Experimental stability relations of the hornblende magnesiohastingsite: Summary
MICHEL P. SEMET* Department of Earth and Space Sciences, University of California, Los Angeles, California 90024 W. G. ERNST
Phase relations for the bulk composition TABLE 1. Mh DECOMPOSITION
Na20-4CaO 8MgO- Fe2CV12SiC>2-2A1203 f0! buffer Fluid pressure in bars + excess H20 have been determined as a 300 500 1,000 function of fluid pressure (P), temperature (T), and oxygen fugacity {f,,.,) using conven- Iron-quartz-fayalite (IQF) 692 °C 785 °C 944 °C tional hydrothermal apparatus and the Fayalite-magnetite-quartz (FMQ) 860 °C 925 °C 1,011 °C oxygen buffer method. Magnesio- Manganosite-hausmannite (MMO) 960 °C — — 3+ Hematite-magnetite (HM) 968 X 1,018 °C — hastingsite, NaCa2Mg4Fe Si6
A12022(0H)2, abbreviated as Mh, is stable Note: Conventional Pnui
olivine, respectively, with increasing f0,. Conversion of the high-termperature as- semblages to 100% Mh was not effected : 600 even 100 °C below the first appearance of amphibole. This is thought to result at least in part from the fact that production of an equilibrium cation distribution at a given P , 400 and T is a very slow process. A highly ordered amphibole should be stable to higher temperatures than a chemically equivalent disordered phase — thus 200 presumably relatively disordered Mh was synthesized from and coexists with the breakdown products. Mh decomposition in the presence of excess aqueous fluid occurs 750 800 under the conditions shown in Table 1. Temperature in Conventional Pn i The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 92, no. 2, p. 274—357. Geological Society of America Bulletin, Part I, v. 92, p. 71-74, 8 figs., 1 table, February 1981, Doc. no. S10203. 71 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/2/71/3444494/i0016-7606-92-2-71.pdf by guest on 29 September 2021 Fayalite- quartz- magnetite Mangonosite-hausmonnite 1000 -1 O—O—CO-COT 600r 500- 800 dJduaa Magnesiohastingsite Magnesiohastingsite + + -400- Cpx + 01 Fluid : 600 Fluid Cpx + 01 + Ne + Sp +Ne+Sp + Mt + L ^ 300 - V + L + Fluid + Fluid 400 .2 200 - • • 200 100- » A m A' 750 800 850 900 950 1000 1050 1100 750 800 850 900 950 1000 1050 1100 Temperature in °C Temperature in Figure 3. /Vmi -T diagram for Mh + fluid bulk composition with f0, defined by the Figure 2. Pnu-,d-T diagram for Mh + fluid composition withf{H defined by the FMQ MMO buffer. Symbols follow usage of Figures 1 and 2 cxcept that Mt has joined the high- buffer. Symbols follow usage of Figure 1, except as follows: triangles indicate runs-where temperature assemblages. Half-filled symbols indicate that the assemblage Mh + Cpx + 01 the assemblage Cpx + 01 + Ne + Sp ± Mt was analyzed, and inverted triangles, run + Ne + Sp + Mt + F persisted. Runs at 100 bars do not represent equilibrium with respect where melt was present after reaction. to amphibole. Hematite-magnetite 600 500 Magnesiohastingsite + : 400 Cpx + 01 Fluid +Ne+Sp + M1 + L Figure 4. PnuW -T diagram for Mh + fluid bulk ) 300 + Fluid composition with^,2 defined by the HM buffer. Symbols follow usage of Figures 1 and 2. 200 100 750 800 850 900 950 1000 1050 1100 Temperature in °C Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/2/71/3444494/i0016-7606-92-2-71.pdf by guest on 29 September 2021 1000 Pfluid = 400barS 0.00 -5.00 2.-10.00 -15.00 -20.00 750 800 850 900 950 1000 1050 Temperature in °C -25.00 Figure 5. Magnesiohastingsite and pargasite dehydration curves: (1) 650 700 750 800 850 900 950 1000 1050 1100 Mh, IQF buffer; (2) Mh, FMQ buffer; (3) Mh, HM buffer; (4) pargasite Temperature in °C (Boyd, 1959). Figure 6. Isobaric Pnuid (Ptotai = 400 bars) log/,.,-7' diagram for Mh + fluid bulk com- position (the abscissa is linear for IIT ( K)). Field I is for the assemblage Cpx +01 + Ne + 3+ Fe(3+) +T¡/AI + Fe( '+Ti Sp + Mt + F. Field II is the high fo2 area where Cpx + Ol + Ne + Sp + L + F is the stable .10 .20 .60 .80 - 30 .70 .90 1.00 assemblage. See text for discussion. 1.00 -1 ~1— ~1— .90 80 + c + .70 o 4- I o ,amphiboles that closely approximate Mh and occur in igneous rocks. Analyses are from Leake (1968). Open circles are analyses from dioritic Pargasite Magnesiohastingsite and andesitic rocks. Filled circles represent amphiboles from gabbroic and basaltic rocks. .40 Ferropargasite Hastingsite .30 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/2/71/3444494/i0016-7606-92-2-71.pdf by guest on 29 September 2021 74 SEMET AND ERNST 3+ pe(3+) +Ti/AI+Fe< '+Ti H20-rich volatiles will raise the prevailing ,0 JO 20 .30 40 .50 .60 70 .80 .90 LOO ft>2 relative to the totally anhydrous system) 1 1 1 1 i • i i i may leave Mh-rich amphiboles as a refrac- • • tory residual phase. However, the over-all • • paucity of hornblendes in upper-mantle • • • parageneses attests to low fH,0, high total pressure and, most importantly, relatively • low fo, . - • . • • • REFERENCES CITED Allen, J. C., Boettcher, A. L., and Marland, G., 1975, Amphiboles in andesite and basalt: I. Stability as a function of P-T-f0, : American Mineralogist, v. 60, p. 1069-1085. Pargasite Magnesiohastingsite Boyd, F. R., 1959, Hydrothermal investigations of amphiboles, in Abelson, P. H., ed., Re- searches in geochemistry: New York, John Wiley 8c Sons, Inc., p. 377-396. Cawthorn, R. G., and O'Hara, M. J., 1976, Am- phibole fractionation in calc-alkaline mag- mas genesis: American Journal of Science, Ferropargasite Hastingsite v. 276, p. 309-329. Conquéré, F., 1977, Petrologie des pyroxéxnites litées dans les complexes ultramafiques de Figure 8. Compositions of amphiboles that occur as primary minerals in peridotite l'Ariège (France) et autres gisements de lherzolite à spinelle. I. Compositions nodules from basaltic rocks and in rocks of the lherzolite clan. minéralogiques et chimiques, évolution des conditions d'équilibre des pyroxénites: Bul- letin Société français Minéralie et Cristal- lographie, v. 100, p. 42-80. Incongruent melting is initiated on the solid solutions from Leake's (1968) Ernst, W. G., 1968, Amphiboles, crystal chemis- amphibole reaction curve at a minimum catalogue occurring in basalts, gabbros, try, phase relations and occurrence: New York, Springer-Verlag, Inc., 125 p. fluid pressure of 420 bars at 1002°C (HM andesites, and diorites are presented in Fig- 1978, Petrochemical study of some lherzoli- buffer). Nepheline probably dissolves in the ure 7. These are thought to be representa- tic rocks from the Western Alps: Journal of silicate liquid at temperatures and pressures tive of the compositional variability of am- Petrology, v. 19, p. 341-392. slightly in excess of the investigated range. phiboles stable with mafic + intermediate Fudali, R. F., 1965, Oxygen fugacities of basaltic Mh is thus stable well within the magmatic melt under crustal conditions. Primary and andesitic magmas: Geochimica et Cos- range. As evident from Figure 5, at high hornblendes are relatively well represented mochimica Acta, v. 29, p. 1063-1075. Holloway, J. R., and Ford, C. E., 1975, Fluid- oxygen fugacities Mh is the most refractory in igneous rocks of the alkali basalt and absent melting of the fluoro-hydroxy am- hydroxyl-bearing, Ti-free amphibole end calc-alkaline series (for example, see Allen phibole pargasite to 35 kilobars: Earth and member yet studied experimentally. Its and others, 1975; Cawthorn and O'Hara, Planetary Science Letters, v. 25, p. 44-48. upper thermal stability limit is raised by 50 1976). However, primary hornblendes are Leake, B. E., 1968, A catalogue of analyzed cal- ciferous and subcalciferous amphiboles to- to 100 C° compared to pargasite (Boyd, virtually absent from oceanic tholeiites, gether with their nomenclature and as- 1959). Enhanced thermal stability of probably as a consequence of low values of sociated minerals: Geological Society of :i+ hornblendes containing Fe instead of A1 /H,Kaersutite and kaersutite eclogite from Kakanui, New strated (for example, see Ernst, 1968; conditions at H20 fugacities less than about Zealand — Water-excess and water- Thomas, 1977, 1981). The clear diminution 500 bars. deficient melting to 30 kilobars: Geological of the thermal stability limit of this ferric Analyzed Mh from lherzolites and Society of America Bulletin, v. 86, p. 555- iron-bearing hornblende at lower oxidation peridotite nodules are shown in Figure 8. 570. states, however, is illustrated in the 400-bar Even though occurrences are relatively rare Thomas, W. M., 1977, Preliminary stability re- lations of the hornblende hastingsite, and isobaric log F(1,-T section presented as Fig- in high-temperature peridotites, those that 3+ the effect of Fe for aluminum replacement ure 6. have been reported (for example, see Con- in amphiboles [abs.J EOS (American Standard enthalpy and entropy of reac- quéré, 1977; Ernst, 1978) indicate that Geophysical Union Transactions), v. 58, tion for the equilibria Mh = Cpx + Ol + hornblendes of the pargasite-magnesio- p. 1244. Thomas, W. M., 1981, Stability relations of the Ne + Sp ± Mt + F range from 50 to 30 hastingsite series may be stable under cer- amphibole hastingsite: American Journal of kcal/mol and 37 to 35 cal/K/mol, respec- tain upper-mantle conditions. Characteris- Science, v. 281 (in press). tively, from low to high fo,. tically, such hornblendes are enriched in F, Because of their extensive P-T stability Ti, and Fe:i+; experiments conducted by fields, Mh-rich hornblendes should be pres- Holloway and Ford (1975) and Merrill and ent in igneous and high-rank metamorphic Wyllie (1975) demonstrated enhanced MANUSCRIPT RECEIVED BY THE SOCIETY DE- rocks whose norms contain major magne- maximum P-T ranges for such amphiboles. CEMBER 26, 1979 sian Cpx + Ol and aluminum phases. Possibly the incipient melting of a fluid- REVISED MANUSCRIPT RECEIVED JUNE 6, 1980 Compositions of 65 pargasite-hastingsite modified upper mantle (stoichiometric MANUSCRIPT ACCEPTED JULY 7, 1980 Printed in U.S.A. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/2/71/3444494/i0016-7606-92-2-71.pdf by guest on 29 September 2021