American Mineralogist, Volume 60, pages 1069-1085, 1975

Amphibolesin Andesiteand : I. Stability as a Function of P-T-fo,

J. C. Alt-nN, Departmentof Geologyand Geography,Bucknell Uniuersity Lewisburg,Pennsyluania 17837

"i,Tl De p a r t m e n t oyc o,,i):,,!, ) ;, uq n ia s ta t e uniu e rs i ty Uniuersitv" Park. Pennsvluania16802

nNn G. M.c,nr-A.No

Department of Geography and Geology, Indiana State Uniuersity Terre Haute. Indiana 47809

Abstract

The stabilitiesof amphiboleshave been determined in an ,three , and an olivinenephelinite in thepresence of HrO vaporat valuesof oxygenfugacity approximating thoseof FerOo-FerOs,Ni-NiO, and FerOr-FeO.The thermalstability decreases with increas- ing activityof silicain the parentrocks. Maximum thermal stability of amphiboleoccurs at 1090"Cat l3 kbar in the Hualalaialkali basalt.Maximum pressure stability occurs at 31.5 kbar at a temperatureof 1030'Cin the olivinenephelinite. Amphibole-bearing assemblages convertto -bearingassemblages at pressuresabove l8 to 30 kbar. The amphiboles straddle the calciferous-subcalciferousboundary, and all of them fall in the pargasite- tschermakite-tschermakitichornblende category. No orthopyroxenewas found in any of the basalticcompositions under any of the conditionsinvestigated, although it did occurin the andesite.This restrictsthe potentialP-T-X conditionsunder which fractionalcrystallization of orthopyroxeneis an important mechanismin the derivationof SiO2-undersaturated . Our resultsare consistentwith thosehypotheses in which someandesitic magmas derive from basalticcompositions through amphibole-liquid equilibria. Fractionation of alkalisby amphibolesmay contribute to theproposed gradient in KrO acrosssubduction zones at island arcs and continentalmargins. The shallowdips and depthsof subductingslabs beneath orogenic-zonevolcanoes is in concertwith this model.

Introduction igneousrocks may increasecontinentward from the Knowledge of the P-7 conditions under which oceans, and the fractionation of alkali-bearing hydrous minerals, such as amphiboles,exist is fun- minerals, such as amphibole, at depths may play a damentalto our understandingof melting and related role in this postulated trend. Fractionation of such processesin the crust-mantle system. As is well amphiboles from a basaltic may give rise to known, water dissolvesin silicateliquids under pres- andesiticmagmas as proposedby Bowen (1928) and sure and lowers the temperatureof the beginningof others. melting. lf all of the water is bound in amphibolesor To solve these problems, we experimentally es- other hydrous phases,the chemicalpotential of water tablished the stability of amphibolesin an andesite, would be less than if the water were present as a three basalts, and an nephelinite in the nearly pure aqueous fluid, and temperaturesof the presenceof HrO vapor at values of oxygen fugacity beginning of melting would be higher (Burnham, ffi,) approximating those of FerOa-Fe2Os,Ni-NiO, t967\. and FerO.-FeO, from l0 to 36 kbar, determining the In addition, the KrO contents and KrO/NarO of pressuresat which amphibole-bearingassemblages 1070 ALLEN, BOETTCHER, AND MARLAND

transform to garnet-bearing assemblages,i.e., al Tnsle la. Chemical Compositions of Rocks depths(<75-100 km) in the Earth. Used in the Experiments

Andesite l)tattz Ollvine AIkaIl Nephe-Linite Experimental Methods tholeiite tholeiite basalt si02 59.10 50.71 49.71 46.01 38.57 Starting Materials Ti02 o.94 r.70 2.5I 2.I0 2.79 Starting mixes were prepared from a Mt. Hood A1^0^ 17.8 L4.48 L2.14 13.89 11.71 andesite,a -normativePicture Gorge tholeiite, cr" 0^ 0. 065 0. 06 a l92l Kilauea olivine tholeiite, a - Ft2o3 1.78 4,89 3.23 4,29 5,2r FeO 4.83 9.07 8.40 8.96 7.18 normative prehistoric Hualalai , and a Mn0 0.10 0.22 0.r7 0.r9 0.11 Honolulu olivine nephelinite(Table I ). These rocks .ntg0 3.05 4.68 10.31 10.01 13.08 were chosenbecause they encompassa wide range of Ca0 6.85 8.83 10.73 10.36 r2,A4 SiO, and alkali concentrations, and experimental Sr0 0.05 work had previously beenaccomplished on some of Ba0 0. 03 0, 08 theserocks at other conditions (Eggler, 1970,1973; Li,20 < .01 Hill and Boettcher,1970; Holloway, 1970,1971; Hol- Na20 4,27 3.16 r.97 2.59 4.22 loway and Burnham, 1969, 1972;Yoder and Tilley, *zo 1. 08 0.77 0.49 0,75 r.20 Rb20 0.00186 1962;Fudali, 1965;Tuthill, 1968).All sampleswere Hzo* 0.1 I 0.07 0.17 0.59 crushed to -200 mesh under acetone, dried in an lr.o+ 0.00 0.00 0.19 oven at I l0'C, "20- I and stored in sealedvials over KOH P^0- o.22 0.36 o,21 0.33 1.11 in a dessicator. 0.05 0.27 "oz Capsules F 0.04 100.12 99.91 100.00 99.89 99.81 Because the T-fs, in our furnace assemblies is 0=F -0. 02 buffered at values approximating those of the Ni- 99.87 NiO (N-NO) assemblage(Hill and Boettcher, un- Msl (Ms+Fe2+) 0.53 0,48 0.69 0.66 0.75 publisheddata), runs made under N-NO conditions Ms/ (Ms+tFe) O.46 0.38 0.62 0.58 0.65 could be made with larger capsules,for an external r,62 0.54 solid buffer was not required. Thus, for runs under theseconditions, approximately 35 mg of sampleplus 19-26 wt percent HrO were sealedin welded AguoPduo (1975) report 30 and 40 wt percent iron loss from tubing of 2.2 mm I.D. For runs made under andesite-HrOmixtures at 1100'C and 30 kbar in cap- FesO.-FeO (M-W) or FerO.-FerOs (M-H) condi- sulesof AgruPdzuand AgoPdTo,respectively, but find tio-ns, the double-capsule technique (Boettcher, that iron lossis not significantat 1000"Cin runs of l0 Mysen, and Allen, 1973)originated by Eugster(1957) hours. However, our data for runs with the andesite and used previously in piston-cylinderapparatus by in 2.2-mm l.D. AguoPds.capsules under N-NO con- Ganguly and Newton (1968), Lindsley and Munoz ditions at temperaturesbelow 1000'C suggestiron (1969), and Gilbert (1969) was employed. With this lossas high as 40 percent.Run number 50 (Table 2) technique,only about 1.5mg of sampleplus 20-25 wt contained approximately 80 percent glasswith l.0l percent HrO were sealedin AgroPduotubing of 1.5 percenttotal Fe (Table 7),20 percentamphibole with mm I.D. Thesesmaller capsulesplus either the com- 10.57 percent total Fe (Table 6), ( 1.0 percent ponents for the M-W or the M-H buffer were in turn orthopyroxene and a trace of an opaque mineral. sealedin 3-mm O.D. Pt capsules.Possible water loss Thesefigures account for at least58 percentofthe 5.0 during preparation was determined by weighing the percent total Fe in the starting material (Table l). capsulesbefore and after welding. Potential leaks in Our runs with the andesiteunder M-W and M-H these capsuleswere detected by heating the capsules conditions were made with smallercapsules (1.5-mm at I l0'C for 12-24 hours and reweighingthenr. Op- LD.) and afford a bettercomparison of our data with tical and X-ray diffraction techniques were used to that of Stern and Wyllie. For example,run number determine whether the M-W or M-H assemblages 68 (Table 2) contains at least 85 percent glasswith lasted the time of the experiments. 1.32 percent total Fe (Table 7), at least 5 percent The sampleswere sealedin AguoPduorather than Pt amphibole with 6.4 percent total Fe (Table 6), and in order to preventloss of iron from the sampleto the ( l0 percent opaque mineral. These figures indicate capsule(Merrill and Wyllie, 1973).Stern and Wyllie no absorption of iron by the capsule.This is ex- AMPHIBOLES IN ANDESITE AND BASALT l07l

TneLr lb. Normative Compositions(C.l.P.W.) of Tngr-E 2. Run Data Rocks Used in the Experiments @wtzDura- Phase NepheliniEe kbars 'c /l Mterier It20 tion e6sdblage nr 9. (OP) 10.14 3 .82 LO 920 140 andeslte N-NO 25.0 7.0 L,V,Am, qM' ' ?opx 757 andesite N-NO 25.0 7.O r,V, An, qM or Ehoclase 6.38 2.9D 4.43 IO 940 l0 960 r9Z andeslte N-No 20.2 6.5 1,?V,?qM albite 36.13 26-14 L6 67 21.54 900 50 andesite N-No 20.0 12.0 L, ?V,h,opx,OP 920 sz andeslte N-No 20.0 12.5 L,v,An, Opx,0p,qM 26 22 23 05 24,4t 24.06 947 940 54 atrd es iEe N-No 19.9 12.5 L, ?v,An,opx,qM 0 .20 19.34 13 960 56 andeslte N-No 20.0 13.3 L,?V qM,Ru 18 920 102 andesi!e N-No 20.0 1r.3 L, ?V,An,Opx,Ga'OP r leuc ite 5.56 18 940 110 andeslte N-NO 22.6 12.5 L,V,An,Ga,Op,qM 15.18 21.67 20.28 26.1-6 18 960 104 andesite N-No 25.0 12.0 L, V , q!,1 I9 850 130 andesite N-No 25.0 22 0 1-,V,An,Ga,opx 'Ru (2.65) (7.69) (11.28) (r0.52) (13.80) (Op),qM L9 920 \26 andesite N-No 25. 0 23.0 L,v,Ga, 22.3 L, ?V,Ga Op,qM (1.43 (3. (7.1r) (7.00) (r0 56) 20 920 l!4 andesite N-No 24.9 , ) ee) (Op),qM 20 960 425 andesite N-No 20.7 26.5 L,v,Ge, (1.8r) (r.r3) (3.s0) (2.68) (2.75) 20 975 230 and es i!e N-No 33.3 3.5 L,v,Ga,(Op),qM 20 980 423 and es 1te N-No 20,6 22.5 I,v, (0p) 'qM 11.07 (0P),qM 23 1000 429 andeslte N-No 21.0 22.5 L,V,Ga' (op),qM (6.17 ) Q .61) (r7.80) 23 to20 424 andesice N-NO 22.4 22.5 L,v, (oP) 10 940 202 andesl! e M-W 2t.7 4.5 L,V,An, fs (4 90) (6.7 2) (6.19) 10 960 209 andesite M-W 20.2 4.5 L,V,Ao,Op,qM olivine 0.16 18.00 22.6a l0 970 233 and es l!e M-W 24.8 4.5 L,v,qM L3 900 94 andesite M-W 20.0 4.5 L,?V,AD, opx,OP,SP,Ru to (0.12) (r2.56) (1s.43) (0p) I3 940 88 andeslte M-W 20.0 4.5 L,?V,Am, qM fa (o.04) (s.44) (2.91) 13 950 93 andeslte M-W 20,O 4.5 L, V, M-H 25.0 3,0 L,V,An,Op,qM (4.34) 10 940 227 andesi!e 10 960 222 andesite M-H 25 0 3.0 t ,v,op 20.0 5.2 L,?V,Am,P1'OP magnetite 2.58 7.09 468 6.22 7.55 880 12 and es lte M-H 13 920 68 andesite M-lt 20.0 3,2 r, ?v,h,0P iLnenite 1.7 9 3.23 4 -71 3 .99 5.30 13 940 70 and esite M-H 20,0 4.o L,V,An,Op,qM L,V,0p,qM -77 2.51 I3 960 223 andesite M-rl 25.0 4.3 10 920 r79 qz thol N-No 21.0 15.5 L,V,h,Cpx,0P'qM qM 10 940 r94 qz thof N-No 20.0 5.0 L,V,An, Cpx' OP' itlt. Haad andesite See Wlse (L969, p. 1A02, #155)- 10 960 )-73 qz tbol N-No 20,1 6.5 l-,v,Cpx,Op,qM qM 10 980 r78 qz Ehof N-N0 20.2 7 .0 L,V,Cpx, Picture CarEe qudrtz thal,eiite, AnaLUst: C,0. Ingffiells, The PennsltL- IO 1000 142 qz thol N-No 25.0 7.0 l,,V,qM Danis State UniDe?s;.t!. (See also Hmilton et al., 1964, p. 23). qM 13 980 101 qz thol N-No 20.0 1.5 L,V,An, Cpx' 0P' I,V,AE,Cpx,qM 192L KiLeea, Haoaii oliaine thoLeiite. Analllst: C,0 IrLgmeLLs, lhe 13 1000 45 qz thol N-No 19.0 14,3 qz N-No 18. 2 12.0 L,V,qM Pemsrltan'ia State UniDersit!, (See also fUdali, 1965, p. 1A65) L3 1020 53 Ehol 13 1040 42 qz thol N-No 19.0 12.0 L, V, qM,qAn qM' qh Prehistaric Hualalai, Hmaii aLlaL'L basalt. Analljst: J,B. Bodkin, 15.5 1000 170 qz lhol, N-N0 20.0 7 .o L,V, Cpx,An ' OP, (See The Pennsylrania State UnitetsitlJ al6o Yoder and ?iLLe,y, 1962, I8 -060 108 N-No 23.4 14.3 L,V,Cpx,An'OP,qM'qh qMj p. 36A). 18 980 120 qz thol N-N0 25.0 7.7 L,V, Cpxrh, oP, 18 1000 105 qz thol N-No 25.0 11.8 L, V, qH Honolulu olivine nephelinite. See Winchell (194?, p. 30-31, analgsi-s qM 20 960 7)-2 qz thol N-No 24.8 22.7 L,V,Ga,Cpx, Op, #ls). qM 2t 960 116 qz thol N-NO 25.0 23. 0 L,V,Ga,Cpx,Op, qM,?qM 10 960 215 qz thol M-W 20.8 4.5 L,V,h, cpx ' 0P' qM t0 980 228 qz thol M-W 20.0 4.5 L,v,Cpx, Op, plicable in terms of the high low activity of 10 1000 197 qz rhol ll-il 19.6 4,5 L,V,Cpx,qM,qh -fozand 10 ro20 289 qz lhol M-W 24.4 3,0 I, V, qM (0P) 13 940 400 qz thol M-W 22.2 4.O L,V,Cpx,An' 'qM metallic Fe (cp"") in the samplewhen using the M-H (oP) 13 960 396 qz thol M-W 25.3 4.O r,v,cpx,h, 'qM buffer. 13 980 89 qz thol M-W 20,0 4.5 L,V,Cpx,An,op,qM 13 1000 86 qz thol M-W 20.0 4.5 L,V,cpx,Am,qM 13 1020 100 qz Ehol M-W 20,0 4.5 L,v,(op),qM Apparatus l-3 1060 98 qz thol M-W 20,0 4.5 I,v,qM l-0 940 226 qz thol M-H 25.0 4.5 L,V, Cpx,h,Op 10 960 235 qz Ehol M-H 25.0 4.5 I,V, Cpx,An,0p,qu, qAD All experimentswere in piston cylinder apparatus 10 980 288 qz thol M-H 25.4 3 0 L,V, Cpx,0p,qM, qh 10 ro00 224 qz Ehol M-H 25-O 4.0 I,V, Cpx,Op, qM, qh similar to that described by Boyd and England IO 7040 225 qz thol M-11 25.O 4-5 l, ?V,Cpx,0p,qM (1960). between20 and 36 kbar 10 1060 234 qz lhol M-H 25.0 4.5 L, V, op, qM Severalruns made 13 910 395 qz rhol M-H 25.3 4.0 l-,V, Cpx,Arn,op,qM l2.l-mm diameter furnace assembly and 13 920 76 qz thol M-H 20.0 4.5 I, ?V, Cpx,Op,An used a l3 940 78 qz lhol M-H 20.0 4.8 l,?v,Cpx,0p,An piston (Boettcherand Wyllie, 1968);in the remaining 13 960 77 qz thol M-tt 20.0 4.0 l, ?V,Cpx,0p 13 1000 57 qz thol M-H 20.0 3.0 L, ?V,Cpx, op, qM runs we used a 25.4-mm diameter furnace assembly 13 1020 59 qz Ehol M-H 20.0 6.0 L,V,Cpx,op,qM 13 1040 67 qz thol M-H 20.0 4.5 L, ?v, Cpx,0p and piston (Boettcher,in Johanneset al, l97l). Cap- 13 1060 84 qz thol M-H 20.0 4.5 L,V, Op,qM, qcpx, qAn l3 1080 81 qz thol. M-H 20.O 4.5 L, ?v,Op,qCPx suleswere placed horizontally in the notchedtop sur- 10 1020 196 oI thol N-No 20.3 5. 5 r,V, Cpx, h, qM,qAn 10 1040 198 01 thol N-N0 20.1 9.5 L,V, Cpx,qM, qh face of a cylinder of boron nitride that overlay a 10 1060 181 01 thol N-No 20,4 6.5 L,V, Cpx,qM 10 1080 193 01 rhol N-No 20.1 6.5 r, V, qM cylinder of talc and a disc of pyrophyllite. To avoid I3 1050 44 01 thol N-No 19,0 12.0 l,V,Cpx,An,qM 1070 L7 01 thol N-No 19.0 12.0 I,V,Cpx,qM contaminationof the thermocouple,a strip of Pt foil l3 1085 51 01 rhol N-No 18,9 12.0 r, ?v, qM 1000 103 01 Eho N-No 20.0 11,8 I,V, Cpx,An,Op,qM was placed betweenthe capsuleand the thermocou- 15 1065 210 o1 lho N-No 20.5 7.0 L,V, Cpx,qM 18 1000 107 oI tho N-No 24.1 12.0 L,V,Cpx,h,Op,qM ple, which passedsuccessively through cylinders of 20 1000 111 ol tho N-No 25. 0 12.4 L,V, Cpx,An,Ga, Op, qM 20 1020 118 oL tho N-No 25,1 22.5 L,V, Cpx,An,op, qM Solenhofen limestone, pyrophyllite, and boron 20 1060 123 o1 lho N-No 25.0 7 .O L,V, Cpx,h, qM 20 1080 131 oI tho N-No 25.0 7.O L,v, qM,qcpx nitride in the upper part of the furnace assembly. 20 to97 Lza 01 tho N-No 25.0 7.O L,V, qM,qcpx, qh graphite fur- 21 1000 115 ol tho N-No 24. 9 22.5 L,V, Cpx,Ga,Op,qM Thesewere surroundedby a cylindrical 10 960 205 ol tho M-W 20.6 4.5 L,V, Cpx,Am,0p,01,qM nace inside a cylinder of talc with a ring of fired 10 980 204 ol tho M-W 21.5 4.5 L,v,cpx,h,0B(01) , qM 10 1000 208 oI lho M-W 21-,3 4 -7 L,V, Cpx,h, op, 01, qM,qh pyrophyllite at the baseof the talc cylinder. This fur- IO 1020 203 oI tho M-W 2r.6 4.5 L,V,Cpx, (Op),qM IO 7060 229 01 tho M-W 20.0 4.8 L, V, Cpa, qM nace assemblywas surroundedby a sheetof lead foil 10 1080 236 oI tho M-W 25.0 4.5 L,V,0p, qM_ 1072 ALLEN, BOETTCHER, AND MARLAND and lubricated with a slurry of MoS, and Tesr-E 2. Continued perfluorokerosene. 25 IO20 285 nephelinire N-N0 20.7 22.O L,V,01, Cpx,An,0p, qM 2a 1020 310 nephelinite N-N0 25.2 22.5 I,V,01, Cpx,tu, Op,qM Temperatureswere measuredby Pt-PtroRhrother- 28 rO40 326 nepheliniEe N-No 24.9 4.3 I, ?v,01, Cpx,An,0p,qM, mocouples,but they were not correctedfor the effect 28 1060 319 nephefinite N-No 24.9 7.O L, ?V,01,Cpx,op,qM,qh of pressure -12 28 1080 320 nephellnlte N-No 25,1 4,0 L, ?v,01, Cpx,Op,qM,qAm on the emf. Friction correctionsof 28 1125 309 nephelinlte N-N0 2I.2 6.3 L, ?v, cpx,Op,qM, qAn, qcpx percent for the 12.7-mmfurnace -5 28 1155 316 nephelinite N-N0 24.8 4 .3 L, ?V,qM, qAn, qcpx assembliesand 30 850 327 nephelinlte N-N0 25.5 23.r l,V,01, cpx, Ga,h, op, qM percentfor the 25.4-mm furnaceassemblies were ap- 30 900 3L3 nephelioi!e N-N0 24.5 22.2 l, ?V,01,Cpx,Ga, An, Op, ql'l 30 LO20 281 nephelinite N-N0 20 0 22.3 I,V,01, Cpx,Ga, An, Op, qM 30 1100 323 nephelinlte N-No 26.0 4 0 L, ?V,( ?01) , Cpx,op, qM,

31 1020 321 nephelinite N-No 26.4 23.0 l, ?V,01,Cpx,Ga, AD, Op, qM Tlulr 2, Continued 32 900 328 nepehllnlre N-No 25-8 23 0 l, ?V,01,Cpx,Ga , Op,qM, qh ro20 293 nephelinlte N-No 24.4 24,O l, ?V,01,cpx, Ga,op, qM Run Starllng 900 337 nephelinite N-No 25.4 25,3 L, ?V,01,Cpx,Ga,0 p, qM, kbars tl Mterlal Hzo Elon assmbla8e qAn hr s. 900 339 nepheLinile N-N0 25.3 20.O L,V,0l,Cpx,Ga,Op,qM,qh 1020 336 nephellnlte M-W 23-O (0p),qM 13 980 401 oI thol M-w 22.7 4.0 L, V, Cpx,Ao,01,qM 6-5 L,v,01,Cpx,h, 1040 340 nephelini!e M-W 22.3 1.O L,v, 01,cpx,(op),qh,qM 13 1020 397 01 thol M-W 25.O L,v , Cpx, An, qM 1020 330 nepehlinite M-w 25,6 4.O L,V,0l,Cpx,h, (op),qM 13 1040 96 ol rhol M-!I 20,0 L,V,Cpx,An,01,qM,qAn 1040 nephelini!e M-W (op),qM,qh 13 1060 85 oI rhof M-w 20,0 L,V,Cpx,Ah,qM 334 25 0 6.1 L,v,01,Cpx, 1050 332 nephelinire M-H 25.r 4.r L,V,01,Cpx,(op),qM 13 1080 146 01 thol M-l,l 20.O L, V, qU 1040 335 nephelinite M-H 24.9 7.O r,v,Cpx,A,\,0p, qM 13 1097 92 oL thol u-W 20.0 L,v , qM 1060 338 nephellnlte M-H 25.7 7.0 L,V, Cpx,Op, qM, qAn r0 980 248 ol Ehol M-H 25.O L, V, Cpx,Am,Op, qM 1020 329 nephelinite M-tr 25.5 4.0 r,V, Cpx,An, op, qM r0 to00 272 oI Ehol M-It 25.4 L,V,Cpx,op,qM 1040 333 nephelinire M-H 25.3 6.0 L, V, Cpx,h, Op,qM 13 950 394 o1 thol M-H 2O.2 4.0 L,V, Cpx,An,0p, qM 1060 331 nepheliniEe M-H 25.r 4.O L,V, Cpx,0p,qM, qAn 13 980 77 01 thol M-H 20.0 4.5 L,?V,Cpx,As,Op 13 1000 13 01 thol M-H 20.0 4.9 L, ?V,Cpx, Op 13 1020 69 ol thol M-H 20,0 4.8 L, ?V,Cpx,Op AbbveDiqtions: qz thal = qua"tz thaleiiLe; aL bhal, = oLi,ine thaleiite; 13 1040 61 oI thol M-H 20.0 5.0 L,V, Cpx,Op,qM alk bas = aLkali basalt; L = gLass interp?eted ta be 13 1060 82 01 thol M-H 20,0 L,V,Cpx,0p, qM,qArD quenehed Liquid; V = gldss interpreted to be quenched 13 1080 80 oI !ho1 M-H 2o.0 L,V, Op,qM, qcpx, qlo Dapay; A,a = @rphiboLe; Cpr : clinaplrocene; jpr = ortha- t0 980 201 alk bas N-No 20.4 12.0 L,V,0L,Cpx,An,Op,qM pA?@ene; 0L = oLiDine; Op = opaque nineral; Ga : gu- 10 1000 r74 a1k bas N-No 20.2 6.5 L,v,0L, cpx,An,qM, qh net; Bi : Biatite; Sp = sphene; Ru = rutilei M = nica- 10 1020 186 alk bas N-No 20.3 22-0 qM,qAn L, V, 01, Cpx,Am, ceas mineral; PL - ; q = interyreted ta haDe 10 ro40 L72 alk bas N-NO 20,0 6.5 l-,V,01, qM,qAn Cpx, erystallized during the qlench; ? = questi.onable; ( ) = 10 1060 r7r a1k bas N-No 20.0 6.1 r,V,01,Cpx,qM,qAn trace mount 10 1080 1.39 alk bas N-No 25,0 7.1 I,v,01,Cpx, qM 10 lr00 177 alk bas N-No 20.1 L, V, qM,qAn 13 1080 39 alk bas N-No 19.0 11.5 L,V,0I, Cpx,AE, qM, qh 13 1105 46 a1k bas N-No 18.9 10.5 L, V, qM,qcpx plied, 18 1040 109 alk bas N-No 24.5 13.1 L,V,0I, Cpx,An,qU all runs usedthe piston-in method (Johannese/ 18 1097 106 a1k bas N-No 25,0 10.7 L,V, qM,qcpx, qh al, l97l). 20 1040 113 aIk bas N-No 26.0 L,V,01, Cpx,M, qM 20 1080 138 a1k bas N-No 25.0 7.O L,V,01, qM,qcpx, qAn 22 1040 117 alk bas N-No 25.0 223 L, V,01, Cpx,h, qM 22 1060 r2r elk bas N-N0 25,0 23.0 L,V,01, Cpx,h, qM Run Procedure 22 l-O97 125 a1k bas N-No 25,0 1,O L,V, qM,qh, qcpx 24 1040 r29 a1k bas N-No 25,0 30.0 L, V,01, Cpx,An,Ga,qM A pressureof at least one kbar below that to be 26 1040 r33 alk bas N-No 25.0 27. s L, V,Ga,Cpx,01, qM, qAn 10 980 21r a1k bas M-W 21.9 4-7 L, ?V,01,Cpx,h maintainedduring the run was first appliedto the fur- l0 1000 212 alK bas tr-u 2t.\ 4-5 L,V,0I,Cpx,k,qM, qh 10 1020 2r3 a1k bas M-l{ 2)-.3 L,v,0I, Cpx,qM, qAn nace assembly.Next, the temperaturewas increased 10 1040 218 a1k bas M-W 20,0 L,V,0I, Cpx,qM, qAn 10 1080 290 a1k bas M-W 22.7 .0 L, V,0t, qM to the appropriate value over a period of about ten 13 1020 95 a1k bas M-W 79.7 .5 L,V,0l, Cpx,An,qM, qAn 13 1040 91 alk bas M-U 20.3 ,5 L,v,01, Cpx,An,qM minutes, and the pressurewas then increasedto the 13 1060 90 alk bas M-W 20,0 .5 L,V,0I, qM,qAn, qcpx 13 1080 145 alk bas M-!l 20.0 L,V,01, qM,qh, qcpx desired value. Synthesisruns lasted between three 13 rO97 87 alk bas M-W 20.0 4.8 L,V,?Op,qM,qcpx 10 1000 270 a1k bas M-H 25.7 4.3 L, V, Cpx,An,0p,qM and 3l hours. To reversethe high-temperaturepart of 10 1020 277 alk bas M-H 25.6 L,V, Cpx,An,Op, qM l0 )-o40 257 alk bas M-H 25.0 3.0 L,V, Cpx,Op,qM the amphibole-outcurves, two-stage runs were made. 13 1020 74 alk bas M-H 20.0 4.5 L,v,Cpx,An,op 13 1040 75 alk bas M-H 20.0 4.9 L, ?V,Cpx,h, Op That is, a run under conditionsknown from previous 13 1060 63 aIk bas M-H 20.0 4.0 L, ?v, Cpx, Op,qM, qAD runs 13 1097 83 a1k bas M-l{ 20.0 L,V,0p, qM,qh, qcpx to be sufficient to melt the entire charge was 13 1100 79 a1k bas M-lt 20.0 5.3 L, ?V,Op, qh, qcpx readjustedto within a few degreesless 10 7020 283 nephelinite N-No 19.9 5.0 L,v,01,cpx,h,qM than the condi- 10 1040 286 nepheliniEe N-No 19.1 5.5 L,v,01,Cpx, (An),qM tions known from previoussynthesis runs to produce 10 1060 292 nephelinile N-No 19.9 4.2 L,V,01, Cpx,qM 13 rO20 265 nephellnite N-No L9. 9 6.0 L,v,01,cpx,An, (op),qM, amphiboleand held at thoseconditions from 2l to 40 qAn 13 rO40 269 nephellnlle N-No 19.9 4.2 L, V,01, Cpx,?h, qM,qAn hours. A similar procedure was used to reversethe 13 1060 261 nephelinlre N-No 20.0 6.0 L, V,01,Cpx,qM 13 1080 275 nephellnlte N-No 19.9 4.8 L,V,01, Cpx,qM high-pressurepart of the samecurves. In these,a run 13 1100 263 nephelinite N-No 20.0 6.5 L, v, 01, qM 13 Lr20 267 nephellnlte N-No 20.0 4.O L,V, qH at a half kilobar above the pressuresufficient to form 15 1-040 278 nephellnlle N-No 20,0 5.0 L,v,01,Cpx,qM 18 1720 29r nephellnlte N-No 20.0 4.5 L,V , qM a garnet-bearing, amphibole-free assemblagewas 19 rO20 276 nephelinile N-No 20.3 7.O L,V,01,cpx,An,Op,qM 20 1040 303 nephellnite N-No 20.1 6.2 L,V,01, Cpx,An,Op, ?Bi, qM lowered to a half kilobar below the highestpressure 20 1060 305 nepheliniEe N-No 20.1 5.3 L, V,01, Cpx,An,Op,qM,qAn, qcPx at which amphibole was synthesizedand held at these 20 1100 282 nephelinite N-No 20,0 6.2 L, ?v, ( 01) , qM,qcpx, qh 22 7020 284 nepheliniEe N-No 20.1 21.0 L,V,01, Cpx,An, Sp, Op, qM conditions from 22 to 40 hours. 22 1080 304 nephellnite N-No 20.0 6.0 L, V,01, Cpx,op, qM,qh 24 1060 300 nephelinlte N-No 20,1 7.L L,V,01, Cpx,Ao,Op, ?Bl, qM 24 1080 308 nephellnlre N-No 20.0 5.0 L,V,01, Cpx,Op,qM, qb, Identfficalion and Description of Phases qcPx 24 1105 305 nephellnlte N-No 20.0 6.0 L, V,01, Cpx,Op, qM, qh The phaseassemblage at run conditionswas 24 1115 307 nephelinlEe N-No 20.0 6.0 L,v, (01), (?op),qM,qAn, deter- qcpx mined by standard optical and X-ray diffraction AMPHIBOLES IN ANDESITE AND BASALT 1073 study of the quenchedproducts. Mineral phasesin Hill and Boettcher,1970; Essene, Hensen, and Green, amountstoo stnallto be detectedby X-ray diffraction 1970).This study accuratelyestablishes these fields techniques could usually be detected with the and shows the effectsof varying bulk composition petrographic microscope, using the following (from the olivine nephelinite with 38.5'7VoSiO, to the characteristics. andesitewith 59.107oSiOz) and fs, throughout the Forsterite occurred as colorless,euhedral to sub- stability range of magnetite.The results of varying hedral, equidimensionalcrystals. ,fHzowill appearin a following paper. Although HrO Clinopyroxene generally appeared as euhedral, was the only volatile added to the starting materials, colorless to faintly green prisms of moderate the mole fraction of HzO in the vapor (X]r,s) was birefringence. slightly lessthan 1.0 becausesome solids dissolvein Orthopyroxenewas characteristicallyin subhedral, the vapor and also becauseX]1, ranges from 0'05 for medium-brown, squat prisms, showing faint M-W conditions to 0.01 for M-H conditions,assum- pleochroismin some runs. ing ideal mixing (Mysen and Boettcher, 1975a,b). Amphibole occurred as slightly pleochroic, light- Amphibole in the Mt. Hood andesite,under ap- green to slightly yellow-green,tabular, subhedralto proximately N-NO conditions (Fig. lA), is stableon euhedralcrystals with an extinctionangle of 12"-25o, the vapor-saturatedliquidus with or without spinel, often with twinning. to a temperature of about 950'C over a pressure Plagioclase occurred as subhedral prisms with range from at least as low as 10 kbar to as high as albite twins. about 18 kbar. Our value of 950'C compares Garnet, although amber to rusty red under a low- favorably with the work of Eggler(1970) on the same power binocular microscope, formed colorless, andesiteat 5.5 kbar. Under water-saturatedcondi- isotropiceuhedra. tions (9.7 wtTo HzO in the liquid), he reported a Sphene occurred as brownish-red subhedral temperature of 955"C for the breakdown of crystals,and rutile occurred as acicular dark reddish amphibole. The amphibole-out curve acquires a brown crystals of high birefringence. rather small slope (dPldZ) at about 18 kbar where Opaque oxides,although not identifiedfurther, oc- garnet becomesstable at the expenseof amphibole. curred as subhedral to euhedral crystals. In the vicinity ofthe changein slopeat about l8 kbar, positive slope Quenched silicate melt was a transparent,faintly the liquidus has a moderately steep lilac, isotropicglass. (dP/dT) extending to our last datum point at 23 Quenchedvapor in most casesoccurred as spheres kbar and l0l0'C; garnet,with a minor amount of ("fish roe"), sometimesarranged on fiberslike beads, spinel,remains stable on the liquidus throughout this but was also as irregular masses.It was not easilydis- pressurerange of about 5 kbar. Thus at temperatures tinguished from quenched liquid at pressuresabove of the amphibole-out curve, amphibole melts in- 27 kilobars. Quench rninerals include amphibole, congruently; and at pressuresgreater than those of clinopyroxene,and mica. The quenchamphibole and the curve, amphibole-bearingassemblages transform quench clinopyroxene occurred as ragged skeletal to garnet-bearing assemblages.This general con- crystalsand raggedovergrowths on primary crystals. figuration is repeatedin eachof the other four rocks no detect- Quenchmicas occurredas colorlessplates, often with (Figs. lB, lC, 1D, and 3). In the andesite raggededges. A brown mica, which occurredin some able changeoccurred in the position of the liquidus runs with the alkali basalt and nepheliniteat pres- or the stability of amphibole when using either the suresof 20 kbar and above,may be primary. M-H or M-W buffersat 13 kbar, or the M-H buffer at l0 kbar; however,amphibole remainedstable to a Results slightly higher temperature,965oC, when using the M-W buffer at 10 kbar (Fig. 2A). It is of some,in- Phase Relationships terest to note that in two runs in which the capsule The results of experiments on the five starting leakedand most of the water was lost, clinopyroxene materials are presented in Table 2. The phase instead of amphibole was the primary crystalline relationshipsare shown in P-T projection in Figures phase. l-6. It was possiblefor us to predict the approximate Amphibole and clinopyroxene, with or without configuration of the stability field of amphibolesin spinel, coexist on the liquidus from about l3 kbar Figures l-6 as a result of earlier work at high pres- and l0l0"C to 19 kbar and 980'C in the Picture sures by Lambert and Wyllie, 1968; Gilbert, 1969; Gorge quartz tholeiite under approximately N-NO 1074 ALLEN, BOETTCHER, AND MARLAND

Vapor-Satu rated Liquidus

Amphibole Out o ;ro -o Y o :2s o an o L o. Amphibole Out

uuu wu 1000 1100 800 900 1000 ltoo Temperature, oC

Ftc. l. Crystallizationsequence for: (A) Mt. Hood andesite,(B) PictureGorge quartz tholeiite,(C) l92l Kilauea olivine tholeiite,(D) Hualalai alkali basalt.All assemblagescoexist with liquid and vapor Legend:

(A) Andesite (B) Quartz Tholeiite (C) Tholeiite (D) Alkali olivine basalt Cn. O cp" O cp* Orn*Cpx*Ga+OpCot A Am+Opx A Cpx+Op A A ot+Cpx Am*Cpx*Op E em+Opx+Op fl ,qrn* Cpx * op L l I e.+ol+cpx O Am*Op*Ga O Am*Cpx O Am*Cpx O Am+Ol+Cpx*Ga nm+Opx*Op*Ga A Cpx*Ga+Op O A Ca+CpxfOp O Ot+Cpx*Ga A Am+OpxfGa I e bou.liquidus I Abou"li-quidus A Ol+Cpx*Ga*Ru*Op O Op*Ga I Above !A.+Ol+Cpx*Ga*Ru*Op liquidus I Aboueliquidus conditions(Fig. I B). At l0 kbar, clinopyroxeneis the stablerelative to the garnet-bearingassemblages. The stableliquidus phase at a temperatureof 990oC,and slope of that part of the amphibole-out curve amphibole appears as an additional phase below representingthe upper-pressurestability probably ex- 950"C. At pressuresabove l9 kbar, amphibole is un- tends to lower temperatures with a small dp/dl AMPHIBOLES IN ANDESITE AND BASALT 1075 basedon the reversedbrackets at 12 and24kbar and Figure lC. Clinopyroxene is the sole crystalline li- 700"C establishedby Esseneet al (1970).The quidus phase up to 20 kbar, at which pressureit is stabilityof amphiboleat 10kbar is increased20o by joined by amphibole. At pressuresabove 20 kbar, a employingeither the M-W or M-H buffer(Fig. 2B); garnet-bearingassemblage replaces the amphibole- the stability of clinopyroxene(not shown) is in- bearing assemblage.Hill and Boettcher(1970) deter- creased20o using the M-W bufferand by 60" using mined the stability of amphibole at lower M-H. At 13 kbar, amphiboleis stableon the li- temperaturesfor this rock, but their value of 26 kbar quidus to about 1010'C with M-W as with the at 700'C is too high becauseof the metastableper- N-NO buffer;however, under conditions of theM-H sistence of amphibole in nonreversed runs. The buffer, the stability is decreased60o at 13 kbar, stabilityof amphiboleis increasedby l0'at l3 kbar whereasthe stabilityof clinopyroxene(not shown)is using M-W buffer (Fig. 2C), but increasingthe fs, increasedby 40". with M-H reducedthe stability by 70'; however,the Melting relationsfor the 1921 Kilauea olivine clinopyroxeneliquidus remained at 1070'C for both tholeiiteat approximatelyN-NO conditionsare in buffers.Decreasing the pressurefrom l3 to l0 kbar

20

15

o h10 .o I o =25 o .n o o.L

15

d I 800 900 1000 1100 800 900 1000 1100 Temperature, oC

Frc. 2. Amphibole-out curvesfor (A) Mt. Hood andesite,(B) PictureGorge quartz tholeiite,(C) l92l Kilauea olivine tholeiite. and (D) Hualalai alkali basalt under M-W, N-NO, and M-H conditions. r076 ALLEN, BOETTCHER, AND MARLAND

maximumstability of amphiboleby 40'C (Fig. 2D). Olivine is also the liquidus phasewith the M-W buffer,but no olivinewas found under M-H condi- tions;instead clinopyroxene was the liquidusphase. Melting relationsin the olivinenephelinite at ap- proximatelyN-NO conditionsare shown in Figure3. Olivineis the solecrystalline liquidus phase from 10 to 24 kbar, abovewhich it is replacedby clinopyrox- ene. The amphibole-outcurve has a temperature H2O-Saturated maximumof 1070'Cat 22 to24 kbar,but decreases Olivine Nephelinite ----lClinopyroxene Out f- to 1050"at 28 kbarand 1040'at l0 to l3 kbar.The pressuremaximum for this curveoccurs at 31.5kbar and 1030'C.This representsthe highestpressure for any reuersedamphibole-out curve yet reported. Loweringthef6" with the M-W buffer decreasedthe stabilityof amphiboleby l0o at l0 and 13 kbar (Table2); increasing/6,with the M-H increasedthe

800 900 1000 il00 tzoo Temperature,oC FIc.3. Crystallizationsequence for Honolulu olivine nephelinite All assemblagescoexist with liquid andvapor. Legend o Alkali basalt for Fig 3 (for L abbreviations,see Table 2): (lt E Ol, Cpx,Am I Aboveliquidus \ Cpx, Op 4t A Ol, Cpx I Ol, Cpx, Am, Op v Ol, Cpx, Am, Ga, Op Y O Ol v Ol, Cpx, Op A Ol, Cpx, Ga, Op qt Olivine tholeiite L f o causedthe stability of amphibole q to decreaseby 30'C o for the N-NO buffer, by 60'C for the M-W buffer, L but to remain unchangedunder M-H conditions o. (Fig.2C). The most refractoryamphiboles in this studyare thosein the prehistoricHualalai alkali olivinebasalt (Fig. lD). Olivineis the liquidusphase at all pres- suresfrom l0 to 26 kbar at approximatelyN-NO conditions.Amphibole was stable to the highest temperature,1090'C, at l3 kbar and approximately N-NO conditions.The upperpressure stability limit 800 900 1000 1100 is about 25 kbar, at which pressurean amphibole- oC bearingassemblage is replacedby a garnet-bearing Temperature, assemblage,and the amphibole-out curve proceeds to Ftc. 4. Pressure-temperature projection showing the relative lowertemperatures with a smallpositivedp/dT.Our positions of the amphibole-out curves under N-NO conditions for resultsfor this curveare in agreementwith thosees- the Mt. Hood andesite, Picture Gorge quartz tholeiite, l92l Kilauea tablishedat 700oto 900"Con a similarrock by Es- olivine tholeiite, Hualalai alkali basalt, and Honolulu olivine nephelinite along with the shield geotherm seneet al (1970).At 13 kbar, lowering/6, from Clark and with the Ringwood (1964) and oceanic geotherm from Ringwood, M-W buffer or raising-fo" with M-H reduce the MacGregor, and Boyd (1964). AMPHIBOLES IN ANDESITE AND BASALT 1077

with an ARI--nltx electron microprobe analyzer Al203 (Tables 3-7). Although results are shown up to as many as four significant figures, they almost certainly are accurateonly to two or three. Salient featuresof these determinationsare as follows: Oliuine. Olivine occurs in significantamounts in two of the rocks, the alkali basalt and the Mso only nephelinite,although therewere traceamounts in the olivine tholeiite.The four analysesof theseolivines in Table 3 indicate that the Mg/(Mg+>Fe) of the olivine increaseswith that of the starting material. Gqrnet. Garnet occurs at high pressuresin all five rocks. The chemistry of these (Table 4) in- dicates that the Mgl(Mg+>Fe) is proportional to that of the starting material and that the pyrope con- CoO tent increaseswith pressure. FeO Clinopyroxene. Near liquidus temperatures, clinopyroxene is the most abundant mineral in the three basalts and in the nephelinite, but is undetected CoO in the andesite.Mgl(Mg*)Fe) of the clinopyroxene increaseswith that of the starting material (Table 5), No20 but it decreaseswith increasing pressurein the quartz FeO tholeiite and the olivine tholeiite and appears to be independent of pressure in the nephelinite. Clinopyroxenes synthesized under approximately Fe2O3 N-NO conditions show larger values of AlzOg and NarO compared to those at higher or lower valuesof { -40 J oz' 45 50 55 60 62.865 70 Amphibole. Although present in only minor Percent SiO2

Ftc. 5. Subtraction diagram. Mt. Hood andesiteminus approx- Teet-n 3. Olivine Compositions imately 20 percent amphibole yields composition in right column. Analysis 1 2 3 4 Rock Alka11 A1ka1i A1kali Nephelinite basalt basalr basalt stability l0'. Olivinewas phase a stablecrystalline T'c 1020 1040 1040 1060 with only the N-NO and M-W buffers.Our results P, Kbar 13 18 24 24 on the stabilityof thepargasitic amphiboles (Table 6) Buf fer M-W synthesizedin the alkali basaltand the nephelinite si02 39.38 39.31 39.L4 39.1 areconsistent with the work of Boyd(1959) and Hol- A1203 o.25 0.23 0. 34 0. 04 loway(1973) on pargasiteat pressuresfrom 250bars Fe0* 2t.5t 24.58 23.64 18.2 to 8 kbar. Mco 40.03 36.68 36.54 40.8 We performeda brief investigationof the stability Ca0 of amphibolein a (Winchell, 1947, p. 30-31, 101. 17 101.20 99.87 98.54 anafysisno. 13 with 42.86VoSiO, and 10.517onor- mative nepheline)at approximatelyN-NO condi- Cations/4 Oxygens Here, (with sl 1 .004 1.017 LOLZ I .006 tions. amphibole olivineand clinopyrox- A1 0. 008 0.007 0.011 0.00r ene)is stableto 1080"Cat pressuresof l3 to l6 kbar, Fe 0.459 0.532 0.537 o.392 Mg r.522 L.4r4 L.479 1.566 althoughit is not presenton the liquidus. n.d. 0.011 0.006 0. 011 ChemicalAnalyses XX 1, 981 L.946 2.016 1.958 Itgl (Mg+Fe) o.77 0.73 o.73 0.80 The chemicalcompositions of ofiyi,nes,pyr_oxenes, ^Iotal, amphiboles,garnets, and glasseS-weredetermined -- iron as FeA, 88,0. Wsen, ArcLAst. 1078 ALLEN, BOETTCHER, AND MARLAND

TnsLr 4. GarnetCompositions amphibole in peridotitesat high pressures.However,

Analysis | 2 345 the amphiboles in the quartz tholeiite, the alkali Rock Andesite Andesite Quartz Quartz Alkali basalt,and the nephelinitemaintain a rather constant rholeiite tholeiite basalt Me/(Me+)Fe), which is lower than that of the T"C 920 920 960 960 1040 P, Kbar 18 20 20 21 26 clinopyroxenes. Buffer N-NO N-NO N-NO N-NO N-NO Glasses. Two representative chemical analysesof si02 4t.59 45.40 38.41 39.19 40.47 glasses(quenched liquids) from the andesiteare listed Ti02 l. 68 1. 55 r,24 L.64 0.64 in Table 7. These glassesare highly aluminous and ALzOz 19.88 17.51 r9.4r 18.95 21.20 quartz-normative.They contain lessNarO and more Fe0* 19.18 19.00 23.32 22.L6 15.86 KrO than their associatedamphiboles. Mgo 8, 06 8.48 6.53 9.04 13,82 't0-55 CaO 9.44 9 .43 R-li A oo Discussion TOrAL 99.83 101.37 99.47 99.10 As suggestedby Lambert and Wyllie (1968), the stability of amphiboles in andesitesand basalts is Cations/24 Oxygens limited by melting reactionsat high temperaturesand si 6.277 6.696 6.003 6,L36 6.535 Ar 3.537 3.044 3 .577 3.502 4.Orr by transformation to garnet-bearing assemblagesat Tl 0.191 0.171 0.146 0.193 0.O77 high pressures.Amphiboles are stableto higher pres- Fe 2.42L 2 .343 3 .049 2,906 2.t29 Mg 1.812 1.865 1,520 2,rr3 3.307 sures and, with the exception of the nephelinite,to Ca 7.527 1 ,491 7.768 1. 365 r.376 higher temperaturesin order of decreasingactivity of Mg/ (Ms+IFe) o,43 o.44 0.33 0.61 0.42 silica of the starting materials (Fig. 5). Although *Total iron as FeO, Mysen and Boettcher(1975a) found that the stability of amphibole in peridotitesincreases with increasing Mgl(Mg+Fe'+) of the starting material, this is not amounts near the high-temperature part of the true for thesefive rocks. The ratios Mg/(Mg*Fe2+ amphibole-outcurve, amphibole rapidly increasesin *Ca) and Mgl(Mg+>Fe*Ca) of the starting amount at lower temperatures. In computing the materials were also examined in this regard, but no chemicalanalyses, the raw counts were correctedby obvious relationshipswere found. All mineralsin the the method of Bence and Albee (1968) with an APL five rocks, for which we have comparativeanalyses, program prepared by V. J. Wall. This program uses show a strong positive correlation between the an estimatedFerOr/FeO and HrO content. HrO was Mgl(Mg+ ) Fe) of the mineral and that of the parent arbitrarily taken as 2.00 percent; FerOr/FeO values starting material. of 0.3, 0.4, and 0.8 were selectedfor M-W, N-NO, No orthopyroxene was found in any of the three and M-H buffers,respectively. These values are not basalts,nephelinite, or the basaniteunder any of the inconsistent with the range of values listed in the conditions investigated by us. Green (1970, 1971, amphiboleanalyses in Leake (1968). 1973)and Bultitude and Green (1968, l97l), have In view of the diversityin chemistryof the starting proposed a major role for orthopyroxene in the materialsand the rangeof pressure,temperature, and derivation of SiOr-poor magmas, such as olivine /6, covered in this investigation,the amphibolesare nephelinites,from a hydrous pyrolite mantle. They remarkably similar in composition (Table 6). Ac- report a small stability field of orthopyroxene in cording to the classificationdevised by Leake (1968), basanitesand olivine nephelinitesin the presenceof 2 they straddle the calciferous-subcalciferousboun- to 7 percent water. It is difficult to compare our dary, and all of them fall in the pargasite- results with theirs, for their runs were not under tschermakite-tschermakitic hornblende category. water-saturatedconditions and were at different(un- Only seven contain enough titanium to justify the controlled) conditions of -for however, the lack of prefix titaniferous()0.25 Ti per 24 oxygens).Except orthopyroxenein our basalticrocks certainlyrestricts for the amphibolesfrom the olivine tholeiite(possibly the potential P-T-X conditions under which frac- due to an error in the analysisof MgO and FeO in tional crystallizationof orthopyroxene is an impor- analysis2l,Table 6), Mgl(Mg+)Fe) increaseswith tant mechanism. We are now investigating the that of the starting material. The amphibolesin the stability of amphibolesin the andesiteand the olivine andesitebecome less magnesian with increasingpres- tholeiite under vapor-saturatedconditions in which sure at conditions approximately N-NO, similar to Xfi,o is varied by dilution with COr. No ortho- the trend found by Mysen and Boettcher(1975b) for has been found in runs made with the AMPHIBOLES IN ANDESITE AND BASALT

Tlsr-E5. ClinopyroxeneCompositions

T o^ o2n 980 980 960 960 960 960 980 1050 1000 P,Kbar 13 13 i1 l8 20 ZI 10 13 13 18

Buffer M-H M-W N-NO N-NO N-NO N-NO l,t-I,I M-ir N-NO N-NO si02 5L.46 51,87 48.03 48.99 49.74 s0.56 50.58 5L.47 52.95 1102 0.39 0.53 0.49 0.35 0.68 0.57 0.79 Q.77 0.87 0.75 A1203 6.20 2.8Q 2.L7 4.62 4.39 5.83 2.76 z.)4 2.15 2.57 Fe0x 9.06 11.36 9.37 LO.23 12.50 9.O4 ). oc 6.35 5.51 6.68 MCo IL.72 t2.35 L4.86 13.69 13.01 11.73 Lt.45 L7.76 L).5 t 16.51 CaO L9.57 19.90 20.66 19.9l 18,83 2L,84 19.95 19.59 20.27 20.7L Na20 0.87 0.29 0.29 0.60 9.76 o.74 0.28 0.27 o.24 u. zo TOTAL 99,27 99.48 99.70 97.44 99.15 99.48 97.6r 97.41 96.44 IoU,44

Cations/6 oxygens si 1.914 1.963 L.937 1, 883 1.899 1.868 1.900 1. 900 1.960 1.935 A1 0,212 o.L24 0.096 o.216 0.20r o.258 o.t22 0.117 0.L27 0,111 Ti 0.011 0.015 0.0r4 0. 011 0.020 0. 016 0.Q22 0.023 0.025 0.021 Fe 0.282 0,357 0.293 0.339 0.407 o.284 0.184 o.207 0.180 o.204 Mg 0.649 0.691 0.827 0.808 0.755 0. 656 0.976 1.032 0,894 0.899 Ca 0.780 0.801 0.827 0.837 o.793 0.879 0.803 0.819 0.827 0.811 Na 0.063 0.02L 0.021 0.046 0.058 0.054 0.021 0.02r 0.019 0.019

MB nrn 0.83 0.83 o.82 (ilg+DFe)"" " 0.66 0.74 0.70 0.6s 0.70 0.84

Mg 37.9 cz.) 40.7 38.6 36.1 49.7 50.r 47.0 47.O Fe 16.5 19.3 15.0 17.1 20.8 15.6 9.4 10,1 9.5 10.7

ua 4). o 43.3 42.) 42.2 40.6 48.3 40,9 10 R 43.5 42.3

T,'c tooo 1020 r040 1040 1040 1040 1040 1015 1015 1060 P,Kbar 2L 13 l3 18 20 24 26 19 t9 24 fer si02 49.64 49.67 49.3L 51.33 5L.71 48.90 51.11 50,55 49.94 52.8 Tr02 0.74 1 ?O t,.2L 0.45 0. 69 0.85 n.d, n.d. 0.8 A1203 5,14 3.94 6.64 3.60 2.tL 4.66 ).+I 3.25 3.42 3.6 Fe0* 8,30 8.47 8.61 6.33 5.88 5.69 8.77 5.11 4.93 4.6

Mgo ]-5.23 13.69 L6.23 L6.71 T).1U 15.93 l-3.91 16.41 16,03 14.8 Ca0 20.76 21.54 18.91 2L.L2 23.72 20.75 20.03 23.44 23.98 22.L Naro 0.46 0. 45 0.45 0.29 0.3s 0.43 0.83 0.43 0.36 o.4 100.2 6 99.15 r01.40 99.93 99.93 97.05 100.97 99.19 98.66 99.1

Catlons/6 0xygens

si 1.841 r.872 1.801 1.890 1. 916 r,oo7 1.876 1.883 1.873 L.952 A1 0.225 0.175 0.286 0.156 0. 092 0.210 o.237 0.143 0.151 0.153 T1 0.021 0.039 0.033 0. 015 0.013 0.020 0.o24 0.o22 Fe 0.258 0.267 o.265 0.195 0,182 0.187 o.269 0.159 0.155 0. 140 o.842 o.769 0.883 0. 917 0.867 0.934 o.76l 0,911 0.895 0. 800 Ca 0.825 0.870 0.740 0.833 0.942 0.849 0.788 0.935 o.964 0. 859 Na 0.033 0.033 0.032 0.021 0.025 0.033 0.059 0.031 0.026 o.026

.-u.ll 0.74 o.77 0.82 0.83 0.83 0.74 0.85 0.85 0. 85 (r,lg+u r e,

Mg 43.7 40.3 46.8 4t.L 43.5 47.4 4t.9 4).4 44.5 4q.)

?o t.d !e L).c 14.0 14.0 10.0 9.1 14.8 ca 42.9 45.7 39.2 42,8 47,3 43.L 43.3 46.7 47.8 47.7 eAmLAst, B. O. I4Asen, *Iotal ITon aB FeO. 1080 ALLEN, BOETTCHER, AND MARLAND

Trnle 6a AmphiboleCompositions

And ea 1t e Quertz tholellte oc T, 92o 92o 900 900 g2o g4o 880 880 9oo g2o gzo g2o 960 P,Kbar 10 10 13 13 18 r0 13 13 13 13 10 10 10 Buffer N-No N-No N-No N-No N-No M-w M-H M-H !r-!J lt-H N-No N-No M-w s102 45,54 45.00 46.68 45.55 43,81 44.63 47.07 46,59 44.59 43.2 42.Or 4t.54 42.6L Tt02 2.01 2.tO 1.36 1.33 L.zL 2.30 0.39 0.44 L.25 0.63 2.t9 r.93 2.75 dzo: 12.50 12.55 11.71 11.34 L4.69 1r.66 14.04 13.59 12,81 t4.6 L2.69 L2.87 L2.36 Fer0a* 3. 70 3. 40 3. 83 4.00 4.L2 2,42 6.99 7.87 3.06 3.82 4.55 4.7A 3,47 FeO 9.24 8.50 9.58 10,01 10.30 8.06 8.74 9.83 10.20 4,78 11.38 Lr.g5 1r.57 Mgo 14.13 15.14 13.42 13.95 11.68 14.75 LO.47 10.0r L4.L4 14.05 LL.77 12,74 13,35 cao 8.18 9.16 9.05 Lo.26 9.34 10.84 7.80 8.49 9.49 9.8 9.oo g,g2 10.02 Ne20 2.Li L92 2.38 2.ZZ 2.52 2.O4 2.34 2.24 I.7g 3.3 2.48 2,40 2,OO KzO 0'35 o'34 0.33 0.32 0.55 0.33 0.24 o.23 o.5l 0,45 0.51 o.47 0,73 H"0* 2'00 2'00 2.00 2.00 2.oo 2.oo 2'00 2.oo 2,00 2.oo 2,00 2,00 TorAL 99,82 100.11 100.34 100.99 too.22 99.03 100.08 lor.29 99.84 94.65 98.58 100.61 100.86 Mg (TIE) 0.67 0,70 0.b5 0.ds 0.60 o.7z 0.s5 0,st 0.66 o..ts 0,58 0.58 Ug 0,62 (rlg+Fe2-) 0.73 o.i6 0.71 0.71 o.6i o.77 0.68 0.64 0.71 0.84 0.65 0.66 0.57

19 20 52627

T"C 910 910 950 950 950 980 980 1000 lo00 980 980 980 9EO 1040 P,Kbar 13 13 13 L3 13 13 13 t8 20 10 10 r0 10 IJ Buffer M-H M-H u-H M-H M-H M-w u-w N-No N-No M-w N-No N-No N-No M-H si02 43'19 43.77 42.55 43'02 43.95 46.L2 45.47 42.39 40,05 45.12 43.58 43.37 4r.72 4L6t T1O2 2,r0 1.87 7.45 L40 I.25 3.76 4,28 2.42 1.38 2.5g 2.88 2.88 2.35 I.52 A1203 15'15 15'65 13.88 13.9s L3.25 13.91 13.54 17,09 15.42 75.75 13.06 13,04 13.28 L3.42 Fer0r* 6.08 5.43 3.36 4.11 3,60 2.58 2.9L 5.L7 2.L7 1.96 2.9g 3.15 2.97 7.32

Fe0 7,6L 6,74 4.20 4.49 8.58 9.7r L2,92 5.41 7.47 Mgo 12.11 16.58 16.44 10.26 9.42 10,05 t4,76 L2.70 14,13 L4,43 t4,76 ca0 IO.11 9.25 12.55 tt.1 6 rt,59 10.55 4.79 l7 ,L4 10.07 11.89 L0.76 9.86 Na20 2,ro 1.81 2.34 r.59 1.53 0.12 0.61 1.7s 2,L2 2.34 *20 0.56 0.50 0.53 0.53 0.58 u.)) 0.37 0,01 0.73 0.72 o.72 0.78 o.74 H 2.00 2.00 2.OO 2.00 2.OO 2.OO 2.OO 2,OO 2,00 2.OO 2.OO 2.00 2.00 2,00 I0I.0l r00. 100.68 98,91 99.70 100,88 101.33 LOO.47 98.38 (Mg+tPe) 0.62 0.67 0.80 0.77 0.79 0.63 0,58 0.51 0.78 0.73 0.71 0.71 0.72 0,61

o.74 0,78 0.88 0.85 0.87 0.58 0.63 0,58 0.83 0,78 0.77 0,71 0.78 0.73

Analysl6 ..x

NeDheltalte T, "C 1020 1015 r015 1015 1015 1060 1020 1020 1020 P,Kbar 11 10 19 19 19 t9 24 2A I3 l3 Buffer N-No N-NO N-Ng N-NO N-NO N-NO N-NO N-NO M-[I M-II si02 42'o 42,60 42.0L 43.14 42.49 42.38 42.20 39.3 Ti02 L,62 1.73 1.56 1,70 L.62 1.55 2,LO 0,78 2,L8 1.18 dz0: 13.4 15.81 15.43 LJ,44 L3.42 L4,23 13.8 15.3 13.8 L3.32 Fer0r* 2.23 2.I0 2,06 2.L2 2.I4 2.LO 1.94 2.L2 3.40 leu ).J/ ).ro 5.29 5,34 5.26 4.86 7 ,08 7,25 I'1C0 16. 3 16,05 16.45 16.95 16.83 ro. b L4,9 15.0 L7,6 Cao 11.4 10.71 11.78 1r.68 r0.85 11.3 r0.93 11.38 10.84 Na20 2,7 2.01 2.29 2.46 2,L5 2.6 2,84 3.4 K20 L.22 L.48 r, 35 L.20 1.51 0.84 I.JJ H2o* 2.00 2.00 2.00 2.00

4 (r.{s+tFe) 0.79 0.80 o.7g o.8o o.8l 0.81 o.az 0.75 o.75 0.75 Mg GrFlFe'?+) 0,84 0,84 0.84 0.85 0.85 0,85 0.85 0.81 0.79 0.81 tEstircte, See Iat. sAnelyBt! B,0. Ween, EArclyst, B. 0, Waa AMPHIBOLES IN ANDESITE AND BASALT l08l

Tnslr 6b. ChemicalFormulae (O-23) for Amphibolesin Table6a

123 910 13 14 15 16 L'| 18 19 6,536 6.438 6.683 6.54t 6.342 6.472 6.119 6.643 6.450 6,352 6.261 6.110 6.209 6.t77 6.2Lr 6.L21 6.t29 6,3r2 6.594 rv ^, L,464 L.562 L.3L7 1.459 1.658 r.528 1.281 1.357 1,550 1.648 1.739 1,890 L.791 L.823 1.7891.873 1.871 1.688 1.406 otVr 0.652 0,556 0,660 0.461 0.850 0.467 1.083 0,929 0.636 0.884 0.492 0.343 0.333 0.733 0.830 0.485 0.473 0.557 0.940 T1 o.2L7 0.226 0.L46 o.144 0.].32 0.251 0.042 0.047 0.135 0.070 o.245 0.2L' 0.301 0.226 0.200 0,157 0.150 0,135 0.404 -3+ fe 0.400 0.366 0.413 o.432 0.449 o.264 0.75L 0.845 0.333 0.423 0,5110,529 0.38r 0.655 0.580 0.364 0.441 0.389 0,278 !tg 3.O22 3.228 2.863 2,985 2.520 3.L88 2.227 2.127 3.048 3.081. 2.6L4 2.793 2.899 2,58L 2.859 3.558 3.490 3.562 2.L86 Fe-' 1,109 1.017 1,147 L.202 L.247 0.978 1.043 L,L72 1.234 0,588 1.4r0 0.910 0.805 0.506 0.611 0,539 1.026 CE 1.258 1.404 1.388 t.574 L.449 1.684 r.193 L.297 L.47L r.544 L.437 L.563 1.565 1.549 L.406 r.952 1.795 1.784 1.618

Na 0,504 0,533 0.66r 0.618 0.707 o,574 0.648 0.619 0.502 0.941 o,7L7 0.684 0,565 0.582 0.515 0.572 o.646 0.443 0.374 0.064 0,062 0.060 0.059 0.102 0.061 0.044 0.042 0.094 0.086 0.097 0.088 0.136 0.102 0.r09 0.097 0.096 0.10r 0,106

ca+Na+K 1.926 1,999 2,LO9 2.255 2.258 2.3L9 I.885 L958 2.067 2,57L 2.25L 2.335 2.266 2.233 2,O3O2.62L 2.537 2.328 2.098 os 0,67 0.70 0.65 0.65 0.60 0.72 O.55 0.51 0,66 0.75 0,58 0.58 0,62 0.62 0.67 0.80 O,77 O.79 0,63 classl- t.h. t, t.h. t.h, t. t. t.h. t.h. t. t, t. t. t' t' t. p. t. t' t.h. flcat lon* Fe/Mg 0.50 0.43 0,54 0.55 0.67 0,40 0,81 0.95 0.51 0.33 O.74 O.72 0.62 0,6L 0.48 0.24 0.30 0.26 0,60

Fomula 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 6.500 6.092 5.853 6.445 6.3L2 6.r95 6.r39 6.O24 6.145 6.090 6.O87 6,2L8 6.146 6.273 6.r5r 6.121 5.906 5.970 ^,rv t.5oo t.908 2.147 r,555 1.688 r.805 1.861 1.9?6 1.855 1,910 1.913 1,782 L.854 r.727 1.849 r.8?3 2.O94 2.030 vr ., 0.783 0.989 0.5I1 I.100 0,543 0.392 0.444 0.316 0.457 0.756 0.724 0.503 0.436 0.639 0.5r3 0.747 0,352 0.225 0.460 0.262 0.L52 0.278 0.314 0.309 0.260 0.166 0.178 0.186 0.181- 0.184 0.1,76 0,175 0.229 0.Oas 0.246 0.L27 F"3r 0,313 0.559 o.23g o.zLr 0.326 0,339 0.329 0.79s o.246 o.226 o.225 o.z3o o.233 o.223 o.zL2 o.280 o.24o o.367 Mg 2,OO7 2.L55 3.2L5 2.104 3.O50 3,012 3,237 2.932 3.554 3.422 3.270 3.534 3.656 3.535 3.590 3.224 3.360 3.764 !e 1.161 1,553 0.661 0.781 0.905 0,940 0.9r3 1.108 0,682 0.628 0.625 0.638 0,646 0.620 0.590 0.777 0.890 0.870 UA L.757 L.354 2.684 r.462 1.563 1.820 1.697 1.530 L.787 L.64L 1.829 1.804 1.816 1.639 L.757 L.70L 1.833 1.667 Na 0.424 0.033 0.L73 0,485 0.660 0.587 0.668 0.702 0.766 0.574 0.643 0.651 0.690 0,588 0.732 0.800 0.991 0.979 K o.roo 0.068 0,002 0,133 0.133 0.131 0.146 0.r37 0.228 0.270 0.255 0.248 0,221 O,272 0,156 0.250 0.r82 0,167

Ca+Na+K 2.28L L.455 2.859 2.080 2.356 2.538 2,5Lt 2.369 2.78t 2,485 2.727 2,703 2.727 2.499 2,645 2.75L 3.006 2,813 ng 0.58 0.51 0.78 0,73 0,71 0.71 0.72 0,61, O.79 0.80 0.79 0.80 0.81 0,81 0.82 0.75 0.75 0.75 claesifl- t. t. p. t. t, t. t. t. p. t. P. p. p. t' p. p. P. P. cation* Felue 0,73 0.98 0.28 0.37 0.40 O.42 0.38 0.65 0,26 O.25 0,26 O.25 0.24 0.24 O.22 0.33 0.34 0.33 p. = potgasite; t. = tschemkite; t.h. = tschemakitic homblend.e *0n basis of 24 eugens, according to cl,ageification of Leake (L968)

olivine tholeiite with )ftro : 0.75, 0.50, and 0.25; ample by crystals sinking into hotter regions of a however, orthopyroxene is stable in the andesite basalticmagma, can enrich alkalis in the liquid and under these same conditions and occurs in greater develop nepheline-richcompositions. proportions than in the runs made with pure H2O. Fractionation of amphiboles,such as the one listed Under all conditions investigated,amphiboles in in Table 6, analysis 2, from a magma of andesitic the three basalts and in the nephelinite are stable to composition could lead to the development of a temperatures greater than those of the vapor- residual liquid intermediate between quartz diorite saturatedliquidus for the andesiticcomposition. The and granodiorite (Fig. 6). This subtraction diagram SiOz content of the amphiboles formed from these suggeststhat when MgO has diminished to zero by basaltsranges from 39.3 to 46.1 percent.Separation removal of approximately 20 percent amphibole of the low-silicaamphiboles from basalticmagmas by crystals,the liquid remainingwould contain 62.8per- partial melting or fractional crystallization in the cent SiOz. presence of water would be an effective process The garnetsformed in the Mt. Hood andesitecon- leading toward silica enrichment, such as proposed tain about 32 percentpyrope component, l9 percent by Bowen (1928). )Fe, and 9.4 percent CaO. Fractionation of such a Theseresults are also consistentwith the proposal garnet in magmasat pressuresabove the breakdown set forth by Bowen (1928)and developedin detail by oI amphibolewould lead to enrichmentof alkalisand Tuthill (1968) that resorption of amphiboles,for ex- silica in the residual liquid. Essentially the same 1082 ALLEN, BOETTCHER, AND MARLAND

Tlslr 6c. NormativeCompositions (C.l.P.W.) of Amphibolesin Table6a

orlhoclase z,o1 2.ol 1.95 1.89 3,25 r,95 L.42 1.36 3,01 2.72 3.01 2.74 4.3L sLblte 18.35 L6.25 20.L4 L7.55 15.48 !4.4L 19,80 18.96 15.15 tt,22 14.31 4.28 9.31 anorthlte 23.34 24.62 20.30 20.03 27.L5 2!.69 27.tO 26.35 25.41 2t.99 22.96 22.59 nepheline u. oo 5. Lt 9.05 3.61 6.52 4.O9 leuclte corunduD dlopside 13,79 16.58 L9.76 24.80 15.51 25.56 9.32 t2.72 L7,44 L9.70 18.31 2t,22 21.85 vo (7 .2O) (8.69) (r0,27) (12.89) (8.01) (13.40) (4.84) (6.s8) (e,05) (r0.42) (9.46) (10.e6) (11.32) en (5.06) (6.32, (6.95) (8.69) (5.12) (e.76) (3.27) <4.28) (5.99) (8.14) (6.06) (6.ee) (7.43) f6 (1.53) (1.57) (2.54) (3,22) (2.38) (2.40) (1.21) (1.85) (2,40) (1.14) (2.7e) (3,21) (3.10) hypersthene 10,51 6.11 7.35 22.57 1.70 en (8.08) (4.90) \). Jv, (r8.48) (r5.74) (1.21) fs (2,43) (1.2I) (1.97) (6.82) (6.83) (0.49) olivlne 20.58 23,62 20.70 25.72 25.40 24.0L 4.27 5.09 28.32 2L.73 24.59 26.26 26.39 fo (15.4s) (18,s6) (r4,77) (18.26) (16.80) (r8.eo) (3.04) (3.44) (r9.63) (18.83) (16.30) (17.33) (r8.09) fa (5.13) (5.06) (5.e3) (7.46) (8.50) (5.11) (1.23) (1.6s) (8.69) (2.90) (8.29) (8.93) (8.30) cs Mgnetite 5.37 4.93 5.55 5.80 5.97 3.51 10.14 11.41 4.44 5,54 5.60 6.93 5.03 llmenite 3.82 3,99 2.58 2.53 2.30 4.37 0.74 0.84 2.37 r.20 4.76 3.67 5.22

I4 I7 23 orthoclase 3.31 3.55 3.13 3.13 3.25 3.43 3.25 2.r9 4.31 4.26 4.26 e1b1te 15. 68 15.82 0.77 3.60 8.83 11,42 12.95 r.O2 14.81 t2.2r 5,32 anoEthlte 30.26 32.54 27.rr 26.00 27.39 30.18 28.46 43.67 39.31 32.99 22.96 23.94 nepheline 1, 13 8.98 8.78 2,5L 2.80 Ieucite 0.05 corunduo 0.51 diopslde 15.76 r0.54 27.93 25.42 23.63 L7.74 22.83 16.81 11.41 21.39 27.80 wo (8.31) (5.57) (14.88) (13.51) Q2.57) (9.27) (11.88) (8.88) (6.01) (Lr.27) (14.63) en (6.31) (4,3t) (r2,29) (10.92) (10.26) (6.56) (8.07) (6.84) (4.53) (8.s4) (11.00) fs (I. 14) (0.63) (0.76) (0.9e) (0.80) (r-.el) (2 ,88) (1.09) (0.87) (1.58) (2,r7)

hwhaieih6h5 22.89 14.39 38.33 8.27 en (3.10) (r7.1 4) (10.61) (23,70) (6.93) fs (0.4s) (3.78) (14.63) (1.34) olivine 20.06 2r.3L 2I.70 4.66 1.59 32.23 17.15 22.47 2L28 fo (16.7I) (18.38) (20.32) (21.04) (21.85) (0.87) (3.35) (o.es) (2O.97) (14,14) (18.67) (17.48) fa (3.35) (2.93) (1.38) (2.10) (1.86) (0.28) (1.31) (0.64) (3.69) (3.0r) (3.80) (3.80) (7.57 ) mgnelite 8.82 7.81 4.87 5.96 5.22 3.74 4.22 7.50 2.84 4.34 4.57 i.Inenlte 3 ,99 3.55 2.75 8 .13 4.60 4.92 5,41 5.47

Norn 26 27 28 29 30 31 32 33 34 35 36 37 orEhoclase 4.61 4,37 6.00 2.65 8.92 4.96 albite 3.74 7.79 1.59 0.30 anorthiLe 23.43 23.2I 20.84 29.48 27,75 22,23 22.03 24.72 23.51 25.07 19.59 17.86 nepheline 8.70 7 .24 12.38 9.49 10.50 10.68 rr.28 8.99 rr.76 73.02 15.59 16.14 Leuclte 5.65 2.r5 6.40 4.r8 5.56 6.26 4.40 4,22 corundun diopside 23.16 20.47 26.68 18.65 23.08 28.16 26.95 22.95 25.60 22.9L !7.42 25.24 wo (12. 51) (10.73) (14.10) (9.88) (L2.22) (14.91) G4.27) (12.16) (13.59) (r2.03) (9.31) (13.43) en (9.39) (7.83) (10.97) (7.79) (9.58) (1r.76) (11.25) (9.60) (10.94) (8.83) (6.96) (10.94) fs (1. 86) (1.e1) rl_ut' rl_*' rl_ru' r1-r" (1.43) (r.19) (1.07) (2.05) (r.49) (0.87) hyperothene

fs

olivine 23.37 23.13 24.70 25.77 23.05 23.30 25.35 25.74 23.59 25.02 30.78 26.25 fo (19.18) (18.23) (20.76) (22.57) (19.70) <20.46) (2r.77) (22.64) (2r.30) (19.81) (21.30) (23.0s) fa (4.1e) (4.90) (3.34) (3.14) (2.90) (2.84) (3.04) (3.10) (2.2e) (5.08) (5.01) (2.04) cs (0.60) (0.45) (0.60) (0.13) (4,47) (1,16) @gne!ate 4 .37 10.61 3.05 2.99 3.07 3.10 3.05 2.81 3.7L 3.07 4.93 llnenire 4,46 2.89 3,08 3.29 AMPHIBOLES IN ANDESITE AND BASALT

Tnet-s 7a. Composition of Quenched Liquids andesiticand basalticcomposition would be released (Glass) by melting at thesedepths. Amphiboles have higher NazO/KzO and lower alkali/HzO than do micas;thus, Analysis L' 2" when phlogopite or biotite forms from amphibole, Rock Andesite Andesite 900 920 excessH2O and NarO will dissolvein a silicatemelt. P, Kbar 13 rJ Phasessuch as phlogopite would remain stable to Buffer N-NO M-H great depths, 100-175km (Modreski and Boettcher, slo2 68.3 ov.r 1972. 1973 Forbes and Flower, 1974),before they Tio2 u.o u.4 releaseadditional HzO and KzO. Ar203 2L.O It has been proposed that the KrO content in- Fer0r** o,37 Q.76 creases while K/Rb decreasesin volcanic rocks n 02 FeO 0,94 acrossisland arcs towards continental areas(Kuno, uco 0.4 0.4 1966; Hatherton and Dickinson, 1969; Dickinson, qo Ca0 1970;Jake3 and White, 1970);and similar observa- Na20 0.8 0.7 tions have been made across the Andes (James, K2o 0.8 0.7 l97l ). across the Southern California batholith Total 99.9 99.9 (Baird, Baird, and Welday, 1974) and the central Mg/ (Mg+[Fe) 0. 35 0.30 SierraNevada batholith (Batemanand Dodge, 1970). Partial fusion of eclogite in deeper parts of the Mysen aAnalyst, B.O. Iithosphere could contribute to these variations *xEstimate, see test, (Green and Ringwood, 1968). Jake3 and lilhite (1970),Fitton (1971),and Allen et al (1972)also sug- process would work in the three basalts and the gest that fractionation involving amphibole-liquid nephelinite, but the enrichment would not be as and mica-liquid equilibria during the partial melting pronounced, for clinopyroxene with or without of a subducting slab of oceaniccrust could account olivine would also be fractionated. for theselateral variations. Ringwood (1974, p. 190) states that amphibole Wyllie (1973)and othershave criticized our models fractionationwould not "greatly alter the Na/K ratio involving amphiboleson the basisthat they can play of residual liquid or partial melt" becauseNa/K of no part in the formation of magmasat distancesmore amphibolesand the liquids from which they crystal- than 100 km from a trench because-thedepths wou!$ lize are comparable.Comparison of the Na/K ratio be greater than those at which'amphibolesare stable of the starting m aterials (Table I ) with the (assuming a 45" dip subduction zone). Many amphiboles(Table 6) suggeststhat this is not true in petrologists-and-some geophysicists apparently are all cases;most of the amphibolesin the three basalts unaware that the dip of Benioff zones beneathcon- and some island arcs is considerably and the nephelinite have lower Na/K than the --.tiiental areas starting materials. Ringwood (1974) also statesJhat less. In his original description, Benioff (1954) amphibole fractionation would not "produce the describedthese seismiczones as having three parts: tholeiiteearly iron-enrichmenttrend," for the fe/Mg (1) a shallow zone extendingwith flat dips under the of amphibole is comparable to the FelMg of the continents to depths of about 60 km, (2) an in- magma. However, .our ddta (Tables I and 6) show that the \f /Vtg of all but two of the amphibolesis TleLs 7b. Normative Compositions(C.I.P W.) of lower than that of the starting material. Thus frac- Liouids in Table 7a tionation of theseamphiboles could contribute to the tholeiitic trend of early iron enrichment. Norm

The maximum depth to which amphiboles syn- Quartz 47.16 48.83 thesizedfrom theserocks can exist is about 75 to 100 0rthoclase 4.13 4.L4 Albite 5.92 km (Fig. 5). However, igneous amphiboles contain Anor thlte 29.27 29.27 (F-) (OH-) some in the sites,and recentexperimen- H)rpersthene T.4I 1.62 talwork by Hollowayand Ford (1975)reveals that F- Fs (0.4r) (0.62) (1.00) (1.00) greater pressures En bearing amphiboles are stable to Magnetite 0.54 1.10 (and temperatures)than their hydroxy analogs.Thus, Ilnenlte 1 .14 o.76 H2O, as well as alkalis, in amphibolesfrom rocks of Corundum 8.89 6.JO 1084 ALLEN, BOETTCHER, AND MARLAND

termediatezone with shallow dip (22-23" under Peru In, P Abelson, Ed., Researchesin Geochemistry John Wiley and and Chile) extendingto 200-300 km depth,and (3) a Sons, Inc, New York, p. 377-396. -, AND J. L. ENcleNo (1960) phase- deep zone with a dip of about 60o. Apparatus for equilibrium measurementsat pressuresup to 50 kilobars and Recent evidenceis that the dips of seismiczones, temperatures up to 1750'C. J. Geophys. Res. 65, 741-'148. such as thosealong the continentalmargins of South Bulrrruor, R. J., ,ruo D. H. GnseN (1968)Experimental study at America, are certainly small ((20'), although they high pressureson the origin of olivine nephelinite and olivine are difficult to determineat depthsless than about 70 melilitenephelinite magmas. Earth Planet.Sci. Lett.3,325-337. -, AND _ (197 Experimental km (seeIsacks and Molnar,1969; l97l;also Molnar, 1) study of crystal-liquid relationships at high pressures in olivine nephelinite personal and communication, 1974). In addition, the basanitecompositions. J. Petrol. 12, l2l-147. seismic zone probably does not define the upper BunNHnv, C WnyNs (1967)Hydrothermal fluids at the magmatic boundary of a subductingslab at depthson the order stage. In, H. Barnes, Ed,., Geochemistry of Hydrothermal Ore ol 70 km, but rather is within the slab (Isacks and Depo.sits Holt, Rinehart, and Winston, New York, p. 34-76. Cunr, S. P., nNo A. E. Rrncwooo (1964) Molnar, 1969).Thus, there is ample evidencethat in Density distribution and constitution of the mantle. Reu. Geophys.2, 35-88. areasof active calc-alkaline volcanismsuch as South DtcxrNsoN, W. R. (1970) Relations of ,granites, and America, the depths to the subduction zone may be derivative sandstones to arc-trench tectonics. Rers.Geophys. E, lessthan 60 km. Conditions at such depthswould be 8 I 3-860. well within the stability ranges of our amphiboles, Ecclrn, D H. (1970) Water-saturated and undersaturated mefting relations in two (abstr.). supporting our concept (see Boettcher, 1973) that natural andesites Geol. Soc. Am Abstr.Progr.2,544. crystal-liquid equilibria involving amphiboles is im- -- (1973) Crystallization and fractionation trends in the portant in the genesisof the andesite-basaltclan in system andesite-Hro-Cor-O, at pressuresto l0 kb. Geol. Soc. orogenic zones. Am. Bull. 84, 2517-25i2. EsseNr,E J., B. J. HpNsrN, rNo D. H. Gnrrr (1970)Experimen- Acknowledgments tal study of amphibolite and eclogite stability. Phys. Earth Planet. I nteriors, 3, 378-384. Conversations with Peter Molnar of the MassachusettsInstitute Eucsren, H P. (1957) Heterogeneousreactions involving oxida- of Technology regarding the nature of subduction zones provided tion and reduction at high pressuresand temperatwes. J Chem. valuable help This researchwas supported by NSF through grants Phys. 26, 1760-1761. CA-12737, Ca-41203,and GY-8536 to Boettcher. FIrrou, J G. (1971) The generation of magmas in island arcs. Earth Planet. Sci. Lett 11,63-67. References Fonsss, W. C., ero M. F. J. Fr-owen (1974) Phaserelations of titan-phlogopite, K,MgoTiAlrSi"Oro(OH).: A refractory phase in Alr-rN,J. C., P. J. Moonrsrr,C. Hnycoon,nNo A. L. Borr- the upper mantle?Earth Planet. Sci. Lett.22,60-66. rcHrn (1972)The role of water in the mantle of the Earth: The Fuoelt, R. F. (1965) Oxygen fugacitiesof basalticand andesitic stability of amphiboles and micas. 24th Int. Geol. Congr 2, magma. Geochim. Cosmochim Acta, 29, 1063-1075 23t-240. G,qNcur-v,J., rNp R. C NrwroN (1968) Thermal stability of Betno, A. K., K W. Bnrno, .rNo E E. WBr_oly (1974) Chemical chloritoid at high pressureand relatively high oxygen fugacity. "/. trends acrossCretaceous batholithic rocks of Southern Califor- Petrol.9, 444466. nia. Ceology, 2, 493-495. GIlnnnr, M C. (1969) Reconnaissancestudy of the stability of Brtrneu, P C., llo F C. W. DoocE (19?0)Variations of major amphiboles at high pressure. Carnegie Inst. Wash. Year Book, chemical constituents across the Central Sierra Nevada 64, t67-170. batholith. Geol Soc. Am. Bull.81,409-420. GnesN, D H. (1970) The origin of basaltic and nephelinitic BrNce, A. E., nNo A. L. Ar-sEE(1968) Empirical correction factors magmas. Trans. Leicester Lit. Phil. Soc. 64, 28-54. for the electron-microanalysisofsilicates and oxides.J. Geol.16, - (1971) Composition of basaltic magmas as indicators of 382-403 conditions of origin: Application to oceanic volcanism. Pftr7. BrutonR, H. (t954) Orogenesis and deep crustal structure- Trans. Roy. Soc. London A,26E,707-725. additional evidence from seismology Geol. Soc. Am. Bult. 65, - (1973)Conditions of meltingof basanitemagma from gar- 385-400 net . Earth Planet. Sci. Lett. 17,456-465. Boettcnrn, A. L. (1973) Volcanism and orogenic belts-the GnerN, T. H., nNo A. E. RrNcwooD (1968) Genesis of the origin of andesites. Tectonophysics,17, 223-240. calcalkaline suite. Contrib. Mineral. Petrol 18, -, B. O. MvsrN, nr.roJ. C. Alr_nN (1973)Techniques for the 105-t62. control of water fugacity and oxygen fugacity for experimenta- HlvrlroN, D L., C. WeyNr Bur.NsAM,AND E. F OsronN (1964) tion in solid-media high-pressureapparatus. J. Geophys.Res.78, The solubility of water and effects of oxygen fugacity and water 5898-5902. content on crystallization in mafic magmas. J Petrol. 5,2l-39. -, AND P. J. Wylr-rE (1968) The calcite-aragonite transition HernrntoN, T., lNo W. R. DrcxrNsoN (1969) The relationship measured in the system CaO-CO,-H,O. J Geol. j6,314-330. between andesitic volcanism and seismicity in lndonesia, the BowsN, N. L (1928) The Euolution of the lgneous Rocks. Lesser Antilles, and other island arcs. J. Geophys. Res. 74, Princeton University Press, Princeton. 5301-53l0 Bovo, F. R (1959) Hydrothermal investigationsof amphiboles. Hrr-1,R. E. T., lNo A. L. BoErrcHrn (1970)Water in the Earth's AMPHIBOLES IN ANDESITE AND BASALT 1085

mantle: Melting curves of basalt-water and basalt-water-carbon along the join hedenbergite-ferrosilite. lz J Sci 261-4, dioxide. Science, 167, 980-982. 295-324. Horrowny, J. R. (1970) Phase Relations and Compositionsin the MrnnrLr-, R. B., nNo P. J. Wvllrr (1973)Absorption of iron by Basalt-COr-HrO System at High Temperatures and Pressures. platinum capsulesin high pressurerock melting experiments. Ph.D. Thesis, The Pennsylvania State University, University Am Mineral 58, l6-20. Park, Pennsylvania. Moonssrr, P. J., nNo A. L BosrrcHrn (1972) The stability of -- (197I ) Composition of fluid phase solutes in a phlogopite and enstatite at high pressures:A model for micas in basalt-H,O-CO2 system. Geol. Soc. Am. Bull.82,233-238. the interior of the earth. Am J. Sci. 212,852-869 - (1973) The system pargasite-H,O-COz: A model for -, AND- (1973)Phase relationships of phlogopitein the melting of a hydrous mineral with a mixed-volatile fluid-I. Ex- system K,O-MgO-CaO-ALOg-Sior-H,O to 35 kilobars: A bet- perimental results to 8 kbar. Geochim. Cosmochim. Acta,37, ter model for micas in the interior of the earth. Am. J. Sci.273, 65l-666. 385-4I 5 -, ANDC. We,yxn BURNHAM(1969) Phase relations and com- MvseN, B. O., nrn A. L. Borrrcsen (1975a)Melting of a hydrous positions in basalt-H,O-COz under the Ni-NiO buffer at high mantle: l. Phaserelations of natural peridotite at high pressures temperatures and pressures (abstr.). Geol. Soc. Am Abst. and temperatures with controlled activities of water, carbon Progr l,104-105. dioxide, and hydrogen;J Petrol 16 (in press) -, AND - (1972) Melting relations of basalt with -, AND - (1975b) Melting of a hydrous mantle: IL equilibrium water pressureless than total pressure.J. Petrol.13, Geochemistry of crystals and liquids formed by anatexisof man- t-29 tle peridotite at high pressuresand high temperatures as a func- -, AND C. E. Fono (1975) Fluid-absent melting of the tion ofcontrolled activitiesofwater, hydrogen,and carbondiox- fluorohydroxy amphibole pargasite to 35 kilobars. Earth Planet ide. J. Petrol. 16 (in press). Sci. Lett.25,44-48. Rulcwooo, A. E. (1974) The petrologicalevolution of island arc IsAcKs, B, lNo P. MorNln (1969) Mantle earthquake systems.J. Geol. Soc London,130, 183-204 mechanismsand the sinking of the lithosphere.Nature,223, -, I D. MlcGnrcoR, AND F. R. BoyD (1964) Petrological ltzt-l t24. composition of the upper mantle. Carnegie Inst. Wash. Year -_, AND _ ( l97l ) Distribution of stresses in the Book,63, 147-152. descendinglithosphere from a global survey of focal-mechanism SrenN, C. R., nno P. J Wvt-lre (1975)Effect of iron absorptionby solutions of mantle earthquakes. Reu Geophys.Space Phys.9, noble-metal capsules on phase boundaries in rock-melting ex- 103-I 74. perimentsat 30 kilobars Am. Mineral.60, 681-689. Jers5, P., ANDA J. R. WHrrE (1970)K/Rb ratios of rocks from Turnrrr, R. L. (1968) The Hydrothermal Behauior of Basalts in island arcs. Geochim. Cosmochim. Acta. 34. 849-856. Their Melting Range at 5 Kilobars. M. S. Thesis, The Penn- (1971) hrrrrs, D. E. Plate tectonic model for the evolution of the sylvania State University, University Park, Pennsylvanra. Central Andes. Geol. Soc. Am. Bull. E2,3325-3346. WINcurrl, H (1947) Honolulu series,Oahu, Hawaii. Geol. Soc. JoruNNrs, W., P. M. BgLL,H K. Mlo, A. L. Botrrcnrn, D. W. Am Bull 58. l-48. CHrelrlN, J. F. Hnys, R. C. NrwroN, ANDF. Ssrrenr (1971)An Wrsr, W S (1969)Geology and petrology of the Mt. Hood area: interlaboratory comparison of piston-cylinder pressurecalibra- A study of high Cascade volcanism. Geol. Soc. Am. Bull 80, tion using the albite-breakdown reaction. Contrib. Mineral. 969-r006. Petrol. 32. 24-38. WyLLIE, P J. (1973) Experimental petrology and global KuNo, H. (1966) Lateral variation of basalt magma type across tectonics-a preview Tectonophysics,11 , 189-209. continental margins and island ar cs.Bull Vol canol. 29, 195-222. Yooen, H. S, JR, nNo C. E. Trrrry (1962)Origin of basalt Lnlrrenr, I. B., lxo P. J. WvLLTE(1968) Stability of hornblende magmas: An experimental study of natural and synthetic rock and a model for the low velocity zone. Nature,2l9,1240-1241 svstems.J Petrol 3.342-532. Lrlrr, B. E. (1968) A catalog of analyzedcalciferous and sub- calciferous amphiboles together with their nomenclature and as- sociated minerals. Geol. Soc. Am. Spec. Pap. 68. Manuscript receiued, February 5, 1975; accepted LINosLey, D. H., AND J. L. Muroz (1969) Subsolidusrelations for publication, July 7, 1975.