Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.538l&link_type=abstract
Meteoritics, vol. 30, no. 5, page 538
Computer Science
Angrite, Fassaite, Fractional Crystallization, Meteorites, Allende, Lewis Cliff 86010, Lewis Cliff 87051, Partial Melting, Volatile Depletion
Scientific paper
Experiments [1] have shown that partial melting of devolatilized carbonaceous chondrite source regions can produce eucrite-like liquids at low-fO2 (IW-1) and low-SiO2, angrite-like liquids at higher fO2 (IW+2). The reason for this different behavior is reduction of ferrous iron at the lower fO2, which changes the normative character of the silicate source. Although the low fO2 experiments produced liquids closely resembling known eucrites, the match between liquids from the "high" fO2 runs and known angrites was not as close and aluminous augite (fassaite), a key mineral in angrites, was observed only at the solidus. Figure 1 is a depiction of the olivine liquidus surface projected from the olivine component with boundary curves drawn to agree with the liquid data (open squares) of [1]. The diopside solid solution field is taken from the data of [2,3] in the CMAS system as an approximation. These latter data show a dramatic increase in A1203 concentrations in coexisting diopside as SiO2 in the liquids decrease (~3 % A12O3 in di at the thermal divide M3 increasing to 10.5 % (=fassaite) near the liq+ol+di+an+sp invanant point). Tielines between diopside (augite) and anorthite divide the diagram into a number of potential partial melting fields (I - IV). Bulk compositions in field I will begin melting at the liq+ol+di+an+opx invanant point; those in field IV begin melting at the liq+ol+di+an+sp point; and those in fields II and III begin melting along the liq+ol+di+an boundary curve. These latter melts will have crystallization paths in opposite directions as determined by the M3 thermal divide. A vector, cutting across the melting fields, shows the direction and magnitude of Fe-loss from an Allende-like source. The heavy shaded curve A shows the "high" fO2 Allende/Murchison melting path from [1], which passes to the low-silica side of the Lewis Cliff angrites [4]. A second shaded curve (B) shows the melting path for an Allende-like source that has lost ~15% of its FeO. Curve C shows the fractional crystallization path of a partial melt from an Allende-like source with <= 10% FeO loss. Clearly, liquids along this path that would closely match the compositions of the Lewis Cliff angrites (+/-ol ) and would crystallize fassaite after ol + sp (optional) and ol + an. References: [1] Jurewicz A. J. G. et al. (1993) GCA, 57, 2123-2139. [2] Longhi J. (1987) Am. J. Sci., 287, 265-331. [3] Libourel G. et al. (1989) Contrib. Mineral. Petrol., 102, 406-421. [4] Mittelfehdlt D. W. and Lindstrom M. M. (1990) GCA, 54, 3209-3218.
Houston Jones Jane
Jurewicz Amy J. G.
Longhi John
Mittlefehldt Dave W.
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