for olivine, pyroxene, and plagioclase we find an over- very important, it is not the only coupled substitution arching fingerprint of planetary parentage, in any indi- in play. The second most important is Al (M1) – Al vidual suite there is significant scatter along the aver- (tet.). Therefore, the competing effects of variable Al age trend line. The purpose of this paper is to explore vs. Ti content of pyroxene can change the positioning the dispersion of Mn/Fe ratios of pyroxene in lunar of an individual point. Substitution of Ti into pyroxene basalts and to illustrate the effect of crystallization leads to increases in the Mn/Fe ratios because Ti4+ sequence upon this parameter. preferentially replaces Fe relative to Mn because its Lunar mare basalts are divided into three main ionic radius is closer to that of Fe than Mn. types: high-Ti (from Apollo 11 and 17 landing sites); Methods: Data for pyroxene from high-Ti, low-Ti, low-Ti (mainly from the Apollo 12 and 15 landing and L-24 VLT basalts were taken from the literature sites); and very-low-Ti basalts (VLT; from the Soviet [8-11]. The L-24 data were included in this study to Luna 24 (L-24) mission and from the Apollo 17 (A-17) examine the potential variability in mantle Mn/Fe out- drill core). All of the Apollo samples were collected side of the PKT. The A-17 VLTs considered here are within or immediately adjacent to the Procellarum those studied by [12]. New analyses of pyroxene in KREEP Terrane (PKT; [4]), whereas the L-24 basalts these VLT samples were obtained with a JEOL 8200 represent melts derived from outside this terrane. Most fully automated electron microprobe at the University of the analysis of lunar samples indicates a fairly lim- of New Mexico, operated at 15 kV. Counting times on ited range of oxygen fugacity, generally close to IW-1 peaks ranged from 10 to 30 s; background count times (e.g., [5]). Thus, Ti is mainly in the 4+ oxidation state equaled the peak counting times for each analysis. Pure [6]. Rather than wide ranges in fO2 or analytical error, oxide, synthetic glass, and natural mineral standards the variability of Mn/Fe ratio in lunar basalt pyroxene were used. Average relative errors are ~6% for Ti and is directly or indirectly dependent upon Ti concentra- Mn, 2% for Al, and 0.5% for Fe. Fits to the data, both tions in their mantle sources, as shown below. linear and polynomial, were generated by curve-fitting There are many possible coupled substitution algorithms built into the KaleidaGraph v4.5 plotting mechanisms in pyroxene, and these can be summarized software program. The polynomial fits are intended to using an equation assuming stoichiometric formulae be illustrative rather than a rigorous fit to the data. (where coefficients indicate the magnitude of the ex- Results: Individual analyses are plotted in Fig. 1a; cess/deficiency, such that, for example, VIAl3+ is polynomial fits are shown in Fig.1b; and linear fits in equivalent to a +1 charge excess, while 2VITi4+ indi- Fig. 1c. Although a single linear slope for lunar pyrox- cates that for every Ti4+ substitution, there is a charge ene analyses has been previously reported (e.g., [13]), excess of +2). For lunar pyroxene, the equation ex- in detail all four data sets have sinusoidal trends with pressing this completely is: Excesses: VIAl3+ + VIFe3+ + positive trajectories (Fig. 1). VICr3+ + VIV3+ + VITi3+ + 2VITi4+ = Deficiencies: IVAl3+ The high-Ti (HT) basalt data have a sharp increase + M2Na+. In lunar pyroxenes, the Fe3+, V3+ and Na in Mn content at low Fe#, a result of rapid, early incor- 50th Lunar and Planetary Science Conference 2019 (LPI Contrib. No. 2132) 1413.pdf poration of Ti into the pyroxene M1 site and conse- however, appear to be significant. The limited data quent displacement of Fe, leading to increasing Mn/Fe available for Mn partitioning in silicates (e.g., [15]) ratios. This near-vertical trajectory continues until the show no correlation with melt structure. Ti-oxides stop crystallizing. Next is a segment (Fe# 0.025 from ~0.5-0.9) with a shallower slope. This is after Mare basalt pyroxene plagioclase crystallizes. The inflection before (at lower 0.020 Fe#) the flattened part of the trajectory occurs when plagioclase comes onto the liquidus and competes with 0.015 pyroxene for Al, causing sharp drops in both Ti and Al 0.010 contents in pyroxene. The last upturn involves a se- High-Ti Low-Ti cond increase of Ti activity in the melt, which enters Mn cations per 6Ox A-17 VLT the late-stage pyroxene and again displaces Fe, causing 0.005 L-24 VLT an increased Mn/Fe ratio and resulting in the crystalli- a 0.000 zation of late-stage ilmenite and Ti-rich spinel. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 As observed for the high-Ti pyroxene, a sinusoidal Fe/(Fe+Mg) 0.025 pattern with three main segments is observed for low- Ti basalt pyroxene on a plot of Mn cations against Fe# 0.020 HT (Fig. 1a,b). The first segment reflects co-crystallization of olivine and pyroxene, with pyroxene incorporating 0.015 Ti and Al in a ratio of ~¼ and Mn increasing sharply A-17 VLT over a narrow range of Fe#. The second is a flattening L-24 VLT 0.010 when plagioclase comes onto the liquidus. The con- LT tents of Al and Ti cations are not as strongly correlated Mncations per 6Ox High-Ti 0.005 Low-Ti in the low-Ti basalts as they are in the high-Ti basalts A-17 VLT because, with the lower initial Ti/Al in the parental L-24 VLT b 0.000 melts of the former, two substitutional couples are in- 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 4+ 3+ Fe/(Fe+Mg) volved. The first is, as above, Ti (M1)- 2Al (Tet site, 0.025 tetrahedral). The second is Al3+(M1)- Al3+(Tet-site). Sinusoidal crystallization trajectories are also ob- 0.020 HT served for both A-17 VLT and Luna 24 VLT pyrox- enes (Fig. 1a,b). A rise in the activity of Ti in late- 0.015 stage liquids due to fractional crystallization of plagio- A-17 VLT clase and Ti-poor pyroxene causes the upswing of all L-24 VLT 0.010 LT trajectories and leads to the crystallization of Ti-rich Mn cations per 6Ox High-Ti spinel and ilmenite, which preferentially take up Fe 0.005 Low-Ti relative to Mn. The HT and L-24 trends are the most A-17 VLT L-24 VLT c strongly sinusoidal, with the other suites having flatter 0.000 trends. The linear fits are illustrated in Fig. 1c. The HT 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Figure 4 Fe/(Fe+Mg) basalts have a relatively steep slope (0.022) and the other suites have more typical lunar slopes of ~0.013. References: [1] Papike J. et al. (2003) Am. Min., These data imply that the activity of Ti in the bas- 88, 469. [2] Papike J. et al. (2017) Am. Min.,102, 1759. alts, their crystallization sequences, and the nature of [3] Mittlefehldt D. et al. (1998) Revs. Mineral. 36, 4-1. the M1 site in pyroxene result in minor variations in [4] Jolliff B.
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