Fluid-Mineral Interactions: A Tribute to H. P. Eugster© The Geochemical Society, Special Publication No.2, 1990

Editors: R. J. Spencer and I-Ming Chou

The aluminum content of hornblende in calc-alkaline granitic rocks:A mineralogic barometer calibrated experimentally to 12 kbars *

WARREN M. THOMAS and W. G. ERNST'Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90024-1567, U.S.A.

Abstract-Partial melting experiments on a natural hornblende-bearing tonalite have been conductedin a piston-cylinder apparatus in order to determine the total aluminum content of hornblende inequilibrium with quartz + K-feldspar + plagioclase + biotite + epidote + sphene + Fe-Ti oxide+ melt + fluid at 750°C as a function of pressure in the range 6-12 kbar. A hornblende of similarcomposition to that in the tonalite itself, but possessing a higher aluminum content, was added tothe starting material. In runs of one week duration, rims formed on both types of hornblende grains,with the compositions converging from initially different values in the two hornblende populations.

Using this technique, a calibration for the total Al content of hornblende as a function of aqueousfluid pressure at a constant, near solidus temperature was derived: P (±1.0 kbar) = -6.23 + 5.34AlT. The pressures calculated from this equation are appropriate for granitoid melts crystallizing atapproximately 750°C. They agree well with a previous experimental calibration, but are too low bymore than a kbar when extrapolated to low pressures.

INTRODUCfION measure convergence in rim compositions of thetwo hornblendes as a function of pressure.

STARTING MATERIAL

DETERMINAnON of the pressure of crystallizationof calc-alkaline plutons has historically been diffi-cult; most efforts have relied largely on constraintsgleaned from appropriate mineral assemblages, ifany, in the contact aureole. HAMMARSTROM andZEN ( 1986) and HOLLISTER et al. (1987) empiri-cally calibrated an approximately temperature-in-dependent barometer based on the Al content ofhornblende coexisting with quartz + K-feldspar+ plagioclase + biotite + sphene + Fe-Ti oxide(+ melt) in calc-alkaline granitoids. In practicalterms, the nearly isothermal nature of the solidusat middle and deep continental crustal levels allowsAIT in the amphibole to be employed as a miner-alogic barometer. RUTTER and WYLLIE (1988)published a note describing agreement with thisnaturally-calibrated barometer of three experimen-tally altered hornblende compositions in a tonalite,but under vapor-absent conditions and with garnetin the subsolidus assemblage. JOHNSON and RUTH-ERFORD (1988, 1989) have recently calibrated thisbarometer with reversed experiments in the pressurerange 2 to 8 kbars using natural starting materialand an internally heated gas apparatus.

The purpose of the present study was to calibrateexperimentally this hornblende barometer at higheraqueous fluid pressures using a piston-cylinder ap-paratus. The basic approach to the problem was tomelt partially a natural hornblende-bearing tonaliteto which a second, chemically similar but morealuminous hornblende had been added, and to

The starting material for the experiments wasa tonalite from the Late Cretaceous JosephineMountain Intrusion from the San Gabriel Moun-tains, California. The mineral assemblage of thissample is quartz, plagioclase, K-feldspar, horn-blende, biotite, epidote, magnetite, sphene, and zir-con; the rock is fresh and unaltered. Because theamount of alkali feldspar is somewhat variable fromsample to sample, 10 wt% natural orthoclase ofcomposition Or95Ab05Anoo was added to thecrushed sample to insure its presence in each ex-perimental charge. The natural granitic material waslightly crushed to a maximum grain size of 0.1 mm,but was not sieved to avoid eliminating mineralswhich might be preferentially represented in thefiner-grained fractions. For most of the critical ex-periments, the starting material was altered by add-ing a second hornblende similar to that in the ton-alite and crushed to the same grain size. This in-troduced hornblende, which has a higher aluminumcontent and lower total iron content than that ofthe indigenous hornblende, is from a high-pressuregarnet hornblendite from Santa Catalina Island,California (SORENSEN, 1988; JACOBSON and So-RENSEN, 1986). Although the coexisting mineralassemblage of this rock consists primarily of garnet+ zoisite, it was chosen in order to obtain a horn-blende with a very high aluminum content to con-trast with that from the tonalite. The two horn-blendes are easily distinguished in the experimentalcharges, both optically and by the compositions oftheir cores. The hornblende from the tonalite isslightly zoned, and inasmuch as spatial relationships

59

* Institute of Geophysics and Planetary Physics Publi-cation No. 3241.

I Present address: School of Earth Sciences, StanfordUniversity, Stanford, CA 94305-2210.

60 W. M. Thomas and W. G. Ernst

Table I. Composition of starting material, in weightpercent

Weight Natural Added Introduced Startingpercent tonalite* K-feldspar hornblende material

Si02 57.2 64.81 42.84 56.7Ti02 0.78 0.02 0.58 0.69AhO) 18.0 18.15 17.59 18.0Cr20) n.a. 0.04 0.29 0.06FeO** 6.06 0.05 9.91 5.84MnO 0.12 0.Q2 0.05 0.11MgO 2.7 0.00 10.98 3.13CaO 6.42 0.00 11.42 6.26Na20 3.9 0.53 2.05 3.47K20 1.89 16.55 0.56 3.07P20S 0.43 n.a.j+ n.a. 0.36tCO2 0.28 n.a. n.a. 0.23tH20t 0.69 n.a. n.a. 0.57tTotal 98.47 100.17 96.27 98.49t

of this modified starting material is given in Table1. Chemical analyses and cation proportions ofhornblendes are presented in Table 2. Except forAl and Fe content, the amphiboles employed in theexperiments are closely comparable. Analyses arereported with all iron treated as ferrous, althoughthey are compatible with ferric iron contents rangingfrom 5-20%. Changes in the estimated amount offerric iron generally affects the calculated AlT byless than 2% relative. This starting material was runin sealed capsuleswith 10wt%distilled H20, enoughto saturate the melt experimentally generated bypartial fusion.

EXPERIMENTAL PROCEDURE

All experiments were conducted in a piston-cylinderapparatus at UCLA using the NaCl cell furnace assemblyof BOETTCHERet al. ( 1981); no pressure corrections weredeemed necessary. The press was calibrated employing theequilibrium albite = jadeite + quartz (JOHANNESet al.1971); nominal pressure is considered accurate to 200bars. Temperatures were measured with Pt-~oRhIO ther-mocouples and are thought to be known to ±5°C.

All experiments were run in Pt or Ag capsules of 11.4mm length and 2.79 mm o.d. Because as large a chargeas possible was desired, these experiments were performed

* XRF analysis by A. Barth.** All Fe reported as FeO.t Minimum values.tt Not analyzed (n.a.).

are partially lost through crushing, the zoning con-tributes to the uncertainty in original Al content of±0.05 (ItT)per 23-oxygen formula. The composition

Table 2. Compositions of original and experimental hornblendes (wt. %)

Original 6 kbar 8 kbar 10 kbar 12 kbar

A* B* A B A B A B A B

Si02 44.12 42.84 40.66 41.86 39.44 40.33 38.68 38.71 39.00 39.40Ti02 1.24 0.58 0.76 0.98 1.16 1.64 0.87 1.01 0.61 0.65AhO) 9.68 17.59 13.26 12.81 15.02 14.55 16.82 14.72 20.00 19.61Cr203 0.02 0.29 0.08 0.11 0.00 0.00 0.05 0.02 0.00 0.D7FeOt 17.72 9.91 15.44 13.78 18.11 18.47 16.74 16.66 17.06 16.30MnO 0.54 0.05 0.40 0.30 0.36 0.24 0.33 0.27 0.20 0.24MgO 10.48 10.98 9.28 11.18 7.90 8.15 8.57 8.48 6.34 6.66CaO 11.76 11.42 11.88 11.96 11.45 11.50 11.34 11.31 10.86 11.04Na20 1.39 2.05 1.47 1.53 1.92 1.76 1.95 1.93 2.08 2.13K20 1.07 0.56 1.92 1.75 1.58 1.70 1.77 1.56 1.78 1.72Total 98.02 96.27 95.15 96.26 96.94 98.34 97.12 94.67 97.93 97.82

Cations per 23 oxygens

Si 6.64 6.26 6.30 6.34 6.07 6.11 5.90 6.07 5.86 5.91AIT 1.72 3.03 2.42 2.29 2.72 2.60 3.03 2.72 3.54 3.47Al'v 1.36 1.74 1.70 1.66 1.93 1.89 2.10 1.93 2.14 2.09Alv, 0.36 1.29 0.72 0.63 0.79 0.71 0.93 0.79 1.40 1.38Ti 0.14 0.06 0.09 0.11 0.13 0.19 0.10 0.12 0.D7 0.D7Cr 0.00 0.03 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.01Fe2+ 2.23 1.21 2.00 1.75 2.33 2.34 2.14 2.18 2.14 2.04Mn 0.07 0.01 0.05 0.04 0.05 0.03 0.04 0.04 0.02 0.03Mg 2.35 2.39 2.14 2.52 1.81 1.84 1.95 1.98 1.42 1.49Ca 1.90 1.79 1.97 1.94 1.89 1.87 1.86 1.90 1.75 1.78Na 0.41 0.58 0.44 0.45 0.57 0.52 0.58 0.59 0.61 0.62K 0.21 0.10 0.38 0.34 0.31 0.33 0.34 0.31 0.34 0.33

* A = indigenous hornblende; B = introduced hornblende.t All Fe reported as FeO.

Aluminum content of hornblende: barometer

unbuffered with regard to oxygen; 10, imposed by the fur-nace assembly and press is between Ni-NiO and hematite-magnetite buffers, as corroborated by running charges ofthose assemblages at the conditions of the experiments.Moreover, JOHNSON and RUTHERFORD (1988, 1989)stated that they found a minimal effectin their experimentsby variation of oxygen fugacity between the Ni-NiO andthe MnO-Mn)04 buffers. No evidence was found in anyof the runs of oxidation or reduction of Fe in the charges.All experiments were of one week duration. Initially, a

variety of temperatures were used, ranging from 6S0 to900°C, but the final critical experiments were conductedexclusively at 7S0°C to increase reaction rates. Experi-ments were performed at pressures of 6, 8, 10, and 12kbars, with one 2-kbar experiment conducted hydrother-mally in a cold-seal pressure vessel.At the conclusion of each experiment, the capsule was

mounted on a glass slide, and ground and polished for theelectron microprobe. Even small degrees of partial meltingcaused the sample to cohere and behave like a typical rockin the thin-sectioning process. Each experimental productwas examined optically and subsequently with the electronmicroprobe. Amphiboles were chemically analyzed usinga Cameca Camebax microprobe employing IS kvaccel-erating voltage, 10 nanoamp sample current and a 1 µmbeam diameter. ZAF corrections were applied and majorelements are considered accurate to 1-2% relative, andminor elements to 2-10% relative.Zoned amphiboles were analyzed-and portrayed in

Fig. I-for only those well-constrained runs in which allstarting phases were present in addition to melt at theconclusion of the experiments.

RESULTS

In all experiments, melt was generated and rimsof distinctive composition up to 30 µm wide wereproduced on hornblendes in the charge. In mostcases, the change in composition appeared texturallyto have been accomplished by diffusional modifi-cation of the original grain rim; in a few samples,overgrowths were observed, as evidenced by eu-hedral rims in contact with melt. In all cases, therims exhibited compositions with allowed amphi-bole stoichiometries. The cores retained their orig-inal compositions; thus the two populations ofhornblendes were readily distinguished. The totalaluminum content in the indigenous hornblendewas altered from an original value of 1.72 (5) per23-oxygen formula unit to a rim maximum of3.54;for the introduced hornblende, the original valueof 3.03 (3) changed to a minimum of 2.29 and amaximum of 3.47. The aluminum content of therims is a clear function of pressure, with the highervalues associated with higher pressures, as seen inthe empirical and earlier experimental calibrations.Note also that, although the two initial amphibolespossessed contrasting Fe as well as Al contents, Ta-ble 2 demonstrates that experimentally producedrim compositions are sensibly identical-hencechemical equilibration is indicated.

61

3.4

~duced hornblende

3.0f- r I AI

~ I I 1/ IQ)C>>-~ 2.6(')N,_'

62 W. M. Thomas and W. G. Ernst

(J)CQ)0>>.>

Aluminum content of hornblende: barometer

Table 3. Comparison of barometers

AlTj23 oxygens This study J and R*

1.52.02.53.03.5

1.8 kbar4.57.19.812.5

2.9 kbar5.07.19.211.4

* JOHNSONand RUTHERFORD(1988, 1989).

Al-rich value for the indigenous hornblende andthe least AI-rich value for the introduced hornblendewould result in brackets that have apparently"passed" each other. Thus, this feature of the plotas presented may not necessarily imply disequilib-rium. On the other hand, the indigenous hornblendewas much closer to the composition in equilibriumwith the melt than was introduced hornblende, andthe latter may have initially adjusted to a more AI-poor, metastable composition. Inasmuch as all ex-periments in the present study were of the sameduration, it cannot be determined from them if theintroduced hornblende would have readjusted tomore aluminous compositions with time.

Consideration of the compositions of experi-mentally produced amphibole rim compositionsshows that both Al IVand Al VI increase with pres-sure. Fitting AIVI against AI IVfor individual analysesin Table 2 results in the equation

AlvI = -1.74 + 1.38AIIV, r2 = 0.65,suggesting that some of the variability in aluminumcontent is accounted for by a Tschermak type sub-stitution, R2+ + Si µ AIIV + AlvI. Fitting the rimcompositions of the indigenous hornblende and itsaverage initial composition yield

The same treatment for rim and initial compositionsof the introduced hornblende yields no significantcorrelation, suggesting that the introduced horn-blende may be adjusting its composition by a dif-ferent exchange mechanism.

In conclusion, results of the present study areconsistent with the calibration of JOHNSON andRUTHERFORD (1988, 1989) and extend the exper-

63

imental evidence for increasing aluminum contentof hornblende as a function of pressure to 12 kbar.

Acknowledgements-This experimental study was con-ducted at UCLA, supported by the Institute of Geophysicsand Planetary Physics and by the National Science Foun-dation through grant EAR86-16624. Valuable reviews ofthe manuscript and helpful comments were provided byM. Cho, J. M. Hammarstrom, Art Montana, M. J. Ruth-erford, S. S. Sorensen, E. D. Young, and E-an Zen. Theauthors thank the above-named institutions and research-ers for their support. The sample of tonalite was suppliedby A. Barth and the aluminous hornblende from CatalinaIsland by G. Bebout.

REFERENCES

BOETTCHERA. L., WINDOMK. E., BOHLENS. R. andLUTHR. W. ( 1981) Low-friction, anhydrous, low- tohigh-temperature furnace assembly for piston-cylinderapparatus. Rev. Sci. Instr. 52, 1903-1904.

HAMMARSTROMJ. M. and ZENE-AN(1986) Aluminumin hornblende: an empirical geobarometer. Amer. Min-eral. 71, 1297-1313.

HOLLISTERL. S., GRJSSOMG. c., PETERSE. K., STOWELLH. H. and SISSONV. B. (1987) Confirmation of theempirical correlation of aluminum in hornblende withpressure of solidification of calc-alkaline plutons. Amer.Mineral. 72,231-239.

JACOBSONC. E. and SORENSENS. S. ( 1986) Amphibolecompositions and metamorphic history of the RandSchist and the greenschist unit of the Catalina Schist,southern California. Contrib. Mineral. Petrol. 92, 308-315.

JOHANNESW., BELLP. M., MAO H. K., BOETTCHERA. L., CHIPMAND. W., HAYSJ. F., NEWTONR. C. andSEIFERTF. (1971) An interlaboratory comparison ofpiston-cylinder pressure calibration using the albitebreakdown reaction. Contr. Mineral. Petrol. 32,24-38.

JOHNSONM. C. and RUTHERFORDM. J. (1988) Exper-imental calibration of an aluminum-in-hornblendegeobarometer applicable to calc-alkaline rocks (abstr.)EOS69,1511.

JOHNSONM. C. and RUTHERFORDM. J. (1989) Exper-imental calibration of the aluminum-in-hornblendegeobarometer with application to Long Valley caldera(California) volcanic rocks. Geology, (In press).

RUTTERM. J. and WYLLIEP. J. (1988) Experimentalcalibration of hornblende as a proposed empirical geo-barometer (abstr.). EOS 69,87.

SORENSENS. S. (1988) Petrology of amphibolite faciesmafic and ultramafic rocks from the Catalina Schist,southern California: metasomatism and migmatizationin a subduction zone metamorphic setting. J. Meta-morphic Geol. 6,405-435.

YORKD. (1969) Least squares fitting of a straight linewith correlated errors. Earth Planet. Sci. Lett. 5, 320-324.

Page 1TitlesThe aluminum content of hornblende in calc-alkaline granitic rocks: A mineralogic barometer calibrated experimentally to 12 kbars *

Page 2TablesTable 1Table 2

Page 3Titles1.8 ~ Indigenous hornblende 12 8 6 ~duced hornblende 3.0f- r I AI ~ I I 1/ I