Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.335c&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 335
Mathematics
Logic
Cais, Cooling Rates, Kamacite, Leoville, Metal, Platinum-Group Elements, Taenite, Vigarano
Scientific paper
Metal particles in Vigarano 1623-8, a Type B2 CAI [1], underwent virtually no sulfidation, as is typical of opaque assemblages from Ca, Al-rich inclusions in the reduced CV3 chondrites [2]. In this study, we have identified two large metal grains (M1 and M2) with chemical and mineralogical features that may indicate cooling under different conditions and are, therefore, difficult to understand in the environment of a single CAIs thermal evolution. M1 is an almost spherical, kamacite+taenite-bearing particle included in a fassaite grain of the CAI host; a 17.5 micrometer-long (0.5 micrometer steps) microprobe traverse along M1 shows that Ni and Ru contents in the taenite (31.5 and 1.1 wt%, respectively) are uniform, and differ from those in the adjacent kamacite (Ni=4.5, Ru=0.7 wt%). M2 is a 20 micrometer, irregularly-shaped taenite particle, embedded in a fine-grained (spinel-rich) portion of 1623-8. It has a homogeneous composition with 10.5 wt% Ni, 0.4% Co, 0.7% Re, 0.6% Pt and high concentrations of Ru (6.5 wt%), Os (4.3 wt%) and Ir (8.2 wt%), as previously recognized by [1]. The composition of M2 is such that it should have undergone exsolution at 800 = T >= 600 degrees C (according to experimental data by [3]) to form at least two (alpha+gamma-NiFe), or probably three (+epsilon-RuFe) different phases. Lack of exsolution features in this large grain is therefore indicative of equilibration at relatively high temperatures (T>600 degrees C) followed by rapid cooling. Other metal particles of similar bulk compositions in CAIs from the Leoville chondrite (also a reduced CV3) show extensive exsolution features that have been interpreted as the result of low- temperature equilibration of the CAI and their constituents after incorporation into their parent body [4, 5]. The relatively high equilibration temperature of M2 is, however, inconsistent with the existence of kamacite in M1. From the phase relations in the Fe-Ni binary, a grain like M1, with 25 wt% bulk Ni, would barely start to exsolve kamacite at 550 +- 22 degrees C (for a 25 +- 10 vol% kamacite fraction, assuming that the effect of 2.5 wt% Co and ~3 wt% total PGE is not substantial). Therefore, exsolution of kamacite in M1 must have occurred prior to its incorporation into 1623-8. Then, in order to preserve the sharp taenite-kamacite boundary observed in M1, the cooling rate of the inclusion at T <- 1200 degrees C must have been fast since diffusion of FeNi at 50 degrees C/hr in the 1200-550 degrees C range would result in appreciable reequilibration of kamacite and taenite after just ~1hr. This is in conflict with previous experimental results that suggest that the reverse zoning observed in melilites from Type B CAIs formed at cooling rates <- 50 degrees C/hr [7]. Then, M1 could not have been incorporated into the CAI during the melting event that produced the primary melilites in 1623-8 (with reverse zoning [1]) but could have entered the inclusion during a second melting stage, such as proposed by [1], followed by very fast cooling (>>50 degrees C/hr). After kamacite exsolution, the cooling history of M1 (at T <- 550 degrees C) must have been slow. Calculations based on D^gamma- NiFe(sub)Ni~10^-18 cm^2s^-1 [6] show that the homogenization of Ni in taenite over distances of 5 micrometers (as observed in M1) would take about 8000 years at 550 degrees C (or require a cooling rate ~0.1 degrees C/yr). This should be considered an upper limit to the cooling time scale since the probable effect of the few percent levels of Co and PGEs in this grain will be to accelerate Fe-Ni interdiffusion, and taenite may homogenize substantially faster than in the pure binary system. But, if such slow cooling had taken place in the CAI, M2 (just a few hundreds of microns apart from M1) would have undergone exsolution [3]. This means that not only kamacite exsolution but also production of homogeneous taenite in M1 predate its incorporation into the CAI. The phase assemblages in M1 and M2 cannot be reconciled with a single thermal history and thus require separate environments for their early thermal evolution. References: [1] MacPherson G. J. and Davis A. M. (1993) GCA, 57, 31-243. [2] Casanova I. and Grossman L. (1993) LPSC, XXIV, 257- 258. [3] Blum J. D. et al. (1989) GCA, 53, 483-489. [4] Simon S. B. and Grossman L. (1992) EPSL, 110, 67-75. [5] Blum J. D. et al. (1989) GCA, 53, 543-556. [6] Dean D. C. and Goldstein J. I. (1986) Met. Trans., 17A, 1131-1138. [7] MacPherson G. J. et al. (1984) J. Geol., 92, 289-305.
Casanova Ignacio
Grossman Lawrence
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