Basalt-andesite-rhyolite-H2O: Crystallization intervals with excess H2O and H2O-undersaturated liquidus surfaces to 35 kolbras, with implications for magma genesis

Earth and Planetary Science Letters, 28 (1975) 189-196 © Elsevier Scientific Publishing Company, Amsterdam

Printed in The Netherlands

BASALT-ANDESITE-RHYOL1TE-H20: CRYSTALLIZATION INTERVALS WITH EXCESS H=O AND H20-UNDERSATURATED LIQUIDUS SURFACES TO 35 KILOBARS, WITH IMPLICATIONS FOR MAGMA GENESIS C t t A R L E S R. S T E R N , W U U - L I A N G H U A N G and P E T E R J. WYI_LIE

Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y. (USA) Department of" Geology, National Taiwan University, Taipei (Taiwan) Department of Geophysical Sciences, University of Chicago, Chicago, Ill. (USA) Received May 22, 1975 Revised version received September 8, 1975

Three rocks representing the calc-alkaline rock series gabbro-tonalite-granite or basalt-andesite-rhyolite were reacted with varying percentages of water in sealed capsules between 600 and 1300°C and pressures to 35 kbars, corresponding to depths of more than 120 km within the earth. For each rock we present complete P T diagrams with excess water, and the water-undersaturated liquidus surface projected from P 7"- Xll20 space mapped with contours for constant H20 contents and with the fields for near-liquidus minerals. All changes in liquidus and solidus slopes can be correlated with changes in mineralogy from less dense to more dense, or with expansion of crystallization fields, without appeal to changes in molar volume of If20 in liquid and vapor phases. The results indicate that tholeiites and andesites of the calc-alkaline series with compositions similar to the rocks studied are not primary magmas from mantle peridotite at depths greater than about 50 km. Primary andesitic magmas from shallower levels would require very high water contents and we do not believe such magmas could normally reach the surface. The liquidus results are consistent with file derivation of andesites with little dissolved water as primary magmas from subducted ocean crust (quartz eclogite), but multi-stage models are preferred. Temperatures required for tile generation of andesites by fusion of continental crust are higher than considered reasonable. The evidence precludes the generation of primary rhyolites or granites from the mantle or subducted oceanic crust at mantle depths. Primary rhyolite or granite magmas with moderate water contents (saturated or undersaturated) can be generated in the crust at reasonable temperatures, and could reach near-surface levels before vesiculation. Water-undersaturated granite liquid with residual crustal minerals could constitute plutonic magmas of intermediate composition.

1. Introduction

including variable water c o n t e n t , carbon dioxide content, or oxygen fugacity. Many partial studies have been published previously for individual rocks. Here, we present for three rocks, representing basalt, andesite and rhyolite, the c o m p l e t e phase relationships to 35 kbars with excess water, and the three c o m p l e t e waterundersaturated liquidus surfaces, with liquidus or nearliquidus minerals. The compositions o f the basaltic intrusion, the tonalite and the granite listed in Table 1 correspond fairly well with basalt-andesite-rhyolite o f the calcalkaline series. The melting relationships of these rocks have been the subject o f a progressive experimental program, first with excess water at low pressures, then near-solidus at high pressures, and then with water-

The origin and evolution o f m o u n t a i n ranges and c o n t i n e n t s are intimately related to the origin o f the calc-alkaline rock series, which is p r o m i n e n t l y displayed in the volcanoes o f island arcs and continental margin arcs, and in batholiths. Calc-alkaline magmas or their precursors may be generated in deep continental crust, in s u b d u c t e d oceanic crust, in the mantle wedge above a subducted lithosphere slab, or in c o m p l e x processes involving material from all three environments. One approach to the petrogenesis o f the rocks is e x p e r i m e n t a l d e t e r m i n a t i o n o f their phase relationships through a range of pressures and temperatures, and under a variety of other conditions, 189

190 TABLE 1 Chemical analyses, CIPW norms, and approximate modes of the rock samples

SiO2 TiO2 AI203 l:e203 FeO MnO MgO CaO Na20 K20 1t20+ ft20 P2Os (702

Tholeiite (gabbro)

Tonalite (andesite)

Granite (rhyolite)

45.9 0.94 17.2 2.3 7.7 0.22 7.5 13.5 1.6 0.14 1.8 1.3 0.04

59.1 0.79 18.2 2.3 3.6 0.11 2.5 5.9 3.8 2.2 0.82 0.04 0.30 0.01

75.4 0.15 13.5 0.0 0.64 /).04 0.10 1.0 4.0 4.6 0.35 0.04 0.07 0.05

11.7 12.9 32.2 26.2 0.9 9.4

31.6 27.2 34.1 5.0

CIPW norms

Qz Or Ab An Di lly Ol Mt II Ap

0.83 13.8 39.1 22.6 7.0 8.6 3.4 0.78 0.10

0.6 -

3.4 1.5 0.7

0.3

lished data, permit completion of the phase diagrams illustrated in Figs. l through 6. Finely crushed rock samples (passed through 200mesh sieve) were reacted with measured amounts of water within sealed noble-metal capsules (AgvsPd2s and Ag3oPd7o). Experiments above 10 kbars were performed in piston-cylinder apparatus using a hot pistonout procedure, with standard furnace assemblies and techniques [3]. Run durations are consistent with those in similar studies front this laboratory [3,5-7,28]. Nominal pressures are believed accurate to -+5~7~,and temperatures precise to -+5°C, with maximum error -+13°C (Pt/Pt-10%Rh thermocouples). There are many experimental uncertainties in rock-water melting studies, one of the worst being absorption of iron from the sample by the noble-metal capsules. Many phase boundaries in complex rock systems defy close reversals [5, 27]; the results shown are therefore reaction, or synthesis diagrams rather than phase equilibrium diagrams. We are satisfied, however, that many of the phase boundaries are close to their equilibrium positions [16,56]. These and other problems relevant to experimental studies with these rocks have been reviewed elsewhere [4 7,9,18]. Run products were analyzed using standard microscopic, X-ray diffraction, and electron microprobe nrethods [52]. Data tables will be presented with the report of the complete experimental study, including the water-undersaturated crystallization interval (manuscript in preparation).

Modes

Oz KSpar P1 Cpx Bt lib Ol Opaque

47 47 3(serp.) 3

13 4.5 59 12.5 9

/

34.8 29.0 31.5

2. Gabbro-tonalite-granite melting relationships with excess water

4.7 (mostly Bt)

Fig. 1 shows results for gabbro-water. The subsolidus phase relationships, the solidus to 35 kbars, and some reactions above the solidus up to 25 kbars were presented and discussed by Lambert and Wyllie [6,8] ; they incorporated near-liquidus data from several published studies on similar rocks [ 9 - 1 3 ] to estimate schematic phase relations through the melting interval. They tentatively identified orthopyroxene as a subsolidus phase at high pressures, but this is now recognized as kyanite (microprobe). The earlier preliminary diagrams are here confirmed in part and extended by additional runs at 15 kbars (squares, [2]), and 30 and 35 kbars (circles, [1]) for this specific gabbro. The phase boundary for amphibole between

2

Tholciite is from a fine-grained basaltic sill from Scotland [6]. Tonalite and granite are from the Sierra Nevada batholith, kindly provided by P.C. Bateman and F.C. Dodge. These are tonalite 101 (or M127) and granite 104 (or HL29) in Piwinskii's papers [16,191.

undersaturated conditions at high pressures (references cited below). Results in the 1973 dissertations of Stern [1] and Huang [2], combined with the previously pub-

191

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Fig. 1. Phase relationships of gabbro (Table 1) with excess water (reaction diagram). See text for sources of data. Dashed lines: phase boundaries estimated or uncertain. Abbreviations for phases in this and other figures: PI = plagioclase; Or = alkali feldspar; Qz = quartz; Ct = coesite; O1 = olivine; Px = pyroxcne; Cpx = clinopyroxene; Jd = jadeite; Ga = garnet; Ky = kyanite; Hb = amphibole; Bt = biotite, L = liquid; V = vapor. For symbols, see text.

15 and 20 kbars is from Allen et al. [17]. The triangles at 20 and 25 kbars are hyperliquidus runs for an olivine tholeiite composition (22.7% normative olivine) with excess H20 published by Nicholls and Ringwood [14] after the previous reviews [6,8]. For the tonalite, the subsolidus and the nearsolidus phase relationships to 25 kbars in Fig. 2 were determined by Lambert and Wyllie [6,15], who also estimated schematic phase relationships through the melting interval by incorporating Piwinskii's [ 16] data at 1,2 and 3 kbars, and the liquidus results compiled from several experimental studies on andesites [ 13,17,18]. The preliminary diagram is here con firmed in part and extended by additional runs at 15 kbars (squares, [2,32]) and 30 kbars (circles, [1]). The phase b o u n d a r y for amphibole near 1 8 - 1 9 kbars is from Allen et al. [ 17]. For granite-water, a preliminary version of Fig. 3 was presented by Stern and Wyllie [20], incorporating Piwinskii's [16] results to 3 kbars, and other nearsolidus results to 30 kbars [21]. The results are n o w consolidated and extended by additional runs [ 1 ] plotted in Fig. 3. The solidus temperature is identical

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Temperature

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Fig. 2. Phase relationships for tonalite (Table ]) with excess water (reaction diagram). See text for sources of data. Dashed lines: phase boundaries estimated or uncertain. For abbreviations see Fig. 1.

4°1

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Temperature

I I000 °

°C

Fig. 3. Phase relationships for granite (Table l) with excess water (reaction diagram). See text for sources of data. Dashed lines: phase boundaries estimated or uncertain. For abbreviations see Fig. 1.

192 within limits of experimental error with the solidus temperature in the system NaA1Si3Os KA1Si3Os SiO2--H20 [22,23]. Piwinskii [19] presented detailed phase relationships with excess water to 10 kbars for the same tonalite and granite; there are differences of about 25°C between the temperatures of our respective phase boundaries at 10 kbars, for which we have no explanation. The basic phase relationships, however, are consistent with results in Figs. 2 and 3. Uncertain boundaries in Figs. 1,2, and 3 are indicated by dashed lines. In particular, precise deterruination of the garnet and amphibole boundaries below 1000°C is difficult (see reviews in [3,27,28]). Alkali feldspar dissolves in the subsolidus aqueous vapor phase at high pre}sures, and the appearance of kyanite in all three rocks could well be due in part to preferential solubility of alkalies and silica [29]. Deepseated aqueous fluids are dense, concentrated solutions [20,29].

3. Water-undersaturated liquidus surface for basaltandesite-rhyolite The phase relationships for rocks ranging from dry to water-saturated can be illustrated in various projections and sections through space. Preliminary isobaric T Xlt20 (rock-water) diagrams for each of the three samples in Table 1 have been presented at 30 kbars [30], and similar diagrams are available for these and related compositions at 15 kbars [31,32], 8, 5, and 2 kbars [ 3 3 - 3 6 ] . Preliminary phase relationships have been presented for the complete rock series basalt-andesite-rhyolite, dry and with fixed water content at 27 30 kbars [30,37, 52]. Results have also been shown for isoplethal P - T sections for individual rocks with fixed water content [18,31,38 40]. Figs. 4, 5, and 6 show for each of the rock compositions listed in Table 1 the water-undersaturated liquidus surface extending from the dry (0% H2O) liquidus to the water-saturated liquidus. The excesswater liquidus curves are reproduced from Figs. 1, 2, and 3. The water contents of the saturated liquids along these curves increase from essentially 0% at 1 bar to about 27 wt.% at 30 kbars [20,41]. The anhydrous liquidus curves are estimates, based on pub-

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THOLEIITE

LIQUIDUS O~

Ntg.,.r°i~ i 1200 1400

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TEMPERATURE °C Fig. 4. Contoured map of water-undersaturated liquidus surface for tholeiite (gabbro, Fig. 1, Table 1), showing liquidus and near-liquidus minerals. For abbreviations see Fig. 1.

lished data for similar compositions [18,23,34,37,42 45]. Each surface is contoured by lines of constant water content drawn from the known points on the excess-water curves, through corresponding points on the liquidus boundaries for the rock-water isobars at 15 and 30 kbars [30,32]. The maps of the surfaces inP T - X H 2 0 space also show the fields for the liquidus and near-liquidus

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I000 1200 NO0 1600 TEMPERATURE °C Fig. 5, Contoured map of wateriundersaturated liquidus surface for andesite (tonalite, t:ig. 2, Table 1) showing liquidus.and near-liquidus minerals. For abbreviations see Fig. 1

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sensitive to small changes in composition (Figs. 2 and 4); [14,17-19,34,42]. Figs. 4 and 5 show that amphibole crystallizes at near-liquidus temperatures in basalts and andesites only for magmas with abnormally high water contents (12-20%). High water contents are not required to stabilize amphibole, however. From liquids with low water contents (probably <1%, see [34]), amphibole begins to precipitate below the liquidus, well within the crystallization interval at temperatures close to those shown in Figs. 4 and 5 [32,34,42]. Similar relationships hold for biotite and muscovite in granitic liquids [31,36].

0

1400

TEMPERATURE °C

Fig. 6. Contoured map of water-undersaturated liquidus surface for rhyolite (granite, Fig. 3, Table 1), showing liquidus and near-liquidus minerals. For abbreviations see Fig. I.

minerals. These are our best estimates based on Figs. 1, 2, and 3, rock-water isobars at 15 and 30 kbars [30,32], the results of dry experiments cited above, and the results of water-deficient experiments below 10 kbars •34,42]. Kyanite, a minor accessory mineral in Fig. 3, has been omitted from Fig. 6. The relative positions of the phase boundaries in Figs. 4, 5, and 6 are correct for most rocks called basalt, andesite, and rhyolite, but their absolute positions may change significantly with changes in some chemical paraineters. Nicholls and Ringwood ]14,461 explored near-liquidus phase relationships below 1200°C for water-undersaturated olivine tholeiite and SiO2-saturated tholeiite compositions; these contain 23% and 0% normative olivine, respectively, compared with 9% in the tholeiite of Fig. 4. They did not observe primary amphibole in water-saturated runs near the liquidus. The maximum pressure of the field of olivine crystallization on or near the liquidus differs considerably among the three compositions but, for each, it increases with increasing water content. There is no olivine on the andesite liquidus in Fig. 5, but Nicholls [47] produced extensive fields for primary olivine in andesitic compositions by adding olivine components. The distribution and relationship of orthopyroxene to clinopyroxene near the waterundersaturated liquidus for tholeiites and andesites remains somewhat uncertain, and is probably quite

4. Discussion

The general features of the completed excess-water phase diagrams in Figs. 1,2, and 3 confirm and extend previous accounts [6,7,17,20,24]. The diagrams for gabbro and tonalite are dominated by the transition from low-pressure feldspar-amphibole mineralogy to high-pressure eclogitic mineralogy. It is this mineralogical transformation that is responsible for the change in slope of both solidus and liquidus [21], and not the relative values of the partial molar volumes of H20 in vapor and liquid phases, as proposed by Carmichael et al. [25, pp. 120-121]. Similarly, we conclude that the change in slope of the amphibole stability at about 18 kbars [17] is due to its involvement with the reaction producing garnet, and not due to the compressibility of H20 in the vapor [25, p. 121]. The compressibility effect is certainly real, but it seems that higher pressures must be reached to prove it. All changes in solidus or liquidus slopes reported so far can be correlated directly with mineralogical transitions, or with changing liquid compositions, as exemplified by granite-water in Fig. 3. The change in slope of the quartz-out curve between 3 and 5 kbars is caused by expansion of the field for the crystallization of quartz with increasing pressure I19, 20,23,26]. Figs. 1 through 3 outline conditions for the beginning of melting of crustal rocks and subducted oceanic crust in the presence of aqueous pore fluid, and the closing stages of crystallization of members of the calc-alkaline plutonic rock series. The maximum pressure of 35 kbars corresponds to a depth of more

194 than 120 km. Applications of these near-solidus results to metamorphic and magmatic processes have been reviewed elsewhere [6,7,20,31 ]. Figs. 4 through 6 place constraints on possible source materials for the generation of basalts, andesites and rhyolites, illustrate the effect of water on the liquidus relationships, and limit the depth-temperature range for the crystallization of specific phenocrysts from the magmas. A primary magma must be in equilibrium with the residual minerals in the source rock at its place of origin. Therefore, the near-liquidus minerals for a primary magma must correspond to major minerals in the source rock at, the pressure and temperature of origin. Residual minerals with a reaction relationship, of course, do not crystallize from the separated liquid. These and related criteria and arguments for primary magmas have been reviewed in detail by Nicholls [47] and Mysen et al. [48]. The high-pressure near-liquidus mineralogy of basalts and andesites with compositions represented by Figs. 4 and 5 (Table 1) is dominated by garnet and clinopyroxene; there is no olivine or orthopyroxene at subcontinental mantle pressures for any water content. The absence of olivine could be attributed to a reaction relationship, but it has been demonstrated that much higher normative olivine contents are required to extend the olivine crystallization field to mantle pressures, especially for low water contents [14,47]. We conclude that basalts and andesites with compositions similar to those of Figs. 4 and 5 are not derived as primary magmas from the mantle peridotite. Primary magmas would have to be nmch more magnesian, and would probably fractionate olivine during ascent, as proposed first by O'Hara [49]. Only in oceanic regions, where magma may separate from mantle peridotite at shallow levels, do we expect primary magmas to reach the surface without fractionation. The near-liquidus mineralogy of the basalt and andesite is consistent with their derivation as primary magmas from quartz eclogite in subducted oceanic crust, and this has been a popular interpretation for andesites [37,50]. Several recent reviews, however, find multi-stage processes more appealing [14,46,51,52]. The liquidus mineralogy of rhyolite in Fig. 6 is dominated by quartz. The liquid could therefore be produced as a primary magma from crustal rocks, but

we see no prospect of generating rhyolite as primary magmas from mantle material. This conclusion extends to subducted oceanic crust at mantle pressures, but for other reasons. Expansion of the field for the primary crystallization of quartz at pressures above about 5 kbars (Fig. 3) causes the generation of liquids less silicic than rhyolite or granite [20,23]. Experimental evidence indicates that the only prospect of generating quartz tholeiites and andesites as primary magmas from mantle peridotite is if the liquids are saturated, or nearly saturated with water [47,48, 51]. Figs. 4 and 5 show that such liquids would contain 10--20 wt.% dissolved water at their mantle sources. We are aware of no evidence that maglnas contained such high water contents before eruption [53,54], and there is evidence that some andesites may contain between 0.5 and 2% water [42,50,53]. If water-saturated andesitic magmas were generated in mantle peridotite, Figs. 5 and 2 show that crystallization and evolution of vapor would begin as soon as they moved upwards, and it seems unlikely that they could reach the surface without undergoing considerable fractionation. According to Fig. 5, the generation of primary andesite magmas with 2% dissolved H20 by partial fusion of crustal materials between 30 and 50 km depth would require temperatures of 1100°C. Although these temperatures could exist [57], we consider them too extreme for this to constitute a normal source for andesites. Fig. 6 shows that rhyolite magmas with moderate water contents could be generated in the crust at temperatures attained during regional metamorphism [55], and they could reach near-surface levels before vesiculation. Magmas of intermediate composition composed of water-undersaturated rhyolite liquid and residual crystals could similarly be generated by partial fusion of crustal materials [39,56].

Acknowledgments This research was supported by the Earth Sciences Section, National Science Foundation, NSF Grant DES 73-00191 AO1. We wish to acknowledge also the general support of the Materials Research Laboratory by the National Science Foundation. We thank P.C. Bateman and F.C. Dodge for the tonalite and granite,

195 and analyses o f the rocks, and A.T. A n d e r s o n for critical review o f the m a n u s c r i p t .

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