Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz, Colombia, November 13, 1985

Journal of Volcanology and Geothermal Research, 41 (1990) 97-126 97 Elsevier Science Publishers B.V., Amste rdam-Pr in ted in The Netherlands

Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz,

Colombia, November 13, 1985

WILLIAM G. MELSON a, JAMES F. ALLAN b, DEBORAH REID JEREZ c, JOSEPH N E L E N a, MARTA LUCIA CALVACHE d*, STANLEY N. WILLIAMS e, JOHN

FOURNELLE a and MIKE PERFIT f

a Department of Mineral Sciences, Smithsonian Institution, Washington DC 20560, U.S.A. b Ocean Drilling Program, Texas A&M University, College Station, TX 77840, U.S.A.

c Weston Inc., 955 L'Enfant Plaza, Washington DC 20024, U.S.A. d INGEOMINAS, Observatorio VulcanolSgico de Colombia, Apartado Aereo 1296, Manizales, Colombia

e Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803-4101, U.S.A. f Department of Geology, University of Florida, Gainesville, FL 32611, U.S.A.

(Received December 1, 1989)

Abstract

Melson, W.G., Allan, J.F., Jerez, D.R., Nelen J., Calvache, M.L., Williams, S.N., Fournelle, J. and Perfit, M., 1990. Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz, Colombia, November 13, 1985. In: S.N. Williams (Editor), Nevado del Ruiz Volcano, Colombia, I. J. Volcanol. Geotherm. Res., 41: 97-126.

The petrology of the highly phyric two-pyroxene andesitic to dacitic pyroclastic rocks of the November 13, 1985 eruption of Nevado del Ruiz, Colombia, reveals evidence of: (1) increasingly fractionated bulk compositions with time; (2) tapping of a small magma chamber marginally zoned in regard to H20 contents (1 to 4%), temperature (960-1090 ° C), and amount of residual melt (35 to 65%); (3) partial melting and assimilation of degassed zones in the hotter less dense interior of the magma chamber; (4) probable heating, thermal disruption and mineralogic and compositional contamination of the magma body by basaltic magma "underplating"; and (5) crustal contamination of the magmas during ascent and within the magma chamber. Near-crater fall-back or "spill-over" emitted in the middle of the eruptive sequence produced a small pyroclastic flow that became welded in its central and basal portions because of ponding and thus heat conservation on the flat glaciated summit near the Arenas crater. The heterogeneity of Ruiz magmas may be related to the comparatively small volume (0.03 km 3) of the eruption, nearly ten times less than the 0.2 km 3 of the Plinian phase of Mount St. Helens, and probable steep thermal and PH20 gradients of a small source magma chamber, estimated at 300 m long and 100 m wide for an assumed ellipsoidal shape.

* Present address: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803- 4101, U.S.A.

0377-0273/89/$03.50 (© 199 Elsevier Science Publishers B.V.

98 w ( ~ MELSON ET AL,

Introduction

Around 21:30, November 13, 1985, pumice lumps began to fall over a large region to the northeast of Nevado del Ruiz Volcano, Colom- bia, marking the beginning of a tragic evening in which about 22,000 people perished in lahars triggered by this eruption. The geology and geomorphology of this deeply dissected volcano have been described by Herd (1982) and the events on the night of November 13, 1985, have been reviewed by many authors (e.g. Williams, 1987; Voight, 1988). There are a number of estimates of the total volume and mass of the November 13, 1985 ejecta which do not include the relative amounts of magmatic and lithic components. Carey et al. (1986) estimate the mass at 3.5 × 10 l° kg and an isopach volume of 0.03 km 3 has been calculated by Calvache et al. (1987). The volume is much smaller than that of two other recent Plinian eruptions that will be mentioned below: the May 18, 1980 eruption of Mount St. Helens (0.2 km 3, Chris- tiansen and Peterson, 1981) and the 1982 eruption of E1 Chichon (0.4 km 3, Varekamp et al., 1984).

The eruptive mode of Ruiz over the past 2100 years has involved periodic Plinian eruptions leaving clear tephra sets mainly to the north of the volcano which consist of a basal pumice layer on paleosol. The relative volumes of the largest eruptions and their dates are 0.14 km 3 (around 1595 A.D.), 0.12 km 3 (1080 yr B.P.), 0.28 km 3 (2150 _+ -100 yr B.P.) based on the volume estimates of Calvache et al. (1987) and radiocarbon dates of Herd (1982). The volume of the 1985 tephras is thus about a fourth of the smallest of the earlier eruptions and about a tenth of the 2150 yr B.P. deposit. The 1985 eruptives are being rapidly eroded around the volcano (Eduardo Parra, pers. commun., 1986) and will leave little if any geological record.

Our work here focuses on: (1) magma chamber compositional zoning; (2) the compositions, eruption temperatures, water contents, and crystal-melt ratios of the Plinian pumices;

(3) the summit welded tufts; (4) magma mixing and assimilation.

Samples and methods of analysis

In January, 1986, Dr. Victoria Funk (botanist), Dr. Gary Graves (ornithologist), and three geologists (Ms. Deborah Jerez, Drs. James F. Allan and William Melson) conducted a field study of the eruption and its impact. A wide variety of samples were collected, including a suite from the pyroclastic rocks near the summit crater during an ascent led by Ms. Marta Calvache on January 29, 1986). In addi- tion, S.N. Williams provided airfall pumice samples that previously were described (Melson et al., 1986) and Marta Lucia Calvache provided a suite of samples from two summit stratigraphic sections (MC-1 and MC-2) described in this volume (Calvache, 1990). The samples described here are in the Smithso- nian's Collection of Rocks and Ores and may be requested for additional study by referring to their museum numbers that are appended to table and figure labels.

Some of these samples were analyzed (Tables 1 and 2) by classical wet chemical methods (J. Nelen), the electron microprobe (Jerez, fused powder analyses; Melson, minerals and glasses), INAA (J. Allan), isotopic dilution (M. Perfit) and petrographically (W. Melson and J. Fournelle). The electron microprobe work in- cludes a short survey of phenocryst compositions (Table 3) and focuses mainly on matrix glasses, melt inclusion and Fe-Ti oxides for calculation of temperatures and oxygen fugacities (Tables 4, 5 and 6). Sodium loss from glasses was minimized by moving the beam during electron microprobe analyses. Still, the presence of corundum in some of the norms reflects sodium loss. This loss has been cor- rected in the analyses used in the tables and figures by adding sufficient Na20 to convert the excess A120 3 to normative albite as described in Merzbacher and Eggler (1984).

All the samples examined were rapidly quen-

WATER CONTENTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RUIZ 99

ched on eruption both by rapid ejection and by exsolution and adiabatic expansion of dissolved volatiles and all thus have mainly glassy matrices. Est imates of pre-eruption H20 contents of these quenched glasses are based largely on the "geohygrometer" proposed by Merz- bacher and Eggler (1984), which is in substan- tial agreement with the experimental studies of Rutherford et al. (1985). This involves the experimental observation that dacitic magmas show increasing plagioclase solubility with increasing PH20. The method of Anderson (1974) (i.e., precise analyses of melt inclusions and determining H20 contents by the difference between their sums and 100%) indicates water contents as high as 3 - 4 % in the Ruiz pumices. This is not a satisfactory method for most of the melt inclusions because of their high sums, commonly greater than 97%.

The amount of residual matrix glasses was calculated from the bulk and matrix glass K20 contents assuming all K20 is parti t ioned into glass. The potassian phases biotite and hornblende occur, but in quantit ies of less than 1%. This method was found to be more rapid and probably more accurate than petrographic modal analysis because of the fine grain size of the groundmass and the abundance of small vesicles in most samples.

Petrologic overview

All the rocks we examined from the 1985 sequence are augite-hypersthene andesites to dacites which have a wide range of bulk, minor- and trace-element compositions, textures, accessory minerals, and xenoliths. The amount of matrix glass ranges from about 35 to 65% and correlates inversely with the silica content (Table 5) and reflects the extent of pre-eruption crystallization.

The dominant phenocrysts are plagioclase that commonly contain melt inclusions and are followed in order of abundance by hypersthene and augite with accessory amounts of olivine, biotite, hornblende, magnetite, i lmenite and,

very rarely, a silica polymorph, probably tridymite. Olivine commonly contains small inclusions of chromian spinel and has partially resorbed idiomorphic forms (Fig. 1B) commonly surrounded by hypersthene. Hornblende, biotite and olivine do not occur in all thin sections. Banded pumices and dark-colored pumices with angular white pumice fragments are common. Some rocks contain diverse xenolithic assemblages, one including stringers of anhydrite (Fig. 1A). All the rocks analyzed here (Tables 1 and 2) fall into the high K20 andesite group of Gill (1981). Mineral compositions for some of the samples are in Table 3.

The bulk rocks show considerable scatter on MgO variation diagrams (Fig. 2). Nonetheless, maximum major-element differences can be ac- counted for by about 25 wt. % normative plagioclase and 9 wt. % combined hypersthene and olivine in the most magnesian sample as compared to the most evolved sample.

Eruption chronology and stratigraphic compositional zoning

The eruptive sequence began on September 11, 1985 with six hours of phreatic eruptions from the summit crater which spread fine tephra over much of the region to the northwest of the Arenas crater. No magmatic component has been recognized in this premonitory phase. Considerable seismic activity and gas emission from the summit crater accompanied this phase, but no other significant eruptions occurred until the night of November 13. The cataclysmic sequence began around 21:08 and was over by around 23:00.

The specific events of that night can be cor- related tentat ively with our studied samples (Tables 1 and 2). The eruptive events have been reconstructed from seismic records (Dave Harlow, pers. commun., 1988) and from the strat igraphy of near-crater sections and the t iming and distribution of airfall pumice (Calvache, 1990-this volume):

(1) An initial intense blast, probably cor-

100 W(~. MELSON ET AI,

TABLE 1

Bulk ana lyses of pyroclast ic rocks from the November 13, 1985 erupt ions by electron probe ana lyses of fused powders, INA and isotopic d i lu t ion (asterisks). Es t ima ted average one s igma errors for INAA in ppm are Sc, 0.2; Cr, 2; Co, 0.2; Ni, 19; Rb, 5; Sr, 40; Zr, 27; Sb, 0.03; Cs, 0.06; Ba, 24; La, 0.2; Ce, 0.5; Nd, 2.2; Sm, 0.07; Eu, 0.016; Tb, 0.029; Yb, 0.03; Lu, 0.007; Hf, 0.09; Ta, 0.019; Th, 0.12; U, 0.18.

Sample: ANP585 ANP681 ANP682 ML76T ML77 ML781 ML782

SiO 2 63.88 61.12 60.80 66.43 58.04 60.60 59.80 TiO 2 0.56 0.66 0.60 0.72 0.66 0.67 0.73 A1203 17.14 17.10 16.70 16.69 18.85 16.40 16.47 FeO* 4.37 5.17 5.18 4.90 5.40 5.49 5.76 MgO 3.18 4.75 5.14 1.79 5.12 5.00 5.42 CaO 5.01 6.06 6.30 3.26 6.18 5.99 6.15 Na20 4.00 4.18 4.00 3.24 3.39 3.72 3.59 K20 2.56 2.24 2.08 2.30 1.96 2.12 2.09 P205 0.19 0.26 0.24 0.19 0.22 0.19 0.20

Total 100.89 101.54 101.04 99.52 99.82 100.18 100.21

Sc 11.94 15.77 17.18 11.27 16.80 16.50 17.40 Cr 48.50 188.60 197.70 58.10 201.00 190.00 200.00 Co 14.86 21.05 22.04 21.34 22.30 21.70 23.30 Ni 28 67 67 36 82 78 103 Rb 81 64 61 58 61 66 61 Rb* 87.7 68.7 61.5 - 60.2 - -- Sr 570 580 610 520 499 524 494 Sr* 535 547 549 - 512 - - Zr 222 199 199 198 169 171 178 Sb 0.80 0.61 0.56 2.01 0.65 0.56 0.59 Cs 4.88 3.74 3.48 3.32 3.27 3.76 3.53 Ba 1216 1041 1008 1121 935 983 947 La 20.12 19.89 19:53 18.22 18.70 20.00 19.30 Ce 44.20 40.00 38.70 35.10 38.40 39.30 39.40 Nd 18.70 16.40 17.20 13.80 16.10 18.00 18.30 Sm 4.07 3.98 3.92 2.75 3.92 3.93 3.89 Eu 0.908 0.958 0.975 0.726 0.932 0.943 0.944 Tb 0.414 0.412 0.434 0.264 0.425 0.402 0.414 Yb 1.42 1.40 1.33 1.00 1.39 1.37 1.49 Lu 0.224 0.209 0.213 0.138 0.203 0.205 0.210 Hf 4.56 3.94 3.88 3.68 3.51 3.89 3.79 Ta 0.544 0.459 0.426 0.423 0.430 0.449 0.414 Th 12.78 10.25 9.54 9.85 9.18 10.30 9.60 U 5.72 4.35 4.14 4.33 3.83 4.47 4.05 87Sr/86Sr 0.704394 0.704336 0.704317 - 0.704306 - -

1. ANP585. Air fa l l pumice. 116185-66 . 2. ANP681. L igh t band in pumice 116185-68 . 3. ANP682. Da rk band in pumice 116185-68. 4. ML76T. F ine ash from base of Calvache section MC-1 116185-11 . 5. ML77. Pumice near base of Calvache section MC-1. 116185-12 . 6. ML781. Pumice above 77. Calvache section MC-2. 116185-13a . 7. ML782. Pumice. Calvache sn, Same zone as ML781. 116185-13b.

WATER CONTENTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RUIZ

TABLE 1 (continued)

101

Sample: MLWT79 WT2 WT6 MLWT80 ML82 ML83 ML84

SiO 2 59.93 60.10 59.65 60.11 45.07 62.82 61.71 TiO 2 0.68 0.69 0.66 0.69 0.60 0.66 0.66 A1203 16.57 16.73 16.76 16.27 38.81 16.59 16.93 FeO* 5.51 5.37 5.64 5.69 4.39 4.60 4.54 MgO 5.12 5.43 5.39 5.40 3.75 2.60 2.73 CaO 6.21 6.16 6.35 6.34 4.62 5.86 5.87 Na 2 3.83 3.66 3.77 3.69 2.57 3.40 3.34 K20 2.04 1.94 1.53 1.97 1.48 2.36 1.98 P205 0.20 0.27 0.26 0.16 0.20 0.18 0.15

Total 100.09 100.35 100.01 100.32 101.49 99.07 97.91

Sc 17.40 17.06 16.98 18.00 13.10 11.30 12.30 Cr 198.00 185.60 192.30 213.00 122.00 69.00 79.00 Co 23.00 23.28 22.01 24.10 16.80 15.40 17.70 Ni 86 108 102 73 52 51 60 Rb 61 65 60 61 49 63 51 Rb* . . . . . . 54.3 Sr 558 570 630 542 354 484 560 Sr* . . . . . . 577 Zr 190 207 200 187 152 199 171 Sb 0.53 0.58 0.57 0.55 0.56 1.22 1.19 Cs 3.43 3.66 3.43 3.41 2,84 4.00 2.92 Ba 991 1001 985 981 762 1066 976 La 19.70 19.31 19.18 19.40 15.40 20.00 18.30 Ce 39.00 38.50 38.20 39.70 31.60 40.10 36.60 Nd 17.90 19.45 16.60 17.50 13.60 15.60 16.00 Sm 3.90 2.82 3.86 3.88 3.21 3.74 3.50 Eu 0.987 0.968 0.954 1.000 0.739 0.852 0.894 Tb 0.414 0.438 0.409 0.406 0.330 0.350 0.352 Yb 1.30 1,45 1.38 1.42 1.04 1.16 1.09 Lu 0.205 0.215 0.211 0.210 0.163 0.185 0.176 Hf 3.73 3.80 3.63 3.67 3.19 3.84 3.44 Ta 0.429 0.426 0.441 0.423 0.386 0.438 0.418 Th 9.51 9.64 9.51 9.51 7.65 10.34 8.62 U 3.95 4.30 3.94 4.04 3.12 4.51 3.78 87Sr/86Sr . . . . . . 0.704347

8. ML79. Welded tuff from base of welded section. 116185-14. 9. WT2. Summit welded tuff, mid-section. 116185-2.

10. WT6. Summit welded tuff, mid-section. 116185-6. 11. MLWT80. Welded tuff. Near top Calvache MC-1. 116185-15. 12. ML82. Fine tephra. Surge near top Calvache MC-1. 116185-17. 13. ML83. Pumice lump. Near top of Calvache MC-1. 116185-18. 14. ML84. Pumice lump. Near top of Calvache MC-1. 116185-19.

r e l a t e d wi th a 21:08 s t rong se i smic episode,

depos i t ed a t h i n sequence n e a r the s u m m i t

c r a t e r of p h r e a t i c m a t e r i a l , much of i t f ine ash.

S a m p l e ML76T (Table 1) is such fine t e p h r a

f rom the base of a s u m m i t sect ion, and has low

Cr, Ni and Co and is s i m i l a r to the pumice

l u m p s f rom the top of the sect ion. Th i s i nd i ca t e s

l i t t l e abou t m a g m a c h a m b e r zoning, however ,

Fig. 1. A. Anhydrite xenolith (AN) in matrix glass and near oxidized biotite (BZ, right). Sample ML83. B. Partially resorbed olivine (OL, Fos,) with minute octahedra of chromite (CR). Sample 116170. C. Euhedral, broken plagioclase phenocrysts with melt inclusions. Low-temperature Plinian pumice ML77. D. Partially resorbed plagioclase phenocryst with melt inclusions. High-temperature Plinian pumice 116170. E. Welded tuff from near the Arenas crater contains light-colored highly crystallized collapsed pumice in dominantly dark-colored collapsed pumice (welded tuff sample WT2). F. Resorbed hornblende (welded tuff sample WT2). Width of field in A-D is 0.9 mm, in E 2.5 mm, and in F 0.3 mm.


TABLE 2

Bulk composition of Ruiz airfall pumice lumps from the ca. 1595 AD and November 13, 1985, eruptions. Fluorine and chlorine (Table 2) were determined using ion-selective electrodes (Ingrain, 1970; Haynes and Clark, 1972) and sulfur was analyzed by the method of Peck (1970). J. Nelen, Analyst.

1 2 3 4 5 6 Eruption: 1595? 1595? 13NOV85 1 3 N O V 8 5 1 3 N O V 8 5 13NOV85 USNM # 116159 116160 116158 116165 116166 116170

SiO 2 59.58 59.85 60.10 60.33 59.92 62.41 TiO 2 0.65 0.69 0.70 0.70 0.69 0.60 A1203 16.42 15.94 16.17 15.84 16.12 15.92 Fe203 1.90 1.84 1.93 1.59 1.92 1.62 FeO 3.31 3.79 '3.51 3.82 3.75 3.13 MnO 0.10 0.10 0.08 0.10 0.10 0.09 MgO 4.87 5.19 5.18 4.99 5.02 3.75 CaO 6.12 6.18 6.31 6.04 5.95 5.28 Na20 3.87 3.82 3.72 3.74 3.75 4.01 K20 2.02 2.06 2.15 2.13 2.15 2.46 P205 0.17 0.18 0.21 0.17 0.21 0.17 H20÷ 0.58 0.14 0.25 0.40 0.62 0.34 H20- 0.16 0.05 0.05 0.08 0.02 0.03

Total 99.75 99.83 100.36 99.93 100.22 99.81

F 0.041 0.044 0.043 0.037 0.040 0.046 C1 0.032 0.032 0.035 0.028 0.034 0.054 S < 0.01 0.01 < 0.01 < 0.01 < 0.01 0.02

because of possible wind or blast sorting. The sample is also higher in Sb than any other sample. The high Sb, an element forming volatile compounds at low pressures and magmatic temperatures, may reflect condensation from a vapor phase onto the fine ash. Included in this sequence are ballistically emplaced blocks. Grading upward there are increasing amounts of, and increasingly larger, pumice fragments (analyses ML77, ML781, ML782, Table 1). This event marked the first breaching of magma. The bottom section pumices (open squares in Fig. 2) all fall in the high-MgO region.

(2) Around 21:31 a still stronger seismic event occurred, probably coincident with emission of a pyroclastic flow which covered much of the summit, and became welded in some areas after deposition (analyses MLWT79, WT2, WT6, and MLWT80, Table 1). The pyroclastic flow may have been the main cause of melting of summit

glacial ice and the ensuing mudflows. The bulk compositions of the welded tuffs show no systematic stratigraphic zoning (Fig. 2), and, like the initial pumice, cluster in the high-MgO portion of the compositional array.

(3) Finally, a base-surge occurred (tephra analysis ML82) jus t before the main intense Plinian eruption which draped the country-side with airfall pumice (ANP585, ANP681, ANP682, Table 1, analyses 3 -6 , Table 2), probably beginning around 21:36, the time of the third and most intense seismic event. Like sample ML76T, fine tephra sample ML82 is strongly enriched in plagioclase fragments and is not representative of magma composition. Analy- ses ML83 and ML84 (solid triangles, Fig. 2) are pumice lumps from the top of a summit stratigraphic section (section B, MC-1, Calvache 1990-this volume). The airfall pumice samples from the northwest slopes (asterisks,

104 w ~ MI~:LS()N ET AL

T A B L E 3

M i n e r a l compos i t i ons in p u m i c e f r o m N o v e m b e r 13, 1985 a n d 1595 AD(?) (Sample 116159). E l e c t r o n mic rop robe ana ly se s . N u m b e r in p a r e n t h e s e s r e f e r s to coex i s t i ng p h a s e s .

SiO 2 TiO 2 AI203 FeO* MgO CaO Na20 K20 Cr203 NiO SUM

Hypersthene phenocrysts. 116158 1. 53.12 0.17 0.86 20.43 24.62 1.17 0.00 0.00 - 100.37 2. 52.73 0.18 0.76 20.70 24.59 1.17 0.00 0.00 - 100.13 3. 52.95 0.23 0.91 21.26 23.63 1.16 0.00 0.00 - 100.14

Magnesian hypers thenejacket ing olivine. 116158 4. 48.08 0.21 5.45 13.88 31.97 1.07 0.00 0.00 100.66 5. 52.08 0.30 5.92 12.59 28.33 1.38 0.00 0.00 100.60

Hypersthene overgrowth on O1(27) + Au$17). 116159 6. 51.00 0.28 4.01 12.57 28.38 1.70 0.00 0.00 97.94

Ptagioclase phenocrysts. 116158. 7. 57.60 0.00 27.19 0.43 0.07 8.84 5.78 0.67 t00.58 8. 52.48 0.00 30.20 0.65 0.10 12.36 3.99 0.23 - 100.01 9. 51.04 0.00 30.60 0.63 0.10 13.05 3.87 0.24 99.53

10. 58.04 0.00 26.27 0.39 0.03 8.51 6.06 0.57 99.87

Plag euhedra in clot of O1(26) + Hb(23) + Opx. 116158. 11. 53.12 0.06 29.28 0.66 0.09 11.95 4.46 0.25

Plagioclase in H b + Opx clot 116158. 12. 58.07 0.07 26.27 0.39 0.03 8.51 6.06 0.00

Plagioclase phenocrysts. 116159. 13. 52.68 0.05 29.17 0.47 0.01 11.82 4,18 0.32 14. 57.03 0.00 27.22 0.45 0.02 8.77 5,70 0.56

Augite phenocrysts(15, 116170)and High-Cr augite (16, 116166) 15. 53.60 0.32 1.41 8.63 14.85 20.73 0.40 0.02 16 52.50 0.59 4.03 6.00 16.98 20.51 0.35 0.02

Augite (Fs: En :Wo = 1 0 : 4 7 : 4 2 ) in O1(27) + Opx(6) clot. 116159. 17. 51.53 0.35 3.26 6.71 17.10 21.11 0.41 0.00

Chromian spinel(18) in olivine (19, Foss,5). 116165. 18. 0.89 0.69 24.00 26.60 13.23 0.26 0.00

High and low Ni olivines. 116158. 19. 40.40 0.02 0.00 11.27 48.49 0.00 0.00 0.00 20. 39.65 0.02 0.00 11.62 48.94 0.17 0.00 0.00 21. 39.30 0.02 0.00 18.78 42.39 0.13 0.00 0.00

Hornblende phenocryst. 116158. 22. 42.02 2.85 13.66 10.16 15.04 11.52 2.54 0.57

Hornblende in O1(26) + Pl( l l ) + Opx clot. 116158. 23. 40.97 2.78 15.35 9.88 14.89 11.19 2.39 0.69

Hornblende in P1 + M ~ O p x + I lmclot . 116158 24. 42.22 3.49 13.99 11.24 14.70 11.86 2.79 0.47

Partially resorbed biotite. 116170. 25. 38.05 5.15 14.01 11.76 16.86 0.04 8.42

Olivine (Fos6) in Hb (anal 23) + Opx + P l (ana l l l ) clot. 116158 26. 40.54 0.00 0.00 12.95 46.05 0.16 0.00 0.00

Ol iv ine (Fo~) in Aug (anal 17) + Opx (anal 5) clot. 116158. 27. 39.77 0.00 0.00 15.63 45.24 0.12 0.00 0.00

99.87

99.40

98.70 99,75

0.04 0.09 100.09 0.94 101.92

100.47

33.34 99.01

0.05 0.31 i00.54 0.02 0.31 100.73 0.01 0.19 100.82

- 98.36

98.14

• - 100,76

0.09 0.05 94.43

99.70

100.76


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Fig. 2. MgO-variation diagrams for the November 13, 1985 eruption and 1595? eruption of Nevado del Ruiz (bulk rock compositions from Tables 1 and 2). Dashed line shows trend among analyzed Plinian pumices (Table 2).

Fig. 2) range widely in composition, and along with the summit uppermost pumices uniquely include the most acidic, lowest MgO samples in the sequence. Remarkably, two pumice lumps

erupted about four hundred years ago are in- dist inguishable compositionally and petrographically from some of those of 13 Nov 1985 (Table 2, analyses 1 and 2, and Fig. 2). They

106 W,G. MELSON ET AL.

also contain dispersed olivine with chromium spinel such as noted below in the 1985 sequence.

Calvache (1990-this volume) found that six samples with stratigraphic control showed an increasing amount of glass up section, that is, the eruptives appeared to be becoming more melt-rich through time, and thus of decreasing viscosity. These conclusions were based on Smithsonian electron-probe analyzes of matrix glasses done under identical analytical setups as reported here. These data are augmented here and indicate that the amount of glass again drops in some of the Plinian pumice and that the spread of residual glass contents is large in clasts in some of the welded tuffs.

In summary, there is a suggestion of a trend toward increasingly more acidic compositions during the eruption (Fig. 2) in the eruptive sequence, a t rend that is the reverse of many zoned eruptive sequences, which normally become more basic with time. However, basic andesites occur in the final Plinian pumices and thus con- t inued to be erupted throughout the sequence.

Pl in ian pumices: temperatures and water contents

A series of Plinian pumice erupted in the final phase show the following features:

(1) Probable eruption temperatures were between 960 to 1090°C based on Fe-Ti oxide geothermometry (Table 4). One of the samples, ANP681, has a glassy groundmass that is inhomogeneous at the 2 micron level, and the "glass" analysis is the average of this glassy mixture, rich in less than 2 micron quench chrystals of plagioclase. The remaining samples have clear, homogeneous glass septa.

(2) There is an approximately linear positive trend between temperature and amount of matrix glass and a corresponding decrease in the degree of fractionation of matrix glass (Table 5 and Fig. 3A).

(3) There is a suggestion of a decreasing bulk SiO 2 and total alkalies with increasing oxide

temperatures (Fig. 4) but little correlation with MgO and some other oxides.

(4) Pre-eruption water contents probably rang- ed from 1 to 3 weight percent (Fig. 5A).

(5) A banded light- and dark-colored pumice yields distinctly different eruption temperatures and matrix melt contents and much smaller differences in bulk composition (ANP681 and ANP682, Tables 1 and 3, and Figs. 4, 5A and B) between light- and dark- colored bands.

These observations are consistent with tapping of a magma body with a wide range of temperatures, water contents, amount of matrix glass and thus viscosities during the Pli- nian phase, and with the intimate mixing of these distinct magma parcels during the eruption. Imposed on this mixing, there is a suggestion of a t rend toward more fractionated, more crystalline, and lower temperature pumices with time. However, sample ML77, a pumice lump from near the base of a summit section, is the lowest temperature pumice so far iden- tified, and also the lowest in bulk SiO 2.

Melt inclusions are common in the phenocrysts. In most Plinian pumices from other volcanoes these are found to be less evolved compositionally than the matrix glass with which they co-exist. This is the usual case because melt inclusions normally enclose samples of the residual melt at an earlier stage of crystallization and degassing than the stage of evolution of the residual melt at the time of eruption, a relationship that is clear for the 1980-1981 eruption sequence from Mount St. Helens (Melson, 1983) and for many other Pli- nian tephras (Devine et al., 1984). We find the reverse to be true for the higher-temperature Ruiz pumices: the melt inclusions are more evolved than the matrix glasses (Table 5, Figs. 3B and 3C). In the lower-temperature pumices, the melt inclusions and matrix glasses have similar compositions, probably reflecting plagioclase growth continuing to just prior to eruption. The melt inclusions appear to have water contents that are the same or higher

WATER CONTENTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RU1Z 107

A. M A T R I X G L A S S

- 7 5 . S i02

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Log F02 116162

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A N P 6 8 2 ,

A N P 5 8 5 ~ / / ~ - + - ~ - ~ / ~ T 4 1 1 6 1 7 0

-9 ~ ~ ~ _ _ ~ I M L 7 7 ~ A N P 6 8 1 . . . . . ~ . . . . . . . . I

( , t , ~ I L i , , I I , , t l 9 5 0 1 0 0 0 T , ° C 1 0 5 0 1 l O 0

B. M A T R I X A N D M E L T I N C L U S I O N G L A S S

~. .... S i O 2 7 5 -

4

"4'o'~ ,5 ,, : -1 \ ' \ . : : v

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| - -0 • - - ~I'TF~ESD * ,~ o o

l 1 L 1 L L t I I I , , , lOOO T ,°C lo5o

l 1100

Fig. 3. A. Fe-Ti oxide calculated temperatures and oxygen fugacities, weight percent melt and matrix glass SiO 2 contents of Plinian pumices. Lines indicate range of values for pairs for given sample. For sample 116166, only one oxide pair was found. B. Compositional contrasts between matrix glass and melt inclusions in plagioclase. Same samples as in A. Line shows schematic path of postulated remelting trend. Black circles are melt inclusions paired with appropriate matrix glass.

108 w.G. MELSON ET AL

TABLE 4

Composi t ion and ca lcula ted t empe ra tu r e s and oxygen fugaci t ies of co-existing magne t i t e a n d i lmeni te pa i rs in P l in i an pumices. Cat ion ca lcula t ions follow methods of S to rmer (1983) and fo~ and T calculat ions from equa- t ions of Spencer and Lindsley (1981).

TiO 2 A1203 Cr203 Fe203 FeO MgO Total T(°C) Log/o,~

Sample ML77 7.93 1.74 0.50 50,71 34.60 2.17 97.71

37.08 0.15 0.20 27.90 28.36 2.80 96.48 945 -9 .58

8.68 1.67 0.52 49,13 35.28 2.15 97,45 36.41 0.16 0.22 28.72 27.70 2.83 96.04 978 - 9 . 1 7

9.20 1.61 0.50 48.08 36,03 1.97 97.39 36.78 0.20 0.18 28.96 28.00 2.85 96.97 996 - 8 . 9 8

7.50 1.66 0.50 52.65 34.77 2.11 99.23 36.23 0.12 0.23 29.75 27.77 2.70 96.80 941 - 9.52

7.94 1.75 0.51 52.48 35.58 2.09 100.40 36.34 0.17 0.19 30.76 27.55 2.88 97.89 959 - 9 . 2 8

Sample 116185-68L 11.47 2.01 0.53 46.79 38.26 2.82 101.94 39.58 0,50 0,16 25,09 29.41 3.47 98,21 1007 -9 .13

10.86 1.83 0.29 47.60 37.56 2.66 100.86 38.21 0.39 0.14 26.60 28.02 3.56 96.91 1009 -8 .98

11.92 1.86 0.35 45.55 38.51 2.71 100.95 39.92 0.32 0.15 24.75 29.77 3.44 98.35 1015 --9.07

Sample 116158-68D 8.31 2.78 0.50 47.79 33.66 2.76 95.86

31.89 0.52 0.29 41.24 23.55 2.88 100.37 1088 - 7 . 4 9

11,85 2,05 0.40 43.86 37.44 3.02 98.73 39.00 0.30 0.19 27.59 28.62 3.62 99.32 1048 -8 .56

Sample 116158-66 7.69 1.73 0.51 53.45 35.55 2.53 101.79

35.00 0.11 0.19 36.78 26.26 3.00 101.45 983 -- 8.78

8.32 1.75 0.48 51.83 35.97 2.10 100.47 35.37 0.08 0.21 36.36 26.93 2.74 101.68 1007 -8.57

7.62 1.96 0.47 50.94 34.24 2.27 97.57 33.28 0.17 0.20 36.36 24.97 2.97 98.23 1004 - 8 . 5 3

Sample 116158B 10.12 1.90 0.59 49.64 37.22 2.73 102.25 32.88 0.42 0.27 36.35 26.02 1.99 97.93 1073 - 7.85

9.76 1.87 0.67 49.50 36.54 2.80 101.23 31.71 0.45 0.27 39.45 24.57 2.27 98.80 1099 - 7 . 4 6

Sample 116166 8.31 3.05 0.53 50.91 34.46 3.23 100.52

30.68 0.50 0.26 42.01 23.35 2.38 99.18 1078 - 7 . 5 3


TABLE 4 (continued)

109

TiO 2 A1203 Cr203 Fe203 FeO MgO Total T( ° C) Log f02

Sample 116170 9.26 2.52 0.44 49.39 35.85 2.79 100.28

36.95 0.28 0.18 33.13 27.24 3.36 101.14 1017 -8.61

9.36 2.55 0.16 47.16 35.33 2.79 97.66 37.10 0.48 0.21 30.82 27.28 3.52 99.58 1013 -8.73

TABLE 5

Composition and amount of residual melt (matrix glass, Mg), Fe-Ti oxide temperatures and oxygen fugacities in airfall pumice. Bulk analyses are in Tables 1 and 2. Mi is average composition of melt inclusions in plagioclase. KBU is the bulk K20 content of sample. Residual melt calculated from ratio of bulk K20 to K20 in residual melt. Na-losses during electron microprobe moving-beam analyses of glass septae have been cor- rected (see text).

SiO 2 TiO 2 AI203 FeO* MgO CaO Na20 K20 KBU % Melt Log [02 T°C

1. ML77. Pumice lump. Base of section MC-1 Mg 74.25 0.35 13.35 1.65 0.30 Mi 72.36 0.33 13.50 1.14 0.26

2. ANP585. Light-colored airfall pumice. Mg 72.58 0.33 13.18 1.29 0.27 Mi 71.85 0.37 12.84 1.24 0.22

3. ANP681. Intermediate-colored band Mg 71.52 0.55 13.06 Mi 72.28 0.38 13.55

4. 116170. Airfall pumice Mg 68.43 0.62 14.83 Mi 67.77 0.47 13.36

1.80 1.42

5. ANP682. Dark-colored pumice band Mg 64.10 0.75 16.17 Mi 67.92 0.61 13.28



1.10 3.66 5.05 1.96 38.8 -9.3 964 1.30 3.81 4.54 43.2 964

1.07 3.61 4.94 2.56 51.8 -8.6 998 1.02 3.55 4.77 53.7 998

in same hand sample as 6 0.40 1.31 3.45 4.64 2.24 48.3 --9.1 1008 0.28 1.03 3.88 4.88 45.9 1008

2.11 0.58 2.06 3.92 4.28 2.46 57.5 -8.67 1015 1.59 0.27 1.36 3.93 4.08 60.3 1015

in same hand sample as 4. (Inhomogeneous Mg) 3.48 1.24 3.65 3.85 3.27 2.16 66.1 -8.0 1068 3.13 0.60 1.88 3.63 4.13 52.3 1068

4.13 1.57 4.12 3.82 3.36 2.15 64.0 -7.5 1078 0.97 0.18 1.61 4.50 4.25 50.6 1078

4.05 1.52 3.85 3.52 3.50 2.15 61.4 -7.7 1086 0.93 0.15 1.76 3.37 3.87 55.6 1086

t h a n the ma t r ix glasses wi th which they co- existed jus t prior to e rupt ion (Fig. 5B and C).

M a n y of the plagioclase phenocrys t s in the three h igher t e m p e r a t u r e pumices are par t ia l ly resorbed. This indicates t h a t the absence of

more basic mel t inclusions resul ts f rom resorp-

t ion and/or no plagioclase growth at these t empe ra tu r e s (ca. > 1040°C, Fig. 3B). Melt in-

clusions which r ema in are s imilar in composi- t ion to those in the lower t e m p e r a t u r e pumices (Table 5, Fig. 3B) and thus were probably in-

her i ted f rom an ear l ier more advanced, low-

110 W G M E L S O N E T A I .

Bsl

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L_

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r 1

4"

t l l

T I

SiO 2

t t

' I

TOTAL ALKALIS

6

I

o5o T °C , 1050 1100 10~30 5_ i 1 I $ l l 1 { 1 1 { t

Fig. 4. Bulk compositional parameters compared to Fe-Ti Dotted line shows trend suggested by samples other than

I I I I I I i I "T [ W - I

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oxide calculated temperatures for Plinian pumices. ML77. Sample symbols are same as in Figure 3A.

temperature episode of crystallization. This is an important line of evidence for convective overturns leading to remelt ing of marginal zones (Fig. 3B), possibly enhanced or even triggered by intrusion of basaltic magma into the base of the pre-eruption magma chamber, as supported by other features noted below.

Plagioclase-melt equilibria (Kudo and Weill, 1970) applied to the matrix glasses, melt inclusions and co-existing plagioclase (Table 6) in some Plinian pumices indicate a PH20 range from about 1.5 to 0.5 kilobars and there is a wide range for some samples (e.g. 116170). This approach requires special explanations about such internal disparity, as well as with the suggested H20 contents from the Merzbacher and Eggler (1984) projections (also in Table 6). Such disparity is in part a result of the common

strong and small-scale zoning of plagioclase, making it difficult for microprobe analysis of plagioclase actually in contact with liquid just prior to eruption. Nonetheless, the data indicate a decrease in PH20 with increasing temperature. The lowest temperature pumice, ML77, has an indicated PH20 of around 1 kilobar for both matrix glass and melt inclusions (Table 6).

Determination on petrologic grounds of the depth or depths at which the residual glasses evolved depends on relating the inferred pre- eruption PH20 to lithostatic pressure. This can be rigorously done only for water-saturated magmas in the presence of a vapor phase that is mainly water. The maximum indicated water content for the Plinian pumices is around 4%. This implies a PH20 of about 1000 bars at

WATER CONTF, NTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RU1Z 111

/ / ~+.A7 L77 opx qz-or/ --~

/ ~ ' > , NPsB5 1

9 9 8 ° 9 ] A N P 6 8 1

~log Z _ ~

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2 ~ 1078(1),

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/ , J i,,o,,, \x o / ¢ ~<>>:t \ 50

\ / / , ( ~ D R ¥ ,, , , \X / . . J 6Z~ /~ . . j / ] 116168 & /i 0̀ oo o :" ; , : /. - \ / A b AblAn = 6/ I O r \

~1o1~(4): , \ ~,~9s151

1008(6) 964(7 )

M S - 3 0 S - 9

Fig. 5. A. Water contents (contours in weight percent) of pumice samples of Table 5 projected into (A) plagioclase (CAS2-hypersthene(MS)-silica + orthoclase(S) following the method of Baker and Eggler (1983), and water content isopleths are in weight percent (Merzbacher and Eggler, 1984). Temperatures from oxide geothermometry are in degrees C. Sample numbers are in parentheses: 1 = 116166; 2 = ANP682; 3 = 116158; 4 = 116170; 5 = ANP585; 6 = ANP681; 7 = ML77. B. Same as A but with projections of melt inclusions (circles). Lines connect these with co-existing matrix glasses, and arrows indicate possible remelting trend. C. Projections of bulk rock (asterisks), matrix glasses (triangles) and melt inclusions (circles) of Plinian pumices into the haplogranite system, albite-orthoclase-quartz-H20 (Tuttle and Bowen, 1958) for each of the samples in Table 5. The dark line in each triangle of Figure 5C traces the compositions of liquids participating in solidus reactions as a function of water pressure, at dry and from 0.5 to 10 kilobars indicated by numbers along trend and under dry conditions (Luth, 1976). Ruled area shows successive approach of matrix glasses to the dry granite-minimum with decreasing temperatures. Thin solid lines ending in triangles (eutectic points) show ex- panded field of plagioclase crystallization resulting from normative Ab/An values of 6 (most evolved glasses) and 3.8 (less evolved glasses) at 2000 bars PH20 (Von Platen, 1965).

112 w(~ M~.:LSON ~;T A~.

TABLE 6

Co-existing matrix glass (MG), plagioclase phenocryst margins and melt inclusion (MI) and plagioclase capsule- wall compositions. PH~O from algorithm of Kudo and Weill (1970); "H20" is weight percent water of glass estimated by projectiofi of Merzbacher and Eggler (1984). This projection is not experimentally calibrated for glasses as acidic as those in ML77 and ANP585, and thus values for H20 are probably inaccurate.

TY PH20 H20 SiO 2 A1203 FeO* MgO CaO K20 NA20 TiO 2 Total

ML77, T = 9 6 4 ° C 1. MG 1180 3% 73.72 13.29 1.24 0.25 1.00 5.18 3.66 0.30 98.64

PL(An41) 57.72 25.03 0.40 0.05 7.75 0.74 6.27 0.06 2. MI 1080 2% 71.61 13.06 1.11 0.26 1.22 4.98 3.46 0.33 96.03

PL(An42) 57.58 25.99 0.39 0.06 8.99 0.70 6.45 0.02 3. MI 1020 2% 72.52 13.29 1.18 0.28 1.23 4.31 3.88 0.36 97.05

PL(An41) 59.41 25.15 0.42 0.05 8.18 0.70 6.61 0.02

A N P 5 8 5 , T = 998 o C

1. MG 840 3% 73.05 13.58 1.42 0.27 1.04 5.04 3.79 0.31 98.50 PL(An41) 58.18 26.01 0.40 0.00 7.83 0.77 6.65 0.06

2. MI 460 3% 73.51 12.94 1.10 0.21 0.99 4.82 3.60 0.33 97.50 PL(An38) 59.68 24.83 0.41 0.05 7.01 0.82 6.81 0.05

3. MI 800 3% 72.24 12.71 1.07 0.14 0.90 4.76 3.60 0.36 95.78 PL(An39) 58.66 25.10 0.39 0.03 7.53 0.72 6.46 0.04

116170, T = 1 0 1 5 ° C 1. MG 1520 1% 69.55 15.20 2.09 0.59 2.13 4.14 4.16 0.66 98.52

PL(An54 53.61 28.40 0.58 0.03 11.38 0.34 4.94 0.02 2. MG 950 3% 70.14 14.07 1.96 0.52 1.90 4.28 3.96 0.57 97.40

PL(An6o) 55.27 27.97 0.56 0.03 10.35 0.47 5.58 0.08 3. M! 900 1% 67.77 13.36 1.59 0.27 1.36 4.08 3.93 0.47 92.83

PL(An45) 57.05 26.75 0.45 0.06 9.01 0.56 6.12 0.00

116166, T = 1 0 7 8 ° C 1 MG 1090 4% 64.30 16.35 3.85 1.47 4.00 3.42 4.16 0.83 98.38

PL(An61) 53.50 29.06 0,72 0.14 11.99 0.29 4.22 0.10 2. MI 520 1% 70.64 15.61 1.18 0.27 1.71 4.20 4.84 0.34 98.79

PL(An45) 58.66 26.88 0.42 0.02 9.12 0.56 6.16 0.08

1. MG 430 3% 64.25 16.46 4.13 1.51 3.93 3.53 3.95 0.78 98.54 PL(An52) 55.62 27.59 0.85 0.13 10.84 0.35 5.33 0.09

2. MG 500 2% 63.97 15.80 4.00 1.56 3.86 3.62 3.61 0.78 97.20 PL(An57) 54.74 28.60 0.70 0.11 11.70 0.25 4.89 0.05

1000°C for a p u r e w a t e r v a p o r p h a s e (Fig. 2, Eggler , 1972, for a Mt. Hood andesi te) , in- d i ca t ing a m i n i m u m dep th of l as t equ i l i b ra t ion of abou t 3 kms , or a g r e a t e r dep th for coex i s tence wi th a CO2-rich v a p o r phase . The re is some ev idence for w a t e r con ten t s as h igh as 6% (sum of m a t r i x g lass sum, p a i r 1, m i n u s m e l t inc lus ion sum, pa i r 3, s a m p l e 116170, Tab le 6). Fo r th i s content , a m i n i m u m dep th of l as t equ i l ib ra t ion is abou t 6 k m for a wate r -

s a t u r a t e d m a g m a . However , both the Merz- bache r and Egg le r (1984) project ion and the

p lag ioc lase -mel t equ i l ib r ia (Table 6) d i sagree wi th the h igh H 2 0 conten t ind ica ted by the low s u m of th is m e l t inclusion. For those s amp le s wi th w a t e r con ten t s of abou t 1%, p robab le m i n i m u m dep ths of las t equ i l ib ra t ion are as low as 1 k m (a round 300 ba r s PH20). In sum- m a r y , a l t h o u g h t he r e a re some indica t ions of h ighe r w a t e r con ten t m a g m a s in the e rup t i ve

WATER CONTENTS. TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RUIZ 113

sequence, most have water contents of about 1 - 4 wt. %, indicating last mineral-melt equilibration at depths as shallow as 1 to 3 km.

Other volati le components

Estimates of CO 2 contents in the Ruiz magmas or co-existing vapor phases are not possible from the data given here. However, est imates of some other volatiles can be made from electron microprobe analyses and from Table 2. Chlorine and fluorine contents in the airfall bulk pumice (Table 2) appear to correlate with the extent of fractionation. These values are clearly lower than those of pre-eruption magmas because of some degassing during eruption. Melt inclusions in an airfall pumice sample (116158, high-temperature Plinian pumice, Table 5) give a range of F, C1 and S contents of 0 -1100 , 600-1500 ppm, and 100-600 ppm, respectively, and averages of 500, 1000, and 200 ppm, respectively, for 14 melt inclusions. The high average value for S in melt inclusions in pyroxene phenocrysts (675 ppm, Sigurdsson et al., 1986) probably reflects the earlier crystallization of pyroxene. Our average for melt inclusions in plagioclase falls close to the 275 ppm value found by Sigurdsson et al. (1986). In this same sample, the matrix glass has F, C1 and S contents of 500, 600 and 300 ppm, respectively. The higher S in the matrix glass is analytically significant and suggests that, for this sample, which falls into the higher temperature group, the matrix melt at the t ime of eruption was richer in S than the glass included at some earlier t ime in some of the melt inclusions. This peculiar feature may indicate a high flux of S into the melt of this part icular sample during an episode of remelting, as outlined below. It is remarkable that some portion of this higher S content was retained even after degassing during eruption.

The F, C1 and S contents of a single crystal of biotite are 5600, 1100, 800 ppm; of hornblende, 1300, 300 and 600 ppm; and of apatite, 22000, 8100, and 4700 ppm, respectively. These high

values probably reflects high partition coeffi- cients for these three volatiles into these phases as well as crystallization prior to some degassing.

The calculations of Williams et al. (1990- companion volume) indicate that the volume of magma required to account for the large volume of SO 2 emissions, est imated at 3.4 x 106 g, require at least 0.92 km 3 of undegassed magma using the high S values in melt inclusions in pyroxene. The presence of anhydrite that is probably nonmagmatic, either xenolithic or a quenched vapor phase, raises the possibility that some of the large volumes of SO 2 emitted from Nevado del Ruiz (Williams et al., 1990-companion volume) may be from an evaporite source (Fournelle, 1990-companion volume). Sulfur isotopic analyses conducted on samples from the summit crater gas column and from the hydrothermal system, however, argue against any significant role for marine evaporites as a source of SO 2 (Williams et al., 1990-companion volume). As yet, anhydrite phenocrysts have not been noted but the mineralogical diversity of the samples is such that some may yet be found. The absence of anhydrite phenocrysts and the presence of reaction rims around the anhydrite xenoliths in ML83 indicate that the magmas were not sa turated with anhydrite as were the magmas of the 1982 eruption of E1 Chich6n (Luhr et al., 1984). Basaltic magmas normally contain con- siderably higher pre-degassing S contents than more acidic ones (e.g. Devine et al., 1984) and may be one source of the excess S, if in fact there was underplating of the andesitic magma by basaltic magma.

Welded tuf fs

The welded tufts in a number of near-crater tephra sections (Calvache, 1990-this volume), all within about 1 km of the Arenas crater, are a remarkable feature of the November 13, 1985 rocks. Some show well-developed eutaxitic textures (Fig. 1E). The welded portions may reach

114 W.G. MELSON ET AL

up to 4 m thick and compose up to a half to two thirds of the basal portion of a given section. However, the stratigraphic sections range widely in lithology and not all sections contain the welded unit. In the summit region, tephra sections are visible only in major post-eruption glacial crevasses and as yet a detailed isopach map of the summit pyroclastic flow could not be made.

Such welding is extremely rare in small- volume pyroclastic flows because of heat loss as the flows cascade down the steep sides of a "normally shaped" stratovolcano. However, Ruiz is exceptional in tha t the glaciated summit to the south of the Arenas crater is nearly flat (Fig. 6). Thus, pyroclastic flows ponded and filled crevasses on the glacier near the crater with minimal heat loss and with temperatures sufficient to cause collapse and welding of

pumiceous fragments near the base of the flow. Temperatures sufficient to cause welding of the dacitic to rhyolitic matrix glasses of these magmas need be no higher than around 990°C, the Ab-Or-Qz minimum (anhydrous granite minimum, Tuttle and Bowen, 1958), and are well within the Fe-Ti oxide values of the pumice (Table 4). McBirney (1968) has proposed that welding of such dense tufts is commonly a result of partial remelting by the fluxing action of trapped water. Post-depositional oxidation is reflected by the dark red color of much of the welded zones. The abundance of phenocrysts in all the tufts argues against any model involv- ing super- or even near-liquidus eruption temperatures. It is also possible that the welded tufts are in fact a welded agglomerate produced by near-crater fall-back ejecta and not by a pyroclastic flow. This is deemed unlikely

Fig. 6. Flattish summit area looking north from about 500 m north of the rim of the Arenas crater at about 4700 m. elevation. Welded tufts are exposed in fissures near the crater in this region.

WATER CONTENTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RU[Z 1 1 5

because even low ejection heights would probably lead to sufficient heat loss to preclude welding after accumulation. The Fe-Ti oxides in the welded tuff are commonly oxidized and/or show exsolution, prohibiting their use as temperature indicators.

Some welded tuff samples from the near- summit sections contain rare, light-colored collapsed pumice fragments in darker, glass-rich collapsed pumice (WT2, Fig. 1E). These light- colored clasts have matrix and melt inclusion glasses tha t are highly evolved compared to the dark-matrix glasses, and some contain biotite microphenocrysts. The greater crystallinity of the light-colored clasts may reflect more extensive degassing and cooling prior to eruption (Fig. 7). The biotite is undecomposed in the light-colored collapsed pumice suggesting that some water was retained during crystallization. However, it contains about 0.5 wt. % fluorine which may have increased its stability to lower PH20.

The heterogeneity in regard to pre-eruption

op•o, 0AS2-60

BROW

MELT IN ~ WHITE MG "~\

/MS-50 . . . . . . S-90 \ Fig. 7. Probable pre-eruption water-contents in matrix glass and melt inclusions in plagioclase in light-colored collapsed pumice clast, and in the matrix glass in dark-colored collapsed pumice in a welded tuff (WT2, Fig. 1E). Projection as in Fig. 5A; stippled area is postulated stability field of hornblende (Merzbacher and Eggler, 1984). Point within each circle is average value and circle indicates range in composition for each glass group.

PH20 shown in the single sample WT2 (Fig. 7) characterizes some of the other welded tuff samples from the summit section (e.g., WT80, Table 7) and is clearly a result of int imate mixing of water-poor and water-rich magma parcels at different temperatures as we found in one of the Plinian pumices (Fig. 5). Probable water contents range from 1 to possibly in excess of 4 wt. %. Some extend into the inferred hornblende liquidus field, suggesting water contents in excess of 6 weight percent (Merz- bacher and Eggler, 1984), although Rutherford and Devine (1988) found that hornblende grew stably in dacitic melts at water contents as low as 5 wt. % at 920°C.

Hybridization, magma mixing and crustal contimination

Some samples are hybrid rocks, consisting of distinctively different phyric andesitic to dacitic magmas mixed while still partially molten. One of the most striking of these (ML83) consists of a mixture of: (1) hornblende- biotite-pyroxene andesite with a variety of micro-xenoliths; (2) a range of pyroxene andesite "magma parcels" with distinctly different phenocryst and microlite sizes; and (3) scattered stringers and pods of anhydrite with opaque sulfide-bearing reaction rims (Fig. 1A). Fine-grained recrystallized xenoliths of a variety of sedimentary and metamorphic rocks occur in small amounts in some thin sections.

Olivine occurs as an accessory mineral (< 2 vol. %) in 14 of 15 thin sections of airfall pumice, and in all 8 thin sections of welded tufts examined here, and ranges from Fo74 to Fos8. It occurs as (1) rounded, partially resorbed phenocrysts, and microphenoczwsts, some containing chromian spinel (Fig. 1E, and analysis 18, Table 3); (2) hypersthene-mantled rounded grains; and (3) conglomero-porphyritic clots with hypersthene, augite, plagioclase and other minerals. Magmas of a composition like most of those analyzed here have a small amount of olivine on a calculated l-atm

116 W G MELSON ETA[,.

TABLE 7

Electron probe analyses of fused bulk rock-powders (BR) and analyses of matrix glasses and melt inclusions by moving beam analyses of summit section welded tufts.

Sample SiO 2 TiO 2 A1203 FeO* MgO CaO Na20 K20 P205 Total

BR-WT2 59.66 0.71 16.62 5.59 5.21 6.26 Dark-colored matrix glass (N = 3)

64.68 0.60 17.01 3.50 1.28 3.80 Clear homogeneous matrix glass in collapsed acidic clast (N = 2)

75.84 0.22 13.11 1.03 0.19 Melt inclusions in plagioclase in collapsed clast

72.14 0.52 14.13 2.27 0.44

BR-MLWT79 59.93 0.68 16.57 5.51 5.12 Homogeneous matrix glass (N = 17)

64.55 0.80 16.22 3.85 1.47

BR-WT5 59.93 0.69 16.51 5.56 5.21 Homogeneous matrix glass (N = 4)

64.06 0.83 16.67 3.71 1.43

BR-MLWT80 60.11 0.69 16.27 5.69 5.40

3.51 1.82 0.17 99.77

4.19 3.76 0.27 99.09

0.78 4.13 4.66 0.06 100.00

1.43 4.11 5.17 0.03 100.24

6.21 3.83 2.04 0.20 100.09

3.68 4.09 3.37 0.25 98.28

6.26 3.83 2.02 0.19 100.20

4.14 4.89 2.63 0.32 98.68

6.34 3.69 1.97 0.16 100.32

4.36 4.80 1.99 0.20 99.56

2.85 3.60 5.53 0.24 98.91

1.53 3.20 6.36 0.08 101.32

6.20 3.77 1.87 0.27 99.70

4.00 5.15 2.12 0.22 99.02

1.46 2.92 5.38 0.09 97.82

6.21 3.47 2.04 0.30 99.67

3.77 3.33 3.40 0.25 98.99

6.19 3.85 1.95 0.22 100.20

3.55 4.83 2.90 0.26 98.79

6.15 3.46 1.97 0.26 99.19

3.02 4.23 3.43 0.27 98.69

Heterogeneous plagioclase-quench crystal-rich glass (N = 3) 65.68 0.83 16.72 3.61 1.37

Evolved matrix glass (N = 4) 64.72 0.82 16.62 2.89 1.64

Highly evolved matrix glass (N = 1) 71.97 0.56 14.98 1.99 0.65


65.40 0.87 15.96 3.90 1.40 Melt inclusion in plagioclase

71.78 0.29 14.69 1.03 0.18


65.55 0.89 16.19 4.03 1.58


65.95 0.82 15.67 3.55 1.26

BR-WT9B 59.11 0.67 16.71 5.51 5.35 Homogeneous matrix glass (N = 4)

65.77 0.86 15.84 3.86 1.41

a n h y d r o u s l iquidus (Roger Nielsen, pers. commun. , 1987) bu t it is soon replaced by h y p e r s t h e n e wi th fa l l ing t e m p e r a t u r e . Thus , the resorbed and h y p e r s t h e n e - m a n t l e d ol ivine could be a p r i m a r y phase . Some of the ol ivine (Fo86, ana ly s i s 26, Tab le 3) is close in composi- t ion to t h a t p red ic ted by the d ry l iquidus phase

ca lcu la t ions for bu lk composi t ions l ike t h a t of the rock in which it occurs (Roger Nie lsen , pers. commun . , 1987). For those s a m p l e s wi th k n o w n FeO and F e 2 0 3 (Table 2), ca lcu la ted Fo con- t en t s for f i rs t ol ivine r a n g e f rom 87 to 89 us ing the a l g o r i t h m of Roeder and Emsl i e (1970), w h e r e a s the observed r a n g e is f rom 74 to 90.

WATER CONTENTS, TEMPERATURES AND DIVERSITY OF MAGMAS OF NEVADO DEL RUIZ J_17

Some of the olivine could thus be in equilbrium in regard to Fo content with the liquids in which they occur, but some, especially the more iron-rich, could not. Some olivine grains are strongly zoned, ranging from interiors with Foss and margins to Fo74, and it is unlikely that these grew in the Ruiz andesitic magmas.

Olivines containing chromian spinel are common, and the high-chromium spinels are unlikely to have crystallized from a magma of the bulk composition of the Ruiz andesites and dacites. Such chromian spinel-bearing olivines commonly are associated with high-chromiun augite, ranging up to 0.94 weight percent Cr20 3 (anal. 16, Table 3) whereas "normal" augite phenocrysts contain about 0.04% (anal. 15, Table 3). The Ni content of olivine ranges from 1700 ppm (Fos0) to 2800 (Foss) in one sample (116170). Calculated olivine Ni contents based on extrapolations of Ni partitioning between melt and olivine using the algorithm of Har t and Davis (1978) give values as low as 700 ppm for the least Ni-rich sample and 2600 ppm for the highest Ni sample (28 ppm, ANP585, and 108 ppm, WT2, Table 1, respectively), and olivine Ni contents of about 2000 ppm for the average bulk MgO and Ni contents (4.3% and 71 ppm, Table 1). The highest Ni olivines are thus higher in Ni than predicted, but still close and probably within the combined analytical and algorithm errors, and do not in themselves rule out a cognate origin for the olivine.

Plagioclase phenocrysts show a wide range in composition, ranging from An35 to An65. Re- cent experiments by Rutherford and Devine (1988) yield a range of plagioclase compositions from about An30 (dry) to An60 (water-rich, CO 2- bearing vapor phase at a total vapor pressure of 2.2 kbar at 920°C). These experiments were done on a Mount St. Helens dacite close in composition to the most fractionated bulk rocks and similar to many matrix glasses in the 1985 Ruiz samples. It is thus probably not necessary to invoke mixing of a more basic magma (Gourgaud and Thouret, 1990-this volume) to explain the plagioclase compositions but such

an origin by mixing cannot be ruled out. Sodic plagioclase (An35) is partially resorbed in one of the high-temperature Plinian pumices (116166), and was unstable in this residual melt at a temperature around 1080°C and a water content of around 3 wt. % (Table 4, Fig. 5A).

Strontium isotopes for five samples (Table 1, Fig. 8) range between 0.704306 and 0.704394 and fall close to the values determined by James and Murcia (1984) at Ruiz although the fractionated Plinian pumice sample ANP585 has both a higher 87Sr/86Sr and higher Rb/Sr than those previously reported values (Fig. 9). The values for this single eruption span a range as wide as those of James and Murcia (1984), presumably samples from a number of separate earlier eruptions.

The S7Sr/86Sr ratio directly correlates with a number of incompatible element contents (Fig. 9). This correlation is most simply explained by wall-rock contamination and/or magma mixing processes. Crustal contamination of more

ANP585HI~

--0.70438

oo

-0170434

-0.70430

ML84~

ANP682

~ ANP681 ,&&

Rb/Sr 0.06 0.10 0.14 0.18 I I I I I I

Fig. 8. Rubidium/strontium and 87Sr/86Sr from the 1985 eruption (squares) cluster near values determined by James and Murcia (1985, triangles) for a samples from a number of Pre-historic eruptions of Ruiz. Bars give errors in determinations.

118 W.C. MI;;LSON ET AI,

1 1 0 0

1 0 5 0

1 0 0 0

9 5 0

6 5

6 0

8 0

5 0

i i ' I

T°C

/ /

- /

• L f i

O ~ L 7 7

A N P 6 8 2

I i f w i

/ $ i 0 2 / • -

I r l

NiO(ppm)

~ O A N P 6 8 1

~ l M L 8 4

A N P 5 8 5 • i i

K 2 0

, ~ r" F J

I

/

- - /

L* I t t

/

i / "

1 t I 0 . 7 0 4 4 0 0 . 7 0 4 3 0

/ /

R b ( p p r n )

/

I I I I

U(ppm)

/

~ o

i i

/ /

/

f

/ • 2 5

4

J - 2 0

/ 8 0

70

-J6o ! f

I I I 6 " l

/

- - 5

4

• i 2 0 ,- i i = I , I I I I I I I

0 . 7 0 4 3 0 0 . 7 0 4 3 5 0 . 7 0 4 3 5 0 . 7 0 4 4 0

8Zsr/S6s, 8 7 S r / S 6 S r

Fig. 9. Some incompatible element contents correlate directly with 87Sr/S6Sr. Analyses in Table 1.

"primitive" liquids was used by James and Murcia (1984) to account for their Ruiz isotopic data. However, mixing between an andesitic to dacitic magma body, possibly already contaminated by wall-rock assimilation, and a basaltic magma, consistent with the lines of evidence outlined above, is yet another mechanism to account for the incompatible element-S7Sr/S6Sr correlations. Correlations between those elements affected by plagioclase fractionation, such as Eu, A1 and Na, show poor or no correlations with 87Sr/S6Sr.

The trace-element geochemistry (Table 1 and Fig. 10) also raises the possibility of crustal contamination and/or magma mixing. The high to medium K contents of the 1985 eruptives are similar to many other eruptives from the Andes (e.g. Lopez-Escobar et al., 1977; Dostal et al., 1977a,b; Kay, 1987; Hildreth and Moorbath,

1988) and elsewhere (Gill, 1981). They are relatively enriched in the LREE, with (La/Sm)ch ranging from 2.9 to 4.1 and (La/Yb)ch ranging from 8.8 to 11.4. The Ruiz samples are enriched in Ba relative to other Andean andesites to the south. Ba/La ratios range from 49 to 62 in the Ruiz pyroclastics, whereas the lavas of other Andean andesitic centers typically have Ba/La ratios of 12-30 (Dostal et al., 1977a; Hickey et al., 1984; Lopez- Escobar, 1984; Hildreth and Moorbath, 1988).

The Ruiz pumice and tuff samples are unusually high in U and Th, and are similar to the trachyandesitic pumices erupted from E1 ChichSn volcano in 1982 (Luhr et al., 1984), although ChichSn samples are somewhat higher in Na20, K20 and other incompatible elements, with the exception of Ba. Th/U ratios in the Ruiz samples range from 2.2 to 2.5, lower


1000 MC-1 SUMMIT SECTION 1000

"*" ML84

100 I - I- ML82 1 0 0

X / " ~ f X i~'~. ' "~" MLWTS0

1 l I I I I I l I I I I I I I I l I

CS F~ K Ba Sr U Tn Ht Ta La Co Nd Sm Eu Tb YO Lu Cs Fib

119

MC-2 SUMMIT SECTION - o - WT6

-o- WT2

\ i \ / \ i \ / ' \ / \ I'-M~" " - ~ / " ~ 1 ~ -x- ML76T

x

I I I I I I I I I I I I I I I I

K 13a Sr U Th Hf Ta La Ce Nd Sm Eu Tb Yo Lu

1000 WELDED TUFFS 1000

o : 7 °1

x iV",, I--T. 1 ___ -o- ML79

D

Cs Rb K Ba Sr U Th Hf Ta La Ce Nd Sm Eu Tb ~ Lu ELB4ENr

100

10

OTHER AIRFALL PUMICE

• . z . ~ , ~ . ].- ANP~81 .o_ AN,.O

,oo \ i.,_ ANPS

8

1 I I I I I I I I I I I I I I I I

Cs RIo K Ba Sr U Th Hf Ta La Ce Nd Sm Eu Tb Yb Lu

ELEMENT

Fig. 10. Normalized trace-element contents of the 1985 Ruiz rocks. Elements normalized to Leedy Chondrite and basalt KDll (Kay and Hubbard, 1978; Kay, 1987). Normalization factors are: Cs,0.013; Rb,1.94; K,116; Ba,3.77; Sr,14; U,0.015; Th,0.05; Hf,0.22; Ta,0.22; La,0.378; Ce,0.976; Nd,0.716; Sm,0.23; Eu,0,0866; Tb,0.0589; Yb,0.249; Lu,0.0387. Note the enrichment in Ba, U, Th and in the light rare earth elements.

than the 2.9 to 4.9 ratios exhibited by Andean volcanoes between 33 and 38°S in Chile (Hildreth and Moorbath, 1988), and lower than typically observed for other medium- to high-K andesites (Gill, 1981). Uranium contents of volcanic rocks from two transects in the Cen- tral Andes (0.70-4.63) correlate with crustal thickness, and indicate tha t the U contents of the lavas may reflect the degree of crustal con- taminat ion of the magmas during ascent (Zen- tilli and Dostal (1977).

Hildreth and Moorbath (1988) noted a strong positive correlation between crustal thickness and Th, K, Rb, Cs, Ba, and 87Sr/S6Sr, and an in- verse correlation of crustal thickness with 143Nd/144Nd along the volcanic front in central

Chile. They interpreted these variations as reflecting differing amounts of crustal con- taminat ion of the andesitic melts, with increased amounts of crustal contamination where the crust is thickest. Similarly, the high Ba, Th and U contents of the Ruiz 1985 eruptive rocks may reflect contamination by continental crustal rocks during magmatic ascent, an inference in accord with Sr, Nd and O isotopic studies of older Ruiz lavas by James and Murcia (1984) who concluded tha t the Ruiz magmas had been contaminated by continental crust. This suggestion is also in accord with the K/Rb ratios of the lavas (210-330) which approach those of average continental crust (Jakes and White, 1970).

1 2 0 w.(~. MELSON ET AL

C o m m e n t s on the pre -erupt ion m a g m a c h a m b e r

The only constraint on magma chamber volume is given by assuming that the chamber was totally emptied during the eruption and thus equal to about 0.012 km 3 of magma of a density of about 2.5 gcm -3 from the 0.03- km 3 isopach volume of Carey et al. (1986). For an elongate ellipsoidal magma chamber with a radius of 100 m, the vertical length of the chamber would be about 300 meters. Int imate mixing of marginal and interior magmas would be favored by such a small magma chamber of narrow cross-section and deep vertical extent. The presence of the viscous, rhyolitic residual melt-bearing pumices late in the sequence might be explained by the entra inment mechanism of Freundt and Tait (1986). In this model, marginal viscous magmas in a vertically elongate magma chamber are initially "pierced" by less-viscous interior magmas but become entrained in them near the end of the eruption.

Rapid development of a marginal "crystal capture front" (Marsh, 1988) provides an alter- native mechanism to entra inment for the

stratigraphic evidence of a trend from least to most evolved rocks through time. In this model, more mafic, less-fractionated magmas are retained at the top of the magma body, and are thus likely to be first out on eruption. Plagioclase settling, one method to account for the fractionation trends in the bulk compositions, would occur at a negligible rates even over 500-year intervals in these crystal-rich viscous magmas (Table 8). Up and down velocities resulting from convection are likely to be on the order of 10 m yr -1 (Shaw, 1965), far larger than likely crystal settling velocities. Both these features favor development of a crystal capture front as does rapid increase in the viscosity at the magma chamber margins due to cooling and degassing.

The matrix glasses and bulk rocks show a bulk density reversal which may have played an important role in convective overturn as well as possibly in triggering the eruption (Fig. 11). For a density-stratified magma chamber, the matrix glasses indicate melts that are increasingly dense with increasing abundance of melt and increasing temperature, which suggests a stably stratified magma body. Note though that the calculated bulk magma den-

TABLE 8

Temperature, matrix glass density (GD, g cm 3), and viscosity (GV, log(10), poises) using analyses of Table 5, and H20 contents estimated from Figure 5A (from equations of Shaw, 1972 and Bottinga and Weill, 1970, respectively), Stokes settling rate in m/yr -1 for a 1-mm plagioclase crystal (P1Mg) in matrix glass free of crystals, weight fraction crystals (FrXls, from Table 5), least-squares calculations of weight fraction plagioclase (FP1) and hypersthene (FPx), calculated bulk rock density (RD) and viscosity (BV, log(10), poises) of bulk mixture of crystals plus matrix glass calculated by GDX(1-FrXls) -2'5 (see Shaw, 1965, p. 130, for discussion of effects of crystal size distribution and abundance on bulk viscosity). Settling velocities of phenocrysts in magmas of ef- fective viscosities like those for the bulk magmas (BV) are insignificant.

Sample T°C GD P1Mg GV FrXls FP1 FPx RD BV

ML77 964 2.19 0.03 6.55 0.64 0.45 0.19 2.66 85.3 ANP585 998 2.23 0.05 6.21 0.48 0.34 0.14 2.57 32.1 ANP681 1008 2.25 0.07 6.03 0.52 0.33 0.19 2:62 37.2 116170 1015 2.24 0.24 5.51 0.47 0.32 0.15 2.57 27.5 ANP682 1068 2.27 1.73 4.63 0.34 0.16 0.18 2.55 13.1 116166 1078 2.24 1.94 4.61 0.36 0.21 0.15 2.51 14.1 116158 1086 2.29 1.20 4.77 0.39 0.24 0.14 2.55 16.1


2 . 3

2 . 2

2 . 6

2 . 5 (

D E N S I T Y ( g c m - 3 )

' ' ' ' I ' ' ' ' I ' ' '•' A. M a t r i x M e l t ~.~'~

.~'~ _

1 II V / /

/ i v + "IF ¢ +

2 . 4 , , , , I , , 9 5 0 t O 0 0

, , , t . . . . i i I ' ~ ' ' ' I ' ' ' '

%%% "I"

%%. %

% %'~ A I

, - + B. B u l k M a g m a

t O 5 0 9 5 0

T o c

V I S C O S I T Y ( L o g ( 1 0 ) , P o i s e s )

' 1 ' ' ' ' I . . . . I . . . .

i "~ '~ ' - .11 + C. M a t r i x M e l t

• . . . 6

+ + + 4

I , , , , I I ' : : t I . . . . t I I I I

' 8 0 I D. B u l k M a g m a

\ 5 0 %

%

~ 1 0 . . . . 1 , , , , I , & ~ ' ~ ,

t O 0 0 / . 0 5 0 I t O 0

Fig. 11. Calculated densities and viscosities of matrix glasses and bulk rocks (data from Table 8) for Ruiz pumices as a function of temperature. Symbols are same as in Figure 3. The density and viscosity calculations are strongly dependent on the estimated H20 contents: arrows show density and viscosity decreases in response to increase of H20 content by 1% in the most mafic matrix glasses. Sensitivity of viscosity and density calculations to temperature errors are shown for sample ML77 (asterisk, for + 40°C error).

sities (crystals + melt) are inverted: the low melt content, cooler and presumably marginal magmas have higher densities then do the hotter presumably interior magmas. The low to basically non-existent rates of crystal settling would retain these bulk density contrasts. These density trends are far more sensitive to bulk composition, particularly water content, than they are to temperature ranges likely in these Ruiz magmas.

The wide-spread but small amounts of chromian-spinel-bearing olivine and other xenocrystic phases of likely basaltic parentage, the evidence of infux of S in some samples, and the isotopic evidence of some mixing point to possible underplat ing of the andesitic magma chamber by basaltic magma. This is an attrac- tive process for it provides a heat source for the common evidence of remelting in the high- temperature Plinian pumices noted above and could assist in intensifying the already pro-

bable unstable density stratification noted above both by heating and possible upward water flux. Also, magma heated by such basaltic underplating may grow by crustal melting, a process evidenced at Ruiz by small xenoliths of a wide variety of metasedimentary and other recrystallized rocks, some of them biotite and anhydrite bearing, and the stron- t ium and oxygen isotope variations (James and Murcia, 1984).

J.C. Eichelberger (e.g. 1980) has explored many aspects of the intrusion of basaltic magmas into more acidic magma bodies. The "mafic foam" xenoliths he reports from many such instances were not noted in the Ruiz samples. Eichelberger (1980) points out that intrusion of water-rich basaltic magmas at depths of less than 12 km will create rapid mixing by formation and rise of low-density 'mafic foam" whereas dry basaltic magma will remain at the base, forming a stably stratified magma body

122 W G M E L S O N E T A L

with interchange limited by diffusion rates. The intimate mixing of the 1985 Ruiz andesites and dacites with basaltic xenocrysts and xenoliths suggest mixing that would be facilitated by the rise of such a "mafic foam". However, if such foam were produced beneath Ruiz, the glassy portion has largely equilibrated with its more acidic hosts.

The 1980 pumices from Mount St. Helens show a crystallization trend strongly domi- nated by H20 degassing (Fig. 12A) which contrasts with that at Ruiz. The amount of matrix glass decreases from about 65% to 35% and there is a suggestion of a slight temperature in-

c rease with increasing crystallization (Fig. 12A) at Mount St. Helens. The Ruiz sequence shows a similar decrease in percentage of melt over a temperature interval of about 120°C during the proposed remelting. Figure 12D compares melt inclusions with co-existing matrix glasses at Ruiz and melt St. Helens in terms of MgO contents. Compared to matrix glasses, melt inclusions are less evolved in Mount St. Helens

pumices and more-evolved at Ruiz, a principal line of evidence for a remelting sequence at Ruiz. At Mount St. Helens, about 0.2 km 3 of homogeneous magma with a water content of about 4% (Merzbacher and Eggler, 1984; Rutherford and Devine, 1988) was emitted in less than a day on May 18, 1980 (excluding the first out proposed cryptodome samples), and degassed magmas later, from the June 12 dome and on. At Ruiz, magmas with a large range in temperatures and water contents were erupted in about two hours. This probably reflects the much smaller volume and hence much steeper thermal and PH20 gradients in the source body at Ruiz. Heterogeneity was perhaps further enhanced at Ruiz by the possible underplating and limited mixing with basaltic magma.

C o n c l u s i o n s : a t e n t a t i v e m o d e l

The following scenario is consistent with the above data and interpretations. The November 13, 1985 eruption began with disruption of a

70

60

j J f i i

+

~:,g B. % M E L T • -

+ D °

40 ~ ~

I , , + , , I , , , ~ I , L I I I

_ 8 , o 0

I ~ t ~ o + -10~ -+ +

' io o6 '

I ' ' ' ' I 0 f f ' ~ I I ' ' ' '

C. % M E L T • T.,, , ,e , , = , , , , , , , ~ , ~ ~

i:

\o ' ' __11 I I + I I I I I I , I , I I , , ~ , = , ' ' '

D. M g O / = = i t i

i t - - i l - o I I , ,

i i - ÷

, e l . . . . . . . . . . . . o

1 0 0 5 9 5 0 1 0 0 0 1 0 5 0 1 O0 T%

70

60

- 50

40

1.0

0 .5

Fig. 12. Contrasting degassing crystallization trend (solid line on left with arrow) in matrix glasses for May 18 (circled asterisk) to August 7, 1980 (crosses) pumices from Mount St. Helens and proposed remelting trend (solid line to right with arrows) for the Ruiz Plinian pumices (symbols same as in Fig. 3). Mount St. Helens gray dacite ("cryptodome") indicated by square. Average compositions of melt inclusions are solid circles for Ruiz and open circles for Mount St. Helens in C and D.


comparatively small compositionally zoned and possibly density inverted magma body, probably vertically elongate and extending to about 6 km beneath the summit crater. Prior to the eruption, a partially degassed and possibly cooler carapace had developed, in some regards similar to the cryptodome which developed at Mount St. Helens prior to the May 18, 1980 cataclysmic eruptions (Christiansen and Peter- son, 1981), but still partially molten.

Underplating of the magma chamber by intrusion of basaltic magma may have played a key role in partial remelting and compositional disruption of the andesitic to dacitic magma chamber and its carapace, and may have triggered the eruption by causing boiling in the overlying magma. The evidence of pervasive but small scale mixing of basaltic and andesitic magmas reflects a significant although uncer- tain amount of time between intrusion of the basaltic magmas and the eruption, and/or multiple periods of basaltic intrusions. Dispers- ed chromium-spinel-bearing olivine in the ca. 1595 AD pumice indicate recurrent episodes of basaltic intrusion and mixing with andesitic magma bodies. Degassing, fractional crystallization, and wall-rock contamination of mixed andesitic and basaltic liquid which remains in the magma chamber between eruptive episodes may have given rise to the andesitic to dacitic magmas which existed before this and each of the Holocene Plinian eruption sequences. Variations in strontium isotope ratios in the andesites reflect interaction with continental crust and with basaltic magmas during repeated episodes of intrusion.

The low-volatile contents of melt inclusions and matrix glasses record degassing of a magma body which partly fed the large and well-developed geothermal system beneath Ruiz (Giggenbach et al., 1990-companion volume). Degassing of the November 13, 1985 magmas alone (Williams et al., 1986) accounts for only a small part of the large amount of SO 2 that was emitted and is still being emitted. Instead, the SO 2 and other volatiles were

derived in part by discharge into the conduit of some of the extensive geothermal system which previously had been fed by a much larger volume of degassing magma (Giggenbach et al., 1990-companion volume), possibly by a still ac- tively degassing and much larger and deeper magma body (Williams et al., 1990-companion volume), by interaction with sedimentary gyp- sum, and by a sulfur-rich gas phase which moved upward from a possible basaltic "underplate".

The November 13, 1985 eruption tapped one of possibly many small residual magma bodies distributed in the central portion of conduits originally fed over a million years ago (Thouret et al., 1990-companion volume; Williams et al., 1990, et al., volume, Giggenbach et al., 1990- companion volume) during the cone-building phase of Nevado del Ruiz. Long-term cooling, degassing and possible replenishment by basic intrusions led to the creation of the large, "mature" geothermal system. Successive pockets of residual magmas, possibly repeated- ly intruded and partly regenerated by basaltic magma, produced eruptions that deposited a number of pumiceous tephra sets over the past 2000 years or so. The widely distributed microearthquakes beneath Ruiz before and after the 1985 eruption (Munoz et al., 1987) indicate the broad extent of the pressurized geothermal system which continues to undergo "readjustment" after disturbance by the November 13, 1985 eruption. Additional intrusion of basaltic magma at depth may also be contributing to the ongoing high level of seismicity and gas emissions.

Acknowledgements

We are grateful for the assistance of Gene Jarosewich and Jim Collins for electron microprobe usage, to Frank Walkup and Richard Johnson for thin section preparation, to Roger Hubell for drafting and editing, and to M. Linstrom, P. Castillo, L. Haskin, and R. Korotev for help with obtaining the INA data.

124 W.G, MELSON ET At,

The Smithsonian field study was made possible by a grant from the Smithsonian Research Op- portunities Fund. Acquisition of the INA data and support of J. Allan were funded by NSF grants OCE-8415270 and OCE-8508042 (R. Batiza), and ONR Grant 10-14-80-C-0856 (R. Batiza) and an NSERC Strategic Operating Grant 5-55847 (D. Chase). S.N. Williams acknowledges the NSF (INT-8608977, INT- 8714954, EAR-8601458 and EAR-8721206) and NGS. Clifford A. Hopson made many useful comments on the petrography of the pumices. We wish to thank Eduardo Parra of IN- GEOMINAS, Manizales, Colombia, and his many colleagues for their assistance during the field studies. The manuscript was improved by the reviews of James Luhr, Gary Byerly, Amitava Roy, Richard Stoiber, and John Stix.

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Water contents, temperatures and diversity of the magmas of the catastrophic eruption of Nevado del Ruiz, Colombia, November 13, 1985