Modelos de alteración de cromititas ofiolíticas durante el(...).pdf

Modelos de alteración de cromititas ofiolíticas durante el metamorfismo

Colás Ginés, Vanessa
Fanlo González, Isabel (dir.) ; González Jiménez, José María (dir.)

Universidad de Zaragoza, 2015

Resumen: The composition of chromite is commonly used to interpret the petrogenesis and the geodynamic setting of its host ultramafic rocks. Its high resistance to alteration compared to the primary silicates has made this oxide particularly useful as a petrogenetic indicator in ultramafic rocks in which metamorphic alteration has obliterated other primary fingerprints. However, a great body of work, based on the study of chromitites from ophiolites, layered complexes, Alaskian-Ural complexes and komatiites, have shown that the chemistry and structure of chromite can be also significantly modified during both prograde and retrograde metamorphism. One of the main targets of the scientific community working on chromite in ultramafic host rocks is to know what processes may produce the alteration of chromite, and more importantly, what are the implications for the petrogenetic interpretations derived from them. Of particular interest is to develop a general alteration model to constrain the processes, mechanisms and conditions (temperature, pressure and chemical potential of the species) able to alter the chromite and to test if the alteration occurs during the prograde and/or retrograde metamorphism. In this work I will use a combination of petrological and geochemical tools to model the mechanisms that produce the alteration of chromite during metamorphism. A set of chromite-bearing ophiolite complex showing variable metamorphic pathway were selected to undertake this study. The chromitite samples were selected from chromite deposits of three ophiolite complexes: i) the Eastern Rhodope in Bulgaria (retrograde metamorphism from eclogite- to amphibolite-facies), ii) Tehuitzingo serpentinites in Mexico (retrograde metamorphism from eclogite- to greenschist-facies), and iii) Los Congos and Los Guanacos ultramafic massifs in Argentina (prograde metamorphism up to granulite-facies with amphibolite-facies overprint). Additionally, chromitite samples of unmetamorphosed ophiolites from Mercedita (Eastern Cuba) and Dyne (New Caledonia) were used as examples of unaltered chromites for comparison. Chromite in the metamorphosed chromitites from Eastern Rhodope and Tehuitzingo exhibits up to four textural types: i) partly altered chromite, with primary cores surrounded by porous chromite enriched in Cr2O3 and FeO and depleted in Al2O3 and MgO; ii) porous chromite, with a well-developed porosity and chlorite filling the pores; iii) zoned chromite, characterized by primary cores surrounded by non-porous chromite enriched in Fe2O3 (i.e. ferrian chromite), occasionally rimmed by magnetite; and iv) non-porous chromite, with polygonal mosaic-like texture. The different pattern of zoning are interpreted as a consequence of two-stage processes associated with the infiltration of reducing and SiO2-rich fluids which evolve to more oxidizing and SiO2-rich conditions. P-T-X diagrams performed for these chromitites predict that the first alteration stage took placed as a result of the reaction of primary chromite with highly reducing SiO2-rich fluids. This would result in the formation of porous chromite in equilibrium with chlorite. The theoretical model reproduces well the changes of Cr# and Mg# observed from primary to porous chromite in the natural samples. It suggest that porous chromite take place from ca. 700 to 450ºC in Eastern Rhodope while its occurs from ca. 700 to 250ºC in Tehuitzingo, during the retrograde metamorphism from eclogite- to amphibolite- and greenschist-facies, respectively. T-¿SiO2 pseudosection suggests that the addition of highly reducing SiO2-rich fluids was crucial for the alteration of chromite. I suggest that such kind of fluids could emanate from the surrounding peridotite during serpentinization when silica minerals (olivine and pyroxene) were hydrated and dissolved. The temperatures for the second stage were estimated using isotermal Al3+-Cr3+-Fe3+ sections and range between 450 to and 600ºC. These overlap the range of temperatures estimated for the first stage. This suggests that oxidizing fluids evolved from previous reducing fluids, stabilizing magnetite and form non-porous ferrian chromite by infiltration through the network of pores in the porous chromite. The analysis of a suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the chromite grains from Eastern Rhodope and Tehuitzingo using LA-ICP-MS in this work is revealed as a powerful tool to identify metamorphic fingerprints. Thus when the distribution of these elements in the altered chromite grains is compared with those from unmetamorphosed chromitites, I observed that partly altered chromites after primary high-Cr chromite are enriched in Zn, Co and Mn (ZCM-anomaly) but strongly depleted in Ga, Ni and Sc. This distribution of minor and trace elements is related to a decrease in Mg# [Mg/(Mg+Fe2+)] and Al2O3, produced by the crystallization of chlorite in the pores of porous chromite. Non-porous chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned chromites have cores depleted in Ni, (Ga) and Sc but are progressively enriched in Zn and Co as Mg# and Al2O3 decrease toward the rims. They have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian chromite. Magnetite rims of zoned chromite from Tehuitzingo are strongly depleted in all minor and trace elements, denoting their subsequently formation. The complex variation of the minor and trace elements vs Fe3+/(Fe3++Fe2+) in the different types of chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims. The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear ¿unaltered¿ in terms of major elements. In the samples from Los Congos and Los Guanacos (Argentina) there are different textural types of chromite: i) Type I homogeneous Fe2O3-rich chromite in contact with relicts of clinopyroxene; ii) Type II, with spinel grains hosting a complex intergrowth of blebs and very fine lamellae of magnetite; iii) Type III, with symplectic texture composed of variable proportion of magnetite and spinel. Isochemical phase diagrams (pseudosection) computed for chromitite samples from Los Guanacos predict that the formation of non-porous ferrian chromite take place as a result of the infiltration of more oxidizing fluids through porous chromite probably, during seafloor metamorphism (ca. 300ºC and 4 kbar). According to the calculated phase relations, prograde metamorphism produces the reaction of non-porous chromite with chlorite to form a homogeneous chromite with higher Al2O3, Fe2O3 and MgO at ca. 780ºC and 10kbar, which according to regional studies can be associated with the granulite-facies peak conditions. The blocking temperature obtained in Type III chromite from Los Guanacos (ca. 600ºC) suggests that the exsolution of non-porous chromite in Type II and Type III chromite grains, which have higher Fe2O3 and lower Cr2O3 than those from Type I take place during the amphibolite-facies metamorphism. Partition coefficient of major, minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the equilibrium pair spinel-magnetite in Type III chromite grains from Los Guanacos indicate that Ga3+, Co2+ and Zn2+ are compatible, while Ti4+, Ni2+, V3+, Sc3+ and, lesser extent Mn2+ are incompatible in spinel. I suggest that such distribution is the consequence of the affinity of these metal ions for an octahedral or tetrahedral coordination site in the spinel structure and, therefore, their preference to form normal or inverse spinel structure. The ideal behavior of minor and trace elements in the equilibrium pair spinel-magnetite explains why the formation of non-porous chromite rims, with inverse spinel structure, modified the composition of cores from zoned chromite, with normal spinel structure.