Lunar and Planetary Science XXIX 1302.pdf

A PRELIMINARY STUDY OF ZONELESS PLESSITE IN ORDINARY . R. J. Reisener1 and J. I. Goldstein2. 1Dept. of Geosciences, University of Massachusetts, Amherst MA, 01003 2Dept. of Mechanical and Industrial Engineering, University of Massachusetts, Amherst MA 01003. isolation from other metal particles during cooling. The Introduction: Metal assemblages in ordinary concept of chemical isolation is consistent with the chondrites often contain zoned . Zoned taenite has observation that ZP particles are often “sealed” inside received much attention because the Ni profiles are useful silicates. In any case, it is important to understand why ZP for calculating metallographic cooling rates. An unusual metal transformed to martensite rather than to zoned taenite form of plessitic metal ( + taenite intergrowth) and kamacite. It is possible that ZP particles had an was identified in Barwell (L5) and called "zoneless plessite" independent thermal history and were accreted at low [1]. Only a few studies have considered zoneless plessite temperatures, i.e., they are aberrant grains [6]. We tested (ZP) metal [1-3]. In this work, we searched for ZP in this hypothesis by measuring the compositions of chondrites from the H, L and LL groups and various olivines which enclose ZP particles. These olivines have petrographic types (3.4-6). ZP was identified in 7 ordinary compositions identical to the average, indicating chondrites (Table 1). Here we report our observations and the olivines were equilibrated in situ and the enclosed ZP give some consideration to the origin of these particles. particles are not aberrant. An alternate possibility is that ZP metal and zoned metal shared the same thermal history, Occurrence and Description: All of the chondrites but were controlled by different chemical systems. The which contain ZP are equilibrated (types 5-6) and are of low most obvious chemical difference between ZP and zoned shock (S1-S2). In Estacado, Guarena, and Kernouve, ZP is metal is the lack of troilite blebs inside ZP. Experiments as abundant as zoned taenite. We recognize two are in progress to evaluate the effects of troilite and S- petrographic occurrences: between silicates in saturation on taenite transformations during cooling. recrystallized matrix, and inside silicate fragments and surviving silicate . ZP is especially common It is not clear whether ZP bulk compositions were inside the outer olivine shell of barred olivine chondrules established before accretion (i.e., during a - (Figure 1). In contrast to zoned metal phases, ZP particles forming event) or during metamorphism. The are spheroidal or ellipsoidal and have a narrow size first possibility was favored by [3] who recognized bulk distribution (generally < 100 um). ZP particles in type 6 Ni/Fe variations among Kernouve ZP particles and cited chondrites are generally larger than those in type 5. It has these variations as evidence against intergrain been reported [2] that ZP is not associated with troilite, but homogenization during metamorphism. In addition, these we observed several examples of troilite attached to ZP in authors reported that each ZP particle has a cosmic bulk Allegan, Guarena, and Kernouve. Small troilite blebs and Co/Ni value (~0.05), regardless of its bulk Ni/Fe value. chromite grains, which commonly occur inside zoned Analysis of variance of our broad-beam analyses indicates taenite and kamacite, have not been observed inside ZP. that apparent intergrain variations may not be statistically Two types of ZP microstructures were recognized by [3]: 1) significant. We also find that for a given chondrite, ZP taenite + kamacite intergrowths which superficially particles which have cosmic Co/Ni values have Ni/Fe resemble Widmanstatten structure (i.e., Figure 2) and 2) values which are similar to the chondrite bulk metal value. blocky taenite + kamacite intergrowths. Careful EPMA ZP particles which have high Ni/Fe values tend to have low indicates that ZP consists of low Ni kamacite (as low as 3 Co/Ni values, and vice versa. These variations may be due wt.% Ni) and tetrataenite (~50 wt.% Ni). The bulk to real bulk composition variations among ZP particles, or compositions of homogeneous ZP particles were to preferential sampling of ZP tetrataenite or kamacite. determined using broad-beam (10-25 um spot size) EPMA, and averages for each chondrite are reported in Table 1. In Although ZP particles must have been chemically isolated addition to the microstructures 1) and 2) above, we during cooling, they were not necessarily isolated during observed kamacite particles which contain large blebs of prograde metamorphism. The large ZP particles in some tetrataenite at metal-silicate boundaries. It is difficult to type 6 chondrites may suggest a partly metamorphic determine bulk compositions for these particles because origin. The amount of metal atom diffusion between the tetrataenite and kamacite phases are coarse and separate metal particles during metamorphism is not heterogeneously distributed. known. Two end-member possibilities exist for chondrites which were heated into the taenite field: 1) each metal Origin of Zoneless Plessite: ZP microstructures are particle remained isochemical during metamorphism, and entirely consistent with formation by a martensitic 2) all metal particles homogenized with one another. It is reaction followed by martensite decomposition. When often difficult to compare the bulk chemistry of different supersaturated taenite with 10-33 wt.% Ni cools below the metal particles using EPMA because either the particles are Fe-Ni martensite-start line, the diffusionless reaction chemically zoned due to communication between taenite taenite -> martensite occurs [4]. During further cooling, or and kamacite, or the particles have transformed to coarse during reheating into the taenite + kamacite field, and heterogeneous ZP. These complications do not apply martensite transforms to a taenite + kamacite mixture [5]. to ZP particles with homogeneous structures (i.e., Figure The bulk composition of each ZP particle we studied is 2). within the martensite composition range. The compositions of ZP tetrataenite and low Ni kamacite indicate intragrain equilibration at low temperatures. The absence of edge to edge zoning indicates chemical Lunar and Planetary Science XXIX 1302.pdf

STUDY OF ZONELESS PLESSITE IN ORDINARY CHONDRITES: R.J. Reisener and J.I. Goldstein

The Special Case of Richmond: Richmond metal (1000 um across). Troilite occurs at metal surfaces, but it has an unusual unzoned microstructure which consists of has not been observed inside metal. Metallographically, coarse tetrataenite (up to 20 um across) in a kamacite Richmond metal is a form of zoneless plessite. However, matrix. No zoned metal phases were observed in our the metal microstructures are considerably coarser than sample. We used point counting (n=2000) to determine a experimental martensite microstructures and we are not bulk composition of 17 wt.% Ni (well within the certain that Richmond metal structures formed via a martensite-formation range) for the largest metal particle martensitic reaction.

Table 1. Zoneless Plessite in Ordinary Chondrites.

Chondrite Class # ZP Mean ZP Bulk Chondrite Mean Size Studied Ni/(Fe+Ni) Ni/(Fe0+Ni)*[7] (sq. um) † Allegan H5(S1) 9 0.10 0.09 646 Estacado H6(S1) 10 0.10 N/A 5067 Forest City H5(S2) 1 †† N/A 2867 Guarena H6(S1) 17 0.10 0.10 5408 Kernouve H6(S1) 11 0.10 0.09 2237 Nuevo Mercurio H5(S1) 12 0.11 N/A 1007 Richmond LL5(S2) 1 0.17 N/A massive grain *H Chondrites have a narrow Ni/(Fe0+Ni) range: 0.09-0.12 for unweathered chondrites [7]. †Mean sizes favor large ZP particles which are easiest to identify and analyze. ††Heterogeneous ZP particle.

Fig. 1. BSE image of BO chondrule in Nuevo Fig. 2. SE image of zoneless plessite in Kernouve Mercurio. Zoneless plessite occurs inside the showing tetrataenite plates in a kamacite matrix. outer olivine shell.

References: [1] Sears and Axon (1975), 10, 486. [2] Willis and Goldstein (1983), Proc. Lunar Planet. Sci. Conf. 14th, B287. [3] Hutchison and Bevan (1983), Chondrules and their Origins, 162. [4] Kaufman and Cohen (1956), Journal of Metals, October, 1393. [5] Zhang et al. (1994), Metallurgical Transactions A, 25A, 1627. [6] Rubin (1990), GCA 54, 1217. [7] Jarosewich (1990), Meteoritics 25, 323.