| Experimentalp hasee quilibriah ave been investigatedo n three medium-K silicic andesite (60-61 wt % SiO2) samples from Mount Pelhe at 2-4 kbar, 850-1040øC, under both vapor-saturatedC O2-freea nd vapor-saturatedC O2-bearingc onditions.M ost experiments were crystallizatione xperimentsu singd ry glassesp reparedf rom the naturalr ocks.B oth normal-a nd rapidq uenche xperimentsw ere performed.T wo rangeso f oxygenf ugacity( fo2) were investigatedN: NO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderatelyo xidizing conditionsp, lagioclase( pl) and magnetite( mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature( T)-H20 range( 875-950øC and 5-7 wt % H20 in melt). Amphibole (am) is stable undern ear vapor-saturatedC O2-freec onditionsa t 876øC. At 900øC, ilmenite (ilm) is found only in experimentsle sst hano r equalt o NNO. Upon increasingp ressure( P) underv aporsaturatedC O2-freec onditionsp, l + mt is replacedb y am + mt on the liquidusa bove3 .5 kbar. For highly oxidizingc onditionsm, t is the solel iquidusp hasea t 2 kbar, followed by pl and opx, except in the most H20-rich part of the diagram at 930øC, where opx is replaced by Carich pyroxene( cpx) and am. Compositionso f ferromagnesianp hasess ystematicallyc orrelate with changingfo2E xperimentalg lassesr angef rom andesitict hroughd acitict o rhyolitic, showings ystematicc ompositionavl ariationsw ith pl + opx + mt fractionation( increaseo f SiO2a ndK 20,d ecreasoef A1203C, aO,F eOt, andM gO).F eO*/MgOm oderateilnyc reases with increasingS iO2.F orfo2 conditionst ypical of calk-alkalinem agmatism( approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas,a nd this shoulds timulater eexaminationfo r the mechanismso f generationo f andesitesb y fractionationf rom basalticp arents. |
Export this paper
