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A Modified Cipw Norm Calculation for Lunar Mare Basalts

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A Modified Cipw Norm Calculation for Lunar Mare Basalts Lunar and Planetary Science XXXI 1427.pdf A MODIFIED CIPW NORM CALCULATION FOR LUNAR MARE BASALTS. R. E. Milliken and A. Basu, Department of Geological Sciences, Indiana University, Bloomington, IN 47405 ([email protected]) Introduction: CIPW norm calculations of lunar amount of FeO remaining after the formation of il- mare basalts yield normative compositions that differ menite. considerably from modal mineralogy [1]. This anom- Adding reserved amounts of these elements to FeO aly arises because oxygen fugacity of such magmas is effectively changes the Fe/Mg ratio of the system, al- low, near the iron-wustite buffer. This causes transi- lowing normative pyroxenes to form in the forbidden tion elements to prefer reduced valence states and pro- field. This increases the normative amount of pyrox- hibits Fe3+ bearing oxides from forming [2]. Fe2+ goes ene produced while also having the effect of reducing into pyroxenes which accept greater amounts of Al3+, normative ilmenite and plagioclase. The above men- Ti3+, and Cr2+ than those of terrestrial basalts, due tioned percentages were chosen on a trial and error partially to their reduced valence states. Lunar basis, observing all mineralogical constraints and pyroxenes also accept large amounts of Ca and Fe minimizing the differences between modal and revised simultaneously, crystallizing in the Ca-Fe rich forbid- normative mineralogy. We have used chemical and den field. In such pyroxenes, the M2 sites accommo- modal compositions from the compilation by Papike date Ca, Na, Mn, and Fe2+, whereas the octahedral M1 [4] for our revised calculations. The results are given sites may contain Mn, Fe2+, Mg2+, Fe3+, Cr3+, Cr2+, in the table below. 4+ 3+ 3+ Ti , Ti , and Al [2]. Some lunar pyroxenes are Conclusions: Allocating pre-selected amounts of reported to contain 6% TiO2, 7% Al2O3, and 1.5% CaO, TiO2, and Al2O3, to go into normative pyroxene Cr2O3. For lunar basalt compositions, standard CIPW composition (e.g., STEPS 2,3) produces closer calculations produce normative mineralogy low in matches between modal and normative mineralogy of pyroxene and high in ilmenite if compared to their lunar mare basalts. Allocations mentioned above work modal compositions. Our goal is to achieve a closer best for most Apollo 11 high-Ti basalts. Different match between modal and normative mineralogy by allocations produce closer matches for low-Ti or VLT modifying the standard procedure of the CIPW norm basalts but will produce greater discrepancies for other calculation (e.g., Malisetty [3]). Our revision allows basalts. It should also be noted that the CIPW norm Ca + Fe, Ti, Al, and Cr to go into pyroxene, effec- tests for silica saturation and is only able to produce tively modifying the chemical compositions of norma- either normative olivine or quartz. Normative miner- tive di' and hy', a condition not permitted in the stan- alogy will never show olivine and quartz co-existing, dard CIPW calculation. although the modal mineralogy may, in fact, show Methodology: With these conditions in mind, we quartz in the mesostasis of an olivine basalt. apply the following revisions: References: [1] Riegsecker, S. and Basu, A. STEP 1: All S present forms troilite (FeS). (1999) LPSC XXX Abstract #1759. [2] Papike, J. J. STEP 2: Reserve 20% of bulk CaO and 10% of bulk (1980) in Reviews in Mineralogy, 7, Pyroxenes, TiO2 for later use in the formation of pyroxene. Charles T. Prewitt, ed. [3] Malisetty, M. R. et al. STEP 3: Total amounts of bulk Cr203 and MnO and (1992) J. Geo. Edu. 40, 237-240. [4] Papike, J. J. et 5% of bulk Al2O3 are added to the total amount of al. (1976) Rev. Geoph. Sp. Phys. 14:4, 475-540. FeO remaining after STEP 1, assuming that these Acknowledgement: Dr. G.D. Garlick kindly sup- elements may go into ilmenite in the unlikely case plied CIPW norm calculation on EXCEL. This re- where Ti overwhelms Fe. search is supported in part by NASA Grant NAG5- STEP 4: Calculate normative apatite, orthoclase, al- 4018. bite, and anorthite in standard fashion. STEP 5: Before calculating normative diopside, the reserved amounts of TiO2 and CaO are added to the Lunar and Planetary Science XXXI 1427.pdf MODIFICATION OF CIPW NORM CALCULATION: R. E. Milliken and A. Basu Table. Comparison of modal compositions, standard CIPW norms, and revised norms. Sample # ol pl px il tr qz ksp ap oth 10003 Modal 0 29 51.7 18.2 0 0.3 0 0 0.2 CIPW 0 29.9 48 19.9 0 0.8 0.4 0.3 0 This 1.5 28.4 50.2 17.9 0.6 0 0.4 0.3 0 10050 Modal 1.2 29.5 50.2 14.8 0 2.8 0 0 1.5 CIPW 0 30.9 48.8 18.3 0 0.4 0.4 0.2 0 This 2.7 29.4 49.5 16.4 0.4 0 0.4 0.2 0 10057 Modal 0 19.2 50.9 15.7 0 0.1 0 0 3.3 CIPW 0 30.8 45.5 21.7 0 0.05 1.9 0.4 0 This 3.9 29.3 44.7 19.6 0.7 0 1.9 0.4 0 10017 Modal 0 21.5 51 20.2 0 1.1 0 0 6.1 CIPW 0 22.9 50.1 22.4 0 2.1 1.7 0.4 0 This 0 21.9 54.3 20.1 0.7 0.6 1.7 0.4 0 75055 Modal 0 29.1 51.4 13.4 0 3 0 0 3 CIPW 0 28.1 49.4 19.3 0 2 0.5 0.2 0 This 0 26.8 53.4 17.4 0.6 0.7 0.5 0.2 0 70215 Modal 7 29 42 18 0 4 0 0 0 CIPW 0.6 25.4 48.1 24.6 0 0 0.3 0.2 0 This 5.5 24.2 46.3 22.2 0.6 0 0.3 0.2 0 71055 Modal 1.8 20.8 50.9 25.1 0 1.4 0 0 0 CIPW 0 24.6 49 25.5 0 0.3 0.4 0.2 0 This 4.4 23.4 48.2 22.9 0.6 0 0.4 0.2 0 74275 Modal 10.1 13.7 45.7 30.4 0 0 0 0 0.1 CIPW 2.3 24.5 47.8 24 0 0 0.4 0.2 0 This 7.5 23.3 46 21.6 0.5 0 0.4 0.2 0 12009 Modal 21.7 0 9.9 0.6 0 0 0 0 67.8 CIPW 11.9 24.2 57 5.5 0 0 0.4 0.2 0 This 14 23 57.1 5 0.2 0 0.4 0.2 0 12040 Modal 29.3 19.8 46 4.3 0 0 0 0 0.6 CIPW 28.8 20.4 45.5 4.7 0 0 0.3 0.1 0 This 30.4 19.4 45.4 4.2 0.1 0 0.3 0.1 0 12021 Modal 0 30.7 62.6 5.6 0 0.3 0 0 0.8 CIPW 0 30.4 60.2 6.7 0 1.8 0.4 0.2 0 This 0 29 62.8 6 0.2 1.2 0.4 0.2 0 12022 Modal 16.5 12 58.6 11.2 0 0 0 0 0 CIPW 16.4 26.1 47.4 9.2 0 0 0.4 0.3 0 This 18.8 24.8 47 8.3 0.3 0 0.4 0.3 0 12064 Modal 0 33.1 57.2 6.7 0 2.3 0 0 0.7 CIPW 0 30.2 59.3 7.6 0 2.2 0.4 0.1 0 This 0 28.8 62 6.8 0.2 1.6 0.4 0.1 0 15555 Modal 5 25 65 3.5 0 0.3 0 0 0.4 CIPW 18.5 24.7 51.8 4 0 0 0.2 0.1 0 This 19.8 23.5 52.1 3.6 0.2 0 0.2 0.1 0 15499 Modal 0.8 0 41.8 0 0 0 0 0 57.3 CIPW 0 26.4 69 3.4 0 0.1 0.4 0.2 0 This 0.8 25.1 69.7 3.1 0.2 0 0.4 0.2 0 15076 Modal 0 28.5 66.3 2.4 0 2.1 0 0 0.6 CIPW 0 25.6 68.3 3.6 0 1.5 0.4 0.2 0 This 0 24.4 69.9 3.3 0.3 1.2 0.4 0.2 0 .
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