Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Olivier Bachmann; Michael A. Dungan

doi:10.2138/am-2002-8-903

Abstract

An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 μm steps) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P-T-f_H₂O evolution of the magma, and (2) a means of evaluating recent calibrations of the Al-inhornblende barometer (Anderson and Smith 1995; hereafter AS1995) and thermometers (Blundy and Holland 1990; thermometers A and B of Holland and Blundy 1994; hereafter BH1990, HB1994TA, and HB1994TB).

Hornblende phenocrysts are variable for most major elements (e.g., 5-9 wt% Al₂O₃ and 44-50 wt% SiO₂). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [^TSi + ^A■ = ^TAl + ^A(Na + K)] and another 25-30% is the consequence of a Ti- Tschermak exchange (^TSi + ^M1-M3 Mn = ^TAl + ^M1-M3Ti). In contrast, the pressure-sensitive Al-Tschermak substitution (^TSi + ^M1-M3 Mg = ^TAl + ^M1-M3Al) did not play a significant role, as ^M1-M3Al does not correlate with ^TAl and is always <0.2 apfu.

In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein Al₂O₃ contents are within analytical error (5.95 to 6.05 wt% Al₂O₃, N = 78 and 7.7 to 7.8 wt% Al₂O₃, N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 ± 0.05 for the high-Al population at 760 °C, which is indistinguishable from the 2.4 ± 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population (<1 kbar at 760 °C). The three thermometric formulations give reasonable results between 706 and 814 °C, but the HB1994TA calibration gives a mean temperature higher by ~50 °C and is more sensitive to small analytical differences (~100 °C spread for each population). HB1994TB is considered the most reliable calibration of the Al-in-hornblende thermometer as it most precisely reproduces T estimates determined by independent methods.

Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in ^TAl, A(Na + K), and ^M1-M3Ti, compensated by decreases in ^TSi, and ^M1-M3Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715 °C, which is ~35-45 °C above the water-saturated granite solidus at 2-2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760 °C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40 °C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-SiO₂, low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Abstract

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