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The Stability of Amphibole in Andesite and Basalt at High Pressuresr

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The Stability of Amphibole in Andesite and Basalt at High Pressuresr American Mineralogist, Volume 68, pages 307-314, 1983 The stability of amphibole in andesiteand basalt at high pressuresr J. C. Alt-BN Department of Geology and Geography Bucknell University Lewisburg, Pennsylvania 17837 exo A. L. Bonrrcsen Institute of Geophysics and Planetary Physics and Department of Earth and Space Sciences University of Califurnia, Los Angeles Los Angeles, California 90024 Abstract Some of our earlier work (Allen et al., 1975)on the stability of amphibolesin andesite and basalt at high pressuresis subject to criticism becauseofloss ofiron from the starting material to the walls of the capsules(AgsoPdso) during the runs of/6, bufered over the range of stability of magnetite.Analyses of fused run products show substantialloss of iron from runs at magnetite-wiistite and nickel-nickel oxide conditions, but none from those at magnetite-hematiteconditions. We have now redone our earlier work on the Mt. Hood andesiteand l92l Kilauea olivine tholeiite under N-NO conditions and with silver capsules.Analyses of fused run products show no iron loss, and the reversed curve representing the maximum stability of the amphibolesin the Mt. Hood andesiteshows no changein location, although we now have better control on the high-pressurepart of this curve. The revised curve for the appetrance of garnet is significantly lower in pressure, passing through 15.5 kbar/940'C and 14.5 kbar/9fi)'C. No orthopyroxene appearedin the run products, in contrast to the results of our earlier work. The high-temperature segment of the amphibole-out curve for the tholeiite is at least as high as 1040'Cat 13kbar and 1050"Cat 16kbar, and the high-pressure part of this curve is at about 27 kbars, about 6 kbar higher than in our earlier work. Amphibole is the sole silicate phase on the vapor-saturatedliquidus in the andesite at pressuresbelow the garnet-in curve (<15.5 kbar), and it is accompaniedby a minor amount of an Fe-rich oxide. The revised phase relationships are consistent with the derivation of andesite by amphibole-liquid equilibria in basaltic magma, and they reveal that the amphibolesare stableto depths of nearly 100km. The great depthsto which amphibolesare stable make it all the more probable that they are a source of H2O for partial melting in deeply subductedoceanic crust. Introduction FezOr (M-H), Ni-NiO (N-NO), and FerOrFeO (M-W) from l0 to 36 kbar. Later, we (Allen and Boettcher, 1978) To understandthe role of amphibolesin the genesisof extended that investigation to lower and more realistic andesites,e.g.,by the fractionation of amphiboles from values of by using H2MO2 vapors and made all hydrous basaltic magmasin orogenic zones, we (Allen el /sr6 runs at an;fs, approximately that of M-H. Our earlier al., 1975) experimentally established the stabilities of work (1975), conducted in the presence of nearly pure amphiboles in a Mt. Hood andesite, a l92l Kilauea H2O vapor, was subject to criticism (Stern and Wyllie, olivine tholeiite, two other basalts,and an olivine nephe- 1975)because of probable loss of iron from the starting linite in the presenceof nearly pure H2O vapor at values material to the walls of the capsules (AgsoPdso).As a of oxygen fugacity (fo,) approximately those of Fe3Oa- result, we determined the amount of iron lost in our earlier experiments by analyzing the glass (quenched I Institute of Geophysicsand Planetary Physics Contribution liquid) with an electron microprobe analyzer.The results No. 2297. indicated almost total loss of iron from those experiments m03-004x/83/0304-0307$02.00 307 308 ALLEN AND BOETTCHER: AMPHIBOLE IN ANDESITE AND BASALT AT HIGH PRESSURE Table l. Experimental run data silver does not absorb iron from starting materials. The experimentswere accomplishedwith excessH2O and at P, kbar T,'c tur a Dura- PlEee ass@bla8e* an/o, approximately that of N-NO. The sametechniques hout a describedin our earlier papers(1975 and 1978)were used to analyzethe run products. No loss of iron was detected. Ande6ite All experimentsused a piston-cylinder apparatussimilar IO 940 A 609 3.0 L, Ao,0p, QU 10 960 A 614 L,oP,QM to that described by Allen et al. (1975) and the same 13 940 A 561 L, Io, Op,qM 960 A 564 L,QM furnace assembly and starting materials described by IJ 980 A 563 L,QM Allen (1978).To reversethe amphibole-out T4 900 A 611 24.0 L, Ao, Op, QU and Boettcher 900 A 608 24.2 L,AE, Gs, Op,er, QM curves, two-stageruns were made in which temperatures 940 A 593 24.0 L, An, Op,QU 900 A 607 24.0 L, An, c€,Op,Ru,QM were initially held above and then lowered into the 16 940 A 555 )-) L, An, @, Op, qM 940 L 567 ),) L, .AD,Ga , Op , Qil stability field of amphibole, as tabulated in Table l. The t7 960 A 603 3.0 L, op, Qlt phases 18 900 L 622 100.0 L, Am,C€, Cpx, Op,hr, QIt were identified by the techniques described by 18 940 A 60r 24.0 L,Ao,Ga,Op,QM Allen er al. (1975,p. 1072). 19 900 A 62r 99.0 L,Ga,Cpr,Op,R,QM 19 940 A 598 24.O L, Ca, Op, QIt 20 940 A 596 24.0 L, G6,0p, qlt 20 980 L 32 24,0 L, ee, QM Results 20 1000 A 33 24.O L, QU 23 900 a 617 24.0 I,V,Ga,Ru,Op,qU 23 1020 A 28 8.0 Lrca,QU Phase relationships 23 1040 a,29 8.0 L, QM 25 1040 A 30 8.0 L, Ga,QM The phaserelationships for the Mt. Hood andesiteare 1060 A 3I 8.0 L, Ga, Qlt 25 1070 A 34 8.0 L,QU given in Table I and in Figure 1. A comparison of these **Reversals 965 A 600 6.3 (see run #A 564) 13 935 16.6 L, Ae, Op, Qlt MT.HOOD ANDESITE 900 612 24.0 (see tun #A 608) 13.5 900 24.5 L,An,op, (ca),Qu 20 940 623 24.0 (see run #A 596) 900 100,0 L,Ao, ca, Cpx, Op,Qlt Baealt I 13 1020 566 L,Cpx,dm,Op,QU 25 1030 512 5.5 L, Cp(,Ad,0p,QM Ga+[ 16 1040 ).) L,Cpx,AD,0p,QM 22 1040 24.O L,Cpx,An,Op,QU 1040 597 24.O L, Cpx, An, cs,Op, qM 23 r070 619 L, Cpr,!D, ce, Op,QM 25 1040 602 24.O L, Cpx, AD,ca, Op,qM 26 1020 2015 24.O L, Cpx,Ar,Ca,Ru,Op, QU 27 1045 20t6 24.O L,Cpx,Ga,0p,QU €L 28 1020 ttg4 24.O L, Cpx, Ca,Op, &r, qU -o 20 28 r060 tr90 24,O L, Cpx, Ga,Op, QM **RweraaLa u e, r020 R 2034 2o.o (see En #B 1194) 26 LO20 24.0 L,Cfn,AD,ca,Op,qM U e, Abb"eoiatime: hn = @nphibole; Cpc = cli.tupVrorere; Ga : gwt; 7, - g1aes inte?Weted to be qrerched Liquil.; M = tiweow mLrercL; op = opaque ninetal; Ru = rutile; Q = inte?pteted to lw|e c"Us- talli.aed dtrlng the quach; ( ) = twee mwt t5 * ALL avs@blageo i.rcLude uapor f ktn wbere preeeded by A anployed. andeeite @ a st@bing tute"iaLj tToee preeeded bg B used. baeal.t. ** See teet for deeeLptin of wereal proce&ze Am+Op+L at M-W conditions, significant loss at N-NO conditions, but no observable loss at M-H conditions. Thus, the ,t0 phase relationships reported in our recent investigation (Allen and Boettcher, 1978)are satisfactoryfor they were 900 ,t000 1100 done at a sufficientlyhigh/er, i.e.,M-H, but that part of TEMPERATURE('C) our earlier work (1975)done at anl6, lower than approxi- mately M-H was in need of reinvestigation. Fig. l. Crystallization sequencefor Mt. Hood andesite. All assemblages coexist with vapor. Abbreviations: Am = : : = Experimental methods amphibole; 6a garnet; Op opaque mineral; Cpx clinopyroxene;Ru = rutile; L = liquid. E Above liquidus;Z Op To preventloss of iron, we haveused silver capsulesin + L; lGa +!;OAm + Op + L;OAm + Ga* Op + L. OGa our reinvestigationof the stability of amphibolesin a Mt. + Op + L; OAm -f Ga * Op + Ru + L; AAm + Ga + Op Hood andesite and a 1921 Kilauea olivine tholeiite, for +Cpx + Ru + L; AGa + Op + Cpx + Ru + L. ALLEN AND BOETTCHER: AMPHIBOLE IN ANDESITE AND BASALT AT HIGH PRESSURE 3(D I92I KILAUEAOLIVINE THOLEIITE was found in this study with Ag capsules,in contrast to our earlier work. The phaserelationships determined in our reinvestiga- tion of the l92l Kilauea olivine tholeiite are given in Table I and in Figure 2. The melting of silver prevented a A complete determination of the melting relationships in this system; however, the high-temperaturepart of the 25 o amphibole-outcurve for the tholeiite is at least as high as Cpx+Am+Ga+OP+L l(X0'C at 13 kbar and 1050"C at 16 kbar. The actual oo location of this part of the amphibole-outcurve is proba- bly not significantly higher, if at all, than the position oL .o determined in our earlier study using AgsoPdsocapsules, J for, in that study, there was only a small amount of U e, Cpx+Am+Op+ L amphibole in the products from the runs at 1030'Cand 13 kbar, and at 1040"Cand 16 kbar. The high-pressurepart ttt 4 of the amphibole-outcurve probably has a relatively flat dPldT slope; amphibole-bearingassemblages start con- verting to garnet-bearing assemblagesat a pressure of o about 6 kbar higher than in our earlier study.
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