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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/7/1373/3432731/i0016-7606-80-7-1373.pdf by guest on 26 September 2021 1374 FISHER AND ENGEL—ULTRAMAFIC AND BASALTIC ROCKS, TONGA TRENCH 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/7/1373/3432731/i0016-7606-80-7-1373.pdf by guest on 26 September 2021 NOTES AND DISCUSSIONS 1375 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/80/7/1373/3432731/i0016-7606-80-7-1373.pdf by guest on 26 September 2021 1376 FISHER AND ENGEL—ULTRAMAFIC AND BASALTIC ROCKS, TONGA TRENCH 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.