Chromium, Nickel, and Titanium Abundances in 74275 Olivines: More Evidence for a High-Pressure Origin of High-Titanium Mare Basalts

EVIDENCE FOR A HIGH-PRESSURE ORIGIN OF HIGH-TITANIUM MARE BASALTS; J. W. Delano and D. H. Lindsley, Dept. of Earth and Space Sciences, State Univ. of New York, Stony Brook, N. Y. 11794 Introduction The depth of origin for the fine-grained, type C [l], mare basalt, 74275, has been a subject of controversy since 1976. Green, Ringwood and coworkers [2] experimentally determined the liquidus phase relations of 74275. Since it is a fine-grained basalt, they [21 had assumed that it represented the composition of a liquid. They reported a multiple saturation point involving olivine + 2 pyroxenes at a pressure of 12 to 13 kilobars and concluded that 74275 had been produced by partial melting deep (% 250 Ian) within the Moon. However, Walker -et al. [3] pointed out a potentially serious weakness in that hypothesis. Specifically, the bulk composition of 74275 was not that of a liquid only because the rock contained a recrystallized dunite fragment [4] and euhedral olivine "megacrysts" that were too magnesian to have crystallized from that composition. Walker et al. [3] concluded that these olivines in 74275 -were exotic fragments that artifically biased the experimental results [21 toward high pressures. We present analyses of these olivines for Cr, Ni, and Ti in order to test the hypothesis [31 of an exotic origin. RESULTS (summary o f data i n Table 1 ) As shown in Fig. 1, the recrystallized dunite contains up to 130 ppm Ni, whereas the olivine megacrysts have $ 30 ppm Ni (detection limit = 20 ppm). Since basalt 74275 contains < 3 ppm Ni [51 and the olivine/liquid partition coefficient for Ni is about 10 [e.g., 61, any olivines that crystallized from 74275 'should have Ni abundances 5 30 ppm. These data indicate that the dunite is a xenolith, but that the megacrysts may be related to 74275, or a slightly more primitive liquid. We found, in agreement with Butler [71 , that the olivine/liquid partition coefficient for Ti is about 0.011. Since the core of the dunite clast contains only 500 ppm Ti (~ig. 21, it equilibrated with a liquid containing about 7.5 w t . % Ti02. In contrast, 74275 has 12.5 w t . % Ti02 [5,81. Once again, we conclude that the dunite fragment is a xenolith. The olivine megacrysts, however, contain 800 ppm Ti indicating that they crystallized from a liquid with about 12 wt.% Ti02 (i.e., identical, within uncertainty, to 74275). Therefore, the megacrysts may be related to 74275, or a slightly more primitive liquid. Figure 3 shows that the core of the dunite contains % 1800 ppm Cr, whereas the megacrysts have % 2500 ppm. If the megacrysts precipitated from a close relative of 74275, as suggested by the Ni and Ti data, then the olivine/ liquid partition coefficient for Cr was about 0.6. This needs to be tested. CONCLUSIONS (A) In agreement with other investigators [3,41, these data demonstrate that the dunfte in 74275 is a xenolith. (B) The euhedral olivine megacrysts crystallized from a high-Cr, low-Ni, and high-Ti mare liquid that, we believe, was a slightly less differentiated relative of 74275. That liquid would have had a Mg# of 52.8 (for KD = 0.2681, rather than the value of 50.6 [5,81 in 74275. The major-element composition can be estimated as the following: Si02 = 38.5%; Ti02 = 12.2; ~1203 = 8.35; *~r203 = 0.59; FeO = 18.2; MgO = 11.5; CaO = 10.1. Using the empirical relation for estimating the pressure of multiple saturation [31 in mare basalts, a value of about 12 kbars is obtained. This is identical with the value reported by Green -et al. [21. Consequently, the issue raised by Walker et al. -[31 was important and valid but our data show that the high-pressure origin advocated by [21 remains intact.