EXCESS ARGON IN MELT INCLUSIONS IN ZERO-AGE ANORTHOCLASE FELDSPAR FROM MT. EREBUS, ANTARCTICA, AS REVEALED BY THE AR-40 AR-39 METHOD/

Historically erupted (1984) anorthoclase phenocrysts from Mt. Erebus y ield K/Ar and Ar-40/Ar-39 apparent ages as old as 700 ka indicating th e presence of excess argon. Ar-40/Ar-39 furnace step heating results f rom anorthoclase reveal a positive correlation between the Cl/K ratio and apparent age. Because chlorine (up to 1700 ppm) is present in melt inclusions but not in the anorthoclase crystal lattice, this correlat ion suggests that the excess argon is associated with melt inclusions trapped within the anorthoclase during rapid crystal growth. Confirmat ion of the source of excess argon comes from step-heating experiments on multiple anorthoclase aliquots separated from two phenocrysts and o ne glass aliquot prepared from the matrix of a volcanic bomb. The anor thoclase phenocrysts were crushed and HF etched to yield aliquots cont aining 30%, 10%, and 1% melt inclusions. The step-heated anorthoclase with 30% and 10% melt inclusions yielded the highest Cl/K ratios and a pparent integrated ages of 640+/-30 ka and 179+/-16 ka, respectively. The anorthoclase with 1% melt inclusions yielded significantly lower C l/K ratios and apparent integrated ages of 48+/-8 ka and 50+/-30 ka, T he step-heated volcanic glass yielded the least variable Cl/K ratios a nd a total gas age of 101+/-16 ka. Argon released from the anorthoclas e and the trapped melt inclusions can be distinguished by differences in their degassing behavior, allowing geologically more reasonable age s to be obtained. Melt inclusions exposed on the exterior of anorthocl ase grains principally degas during furnace extraction at temperatures less than 1200 degrees C. Inclusions entirely within anorthoclase gra ins principally degas at temperatures greater than 1200 degrees C when incongruent melting of the anorthoclase allows melt inclusion hosted excess argon to escape. Anorthoclase aliquots prepared with less than 1% inclusions can be fitted with a plateau for heating steps below 120 0 degrees C to yield ages as young as 8+/-2 ka, whereas steps above 12 00 degrees C yield ages in excess of 100 ka. However, anorthoclase ali quots containing 10-30% melt inclusions yield ages in excess of 200 ka for heating steps below 1200 degrees C. Minimizing the effects of the excess argon from melt inclusions relies on sample preparation and st ep-heating. Fine crushing and treatment with hydrofluoric acid removes many of the larger melt inclusions. Small melt inclusions which remai n within the anorthoclase degas primarily at temperatures above 1200 d egrees C. Temperatures below 1200 degrees C yield the most accurate ag es. Attempts at post-analytically correcting for the chlorine-correlat ed excess argon are hindered by the variations in Ar-40(E)/Cl within a nd between samples. Elevated Ar-40(E)/Cl ratios in bubbles within the melt inclusions, as deduced from in vacuo crushing experiments, are th e most likely cause for some or all of a sample's total Ar-40(E)/Cl va riation. In addition, relative solubilities of argon and chlorine with in phonolitic melts may be partly responsible for variations in Ar-40( E)/Cl. Copyright (C) 1997 Elsevier Science Ltd.