Ck. Shearer et al., THE ROLE OF ILMENITE IN THE SOURCE REGION FOR MARE BASALTS - EVIDENCEFROM NIOBIUM, ZIRCONIUM, AND CERIUM IN PICRITIC GLASSES, Geochimica et cosmochimica acta, 60(18), 1996, pp. 3521-3530
To investigate models for the generation of lunar high Ti-basalts, we
have analyzed lunar picritic glasses for Zr, Nb, Ce, and Ti at high pr
ecision using ion microprobe techniques. The picritic magmas represent
ed by these glasses have experienced minor crystallization, which has
allowed us to partially eliminate the effects of post-melting processe
s commonly experienced by crystalline high-Ti mare basalts. The Nb/Zr
for these glasses ranges from .05-.11. The high-Ti glasses generally t
end to have higher values of Nb/Zr (.072-.109) than the very low-Ti gl
asses (.048-.085). The crystalline mare basalts tend to have slightly
higher Nb/Zr than glasses with similar Ti from the same site. For exam
ple, the Apollo 17 (A17) high-Ti basalts have Nb/Zr of approximately .
09. whereas, the A17 high-Ti glasses have Nb/Zr of .07. KREEP has Nb/Z
r of approximately .06. Thus, Zr is fractionated from Nb to different
degrees in the various picritic magmas. The concentrations of Zr, Nb,
and Ce increase from the very low-Ti glasses to the high-Ti glasses, a
nd along that trajectory the Nb/Ce and Zr/Ce increase. Nb/Ce (.25-1.6)
and Zr/Ce (4-15) for the picritic glasses overlap with KREEP (Nb/Ce =
.36 and Zr/Ce = 5). Zr/Ti and Nb/Ti show a wide range of variation in
these glasses. Both Zr/Ti and Nb/Ti in the glasses range from approxi
mately .0014 to slightly less than .0003. The Zr/Ti and Nb/Ti for thes
e glasses overlap with that of the crystalline mare basalts. Generally
, with increasing Ti, Nb, and Zr, Zr/Ti and Nb/Ti decrease. The except
ions to this are the Apollo 14 (A14) glasses that exhibit an increase
in Nb/Ti and Zr/Ti. Based on these data for the picritic glasses and e
xperimentally determined partition coefficients for Nb, Zr, and Ce, th
e mantle sources for these picritic magmas are slightly to moderately
fractionated from Cl chondrite and previous estimates of the bulk sili
cate Moon. Our best fit model for our data and this observation is tha
t both the very low-Ti and high-Ti picritic magmas were derived throug
h small to moderate degrees of nonmodal melting of lunar mantle source
s consisting of a mixture of late-stage LMO cumulates (derived after >
95% crystallization of the LMO) and early to intermediate LMO cumulate
s (derived prior to 80% crystallization of the LMO). The early LMO cum
ulates had Nb/Zr, Zr/Ce, and Nb/Ce ratios near Cl chondrite, whereas t
hese ratios were fractionated in the late-stage LMO cumulates. This hy
bridization of mantle sources occurred during large scale overturning
of the LMO cumulate pile. The source for the low-Ti picritic magmas ha
d very minor amounts of ilmenite, whereas the source for the high-Ti p
icritic magmas probably contained less than 6% ilmenite. For all the p
icritic magmas, ilmenite was exhausted from the residua during melting
. Models suggesting that the high-Ti magmas are derived through the as
similation of an ilmenite-bearing cumulate layer or preferential assim
ilation of ilmenite by low-Ti primary magmas are not consistent with t
he Nb, Zr, Ce, and Ti data magmas (Hubbard and Minear, 1975; Wagner an
d Grove, 1993, 1995). In particular, the preferential assimilation by
very low-Ti picritic magmas of ilmenite with expected Nb/Ce (20,000-22
,000) and Nb/Zr (55) signatures would displace the resulting high-Ti m
agma too far from our observed data. Large scale overturning of the LM
O cumulate pile also accounts for the trace element signatures found i
n the A14 picritic glasses. The evolved signature found in these primi
tive very low-Ti picritic glasses is most likely a product of KREEP in
corporation into the LMO cumulate source rather than either contaminat
ion by evolved ilmenite-bearing cumulates or incorporation of higher p
roportions of locally derived intercumulus melt.