Fe-alkali-halogen secondary phases in CV and CO chondrites versus Kudryavy volcano fumarolic incrustations
Among the carbonaceous chondrite group assumed to be one of the most pristine, the CV and CO chondrites show documented evidence of alteration effects (6, 7). Numerous surveys [see recap in (7)] have shown matrix, refractory Ca-Al–rich inclusions (CAIs) and chondrules of CV and CO chondrites have all been affected by a so-called Fe-alkali-halogen metasomatism that has resulted in the formation of a wide range of secondary, dominantly anhydrous minerals. It has been shown that conditions of formation of these secondary phases in CV chondrites encompass the same range of temperature, oxygen fugacity, and silica activity (8, 9), suggesting that reduced (CVR) and oxidized (CVOx) dichotomy would refer to modal abundance of their secondary phases (i.e., magnetite) rather than their metamorphic conditions of formation. The secondary minerals include abundant Ca-Fe–rich silicates: diopside-ferrosalite, hedenbergite, andradite, fayalite, kirschsteinite, wollastonite, rankinite, and larnite; oxides: magnetite, hercynite, and corundum; sulfides: troilite and pentlandite; Ca-phosphates; Fe-Ni-Co metal: awaruite and wairauite; carbides: cohenite; Na-Al-Cl–rich feldspathoids: nepheline, sodalite, and wadalite; and rare phyllosilicates: talc, tremolite, anthophyllite, and serpentines. Ca-Fe-rich pyroxenes (i.e., hedenbergite) are, by far, the most dominant secondary phases and are frequently found in association with andradite, wollastonite, and/or kirschsteinite (3, 8, 10). CO chondrites show also substantial evidence of alteration events, but the degree and extent of alteration are, in general, less than those for the CV chondrites (7, 11). The textural features of these secondary phases are the same in both groups and correspond to (Figs. 1 to 3) (i) altered chondrules or CAIs showing concentration of Ca-Fe silicates around these components and a more sparse distribution in the matrix away from them, (ii) micrometer-sized porous and polycrystalline nodules or patches scattered in the matrix, (iii) fine polycrystalline veinlets forming, in some cases, an imbricated network in the matrix or in dark inclusions, (iv) larger delineated area (veins or dark inclusions) showing radial or lateral mineralogical zoning, (v) polycrystalline rims around preexisting Mg-rich chondrule olivine grains scattered in the matrix, and (vi) incrustations in larger cavities (pores) with nearly euhedral minerals forming negative crystals. When data are available, matrix abundance and bulk porosity of carbonaceous chondrites are highly variable [between 1% porosity in Efremovka and 25 to 30% for Allende and Mokoia; see (10)].
Synopsis of the occurrence and composition of these secondary phases may be found elsewhere [e.g., (6–8, 10–14)] and is not reiterated here. Despite this extensive work, the environment and conditions of their formation remain however controversial. Palme and Wark (15) were among the first to favor a nebular origin for several secondary features (e.g., Fe-alkali-halogen metasomatism of CAIs and formation of fayalitic olivine rims around forsteritic grains). Observed intergrowths of nearly pure hedenbergite and diopside-hedenbergite and the presence of andradite have also been assigned to a high-temperature (≈1000°C) nebular origin (16). Although these minerals assemblages are clearly restrictive in term of temperature, their proposed nebular formation has progressively fallen into disuse. From textural and mineralogical observations and chemical analyses, Bischoff (17) found evidence for these secondary phases to have originated from preaccretionary aqueous alteration processes that occurred in small precursor planetesimals before formation of the final parent bodies, an intermediate model between gas-solid interaction in the nebula and processes that occurred in the parent body. The debate was stirred up again on the basis of the thermodynamic modeling of mineral equilibria and the oxygen isotopic composition when it was suggested that secondary phases (i.e., Ca-Fe-rich pyroxenes, andradite, fayalite, phyllosilicates, and magnetite) resulted from low temperatures (<300°C) aqueous fluid-rock interaction in an asteroidal setting (3, 13), in which reduced and oxidized CV chondrites witness different physicochemical conditions. This low-temperature model has been challenged recently (8) by showing that Ca-Fe–rich phases in reduced and oxidized CV have formed at higher temperature (210° to 610°C, and possibly higher) and in similar reduced conditions near the iron-magnetite redox buffer under low aSiO2 [log(aSiO2) < −1]. In this model, the various CV lithologies, which may be in part applied to CO chondrites, are inferred to be fragments of an asteroid percolated heterogeneously via porous flow of hot and reduced hydrothermal fluids. To sum up, although there is a general agreement that parent body alteration is a fundamental process in the evolution and the formation of secondary phases of CV and CO carbonaceous chondrites, there is, however, no consensus neither on the composition and the nature of the fluids involved nor if we should consider one or several generations of parent bodies (18).
Kudryavy volcano consists of a small cone located in the northern end of Iturup island in the Kurile volcanic arc. The last large eruption occurred in 1883 and produced basaltic andesite flows, but small-scale phreatic eruption occurred more recently in 1999 (19). The Kudryavy basaltic andesite flows are composed of rare olivine, orthopyroxene (hyperstene), clinopyroxene (augite), plagioclase (labradorite to bytownite), and titanomagnetite (20) with porphyritic textures with fine-grained microlithic groundmass (Figs. 1 and 2), its chemical composition being typical of the calc-alcaline serie. Kudryavy volcano probably has a several-decade period of sustained high-temperature fumarolic activity (up to 920° to 940°C measured in 1992) and degassing of magma, which have been, at least for the last 30 years, regularly sampled for gases and condensates (20–23).
Its gas chemistry is typical of high-temperature subduction zone volcanoes with relative uniform H2O/CO2 ratios between 40 ± 10 and 70 ± 15, H2/H2O in the range of 10−2 between the fayalite-magnetite-quartz (FMQ) and nickel–nickel oxide (NNO) mineral buffer curves, CO2/S (or C/S) and S/Cl ratios of 1 ± 0.3 and 4 ± 1, respectively, and HCl concentration amounting to 0.5 to 1 mole percent (mol %) (21, 22).
Despite the complexity of the fumarolic environments, it has been shown that three different types of the alteration of the Kudryavy fumarolic rocks prevail when (i) volcanic gas directly reacts with the rock at high temperatures (900° to 500°C), forming a first generation of incrustations, (ii) oxidized volcanic gas, resulting from mixing with the atmosphere, directly reacts with the rocks at intermediate temperatures (500° to 300°C), and (iii) acidic leaching of the rocks by highly contaminated meteoric water remobilizes previously formed sublimates and/or incrustation deposits together with the precipitation of secondary incrustations (20, 21, 24).
Focusing only on the higher-temperature interactions where no atmosphere mixing exists (as attested by magmatic redox conditions close to the FMQ buffer and oxygen isotopic compositions typical of magmatic vapors associated with subduction-related arc volcanism), the mineralogy of incrustations in representative fumarolic rock samples (20, 21, 23) is strikingly similar to the aforementioned secondary phases of CV and CO chondrites. It includes andradite, diopside-ferrosalite, hedenbergite, Fe oxides, hercynite, wollastonite, cristobalite, and tridymite, here replacing the primary magmatic minerals. Diopside/salite-hedenbergite-andradite associations are found to overwhelmingly dominate the mineralogy of Kudryavy incrustations in the same manner as Ca-Fe–rich secondary phases do in CV and CO chondrites. Similarly, they are intimately associated with assemblages of Na-Al-Cl–rich phases, i.e., albite, nepheline, and sodalite, with the notable addition of davyne, another tectosilicate containing both calcium sulfate and sodium chloride. If silica polymorphs and numerous sulfides are present as secondary phases, the absence of fayalite, kirschsteinite, larnite, metal, and phyllosilicates in the Kudryavy incrustions are, however, noticeable. Last, veins, altered phenocrysts with dissolution pits, and occurrence of numerous cavities in the microlithic groundmass filled with euhedral or layers of secondary phases (Fig. 2) indicate proactive dissolution/precipitation process.
Analogy between CV chondrites parent body hydrothermal activity and Kudryavy high-temperature fumarolic environments
Incrustations from Kudryavy fumarolic rocks have noteworthy similarities with the secondary phases of CV and CO chondrites not only in terms of their mineralogy and chemistry but also in terms of the intensive parameters controlling these several assemblages.
Mineral chemistry. Mineral chemistry of secondary phases is unexpectedly similar in these two different settings (Fig. 4). Ca-Fe pyroxene, wollastonite, andradite, and feldspathoid, analyzed in CV and CO chondrites (3, 13, 25–27), all show the same ranges of chemical variations as those observed in the Kudryavy fumarolic rocks. Ca-rich pyroxenes varying from diopside to hedenbergite compositions in Kudryavy incrustations encompass the same range of compositions as the one observed in CV and CO carbonaceous chondrites. Mineralogical zoning in these pyroxenes is also similar, with a hedenbergite core surrounded at crystal edge by complex and irregular oscillatory zoning of iron-depleted pyroxene. Concentrations of Na2O and Al2O3 in these secondary pyroxenes are also relatively high in both settings. Ca-Fe garnets in both type of environments show also same range of variations from almost pure andradite to grossular-rich compositions (20, 28) with sometimes occurrences of oscillatory zoning depending on their growth location (Fig. 3). In both settings, wollastonite and andradite are intimately associated. Last, Na-Al-silica–poor phase compositions: nepheline and sodalite (Fig. 3), are also matching in the two sites, with similar enrichments in iron (as NaFeSiO4 end member) in both phases and potassium depletions (Na/Katomic ratio > 3) in nepheline (table S1).
Texture. Both settings are characterized by heterogeneous textures of alteration with evidence for metasomatism (secondary phases) generally restricted to distances of a few hundred micrometers. Ca-Fe–rich silicates, Fe oxides, sulfides, Na-Al-Cl–rich feldspathoid phases in both carbonaceous chondrites (6–8, 13, 27, 29), and Kudryavy fumarolic rocks (20, 23, 30) are intimately associated under similar types of texture (Figs. 1 and 2): (i) overgrowth or pseudomorph on preexisting minerals, replacement of glassy or crystallized preexisting phases in matrix, chondrule mesostases, and refractory inclusions or in microlithic groundmass and phenocrysts; (ii) micrometer-sized porous polycrystalline patches dispersed in the fine-grained matrix; (iii) fine veinlets forming imbricated network in the matrix or inclusions; and (iv) larger delineated area (veins or dark inclusions) showing radial or lateral mineralogical zoning. These textural similarities are even more convincing if euhedral zoned crystals of andradite (or hedenbergite) growing in cavities of the volcanic rock or meteorites are considered. Euhedral andradites or hedenbergites often protrude from cavity walls or form negative crystals in micrometer-sized porous polycrystalline patches dispersed in the “unaltered” fine-grained matrix (Figs. 1 and 2) (31).
Intensive parameters. Beyond these similarities, the most convincing features supporting this analogy rely very likely on the fact that the same set of intensive parameters (i.e., aSiO2-fO2-T) controls these several assemblages in both these terrestrial and extraterrestrial environments. Among these, the andradite-hedenbergite assemblage ± wollastonite ± magnetite ± sulfides is of particular interest. From experimental and thermodynamic analyses in the system Ca-Fe-Si-O, Gustafson (32) has shown that andradite and hedenbergite are stable over a range of log fO2 and T at low total pressure. Ganino and Libourel (8) recently complemented this view by establishing that the silica activity of the system is another key parameter controlling the stability field of andradite relative to hedenbergite. If andradite is stable at high temperature in oxidizing conditions at or close to silica saturation, it noticeably expands its stability field toward reducing conditions at low temperature in low-silica-activity environments, andradite being costable with iron and magnetite (8). That is the case for CV and CO chondrites, in which andradite and hedenbergite assemblages are often associated with magnetite ± Fe-Ni alloy [e.g., awaruite; (3)] and sulfide in the presence of reduced carbon-rich material. From the thermodynamic parametrization of these associations (8), Ca-Fe–rich assemblages in CV and CO chondrites are inferred to have formed in reduced conditions near the iron-magnetite redox buffer at low aSiO2 [log(aSiO2) << −1] and moderate temperature (210° to 610°C). In addition to the cocrystallization of Na-Al-Cl–rich feldspathoids (nepheline and sodalite), which are good tracers of silica undersaturated environments in the parent body of CV and CO chondrites, very low silica activity conditions [i.e., log(aSiO2) < −2; see (33)] are also attested by the occurrence of rankinite, larnite, and wollastonite in the case of CV chondrites (8).
Using the same thermodynamic framework (Fig. 5), andradite-hedenbergite assemblages of Kudryavy fumarolic rock are reproduced at high temperature (T > 650°C) and at intermediate silica undersaturation −1 < log(aSiO2) ≤ 0 and redox conditions close to the FMQ buffer, i.e., −17 < log fO2 < −12. These inferred conditions are consistent with fumarole gas measurements and estimates (21, 22), indicative of elevated temperature of crystallization circa 900°C in more oxidizing conditions close to the FMQ buffer curve with log(fH2/fH2O) values in the range of −2.0 to −2.5 that lie between the FMQ buffer and the NNO buffer (21). The mild silica undersaturation reigning in the Kudryavy fumarolic system by comparison with the CV/CO environments is also consistent with a unique T-aSiO2 trend (Fig. 5) close to the nepheline/albite silica activity buffer curve, as indicated by the occurrence of the nepheline-albite-sodalite-davyne assemblages.