ROBERT L. FISHER Scripps Institution of Oceanography, Vniversity of California San Diego, La folia, California CELESTE G. ENGEL L'. S. Geological Survey, La Jolla, California

Ultramafic and Basaltic Rocks Dredged

from the Nearshore Flank of the Tonga Trench

Abstract: Deep dredging in the Tonga Trench (Southwest Pacific Ocean) at a depth of 9150 to 9400 m yielded fresh to granulated and serpentinized peridotite and dumte. Other rocks re- covered there and at three stations deeper than 7000 m include basalts, tuffs, and tuffaceous agglomerates. Chemical analyses of the fresli peridotite, with combined H2O < 0.10 weight percent, indicate that the rock consists of Si, Mg, Fe (6 percent), and Cr + Ni about 0.7 percent. Mineralogically, the peridotite contains forsteritic olivine and enstatite with minor spinels. The ultramafic mass exposed at 9400 m probably is an accumulate exposed by faulting.

Introduction analyses came from NSF Grant GA 800, NASA NsG-321 and NASA-NAS-9-7894. Thanks are The Tonga Trench lies immediately east of due to Harry H. Hess, Wayne E. Hall, and the Tonga Islands in the Southwest Pacific and K. J. Murata for critical reading of the manu- forms the northern portion of the Tonga- script. Kermedec Trench. The trench extends from Samoa southwest to New Zealand (Fig. 1) and Discussion separates the Melanesian island arcs from the Pacific Basin proper. Earlier work (Raitt and During Expedition NOVA (R/V Argo in others, 1955) by the Scripps Institution of 1967) of the Scripps Institution, rock samples Oceanography in 1952-1953 delineated the were obtained from the nearshore flank of the boundaries of the Tonga Trench; that extensive trench near the trench axis (Fig. 1). Figure 2 is bathymetric, seismic-refraction, and magnetic a composite cross section through the area program served as a base for the present study sampled and shows the relative positions and involving bathymetric, magnetic, and seismic- approximate depths of rocks sampled in detail. reflection measurements (by airgun), bottom The dashed line (Figs. 1, 2) is plotted from photography, and rock sampling. Only the earlier Scripps Institution sounding data; the rock-sampling results are reported here. solid line (Fig. 2) is a 1967 track, and the corresponding portion of the section is traced Acknowledgments from a Precision Depth Recorder profile. The Officers and crew of R/V Argo carried out upper cross section of Figure 2, with no vertical arduous and meticulous navigational and ma- exaggeration, shows the gentle over-all but neuvering operations in the location and re- locally extremely steep slopes of the trench covery of rock samples from the very deep flanks. Hard rocks were obtained in three trench walls. A. L. Sutton, U.S. Geological dredge and one camera lowering; the latter, Survey, Denver, Colorado, provided the 78-C, overlaps in part the depths sampled in quantitative spectrographic analysis presented Dredge 88-D. Aside from coarse-grained in Table 1. Support for field work came from ultramafics, Argo s dredge hauls yielded alkali contracts with the Office of Naval Research feldspar-rich volcanics, tuffs, tuffaceous-basaltic and from National Science Foundation Grant agglomerates (conglomerates), and zeolitized GA 776. Additional support for laboratory basalts.

Geological Society <>1 America Bulletin, v. 80, p. 1373-1378, 2 figs., July 1969 1373

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I72°W 18" S +

/ CAPRICORN SEAMOUNT

19° S +

/^'GENERAL LIMIT / OF REGION 20 s + EASTERN PORTION / ^SAMPLED ° / TONGA ISLANDS SOUTHWEST PACIFIC

21'S +

I72°W Figure 1. Regional map showing (1) location of the Tonga Trench and (2) the region surveyed and sampled, with respect to the trench axis.

The deepest successful dredge haul was 88-D, '---'Fogs) and is remarkably fresh. The olivine at 20°25'S., 173°16'W.1 and 9150 to 9400 m grains exhibit strong strain shadows in polarized deep. There dredging recovered about 20 kg light. The orthopyroxene (enstatite) is color- of ultramafic rocks consisting primarily of less and many large grains contain tiny exsolu- peridotite and some dunite. The largest tion lamellae of a clinopyroxene. Much of the coherent fresh sample is a layered and lineated opaque material probably is chromite, as is peridotite rock approximately 12 X 16 X 20 cm. indicated by the high chromium content of the The rock is coarsely crystalline, light gray rock (Table 1). Serpentine occurs along thin green in color, and has a very thin weathered or layers or cracks and as rounded replacement discolored surface. Most of the petrographic areas. Serpentinous alteration is rare in the and chemical work to date has been con- sample that we have studied in detail. centrated on thin sections and rock powders Other pieces of peridotite in 88-D also are from this large piece. Modal analyses of 1000 fresh, but a few samples contain abundant counts were made on each of four thin sections lenses of serpentine which cut across grains of from various parts of the sample. The averages olivine and enstatite. One large piece of for this rock (in volume percent) are: olivine, peridotite is a dark gray green, brecciated rock, 86; orthopyroxene, 13; opaque minerals and with faceted and slickensided surfaces, and it serpentinized alteration material, <1 percent. contains numerous crosscutting veinlets of The olivine is forsterite (Fe, 4.4 weight percent, pale-green serpentine. Another sample of ultramafic rock contains abundant fine-grained 1 Positions given in the text are nominal points taken olivine interstitial to coarse-grained olivine and near the middle of the dredge sampling. pyroxene. This textural feature may be the

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result of granulation in a fault zone or perhaps containing abundant reagent salts and a very of late cooling of pore-space liquid. Microprobe large concentration of magnesia (MgO, 46.35 analyses of individual grains of olivine are in weight percent, Table 1). progress. Hess (1964, p. 171) has suggested that the The chemical composition of the peridotite ratio Al2Oa/CaO is of extreme petrogenic is listed in Table 1. The values for the various interest in the origin of ultramafic rocks. He oxides listed under wet chemical were obtained concludes from the experimental and chemical by gravimetric, photometric, and flame-photo- data that the amounts of alumina in the metric techniques. There are limitations in the pyroxenes of peridotites may increase with chemical analysis of a rock of this skewed depth and that an increase in the ratio Al2Os/ composition. Inspection of Table 1 indicates CaO is indicative of a mantle source. Hess that the peridotite consists essentially of silica, states that a ratio of approximately 1 is magnesia, and iron. The extremely low con- characteristic of peridotites derived from the centrations of Al2Os and CaO are difficult to upper mantle. The analytical difficulties determine and may well be in error. The Al2Oa inherent in obtaining accurate alumina and value was obtained by subtracting the amount calcium concentrations in a peridotite by the of ferric iron from the R2Os group (some Si, methods described above are so great that we Fe, Ti, P, and in this case Ni, and probably conclude that this ratio remains essentially Cr). Further, the weight percent of alumina is unknown. dependent upon weighing Fe in the oxidized Na2O and K2O were determined on a flame state. CaO was determined from a solution photometer. The value for potassium oxide

NA TOR A L SCALE o—, 7-.__^ / Lekeleka I. x / (tulunga Group) 1000—--

.20- W 20-. 2000—\

K///V 3000 —\

VERT. EX AGO. a SO*

25 SO KILOMETERS 6000

DEPTH RANGE - DREDGE ON BOTTOM

Figure 2. Composite cross section of the Tonga Trench. See 2 on Figure 1 for position of the section. The dashed profile is from a 1952 sounding run; the solid portion is traced from a Precision Depth Recorder profile. The upper profile is plotted without vertical exaggeration. Depth ranges sampled by dredge and camera have been projected onto this typical cross section.

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TABLE 1. CHEMICAL COMPOSITION OF PERIDOTITE collection, is gmelinite, a sodium-rich member, DREDGED FROM THE TONGA TRENCH* and the common clay mineral is montmoril- lonite (M. N. Bass and J. Griffin, respectively, Wet chemical* Spectrographic* 1968, oral communs.). (oxide weight percent) (parts per million) Dredge 85-D (Fig. 2; 20°32.2'S., 173° Si0 44.62 Ba 6 25.O'W., depth 7200 to 7700 m) recovered 2 small fragments of medium- to coarse-grained A1203 0.16 (?) Co 130 Fe203 0.51 Cr 3700 augite-hornblende gabbro. In thin section, FeO 7.31 Cu 8 lathes of plagioclase are distinguishable but MgO 46.35 Mn 740 CaO 0.09 (?) Ni 3000 altered. Augite is partially or completely Na2O 0.10 Sc 150 replaced by a blue green-to-brown amphibole, K20 < 0.02 Ti 61 and the rock is cut by veinlets of an alkali H20+ < 0.10 V 28 feldspar. H20- 0.00 P Os < 0.01 Camera station 78-C (Fig. 2; 20°16'S., 2 173°12'W., depth 9000 to 9400 m) yielded a Total" 99.14 small fragment of rock lodged in the camera * Depth of water: 9150 to 9400 m. housing. One face has a chloritic, slickensided f Analyst: C. G. Engel. surface. The sample is composed of alkali * Quantitative spectrographic analysis (+15 percent) feldspars, some basaltic hornblende (in part by A. L. Sutton, U.S. Geological Survey, Denver, altered to chlorite), and rare crystals of sphene. Colorado. Other elements: Ag < 2; B < 50; Be < 10; This rock may be part of a slumped mass, and Cd < 100; Ga < 10; Ge < 20; La < 100; Mo < 10; its origin and environment are unknown. Pb < 50; Sn < 40; Sr < 5; Y < 20; Yb < 5; Zr < 20. However, photographs from this lowering show " Total does not include values for K2O and H2O. locally steep slopes, pockets of angular talus, Oxides followed by a question mark are explained in and outcrops of fragmented or breccia ted rock; the text. its occurrence in a large fault zone would not be surprising. The deepest dredging attempt, 86-D (Fig. 2; probably is less than 0.02 weight percent and 20°30'S., 173°18'W., depth 9100 to 9960 m), very likely reflects errors induced by "dirty" was unsuccessful in obtaining rock, but the reagents and by water used in the solution dredge was on the trench wall within 300 m of chemistry. The total given for the analysis of the flattish, 9970-m-deep floor. It was recovered the peridotite does not include the rather large in a polished and badly battered condition, amounts of Ni and Cr shown in the spectro- with the safety line torn free. This suggests graphic analysis. The trace-element analysis— that hard rock outcrops within a very few by A. L. Sutton of the U.S. Geological Survey hundred meters of the trench floor, although —indicates that the peridotite is essentially photographs and PDR records indicate a small sterile except for large amounts of Cr (3700 sedimented and coarsely ripple-marked apron ppm) and Ni (3300 ppm) and moderate at the very base of the steep flank. amounts of Co, Mn, and Sc. Amounts of Ba Dredge 87-D (Fig. 2; 20°39.5'S., 173° and Ti are very low, and Yb and Zr are lower 27.8'W., depth about 7000 m) recovered than the limits of detection (Table 1 and foot- tuffaceous agglomerates consisting of angular note). The platinum metals (Pt, Pa, and Rh) and rounded pieces of altered basalt as well as will be reported in a detailed paper. Dredge discrete crystals and fragments of plagioclase 88-D also includes some samples of dunite and clinopyroxene. Petelin (1964, p. 82) which probably represent more olivine-rich reported tuffaceous conglomerates, tuffs, and layers in the ultramafic mass. basalts from a Vityaz station nearby, at 20° Dredge 88-D also contained about 5 kg 02'S., 173°08.3'W. and depth ranging from of variously oxidized and zeolitized basalt. One 7500 to 8500 m. NOVA 87-D also included a sample, probably the least altered, is a hydrated sample of highly vesicular pumice containing alkali olivine basalt which contains about 4 euhedral crystals of alkali feldspar and rare weight percent total H2O and 0.55 CO2. crystals of sphene. The silica content is 46 percent, and the Na2O The assemblages of rocks here studied suggest (4.71 weight percent) and K^O (1.27) ratio that on the nearshore flank of the Tonga is 3.7. The most abundant zeolite in this Trench the principal rocks exposed are basalts basalt, and in other basalts in this dredge and derivative alkali-rich volcanics. The

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peridotitc-dunitc mass sampled at depths in 65°42'W., depth 5850 to 6750 m. Their excess of 9100 m is probably an accumulate basalts are grouped into porphyritic, amygdu- formed as sedimented sheaths and mats in rest lar, and diabasic types. The porphyritic and magma chambers and in irregular conduits amygdular basalts are extensively altered, the below the volcanoes (Jackson, 1961, p. 1; 1968, diabasic basalt less so. All of the basalts contain p. 135) or perhaps as part ol a stratiiorm sheet. plagioclase and variously altered augitc, and Hither of these structures might have been the porphyritic and amygdular basalts contain exposed by faulting, for which there is strong altered olivine. evidence on both flanks of the trench. These Chemical compositions for the three altered conclusions are based on two characteristics: basalts are listed in Table 2 of Chase and Ilersey (1) the texture and structure of the ultra- (1968, p. 305), and norms indicate that the mafics and (2) their unique lack of alteration. rocks are olivine tholeiites or undersaturated As to their texture and structure, the constituent nepheline basalts. The Fe2Os/FeO ratio olivme and pyroxene are aligned in distinct varies from about 2 in the diabasic variety to layers and lineated patterns of interlocking 6 in the porphyritic and amygdular types. crystals. The resulting cumulatelike textures Water is present in considerable abundance, are similar to those found in most of the more indicating further extensive alteration. In- uniformly sedimented stratiform sheets. spection of their Table 2 indicates that the In composition and related features the diabasic basalt is probably tholeiitic but that basalts recovered in the NOVA dredge work the porphyritic basalt is probably an alkali approach those found on the exposed volcanoes olivine basalt (K2O, 1.27) and the amygdular of the Tonga Islands which lie immediately to basalt of intermediate composition (K»O, the west of the sites. We arc inclined to the 0.64; NazO, 3.22). The possibility exists that interpretation that the basalts and ultramafics both tholeiites and alkali olivine basalts were that we have dredged are lault-exposed com- recovered in the dredge hauls made by R/V ponents of conduits associated with the Chain. aligned volcanoes of the 'longa arc (Fig. 1). The U.S.S.R. research vessel Vityaz has suc- We see no reason to consider either the alkali cessfully dredged other trenches in the western olivine basalts or the ultramafics as samples of Pacific; the collection includes samples from the subjacent mantle. However, one might the Marianas and New Britain, as well as the argue that extensive portions of the upper Tonga, trenches. Yagi (1960, p. 216) reported mantle may be identical in texture, structure, an augite dolente of tholeiitic composition from and composition to these dredged ultramafics the floor of the Marianas Trench (at H°20.6'N., in localities where ultramafics represent the 142°16.2'E. and depth 10,700 m). Petelin settled residues after removal of basaltic (1964, p. 80) gives the petrographic and chemi- magma. cal composition of a tholeiitic pillow basalt The only other ultramafics reported Irom from the New Britain Trench, at 5°11.6'S., oceanic trenches are serpcntinized peridotites 152°43.4'E. and depth 7300 to 7600 m. Both Irom the northern wall of the Puerto Rico samples are hydrated basalts with K2O content Trench (at 19°58'N., 66°28'W., depth 6800 of 0.31 and 0.32, respectively. Rocks reported to 6900 m). Those rocks arc almost completely from Vityaz s dredging operation in the Tonga serpentinized, but some samples contain Trench include basalts, but the compositions bastite pseudomorphs after orthopyroxene or are not given (Petelin, 1960, p. 82). No ultra- relict grains ol orthopyroxene with line mafic rocks or alkali-rich basalts have been cxsolution lamellae (Bowin and others, 1966, reported from the Marianas or New Britain p. 262, and Table 2, p. 265j. Other ultra- trenches by the Soviet scientists. malic rocks Irom that trench include "talc Tholeiitic basalts have been dredged from rock." the Puerto Rico, New Britain, and Marianas Altered basalts also were dredged from the trenches and alkali olivine basalts have been Puerto Rico Trench offshore flank; these are recovered from the Tonga Trench and possibly described as plagioclase-clinopyroxene basalts from the Puerto Rico Trench. (Bowin and others, 1966, p. 265). Chase and It is noteworthy that seaward from the Ilersey (1968) further reported on the miner- trenches of the Pacific ultramafic rocks are alogy and chemical composition of basalts unknown along either the rise-ridge transform dredged by R/V Chain from the northern fault systems or along the great transcurrent flunk of the Puerto Rico Trench at 20°16'N., faults and fracture zones.

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References Cited Bowin, C. O., Nalwalk, A. J., and Herscy, J. B., 1966, Serpentinized peridotite from the north wall of the Puerto Rico Trench: Geol. Soc. America Bull., v. 77, p. 257-270. Chase, R. L., and Hersey, J. B., 1968, Geology of the north slope of the Puerto Rico Trench: Deep-Sea Research, v. 15, p. 297-317. Hess, H. H., 1964, The oceanic crust, the upper mantle, and the Mayaquez Serpentinized peridotite: p. 169-175 in Burk, C. A., Editor, A study of serpentinite: Natl. Acad. Sci.-Natl. Research Council Pub. 1188, 175 p. Jackson, E. D., 1961, Primary textures and mineral associations in the ultramafic zone of the Stillwater Complex, Montana: U.S. Geol. Survey Prof. Paper 358, 106 p. 1968, Character of the lower crust and upper mantle beneath the Hawaiian Islands: Internal. Geol. Congress, 23rd Session, Sec. 1, v. 1, p. 135-140. Petelin, V. P., 1964, Hard rock in the deep water trenches of the southwestern Pacific Ocean, Internal. Geol. Congress, 22nd Session, Reports of Soviet Geologists: Geology of the oceans and seas, v. 16, p. 78-86 (in Russian). Raitt, R. W., Fisher, R. L., and Mason, R. G., 1955, Tonga Trench: p. 237-254 in Poldervaart, A., Editor, The crust of the Earth: Geol. Soc. America Spec. Paper 62, 762 p. Yagi, K., 1960, A dolerite block dredged from the bottom of the Vityaz Deep, Marianas Trench: Japan Acad. Proc., v. 36, no. 4, p. 213-216.

MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 13, 1968 PUBLICATION AUTHORIZED BY THE DIRECTOR, U.S. GEOLOGICAL SURVEY CONTRIBUTION FROM THE SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIVERSITY OF CALIFORNIA AT SAN DIEGO

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