Evidence from Secondary Minerals for Three Stages of Metasomatism During Thermal Metamorphism in Ordinary Chondrites
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49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1254.pdf EVIDENCE FROM SECONDARY MINERALS FOR THREE STAGES OF METASOMATISM DURING THERMAL METAMORPHISM IN ORDINARY CHONDRITES. J. A. Lewis1 and R. H. Jones1,2, 1Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131. [email protected]. 2School of Earth and Environmental Sciences, University of Manchester, Manchester, M13 9PL, UK. Introduction: Secondary minerals are useful re- zoning of primary plagioclase in types 3.0-3.8 [1,2], corders of the chemical and physical conditions under replacement of calcic plagioclase by sodalite/scapolite which they form. In ordinary chondrites (OCs), the in types 3.2-3.6 [2], development of nepheline in types major secondary minerals resulting from thermal met- 3.4-3.9 [2,5], development of dissolution lamellae in amorphism are feldspars and phosphates. In our recent types 3.6-5 [2,5], and presence of micropores studies, we have shown that feldspar and phosphate throughout the petrologic sequence [1-5]. Also, fine- minerals show a range of textural and compositional scale K-feldspar exsolution lamellae are present in features that indicate the presence of fluids [1-8]. Be- albite in types 3.6-6, particularly in porous regions, cause fluids have significant effects on the chemical indicating infiltration of K followed by rapid cooling to evolution of geologic materials during metamorphism, produce the fine-scale structure [6]. it is important to consider their compositions and the Phosphates. Phosphates form initially by the oxida- timing of their interactions with primary and secondary tion of P exsolved from Fe-Ni metal during thermal chondrite components. Here we summarize our recent metamorphism [7]. Phosphate formation is discussed in work and present an overall three-stage model of meta- detail by [4,7,8] who suggest that merrillite forms first, somatism during thermal metamorphism in OCs. and is then partially altered to apatite by hydrous flu- Observations: Feldspar. Secondary feldspar ids. During retrograde metamorphism, a dry, halogen- forms from the crystallization of fine-grained matrix rich fluid alters the apatite composition to chlorapatite and chondrule mesostasis glass during thermal meta- with a minor F component and no OH. morphism. Historically, the petrologic classification Model: Thermal metamorphism is the result of the scheme of [9] describes fine-grained, μm or sub-μm, energetic decay of short-lived radioisotopes, largely secondary albite in petrologic type 4 OCs that under- 26Al, active during the first few million years after solar goes textural equilibration to grains >50 μm in size system formation. We have developed an overall mod- through type 6. However, we have shown that feldspar el of chemical alteration, i.e. metasomatism, that occurs equilibration in chondrules is more complicated. during thermal metamorphism in OCs, to account for Chondrules in type 3 LL chondrites contain a mix of the development and alteration of feldspar and phos- primary [1] and secondary [2] plagioclase. In Semar- phate minerals. This model consists of three general kona (LL3.00), chondrules contain primary igneous stages: hydrous alteration during prograde metamor- plagioclase within the mesostasis glass [1]. Primary phism, dehydration close to peak metamorphic temper- plagioclase has compositions from endmember anor- atures, and infiltration of anhydrous fluids during ret- thite to endmember albite. Through the type 3 se- rograde metamorphism. In figure 1, the three stages are quence (3.0-3.9), secondary plagioclase becomes in- indicated schematically on a general metamorphic creasingly abundant in chondrules as it crystalizes from time-temperature progression for an onion-shell-like mesostasis glass [2]. The composition of secondary asteroid. plagioclase generally follows the composition of the Stage 1: prograde hydrous alteration. During pro- chondrule mesostasis glass, or primary plagioclase, grade metamorphism, heating occurs relatively rapidly, from the host chondrule [2]. However, albitization of producing the bulk of the observed alteration features. the most calcic compositions occurs in response to alkali metasomatism in petrologic types >3.6 [2-4]. In type 4 L and LL chondrites, mesostasis glass is com- pletely crystallized into secondary plagioclase. Plagio- clase compositions are diverse, though not as extreme- ly calcic (<An85) as is observed for primary plagioclase [3,4]. The equilibration trend continues through type 5 and by type 6, plagioclase is chemically and texturally equilibrated to ~An11 [3,4]. In addition to albitization of calcic plagioclase, a range of textural features in chondrule plagioclase and mesostasis glass indicate alkali metasomatism. These Figure 1: Three stages of metasomatism indicated on a time- temperature diagram for OC thermal metamorphism. Petrologic include dissolution of chondrule mesostasis glass [1-4], type ranges are indicated in each color. 49th Lunar and Planetary Science Conference 2018 (LPI Contrib. No. 2083) 1254.pdf Ices that accrete along with the matrix, metals, sulfides, Stage 3: retrograde anhydrous fluids. The exist- and chondrules, melt and allow for short-range (cm- ence of late-stage fluids has been inferred from two scale) transport of soluble cations between chondrite observations. First, fine-scale K-feldspar exsolution in components. This initially results in the dissolution of albite implies cooling rates that are orders of magni- chondrule mesostasis glass, which increases chondrule tude faster than metallographic cooling rates. If K was porosity allowing for further fluid infiltration and alter- incorporated into plagioclase during initial crystalliza- ation of chondrule mineralogy. tion, or alteration, the exsolution lamellae should de- In low petrologic types (3.0-3.8), primary Ca-rich velop over millions of years and be much coarser. Sec- plagioclase in chondrules becomes zoned in Na-Ca ond, apatite is anhydrous with no compositional equili- where mesostasis glass has been dissolved or replaced bration trend, as would be expected if their composi- by phyllosilicates. Sodalite and scapolite replace calcic tions were set prior to peak metamorphism. plagioclase in petrologic types 3.1-3.6 and nepheliniza- To explain these observations, we invoke a model tion occurs at higher temperatures, types 3.4-3.9. Al- of interior degassing of the parent body that occurs in teration of calcic plagioclase to nepheline has been short duration, high temperature bursts, possible from experimentally shown to be a two-step process [10,11]. degassing of partial melts in the asteroid core, or from Plagioclase is altered to zeolites (fabriesite and anal- impacts. The temperatures achieved are higher than the cime) in a high pH aqueous fluid (pH=13-14) at 200°C ambient temperature of the surrounding material so that [11] and these dehydrate to nepheline at higher temper- cooling occurs relatively quickly, perhaps on time- atures. Micropores and crystallographically oriented scales of days to years that are likely necessary to pro- dissolution lamellae are additional expressions of pla- duce the fine-scale K-feldspar exsolution textures. These late-stage fluids are dry (likely a high gioclase alteration in the presence of an aqueous fluid. H /H O ratio) and rich in alkalis and halogens. The Starting at about petrologic type 3.6, replacement 2 2 fluid moves through the OC material via porosity de- of calcic plagioclase by albite becomes the major mode veloped in Stage 1 and preferentially alters material of replacement alteration. Albitization, an interface- near the pores. This results in K enrichment, and sub- coupled dissolution-reprecipitation reaction, also oc- sequent K-feldspar exsolution, preferentially near pores curs in a basic (pH~12), hydrous environment [12]. within chondrules, and the presence of apatite near Fluids facilitate the introduction of Na and Si and re- pores in crystallized chondrite matrix. moval of Ca and Al. During this process, Fe, Mg, Ti, Implications: This three-stage model of metaso- and Cr that are present in trace amounts in the Ca- matism has important implications for the development plagioclase structure, are removed and form oxides of carrier phases used for chondrite chronology using nearby, typically Cr-spinel, chromite, and ilmenite. In short- and long-lived radioisotopes. For example, feld- lower petrologic types, albitization is limited to μm- spar and chondrule mesostasis glass are the major car- size rims near grain boundaries and in regions of high riers of Al used for Al-Mg dating. Iodine used in I-Xe porosity, but albitization has gone to completion by dating is a highly incompatible element that is likely type 6. mobilized and hosted in secondary phases. Apatite is a A similar replacement reaction is expected for the major host of U for U-Pb and Pb-Pb dating. Because alteration of merrillite to apatite. Since there is evi- each of these phases undergoes multiple stages of al- dence from feldspar for a hydrous fluid, OC apatite teration, the integrity of the systems and interpretation may have originally contained a significant OH com- of the resultant ages require careful consideration. ponent, that was subsequently overprinted (Stage 3). This model also has implications for the volatile Stage 2: dehydration at peak metamorphic temper- budget of OC asteroids. Dehydration and venting of atures. Due to limited fluid availability and increasing fluids from the asteroid core could reduce