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Fluid flow patterns and infiltration isograds in melilite marbles from the Bufa del Diente contact metamorphic aureole, north‐east Mexico
Author(s) -
HEINRICH W.,
GOTTSCHALK M.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00028.x
Subject(s) - geology , metamorphic rock , geochemistry , titanite , phlogopite , hornfels , mineralogy , mantle (geology) , biotite , paleontology , quartz
In the inner aureole of the Bufa del Diente alkali syenite (north‐east Mexico), thin calcareous argillite bands horizontally embedded in impure marbles acted as contact‐metamorphic aquifers for hypersaline brines of magmatic origin. Thick‐bedded marbles were largely impervious. From 180 m up to the intrusion contact, argillites were completely decarbonated, resulting in melilite + wollastonite + phlogopite + perovskite‐bearing parageneses. In marbles, this assemblage is confined to a narrow 7‐12‐m‐wide infiltration zone adjacent to the contact. Up to this distance, calcite + wollastonite + diopside + alkali feldspar + titanite was stable, indicating that the fluid evolution in these marbles was internally buffered. Brine infiltration from the metaargillite aquifer into the marbles occurred perpendicular to the marble‐metaargillite boundaries and was confined to a zone 4‐6 cm wide above the boundaries. This is documented by the three reactions Cc + Di = Mel + CO 2 , (1) Cc + Kfs + Di + H 2 O = Phl + Wo + CO 2 , (2) Cc + Ttn = Prv + Wo + CO 2 , (3) Melilites (Ak 32‐45 Gh 13‐32 Sm 32‐40 to Ak 52‐72 Gh 0‐1 Sm 28‐48 ) occur as rims around diopsides and become continuously thicker towards the metaargillite beds. Fluid inclusion observations suggest that the infiltrating brine was hypersaline (NaCl + KCl cq ∼ 65 wt%) and that the reactions took place at the water‐rich side of the H 2 O‐CO 2 ‐salts immiscibility field at about 600d̀ C (2, 3) and 660 to 680d̀ C (1) at P ∼ 1200 bar and X co2 ∼ 0.02. Mass balance calculations show that the amount of brine infiltrated from the aquifer into the marble was very low and decreased continuously with increasing distance from the boundary. The maximum width of brine infiltration was about 6 cm. This confirms that brine flow was largely parallel to the aquifer, not perpendicular to it. The CO 2 produced by the decarbonation reactions probably escaped as an immiscible low‐density H 2 O‐CO 2 fluid of X co2 ≤ 0.5 into overlying marble via grain‐edge flow. The metaargillite‐marble boundary acted as a semipermeable membrane 6 cm in thickness keeping back the brine in the aquifer and losing the in‐situ produced low‐density CO 2 ‐rich fluid.