Hyaloclastite and Lava Flows on Young Seamounts Examined with a Submersible

Hyaloclastite and lava flows on young seamounts examined with a submersible

PETER LONSDALE University of California, San Diego, Marine Physical Laboratory of Scripps Institution of Oceanography, La Jolla, California 92093 R. BATIZA Department of Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, Missouri 63130

ABSTRACT clastite (aquagene tuff) is produced. Flows at two of the volcanos closest to the axis of of this material are not very mobile on level the fast-spreading East Pacific Rise, Sea- Four small seamounts that rise 800 to terrain; in subglacial Pleistocene Iceland, mount Amph D1 at lat 7°50'S (Bonatti, 1,200 m above the flanks of the East Pacific hyaloclastite piled up as "moberg ridges" 1967) and East Seamount at lat 8°45'N Rise at the Pacific-Rivera plate boundary along eruptive fissures (Walker, 1965). (Lonsdale and Spiess, 1979), yielded both were examined and sampled during four Flows of hyaloclastite have not been re- tholeiitic lava and hyaloclastite, together dives of DSV-3 Turtle. They are all volcanic ported from deep submarine spreading cen- with rapidly accumulated iron and man- cones, at least two of them with summit ters, although off-axis drilling has revealed ganese oxides. craters. Samples from three of the volcanos thick beds of this rock type within the We present here geologic results of four are tholeiitic basalt that chemically resem- oceanic crust (for example, Schmincke and dives by the U.S. Navy submersible DSV-3 bles the basalt erupted at the rise crest, others, 1979). Most fissure eruptions at Turtle onto the summits of central volcanos although they are less fractionated. One of mid-ocean rises are type examples of the close to the axis of the East Pacific Rise (Fig. the cones within 15 km of the spreading "quiet effusion" mode of submarine vol- 1). The purpose was to observe the field re- axis has very young flows of sheet and pil- canism (Bonatti, 1967), forming flows of lationships and collect selected samples of low lava on its summit, including some en pillow and sheet lava whose immediate in- several types of volcanic flows and metal- echelon pillow walls. The other seamounts, teraction with bottom water is usually lim- liferous deposits. The only young, plate- farther from the plate boundary, appear ited to formation of glassy crusts. Lonsdale boundary seamounts that have previously extinci and generally have more weathered and Spiess (1980) did describe thin sheet been observed close-up are those whose lava with thicker ferromanganese crusts. flows on the crest of the East Pacific Rise summits have been built to sea level by They also have extensive flows of hyalo- that have shattered into 10-cm cubes along long-continued volcanism (such as Cobb ciastite that probably formed in deep-water regular vertical contraction cracks. How- Seamount — Schwartz and Lingbloom, phreatomagmatic eruptions. Typical ever, the intense physical and chemical in- 1973; Isla Tortuga — Batiza, 1978) or hyaloclastite landforms are stone streams of teraction of basaltic magma and seawater those that rise several hundred metres rocks that have moved down side slopes of that produces deep-sea hyaloclastite is most above the axes of aberrantly subaerial volcanos and been stabilized by precipita- characteristic of eruptions at central vol- spreading centers (Walker, 1965; Barberi tion of volcanogenic ferromanganese ce- canoes, providing many seamounts with at and Varet, 1971). The submersible Turtle ments. least a superficial cover of this rock (for has previously been used to examine appar- example, Bonatti, 1967; Barr, 1974; Nat- ently young seamounts on Cocos Ridge INTRODUCTION land, 1976). (Fornari and others, 1979), but these vol- Although fissure basalts create most of canos are in in an unusual tectonic setting A range of volcanic lithologic units and the newly accreted upper oceanic crust, on an aseismic ridge and consist of alkali landforms is created at accretionary plate large-scale volcanic cones that are the basalt. boundaries, despite the gross chemical uni- products of central volcanism are also im- formity of the tholeiitic magma generated portant components. On the East Pacific METHODS OF STUDY in this tectonic setting. Fissural volcanism at Rise, seamounts are absent from the axial subaerial spreading centers in Afar and Ice- zone of fissural volcanism (Menard, 1969), Turtle is a sister of the heavily modified land forms volcanic plains of both aa and but cones 0.75 to 1.5 km high are common DSRV Alvin (Winget and McCamis, 1967), pahoehoe lavas; although these lava types at the outer margins of the plate-boundary but less effort has been made to upgrade its differ greatly in appearance and viscosity zone of active tectonism. Most, perhaps all, scientific capabilities. During our diving (and hence build different landforms), of these young additions to the oceanic program it carried one observer, plus pilot transformations from pahoehoe to aa are crust are tholeiitic, although eruption of and copilot, to a maximum operating depth common in a single flow unit. Where axial alkali basalt becomes increasingly likely if of 2,000 m for 2 to 4 hr. Bottom photos eruptive fissures underlie shallow water or the volcanos continue to grow as they drift were taken through the observer's port with glacial ice, drastic chilling of basalt magma away from the plate boundary (Engel and a hand-held 35-mm reflex camera, and causes explosive granulation and hyalo- Engel, 1971; Batiza, 1977). Dredge stations samples of loose rock were collected with

Geological Society of America Bulletin, Part I, v. 91, p. 545-554, 7 figs., 2 tables, September 1980, Doc. no. 00907.

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hydraulic manipulators. A pressure gauge ing axis. A pre-dive echo-sounder survey vehicle's lights were regularly reflected from measured depth, and a surveillance sonar improved the bathymetric map and estab- mirror-like glass (as also occurs at the very located rock outcrops. The submersible's lished that at the crest of its 20° to 30° side youngest flows on some spreading centers; launch and recovery positions were deter- slopes is a flatter 8 km2 summit area with a Lonsdale, 1977a). A few extensive flows mined by the satellite navigator and 12-kHz minimum depth of 1,825 m. Turtle's dive were traversed, but over most of the summit echo sounder of the mother ship, but navi- 278 (Batiza, observer) began on the south- area rock crops out on slight elevations in gation of the submersible while on the bot- ern margin of the summit, at a depth of 20- to 30-m patches separated by calcare- tom was by rudimentary dead-reckoning. 1,880 m. The boat moved northwest, at ous ooze that blankets half of the seafloor. Criteria for selecting dive targets were first up a slope that was never more than 5° The sediment surface is smooth, not swept that the volcano summit must rise above and then across a hummocky summit plain, into current ripples as on many other sea- 2,000 m and should be as close as possible toward a depression that might be a crater; mount summits. to the axis of spreading. The spatial pattern however, before reaching this objective, Smooth and ropy sheet flows (Figs. 2A, of seamounts on this and other segments of after traveling about 500 m in 2.5 hr, the 2B) and flattened pillows (Fig. 2D) are the the East Pacific Rise suggests that formation dive was terminated because of deteriorat- commonest lava type along most of the dive of large cones on new crust is delayed 0.2 to ing weather at the sea surface. track. At mid-ocean spreading centers, both 0.3 x 106 yr until the lithosphere is strong Throughout the dive there were abun- types are indicative of high rates of lava dis- enough to support their weight, and then it dant lava exposures of varying freshness: charge and are most often seen near the proceeds rapidly as volcanos grow to some were covered with a thin ferroman- summits of axial eruptive vents (Ballard and heights limited by an isostatic equilibrium ganese crust, while from some outcrops the Moore, 1977; Lonsdale and Spiess, 1980). between the magma column in their con- duits and the rocks of the adjacent thicken- ing lithosphere (Vogt, 1974; Lonsdale and UO'W 109° W 1 08"W 107° W Spiess, 1979). The volcano nearest to the spreading axis that is tall enough (0.85 km) to be within Turtle's diving range is at lat 20°50'N, just 15 km west of the axis on the crust with an estimated age of 0.5 x 106 yr. 2 3°N The rise crest at this latitude is the medium-spreading boundary of the Pacific and Rivera plates and has been the locale for intensive study with deeply towed in- struments (Larson, 1971; Normark, 1976), rock dredging (Moore and others, 1977), and rock-collecting submersibles (Fran- cheteau and others, 1979; Spiess and others, 1980). We hoped to complement these studies of the results of fissure erup- 2 2° N tion, tensional faulting, and hydrothermal activity by examining the products of adjacent central volcanism which rise (just) above 2,000 m depth, but persistent bad weather allowed only a single dive in this region (dive 278). On three other dives (279, 280, 281) we studied seamounts on older crust farther north on the rise. 21 °N Samples of basaltic lava and hyaloclastite were examined petrographically, and by electron microprobe and instrumental neutron activation analysis. Metalliferous pre- cipitates were analyzed by X-ray diffraction and X-ray energy spectrometry.

OBSERVATIONS FROM THE SUBMERSIBLE 20° N

At Rise Crest, Lat 20°50'N (Dive 278)

Larson (1972) mapped a cluster of three seamounts within the central magnetic Figure 1. Location of seamounts near Pacific-Rivera spreading center, and of dive sites. anomaly near lat 20°50'N, and one of them Inset, from Larson (1972), shows near-axis seamount examined on dive 278 and dredge has a summit only 15 km from the spread- stations of Larson (1971) and Moore and others (1977).

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Many of the flattened pillows have a loose fragment; the glassy surface had come low joint blocks at their feet (Fig. 2F), and downslope elongation, and some broken to rest on sediment. they closely resemble the pillow walls de- ones are observably hollow. Turtle col- Bulbous pillows form a few tumulus-like scribed by Fornari and others (1978) from lected one 3-cm-thick fragment of the wall heaps rising 2 to 3 m above the seafloor the submerged east rift zone of Kilauea and of a hollow pillow, a tholeiite with a fresh (Fig. 2C), but the most spectacular land- by Lonsdale (1977b) from the axial rift glassy external surface and a ridged interior forms of this lava type were long narrow zone of the East Pacific Rise. Those pillow encrusted by 2 mm of ferromanganese pillow walls. A parallel en echelon series of walls are thought to be formed by low- oxide; this hand specimen (sample 278-1) is these ridges, each 5 to 10 m high and at volume eruptions from en echelon fissure almost identical in appearance to a rock il- least 20 to 30 m long, but only 2 to 3 m vents in volcanic rift zones; the walls exam- lustrated in Larson (1971) that was dredged wide at their crests, was discovered near the ined on the seamount at lat 20°50'N trend from 2,700 m on the adjacent rise flank. end of the dive. They have steep (50° to 60°) 320° toward the center of the summit plain, The encrusted surface, once the interior of a upper slopes of in situ bulbous pillows (Fig. and are interpreted as the trace of a radial hollow pillow, was the upper surface of the 2E), with a 30° talus ramp of spalled-off pil- rift zone. The glassy, corrugated surfaces of

Figure 2. Bottom photos from summit of seamount examined on dive 278; water depth 1,860 to 1,830 m. Width of near field is about 2 m. (A) Smooth sheet flows dusted with calcareous ooze. (B) Patch of ropy sheet lava. (C) Small heap of bulbous pillow lava. (D) Elongate flattened pillows, near site of sample 278-1. (E) Fresh pillow lava at crest of pillow wall. (F) Joint-block talus at foot of pillow wall.

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pillows on the walls appeared fresher than cally laminated ferromanganese oxides. The that on sample 2781-1 (Fig. 2C), and any seen earlier in the dive, but landing rock streams are short-lived hyaloclastite accretion of the laminated crust must have Turtle for sampling was prevented by the flows, stabilized by ferromanganese depo- preceded collapse of the tubular lava. extreme roughness of this zone, with paral- sition. The rest of the dive was across the sea- lel 10-m-high walls in places only 10 m At 1,645 m, near the crest of the side- mount's summit plateau, a rolling surface apart. slope, the first outcrops of in situ lava were with 100 m of relief between 1,640 and This seamount deserves further study encountered, at the head of a rock stream. 1,540 m. Large areas had as much as 80% with a better navigated submersible. Our They are elongate pillow basalt, with tubes rock cover; all the outcrops are of pillow single dive found basalt outcrops as fresh as that are oriented downslope and are obvi- lava, commonly elongate and with frequent any seen at "actively spreading" rise crests, ously hollow: many parts of the tube roofs collapse exposing hollow interiors. Most of and most of the lava was the type associated have collapsed (Fig. 3B). One such broken the pillow surfaces have been smoothed by with voluminous eruptions. The volcano is fragment of a lava tube was collected (Fig. thickly encrusting ferromanganese oxides probably still in its most active cone- 4; sample 281-2 of Table 1). It is a curved (as in Fig. 3C), but a few outcrops have building phase or has only recently become sheet of basalt 30 cm across and 23 mm well-defined corrugations and fault slivers, quiescent; we cannot determine whether the thick. The outer, convex surface, which was minor features of pillow surface that would small flows and pillow walls of youngest resting on sediment, has a thick, laminated be obscured by crusts more than a few mil- basalts with mirror-like glass mark local crust of ferromanganese oxide, identical to limetres thick. There are no fresh glassy sur- discharges between past and future major that on the hyaloclastite sampled nearby. faces like those seen on dive 278. Pillow eruptions or are the final products of wan- The concave surface, once the nonglassy walls were not encountered either, although ing volcanism. No hyaloclastite flows were inside of a lava tube, has just a patina of fer- the culminating points of the seamount are encountered, but the steep sides of the cone romanganese oxides, although in the de- steep pillowed cones with radii no greater were not examined. tached fragment this was the surface ex- than their heights (20 to 30 m); they consist posed to bottom water (Fig. 3B). The pat- of steeply dipping lava tubes radiating from Dives 281 and 279 tern of encrustation is exactly opposite to the core apex and closely resemble those

Two seamounts on opposite flanks of the East Pacific Rise were investigated on Turtle dives 281 and 279. Both volcanos are built on oceanic crust with an age of 2.0 x 106 yr, according to the magnetic anomaly in- terpretation of Larson (1972), and they seem to be representative of mature seamounts that have drifted out of the plate boundary zone. Dive 281 (Lonsdale, observer) was onto a large seamount that rises 1.6 km to a minimum depth of 1,540 m near lat 21°35'N, long 108°20'W. The dive began southwest of the summit, and the first hour of the 2.5 hr on the bottom was spent climbing the 20° to 30° sideslope from 1,740 m to 1,640 m. Unrippled calcareous ooze covers about 75% of the slope, but this blanket is inter- rupted by 20- to 40-m-wide strips of rock fragments that extend downslope (Fig. 3). These rock streams have well-defined margins elevated about 1 m above the adjacent sediment, but any superficial resemblance to subaerial stone streams of loose rock fragments that move slowly downslope by mass-wasting processes was dispelled by the first attempts to sample. The apparently detached rocks, averaging 10 to 20 cm in diameter, are in fact cemented together by thick ferromanganese crusts. Most sampling attempts were thwarted by crushing of the friable rock and crust in the submer- Figure 3. Sketch of summit area seamount examined on dive 281. Note hyaloclastite sible's manipulators, but eventually rock streams extending from flows of broken hollow pillows at top of sideslopes. (A) Hyalo- sample 281-1 was obtained from a depth of clastite flow (stone stream). (B) Broken hollow pillows. (C) Part of ferromanganese- 1,650 m. It is a piece of hyaloclastite en- encrusted pillow cone. (Locations shown in sketch.) Large fragment in center of photo B is closed in a 15- to 20-mm crust of concentri- sample 281-2 (Fig. 4).

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formed around circular vents (as opposed 60-m-deep depression, only 100 to 200 m from 1,906 m has a core of hyaloclastite to fissures) on the Mid-Atlantic Ridge (Bal- across, that the sonar indicated to be a enclosed in a laminated ferromanganese lard and Moore, 1977). near-circular volcanic crater. Its floor was crust to 10 to 35 mm thick. Dive 279 (Lonsdale, observer) examined just beyond the operational depth limit of and sampled a seamount with a 1,905-m Turtle; we spent 30 min examining the cra- Dive 280 summit at lat 22°10'N, long 109°01'W. ter wall. This is nearly vertical for the upper Turtle landed at 2,000 m and proceeded 20 to 40 m, truncating a series of pillow About 25 km southwest of the Tamayo upslope at 300°, a course maintained, ex- flows and 1-m-thick sheet flows (Fig. 5A), transform fault zone is a pair of seamounts cept for local maneuvering, throughout the and is buried with steep lava talus near its aligned and elongate parallel to the trans- dive. The short section of 20° to 30° outer foot (Fig. 5B). The crater is asymmetric, and form. A dredge haul on the southeast slope climbed to a depth of 1,970 m is simi- eccentrically located: the northwest wall yielded tholeiitic basalt and hyaloclastite lar to that described from dive 281, with rises to 1,930 m, and beyond its crest we (Lopez and others, 1978). The northwest- streams of friable, ferromanganese-en- climbed up an irregular rocky slope to the ern peak has a 5-km-long summit depres- crusted rocks that are probably hyalo- volacno's flat summit (Fig. 5 A). The summit sion, as much as 1 km across and 200 m clastite. Between 1,970 and 1,960 m is a region has piles of ferromanganese- deep, that we identified as an elongate cra- short (100 m), flatter summit area, with encrusted and collapsed pillows, but much ter (Fig. 6). Dive 280 (Batiza, observer) ex- some outcrops of old pillow basalt; the of the surface is strewn with fist-sized cob- amined and sampled this peak, whose submersible then dove into a steep-walled bles. One typical cobble (279-1) sampled summit (1,565 m at lat 23°02'N, long 108°42.5'W) is about 70 km from the OUTER SURFACE OF Pacific-Rivera spreading axis, on crust with an estimated age of 2.4 X 10" yr. Fe-M" C\UST 5 CENT, MET E RS Turtle landed at 1,645 m on a 10° incline near the top of the southwest side of the volcano. The seafloor is strewn with ferromanganese-encrusted cobbles 10 to 20 cm in diameter. Although dusted with sediment, they form a fairly complete cover (Fig. 5C), rather than being restricted to ' LAMINATED rock streams like those seen on dive 281. I Fe-Mn CRUST Most cobbles are cemented together by their crusts, and there are also extensive fer- FRACTURED GLASS SHELL WITH VEINS romanganese pavements (Fig. 5D). As the ' OF ZEOLITES submersible traveled northeast to the crest of the slope at 1,615 m, it passed only two VARI0LITIC LAVA small outcrops that could be identified as broken lava tubes and pillows, despite the NNER SURFACE OF LAVA TUBE WITH NO GLASS LAYER AND crust that obscures their original shapes. 0.1 mm Fe-Mn PATINA Beyond the southwestern summit Turtle fol- lowed the seafloor down 1-m steps that may Figure 4. Broken surface of part of rock sample 281-2, once wall of hollow pillow. be encrusted fault scarps, and then it came

TABLE 1. MAJOR-ELEMENT ANALYSES FOR BASALTIC ROCKS AND GLASSES

Oxide This study1^ East Pacific Rise crest* 278-1 279-la 279-lb 281-la 281-lb 281-lc 281.2 lat 20°56,N L6-B L10-A2 Lll-A

SiO2 49.48 49.72 49.10 50.23 49.54 49.41 50.30 50.30 50.34 50.73 Ti02 1.34 0.97 0.91 1.20 1.05 1.11 1.12 1.40 2.34 1.47 AI2O. 16.28 15.69 16.17 15.68 14.93 14.89 15.49 16.23 14.16 15.15 FeO" 8.60 8.75 8.71 8.89 9.30 9.36 9.14 9.15 12.08 10.11 (8.58) (9.00) (9.27) MgO 8.48 9.41 9.55 7.67 8.79 8.69 8.01 8.05 6.55 7.73 MnO 0.23 0.14 0.16 0.19 0.18 0.16 0.28 CaO 11.90 12.88 12.52 13.18 13.20 13.15 13.19 11.91 10.68 11.83 NazO 2.77 2.36 2.35 2.49 2.48 2.40 2.30 2.75 2.89 2.68 (2.87) (2.48) (2.24) K2O 0.07 0.05 0.07 0.13 0.09 0.09 0.03 0.22 0.14 0.12 P205 0.05 0.02 0.02 0.16 0.22 0.11 Total 99.20 99.97 99.54 99.68 99.58 99.25 99.88 100.17 99.44 99.93 Mg:Mg+Fe 0.637 0.657 0.661 0.606 0.634 0.623 0.610 0.611 0.491 0.577 Note: See text for sample descriptions. Values are percent by weight. Microprobe analyses provided by D. Fornari, with some additional instrumental neutron activation analysis determinations (in parentheses). * Analyses are from Moore and others (1977).

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to the brink of a 150-m scarp trending cobbles, and a dusting of sediments. A few wall is a 30° to 40° rubble slope, similar in 120°: the wall of the summit crater. The ledges with 60° faces are developed on in appearance to the gentler outer slopes of the crater wall is not a sheer cliff. Even the top situ lava (Fig. 5E); their thick crust hampers volcano. has an average inclination of only 45°, a identification of rock type, but some obvi- To conserve battery power, Turtle partial cover of ferromanganese-encrusted ous broken pillows are visible. The lower crossed to the northeast wall at a depth of

Figure 5. Bottom photographs from dives 279 and 280; each shows area about 2 m across in near field. (A) Dive 279, 1,940 m; basaltic pillow and sheet flows exposed at nearly vertical crater wall. (B) Dive 279, 2,000 m; ramp of talus at foot of crater wall. (C) Dive 280, 1,645 m; ferromanganese-encrusted cobbles, probably of hyaloclastite, near top of sideslope. (D) Dive 280, 1,632 m; ferromanganese-encrusted ledges and pavements. (E) Dive 280, 1,680 m; shattered, thinly bedded lava exposed on 60° cliff of crater wall. (F) Dive 280, 1,565 m; looking downward onto flat, overhanging ledge of ferromanganese-encrusted hyaloclastite at northeast crest of summit wall (see inset of Fig. 6).

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1,700 m, out of visual range of the crater ment otherwise detached flakes of basaltic erals, and alteration products (yellow-green floor (Fig. 6). The northeast wall was simi- glass and sheath the external surface, be- smectite and calcite, totaling about 3% of lar to the southwest in being littered with neath the ferromanganese crust. the rock). Vesicles make up 5% to 7% of ferromanganese-encrusted rubble; no The other rocks collected are man- the clast's volume, and are irregular in size definite lava outcrops were seen in the ganiferous hyaloclastites with varying and shape. Many of the glass fragments in 130-m climb to its crest at 1,565 m. At the amounts of lithic fragments. Fresh basaltic this and other samples have the high abun- rim of this wall is a 0.5-m-thick ledge of glass occurs in angular flakes, chips, and dance of tiny vesicles that is characteristic ferromanganese-encrusted rock, which is rough cubes, generally less than 5 mm in of the outermost layer of pillow lava glass, receding by collapse along joints of under- diameter, together with small fragments of but very few of the flakes are curving cut sections (Fig. 5F). The summit of the olivine, plagioclase and opaque minerals. "bubble-wall" shards. seamount, beyond the crater, is a cobble- They are cemented in an open clayey matrix From the morphology of the glass frag- strewn ferromanganese pavement de- of fine altered glass, zeolites, and man- ments and their presence below the esti- veloped on this stratum, which dips away ganese oxides, while angular pieces of lith- mated depth limit for magmatic explosions from the crater at about 10°. It was fol- ified hyaloclastite are themselves cemented (McBirney, 1963), we infer that these lowed to the northeast to a depth of 1,610 by ferromanganese oxide. Sample 280-1 is hyaloclastites were not formed by violent m, where a loose, rounded fragment (280-1) more altered than 279-1, with incipient exsolution of gases from the magma, which was collected; it is a ferromanganese cobble palagonitization of the glass. Sample 281-1 Tazieff (1972) suggested as a general ge- 12 cm in diameter, with a core of hyalo- is virtually a broken pillow breccia (Car- netic mechanism. They are lithologically clastite that is just 2 cm across. lisle, 1963), with several angular pieces of similar to other hyaloclastites thought to be We infer that most of the rock near the lava, 1 to 2 cm in diameter, that are derived from even greater depths, where summit of this seamount is hyaloclastite, sheathed and veined with zeolites. The seawater is above its critical pressure and occurring either as ledge-forming beds that largest fragment (281-la) is holocrystalline the only plausible mechanisms for rock were converted to pavements by ferroman- and has sparse olivine and plagioclase fragmentation are granulation because of ganese plating, or as fragments that became phenocrysts in a fine-grained groundmass the thermal shock when hot rock is intro- the cores of the ubiquitous cobbles. of clinopyroxene, plagioclase, opaque min- duced into cold seawater, and spalling of

IGNEOUS ROCK SAMPLES 108*30 W

Petrography

We recovered two specimens of pillow lava. The hollow, flattened pillow sample 278-1 has very sparse anhedral phenocrysts of olivine (0.2 mm) and plagioclase (0.2 to 0.3 mm) in a glassy to variolitic groundmass. The olivine crystals show very slight oxidation in cracks. The plagioclase

phenocrysts (An70) occur in clots of eight to ten, each with thin rims of quench growth, and as individual equant crystals. Vesicles in the variolitic zone of the pillow are irregular and as much as 1.0 mm long. The pillow has a glassy outer shell, 3.0 mm thick. Most of the glass has sparse, spheri- cal vesicles less than 0.1 mm in diameter. Its external surface is roughened to a fine-scale elephant-hide texture by abundant coalesc- ing vesicles and by intersecting arcuate contraction cracks 0.2 mm wide and as much as 1.0 mm deep. The other lava-flow sample, 281-2 from a broken, hollow pillow (Fig. 4) is petrographically similar, with a moderately altered variolitic zone in which smectite and zeolites partly fill vesicles and HALF-RATE replace microlites and glass. Its glassy shell 29 KM / 10* YRS is thicker (8 mm), and deeply shattered by contraction cracks as much as 0.5 mm 108* 30'* 108*00' W wide. These intersecting cracks penetrate as Figure 6. Bathymetry around Tamayo transform fault (after Kastens and others, 1979), deep as 5 mm and crosscut horizontal frac- with location of dive 280 seamount and dredge haul DH-12 of Lopez and others (1978). tures. Both sets of fractures are infilled with Insets show detailed bathymetry of seamount and sketched section of its summit, each with yellow and orange zeolite veins, which ce- track of dive 280.

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pillow rinds (Bonatti, 1967; Schmincke and samples from rock streams (281-1) and as the principal manganese mineral. Like all others, 1979). broken ledges (280-1), which are about the oxide samples we collected and 90% ferromanganese oxide; the small clots analyzed, they have very low concen- Chemistry of glass and lithic fragments were discov- trations of other transition metals (Co + Ni ered only after sectioning and more careful + Cu + Zn + Pb <0.7%). Fused rock powders from "whole-rock" study. Manganese oxide is intimately mixed samples of pillow lava (278-1; 281-2) and a with the other hyaloclastite constituents DISCUSSION basalt clast in hyaloclastite (281-la) were and also occurs as porous masses of to- analyzed for major elements (Table 1) by dorokite with moderate crystallinity. Fine- Tholeiitic Central Volcanism the electron microprobe technique of For- grained iron oxide precipitated with the to- nari and others (1979). Minor-element dorokite, resulting in Mn:Fe ratios of 2:1 to Our observations and analyses are con- chemistry of these samples (Table 2) was 4:1 for oxides in the interiors of 279-1, sonant with the model in which central vol- determined by instrumental neutron acti- 280-1, and 281-1. canism in plate boundary zones is fed from vation analysis. Table 1 also lists electron The hyaloclastite interiors and the con- the same mantle source as fissure volcanism probe analyses of natural glass flakes vex glassy surface of the lava tube 281-2 at nearby spreading axes, causing rapid (279-la, 279-2b, 281-lb, 281-lc), together (Fig. 4) are covered by 15 to 30 mm of fer- growth of tholeiitic volcanos. Engel and with published data on tholeiites from the romanganese crust that is surprisingly simi- Engel (1971) pointed out the error of early nearby rise crest. The small, altered glass lar in appearance, considering the wide sep- ideas that the igneous petrology of the Pa- flakes of 280-1 proved unsuitable for mi- aration of the sampling sites. It is banded, cific Basin floor could be interpreted by ex- croprobe analysis. Figure 7 shows chon- with alternating chocolate and yellowish amination of its subaerially exposed volan- drite-normalized abundance patterns of laminae, 1 to 2 mm thick; the dark layers, cos, which are mostly alkalic. It increasingly

rare earth elements (REE). which are mainly 8Mn02 and iron oxides seems that oceanic islands may even be mis- All analyzed samples are tholeiitic (Mn:Fe =1), predominate toward the outer leading guides to the chemical composition

basalts, with low Ti02, K20, Hf, and Sr, surface. The light yellowish layers are mix- of the smaller but much more abundant (La/Sm^ of 0.24 to 0.50, and depletion in tures of iron and manganese oxides with submerged volancos. light REE relative to heavy REE. In their fine-grained palagonite, broken plagioclase The tholeiitic magmas of fast-spreading major trace element chemistry, as in their laths, and smectite clay minerals. No rises seem to undergo fractionation in shal- mineralogy, the rocks are indistinguishable biogenic debris was found, despite careful low crustal chambers before eruption at from typical mid-ocean ridge basalt from a searches with a scanning electron micro- axial fissures (Batiza and others, 1977), and depleted mantle source. scope. erupted basalts characteristically have a The thin black crust on sample 278-1 and lower Mg:(Mg + Fe) ratio than lavas from METAL OXIDE DEPOSITS the even thinner patina on the concave sur- slow-spreading ridges (Natland, 1978). face of 281-1 are ferromanganese precipi- Lonsdale and Spiess (1979) noted that sev-

Ferromanganese deposits seen during tates with Mn:Fe of 0.5 to 1.10 and8Mn02 eral tholeiitic seamounts on fast-spreading dives 279, 280, and 281 are so thick that the surface textures of volcanic rocks are generally obscured, and we frequently thought we were traversing bedded and = 20 fractured manganese pavements of the sort o described from San Pablo Seamount (Au- O mento and others, 1968). This impression TL 28 ~ ~ ^-'--l- was supported by cursory study of the O\

LÜ 10- TABLE 2. TRACE-ELEMENT DETERMINATIONS BY INSTRUMENTAL cr NEUTRON ACTIVATION ANALYSIS CO

278-1 281-la 281-2 <1 IN MD- la Hf 2.14 1.72 1.54 5- CO INMD- lb Sc 36.7 40.6 40.5 c Cr 310 379 380 CO SD-8-3 Co 55.7 56.1 94.2 Zn 99 n.d. n.d. _L Sr 91 n.d. n.d. _L La Ce Sm Eu Tb Yb Lu (La/Yb)N 0.66 0.26 0.40 (La/Sir\)N 0.50 0.24 0.39 RARE EARTH ELEMENT (Sm/YbiN 1.32 1.07 1.03 Figure 7. Chondrite-normalized (Haskin arid others, 1968) rare-earth element abun- Note: Technique of Jacobs and others (1977). dances of seamount basalt samples 278-1, 281-1, and 281-2, compared with seamount Values are parts per million; n.d. = not detected. samples INMD-la and INMD-lb from East Seamount (Lonsdale and Spiess, 1979). Also (La/Sm)N is chondrite-normalized (Haskin and plotted are data from sample SD8-3 (Batiza and others, 1977; Johnson, 1979), which is others, 1968) La/Sm ratio. relatively primitive (unfractionated) mid-ocean ridge basalt from Siqueiros transform fault.

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sections of the East Pacific Rise have rocks shallow-water hyaloclastite (Tazieff, 1972). out of magma and hot rock during phrea- less fractionated than those on the adjacent Some of the fragmentation that has created tomagmatic eruptions, and rapidly repre- rise crest, perhaps indicating that their rock streams on steep sideslopes below cipitated. This is shown by their intimate magma bypassed any large, shallow storage summit pillow flows (Fig. 3) no doubt mixture with glass and lava fragments. system. Our new data (Table 1) show a began by mechanical collapse and rockfall However, the metals are not as fractionated similar pattern. The glasses of sample 279-1 from unstable flow fronts. However, this as in some other deposits thought to form in could be unmodified partial melts of the process cannot be responsible for the fine this manner (Bonatti and others, 1972). upper mantle, and the other seamount comminution of the glass and lava incorpo- The laminated ferromanganese and samples are only slightly fractionated, from rated in the hyaloclastite, nor for the sum- palagonite crusts also formed rapidly; this the evidence of their high magnesium-.iron mit caps of this rock seen on dives 279 and is shown by their great thickness on young ratio, absence of negative Eu anomalies 280. Despite some evidence of lateral tran- rocks and, more dramatically, by their

(Fig. 7), and high Cr but low Ti02 abun- sition within a flow unit, which can be ex- much greater thickness than the hydrogen- dances. By contrast, most tholeiites that plained by progressive autobrecciation and ous crusts on longer-exposed surfaces of the have been dredged from the Pacific-Rivera granulation as the flow moves away from same rock (as in sample 281-2, Fig. 4). The spreading center (Moore and others, 1977; its source, the field relations make it clear interlayering of obviously volcanic material Lopez and others, 1978) are moderately to that some eruptions tended to form hyalo- (palagonite and plagioclase crystals) indi- highly fractionated. The seamount rocks clastite, while others on similar terrain cates that the iron and manganese oxides chemically resemble unfractionated tho- tended to form lava. are also volcanogenic. The crusts probably leiites (for example, SD8-3, Fig. 7) from the We favor Bonatti's (1967) suggestion formed by alternating precipitation and Siqueiros fracture zone, a site on a fast- that the dominant control on style of erup- deposition of dissolved and finely particu- spreading rise where rising magma may tion is the viscosity of lava at its vent, which late products of a series of nearby phrea- have avoided the magma chamber thought for magma of constant chemical composi- tomagmatic eruptions, whose readily dis- to underlie the spreading axis (Batiza and tion is determined mainly by the tempera- persed products coat both hyaloclastite others, 1977; Johnson, 1979). ture and thus by the rate of discharge. fragments and adjacent pillow flows. Magma of higher viscosity, conducive to Style of Eruption hyaloclastite formation, may be charac- CONCLUSIONS teristic of waning stages of growth of The rocks we recovered can be arranged tholeiitic volcanos, after the rapid effusion One seamount we examined that is very in a sequence showing increasing physical of conebuilding sheet and pillow lavas: then close to the Pacific-Rivera spreading axis interaction with seawater, from the cracked the vents are more remote from the magma seems to be in a stage of rapid growth by surface of pillow lava 278-1 to the shattered source. It may be significant that the most build-up of lava flows. The other volcanos, glass of pillow lava 281-2, the fragmented complete hyaloclastite cappings we have farther from the plate boundary, showed rock of the pillow breccia 281-1, and the studied, on dives 279 and 280 and with less evidence of recent eruption, although aquagene tuffs 279-1 and 280-1. The pro- deep tow (Lonsdale and Spiess, 1979) are the ferromanganese crusts were very thin on gression of increased chemical interaction, on tholeiitic cones that have summit craters some pillow lava from 50 km off-axis (dive as recorded by the abundance of vol- and calderas. These depressions have likely 281). All the lavas and glass fragments we canogenic zeolites, clays, and manganese been formed late in the eruptive history of sampled from these central volcanos are oxide, is the same. The overwhelming the volcanos also, by summit collapse into tholeiitic basalt that is less fractionated than dominance of hyaloclastite observed on shrunken magma chambers. As the vents most rise-crest fissure basalt. The lava flows some seamounts (dive 280) and its absence grow farther from the magma source, de- on seamount summits resemble those de- among pillow fields on others (dive 278) creasing the discharge temperature, they scribed from active spreading centers, but suggest that spalling from pillows (Rittman, grow into shallower water, where vapori- the older volcanos also have extensive flows 1962) was not the most important zation of sea water is possible, and both fac- of hyaloclastite, which forms distinctive mechanism for producing the lithic and tors increase shattering of the flowing rock. ferromanganese-cemented rock streams glass fragments, but that varying degrees of down the sides of the volcanic cones. The thermal shock granulation were responsi- Ferromanganese Deposits tholetiitic hyaloclastite is thought to be ble. All of the seamount slopes within the made by explosive phreatomagmatic erup- depth capability of the submersible are cov- tions at late stages in the evolution of small The thin, dense SMn02 coatings of 278-1 ered by water at less than its critical pres- and one surface of 281-2 are typical hydro- seamounts, when their eruptive vents are sure, so fracturing of the flows we examined genous ferromanganese crusts, which have still far below sea level but are growing in- could have been enhanced by vaporization accreted slowly by precipitation from sea- creasingly remote from their magma cham- of seawater, although water expansion on water. Their accretion rates probably ex- bers. Extensive metal oxide deposits and boiling near 2,000 m depth may be only ten ceed 1 cm/106 yr, significantly faster than summit craters are probably other products to twenty times. We envisage a potentially growth rates of manganese nodules in the of this explosive activity. explosive chain reaction in a type of deep ocean basins because of the enhanced Research submersibles that allow visual phreatomagmatic eruption where thermal regional supply of volcanogenic Mn and Fe observation and directed sampling are val- shattering increases the surface of hot rock near the active seamount-studded spreading uable tools for geologic study of young available for the steam production that center (Aumento, 1969). seamounts, common features that have causes further fracturing, just as steam ex- The Mn and Fe occurring as oxides in the previously been examined mainly with plosions may abet comminution by expand- matrix of hyaloclastites must have a more low-resolution acoustic sensing and crude ing magmatic gases in the formation of direct volcanogenic origin, being leached rock-dredging techniques. Their summit

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areas are varied enough that they deserve New York, John Wiley 8c Sons, Inc., ports of the Deep Sea Drilling Project, Vol- thorough, acoustically navigated surveys, p. 453-491. ume 54: Washington, D.C., U.S. Govern- Bonatti, E., Kraemer, T., and Rydell, H., 1972, ment Printing Office (in press). not just the reconnaissance tracks we were Classification and genesis of submarine Lopez, M., and others, 1978, Geochemistry and able to make. iron-manganese deposits, in Horn, D. R., petrology of some volcanic rocks dredged ed., Ferromanganese deposits on the ocean from the Gulf of California: Geochemical AC KNOWLEDGMENTS floor: Washington, D.C., National Science Journal, v. 12, p. 127-132. Foundation, p. 149-166. McBirney, A. R., 1963, Factors governing the na- Carlisle, D., 1963, Pillow breccias and their ture of submarine volcanism: Bulletin Vol- For their dedication to the success of the aquagene tuffs, Quadra Island, British Co- canologique, v. 26, p. 455-469. submersible field program we thank the lumbia: Journal of Geology, v. 71, Menard, H. W., 1969, Growth of drifting vol- officers and crews of DSV-3 Turtle and its p. 48-71. canoes: Journal of Geophysical Research, mother ship Maxine D, and many other Engel, A.E.J., and Engel, C. G., 1971, Mafic and v. 74, p. 4827-4837. ultramafic rocks, in Maxwell, A. E., ed., members of U.S. Navy Submarine De- Moore, J. G., and others, 1977, Petrology of The sea, Volume 4: New York, Wiley In- basalt from the East Pacific Rise near 21° velopment Group One. We are grateful to terscience, 791 p. North latitude: U.S. Geological Survey Larry Haskin and D. and M. Lindstrom for Fornari, D. J., Malahoff, A., and Heezen, B. C., Journal of Research, v. 5, p. 753-759. help with trace-element determinations, to 1978, Volcanic structure of the crest of Natland, J. H., 1976, Petrology of volcanic rocks Dan Fornari for electron microprobe Puna Ridge, Hawaii: Geophysical implica- dredged from seamounts in the Line Islands, tions of submarine volcanic terrain: Geolog- in Schlanger, S. O., Jackson, E. D., and analyses, and to Mary Fisk for analyses of ical Society of America Bulletin, v. 89, others, Initial reports of the Deep Sea Drill- ferromanganese crusts. Principal financial p. 605-616. ing Project, Volume 33: Washington, D.C., support was from office of Naval Research 1979, Visual observations of the volcanic U.S. Government Printing Office, p. 749- Contract N00014-75-C-0704, with help for micromorphology of Tortuga Lorraine and 777. sample analyses from National Science Tutu Seamounts; and petrology and 1978, Comparison of chemical and magne- chemistry of ridge and seamount features in tic stratigraphy of basement rocks at DSDP Foundation Grant OCE-7727001 and the and around the Panama Basin: Marine Sites 332 and 395, in Melson, W. G., McDonnell Center for Space Sciences, Geology, v. 31, p. 1-30. Rabinowitz, P. D., and others, Initial re- Washington University. The manuscript Francheteau, J., and others, 1979, Massive ports of the Deep Sea Drilling Project, Vol- was reviewed by James Natland and Ken deep-sea sulphide ore deposits discovered ume 45: Washington, D.C., U.S. Govern- on the East Pacific Rise: Nature: v. 277, Macdonald. ment Printing Office, p. 657-677. p. 523-528. Normark, W. R., 1976, Delineation of the main Haskin, L. A., and others, 1968, Relative and ab- extrusion zone of the East Pacific Rise at solute abundances of the rare earths, in 21°N: Geology, v. 4, p. 681-685. REFERENCES CITED Ahrens, L. H., ed., Origin and distribution Rittman, A., 1962, Volcanoes, and their activity: of the elements: New York, Pergamon New York, John Wiley & Sons, Inc. 305 p. Aumento, F., 1969, The Mid-Atlantic Ridge near Press, p. 889-912. Schmincke, H. U., and others, 1979, Basaltic 45°N; 5. Fission track and ferromanganese Jacobs, J. W., and others, 1977, A well-tested hyaloclastites from Hole 396B, DSDP Leg chronology: Canadian Journal of Earth Sci- procedure for instrumental neutron activa- 46, in Dmitriev, L., Heirtzler, J., and others, ences, v. 6, p. 1431-1440. tion analysis of silicate rocks and minerals: Initial reports of the Deep Sea Drilling Proj- Aumento, F., Lawrence, D. E., and Plant, A. S., Journal of Radioanalytical Chemistry, ect, Volume 46: Washington, D.C., U.S. 1968, The ferromanganese pavement on v. 40, p. 93-114. Government Printing Office, p. 341—356. San Pablo Seamount: Geological Survey of Johnson, J. R., 1979, Transitional basalts and Schwartz, M. L., and Lingbloom, K. L., 1973, Canada Paper 68-32, 30 p. tholeiites from the East Pacific Rise, 9°N: Research submersible reconnaissance of Ballard, R. D., and Moore, J. G., 1977, Photo- Journal of Geophysical Research, v. 84, Cobb Seamount: Geology, v. 1, p. 31-32. graphic atlas of the Mid-Atlantic Ridge rift p. 1635-1651. Spiess, F. 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N., 1979, A pair of MANUSCRIPT RECEIVED BY THE SOCIETY SEP- Pacific Rise and the Siqueiros Transform young cratered volcanoes on the East Pacific TEMBER 1, 1979 Fault: Journal of Geophysical Research, Rise: Journal of Geology, v. 87, p. 157— REVISED MANUSCRIPT RECEIVED FEBRUARY 6, v. 82, p. 265-276. 173. 1980 Bonatti, E., 1967, Mechanisms of deep-sea vol- 1980, Deep tow observations at the crest of MANUSCRIPT ACCEPTED FEBRUARY 13, 1980 canism in the South Pacific, in Abelson, P., the East Pacific Rise, 8°45'N, in Rosendahl, CONTRIBUTION OF SCRIPPS INSTITUTION OF ed., Researches in geochemistry, Volume 2: B. R., Hekinian, R., and others, Initial re- OCEANOGRAPHY, NEW SERIES

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