Other
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p53a..07h&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P53A-07
Other
1025 Composition Of The Mantle, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1065 Trace Elements (3670), 3662 Meteorites, 3672 Planetary Mineralogy And Petrology (5410)
Scientific paper
Martian basalts (shergottite meteorites) display very large variations in their incompatible element systematics, as evidenced by their highly fractionated and variable whole-rock REE patterns. Some samples show a marked depletion in LREE, which is apparent in CI-normalized bulk rock La/Sm ratios: some samples, such as Shergotty and Zagami, have La/Sm~0.9, i.e., relatively flat REE patterns; others, such as QUE 94201, have La/Sm~0.1, reflecting a depletion in LREE relative to the M-HREE. Furthermore, Sm-Nd isotopes in these samples indicate that their source regions are more enriched in LREE relative to M-HREE than the basalts themselves: the 147Sm/144Nd ratios estimated for martian basalt sources (from initial ɛ -Nd143) are 50 to 80% less than the whole-rock 147Sm/144Nd ratios of the basalts (Borg et al. 2001, 2003). This suggests that fractionation of LREE from M-HREE was a relatively recent phenomenon and occurred either in the source region, during magma generation, or during subsequent magma differentiation. This fractionation could reflect the involvement of a LREE-enriched phase in the petrogenesis of the martian basalts. One possibility is that this phase remained in the restite during partial melting, so that the melting process preferentially excluded LREE (Borg et al. 2003). This is supported by modeling of REE partitioning between liquid (=parental melt) and orthopyroxene, olivine and majoritic garnet (Borg and Draper 2003; Draper et al. 2003), which is unable to reproduce the LREE depletions. The phase (or process) responsible for the LREE depletion in the basalts must be able to strongly fractionate La-Nd from Sm-Lu.
Monazite is a LREE-phosphate, generally: (La, Ce, Nd, Th)PO4; it is poor in HREE relative to LREE (Spear and Pyle 2002). Although a common accessory mineral in granites and metapelites (Rapp et al. 1987; Spear and Pyle 2002), to our knowledge, none has previously been reported in basalts.
The Bastar craton of India is transected by multiple generations of mafic dyke swarms, and the emplacement age of one major swarm of tholeiite dykes has now been well established at ~1.9 Ga (French et al. 2004). In searching for minerals suitable for chemical and isotopic dating, a number of late-crystallizing accessory phases were found in these dykes including zircon, baddeleyite, zirconolite, allanite, and monazite. We interpret the monazite to be a primary magmatic phase on the basis of its euhedral morphology, and its occurrence as an inclusion in undoubtedly magmatic clinopyroxene. Furthermore, its Th-U-Pb chemical age is within error of emplacement ages determined for this dyke swarm, indicating that the monazite is not xenocrystic in origin. LREE-oxide concentrations in the monazite are up to 65 wt%, corresponding to ~105 to 106X CI for La-Nd and ~104X CI for Sm-Gd. Nd is fractionated from Sm by a factor of 2; La/Sm~11 and La/Gd~55. As these enrichments are crystal-chemical in nature (cf. whole-rock REE ~100X CI; La/Sm~2; Srivastava and Singh 2003), the monazite from the Bastar dykes represents a phase capable of fractionating LREE from M-HREE in mafic systems.
Recognizing that studies of the stability of monazite in mafic systems at mantle pressures are required to test this hypothesis, we speculate that accessory monazite represents a possible candidate for the fractionation of LREE from M-HREE in the martian basalts.
Borg Lars E.
Chacko Thomas
French J. E.
Heaman Larry M.
Herd C. D.
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