Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 141

27. GEOCHEMICAL DATA ON VOLCANIC ROCKS AND GLASSES RECOVERED FROM SITE 862:IMPLICATIONS FOR THE ORIGIN OF THE TAITAO RIDGE, CHILE TRIPLE JUNCTION REGION1

Randall D. Forsythe,2 James K. Meen,3 John F. Bender,2 and Don Elthon3

ABSTRACT

Geochemical data for volcanic samples recovered at Site 862 of Leg 141 of the Ocean Drilling Program show that a widediversity of magma types were generated in the region of tie Taitao Ridge. We have divided the data into seven groups. Numeroussamples belong to three groups of basaltic glasses—our Groups 1, 2, and 5. Relative few samples of Groups 3, 4, and 6 werecollected. Group 7 includes silicic samples that range from andesites to rhyolites.

Group 1 samples conform in most aspects of their major and trace element and isotopic chemistry and in their liquid line ofdescent to N-type mid-ocean ridge basalts (MORB) saturated in olivine and Plagioclase. Group 2 samples differ from those ofGroup 1 in several features of their chemistry. They have higher contents of K2O and incompatible trace elements and are mildlylight rare earth element (LREE)-enriched. Their Nb depletions and low values of Ce/Pb and K/Pb are consistent with addition ofa subducted sediment-like component. Sr-Nd and Pb isotopic compositions also indicate the presence of a component with highertime-integrated Rb/Sr, U/Pb, and Th/U and lower Sm/Nd than typical for the sources of MORBs. In addition, the Group 2 glassesdefine a trend of very limited Fe-enrichment, as for calc-alkaline rather than tholeiitic suites. Group 5 glasses have compositionsintermediate between those of Group 1 and Group 2 glasses and suggest that there was a broad continuum of primary magmasdepending upon the amount of sediment introduced into the mantle.

Group 1 glasses with MgO = 8.0% fall on the global array of Klein and Langmuir (1987) between Na8 0 and Fe8 0, but lie offthe arrays of Na8 0 and Fe8 0 against axial depth. If the glasses originally did conform to these arrays, the Taitao Ridge has beenuplifted 2000-4000 m.

The glassy plagioclase-homblende-phyric rhyolites are plausibly melts of metamafic sources. Trace-element patterns and Pbisotopic compositions indicate that this source was chemically similar to Group 2 rocks. The higher 87Sr/86Sr of the rhyolitesprobably results from incorporation of seawater-derived fluids in these sources. Incompatible-element enrichments and REEpatterns of the rhyolites are consistent with 10% melting of a Group 2-like source to leave a residue of Plagioclase and hornblende.In this case, phenocrysts in the rhyolites are probably restite.

INTRODUCTION

This study presents additional geochemical data and interpreta-tions for a suite of volcanic samples that were recovered at Site 862of Leg 141 of the Ocean Drilling Program (ODP). The site is locatedon the Taitao Ridge, a topographically anomalous feature along thePacific continental margin of South America within the region of theChile Triple Junction. An initial set of 17 rock samples was analyzedonboard the ship for whole rock compositions using X-ray fluores-cence techniques (Behrmann, Lewis, Musgrave, et al., 1992). Thisstudy also includes microprobe data for glass shards and glassy basaltrinds; additional major- and trace-element data for some of the vol-canic units not analyzed on board the ship, and isotopic analyses (Sr,Nd, and Pb) for a suite of representative samples.

The Taitao Ridge is a bathymetric promontory that extends south-westward into the Pacific Ocean from the continental margin of SouthAmerica. The ridge extends approximately 15 km offshore, essen-tially along the azimuth of the Taitao Fracture Zone. Marine geo-physical data discussed by Behrmann, Lewis, Musgrave, et al. (1992)point out several important characteristics of the ridge. First, despiteits anomalous relief along the continental slope, multichannel seismiclines failed to image a clear deformation front along its boundarieswith either the Antarctic or Nazca plates to the south and north,respectively. Second, the seismic data showed few, if any, coherentinternal reflections, and an internal high seismic velocity. Third, themagnetic data obtained from a series of east-west profiles show twostrongly lineated north-northwest-trending anomalies; and fourth, the

1 Lewis, S.D., Behrmann, J.H., Musgrave, R.J., and Cande, S.C. (Eds.), 1995. Proc.ODP, Sci. Results, 141: College Station, TX (Ocean Drilling Program).

* Department of Geography and Earth Sciences, University of North Carolina atCharlotte, Charlotte, NC 28223, U.S.A.

3 Department of Chemistry, University of Houston, Houston, TX 77204-5641 U.S.A.

free-air gravity data included anomalies with amplitudes up to 200mgal). The seismic, magnetic, and gravity data suggest that the TaitaoRidge consists of dense igneous lithologies formed by seafloorspreading processes. Because of the unknown nature of the bound-aries of the Taitao Ridge with the surrounding oceanic floor, it is notcurrently known whether the ridge is part of the South American,Nazca, or Antarctic plates.

Southeast of the Taitao Ridge, a large ophiolite complex has beenmapped on the Taitao Peninsula (Forsythe et al., 1986). This upliftedcomplex and some associated "near-trench plutons" (Kaeding et al.,1990) are dated by K/Ar methods and microfossils as late Miocene toPliocene in age (Mpodozis et al., 1985; Forsythe et al., 1985). The ageand location of the onshore igneous rocks have tied their formationand uplifting to the late Cenozoic arrival and subduction of the ChileRise Spreading Center along the Peru-Chile Trench. The offshoreTaitao Ridge, because of its proximity to the ophiolite and its geo-physical characteristics, may represent an offshore extension of theTaitao ophiolite (see discussion by Behrmann, Lewis, Musgrave, etal., 1992). If true, then samples recovered from the ridge mightprovide further constraints on the origin of the ophiolite.

TECTONOSTRATIGRAPHIC SETTINGOF THE SITE 862 SAMPLE SUITE

Three holes (862A, 862B, and 862C) were drilled at Site 862 ofODP Leg 141 on the crest of the Taitao Ridge at a depth of approxi-mately 1200-1300 m (Fig. 1). The relative positions and penetrationdepths of the three holes are illustrated in Figure 2. The lithologies ofthe recovered core show that the Taitao Ridge at this location is char-acterized by a 20-m-thick sedimentary layer overlying volcanic rocks(Fig. 3). The upper sedimentary section was cored continuously atHole 862A, but portions of this sedimentary cover also were recov-ered at Holes 862B and 862C. The underlying volcanic units were

331

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

45°50'S - >

45°55' —

46°00' —

46°05' —

46°10'

46°15'76°10'W 76°00' 75°50' 75°40' 75°30'

Figure 1. Location of the Taitao Ridge and Site 862. Bathymetry is derived from sonar profiling; contour interval is 100 m. From Behrmann, Lewis, Musgrave, etal., 1992.

332

GEOCHEMICAL DATA, SITE 862

10m -100 m

1228.6 mbsl

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Vtf * Vtf ' Vtf ' Vtf ' Vtf

" ‰10m

?T| 1pmY T Y Y Y "I

XVflVjfVjYw*W*Vtf'

Figure 2. Topographic profile along the line of hole offsets on the Taitao Ridgeat Site 862, illustrating the depth, core recovery intervals, and the relativepositions of the three holes. Hole 862A is to the northwest; Hole 862C to thesoutheast. From Behrmann, Lewis, Musgrave, et al., 1992.

penetrated approximately 20 m at Hole 862B and between 60 and 80m at Hole 862C.

The overlying sediments and sedimentary rocks are dominantlycomposed of silty clay to clayey silt and silty fine sand. The sedimentsare lithified within 6 m of the seafloor, suggesting that unroofing mayhave occurred. Microfossils (foraminifers and nannofossils) recov-ered from the sediments indicate an age of late Pliocene. In two of theholes, the recovered cores contained sedimentary rock in depositionalcontact with the glassy rinds of phyric basalt flows. Within the sedi-mentary units above and in contact with the volcanic rocks, abundantcopper sulfides of a hydrothermal origin are found (Behrmann,Lewis, Musgrave, et al., 1992).

Volcanogenic material within the cores can be divided into threetypes. These are (1) isolated clasts within the uppermost sediments, (2)glass shards within the sediments, and (3) volcanic rock samples (bothbasaltic and rhyolitic to dacitic) from the underlying volcanic units.

The uppermost cores from all three holes contain rhyolite andrhyodacite clasts. These clasts suggest that elsewhere (presumablyupslope) volcanic units are exposed and are undergoing submarineerosion and transport. These samples are plagioclase-hornblendevitrophyric rhyolite and rhyodacite; they have a glassy groundmass,irregular vesicles, Plagioclase microlites, and hornblende pheno-crysts with embayed margins and opaque-rich coronae. Silicic vol-canic units with identical textures were recovered from deeper levelsin Holes 862B and 862C.

Within the sediments recovered from Holes 862A and 862B, 11layers were noted that had concentrations of pebble to sand-sized darkbrown to black glass shards. In all cases, the glassy shards wereenclosed within much finer, dominantly clayey silt (or siltstone) sedi-mentary lithologies. These sediments are interpreted to have beendeposited during events of submarine glass spallation and dispersalvia bottom currents. The events could represent submarine eruptionsor tectonic disturbances.

Below the sedimentary cover, drilling recovered 103 fragments ofthe volcanic flows. Within these units, however, the core recoverywas poor, ranging from 1% to 18%. This poor recovery results inuncertainty regarding the potential correlation among holes, the rela-tive abundance of types of units within the holes, and even the abso-lute order of units within any one core. Although the majority of thecore fragments were too large to mix within the core barrel, few had

1228.6mbsl

862A 862B 862C

Magmatic/hydrothermal lithologies

^MWM Hydrothermal "ocher" zone (with sulfides)

I Φ Φ j Basaltic volcanics

I " . i ' I Rhyolithic and dacitic volcanics

| Intercalated metasandstone (calcareous)

•– Cooling units (ash/glass layers in Unit I)

Figure 3. General unit stratigraphy for Holes 862A, 862B and 862C on theTaitao Ridge. Note vertical exaggeration. Presence of discrete eruptive eventsthought to be recorded by cooling units within the volcanic rocks are alsoshown. From Behrmann, Lewis, Musgrave, et al., 1992.

clear signs of having been cut by the core bit. The majority of frag-ments, therefore, were likely to have been produced in a zone ofcrushing near the advancing core bit. As the potential for downwardmixing of units from higher stratigraphic level exists, new units weredefined only at the appearance of new compositions or texturalchanges that clearly indicate a separate cooling unit. The applicationof textural and compositional criteria for defining cooling units re-sulted in a breakdown of the rock samples into seven cooling unitswithin the 20 m of penetration of the basement at 862B, and 13 unitswithin the 60 to 80 m of penetration at 862C.

In general terms, the volcanic rock samples are of two petro-graphic types (Behrmann, Lewis, Musgrave, et al., 1992; Kurnosovet al., this volume). One type is represented by plagioclase-clinopy-roxene-olivine phyric basalts, the other by moderately plagioclase-hornblende phyric dacite, rhyodacite, and rhyolite. The basaltic rocksvary from samples with abundant volcanic glass at flow marginsthrough more crystalline textures towards the interiors of the flowunits. The groundmass is extensively devitrified within 1 to 2 cm ofpillow margins. Most of the high silica samples have glomerophyricclusters of Plagioclase and hornblende with a hyalopilitic (or pilotax-itic) to trachytic matrix containing aligned phenocrysts of Plagioclase

333

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

and hornblende and a glassy to microcystalline groundmass matrix ofmicrolites of Plagioclase. Most hornblende phenocrysts are variablyembayed and surrounded by opaque-rich coronae.

ANALYTICAL PROCEDURES

Individual glass shards in sediments as well as the glass rinds onthree basaltic flows were mounted in epoxy for electron microprobeanalysis. The major-element compositions have been determined bywavelength dispersive spectrometers with the JEOL JXA-8600 elec-tron microprobe at the University of Houston. The electron beam wasdefocused to 20 µm in diameter, with a 20 nA beam current. Countingtimes were 50 s on each peak and 25 s on each background position.Repetitive analyses of basaltic glasses under these conditions indicatethat no Na2O migration occurred. Smithsonian Institution glass VG-2was used as the primary standard for all oxides other than TiO2, MnO,K2O, and P2O5, for which ilmenite, rhodonite, sanidine, and apatitewere used. The online Noran ZAF software was used to calculateconcentrations from the intensity data. The reported analyses of glassrinds (Table 1) include the average and one standard deviation for 10individual points; the reported analyses for glass shards (Table 2)typically are the average of three individual points analyzed on agiven shard.

For the major element, trace element, and isotopic analyses re-ported in Tables 3, 4, and 5, glass rinds were removed from basaltsand used to make sample powders. In addition, individual glass chipswere separated from individual sediment layers and powdered. Tomake these powders, individual chips of glass were broken into piecessmaller than 1 mm and were washed ultrasonically in distilled waterand ethanol. Phenocrysts and pieces of glass with surface alterationwere removed by hand picking under a binocular microscope. Thehand selected glass chips were rewashed, and ground to less than 100mesh in an alumina mortar and pestle. Powders for the rhyolitesamples were prepared in the same manner as the basaltic glasses.These basaltic and rhyolitic glass powders were analyzed at Lamont-Doherty Earth Observatory for SiO2, A12O3, CaO, NaO, K2O, FeO,MgO, TiO2, MnO, Sr, and Ba by direct current plasma emissionspectrometry (DCPES). Analytical precision (±1 S.D.) of the DCPESdata, based on replicate analyses of standards, is 0.5% (relative) forSiO2, 1.0% for A12O3, 2.0% for TiO2, 1.6% for MgO, 2.0% for CaO,1.4% for MnO, 1.2% for Fe2O3, 1.5% for Na2O, 1.6% for K2O, 2.5%for Ba, and 2.0% for Sr.

Six samples (4 basalt glasses, 2 rhyolites) were analyzed for REEsby isotope dilution at the State University of New York at StonyBrook on a 9 in. radius, 90-sector mass spectrometer of NBS design.Sample powders (300 mg) were fused with specially cleaned lithiummetaborate flux in graphite crucibles, then dissolved in nitric acid thatcontained the REE spike. Precision is 1% for Ce, Nd, Sm, Dy, Er, andYb and 2% for Eu and Gd. Chondrite normalized values used for theREE plots are listed in Table 4.

For isotopic analyses, 100-150 mg of powdered samples weredissolved in HF in a capped Teflon beaker and, after evaporation todryness, dissolved in the chloride form. One powdered aliquot wasdissolved for analysis of Sr and Nd isotopic compositions; another foranalysis of Pb isotopic composition and content. The latter, afterdissolution in HC1, was aliquoted and one aliquot mixed with a 2 0 8Pbenriched spike. Sr, Nd, and Pb were separated from other elements bystandard ion-exchange chromatography.

Analyses were conducted on a VG Sector 54 8-collector solidsource TIMS at the University of North Carolina at Chapel Hill. Pb wasanalyzed in static mode. Sr and Nd were analyzed using a triple-collection dynamic peak jumping mode. Replicate analyses of NBS987 for 87Sr/86Sr average 0.710245 ± 12 (σ) normalized to 86Sr/88Sr =0.1194. Replicate analyses of Ames Nd standard for 143Nd/144Nd gavea value of 0.512136 ± 18 (σ), normalized to 144Nd/ l46Nd = 0.7219.145Nd/ l44Nd of the standard was 0.348405 ± 9. Pb analyses are normal-ized by 0.10% per mass unit difference relative to the mean value

determined for NBS 981. Scatter in analyses of NBS 981 correspondsto an approximate σ of 0.04% per mass unit difference in the ratio.Within-run errors are always much less than this. Sr analyses used a 3V88Sr signal, Nd analyses used a IV 144Nd signal, and Pb analyses useda 2V 2O8 Pb signal (corresponding to a filament temperature of 1270-1470°C). Blanks are 60 pg for Nd 200 pg for Sr, and 500 pg for Pb.

RESULTS

The compositions of glass rinds from Samples 141-862B-3X-1,95-100 cm, and 141-862B-X-CC, 3-38 cm, are similar in composi-tion to many other mid-ocean ridge basalts (e.g., Bence et al., 1979).The glass rind on Sample 141-862C-W-01, 37-38 cm is distinctlydifferent, however, and is characterized by high SiO2 (53.0%), lowFeO* (6.8%), high K2O (0.41%), and a high Mg-number [100 Mg/(Mg + Fe) = 65.3].

Glass shards were analyzed from seven sedimentary horizonsoverlying the volcanic units (here labeled 1A, 2A, 7A, 8A, 9A, 10A,and 11 A). The sample intervals along with representative composi-tions of the shards are listed in Table 1. The number of glass shards ineach sample varied from <5 to >50. Each analyzed shard was given aseparate number, such that shard number 7A-19 in Table 1 was thenineteenth shard analyzed in sample 7 A. There are three types of glasscompositions that are very common as shards (Groups 1,2, and 5) andfour types of glass compositions that are not abundant (Groups 3, 4,6, and 7). The compositions often representative shards are reportedfor the three most abundant groups in Table 1, whereas all shardcompositions are reported for the remaining groups.

The Group 1 shards are very similar in composition to glass rindSamples 141-862B-3X-01, 95-100 cm, and 141-862B-X-CC, 3-38cm, and are inferred to be part of the same differentiation trend. Theglass rind Sample 141-862C-W-1,37-38 cm, on the other hand, is verysimilar in composition to the Group 2 shards and is considered to bepart of their differentiation trend. The Group 2 shards differ from theGroup 1 shards by having higher SiO2, Na2O, and K2O, with lowerFeO*, MgO, and CaO (Table 1). The Group 5 samples are intermediatein composition between the Group 1 and Group 2 shards. The majorelement chemical relationships between these various groups of ba-saltic glasses are shown in Figures 4A through 4G. Because Groups 1and 2 define the range of basaltic compositions for these samples, mostof the subsequent discussion focuses on their compositions.

The remaining glass shards (Groups 3, 4, 6, and 7) are relativelyrare and are difficult to directly link to local submarine volcanism onthe Taitao Ridge because of the absence of recovered basaltic flowswhose compositions match these groups. The andesites, dacites, andrhyolites (Group 7), might even be related to nearby continentalvolcanism rather than silicic ocean floor volcanism.

The major-element, Sr, and Ba compositions of eight basalticglass samples are listed in Table 3. Also shown in Table 3 is the glassshard group to which the whole-rock analyses are considered to berelated. For the same glass sample, differences between the micro-probe glass compositions (Tables 1 and 2) and the hand-picked glassseparates (Table 3) are attributed to phenocrysts too small to havebeen removed by the procedures used. As with the microprobe analy-ses, the DCPES data also indicate that the Group 1 glasses (Samples141-862B-X-CC, 3-38 cm, and 141-862B-3X-1, 95-100 cm) havehigher CaO and FeO* as well as lower SiO2, Na2O, and K2O at thesame MgO content. In addition, Group 1 basalts are characterized bydistinctly lower Sr and Ba abundances than Group 2 basalts (Table 3).

REE concentrations for four basalts and two rhyolites are shownin Table 4 and have been illustrated as chondrite normalized diagramsin Figures 5A and 5B, respectively. The REE pattern for Group 1basalt, (Sample 141-862B-3X-1, 95-100 cm) is characterized by astrong light REE depletion (CeN/YbN = 0.6), and no Eu anomaly. Thethree basalt glasses related to the Group 2 glasses (Fig. 5A) exhibitlight REE-enriched patterns (CeN/YbN = 1.4-1.5) and slight Euanomalies. The two rhyolite samples illustrated in Figure 5B have

334

GEOCHEMICAL DATA, SITE 862

Table 1. Electron microprobe analysis of glass rinds on phyric basalt samples, Site 862, Taitao Ridge.

Major elements (wt%)SiO2TiO2A12O3FeOMnOMgOCaONa2OK2OP 2 O 5

TotalMg#

141-862B-3X-1,

Average

50.651.19

16.888.420.168.14

11.862.770.050.1

100.2163.3

95-100 cm

1 S.D.

0.310.050.120.120.020.060.080.140.010.01

63.3

141-862B-2X-CC

Average

50.821.2

16.688.440.168.16

11.882.910.040.09

100.38

,3-38 cm

1 S.D.

0.20.060.050.090.010.060.030.030.010.01

65.3

141-862C-1W-1

Average

53.021.08

17.356.780.137.15

10.293.290.410.12

99.59

,37-38 cm

1 S.D.

0.160.050.070.060.010.040.030.030.010.01

Note: S.D. = standard deviation.

REE patterns characterized by high Ce abundances (×45-50 chon-drites), negative Eu anomalies, and heavy REE abundances that areapproximately 25% lower than those of any of the basalts analyzed.

Sr, Nd, and Pb isotopic compositions have been determined for sixsamples (Table 5). The single Group 1 basalt for which isotopiccompositions have been determined (Fig. 6) has Sr-Nd systematicssimilar to those of many MORB from the MAR and EPR (87Sr/86Sr0.7028, εNd = +0.7). The three Group 2 basalts, however, have dis-tinctly different Sr-Nd compositions at 87Sr/86Sr = 0.7033, εN d= +8.0.The two rhyolites analyzed differ in isotopic composition from thebasalts with much higher 87Sr/86Sr (= 0.7041).

Figures 7A and 7B show Pb isotopic variations in the same sixsamples. The Group 1 basalt has the lowest values of 206Pb/204Pb,207p b /204p b a n d 208p b /204p b o f t h e s i x s a m p l e s a n a lyzed. This sample

is displaced from the Northern Hemisphere Reference Line (NHRL)on Figure 7A with higher 207Pb/204Pb than most MORBs. Some otherNazca Plate basalts are similarly displaced to higher 207Pb/204Pbthan the NHRL, although at higher 206Pb/204Pb. The 2 0 8Pb/2 0 4Pb-206pb/204pb r e l a t i o n s o f t h e G r o u p i b a s a l t aj-g n e a r t h e NHRL. The

Group 2 basalts and the rhyolites have similar Pb isotopic ratios (Figs.7A and 7B). They are highter in 207Pb/204Pb than the NHRL or NazcaPlate basalts and have higher 208Pb/204Pb.

INTERPRETATION

Upon consideration of the geochemical data, we consider threequestions related to magmatic activity at the Taitao Ridge:

1. What are the differentiation processes responsible for producingthe geochemical variations within the Group 1 and Group 2 glasses?

2. What do the first-order differences between Group 1 andGroup 2 glasses indicate about magmatism at the Taitao Ridge?

3. What is the petrogenesis of the silicic lavas?

In addition, we discuss the insights that the geochemistry of thesesamples yield into the tectonics of the Taitao Ridge.

Differentiation Processes

Numerous studies of MORBs have shown that low-pressure crys-tallization of olivine, Plagioclase, and (sometimes) augitic clinopy-roxene controls the liquid line of descent in these magmas (e.g.,Bryan, 1979; Grove and Bryan, 1983; Perfit and Fornari, 1983).Because no experimental studies of the crystallization of these TaitaoRidge samples have been undertaken, the 1 ATM liquid lines ofdescent for these samples have been calculated by the CHAOS5algorithm of Neilsen (1988). The calculated liquid lines of descent forestimated parental magmas for glasses from Groups 1 and 2 are listedin Table 6 and are compared with the variations in glass compositionsin Figures 4A through 4G.

The 1 ATM liquid lines of descent for parental magmas for Groups1 and 2 are characterized by a small interval of plagioclase-onlycrystallization, followed by an extended interval of olivine + plagio-clase crystallization, and then olivine + Plagioclase + clinopyroxene.This sequence of crystallization is common amongst MORBs (Bryan,1979; Grove and Bryan, 1983; Grove and Juster, 1989). The plagio-clase-only crystallization interval (Figs. 4A through 4G) is charac-terized by an increase in MgO and other oxides that are notcompatible in Plagioclase, along with a decrease in A12O3 and CaO.The remainder of the liquid lines of descent shown in Figures 4Athrough 4G for the Group 2 parental magma is for olivine + plagio-clase crystallization; the onset of clinopyroxene crystallization inthese parental magmas does not occur until the residual liquid has5.6% MgO (Table 6). The onset of clinopyroxene crystallization inthe parental magma for Group 1 occurs at 7.1% MgO (Table 6), asclearly shown by the decreasing CaO content in Figure 4E.

The initial small interval of plagioclase-only crystallization forthese three parental magmas results in a displacement of the calcu-lated liquid lines of descent away from the trend of the glass shards.This effect is shown for CaO in Figure 4E, where the initial intervalof plagioclase-only crystallization increases the MgO content anddecreases the CaO content, and then olivine + Plagioclase crystal-lization reduces the MgO content while CaO increases slightly. Thecalculated liquid lines of descent are parallel to but slightly lower inCaO than the trends of the glass compositions (Fig. 4E). A compara-ble effect is also seen in FeO* versus MgO space (Fig. 4D), in whichthe calculated liquid lines of descent are displaced to higher FeO*than the trend of the composition of the glass shard.

The three samples of basaltic glass rinds all have abundant olivineand Plagioclase phenocrysts, suggesting that these liquid composi-tions were co-saturated with these two minerals at the time of eruptionand quenching. The glass shards, on the other hand, are typically onlya few tens of microns across and do not have mineral phases associ-ated with them; it is not possible to determine their phenocryst min-erals under magmatic conditions. In light of the petrographicevidence indicating that the Taitao Ridge glasses are at or near co-saturation in Plagioclase and olivine at the time of eruption, it ispossible that this calculated small interval of plagioclase-only crys-tallization is the erroneous result of imperfect knowledge of plagio-clase saturation surfaces in basaltic liquids. It is also possible that asmall amount of water in the natural basaltic liquid (0.3 wt%) wouldsuppress the crystallization of this small amount of Plagioclase so thatthe liquid would actually be co-saturated in olivine and Plagioclaseunder magmatic conditions (fig. 11 of Michael and Chase, 1987).

The usefulness of these calculated liquid lines of descent in inter-preting the chemical variations in the glass shards is that the slopes ofthe lines can be used to determine the minerals that have crystallizedto form the glass trend. The variations in basalt glass compositions areshown in Figures 8A through 8G, in which liquid lines of descent for

335

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

Table 2. Electron microprobe analysis of glasses, Hole 862A, Taitao Ridge.

Core, section:Interval (cm):Shard no.:

3H-127-297 A-19

Major elements (wt%)SiO^TiO;AI20,FeoMnOMgOCaONa.OK20P2O5

TotalMg#

50.641.11

17.088.070.168.55

12.042.860.030.07

100.5965.37

2H-224-262A-6

50.841.02

17.418.090.158..S2

11.822.940.060.08

100.9165.24

3H-I27-297A-4

51.171.05

17.197.890.158.42

11.742.940.08O.I

100.765.54

4H-I17-201 A-1

51.491.14

16.478.410.178.21

11.92.920.040.08

100.8363.5

Group 1

3H-127-297A-18

51.051.15

16.778.280.178.25

11.982.960.040.09

100.7163.96

4H-117-201A-8

51.281.2

16.548.310.158.16

11.862.880.050.09

100.4963.64

4H-117-201A-9

51.341.22

16.598.360.178.22

11.862.930.060.09

100.8163.66

4H-117-201A-6

51.041.26

16.848.390.158.21

11.922.970.050.08

100.8863.56

4H-117-201A-2

51.261.28

16.548.470.168.16

11.942.880.050.11

100.8263.18

4H-117-20IA-3

51.281.3

16.498.410.178.16

11.952.910.050.1

100.863.36

Group 2

3H-16-8

11A-6

51.831

17.246.970.148.14

10.133.20.40.12

99.1567.55

2H-224-262A-2

52.921.04

17.26.850.128.09

10.163.20.40.13

100.0667.8

Note: Group 1 = low K basaltic glass shards, 10 representative samples; Group 2 = high K basaltic glass shards, 10 representative samples; Group 5 = 10 representative samples; Group7 = andesites, dacites, and rhyolites. Groups 3-6: descriptions not available; noted rare but not discussed in any descriptive way.

Table 2 (continued).

Core, section:Interval (cm):Shard no.:

Major elementsSiO^ÜO;A1 2 0 ,

FeoMnOMsOCaONa,OK,OP 2 O 5

TotalMg#

2H-224-262A-3

(wt%)51.89

1.3616.128.790.187.46

11.352.950.220.16

100.4560.19

Group 4

2H-224-262A-4

51.881.39

168.780.197.4

11.312.860.220.14

100.1660.02

3H-16-8

11A-5

50.991.46

15.79.280.187.21

11.252.950.230.14

99.3558.08

3H, 16-8

1 1 A-13

51.351.05

17.237.220.158.38

10.563.120.3().! 1

99.4567.41

3H, 127-297A-6

52.331.03

17.127.280.138.37

10.583.110.310.12

100.3667.21

3H, 127-297A-3

52.071.1

17.047.450.128.15

10.913.040.250.1 1

100.2366.12

3H, 16-8

11A-7

51.391.16

17.337.310.137.97

10.523.250.30.1 3

99.4866.03

Group 5

3H, 127-297A-12

51.771.19

17.367.290.1 57.91

10.933.180.250.1 1

100.1265.93

3H, 16-8

11A-4

51.181.13

17.477.340.137.79

10.513.30.320.1

99.2665.43

3H, 16-8

1IA-16

51.661.17

17.337.180.157.88

10.583.230.30.13

99.5966.16

3H, 160-628A-7

52.881.12

17.357.270.147.63

10.543.380.310.13

100.7565.19

3H, 127-297A-15

52.581.09

17.247.180.137.57

10.453.270.340.12

99.9465.27

3H, 16-8

1IA-14

51.641.16

17.247.430.157.08

10.623.40.330.13

99.1762.94

olivine + Plagioclase crystallization are shown starting from parentalmagmas for the Group 1 and Group 2 glasses (i.e., these liquid linesof descent do not have the initial small interval of plagioclase-onlycrystallization, but retain the appropriate slopes for olivine + plagio-clase crystallization). The A12O3 contents of the Group 1 glassesprogressively decrease as MgO decreases (Fig. 8C), and the FeO*contents increase as MgO decreases (Fig. 8D), comparable in slopeto olivine + Plagioclase co-crystallization. The variations in othermajor element oxides in the Group 1 glasses have similar slopes tothat shown for olivine + Plagioclase crystallization (see Figs. 8A, 8B,8E, 8F, and 8G). The normal proportion of olivine to Plagioclaseduring their co-crystallization from basalts, as deduced from experi-mental studies (e.g., Grove and Bryan, 1983; Juster and Grove, 1985)or from the slope of the liquid line of descent for MgO versus A12O3

(Figs. 4C and 8C) is 2:1.The chemical variations in Group 2 glasses, on the contrary, do not

seem to be explicable in terms of relatively simple low-pressurecrystallization processes. It appears that the trend of Group 2 glasscompositions does not involve olivine + Plagioclase + clinopyroxenecrystallization in normal low-pressure anhydrous cotectic propor-tions because these glasses have nearly uniform CaO contents,whereas the CaO content of residual glasses should decrease substan-tially during olivine + Plagioclase + clinopyroxene crystallization(compare Figs. 4E and 8E). Similarly, the nearly constant A12O3 andFeO* contents of Group 2 glasses appear to preclude the crystal-lization of Plagioclase, olivine + Plagioclase, or olivine + Plagioclase+ clinopyroxene in normal cotectic proportions because these typesof crystallization produce residual liquids with progressively lowerA12O3 contents (compare Figs. 4C, 4D, 8C and 8D). In addition, thetrends in the Group 2 glasses are not likely to result from crystal-

lization at elevated pressures because the principal effect of increas-ing pressure is to cause clinopyroxene to move closer to the liquidusof the basaltic composition. The constant CaO content throughout therange of crystallization of Group 2 glasses rules out this possibility.

Alternatively, the compositional trend for Group 2 glasses wasproduced by crystallization of olivine + Plagioclase in proportions thatdiffer markedly from 2:1. If the proportion is 1:1 by weight for olivine+ Plagioclase crystallization, the A12O3, FeO*, and CaO contents ofresidual liquids remain nearly constant (see Fig. 8 and Table 7).

Magmatic Implications of the Differencesbetween Basalt Groups 1 and 2

There are several fundamental differences between Group 1 andGroup 2 basalts. Major and trace element and isotopic differencesbetween these two groups are so great that the two groups cannot berelated by simple solid-liquid processes and different sources arerequired for Group 1 and Group 2 basalts.

In addition to element-element diagrams (Figs. 4 and 8), Figure 9is used to illustrate major-element variations in the glasses on a con-ventional AFM diagram. Samples within each of the populous groups(1,2,5) show limited variations on this diagram. The most magnesiansamples of each group have similar relative amounts of 'M' but aredisplaced to lower A:F in the order Group 1, Group 5, Group 2. Thisreflects not only the progressively higher K2O and Na2O contents inthe Groups 1,5, and 2 (Figs. 4F and 4G) but also the decrease of FeO*in the same order (Fig. 4D). All samples are more Fe-poor than thetholeiite-calc-alkaline dividing line of Irvine and Baragar (1971) andso all groups are calc-alkaline by this definition. Moreover, theGroups 2 and 5 samples define trends on the AFM that are much

GEOCHEMICAL DATA, SITE 862

Table 2 (continued).

Core, section:Interval (cm):Shard no.:

Major elementsSiOiTio;AI2O3

FeoMnOMsOCabNa,OKoOP2O5

TotalMg#

3H-16-8

11A-2

(wt%)51.87

1.0617.126.880.148.05

10.153.190.390.12

98.9467.61

3H-127-297A-2

52.931.05

17.146.890.137.96

10.223.140.390.12

99.9567.31

IH-373-759A-4

52.61.05

17.456.890.1 37.92

10.283.210.400.11

100.0367.2

Group 2

3H-127-297A-16

52.911.06

17.296.820.137.78

10.283.20.390.11

99.9467.05

3H-127-297A-9

53.571

17.236.750.127.74

10.33.20.390.12

100.467.15

3H-160-628A-1

52.171.07

17.127.060.137.6

10.273.370.360.13

99.2765.73

2H-224-262A-8

52.721.2

17.597.030.136.87

10.313.640.360.11

99.9363.53

2H-224-262A-1

53.281.28

17.267.140.136.7

10.283.670.360.12

100.262.56

3H-127-297A-5

51.84 •1.18

16.87.930 . 1 27.73

11.353.040.190.1

100.2663.48

3H-160-628A-3

51.861.08

16.957.670.157.8

11.43.130.190.09

100.3164.45

Group 3

3H-160-628A-5

52.171.09

16.967.750.157.7

11.363.180.20.1

100.6563.93

3H-160-628A-8

52.251.1

17.027.690.167.85

11.343.170.20.12

100.8964.53

3H-127-297A-8

52.281.19

16.797.850.147.73

11.353.090.180.12

100.6963.72

Table 2 (continued).

Group (

Core, section: 3H-1Interval (cm): 6-8Shard no.: 11A-1C

Major elements (wi%)SiC>T

Tio;A12O3

FeoMnOMgOCaONa,0KooP2°5TotalMg#

Hole:Core, section:

Major elementsSiO,Tio;A1203Fe2O3MnO'MsOCabNa-,0K2O

TotalSrBaFeO*

Group

51.281.58

16.688.30.176.93

10.423.660.180.15

99.3459.81

>

1

2H-224-262A-5

55.611.06

17.036.30.15.569.773.690.590.13

99.8161.15

2H-224-262A-9

58.491.94

16.017.890.162.996.364.261.020.25

99.3540.29

Group 7

3H-127-297A-17

59.931.09

16.555.40.113.756.963.841.120.15

98.8955.32

3H-127-297A-10

60.541.05

16.435.50.083.736.933.841.140.14

99.3654.75

2H-224-262A-7

74.750.36

11.91.470.020.180.842.773.530.04

95.8417.92

Table 3. DCPES analyses of Site 862 volcanic samples.

862A3H-I

(wt%)50.35

1.0616.417.810.138.29

10.073.210.317

97.6512557.7

7.03

2

862A01A-2

52.111.1

16.877.980.1348.44

10.353.280.305

100.3713262.2

7.18

2

862B2X-CC

49.011.05

17.088.560.1418.81

11.652.870.167

99.349518.27.70

1

862B3X-1

49.571.06

16.958.920.1428.61

11.582.940.085

99.86919.88.03

1

862 B4X-2

51.721.17

16.228.560.1249.399.493.40.677

100.7515297.57.7

2

862C1W-I

53.191.12

18.286.570.1025.48

10.83.560.62

99.7215184.65.91

2

862C6R-1

52.031

16.557.740.1288.949.943.220.404

99.9512674.76.97

1

3H-I60-628A-I1

78.380.41

11.591.640.040.190.82.462.850.06

98.4215.51

862X8R-1

51.231.22

16.438.530.1368.439.963.380.389

99.7113558.9

7.68

2

shallower than that of the dividing line (i.e., they have very limitedFe-enrichment). The very limited range of Group 1 basalts appears tohave a steeper trend than do Group 2 and Group 5 glasses. Even thistrend is, however, not steeper than the dividing line.

Figure 9 also shows the fields of samples from the Ecuador Rift-Inca Transform and the East Galapagos Rift (Fornari et al., 1983). Themost magnesian samples in the former field fall near the Group 1glasses but the AFM trend is much steeper and continued to extremeFe-enrichment by the East Galapagos Rift glasses.

Figure 10A shows the variation of FeO*/MgO with SiO2 contentof glasses from Site 862. All samples, except one Group 4 glass, haveFeO*/MgO below the tholeiite/calc-alkaline discrimination line.Group 1 samples exhibit a slight Fe-enrichment as SiO2 increases butGroup 2 and Group 5 glasses have essentially constant FeO*/MgO.

These relations are entirely consistent with relations on the AFMdiagram (Fig. 9).

Figure 10B compares the data for Group 1, 2, and 5 glasses withthose for samples from Ecuador Rift/Inca Transform and from theEast Galapagos Rift. Samples from the former area do straddle thetholeiite/calc-alkaline divide at low SiO2 but move to higherFeO*/MgO as SiO2 rises. The East Galapagos Rift glasses all lie athigher FeO*/MgO than the dividing line and FeO* /MgO increasesas SiO2 rises. In contrast, the Site 862 glasses define a trend of noFe-enrichment with silica increase.

Group 2 BasaltsMajor-element variations of Group 2 glasses differ in several

important particulars from those typical of N-type MORB. At a given

337

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

FeO*

"~• s.

Samples Fields LLDs

Group 1Group 2Group 3Group 4Group 5Group 6

•A

Δ

O

•

D

MgO MgO

Figure 4. MgO variation diagrams for major oxide compositions observed in the glasses analyzed by electron microprobe. Six of the seven groups of data areplotted, along with three liquid lines of descent (LLDs) for estimated parental magmas for Groups 1 and 2. Shaded areas for Groups 1, 2, and 5 glass compositionsare also shown. All values are in weight percent (wt%).

MgO content, Group 2 glasses have higher SiO2 and K2O and lowerFeO* and CaO than N-type MORBs. (Compare Fig. 4 with Bence etal., 1979.) Group 2 glasses show a slight increase in A12O3 as MgOfalls (whereas A12O3 decreases as MgO decreases in MORB) and noFe-enrichment (Figs. 9 and 10). These characteristics are more likethose of a calc-alkaline suite than of a MORB (tholeiitic) suite.

Figure 11 is a spidergram for one Group 1 and three Group 2basalts compared with the N-type and E-type MORB of Sun andMcDonough (1989). Data are from this paper and from Behrmann,Lewis, Musgrave, et al. (1992). The three Group 2 samples have very

similar trace-element-enrichment patterns. These are generally flat atthe right-hand side of the spidergram but relatively more enriched inthe more incompatible elements. That part of the diagram is veryirregular. Pb and K are approximately as enriched over primordialmantle as is Rb. In particular, Pb is enriched over LREE; Nb and, toa lesser extent, Ba are depleted relative to K and Rb.

The pattern of the E-type MORB plotted on Figure 11 (from Sunand McDonough, 1989) is broadly similar to the Group 2 basaltsinsofar as contents at the left-side are 1.5 to 2 times the contents at theright side. There are, however, several differences. The Group 2

GEOCHEMICAL DATA, SITE 862

basalts are much more enriched in K and Rb than E-type MORBs. TheGroup 2 basalts have a positive Pb spike whereas the E-type MORBhas a negative Pb spike. (The negative spike in MORBs on Figure 11results from using a P.M. value of 0.185 ppm.) Group 2 basalts havenegative Nb spikes; E-type MORBs have slight positive Nb spikes.

Group 2 basalts are enriched in Pb and depleted in Nb. Ce/Pb ofMORB and OIB is essentially constant near 25, and MORB and OIBhave K/Nb = 260 and Nb/Pb = 7.8 (Sun McDonough, 1989). Group2 basalts have values of these ratios of 4-5, 1120-1650, and 0.8-1.1,respectively. This must reflect addition of a component enriched in Kand Pb releative to Nb to the mantle source of these liquids. Further-more, because K/Pb of MORB and OIB is near 2000 and the Group2 basalts have K/Pb from 1170 to 1650, the added material wasenriched in Pb more than it was enriched in K.

Group 2 basalts are also depleted in titanium and phosphorous.This set of characteristics is indicative of addition of a subductedsediment-like component to the mantle. Only limited amounts ofsediment need be added to the mantle to produce such trace-elementratios. The average pelagic clay of Taylor and McLennan (their table2.3; 1985) has 30 ppm Pb and Ce/Pb = 2.7, and many pelagic sedi-ments have much lower Ce/Pb. The pelagic clay of Taylor andMcLennan also has K/Nb 1800 and Nb/Pb = 0.5. One major prob-lem with this scenario is the depleted Ba signature in Group 2 glasses.The average pelagic clay of Taylor and McLennan (1985) has 2300ppm Ba so that mantle contaminated with sediment has much greaterratios of B a to other incompatible elements than are exhibited by theglasses of Group 2. Figure 12 shows Ba/Zr-Ce/Zr relations of Group1 and 2 basalts superimposed on the fields of Saunders and Tarney(1991). They used these relations to discriminate between MORBs,back-arc basalts, and island arc basalts. Addition of a subducted sedi-ment component shifts the mantle to higher values of both Ba/Zr andCe/Zr but Ba/Zr increases more rapidly because sediment has suchhigh Ba contents. N-type MORBs have low values of both Ce/Zr andB a/Zr-near those of primitive mantle (0.1,0.09; Ce/B a 11). Arc basaltshave a wide range of compositions that overlap with N-type MORBsat the low end of their Ce/Zr but are displaced to higher Ba/Zr (>l).E-type MORBs define a trend from near the high-Ba/Zr end of theN-type MORB toward the OIB field at higher Ba/Zr and Ce/Zr.

The Group 2 basalts define a trend from near the N-type MORBfield to higher Ba/Zr at essentially identical Ce/Zr. These samples donot overlap with the arc fields denoted by Saunders and Tarney(1991). Displacement of Group 2 basalts from the N-type MORBfield is, however, toward the arc fields.

Group 2 basalts have Sr-Nd isotopic compositions that do not liein the same field as EPR basalts. They are shifted toward the Sr-Ndcompositions of basalts and andesites from the nearby Austral Andesand South Volcanic Zone of the South American mainland (e.g., Sternet al., 1984a, give the average 87Sr/86Sr and 143Nd/144Nd of Mt. Burneyin the Austral Andes as 0.70416, and 0.512754, respectively; andStern et al., 1984b, give average ratios from three Southern VolcanicZone volcanoes as 0.7050, and 0.51260, respectively).

The Sr-Nd isotopic compositions of the Group 2 basalts also lie inthe field defined by five samples of the Taitao ophiolite (Kaeding etal., 1990). Isotopic data for the ophiolite define a trend from valueswithin the EPR field toward the field defined by the felsic TresMontes intrusive suite.

Pb isotopic compositions of the Group 2 basalts also differ fromthose of MORB as shown by Figure 7A. They have much higher207pb/204pb t h a n N a z c a p i a t e b a s a l t s (Tjnmh and Tatsumoto, 1976) butfall in the fields defined by the Southern Volcanic Zone, and bysediments of the Nazca Plate. Similar relations are shown by2 0 6 2 0 4 2 0 8 2 0 4

50

y

Some aspects of the Group 2 basalts' chemistry are consistent onlywith production of the primary magmas from a source containing asubducted sediment-like component. The strongest evidence for sucha component is provided by the relative enrichments in Pb and thealkalis and depletion in Nb and by the Pb isotopic ratios. Furthermore,

CD-t—»

-σco

JZ

.oUJ

10-

Group 1

Group 2 J

^ = = * ^" • - ~ * ^ ^ ^ ^

D B3X01

A C1W01

• B4X02

• C6R01

J A. BasaltsLJ

Ce Sm Gd ErNd Eu Dy Yb

100

&

σ£ =Osz_oUJ

er 10

B. Rhyolites• B4X01

D C5R01

Basalt

field

Ce Sm Gd ErNd Eu Dy Yb

Figure 5. A. Chondrite-normalized REE patterns for four Leg 141 basaltic glasssamples. Samples keyed as follows: Group 1 = 3X1 (Sample 141-862B-3X-1,95-100 cm); Group 2 = 1W1 (Sample 141-862C-W-1, 37-38 cm), 4X2(Sample 141-862B-4X-2, 40-45 cm), 6R1 (Sample 141-862C-6R-1, 25-30cm). B. Chondrite-normalized REE patterns for two Leg 141 rhyolite samples.Samples keyed as follows: 4X1 = Sample 141-862B-4X-01, 21-28 cm; 5R1= Sample 141-862C-5R-01, 11-18 cm.

the trend of Group 2 glasses is calc-alkaline rather than tholeiitic. Themost problematic aspect of interpreting Group 2 basalts as calc-alkaline is their low Ba contents. Such an interpretation is, however,the best fit to the entire dataset.

Group 1 Basalts

Interpretation of the Group 1 basalts as N-type MORBs appears tobe straightforward. As discussed above, the rocks have a major-element trend very similar to those defined by other MORBs. The onesample analyzed for trace-elements and isotopes has a trace-element-

339

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

Figure 6. Variations of Sr and Nd isotopic ratios of oneGroup 1 basalt, three Group 2 basalts, and two rhyolitescompared with fields of Mid-Atlantic Ridge (MAR) andEast Pacific Rise (EPR) MORBs. The field of Taitaoophiolite compositions is from Kaeding et al. (1990).

0.5134

0.5133-

0.5132

0.5131"

0.5130-

0.5129-

0.5128:

0.5127:

0.5126

MAR

EPR

O Rhyolites

• Gp 2 basalts

D Gp 1 basalts

-14

-12

-10

•8

•6

4

-2

o

8[Nd]

0.7020 0.7025 0.7030 0.7035 0.7040 0.7045 0.7050 0.7055

Sr/ Sr

19.5 20.0

204Pb/ Pb

sio.

Figure 7. Variations of 2()6Pb/204Pb with (A.) 208Pb/2ü4Pb and (B.)207Pb/204Pb for one Group 1 basalt, three Group 2 basalts, and tworhyolites. The fields for most MORBs are shown on each diagramand are distributed about the Northern Hemisphere Reference Line(NHRL) in each case. Also shown are the fields of Nazca Platebasalts and sediments (Unruh and Tatsumoto, 1976) and ofvolcanic rocks of the South Volcanic Zone (S VZ) of the SouthAmerican mainland.

15.7

15.6-

15.5

15.4

15.3

O Rhyolites

• Gp 2 basalts

D Gp 1 basalts

B

0n / )

SVZvolcanics

M Nazca Platesediments

H Nazca Plate basalts

17.5 18.0 18.5

206

19.0 19.5 20.0

2 0 VVb

340

GEOCHEMICAL DATA, SITE 862

Table 4. REE analyses of Site 862 Taitao Ridge samples.

Hole:Core, section:Rock type:

REE (ppm)CeNclSinHuGdI)yl j •

Yb

862CIW-1Basalt

Abs/Norm.

15.2/18.710.7/18.03.29/17.11.19/16.54.06/15.74.76/14.72.88/13 52.56/12.3

862C6-1

BasaltAbs/Norm.

13.0/16.09.40/15.72.90/15.11.00/13.93.67/14 24.26/13.12 56/12 02.35/11.3

862B3X-1Basalt

Abs/Norm.

5.74/7.066.91/11.62.65/13.81.03/14.23 74/14 54.35/13.42 74/12 82.51/12.1

862B4X-2Basalt

Abs/Norm.

14.6/17.910.8/18.13.33/17.31.16/16.14 16/16 14.64/14.32 87/13 52.63/12.7

862B4X-1

RhyoliteAbs/Norm.

39.5/48.614.3/24.02.77/14.4

0.625/8.66261/10082.61/8.041 67/7 861.78/8.56

862C5R-1

RhyoliteAbs/Norm.

35.2/43.214.8/24.83.25/16.9

0.882/12.23 31/12.83.32/10.22 06/9 652.05/9.83

Table 5. Sr, Nd, and Pb isotopic compositions for Site 862 Taitao Ridge samples.

Hole, core,section 'Nd/

l44Nd Pb(ppm) "Pb/-

U4Pb 'Pb/-

U4Pb

4 P b

862C-1W-1 0.703355 ± 8 0.513030 ± 7

862C-6R-1 0.703256 ± 12 0.513031 ± 7

862B-3X-1 0.702863 ±9 0.513186 + 6

862B-4X-2 0.703345 ± 10 0.513054 ± 8

862B-4X-1 0.704140 ±9 0.513115 ±9862C-5R-1 0.704015 ±8 0.512985 ±10

3.582.860.7562.723.60

15.4

18.56818.55518.31418.59018.54918.642

15.60915.59015.56015.60015.60315.633

38.54238.51338.17138.58638.50338.695

enrichment pattern very similar to that of many MORB (Fig. 11). Thatpattern shows a gradual decrease from right to left with few spikesexcept for the enrichment in Rb. This anomaly is unexplained but mayreflect some slight hydrothermal alteration of this Rb-poor sample.

The Group 1 basalt has isotopic compositions that are broadlyconsistent with many MORBs, particularly Sr-Nd isotopic composi-tions. The Group 1 sample does not have Pb isotopic compositionsidentical to those of Nazca Plate basalts. It has lower 206Pb/204Pb buthigher 207Pb/204Pb than the data of Unruh and Tatsumoto (1976). Italso has higher 2O8Pb/204Pb than the Nazca Plate basalts, which lie nearthe NHRL on the 206Pb/204Pb-208Pb/204Pb diagram (Fig. 7b).

Many chemical characteristics of the Group 1 basalts are closelysimilar to those of N-type MORBs. They do, however, deviate in sev-eral particulars. We discussed earlier the lack of strong Fe-enrichmentin Group 1 glasses and the fact that AFM and FeO*/MgO-SiO2 rela-tions follow a calc-alkaline trend but are marginal to a tholeiitic trend.The Pb content and Pb isotopic ratios of the Group 1 basalt also differfrom those of most MORBs and those of the Nazca plate basalts. Bothof these deviations from commonly-observed behavior of MORB maybe the result of introduction of small amounts of subducted sedimentinto the source of Group 1 basalts. The low Pb content of the mantleand its very high content, even with respect to other highly incompat-ible elements, in sediments means that Pb contents and isotopic ratiosare highly sensitive to contamination by sedimentary rocks. Additionof an oxidized hydrous sediment to the mantle, even in small amounts,also can result in production of magmas that evolve differently frommagmas formed from uncontaminated mantle.

We earlier defined Group 5 glasses on the basis of their majorelement chemistry as being intermediate between Group 1 and Group2 glasses. As Group 5 glasses are known only from glass shards, wehave no trace-element or isotopic composition data for these rocks.Group 5 liquids define a trend on an AFM diagram between the trendsof Group 2 and Group 1 liquids. The three groups also form a singletrend on an SiO2-FeO*/MgO diagram (Fig. 10). The available datasuggest that there is a continuum from N-type MORB to arc basaltsin the samples from the Taitao Ridge. Presumably this reflects theamount of sediment incorporated in the source of each magma batchproduced during juxtaposition of the ridge and the trench.

Table 6. Calculated 1 atm liquid lines of descent for possible Leg 141

parental magmas.

ParentalMagma

Major elements (wt%)SiO,Tio;AlAFeo*MnOMgOCaONaiOK^O

ΦsTotal

50.601.00

17.008.000.108.50

11.802.850.050.10

100.00

ParentalMagma

Major elements (wt%)SiO ,

no;A 1 2 0 3

Feo*MnOMgOCaONaiOK^OP 2 O 5

Total

52.750.92

17.306.900.108.20

10.203.150.380.10

100.00

After 4.2%Xtln.a

50.591.04

16.278.360.108.85

11.642.860.050.10

99.86

After 4.2%Xtln.c

52.840.96

16.617.150.108.54

10.003.160.390.10

Group 1

After 16.2% After 30.2% After 46.2%Xtln.h

51.081.18

15.159.070.118.40

11.722.960.060.12

99.85

Group 2

After 16.2%Xtln.1

53.601.09

15.667.710.117.899.943.290.440.12

Xtln.c

51.881.41

13.879.920.127.18

12.123.110.070.14

99.82

After 38.2%Xtln.8

55.611.45

13.648.820.135.69

10.193.570.570.16

Xtln.d

52.181.75

13.0911.550.146.141.263.430.080.18

99.80

After 54.2%Xtln. h

56.691.87

12.8910.380.144.428.543.940.740.21

a 4.2% crystallization of An7 5_7 6 (onset of olivine crystallization immediately follows).h an additional 8.8% crystallization of An73_7S and 3.2% crystallization of Fo 8 7 _ 8 8 .c an additional 9.2% crystallization of An69_73 and 4.8% crystallization of Fo 8 4 _ 8 7

(onset of CPX crystallization immediately follows).l l an additional 8.0% crystallization of An 6 ‰ 6 8, 1.5% crystallization of Fo8 ( )_8 4, and

6.5% crystallization of CPX.c 4.2% crystallization of An7 3_7 4 (onset of olivine crystallization immediately follows).' an additional 8.5% crystallization of An7]_73 and 3.5% crystallization of Fo8 8_9 ( ).s an additional 16.0% crystallization of An6 2_7 0 and 6.0% crystallization of F o 8 V 8 8

(onset of CPX crystallization immediately follows).h an additional 8.0% crystallization of A n ? V 6 2 , 0.8% crystallization of Fo77_83 and

7.2% crystallization of CPX.

341

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

Samples Fields

Group 1Group 2Group 3Group 4Group 5Group 6

A

Δ

O

mD

LLDsCalculated ratio of miner-als; plagioclase-only inter-val removed

Assume olivine:plagioclase= 1. Group 2 only.

MgO MgO

Figure 8. MgO variation diagrams of major oxides showing approximated liquid lines of descent for Group 1 and 2 glasses without an early plagioclase-only phase

of crystallization. Also shown is the alternative non-cotectic liquid of descent for a 50:50 olivine-plagioclase crystallization sequence. See text for further discussion.

All values are in weight percent (wt%).

Petrogenesis of Silicic Lavas

On the basis of prior studies of silicic magmatism at mid-oceanridges and the data on the Site 862 samples, four models for the originof the Site 862 rhyolites are evaluated. These models include thefollowing: (1) the rhyolites formed by extreme differentiation ofbasaltic magmas; (2) the rhyolites were produced by liquid immisci-bility with basaltic magmas; (3) the rhyolites result from anatexis ofaltered oceanic crust; and (4) the rhyolites were melts of material thatwas sedimentary or of continental crust origin.

The Nd, Sr, and Pb isotopic differences between the rhyolites andGroup 1 basalts clearly preclude any genetic relationship betweenthese rocks by differentiation, liquid immiscibility, or partial melting.It is also possible to preclude any genetic relationship between theGroup 2 basalts and the rhyolites by extreme differentiation or liquidimmiscibility, principally because of the significantly higher 87Sr/86Srin the rhyolites.

The rhyolites plausibly formed by partial melting of altered oce-anic crust similar to the Group 2 basalts. Isotopically, the only majordifference between the Group 2 basalts and the rhyolites is the higher

342

GEOCHEMICAL DATA, SITE 862

Table 7. Non-cotectic crystallization of olivine in Group 2 parent.

Parent

Major element (wt%)SiO,TiO",A1,0,Feo*MnOMgOCuONa2OK,O

PATotalF of Min.

52.750.92

17.306.900.108.20

10.203.150.380.10

100.00

Olivine

40.380.060.00

11.860.17

47.070.230.000.000.00

99.770.50

Plagioclase

50.440.00

31.780.370.000.31

13.723.300.090.00

100.010.50

Bulk

45.410.03

15.896.120.09

23.696.981.650.000.05

99.890.10

Res. liquid

53.571.02

17.466.990.106.48

10.563.320.420.1 1

100.010.90

87Sr/86Sr in the rhyolites. If oceanic crust of Group 2 compositionwere hydrothermally altered by interaction with seawater, the87Sr/86Sr of the altered oceanic crust would be increased, whereas theNd and Pb isotopic ratios would not change significantly.

Alternatively, the higher 87Sr/86Sr of the rhyolites could reflectincorporation of continental crustal material in the rhyolite sources.The similarity between Pb isotopic ratios of the Group 2 basalts andthe rhyolites indicates that the source of the latter contained littleold continental material as old crust has elevated 207Pb/204Pb and208Pb/204Pb due to high early in the history of the crust and to thehigher Th/U of the crust.

A third possibility is that the elevated 87Sr/86Sr of the rhyolitesreflects in situ growth of radiogenic Sr. Groundmass-rich rhyoliteshave 100 ppm Rb and 110-120 ppm Sr. An upper limit on the87Rb/86Sr of the source of the rhyolites of 2.35 is calculated assumingthat Rb and Sr were not fractionated from each other during anatexis.If the sources formed 23 Ma ago (early Miocene), the sources had(87Sr/86Sr)i 0.7033. Fractionation of Rb from Sr during anatexis—expected for a feldspar-bearing residue—means that the source hadlower Rb/Sr and higher (87Sr/86Sr);. In situ growth can explain some,but not all, of the difference in 87Sr/86Sr between the rhyolites and theGroup 2 basalts.

Trace-element data are also suggestive of a Group 2 basalt-likesource for the rhyolites. Figure 13 is a spidergram ordered in the sameway as Figure 11. The field for the Group 2 basalts is shown shaded.The general pattern of the rhyolites on the left side of this diagram issimilar to, although at higher contents than, the pattern for the Group2 basalts. In particular, the rhyolites are highly enriched in Rb and K,have large negative Nb spikes and are somewhat depleted in Barelative to the alkalis. They also reflect the lesser enrichment in LREEthan K exhibited by the Group 2 basalts. Sample 141-862C-5R-1,11-18 cm, is also highly enriched in Pb, but Sample 141-862B-4X-1,21-28 cm, has a similar Pb content to those of the Group 2 basalts.Given the otherwise similar patterns of the two rhyolites, this plausi-bly reflects retention of a Pb-concentrating phase such as sulfide inthe residue of Sample 141-862C-4X-1, 11-18 cm. Zirconium is en-riched in the rhyolites relative to Group 2 basalts whereas Ti and theHREE are depleted.

The similarities between the patterns of the rhyolites and theGroup 2 basalts from Rb to Ce suggests that, although 87Sr/86Sr of therhyolite source may have been altered by hydrothermal alteration, thetrace-element pattern was essentially unchanged by this alteration.This permits estimation of the amount of melting of the source fromthe enrichments of incompatible elements in the rhyolites over thosein the basalts. Rb is the most concentrated from the basalts into therhyolites requiring 10% partial melting if absolutely incompatible inthe residue. K2O and Ba indicate 18% and 16% partial melting,requiring that small amounts of K2O and Ba remain in the residue(0.23% K2O; 20 ppm Ba suggesting bulk D's of 0.08 and 0.06 respec-tively). Felsic liquids present in amounts much less than 10% areunlikely to escape from their residues (McKenzie, 1985).

D East GalapagosRift glasses

Ecuador Rift &| Inca Transform

samples

F M60 40

M

F M30 70

Figure 9. Conventional AFM diagram for mafic glasses analyzed by electronmicroprobe. A = Na2O + K2O; F = FeO (total); M = MgO, all in wt%. The linedividing the fields of tholeiitic (Th) and calc-alkaline (CA) suites is from Irvineand Baragar (1971). The fields of glasses and fused whole rocks from theEcuador Rift and Inca Transform and of glasses from the East Galapagos Riftare from Fornari et al. (1983). The area containing all the Site 862 samples isenlarged in the lower part of the diagram.

The rhyolites contain corroded phenocrysts that are plausiblyrestitic. An altered mafic source, heated to 700C contains Plagioclase,amphibole, biotite, and quartz. Such an assemblage produces a felsicliquid at temperatures 900°C (P = 0.1 GPa) to 850°C (0.7 GPa) witha residue of Plagioclase and hornblende (Beard and Lofgren, 1991).

Rare-earth element patterns of the rhyolites provide constraints onthe residual mineral assemblage. The similarity of the REE patternsof the rhyolites and that of average upper crust is noteworthy. Averageupper crust (as reflected by average Proterozoic shale) is LREE-enriched (CeN/YbN = 6 to 7), has flat middle to heavy REE, and has anegative Eu-anomaly (Eu/Eu* = 0.7). The rhyolites have lower REEcontents than upper crust (CeN 40 versus 80; YbN 7 to 8 versus 12) buttheir REE patterns have a similar shape. The rhyolites have CeN/YbN

= 4.5, 5.8, flat middle to heavy REE, and Eu/Eu* = 0.7, 0.8. Thisreflects a similar origin, on a small scale, of the rhyolites and theorigin of upper crust (i.e., alteration of basalt, with or without sedi-mentation) and anatexis of the resultant rock.

Group 2 basalts have CeN15, whereas rhyolites have CeN of 43-49; an enrichment factor of three. The HREE contents of the rhyolitesare less than those of the Group 2 basalts and the rhyolites (unlike thebasalts) have negative Eu-anomalies. Formation of the rhyolites byanatexis of rocks derived from Group 2 basalts requires, therefore,that minerals in the residue concentrated HREE and Eu over the liquid

343

R.D. FORSYTHE. J.K. MEEN, J.F. BENDER, D. ELTHON

oU)

o<D

LJL

o

*o

Figure 10. SiO2 against FeO*/MgO for glasses analyzedby electron microprobe from Site 862. A. Samples from allsix groups of mafic glasses, compared with the line sepa-rating tholeiitic (Th) and calc-alkaline (CA) fields (afterMiyashiro, 1974). B. Fields of Groups 1, 2, and 5 glassescompared with those of samples from the Ecuador Rift andInca Transform and of glasses from the East GalapagosRift. (Fields from Fornari et al., 1983.)

East GalapagosRift glasses

Ecuador Rift &Inca Transformsamples

Gp 1 Gp 5 Gp2

50 52 54

SiO,56 58

but excluded LREE. The required minerals are present as phenocrystsin the rhyolites and, if these are restite, in the residue of melting.Plagioclase and hornblende concentrate Eu and MREE/HREE overLREE with respect to felsic liquids, respectively.

We used partition coefficients for REE between Plagioclase andhornblende versus dacitic liquid tabulated by Arth (1976) to calculatemodel liquids. The REE pattern of a model felsic liquid is comparedon Figure 14 with the more LREE-enriched rhyolite and the averageGroup 2 basalt. This models 10% partial melting of a source similarto the average Group 2 basalt with a residue containing 75% plagio-clase and 25% hornblende. Although the modeled pattern is lessLREE-enriched than the rhyolite and has a larger negative Eu-anom-aly, the model and the rhyolite have broadly similar REE patterns.

Three factors may explain the differences between model anddata. First, assumption of a bimineralic residue is an oversimplifica-tion and minor minerals can affect the REE pattern of a coexistingliquid without much changing the major-element contents of thatliquid. Second, the source could have had a somewhat different REE

pattern from that used in the model either because the igneous rockswere more evolved (e.g., andesite) or because there were slight vari-ations in REE patterns between different Group 2 basalts. Rhyolitesonly form from a source that contains some quartz and potassic min-erals, not average crust. Finally, partition coefficients of REE are verydependent upon bulk composition in felsic magmas (compare num-bers given by Arth, 1976, and Fujimaki et al., 1984, for andesitic-rhyolitic rocks).

The interpreted assemblage in the residue of the felsic liquids doesnot place rigorous constraints on the pressure of melting. The assem-blage hornblende-plagioclase-silicic liquid is stable for a mafic bulkassemblage at 0.05 to 2.3 GPa (discussed by Burnham, 1979). Allenet al. (1975) and Allen and Boettcher (1983) show that garnet is astable supersolidus phase in hydrous tholeiitic basaltic systems atPI .5 GPa at T = 900°C and slightly lower pressure in andesitic bulkcompositions. Partition coefficients of REE between garnet and liq-uid increase markedly from LREE to HREE, particularly from Sm toEr. REE patterns of rhyolites are flat in that region. If the rhyloties

GEOCHEMICAL DATA, SITE 862

30

10-

α>

LU

1.0-

0.5

C1W01B4X02C6R01B3X01

N-type MORBE-type MORB

Rb Ba Nb K Ce Pb Sr P Nd Sm Zr Eu Ti Gd Y Yb

Figure 11. Enrichment patterns for trace elements in threeGroup 2 basalts and one Group 1 basalt. Contents arenormalized to primordial mantle (P.M.) values of Sun andMcDonough (1989) using 0.185 ppm for Pb. Data arefrom this paper and from Behrmann, Lewis, Musgrave, etal. (1992). The E-type and N-type MORB patterns fromSun and McDonough (1989) are reproduced for compari-son. Samples keyed as in Figure 5A.

N

O

O RhyolitesΦ Group 2G Group 1

Ba/Zr

formed from a source with a flat REE pattern (e.g., a Group 2 basalt),bulk solid-liquid D's for Sm to Yb were similar and the residuecontained little, if any, garnet (although it may have been in thesource). The rhyolites presumably formed at PI.5 GPa.

Geochemistry and Tectonics of the Taitao Ridge

The primary magmas of Group 1 glasses probably formed in amanner similar to that inferred for other N-type MORBs: polybaric(1-3 GPa) fractional melting of the mantle produces a diverse rangeof primary magmas that are mixed together during ascent or withinmagma chambers (Johnson et al., 1990; Salters and Hart, 1989;Kinzler and Grove, 1992a, b). The Group 1 glasses plot significantlyoff the global array of Klein and Langmuir (1987). They have muchlower Fe8 0 and Na8 0 than the array has for the depth of the TaitaoRidge (Figs. 15a and b). The Group 1 glasses may have been on theglobal array when they formed if the present shallow depth of the

Figure 12. Ba/Zr-Ce/Zr relations for one Group 1 basalt,three Group 2 basalts, and two rhyolites. Fields of N-typeMORB, E-type MORB, ocean island basalts (OIB) andtwo fields of island arc basalts are from Saunders andTarney(1991).

ridge is a tectonic anomaly rather than an original feature. Alterna-tively, the Group 1 glasses may not have been on the array. Wediscussed above the unusual chemistry of the Group 1 glasses, achemistry that may have included different Na8 0 - Fe8 0 relationships.Figure 15C shows the global array with the near axial depth removedas a factor so Fe8 0 is plotted against Na8 0. The Group 1 glasses lie onthis global array, consistent with their dominantly MORB-like chem-istry. Group 2 glasses have lower Fe8 0 than the global array and havehigh Na8 0. Given our interpretation of the Group 2 glasses as calc-alkaline, this deviation from the global array is not surprising. TheGroup 5 glasses are intermediate between the Group 1 and Group 2glasses on Figure 15.

The Na 8 O -Fe 8 O systematics of the Group 1 glasses probablyclosely reflect the conditions of mantle melting of the primary liquidsand were little affected by contamination in the source. The same isnot likely to be true for Group 5 or Group 2 glasses. If the Group 1glasses originally lay on the global arrays of mean axial depth against

345

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

200

100-

Figure 13. Enrichment patterns for trace elements in tworhyolites. Contents are normalized to primordial mantle(RM.) values of Sun and McDonough (1989) as in Figure11. The field of Group 2 basalts from Figure 11 is shownshaded. Samples keyed as in Figure 5B. Rb Ba Nb K Ce Pb Sr P Nd Sm Zr Eu Ti Gd Y Yb

100

α>T3Co

üUJLU

10:

Figure 14. Interpretative REE pattern for rhyolites. Thepattern for the average Group 2 basalt (from Fig. 5A) andfor the more enriched rhyolite (from Fig. 5B) are shown.Also shown is the calculated pattern that one would get ina 10% melt of the average Group 2 basalt if the residuecontained 75% Plagioclase and 25% hornblende, usingpartition coefficients of Arth (1976) for dacitic liquids.

B4X01

Av. Gp 2 basalt

10% melt ofGp 2 basalt.Residue=75% plag,25% hbl

Cθ Nd Sm Eu Gd Dy Er Yb

Na8 0 and Fe8 0, the Taitao Ridge must have been tectonically uplifted2000 m and, possibly, as much as 4000 m (Fig. 15A).

CONCLUSIONS

1. Glass shards in sediments and glass rinds on basalt flows defineseven compositional groups. Group 1 samples are depleted in LREEand other LILE. The compositional variations of this suite are expli-cable by low-pressure fractional crystallization of olivine and plagio-clase in "normal" (1:2) cotectic proportions. This suite does notexhibit significant Fe-enrichment, unlike most MORB suites. Thetrace-element and isotopic compositions of one Group 1 basalt arebroadly similar to those of N-type MORBs. A minor enrichment in Rbpossibly reflects post-magmatic processes. An enrichment in Pb(Ce/Pb = 7.6 compared to 25 in MORB) and the shift in 207Pb/204Pband 208Pb/204Pb to values above the NHRL at the same 206Pb/204Pb are

consistent with addition of a small amount of subducted sediment topart of the source of the Group 1 magmas. Addition of small amountsof oxidized and hydrated sedimentary material to the mantle mayresult in the magmas derived from it following a liquid line of descentnearer to those of calc-alkaline suites.

2. Group 2 samples are enriched in LREE and some LILE. Themost incompatible elements are variably enriched. K, Rb, and Pb arehighly enriched relative to LREE whereas Nb is depleted and Ba ismoderately depleted. The low values of Ce/Pb (4-5), K/Pb (1170-1650), and Nb/Pb (=1 of Group 2 basalts compared to their valuesin most MORB (25, 2000, and 7.8) require addition of material en-riched in Pb and alkalis to the mantle source of the Group 2 samples.

Isotopic compositions of Group 2 samples also differ from thoseof many MORBs. Group 2 basalts have higher 87Sr/86Sr and lower143Nd/144Nd than the Group 1 basalt. They lie in the field of composi-tions from the Taitao ophiolite (Kaeding et al., 1990). Pb isotopic

346

GEOCHEMICAL DATA, SITE 862

compositions also differ from those of most MORBs and are similarto those of arc volcanoes on the South American mainland and thoseof sediments of the Nazca Plate.

The trend of Group 2 glasses is calc-alkaline with no Fe-enrich-ment. Compositional variations are consistent with fractional crystal-lization of olivine and Plagioclase but approximately equal amount ofeach was removed. The apparent lack of augitic clinopyroxene fromthe separating assemblage means that fractional crystallization was atlow pressure.

3. Group 5 glasses are intermediate between Group 1 and Group2 glasses in major-element compositions. The liquid line of descentfor the Group 5 glasses is also intermediate between those defined byGroups 1 and 2. We consider Group 1 samples to have formed frommantle containing a small amount of a sediment-like component;Group 2 samples from a mantle with a much larger amount of thiscomponent. The existence of Group 5 suggests that there is a contin-uum from MORB to calc-alkaline basalts.

The other groups of mafic glasses (3, 4, 6) cannot be relateddirectly to Groups 1, 2, or 5 by fractional crystallization. There was,therefore, a wide range of different primary liquids produced beneaththe Taitao Ridge. Presumably this reflects the variable mantle sourcestapped during interaction of ridge and trench.

4. Parental magmas for the Group 1 glasses appear to have beenproduced by melting processes similar to those for other N-typeMORBs, typically involving polybaric fractional melting of ascend-ing mantle at 1-3 GPa followed by mixing of the various fractions.The Group 1 glasses lie significantly off the MORB global array ofKlein and Langmuir (1987) when plotted against mean axial depth,but lie on the Fe8 0 versus Na80 array, possibly suggesting that thepresent shallow depth is a tectonic anomaly rather than an originalmagmatic feature.

5. The silicic igneous rocks associated with the mafic glassesformed by remelting rocks derived from igneous rocks similar toGroup 2 samples. The Pb isotopic ratios of the rhyolites and Group 2basalts are identical. The rhyolites do, however, have higher 87Sr/86Srthan the Group 2 basalts. The elevation of 87Sr/86Sr is interpreted toreflect interaction of the source rocks with seawater-derived fluids.One rhyolite has l43Nd/I44Nd identical to that of the Group 2 basaltsbut the other has much higher 143Nd/144Nd.

Trace-element enrichments in the rhyolites are consistent withformation from Group 2 basalts. In particular, Rb and K are greatlyenriched over LREE, and Ba and Nb are depleted just as in Group 2basalts. The relative Rb contents suggest that the rhyolites formed by10% melting of Group 2 samples. REE patterns of the rhyolites areconsistent with 10% partial melting to leave a residue of Plagioclaseand hornblende. These minerals occur as phenocrysts in the silicicrocks and are interpreted as restite.

The Group 2 rocks collected in this study could not melt to rhy-olites because they are essentially anhydrous and because they areolivine-normative. Gabbroic equivalents of such rocks are, therefore,quartz-free. Interaction of such rocks with hydrothermal fluids canproduce plagioclase-hornblende-quartz-biotite rocks that melt to rhy-olite liquids at 800°C. The retention of the distinctive incompatible-element pattern of the Group 2 basalts in the rhyolites shows thathydrothermal alteration was not intense. It may have been limited toaddition of water (and radiogenic Sr) to the Group 2 basalts.

ACKNOWLEDGMENTS

The microprobe lab at U.H. is supported by the State of Texasthrough the Texas Center for Superconductivity at the University ofHouston. The isotope analyses were obtained through the generosityof Paul Fullagar and the Department of Geology at the University ofNorth Carolina at Chapel Hill. JKM thanks Jonathan Miller, BrianBeard, and S. Goldberg for assistance in the lab.

Global array of Kleinand Langmuir

Global array of Kleinnd Langmuir

0 1000 2000 3000 4000 5000 6000

mean axial depth (m)

4.0

3.5

3.0

2.5

2.0

1.5

.Global array of Kleinand Langmuir

9Fe

10 11 12

8.0

Figure 15. Plots of mean axial depth in meters against (A) Na8 0 and (B) Fe8 0

for Groups 1, 2, and 5 glasses from Suite 862. These are the contents of Na2Oand FeO* (in wt%) at MgO = 8% and were read directly off MgO-FeO andMgO-Na2O diagrams (Fig. 4). The global array is from Klein and Langmuir(1987). All three groups lie far off the global array. If Group 1 glasses were onthe global arrays on Figure 14A when formed, the Taitao Ridge has beentectonically uplifted 200CM000 m. C. Plot of Fe8 0 against Na8 0 showing theposition of glasses from Groups 1, 2, and 5 compared with the global array ofKlein and Langmuir (1987). The Group 1 basalt, interpreted here as an N-typeMORB, lies on the global array, whereas the Group 2 basalt, interpreted hereas a calc-alkaline basalt, lies at too low a value of Fe8 0. The Group 5 basalt isintermediate between these two.

347

R.D. FORSYTHE, J.K. MEEN, J.F. BENDER, D. ELTHON

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Abbreviations for names of organizations and publications in ODP reference lists followthe style given in Chemical Abstracts Service Source Index (published by AmericanChemical Society).

Date of initial receipt: 1 October 1993Date of acceptance: 4 October 1994Ms 141SR-030