Recent Results of Morse Research

Experimental Studies of the Kiglapait Intrusion

This work is evidently the first systematic, experimental exploration of the liquid line of descent of any layered intrusion at pressure. Samples contained in graphite capsules were melted and crystallized in piston-cylinder apparatus at 5 kb pressure. Liquidus results were reproduced within ± 5-10C. Starting materials were made up from Kiglapait rocks and separated minerals. Run products were analyzed by electron probe micro-analysis. During this study we have located the liquid line of descent (LLD; Fig. 1A,) for the Lower Zone (LZ) of the Kiglapait Intrusion (Sporleder, MS 1998), and roughed out the trend for the Upper Zone (UZ; Peterson, in progress).

We have grown big euhedral crystals of plagioclase and olivine of uniform composition (Fig 2). We achieved the desired crystal compositions (defined by the stratigraphy of the intrusion) by adding Fa and Ab to the chilled margin composition. We also synthesized new liquid compositions by trial and error from analyzed rocks and separated minerals, to achieve the stratigraphically relevant crystal compositions. By this approach we avoided irrelevant compositions previously studied by others. In particular, we avoid extensive subliquidus exploration of individual compositions; rather, we examine the liquidi of successive melts.

Some highlights of the results are as follows. Liquid composition: The liquid line of descent (LLD; Fig. 1A) of the Lower Zone, crystallizing only OL + PL, closely shadows that found from modal analysis (Fig. 1B); note that oxygen normative units are intrinsically more mafic than modal units. The LLD runs from near the PL-OL sideline of the PL-OL-AUG ternary, to saturation with AUG at temperatures from 1245C to 1180C. The temperatures are strongly correlated with plagioclase composition. The LLD is concave toward plagioclase. Its large compositional range and small temperature range imply that troctolitic liquids have a large field of existence for these high-Al compositions at 5 kbar. The lever rule indicates that AUG arrives after about 80% fractional crystallization, closely emulating the volume estimates based on field relations (Morse, 1969; 1979a). The inferred equilibrium saturation with AUG was previously interpreted (Morse, 1979b Fig. 12) to occur near 81 percent solidified (PCS) at a ratio of 38% OL to 62% AUG (Fig. 1b). The experimental result, Fig. 1a, is indistinguishable from the modal result in this respect. The experimental liquidus temperature for the initial magma, 1245C @ 5 kb, may be compared with the earlier estimate of 1250C @ 4 kb (Morse, 1979a).

Element Partitioning. Growth of big crystals has permitted valuable element partitioning studies, using SIMS with the Cameca IMS 3f Ion Microprobe at Woods Hole Oceanographic Institution (WHOI). Partitioning of Sr between plagioclase and melt gave D = 1.63 for plagioclase An68 and 1.31 for An56, compared to 2.17 and 1.95, respectively, obtained on an unmodified Kiglapait marginal sample at 1 atm by Blundy (1997).

Partitioning of other elements includes K in plagioclase: D = 0.39 for An68 and 0.85 for An56, going a long way toward explaining the slow evolution of K in the intrusion. The mean Dfor FeO in plagioclase is 0.042 (4 runs), consistent with literature results.

The phase relations of the experimental LZ-LLD are at odds with predictions from the MELTS algorithm of Ghiorso and Sack (1995). Whereas the assemblage OL + PL is widely observed at 5kb, the MELTS routine predicts pyroxenes (Fig. 3 in the body of the proposal). Calculations of MELTS at lower pressures is required to achieve the observed OL+PL assemblage, an indication that the pressure effect may be incompletely incorporated into the MELTS routine.

Theoretical

This work has been guided by the new theory of linear partitioning in binary solutions(Morse, 1996; 1997; 1998) illustrated in Fig. 4, beyond. This theory has wide application to distillation, melting, and solid state equilibria, with useful applications to the Earth's crust and mantle. It has been successfully applied to show that the lid of the Earth's transition zone can be as thin as 4 km as inferred from phase equilibria as well as from seismic underside reflections (Morse, 1998).

Multiphase Rayleigh fractionation is another topic under development in parallel with linear partitioning. It was used successfully to show that the olivine composition variation in the Lower Zone of the Kiglapait Intrusion is precisely that predicted by theory, and not the result of magma recharge or boundary layer crystallization (Morse, 1996). Extension of the theory is in progress (Morse, 1997 AGU Abs.)

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