Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.387l&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 387
Computer Science
Cao Partitioning Model, Olivine, Chondrules, Cao Content, Partition Coefficient, Cao
Scientific paper
Minor-element contents of olivine are used to make inferences about petrogenetic history. For example, anomalously high CaO contents (>0.5 wt%) are attributed to rapid crystal growth, while low CaO contents (<0.1 wt%) are attributed to the effect of metamorphism or high pressure. In complicated chondrule systems, we often do not consider that the CaO contents of olivine are a function of the partition coefficient, ^s/l^D(sub)CaO, between olivine and liquid. Accordingly, before being considered anomalous, ideal CaO contents in olivine should be calculated from a ^s/l^D(sub)CaO taken from a model. Unfortunately, models for CaO partitioning currently in the literature are largely empirical and derived primarily using terrestrial basalts [1-3]. Our results show that these models vary in how well they can be extended to the more magnesian systems encountered in chondrules. The model of [3] appears to describe the data best, while those of [1,2] have considerably more scatter. We present a new model easier to use than that of [3]; this model is perhaps not quite as accurate, but it underscores that ^s/l^D(sub)CaO is a function of melt structure as well as liquid composition. The dataset we used to derive our model and to compare with [1-3] was compiled from both equilibrium experiments and dynamic crystallization experiments for which it was demonstrated that there was no effect on CaO partitioning with cooling rate [2, 4-9]. Our new model uses the "corrected" calcium partition coefficient of Jurewicz and Watson [2], Kd(sub)90, and the "normative" plagioclase and olivine contents of the coexisting melts, as calculated for an Ol-Si-Pl projection from Di [10]. The empirical parameterization, Kd(sub)90/(PL/OL), was chosen because (1) the Kd(sub)90 is an empirical representation of the effect of relative forsterite/fayalite activity on calcium partitioning and (2) the normative components of the melt generally reflect the melt structure, with the Pl component especially relevant to the complexation of Ca with Al in the melt. When plotted against 1/T, this new parameter describes a variety of iron-free, chondritic, and basaltic compositions. A similar plot of -ln(calculated ^s/l^D(sub)CaO) vs. 1/T using our data in the model of [3] also describes the data well. We note that the arguments of [3] are thermodynamic, whereas our approach concentrates on melt structure, especially the complexation of calcium with plagioclase structures in the melt. Since we can now reasonably estimate CaO partitioning in magnesian systems at least two ways, the results of this study constrain the interpretation of the CaO contents of olivine in chondrules. Both models and experiments indicate that the ^s/l^D(sub)CaO is lower in the high-temperature, high-magnesian, iron-containing melts than in typical basaltic melts. Thus, low CaO contents in olivine may be equilibrium values and should not automatically be interpreted as a metamorphic or condensation effect. Moreover, for bulk melts with less than 8 wt% CaO, rapid growth (cooling >100 degrees C/hr) does not result in higher CaO contents [6,7]. Therefore, high CaO content is not necessarily due to rapid growth, but more likely controlled by bulk composition (greater than 8 wt% CaO in the melt) combined with rapid growth. It is important for interpretation of olivine composition to determine the composition of the liquid from which the olivine crystallized. References: [1] Ford et al. (1983) J. Petrol., 24, 256-265. [2] Jurewicz and Watson (1988) Contrib. Mineral. Petrol., 99, 176-185. [3] Snyder and Carmichael (1992) GCA, 56, 303-318. [4] Watson (1979) Am. Mineral., 64, 824- 829. [5] Lofgren and Lanier (1990) GCA, 54, 3537-3551. [6] Lofgren and DeHart (1991) LPS. XXII, 823-824. [7] Lofgren et al. (1991) Geol. Soc. Am. Abstr. with Progr., 23, A271. [8] Radomsky and Hewins (1990) GCA, 54, 3475-3490. [9] Connolly and Hewins (1990) LPS XXI, 222-223. [10] Walker et. al. (1979) Contrib. Mineral. Petrol., 70, 111-125.
Dehart John M.
Jurewicz Amy J. G.
Lofgren Gary E.
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