Implications for Chondrule Formation

Transcription

1 PROCEEDINGS OF THE FOURTEENTH LUNAR AND PLANETARY SCIENCE CONFERENCE, PART 2 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, SUPPLEMENT, PAGES B559-B566, FEBRUARY 15, 1984 Relict and Other Anomalous Grains in Chondrules' Implications for Chondrule Formation ALFRED KRACHER, EDWARD R. D. SCOTT, AND KLAUS KEIL Institute of Meteoritics, Department of Geology, University of New Mexico We have identified relict olivine and pyroxene grains in chondrules from ordinary and carbonaceous chondrites that probably did not crystallize in situ. Some of these olivines are clear, but others contain fine-grained Fe,Ni ('dusty olivines') and resemble previously described occurrences in ordinary chondrites. There are also chondrules in which all olivine is dusty. We conclude: (l) not all relict olivines are dusty, (2) not all dusty olivines crystallized outside the chondrule in which they occur, and (3) some dusty olivines were produced during chondrule formation by a reduction process that affected the whole chondrule. The occurrence of dusty olivines and relict pyroxenes and olivines in chondrules from carbonaceous as well as ordinary chondrites supports the argument that chondrules from all chondrites had similar origins and histories. We propose that chondrules and mineral fragments were transported across f(o:) gradients in the solar nebula while they were hot, or were reheated in an environment different from the one in which they formed. Partially molten chondrule sometimes incorporated mineral grains or chondrules with different redox states, producing compound chondrules and chondrules containing anomalous grains. Dusty olivines may also have formed when hot chondrules were transported to regions of lower oxygen fugacity. 1. INTRODUCTION Some olivines in chondrules from ordinary and enstatite chondrites contain finely dispersed Ni-poor metal [Fredriksson et al., 1969; Nagahara, 198 la, b; Rambaldi, 1981; Rambaldi and Wasson, 1982; Rambaldi and Wang, 1982]. These grains have been interpreted as relicts of chondrule precursor material that were incompletely melted during chondrule formation [Nagahara, 198 la, b; Rambaldi, 1981], or as grains that formed in an earlier generation of chondrules and were incorporated into a later generation [Rambaldi and Wasson, 1982]. These authors argue that these grains did not crystallize in situ, because they are zoned with FeO decreasing towards the surface, contrary to normal igneous zoning trends, and in many cases they are compositionally unlike other olivines in the same chondrule. Establishing the origin of these grains will constrain models for chondrule formation. Copyright 1984 by the American Geophysical Union. Paper number 3B /84/003B chondrites suggesthat chondrules in both kinds of chondrites have similar origins. We argue that some dusty grains crystallized in situ and were reduced either during further crystallization of the chondrule or shortly thereafter, while the chondrule was hot. Thus 'dusty' and 'relict' are not synonyms. We suggesthat the reduction process that produced Ni-poor metal dust inside olivine was also responsible for creating chondrules with a low Fe/(Fe + Mg) ratio. This process was most probably controlled by the composition of the ambient gas phase in which chondrule formation took place. 2. DESCRIPTION We have studied anomalous grains in thin sections of Semarkona (LL3.0), St. Mary's County (LL3.3), Chainpur (LL3.4), Prairie Dog Creek (H3.8), Murchison (CM2), Ornans (CO3), Most models for chondrule formation assume the existence and Vigarano (CV3). After microscopic examination, objects of solid precursor material (condensation theory is an exception: containing anomalous grains were first surveyed by analyzing Suess [ 1949]; Wood[ 1963]; Blander and Katz [ 1967]). However, for Si, Fe, Mg, and Ca by electron microprobe (ARL EMX, the nature of this material remains controversial, and suggested operated at 15 kv accelerating voltage, na sample curprecursor span the range from a differentiated, coarse-grained rent). Selected objects were analyzed for additional elements rock (Dodd [1981], and references therein) to primitive dust using sample currents up to 50 na and longer counting times to [Scott et al., 1982]. Although the existence of relict grains would attain detection limits of <0.02 wt % for all minor elements. be compatible with chondrule formation by planetary processes Some of the latter analyses were performed on a JEOL 733 (such as proposed by Fredriksson [1963], Dodd[1971, 1981], Superprobe. In addition, some objects were investigated with a and others), we think that the argument summarized by Taylor scanning electron microscope (Hitachi S-450), equipped with et al. [1983] preclude such a model. We shall therefore confine an energy dispersive analysis system. our discussion to a solar nebular setting for chondrule formation. Observations on 11 selected objects representative of the We describe here different kinds of grains with anomalous whole spectrum of our observations are reported in Table 1. textures or compositions (sometimes both) in chondrules from Compositions of pyroxene, glass, and olivine in chondrules ordinary and carbonaceous chondrites. These include dusty containing anomalous grains are given in Tables 2 and 3; these olivines, some of which we believe did crystallize in situ, and are typically averages of 5-25 analyses. In addition, we have both clear and dusty olivines and pyroxenes that appear to have observed many occurrences of dusty olivines in ordinary type 3 crystallized elsewhere. We report the first occurrences of dusty chondrites that resemble those already described in the literaand nondusty relict grains in CM2, CV3, and CO3 chondrites. ture. These are not further described here. Similarities between occurrences in ordinary and carbonaceous B559 Semarkona Figure la shows a pyroxene chondrule (chondrule 50) that appears to contain a relict pyroxene. It contains some olivine

2 B560 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION Chondrite Class* Source t TABLE 1. Properties of Selected Anomalous Grains Sect. No. Obj. No. Anomalous Grain Low-Ni Host Composition Metal Composition Semarkona LL3.0 UNM fs12.0 see text fal.2, fsl.9 St. Mary's County LL3.3 USNM A1 fa3.5 yes fa0.5 Chainput LL3.4 USNM B fa7.2 yes fa7.9 Prairie Dog Creek Murchison H3.8 CM2 USNM UNM A fa0.3 fal.1 no yes fs4-10 : D fa0.9 yes Ornans CO3 UNM fa2.0 no fa33 41 fa2.5 no fa33 Vigarano CV3 BMNH II 1 fa8.3 yes fal.8, fsl.6 3 complex chondrule; see text UNM fa0.8 ** fa5.0 * Subtype assignments by Sears et al. [ 1980, 1982]. t BMNH: British Museum (Natural History); UNM: University of New Mexico; USNM: National Museum of Natural History (Smithsonian Institution). $ Dusty mineral grain in matrix. õ Almost all grains in this chondrule are dusty. II Thin section prepared at UNM from BMNH sample no. 1920, 347. ** No, but low-co kamacite (see text). poikilitically enclosed by pyroxene and resembles pyroxene chondrules described by Scott and Taylor [1983] as type IB. Pyroxene compositions, in general, average fsl (one standard deviation of the distribution), typical of these chondrules. One large grain, however, has a central composition of fs , falling off to about fs5 near the rim. Average compositions of both kinds of pyroxene are given in Table 2 (columns A, B). Most pyroxenes contain very small (<5/ m) metal blebs, although the density of metal is not high enough to regard them as dusty. Most of these metal blebs have Ni contents <0.5%. St. Mary's County both kinds of olivine. produce dusty olivine in this chondrule continued long enough to equilibrate the Fee concentrations in the relict grain with those in the remainder of the chondrule. Prairie Dog Creek This meteorite is more equilibrated than the chondrites discussed above. However, in one chondrule (chondrule 3, Figure lc) we have found an anomalous grain of a different kind: chondrule 3 is a porphyritic pyroxene-olivine chondrule texturally similar to Semarkona no. 50 (see above), although clinopyroxene is richer in Fee (fs4-10; Table 2, column C). The large anomalous olivine has a central composition of fa0.3, but Some porphyritic olivine chondrules consist almost entirely a rim composition of fa18.5. There are also Fee-rich veins of dusty olivines. One such chondrule (A1) is shown in Figure transsecting the low-fa core. We believe the large olivine did not lb. Dusty olivines have narrow (<20/ m) clear margins, like crystallize in situ, because its central Fee concentration is much occurrences of single dusty olivines described by Nagahara lower than that of olivines normally found in this kind of [1981a] and others. The whole chondrule is surrounded by a pyroxene chondrule in ordinary chondrites [Scott and Taylor, fine-grained rim of clear olivine up to 100/ m wide. The rim 1983, Figure 7]. In addition, this olivine is much larger and contains abundant interstitial metal and sulfide. Dusty olivine more regular than the euhedral olivines that occupy the cores of averages fa3.5; the fine-grained clear rim fa0.5. Metal dust is some of these chondrules. The Fee enrichment in the veins and nearly free of Ni (<1%); the large metal blebs around the margin at the rim is probably due to diffusion after crystallization, as in are kamacite with about 6% Ni (EDS analyses). Occasionally, occurrences in Mez6-Madaras [Binns, 1968, Figure 4] and grains of intermediate size (a few microns) and Ni contents Warrenton [Kerridge, 1972]. between those of metal dust and coarse metal are found inside Murchison Chainpur Among a number of chondrules containing dusty olivine, one (chondrule B) was found in which the dusty olivine did not show the usual inverse zoning of Fee. It is an oval porphyritic olivine chondrule, 650 x 920/ m in size, with olivine grains / m in longest dimension, embedded in devitrified mesostasis. One grain contains a dusty, metal-rich core about 90 / m in diameter, surrounded by a clear, 10-40/ m wide rim with hopper faces. This texture is typical of other chondrules with dusty grains, except that in this case the dusty olivine is remarkably similar in fayalite content to the overgrowth and the host chondrule (Table 1). However, like dusty olivines described by Nagahara [198 lb], it has much lower Cae (<0.02%) than other olivine ( %). Apparently the reduction process that Two dusty olivine grains, A and D (Table 1), with low fayalite contents (fal) and Ni-poor metal inclusions were observed. They are texturally and compositionally similar to dusty olivine grains in ordinary chondrites. Grain A is monomineralic, angular, and about 200 x 330/ m in size; grain D is rounded (240/ m diameter) and surrounded by and intergrown with a pyroxene corona. We have also observed a porphyritic olivine chondrule consisting largely of dusty olivines. Ornans Chondrule 21 (Figure 2a) is an irregularly shaped high-fa porphyritic olivine chondrule or chondrule fragment showing the typical features of type II chondrules of McSween [1977], such as strong olivine zoning and devitrified mesostasis. Its

3 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION B561 :....½'.' :-::!. ::-. :?:;:::' ":':'." =:-. T*...::;:..;;.-.:..-?::--.:'.. ß q, '... '... *"',,.'-:'..-.. :';;..s'..,... ' :'.::... '. :.:'..?,:-.: :...,,,I..:......,,,, /..., **::: '?i:'v'<i:.:".:';;,x7'.:.:..-..;., :':'..: :.:':%a:. :,:::..:;:!::'.. -. :::...*,... o.:.'." -;.Z"!.:... ':=':.*"':...:L':...,: " :.:.4'*. :i,;::.i..'...;?--...':. ;!;.½ :'::,:.*.:.'!-:..:":' :/ :!;'!:...,...-.':':' ß ".?.,...,.. :..,, :.,½ --.,;;?...:.;, '.'.:,-'.' :;..:,.-,:"½ " "': '.,.:,, :- ':;:-.-::-r,;½-...- '-". :-:',: '-. ;.;.::..,.....,,½-.,,.,,..:...:.:;..,4. -.:. ::::!.:...':.-. '.' :.": :.:'a;' :.' ;..'.-' }..?;...-.':' '... '*:... ''... '.:.'::: ::' ' r, T-.':::!':;:::;:.; ;:. ':: :....,:.,:..,.**:..,.':../"' ;: :::.;:½:.-?;_....,...,.; '½:,....; '.;;.-.:...: ;.. =...;.-:-':..-:-.....*.;... -?.:.--...:,...:: >, *...,... ' *.. **... 0.,.::-*,... " :': ""',:.".. ::... '...z. :-.**..". :,..,... ß..::...;...::,...:...:,.::.;;.::-;,:..'...?'.':...: ;...,... *..-::..., -'.. ** ' ß..;...**x., :-. :-,...:, ::.- "'Z.' *'"', *'::*';.*" ß ' ' -"' * ß '4... :.;.'.'...:,--.'*'. c..: '..:..,;.:.'..:, ß '?'.%: ß... ß... :::.::.:.½c%...:::' %;:, - :t;-....." ß.7";;.:..i..,,,.'i' :: ;:.,... ;;;; ':*.:::. :...;::;,.:.-',,,.*,c'.:::*...:-'---.,,-'. '-:-: <"'..;:".'": ' ". " r::::;;! " ".?,:,..'...:... :':""'"'::::?.;;: ' '".. "-:...'t;. "'"..--:.;½:. ;. :::' :.;...":%¾.; *...,......::,"':::'..,, **' '"...;.'5?:' ' i ';;-."."....:.-:..,-.'...,.:':'.!:':--".::. "':"': -.,,.;?:,.c:...-.i....., *'..'::?:...(. ':--.:..:.:50:--:...:: m..: ß... Fig. 1. Photomicrographs (plain transmitted light, except where noted otherwise). (a) Semarkona chondrule 50. The large relict pyroxene is marked by arrows. (b) St. Mary's County chondrule A1. All olivine grains except those in the fine-grained margin contain Ni-poor iron, probably as a result of reduction after crystallization. (c) Prairie Dog Creek chondrule 3 (crossed polarizers). Large irregular olivine grain in center of chondrule (nearly in extinction) probably crystallized outside this chondrule, because its shape, size, and core composition (fa0.3) are so different from those of olivines normally found in this kind of chondrule. Diffusion after crystallization made the rims and cracks (lighter grey) richer in FeO (up to fal8.5) than the core. (d) Vigarano chondrule 3. Marked features are: (X) rounded olivine with annular dusty region, (Y) pyroxene poikilitically enclosing olivine, and (Z) location of linescan shown in Figure 3. average olivine composition excluding three anomalous grains is fa (Table 3, column A). The three anomalous olivine grains have cores of much lower FeO content (fa2) and welldefined rims (<10/ m in width) where the FeO concentration rises sharply to fa An analysis of the core lowest in FeO is given in Table 3 (column B); this grain is shown in Figure 2b with the 1ow-fa core in extinction. Chondrule 41 is another high-fa porphyritic olivine chondrule (fa33 + 5; Table 3, column C) like no. 21, but wellrounded in shape (Figure 2c). It contains a skeletal olivine with an FeO-poor core (fa2-5; Table 3, column D). The fa concentration increases to fa20-40 at the outer surface as well as the reentrant inner surface (Figure 2d). Although this grain appears to be slightly dusty optically, this is due to chromite and high-ni metal, and no 1ow-Ni metal has been observed in this chondrule. Similar 1ow-FeO cores in olivines from Mez6-Madaras were described by Hoinkes and Kurat [1974]. These authors considered the object in which these olivines were found to be a rock fragment, rather than a chondrule. Vigarano Chondrule 1 is the porphyritic olivine chondrule shown in Figure 3d of Scott and Taylor [1983]. It has some pyroxene around the margin and contains a dusty olivine grain with a high-fa core (fa ) and a clear, 1ow-FeO rim. The rim has a composition of about fa3, close to other olivines in this chondrule (fal-2). Table 3 (columns E, F) shows that, as in Chainpur chondrule B, the CaO concentration of the dusty olivine core (0.07%) is much lower than that of normal olivine (0.21%). Chondrule 3 consists of olivine and pyroxene grains of various sizes with little mesostasis (Figure ld). The majority of mineral grains, both olivine and pyroxene, contain opaquerich, dusty looking regions. Most of the olivines are optically continuous, including the chadacrysts; a chondrule with a similar texture has been described in Kota-Kota (EH4) by Binns [ 1967]. One large olivine (X in Figure ld) consists of a clear core normally zoned fa (Table 3, column G), a dusty mantle

4 ... ß B562 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION TABLE 2 Compositions of Pyroxene, Glass, and Mesostasis in Chondrules Containing Dusty and/or Relict Grains A B C D E F G SiO2 59. l TiO AI O Cr O FeO MnO MgO CaO Na K Total A -- Semarkona, average pyroxene in chondrule 50. B = Semarkona, center of foreign pyroxene in chondrule 50. C -- Prairie Dog Creek, pyroxene in chondrule 3. D -- Vigarano, pyroxene in chondrule 1. E = Vigarano, pyroxene in chondrule 3. F -- Vigarano, mesostasis in chondrule 1. G = Vigarano, glass inclusion, associated with dusty metal, in olivine (chondrule 3). A-F: microprobe analyses: G: SEM-EDS analysis; in wt %. with small, aligned metal particles (Ni from <0.5 to 2%) associated with a very small amount of SiO2-rich glass (Table 2, column G), and a clear rim. FeO decreases through the dusty zone to fa5 :t: I in the rim, whereas MnO increases continuously from the core to the rim by a factor of 2 (Table 3, column H). Other olivine grains in the chondrule have fa7 :t: I (Table 3, column I). Pyroxene, however, is more reduced (fsl.2 -I- 0.5; Table 2, column œ). The large poikilitic pyroxene shown in Figure ld (Y) is in disequilibrium with the enclosed olivine, which has a central composition of fa6.5 (Table 3, column J). Olivine chadacrysts appear to be zoned like those described by Blander and Abdel-Gawad [1969] and Rambaldi and Wasson [ 1982]. However, they are partially surrounded by small metal/ sulfide grains, which makes it difficult to establish that the zoning is real. Concentration profiles across the chondrule surface were obtained at six locations around the unbroken surface of the chondrule; all six profiles are generally similar. Figure 3 shows a profile taken at the point indicated by (Z) in Figure ld. Olivine is coated by a thin (_<5/ m) pyroxene layer. The Fe/(Fe + Mg) ratio in pyroxene is much lower than in olivine, and decreases toward the surface of the chondrule in both minerals. Olivine, A,..,' '- ".,.;..,... l, '¾'""::' "".?".'2 ' '- d " "'*' ' - ". ' ' ';....'... i: :-,. rn, ß,, -,._. -:....:..:....-:..:, ¾..:.., :... Fig. 2. Photomicrographs of relict grainsin two porphyritic olivinechondrules in Ornans (CO3).(a) Chondrule 21. One ofthe foreign grains is marked, two others are adjacento it. (b) Enlargement of marked grain in (a) (crossed polarizers), with the low-fa core in extinction. Boundary between core (fa2.0) and rim (---fa30) is very sharp. (c) Chondrule 41 with skeletal olivine containing low-fa core. (d) Enlargement of skeletal olivine from (c)(crossed polarizers), with the FeO-poor center (fa2.5) nearly in extinction. Note that the FeO-rich overgrowth (fa20-40) is on the reentrant surface as well as on the outside.

5 i KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION B563 TABLE 3. Average Olivine Compositions in Chondrules from the Ornans (CO3) and Vigarano (CV3) Chondrites Containing Anomalous Grains (Electron Microprope Analyses, wt %) A B C D E F G H I J SiO: Cr:O FeO MnO 0, MgO CaO Total Fa* Comp. var. t A = Ornans no. 21, normal olivine. B = Ornans no. 21, low-feo cores of foreign grains. C = Ornans no. 41, normal olivine. D -- Ornans no. 41, skeletal 1ow-FeO olivine. E -- Vigarano no. 1, normal olivine. F = Vigarano no. 1, dusty olivine. G = Vigarano no. 3, clear core of olivine with annular dusty region. H -' Vigarano no. 3, clear rim of olivine with annular dusty region. I -- Vigarano no. 3, average of normal olivine grains. J = Vigarano no. 3, olivine chadacrysts inside pyro xene. * Some of these averages differ slightly from those given in Table 1, because the latter include results of additional four-element analyses. t Compositional variation expressed as one standard deviation of the distribution (where applicable). pyroxene, and the opaque rim coating the chondrule are all separated from each other by areas rich in fine-grained metal, which is associated with the high Cr contents visible in the concentration profile (Figure 3). Chondrule 13 is an irregular, millimeter-sized porphyritic olivine chondrule that consists mostly of small, granular olivine crystals with a typical grain size of 25 m and a composition of fa One grain is much larger (180 x 240 m) and has a core of fa0.8. The FeO concentration of this grain increases sharply near the rim to about fa3. The 1ow-fa olivine contains a few globules of kamacite (Ni %; Co %); taenite is present in the chondrule groundmass. formation. We favor the idea that chondrules were formed from material like fine-grained, FeO-rich matrix [Scott et al., 1982]. This would be incorrect if the relict grains were an integral part of the chondrule precursor material. However, we know that partially molten chondrules collide œgooding and Keil, 1981], and single mineral grains may be incorporated by an analogous mechanism. It is also possible that entire solid chondrules were incorporated in melt droplets and partially melted, so that only the largest grains of the primary chondrule survived. In this case foreign grains would be true 'relicts' in the sense normally used in terrestrial petrology. This may explain why some chondrules 3. DISCUSSION Anomalous Grains in Chondrules If chondrules formed in the nebula, as we believe œ Taylor et al., 1983], we can envisage two alternative ways in which grains that did not crystallize in situ may have been incorporated into chondrules. 1. The crystals may have been present in the original parcel of matter (e.g., a dustball) that was heated during chondrule formation. They could have escaped melting either because of their higher melting point or because heating was so brief that the largest grains were incompletely resorbed into the liquid. Such grains could have been a constituent of the material from which chondrules initially formed (precursor material), as argued by Rarnbaldi [1981] and Nagahara [1981a, b], or they could be chondrule fragments that were incorporated into precursor material and subsequently remelted. 2. Solid grains may also have collided with chondrules (while these were still liquid) without being completely melted, provided they were sufficiently large. Such grains could be either chondrule fragments, relicts of whole chondrules, or large precursor grains. Rambaldi and Wasson [ 1982] consider it most likely that relict grains originated in precursor chondrules. We will use the term 'relict' to mean any grain that did not crystallize in situ, whether it originated by mechanism I or 2. Although we cannot confidently distinguish these cases at present, the distinction is crucial to our understanding of chondrule interi,_-,,% /\ rim Fi. 3. Stepscan of c,, Si, and Cr across the suvaco of Vi arano chondru]c 3 at the area marked (Z) in Figure ]. O]i nc is coated by pyroxcnc; co decreases towards the suvaco in both o]ivinc and pyroxcnc. Chromium enrichments coincide with areas of fine metal dust.

6 B564 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION (e.g., Ornans 21, Figure 2a) contain several anomalous grains of similar composition that are close together. The skeletal morphology of the foreign grain in Ornans chondrule 41 also suggests that it crystallized from a melt, probably a progenitor chondrule. It is possible that all relict grains are chondrule fragments, but other origins cannot be excluded. Chondrules that collide to form compound chondrules are generally compositionally similar [Lux et al., 1981]. If relict grains were derived from chondrules, they too may have been compositionally similar to the target chondrule in most cases. Thus we may only recognize those (probably rare) relict grains that are compositionally different. This is exemplified by Vigarano chondrule 13 (Table 1), where the compositional difference between relict and indigenous olivine is just slightly larger than could be accounted for by igneous differentiation. Reduction During and After Chondrule Formation We find evidence that not all dusty olivines are relict grains. There are chondrules (e.g., St. Mary's County A 1, Figure lb) in which practically all olivine crystals contain metal dust. In such cases it would be implausible to assume that there are no crystals present that are derived from the chondrule melt. In chondrule 3 from Vigarano, olivine and pyroxene apparently crystallized from melts of different FeO concentrations. The optical continuity of practically all olivine suggests that olivine crystallized in situ. We suggesthat the chondrule was reduced during crystallization, so that FeO does not show normal zoning; MnO was not affected by the reduction and is normally zoned. PoikilitfC texturesuggest that pyroxene also crystallized in situ; olivine may have been partially resorbed, as proposed by Binns [1967]. The decrease in Fe/(Fe + Mg) ratio toward the surface (Figure 3) suggests that the whole chondrule reacted with a reducing gas. The rim of FeO-poor olivine in St. Mary's County A1 (Figure lb) may have formed in a similar way. We sugges that thege chondrules were either reheated above likely to be relicts. It is improbable that such differences in CaO concentrations result from the reduction process itself rather than reflecting primary differences of relict and host material, but given our ignorance of the behavior of minor elements under these circumstances we cannot completely rule it out. It may seem contrived to argue for more than one mechanism to form dusty grains. If, however, as argued below, the redox state of chondrules was imposed by ambient conditions, any brief reheating of olivine in an environment more reducing than that in which it formed will precipitate metal. In other words, we regard the occurrence of dusty olivine as primarily caused by its peculiar response to reheating [Fredriksson et al., 1969; Boland and Duba, 1981], rather than any special property, such as implanted hydrogen or hydrochrbons [Rambaldi and Wasson, 1982] peculiar to those olivine grains that are now dusty. We propose that for some chondrules the duration of the heating/reduction event was long enough to allow metal to coalesce to larger beads, carrying the reduced oiivine past the 'dusty' stage. Metal beads in dusty olivines vary widely in grain size, and in some cases (e.g., St. Mary's County A 1) wide ranges of grain sizes and Ni contents are observed in a single chondrule. More commonly, dusty grains show a restricted range of grain size and spacing between metal particles, but both may differ widely between different grains. Different thermal histories are very likely responsible for this variability. Assuming that metal coalesced by bulk diffusion of neutral Fe, and that diffusion rates of Fe, Ni, and Co are similar, coalescence of metal would eventually be accompanied by partial equilibration with more Ni-rich metal within mesostasis or even in the chondrule rim. Hence we would expect a correlation of grain sizes and Ni contents. This seems to be the case in St. Mary's County A 1, but more data are needed to establish whether this is a general feature of dusty olivines. Reduction Of chondrules while they are partially molten also affects their mineralogy. Since removal of FeO from the remaining melt lowers the O/Si ratio, the melt composition moves towards the pyroxene stability field more rapidly than it would by olivine crystallization alone. Therefore in some cases crystallization of FeO-poor pyroxene follows that of olivine. This effect provides a more straightforward explanation of disequilibrium poikilitic textures like that in Figure ld [in which the Fe/(Fe + Mg) ratio in pyroxene is much lower than in the solidus in an environment more reducing than that in which they originally formed, or that the environment became more reducing while they were in a partially molten state during moderately slow cooling. The reduction caused low-ni metal to precipitate inside olivine while also lowering the FeO concentration of the residual melt. In Vigarano chondrule 3 the metal olivine] than either condensation [Blander and Abdel-Gawad, resulting from reduction of the melt was presumably lost, while 1969] or shock-implantation [Rambaldi and Wasson, 1982]. in St. Mary's County it is present as coarse interstitial metal. Although this scenario does not explain the textural features in Implications for Chondrule Formation every detail, we consider the evidence convincing that these chondrules were reduced while partially molten. Whatever the In conformity with mineralogical abundances in chondrites, reduction process, it is very likely that it took place while these olivine and pyroxene are the most common relict grains in type chondrules were floating in space, as a rim of fine-grained, 3 chondrites, although not the only ones: Cr-poor spinels are FeO-rich material was deposited on them after they cooled. also found as relict grains in chondrules from ordinary chon- Reduction of FeO to metal concomitant with crystallization drites [Kracher and Kurat, 1980; Kracher et al., 1981]. Foreign of silicates has previously been proposed as an explanation for olivine and pyroxene grains may have either lower or higher mineral compositions in igneous, Ca-Al-rich fragments in Fe / (Fe + Mg) ratios than the chondrule in which they occur. Of Lance [Kurat and Kracher, 1980], but in that case reduction the four possible combinations, three have been found in this probably occurred on the parent body. study (Table 1). Relict grains in chondrules with low Fe/(Fe + If we are correct about the formational history of the chon- Mg) ratios may seem less likely to survive, because in this case drules in St. Mary's County and Vigarano, some dusty olivine the relict grain has a lower melting point than the host material. grains may have crystallized in situ and were then reduced However, this case is more easily recognized optically because because of changes in the environment of chondrule formation. of the dusty appearance of the relict grain. We have not yet In those cases where dusty and normal olivine differ signifi- found low-fs relict pyroxene. Such material would be difficult cantly in Ca O content (e.g., Vigarano no. 1, Chainpur B, and to distinguish from a case like Vigarano chondrule 3, where Allan Hills A77011 [Nagahara, 198 lb]), dusty olivines are more pyroxene crystallized after olivine following reduction of the

7 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION B565 whole chondrule. Pyroxene with a much lower Fe/(Fe + Mg) 3. Relict grains may have been introduced into chondrules ratio than olivine in the same chondrule is quite common in some types of chondrules [Scott and Taylor, 1983, Figure 7], but contrary to Rarnbatdi and Wasson [ 1982], we regard poikilitic textures in these chondrules as evidence for in situ crystallization of pyroxene. We suggesthat the redox state of chondrules was controlled in three different ways: as part of the chondrule precursor material, by recycling of chondrule debris with primitive precursor material, or by collisions of partially molten chondrules and chondrule fragments. We regard the last alternative as the most plausible mechanism, but more than one of these processes may have occurred. by ambient conditions, most likely the composition of a sur- 4. Olivines containing metal dust are not necessarily relict. rounding gas phase. In a gas of solar composition, the equilibrium concentration of FeO in silicates is negligible near the liquidus temperature of chondrules [Larirner, 1968]. At the gas densities required to prevent volatile loss during chondrule formation [Tsuchiyarna et at., 1981, 1982], rapid reduction would occur, and all chondrules should have negligible FeO In some cases, partially molten chondrules may have been reduced by reaction with ambient gas, and olivine that had previously crystallized in situ became dusty. 5. No trapped reducing agent inside olivine is needed to precipitate metal dust. Rather, the precipitation of dust is one step in a continuum of reduction reactions that accompanied concentrations. Therefore chondrules with high Fe/(Fe + Mg) ratios may have formed in a gas depleted in hydrogen [Herndon chondrule formation. and Suess, 1977]. Since, unlike Herndon and Suess [ 1977], we Acknowledgments. Chondrule 3 from Prairie Dog Creek was discovered and analyzed by M. Bersch, A. Bly, and S. Recca. G. Conrad do not think that chondrules formed by condensation, we do and J. McKinley provided instruction on the JEOL microprobe. We not require that liquid droplet and ambient gas reached com- thank G. J. Taylor for many helpful comments, and J. Salas and J. plete equilibrium in all cases. If FeO concentrations were controlled by ambient condi- Calhoun for technical assistance. We also thank R. S. Clarke, Jr. and R. Hutchison for the loan of meteorite samples. Reviews by E. R. Rambaldi and R. T. Dodd helped to improve the paper. This work was tions, oxygen fugacities must have varied considerably across supported by NASA grants NGL and NAG the solar nebula. Chondrules apparently were transported between reservoirs of different oxygen fugacities, implying that f(o2) varied locally in the nebula. We believe that these differences existed in space as well as time rather than time alone. At the very least, the fact that we find low-feo grains inside high-feo REFERENCES Binns, R. A., Olivine in enstatite chondrites, Am. MineraL, 52, , chondrules as well as the reverse rules out any simple unidirec- Binns, R. A., Cognate xenoliths in chondritic meteorites: Examples in Mez/5-Madaras and Ghubara, Geochim. Cosmochim. Acta, 32, 299- tional change, such as a nebula becoming gradually less reduc- 317, ing by slow hydrogen loss. Blander, M., and M. Abdel-Gawad, The origin of meteorites and the If it is correct that relict grains were implanted mostly by collisions with molten chondrules, their high abundance would constrained equilibrium condensation theory, Geochim. Cosmochim. Acta, 33, , be evidence for high mass densities of solid material in the Blander, M., and J. L. Katz, Condensation of primordial dust, Geochim. Cosmochim. Acta, 31, , nebula during chondrule formation. High mass densities have Boland, J. N., and A. Duba, Solid-state reduction of iron in olivine-- also been inferred from the abundance of cratered and complanetary and meteofitic evolution, Nature, 294, , pound chondrules [Martin and Mills, 1976; Gooding and Keil, Dodd, R. T., The petrology of chondrules in the Sharps meteorite, 1981; Lux et al., 1981 ] and chondrules containing matrix lumps Contrib. Mineral. PetroL, 31, , Dodd, R. T., Meteorites, a Petrologic-Chemical Synthesis, Cambridge [Rubin et at., 1982], which provide evidence for frequent colli- Univ. Press, Cambridge, 368 pp., sions of liquid or plastic chondrules with other chondrules and Fredriksson, K., Chondrules and the meteorite parent bodies, Trans. N. dust aggregates. Any model of chondrule formation has to account for high Y. Acad. Sci., 25, , Fredriksson, K., E. Jarosewich, and J. Nelen, The Sharps chondfite-- mass densities in nebular regions where chondrules formed and new evidence on the origin ofchondrules and chondrites, in Meteorite Research, edited by P.M. Millman, pp , Springer and variable ambient redox conditions, as well as the apparent ease Reidel, Vienna, with which solid materials were exchanged between these locations. Gooding, J. L., and K. Keil, Relative abundances of chondrule primary textural types in ordinary chondrites and their bearing on conditions of chondrule formation, Meteoritics, 16, 17-43, CONCLUSIONS Herndon, J. M., and H. E. Suess, Can the ordinary chondrites have condensed from a gas phase?, Geochim. Cosmochim. Acta, 41, , The presence of anomalous grains (relict and nonrelict) in chondrules from both ordinary and carbonaceous chondrites Hoinkes, G., and G. Kurat, Chemismus von Spinelien aus dem Mez/J- Madaras-Chondrit, in Analyse Extraterrestrischen Materials, edited strongly supports the notion of a common mechanism of chonby W. Kiesl and H. Malissa, Jr., pp , Kerridge, J. F., Iron transport in chondrites: evidence from the Wardrule formation in all chondrite types [Scott et al., 1982]. Other renton meteorite, Geochim. Cosmochim. Acta, 36, , textural, mineralogical and compositional similarities between Kracher, A., and G. Kurat, Ordinary chondrites--the spinel puzzle chondrules from all kinds of type 3 chondrites are discussed by (abstract), Meteoritics, 15, 319, Scott and Taylor [1983]. Taken together, these data Kracher, A., F. Brandst itter, and G. Kurat, Spinel chondrules: further clues to ordinary chondrite precursor rocks (abstract), Meteoritics, argue against the theory of Dodd [ 1981] that, with few excep- 16, , tions, chondrules in carbonaceous chondrites originated by a Kurat, G., and A. Kracher, Basalts in the Lanc/ carbonaceous chondifferent mechanism from those in ordinary chondrites. drites, Z. Naturforsch., 35a, , Relict olivine and pyroxene may show higher or lower Larimer, J. W., Experimental studies on the system Fe-MgO-SiO2-O 2 Fe/(Fe + Mg) than the host chondrule in which they are and their bearing on the petrology of chondrites, Geochim. Cosmochim. Acta, 32, , embedded. In the latter case the relict grains are generally free of Lux, G., K. Keil, and G. J. Taylor, Chondrules in H3 chondrites: texmetal dust, although some may contain kamacite spherules tures, compositions and origins, Geochim. Cosmochim. Acta, 45, higher in Ni than fine-grained metal dust , 1981.

8 B$66 KRACHER ET AL.: RELICT GRAINS IN CHONDRULE FORMATION Martin, P.M., and A. A. Mills, Size and shape of chondrules in the Bjurb/51e and Chainpur meteorites, Earth Planet. Sci. Lett., 30, , McSween, H. Y., On the nature and origin of isolated olivine grains in carbonaceous chondrites, Geochim. Cosmochim. Acta, 41, , Nagahara, H., Evidence for secondary origin of chondrules, Nature, 292, , 1981a. Nagahara, H., Petrology of chondrules in ALH (L3)chondrite, Mem. Natl. Inst. Polar Res., Spec. Issue No. 20, pp , National Institute of Polar Research, Tokyo, 1981b. Rambaldi, E. R., Relict grains in chondrules, Nature, 293, , Rambaldi, E. R., and D. Wang, Relict grains in enstatite chondrites (abst ract), Meteoritics, 17, 272, Rambaldi, E. R., and J. T. Wasson, Fine, nickel-poor Fe-Ni grains in the oli. vine of unequilibrated ordinary chondrites, Geochim. Cosmochim. Acta, 46, , Rubin, A. E., E. R. D. Scott, G. J.. Taylor, and K. Keil, Silicate matrix material in type 3 ordinary chondrites: implications for the origin of chondrules (abstract), Meteoritics, 17, , Scott, E. R. D., and G. J. Taylor, Chondrules and other components in C, O, and E chondrites--similafities in their properties and origins, Proc. Lunar Planet. Sci. Conf 14th, in J. Geophys. Res. 88, in press, Scott, E. R. D., G. J. Taylor, and K. Keil, Origins of ordinary and carbonaceous type 3 chondrites and their components (abstract), in Lunar and Planetary Science XIII, pp , Lunar and Planetary Institute, Houston, Sears, D. W., J. N. Grossman, C. L. Melcher, L. M. Ross, and A. A. Mills, Measuring metamorphic history of unequilibrated ordinary chondrites, Nature, 287, , Sear's, D. W., J. N. Grossman, and C. L. Melcher, Chemical and physical studies of type 3 chondrites--i: Metamorphism related studies of Antarctic and other type 3 ordinary chondrites, Geochim. Cosmochim. Acta, 46, , Suess, H. E., Chemic der Planeten und Meteoriten-Bildung, Z. Elektrochem., 53, , Taylor, G. J., K. Keil, and E. R. D. Scott, Cosmic setting for chondrule formation, in Chondrules and their Origins, edited by E. A. King, Lunar and Planetary Institute, Houston, in press, Tsuchiyama, A., H. Nagahara, and I. Kushiro, Volatilization of sodium from silicate melt spheres and its application to the formation of chondrules, Geochim. Cosmochim. Acta, 45, , Tsuchiyama, A., H. Nagahara, and I. Kushiro, Conditions ofchondrule formation: Experimental reproduction of texture and volatiliza- tion of sodium from chondrules (abstract), in Papers Presented to the Conference on Chondrules and their Origins, p. 59, Lunar and Planetary Institute, Houston, Wood, J. A., Origin of chondrules and chondrites, Icarus, 2, , K. Keil and E. R. D. Scott, Institute of Meteoritics and Department of Geology, University of New Mexico, Albuquerque, NM A. Kracher, Department of Earth Sciences, Iowa State University, Ames IA (Received May 31, 1983; revised November 9, 1983; accepted November 23, 1983.)