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O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System WHITLOCKITE SATURATION

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O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System WHITLOCKITE SATURATION WHITLOCKITE SATURATION IN LUNAR BASALTS, J.E. Dickinson and P.C. Hess, Dept. of Geological Sciences, Brown University, Providence, RI 02912 As part of a continuing program initiated to evaluate the evolution of late-stage lunar magmas and the possible effects of crystallization of minor phases on their geochemistry, we have determined the saturation surface for whitlockite, Ca (PO ), in a liquid composition produced by crystallization of KREEP basalt 15386 lf~able1). Whitlockite is by far the most abundant phos- phate mineral in lunar rocks and in all probability is more abundant than other minor phases such as zircon, zirccnolite, monazite, and tranquillityite (1). Whitlockite is also an important component of mesosiderites, reported average modal phosphate abundances in mesosiderites ranging from 0.0-3.7% (avg. 1.9%) (2). Even higher phosphate abundances (up to 9.4%) have been observed in basaltic lithic clasts of presumed impact melt origin found in mesosiderites (3). Analyses of lunar whitlockites commonly show Ce203and Y203 contents up to 3 wt% and REE contents that may be as high as 1-2 wt% for La203and Nd203 de- creasing to levels of -0.1-0.2 wt% for the HREE (1). It has been shown that whitlockite is generally more enriched in REE's relative to chondrites than other minor phases in the same rock (4). Uranium and thorium may also be present in significant amounts although other minerals may contain more. Whit- lockite may, therefore, be an important reservoir of rare earth and heat producing elements. In addition, the amount of FeO and MgO in whitlockite is variable (Table 1). An understanding of this variation may give some insight into the major element chemistry of evolved magmas. All experiments were performed in Mo or Fe-foil capsules sealed inside evacuated silica glass tubes and were quenched by dropping the tubes into water. Run times varied from 18-99 hrs. depending on the temperature of the experiment. P205 was added to the initial bulk composition as either a solu- tion of H3P04 which was then evaporated to dryness or as reagent grade CaHP04. Fig. 1 shows the results of the whitlockite saturation experiments. Approximately 4.54 wt% P205 is required for whitlockite saturation at 1200°~, decreasing as a linear function of temperature to 2.44 wt% P205 at 1047OC. Previous work (5) has demonstrated that one of the most important variables controlling the saturation of another phosphate phase, apatite, in basic to intermediate magmas is the SiO, content of the magma. To test this relation- ship for whitlockite we have plotted in Fig. 2 analyses of whitlockite satur- ated and unsaturated experiments. Although these results are from experiments at temperatures ranging from 1220~~-1047~~,it is obvious that there is a strong dependence of the saturation value on Si02 concentration. We would also point out that whitlockite saturation is not only dependent on wt% SiO,, P205 and temperature, but also is a function of CaO content. This conclusion results from analysis of experiments in which P205 was added as H3P04 instead of CaHP04. At 1200~~~adding P205 as H3P04 to the initial composition re- sulted in the development of liquld immiscibility but no whitlockite satura- tion even though the Hi-Fe (-41 wt% Si0,) and Hi-Si (-63 wt% SiO,) liquids contained -13 and 3 wt% P205, respectively. Figs 1 and 2 show this to be in excess of the P,O,concentrations required for saturation. However, when the same percentage of P205 was added as CaHP04, the result was a homogeneous, whitlockite saturated melt. Compositions of whitlockite from experiments at 1200~~~1107O~, and 1047'~ are shown in columns 6-8 of Table 1. Comparison with the natural whit- lockites in Table 1 shows a general similarity between the experimentally grown crystals and the natural ones, although in the experimental samples MgO O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System WHITLOCKITE SATURATION Dickinson, J.E. and Hess, P.C. and FeO values are near the upper values observed for natural whitlockites. The only major difference in composition between the experimental and natural whitlockites is in Ti0,- the experimental whitlockites containing signifi- cantly more Ti02. P205 contents of most lunar basalts range from 0.03% to 0.18% P20, (6). KREEP basalts such as 14310(6.34% P205), 15386(0.86% P 0 ) and 15382(0.5% P205) have higher concentrations of phosphorus. Comparison2.5 with Fig. 1 shows that none of these compositions will have whitlockite as a liquidus phase. Even the black breccia groundmass of 12013 with 1.2 wt% P205 (7) has only 30% of the P205 concentration required for saturation at its liquidus tem- perature (11400C). On the other hand, the felsite in 12013 (P205= 0.13 wt%) (7) with a liquidus temperature of 1080~~may be much closer to whitlockite saturation. Our data do not extend to the high Si02 values of the felsite (-73 wt%), but if we consider the results for apatite saturation in terres- trial granites (8) (-73-78 wt% Si02; T = 750-900°C) to be applicable, then the whitlockite liquidus will be reached after only 7% crystallization. This is an interesting result in that the felsite has a peculiar V-shaped REE pattern (7) that may be an indication of whitlockite (or apatite) fractiona- tion or whitlockite plus some other minor phase. Experimental work is con- tinuing on this problem. The high modal phosphate mineral contents of mesosiderites in comparison to lunar basalts (above) are very intriguing, but our data do not allow us to discriminate between an igneous origin or an origin by redox reactions between metal and silicates. We can say, however, that those with modal phosphate contents above "3% could crystallize whitlockite at reasonable igneous temperatures (>lOOO°C) assuming Si02 contents of 55-60%. References: (1) Frondel, J.W. (1975) Lunar Mineralogy. (2) Prinz --et.al. 1980) PLSC 11, 1055. (3) Nehru --et.al. (1980) LPS XI, 803. (4) Lovering and Wark (1974) LPS V, 463. (5) Watson (1979) Geophys. --Res. Lett. v. 6, no. 12, 937. (6) Taylor (1975) Lunar Science: --A Post Apollo View. (7) Quick s.-a1 . (1977) PLSC 8, 2153. (8) Watson and Capobianco (1981) -MS. TABLE 1 15386 whit.* whit.* vhit.* whit.* exp. exp. start mtl. 12013-10 14310/208 1203619 15475,125 1200~~1:i;6C 1047'~ sio 54.4 ~10~3.8 A1 a3 11.4 ~e8 12.5 MgO 2.9 CaO 9.1 ?a0 1.0 P$~ 1.6 *Analyses do not include reported Y203, Ce203 and REE Fig. 2 O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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