A Biotite Isograd in South-Central Maine, U.S.A.: Mineral Reactions, Fluid Transfer, and Heat Transfer

The biotite isograd in pelitic schists of the Waterville Formationinvolved reaction of muscovite + ankerite + rutile + pyrite+graphite + siderite or calcite to form biotite + plagioclase+ ilmenite. There was no single reaction in all pelites; eachrock experienced a unique reaction depending on the mineralogyand proportions of minerals in the chlorite-zone equivalentfrom which it evolved. Quartz, chlorite, and pyrrhotite werereactants in some rocks and products in others. All inferredbiotite-forming reactions involved decarbonation and desulfidation;some were dehydration reactions and others were hydration reactions.P-T conditions at the biotite isograd were near 3500 bars and400 °C. C-O-H-S fluids in equilibrium with the pelitic rockswere close to binary CO₂-H₂O mixtures with X_CO2 = 0.02–0.04.During the biotite-forming reaction, pelitic rocks (a) decreasedby 2–5 percent in volume, (b) performed – (4–11)kcal/liter P-V work on their surroundings, (c) absorbed 38–85kcal/liter heat from their surroundings, and (d) were infiltratedby at least 0.9–2.2 rock volumes H₂O fluid. The biotite isograd sharply marks the limit of a decarbonationfront that passed through the terrane during regional metamorphism.Decarbonation converted meta-shales with 6–10 per centcarbonate to carbonate-free pelitic schists. One essential causeof the decarbonation event was pervasive infiltration of theterrane by at least 1–2 rock volumes H₂O fluid early inthe metamorphic event under P-T conditions of the biotite isograd.Average shale contains 4–13 per cent siderite, ankerite,and/or calcite, but average pelitic schist is devoid of carbonateminerals. If the Waterville Formation serves as a general modelfor the metamorphism of pelitic rocks, it is likely that worldwidemany pelitic schists developed by decarbonation of shale caused,in part, by pervasive infiltration of metamorphic terranes byseveral rock volumes of aqueous fluid during an early stageof the metamorphic event.     

Reply: Middle Pleistocene sedimentation at Pakefield,Suffolk, England: a response to Lee et al. (2006) P. L. Gibbard,A. Moscariello,H. W. Bailey,S. Boreham,C. Koch,A. R. Lord,J. E. Whittaker,C. A. Whiteman

Journal of Quaternary Science Published online 9 August 2007. DOI: 10.1002/jqs.1118 The above paper was incorrectly titled. The correct title should be: Comment: Middle Pleistocene sedimentation at Pakefield, Suffolk, England J. R. LEE, J. ROSE, I. CANDY and R. W. BARENDREGT (2006) Journal of Quaternary Science 21 : 155–179 The error will be corrected prior to print publication.     

Glaciers and Glaciation by Benn,D. I. & Evans,D. J. A.

2010: Glaciers and Glaciation. Second edition. 802 pp. Hodder Education, London. ISBN 978‐0‐340‐905791. Paperback, Full Colour, £45.00