During Leg 111 of the Ocean Drilling

Program, scientists on the

drilling vessel JOIDES Resolution

studied crustal structure and hydrothermal

processes in the eastern

equatorial Pacific. Leg 111 spent 43

days on its primary objective, deepening

and logging Hole 5048, a deep

reference hole in 5.9-million-year-old

crust 200 km south of the spreading

axis of the Costa Rica Rift. Even before

Leg 111 , Hole 5048 was the deepest

hole drilled into the oceanic crust,

penetrating 274.5 m of sediments and

1,075.5 m of pillow lavas and sheeted

dikes to a total depth of 1,350 m

below sea floor (mbsf). Leg 111 deepened

the hole by 212.3 m to a total

depth of 1,562.3 mbsf (1,287.8 m into

basement), and completed a highly successful suite of geophysical logs

and experiments, including sampling

of borehole waters

Adamson, Andrew C.

Alexandrovich, Joanne

Alt, Jeffrey

Anderson, Roger N.

Becker, Keir

Bideau, Daniel

Gable, Robert

Herzig, Peter M.

Houghton, Simon

Ishizuka, Hideo

Kawahata, Hodaka

Kinoshita, Hajimu

Lovell, Michael A.

Malpas, John

Masuda, Harue

Merrill, Russel B.

Morin, Roger H.

Mottl, Michael J.

Pariso, Janet E.

Pezard, Philippe

Phillips, Joseph

Sakai, Hitoshi

Sparks, Joel

Uhlig, Stefan

OceanRep

on ODP Leg 111 Costa Rica Rift hole deepened and logged D uring Leg 111 of the Ocean Drill-ing Program, scientists on the drilling vessel JOIDES Resolution studied crustal structure and hydro-thermal processes in the eastern equatorial Pacific. Leg 111 spent 43 days on its primary objective, deep-ening and logging Hole 5048, a deep reference hole in 5.9-mill ion-year-old crust 200 km south of the spreading 50 Q> c:: ~ 1 ~ ~ ~ ~ ~ 1.( ~ ~ Costa Rica ~ Rift 11:1 c:: ~ ~ 1.( .... .g 11:1 ::;:, ~ Sites 504, • 677,678 oo 85°W axis of the Costa Rica Rift. Even before Leg 111 , Hole 5048 was the deepest hole drilled into the oceanic crust, penetrating 274 .5 m of sediments and 1,075.5 m of pillow lavas and sheeted dikes to a total depth of 1,350 m below sea floor (mbsf) . Leg 111 deep-ened the hole by 212.3 m to a total depth of 1,562.3 mbsf (1,287.8 m into basement) , and completed a highly ' Isla de Malpelo 80° This map shows locations of Sites 504, 677 and 678 in the eastern equatorial Pacific. Dashed lmes show the 250 m bathymetric contour. (From Ocean Drilling Program) 14 successful suite of geophysical logs and experiments, including sampling of borehole waters. Leg 111 also completed a 5-day pro-gram of coring the 200- to 300-m-thick sedimentary sections at Sites 677 and 678, wh ich are located respectively on a local minimum and maximum heat flow near Hole 5048. There were 2 major purposes for this sediment cor-ing: to recover sediment, especially at the low heat flow site, for high-reso lution studies of Plio-Pleistocene biostratigraphy; and to obtain sedi-ment pore waters for chemical study of advective versus diffusive ex-change between the ocean bottom water and the basement through the sediment cover. Sites 677 and 678 are located 3 km south and 1.6 km south-southeast of Hole 5048, in water 3,461 and 3,425 m deep, respectively. 2 holes were drilled at Site 677 : Ho le 677A reached basal sediments and altered basalt at 309.4 mbsf. Hole 6778, which was offset about 20m from Hole 677A, was terminated at 93.1 mbsf, because time was short . Hole 678A was abandoned after 2 successive failures to obtain a good mudline core. Hole 6788 was cored only at specific stratigraphic intervals at 0 to 7.5, 18.2 to 27.7, 95.5 to 105.0, and 169.5 to 171.8 mbsf; material from intervening intervals was washed down. The last core re-covered fragments of basal basalt. Sediments recovered from Si te 677 are mostly calcareous nannofossil oozes and chalk with some limestone and chert from lower sections. 3 ma-jor sedimentary units and a basal ba-salt unit are recognized. The sedi-mentary section of Site 678 may be divided into 4 similar units, although spot coring at this site makes precise comparison with Site 677 difficul t. The oldest sediment recovered in Hole 677A has an age of 5.6 to 5.9 million years. The rate of sedimentation is surprisingly constant wi th a mean val-ue of 42 m in a million years over the last 5.6 million years. The first 0.3 million years of deposition had a much higher rate . The profiles of sediment pore water composition versus depth differ greatly between Sites 677 and 678. Profiles for Ca and Mg are concave upward at Site 677 and convex up-ward at Site 678. Alkalinity is also convex-upward at Site 678. Profiles for ammonium, silica, and phosphate are dominated by reactions in the sediments. All species except phos-phate show large gradients in the 20 m section above basement at Site 677 and within the top 40 m below the seafloor at Site 678. In the altered basal basalt , pore waters at both sites have almost the same composition. Geotimes Those observations indicate that ocean-bottom seawater flows down through the 300-m-thick sediment into basement at the low heat flow Site 677. Significantly altered seawater formed in basement upwells through 180-m-thick sediment into overlying seawater at the high heat flow Site 678. The rate of the flows estimated from the depth-composi tion profiles is a few millimeters a year at both si tes. However, the similarity in com-posi tion of pore water f rom the basal alteration products at both sites sug-gests that the advective flow rates in sediment are negligible compared to those in basement. 0 40 60 80 100 120 § ~ 140 0. .. '0 E 2 16 0 0 D D .;: 18 0 200 240 260 280 300 .. :.1 u !' .. ., 0 .. ti u 0 ii:: .. ;. .. c u u 0 0: ~ .. .. u c .. " 0 ::;; f ~ L ltnoloo c units lA ,,,,,,,,, 16 NN19 c N'l18 NN17 10 NN1 6 NN1!\I NN14 II A '<N 13 liB NNI2 '''''''''' NN11 This diagram shows 7onal assignments, epoch boundanes, and lithostratigraphy wllhm cores and sub-bollom depth in Hole 677A. Unit / A,B,C: alternatmg clayey b1ogemc calcareous siliceous oozes and clayey biogenic siliceous calcareous oozes. Unit II A,B: s11iceous nannofossil oozes and chalk. Untt Ill A,B: cherty lime-stone and nannofossil chalk. Umt IV: sedi-ments r1ch m 1ron ox1de and smecllle, intermixed w1th glassy basement basalts. Slanted lines represent volcanic ash layers. (From Ocean Drilling Program) March 1987 During the Deep Sea Drilling Project, Glomar Challenger made 4 visits to Hole 5048 between 1979 and 1983, coring to a total depth of 1,350 mbsf. Before Leg 111 , Hole 5048 had pene-trated 275 m of sediment, 575 m of pillow lavas, and 200 m of transition into 300 m of massive dike units. Leg 111 was expected to core mostly with-in the sheeted dike complex that forms Layer 2C of the oceanic crust. The rocks recovered from Hole 5048 during Leg 111 are phyric to highly phyric, fine- to medium-grained, olivine tholeii tic basalts, sim-ilar in chemical composition to the basalts recovered from the shallow basement at the site during previous DSDP legs. Phenocrysts of olivine, clinopyroxene, plagioclase, and rare-ly chromian spinel occur in various combinations, including olivine+ clinopyroxene and plagioclase+ clinopyroxene. Although the basalts were mostly massive units, 5 intrusive dike contacts with chilled glassy mar-gins were also sampled. Most of the recovered basalt is al-tered slightly with o livine totally re-placed by ± chlorite, ± pyrite, ± talc, ± mixed-layer clay minerals, ± mag-netite, or by chlorite ± actinolite. Thin , subho rizontal or subvertical fis-sures and veins are common; some are filled by ± chlorite, ± actinolite, ± sulfides, ± quartz, :±. prehnite, ± laumontite, and ± anhydrite. Actino-lite becomes more abundant deeper in the section, implyi ng higher tem-peratures of alteratio n. Hole 5048 sat undisturbed for 1,233 days after DSDP Leg 92. Before Leg 111 began coring, undisturbed bore-hole temperatures were logged con-tinuously, and borehole waters were sampled. The highest measured tem-perature was 148.9°C at 1,280 mbsf. Deep in the ho le, the temperature gradient is basically linear, decreasing from 116°C/km in the pillow lavas to 61°C/km in the d ikes. The change in gradient suggests a puzzling reduc-tion in heat flow with depth . Four samples of borehole waters were collected from 466, 631, 766 and 1,236 mbsf, at temperatures of 81°, 101°, 115°, and 146°C, respectively. The samples show a strong vertical gradient in major chemical composi-tion : magnesium, sulfate, and alkalies decrease with depth, whi le calcium shows an opposite trend. The sam-ples fall on mixing lines between ocean bottom water and borehole water. This indicates that the chemi-cal composition of the borehole wa-ter is controlled by verti cal convec-tion in the borehole and exchange of borehole water with the ocean bot-tom water that flows downhole into the upper 100 to 200 m of basement. Permeabilities measured in the dike section indicate that the kilometer of basement deeper than 500 mbsf is uniformly impermeable (5 to 20 x 10- 18m2), so that the only permeable section of basement is the uppermost 100 to 200 m of pillow lavas. In fact , temperature measurement indicates that ocean bottom water flows down the hole and into this zone at a rate of 80 liters an hour. Hole 5048 was logged with an ex-ceptionally complete suite of tools: Schlumberger neutron-activation/ gamma-spectroscopy tool, lithoden-sity tool, magnetometer, electrical re-sistivity tool, Lamont-Doherty multi-ODP Leg 111 scientific party "<'" Bt'(~t:r Um,erslt\ ol \lt.lml Co-ch•ct $CI<'nl1l t Hito-.hr S.l~di 0c<'dn R<'..,'drch ln>htute, UntH'r<rtv oi Tokyo. Iapan Co-ch11'i SCI<'nl•.st Ru<'>('ll B \1emll Te~a' A&M l5mwr"tv OOP ,,,,h .scll'nti't Andrew ( Addrn\On Te~a• A&M Univl'r'ilV Joanne Ale,androviCh lamont-Doh~rt\ Geolo~•cal Ol~enatorv lclfre' l Aft \\a<htngton Un"~'"'l'. Ro~er AndNo,on lamont-Doht•rty GeologiCal Ob"-'f\dtOI) Dan•el Btdc,JU IFR£\.\lR Fran<e Robert Gablt• Bureau de Rt•t ht·rche~ Geologrques N Mm1i-rC't1 frctnCt' Prtrr M. ltt•mg Aacht•n UmH·r"tv of Trchnology, frdt•rJI Republic ol Ccrman~ S•mon Houghton (}p('n L!nl\'l?rSttV, UK H•deo hhtzuka Kcxhr Un"c""' , Iapan Hodaka Kdwdh,JI.t Uni\Cr\11\ of Toronto Hapmu Ktno,htla Ch1ba Unr•cr"l~, Iapan M1th.wl A Lo\l'll Um\er"tY ol "'Oittngham, U K. John M,IIPJ' Mcm<>rhll Unl\t'r"tY of /'.('\\lnundi,Jnd HarU<' {1.\,\,U<la Un,wrstt\ of Tokvo. Iapan Roger H ~~~n U.S C..'Oiog~eal Sun<'\ , Demc·r Mtchael I. Mottl Hawarr lnstrtutc of Geoph}'in Janet l. Pall\0 Untvcr~ity ol '¥\.a~hrngton Phrllrppc Pczdrd lamont·Dohcrt} Gt'OiogK at Oh~t'fVatory Jo-eph Phrllip<. Unt\.Cr<rtv ol Texas, Austrn Jod Sp.1rl..s Unt\Crstl\' of \1a,~chuwth Stctan Uhlig UMW>~tv ot Gressen. Federal Rt•publrc or German~ 15 channel sonic log, and U.S.G.S. bore-hole televiewer. When calibrated against properties of recovered ba-salts, the logs yield a nearly continu-ous geophysical, geochemical and litho logical characterization of the basement. The neutron-activation tools re-solved the relative abundances of ma-jor elements AI, Ca, Fe, K, Mg, S, Si, U, and Th , and allowed construction of a normative mineralogy log. The variation in log-determined geochem-istry and minera logy is a response to the original chemistry of the phyric versus aphyric units and to the pres-ence of alteration products such as chlorite, actinolite and clays. The logs show that the alteration products are tightly confined to fractures along boundaries between individual extru-sive or intrusive events. In particular, the basalts beneath the stockwork sampled during DSDP Leg 83 at 910 to 930 mbsf are more phyric and contain more aluminum than basalts above the stockwork. The logs yield a nearly continuous geophysical, geochemical, and lithological characterization. Both sonic and resistivity logs clear-ly distinguish individual lithologic units. Deep in the dikes, compres-sional and shear velocities reach 6.4 and 3.7 km a second, respectively, and electrical resistivity increases to over 1 ,000 ohm-meters. The borehole televiewer revealed major breakouts in this otherwise massive section, suggesting that some of the drilling problems might have resulted from spalling of wall rocks as stresses were relieved around the newly drilled hole. Both fracture and total apparent po-rosity could be determined from elec-trical resistivities; the latter ranges from about 15% in the upper pillow lavas to less than 1% deep in the dikes. The variation in logged abun-dances of alteration products corre-lates with apparent porosities calcu-lated from resistivities, suggesting that some of the apparent porosity may represent original porosity that has been filled by alteration products. The logged abundances of alter-ation minerals also correlate with changes in logged magnetic intensi-ties. The logged magnetic inclination changes clearly at about 800 mbsf, from 15° in the pillow lavas above to 8° in the flows and dikes below. This observation is interpreted to indicate 16 that the boundary between pillow la-vas and dikes in Hole 504B is a relict of early listric faulting between pillows over the dikes in the rift valley. Such a fault might have been the permeable conduit for circulating hydrothermal fluids that produced the heavily min-eralized stockwork at the base of the pillow lavas. A highly successful vertical sound-ing profile experiment was done by shooting a geophone clamped about every 10 m up the hole from 1,535 Oct. 2-Ash plume at Suwanosejima (Ryukyu Islands, Japan); people on an airplane saw a white plume 500 to 600 m high; an ash plume was sometimes seen from 30 km northeast of the summit. Explosions have been fre-quent since the erupt ion began in 1949. Early October and continuing- Re-newed growth of the lava dome in the summit cra ter of Merapi (Java) contin-ued into November; several hundred rockfalls a day have been occurring since mid-October. October and continuing-2 new cones have been growing at Pacaya (southern Guatemala), reaching heights of 3 and 5 m by ovember; explosions alternated between the cones on Nov. 18 and 23. Narrow lava flows as long as 500 m were seen Nov. 16 on the west flank of MacKenney Cone. Activity from several vents has built new cones in and near MacKen-ney Crate r and fed many lava flows since March 1983. Nov. 10 and continuing- Explosions at lliboleng (Lesser Sunda Islands, In-donesia); the first explosions since May 28 occurred Nov. 10, 15 and 24; a gas and ash column emitted early Nov. 24 rose 1,000 m and spread a 1-mm-thick ash layer over 18.7 square kilometers. Nov. 11-The submarine explosion near the Kermadec Islands reported in February Geotimes was probably an earthquake; to people on a yacht, the event sounded like a dynamite blast followed by 2 echoes; no pum-ice, ash or wate r discoloration was seen. P and S seismic waves, appar-mbsf. The results show 2 important reflectors, one of which might be the contact between the dikes of Layer 2C and the underlying gabbros of Layer 3. The reflectors are about 100 and 450 m deeper than the present depth of the hole and may be within reach of the next full drilling leg to Hole 504B. Leg 111 Scientific Drilling Party Ocean Drilling Program, Texas A&M University, College Station, 77843-3469 ently from the event, were recorded at Rarotonga, Cook Islands, and Man-gohao, New Zealand; local magni-tude was 4.5 to 5.0. The yacht's posi-tion at the time of the event was revised with satellite navigation sig-nals to 24.43°5, 175.500W, over the west side of the Tonga Trench. Nov. 12 and continuing-Eruption of Piton de Ia Fournaise (Reunion Island, Indian Ocean); a very short , intense seismic crisis was followed by contin-uous tremor and a day-long ash erup-tion in a pit crater in the summit crater; the pit crater's floor rose near-ly 40 m as 30,000 to 40,000 cubic meters of ash were emitted. On Nov. 17, depth, frequency and magnitudes of seismic events mcreased. 2 days later a shallow seismic crisis shook the summit area; strong seismicity continued through the night Nov. 25-26. The next afternoon an east-west fracture opened on the east flank 200 m below the summit-crater rim, pro-ducing a small amount of lava. On Dec. 6, a fissure eruption began in the bottom of the summit crater. On Dec. 15, a vent was sti ll emitting small quantities of lava, and violent gas ex-plosions occurred from spatter cones. The summit pit crater had formed in March (July Geotimes) ; wall collapse and a brief eruption from the pit crater in July had been the last activity at the volcano. Nov. 12-Big rockslides began at Ma-saya (western Nicaragua) at midday on the south and west sides of Santia-go Crater and the crater floor col-lapsed; part of the southwest wall collapsed, extending Santiago into adjacent Nindirf Crater. The only seis-micity recorded by instruments near Geotimes 

Costa Rica Rift hole deepened and logged

Costa Rica Rift hole deepened and logged

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