JAMES B. THOMPSON JR.’S MODAL SPACE APPLIED TO SERPENTINIZATION AND CARBONATIZATION OF ULTRAMAFICS, STOCKBRIDGE, VERMONT

The reaction history of ultramafic rocks in Stockbridge, VT is understood in the framework of J.B. Thompson Jr.’s modal space. The ultramafics are within phyllite of the Ottauquechee Fm and are part of a ~1000 m long by ~600 m wide, N-S trending, lens-shaped tectonic mélange that is also composed of greenstone, mus-chl schist, gar schist and ab schist. N-S striking and steeply dipping Acadian foliations along the ultramafic margin bound tlc-mgs, tr-chl-chr and chl zones, which cross-cut a serpentinized Taconian core fabric of atg-tlc-dol-chr. Serpentine pseudomorphs observed in the outer core appear to have formed after di across the reaction di + tlc + 6mgs + 5H₂O = 3atg + dol + 4CO₂. Hydration and carbonatization of en or fo may have produced tlc and mgs initially by 3en + 3H₂O = atg + tlc, 6en + 2atg + 6CO₂ = 6mgs + 4tlc or 12fo + 2tlc + 18H₂O = 10atg. Taconian dehydration reactions in adjacent mafic rocks during subduction are viable mechanisms for triggering serpentinization.

The compositional variation of the five major phases observed, AMP-CHL-SRP-TLC-CARB, takes place within the 11-dimensional oxide space, CaO FeO MgO Al₂O₃ SiO₂ MnO Fe₂O₃ Cr₂O₃ TiO₂ H₂O CO_2, which is spanned by 11 independent exchange reactions between phase pairs. These five phases are expressed in terms of their additive components tr-cl-atg-tlc-mgs, respectively. In a closed system, there are 23 independently variable phase components ∑C_{φ ­}among the five solid phases; all 11 oxide components C_s vary independently. The total number of independent reactions N_h = ∑C_φ – C_s (Thompson et al., 1982) in the closed system is 12, eleven pure exchange reactions and one net-transfer reaction, (α) cl = 2atg + Al₂Mg_-1Si_-1 (TK). When the system is opened to H₂O and CO₂ two additional independent net-transfer reactions are derived: (β) 3tr + chl + 6H₂O = 4atg + 5tlc + TK + 6CaMg_-1 (CM) and (γ) 3tr + chl + 6CO₂ = 6mgs + 7tlc + TK + 6CM. These reactions form three orthogonal basis vectors that span the subspace CMASHC. Advancement along β and γ describes the reaction progress after all initial material is consumed, and thus, applies most appropriately to the development of the marginal rocks during the Acadian. Schreinemakers analysis with PERPLEX (Connolly, 1990 database) indicates that the tr-chl-chr zone formed at ~500° C and a maximum of 7 Kb.