An Evaluation of Temporal Geochemical Evolution of the Lo`ihi Summit Lavas: Results from ALVIN ...

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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B1, PAGES 537-550, JANUARY 10, 1993
An Evaluationo f TemporalG eochemicalE volution of Loihi Summit Lavas'
Results From Alvin Submersible Dives
MICHAEL O. GARCIA, BETH A. JORGENSON,A ND JOHN J. MAHONEY
Departmenot f Geologya nd GeophysicsU,n iversitoyf Hawaii at Manoa, Honohdu
EMI ITO
Departmereo f Geologya nd GeophysicUs,n iversitoyf Minnesotaa t Minneapolis
ANTHONY J. IRVING
Departmenot f GeologicaSl ciencesU, niversitoyf WashingtonS,e attle
Stratigraphicallyc ontrolleds equenceso f in situ lavasw ere collectedf rom Loihi Seamountu singt he
Alvb7s ubmersibleto evaluatet he volcano'ste mporalg eochemicael volution. Three sectionsw ith up to
370 m of reliefw eres ampledfr om the twop it cratersa t the summito f Loihi. All of the analysews ere done
on glasss eparates.O ur resultsi ndicatet hat tholeiitica nd alkalic volcanisma t the summito f Loihi has
beenc oeval.T he tholeiitica nd a!kalicl avash aves imilari ncompatiblee lementp atternsa nd O, Pb, Sr, and
Nd isotoper atiosb ut are distincti n somei ncompatiblee lementr atios. Theser esultsa re consistenwt ith
the different Loihi rock typesb eingd erivedb y variabled egreeso f meltingf rom a commons ource. The
crossinga nd light-rare-earth-enrichedr are earth element patterns and variable Sc/Yb ratios of the
tholeiiteinsd icatteh att heirs ourcwea sa garnelht erzoliteT.h er elativelloyw < 5(cid:127)80v alue(s-- -4.9% o)f or
Loihi lavasa re interpretedt o be characteristioc f the Hawaiianp lume.
INTRODUCTION palagoniter im growth rates are affected by many factors,
includingv esicularitya nd compositiono f the glassw, hichv ary
Loihi Seamount is the southernmost volcano in the Hawaiian stronglya mongt he Loihi lavas( e.g.,v esicularitvya riesf rom < 1
chain. It is located 35 km south of the island of Hawaii on the to > 50 vol %) [Mooree t al., 1982].
flanks of Mauna Loa and Kilauea, two active tholeiitic shield This study was initiated to further evaluatet he temporal
volcanoesL. oihi wasd iscoverebdy E mery[ 1955],w ho gavei t evolutiono f Loihi's lavas. Determining the relative age of its
the Hawaiiann amem eaningt he longo ne in recognitiono f its alkalica nd tholeiiticl avasi s essentiatlo decipheringth e melting
elongates hape (Figure 1). It was assumedt o be an old history[e .g.R, ibS,1 988]a ndg eochemiceavlo lutioonf Loihi,
seamountu nrelatedt o the Hawaiianh otspot[ Moorea nd Fiske, whichm ay exemplifyt he earlys tageo f growtho f all Hawaiian
1969] until it had two major earthquakes warmsin 1971-1972 volcanoes[ Clague, 1987]. The temporal evolution in the
and 1975 [K(cid:127)ein, 1982]. A dredgeh aul on Loihi in 1978 geochemistroyf lavasf rom a volcanoi s bestd eterminedu sing
recoveredg lassyla vas,c onfirmingt hat it is an activev olcano. stratigraphicallcyo ntrolleds equenceso f lava. Loihi has two
The rocks obtained in the initial dredge haul are tholeiitic summitp it cratersw ith over 300 m of relief( Figure 2). Three
[Moore et al., 1979], but subsequendt redging in 1981 sequenceso f in situ samplesw ere collectedf rom the walls of
recovereda wide diversityo f rock typesr angingf rom tholeiite these pit craters using the Alvin submersible. This paper
to basanitoid[M ooree t al., 1982]. presentst he resultsf rom a petrologics tudyo f theses amples.
The discoveryo f alkalic lavas on Loihi fundamentally Our results indicate that alkalic and tholeiitic volcanism have
changed models for hotspot volcanism [Clague, 1987]. been coeval at the summit of Loihi and that these lavas are
Previousm odelsf or Hawaiianh otspotv olcanisma ssumedth at probablyd erived from a commonm antle sourceb y variable
each volcanow as composeda lmost entirely of tholeiiticl avas degreeso f partialm elting. The sourceis relativelyu ndepleted
(95-99%) with a thin veneero f alkalicl avasp roducedd uring (comparetdo MORB) andh asm oderate8 7Sr(cid:127)Sr (0.70355)
the waning stageso f shield developmenta nd posterosional and2 ø6pbflø4(p18b. 2-18.5a) ndl ow6 (cid:127)80 (4.9) isotopivca lues,
volcanism[M acdonaM,1 968]. The age of the dredgedL oihi which may be representativeo f the plume componentf or
alkalica nd tholeiiticla vasw as estimatedu singp alagoniteri m Hawaiianm agmas. The chemicacl ompositiono f lavasi n the
thicknessesT.h e alkalicl avasw eref oundt o haves ystematicallys tratigraphics equencesi s quite variable even among the
thickerr imst han the tholeiiticla vasb ut with broado verlap( 1- tholeiites,w hich indicatest hat magma bodiesi n Loihi have
12l (cid:127)m versus0 .5-4l (cid:127)m) [Mooree t al., 1982]. Usingp alagonite been short-lived.
growthr ates for mid-oceanr idge basalts( MORB), average
ageso f 1.6 ka and 0.5 ka were inferredf or the Loilaia lkalica nd REGIONAL GEOLOGY
tholeiiticla vasr, espectivel[yM ooree t al., 1982]. Unfortunately,
The morphologya nd structureo f Loihi haveb eend elineated
Copyright1 993b y the AmericanG eophysicaUl nion. in severalr ecent studies[ Moore et al., 1982; Malahoff et al.,
1982;F ornari et aI., 1988]. Loihi has two prominentr ift zones
Papern umber9 2JB01707. extendingn orth and southf rom the summit;b oth curvet o the
0148-0227/93/9(cid:127)B-01707505.00 east( Figure 1). The southr ift is 19 km long. The northr ift is
537
538 GARCIA ET AL.: TEMPORAL GEOCHEMICALE VOLUTION OF LOIHI SUMMIT LAVA
19 westp it (Figure2 ). Only in situ samplesw ere collectedu sing
05' the submersible's mechanical arm. The suite from the north
side of the east pit includes2 2 glassyla vasc ollectedf rom a
water depthr angeo f 987 to 1316m (dive 1801;F igures2 and
3). The baseo f the southeaspt ortiono f the eastp it is mantied
with talus. Fifteens amplesw ere collectedfr om depthsb etween
977 and 1277m , includinga traverseto a coneo n the northeast
edgeo f thes ummitp latform( dive1 803). The wests ideo f the
west pit is mantiedw ith talus at its base. Five- to ten-meter-
19 ø long rock columnsl ie at the base of the crater wall. Four
00' samplesw erec ollectedn ear the base( 1275-1291m ), but a 190-
m sectiono f the crateri s coveredw ith talus. Ten samplews ere
collectedb etween depthso f 986 and 1084 m, and 13 other
samplesw ere collecteda longt he northernr im of the pit crater
(dive 1802;s eeF igure2 ). Someo f the samplesfr om the upper
portion of the west pit lack glassyr ims. A traversew as also
made alongt he axiso f the upperp ortiono f the southr ift, and
20 samplesw ere collectedb etweenw ater depthso f 966 and
1330m (dive 1804;F igure2 ). Hydrothermavl entsw eref ound
1t3ø
55' on the flankso f twoc onesa longt hist raverse[K arle ta l., 1988].
The thicknesseosf the stratigraphisce ctionsre coveredfr om
Loihi'sp it cratersa re greatert han thosee xposedin the wallso f
Kilaueac aldera( Figure3 ). The age of the Kilaueas ectionis
about 2.8 ka [Casadevalal nd Dzurisin, 1987]. The Loihi
sequenceis probablyo lder than the Kilaueac alderas equence
giveni ts greatert hickness(u p to 370 m versus1 35 m) and less
18 ø
50'
WES T P/T
1805
LOIHI \
18
45' SEAMOUNT
I I I
15520' 155':'15' 155 I0'
Fig. 1. Bathymetric map of Loihi Seamount;s implifiedf rom tile Sea
Beam map of Fornari et al. [1988]. Contoursa re in meters. Dashed
line is an estimate of the location of the contact between Loihi and
Kilauea (stippled)l avas. The dashed-dottedli ne indicatest he axiso f
the main north and south rift zones. Box indicates the location of
Figure2 .
shorter( -8 km) but consistosf two parallelp rongsth at extend
out from the summitp latform. The westernp rongi s the most
prominent. Loihi's summitc onsistso f a broad platformw ith
some prominent cones along its margin and two deep pit
craters( Figure 2). The pit cratersa re steep-walle(ds lopeso f
25ø-42ø)a, nd both are elongatet o the NW. They may be
composite features formed by several collapse events
(especiallyth e easternp it). The westernp it crater has about
300 m of relief and is surroundedb y a ridgew ith cones,e xcept
on the southeasst idew here it is truncatedb y the easternp it
crater. This truncationi ndicatest hat the westernp it crater is
probablyo lder[ Fornarie t al., 1988]. The easternp it craterh as I
about 370 m of relief on its NW sideb ut only about 170 m of SOL/TH t(cid:127)IFT
reliefo n itse asts ide. The wallso f thesep it cratersa re excellent
sitesf or collectings tratigraphicalclyo ntrolleds equenceosf lava. Fig. 2. Bathymetric map of the southerns ummit area and upper
portiono f the southr ift of Loihi Seamounbt asedo n Sea Beamd ata
STRATIG(cid:127)PHIC SECTIONS [after Fornari et al., 1988]. Heavy lines showt he dive tracksf or the
fourAlvin submersibldei ves( 1801-1804). Tick marksa longt he dive
trackss hows amplel ocations.N umbersn extt o ticksr efer to specific
Three sectionsw ere sampled in traversesu p the walls of
samples. Trianglesi ndicatea lkalic samples.C ontour intervali s 100
Loihi'sp it craters: two sectionsin the eastp it and one in the
m.
GARCIAE TA L.: TEMPORAGL EOCHEMICAELV OLUTIOONF L OIHIS UMMITL AVA 539
/OIHI SUMMIT KILAUEA All the samplesin this studyc ontaino livinep henocrysts
WEST CRATER EAST CRATER UWEKAHUNA (>0.5 mm diameter)or microphenocry(s0t.s1 -0.5m m),
NORTH SOUTHEAST althoughth eya re rare( <0.1-0.2v ol %) in the evolveda lkalic
lavas(g lasMs gOc ontentosf 4.6-5.5w t %; Table1 ). Olivineis
2 4 --02 abundan(t- 32 vol %) in twot holeiitisca mple(s1 804-10a nd
IOOO-- 1804-11). Most olivineg rainsa re euhedralb, ut manys how
0.8 resorptionfe atures. Clinopyroxenise p resenitn all of the
samplews ith glassM gO contents< 7.0 wt % but is absenot r
rare( <0.1-0.2v ol %) in samplewsi thM gO content>s7 .0 wt
% (as is typicaol f Hawaiiant holelites[)e .g.,W right,1 971].
Clinopyroxenem icrophenocrysatsn d phenocrysts are
commonly euhedraland stronglzyo ned;s ome grainsh ave
2 IKo
sectorz oningo r overgrowthosn r esorbecdo res.P lagioclasise
LLJ rare in allg lassews ith MgO contentasb ove5 .5 wt % and
absenitn glassewsi th MgO contentasb ove7 .0 wt %. Where
presentp, lagioclasise e uhedrawl, eaklzyo neda nds mal(l< 0.5
mm).
12OO --
ANALYTICAL METHODS
Glass was used exclusivelyfo r all chemicala nd isotopic
analysetso avoidt he effectso f seawatecr ontaminatioann d
accumulationo f mineralss o that a representativem agmatic
1:5OO --
liquidc ompositiocno uldb e determinedG. lassw asc arefully
handpickedu sing a binocularm icroscope. Grains with
adheringm ineralso r any sign of alterationw ere rejected.
1802 1801 180;5 Majore lementws ered etermineuds inga wavelength-dispersive
electronm icroprobaet the Universitoyf Hawaii.T he reported
Fig.3 . Schematsice ctionfso r the threet raverseusp thew allso f
Loihi'sw esta nde astp it craters(d ives1 801-1803).S amplelo cations values( Table1 ) are averageosf sevento 10 spota nalysepse r
are notedb y the tick markso n the columns.L inesi n the sections sampleu singa 25¾tmb eams ize,1 0- to 12-nAs amplec urrent,
separatceh emicaldlyi stincrto ckt ypes.T he alkalicro cksa res hown and2 0-sc ountingti mef or eache lemen(te xcepPt andM n with
byt hes tippledp attern.P ortionosf thes ectioncso veredb y talusa re 50-sc ountingti mes).R elativea nalyticaplr ecisioins 1-2%f or
indicatebdy t her ock.p'va ttern.T het opo f thes outheasset ctionfr om majore lementsa nd5 -10% for minore lements(i .e., <1.0 wt
thee astc rater(d ive1 803)is j usta boves ampl1e1 . Theo thers amples
%).
were collecteda longt lle easternr im of the summitp latform( see
Abundanceos f rare earth elements(R EE; La, Nd, Sm, Eu,
Figure2 ). All sectionasr e shownin metersb elows eal evel. For
comparisotnh,e K ilaueac aldersae ctioant UwekahunBal uffi ss hown Gd,D y, Ho, Yb, andL u), K, Co, Cr, Mn, Cu,N i, Sc,S r,V , Y,
at thes ames cale( summarizefrdo mC asadevaalnl dD zurisin[1 987]). Zn, and Zr abundance(sT able 2) were determinedo n 23
Thea geso f theK ilaueatc phrau nits(i n thousandosfy earsa) res hown samplebs y inductivelcyo upledp lasmae, missiosnp ectrometry
[fromC asadevaalnl dD z(cid:127)trisin,1 987]. (ICP-ES) at the Universityo f Washingtouns ingm ethods
similatro thosed escribbedyX uee ta l. [1990]A. pproximately
750m go f handpickegdla ssc hipsfr ome achs amplwe erer insed
frequenet ruptivea ctivity(b asedo n earthquakaec tivity[)K lein, in aleionizedw ater, then digestedin a mixtureo f concentrated
19821. HF-HCIOq-HNO3. After evaporation,d issolutiono f the
resultingsa ltsi n diluteH CI produceda completes olutionfo r
PETROGRAPHY mosto f the samplesR. esiduaCl r-spineiln severasl amplews as
filtered out, fusedw ith NaOH, dissolvedin dilute HCI, and
Dredged lavas from Loihi have been described combinedw ith the flitrate. All analysews ere done on a Baird
petrographicailnly s everasl tudies[e .g.M, ooree ta l., 1982;F rey PS-1,4 7-channesl pectrometeurs ingb oths ynthetihci gh-purity
andC lague1, 983;H awkinsa ndM elchior1, 983]. We observed solutionsa nd concurrentlyp reparedn aturalr ock solutionsa s
and collectedb oth pillowl avasa nd sheetf lows. Sheetf lows standards.R eportedR EE abundancewse re correctedu sing
were foundo nlyn ear ventsa longt he southr ift zonet raverse. isotoped ilutiond ata for ColumbiaR iver basaltA RHCO-1;
Their lateral extent is unknown. There are no chemical othert racee lementa bundancews erec orrecteda gainsKt ilauea
differencesb etween the different flow morphologiesb, ut the basaltB HVO-I. Analyticapl recisionis 2-5% relativef or all
vesicularitoyf the sheetf lows amplesis higher. elementse xcepHt o (10%).
Vesicularityo f our samplesv ariesg reatly( 0.1-43 vol %). In addition,a ll of the lanthanideR EE, Rb, Cs, Ba, Th, Nb,
The alkalicl avash ave consistentlhyi gh vesicularityw, hich is Y, Hf, Ta, and U were determinedb y inductivelyc oupled
probablyre latedt o the highv olatilec ontento f Loihi alkalic plasma,m asss pectrometr(yIC P-MS) on 13 of the same
lavas[ Byerse t al., !985]. The tholeiitesa re extremelyv ariable sample(sT able3 ) at WashingtoSnt ateU niversityF. or each
in vesicularit(y0 .1-41 vol %). The picritict holeiiticb asalts sample1, 00-200m g of glassw ered issolveodn a hotplatea t
collectedn ear (<10 m) their ventsa longt he southr ift are 110øCu singH F, HNO3, and HCIO4 in an opent eflonv ial.
stronglvye sicula(r-- 40 vol %). Somes ubaerialal vasa re highly After the samplew as evaporatedto incipientd rynessa, n
vesiculanr ear theirv ent and decreasein vesicularitayw ayf rom additionale vaporationw ith HCIO4 at 165øC converted
the vent (e.g., 1947-1948H ekla eruption[ Einarsson1,9 49]; insolublefl uoridesto solublep erchloratesA. n In-Re internal
1984 Mauna Loa eruption). Thus proximityt o the vent may standardw asu sedt o monitord rift. The samplesw erea nalyzed
playa n importanrto lei n controllintgh ev esiculariotyf Loihi's usinga SciexE lan 250 ICP-MS,w ith cross-flowne bulizer,
lavasa nd,p erhapso, f all submarineb asalts. water-cooledto rch,a nd Brooksm assf low controllers.O xide
540 GARCIA ET AL.: TEMPORAL GEOCHEMICALE VOLUTIONO F LOIHI SUMMIT LAVA
TABLE 1. RepresentativMe icroprobeA nalyseso f Major Elementsi n Pillow Rim GlassesC ollectedF rom Loihi VolcanoW ith
the Alvin Submersible
Sample Class SiO2 TiO2 A1203 FeO* MgO CaO Na20 K20 P205 MnO Total
1801-01 Thol 48.77 2.64 13.63 11.73 6.82 11.90 2.36 0.44 0.21 0.20 98.70
1801-04 Thol 48.67 2.75 14.04 11.98 6.18 11.39 2.58 0.52 0.23 0.19 98.53
1801-05 Thol 49.48 2.43 13.55 11.89 6.93 11.46 2.42 0.41 0.18 0.16 98.91
1801-09 Thol 49.52 2.47 13.56 12.06 7.00 11.45 2.37 0.40 0.19 0.16 99.18
1801-13 Thol 49.55 2.42 13.52 11.84 7.07 11.35 2.38 0.41 0.19 0.18 98.91
1801-14 Thol 49.05 2.68 13.56 11.70 6.79 12.01 2.52 0.50 0.18 0.17 99.16
1801-16 Thol 49.13 2.82 i3.86 1i.67 6.49 11.53 2.56 0.60 0.25 0.16 99.07
1801-17 Thol 49.31 2.82 13.70 11.46 6.83 11.94 2.50 0.51 0.22 0.18 99.47
1801-!9 Thol 48.67 2.87 13.75 11.74 6.71 11.69 2.58 0.57 0.24 0.21 99.03
1801-20 Thol 48.62 2.82 13.71 11.78 6.62 12.02 2.52 0.49 0.24 0.20 99.02
1801-21 Thol 49.22 2.79 13.84 11.87 6.48 11.74 2.42 0.46 0.22 0.19 99.23
180i-22 Thol 49.31 2.61 13.84 11.84 6.80 1!.65 2.41 0.42 0.20 0.18 99.26
1802-04B Alk 47.17 3.59 !4.75 12.84 5.01 10.76 3.55 1.02 0..38 0.22 99.29
1802-09A Thol 49.46 2.71 13.83 11.97 6.57 11.74 2.39 0.42 0.20 0.18 99.47
1802-20 Thol 49.39 2.68 13.81 11.60 6.94 11.72 2.41 0.44 0.24 0.17 99.40
1803-14 Alk 47.12 3.88 14.17 13.10 5.49 11.10 3.31 0.84 0.38 0.18 99.57
1803-16 Thol 49.69 2.48 13.26 !1.52 7.50 11.72 2.33 0.36 0.18 0.18 99.22
1804-01 Thol 49.19 2.46 i3.35 11.40 7.29 12.21 2.40 0.49 0.21 0.19 99.19
1804-10 Thol 49.20 2.56 12.93 11.46 8.01 12.02 2.35 0.44 0.21 0.16 99.34
1804-12 Thol 49.20 2.63 13.40 11.27 7.22 12.04 2.48 0.50 0.19 0.18 99.1!
1804-16 Alk 48.38 4.14 13.48 13.57 4.56 9.49 3.53 0.99 0.45 0.22 98.81
1804-19 Alk 48.73 3.76 13.82 12.75 5.17 10.21 3.33 0.89 0.34 0.19 99.19
1804-21 Trans 48.23 2.54 13.54 11.88 7.19 12.30 2.67 0.56 0.21 0.19 99.31
1804-22 Trans 47.97 2.57 13.58 !1.80 7.02 12.41 2.71 0.58 0.23 0.18 99.05
Totali ronr eporteda sF eO*. Thec lass(r ockt ype)f or eachs amplwe asd eterminefdro mF igure4 ;A lk, alkalicT, hol,
tholeiiticT; rans,t ransitionalI.n weighpt ercent.A nalyst:M . Garcia
interferencceo rrectionwse re determinedfo r eachr un using extractedu sing CIF3 [Borthwicka nd Har(cid:127)non, 1982] in a
twos olutionpsr eparedfr oms ingle-elemesntta ndardsS. ample manners imilart o the Br(cid:127) methodd evelopedb y Claytona nd
intensities were first corrected for isobaric oxide interferences Mayeda [1963]. The resultingC O,. gasw as analyzedw ith a
and thenf or instrumentadlr ift usinga mass-weighteadve rage Finnigan MAT Delta E triple collector, 90 sector mass
of the drift of In and Re [afterD oherty,1 989]. The elemental spectrometeart the Universityo f Minnesota.O x3,genis otope
intensitiesw ere calibrateda gainsta curve constructedfr om ratiosa re normalizetdo SLAP( ,(cid:127)80 V.SMOW = -55.0%o)
three in-house standards,w hich were dissolvedw ith each and are reportedr elativet o V.SMOW. The ,(cid:127)80 valuef or
batcho f unknowns.T he Hawaiianb asalts tandard,B HVO-1, NBS-28q uartz on thiss calei s +9.5 _ 0.15%o. Eachs ample
was run as an unknown. Analyticalp recisioni s 1-2% for the wasr un at leastt wice;t he duplicatesa re within _+0.15%o of the
ICP-MS analyses. averagev alue( mosta re <0.06%o;s eeT able5 ).
For Sr, Nd, and Pb isotopicm easurement3s0, -50 mg of
fresh,g lassw erel eacheds uccessiveinly a cetoneu, ltrapure6 N MAJOR ELEMENTS
HCI, and ultrapurew ater in an ultrasonibc ath. Following
digestionw ith HF-HNO3, spikingc, hemicasl eparationa, nd The lavasc ollectedfr om the summitr egiono f Loihi havea
masss pectrometrica nalysesfo r Pb, Sr, and Nd ratios were substantiaral ngei n glassc ompositio(ne .g.,4 .5-8.0w t % MgO).
performeda s describedb y Mahoneye t al. [1991]. Isotopic Three rock types were recovered: tholeiitic, alkalic, and
fractionationc orrectionss, tandardv alues,t otal procedural transitionabla salts(F igure4 ). The vastm ajority( >85%) of
blank levels,a nd experimentaul ncertaintiesa re listedi n Table the new samplesa re tholeiitic. Alkalic lavasw ere recovered
4. from the baseo f the westernp it crater,f rom surfacef lows
For oxygenis otopem easurementcso, arselyg round( >80 alongt he upperp ortiono f the southr ift zone,a nd alongt he
mesh) glassw, hichh ad beenh eatedu nderv acuumf or 2 hours easternm argino f the summit platform( Figure 2). Two
at 350øCto removea nya dsorbemd oisturew, ash and-grounind transitional lavas were taken on the southwest side of the
acetoneu singa n agatem ortar and pestlet o about4 00 mesh summitp latforma t about1 400m waterd epth. The rangei n
and dried to 110øCi n air for 30 min. Sevent o 15 mg of lavac ompositioanm ongt heA lvin-collectesdu iteo f sampleiss
powderedg lassw ere used for each analysis. Oxygenw as smallert han the range for the dredges uite of Moore et al.
GARCIA ET AL.: TEMPORALG EOCHEMICALE VOLUTIONO F LOIHI SUMMFI'L AVA 541
542 GARCIA ET AL.: TEMPO1LALG EOCHEMICALE VOLUTIONO F LOIHI SUMMIT LAVA
[1952]( seeF igure4 ) whichc oversa muchl argerp ortiono f the
volcano and includes basanitoids.
The Alvin suite of Loihi lavasf orms a relativelyc oherent
groupo n MgO variationd iagrams(F igure5 ). Valuesf or TiOz,
FeO, NazO, K20, PzOs,a nd A1203( exceptf or the younger
alkalics amples)in creasew ith decreasingM gO; CaO, and to a
lesserd egree SiO2, decreasew ith decreasingM gO. The
AI20(cid:127)CaO ratioa lsoi ncreasews ith decreasinMg gO, evenf or
the samplesw ith MgO contents> 7 wt % (Figure5 ), which
containv irtuallyn o clinopyroxenoer plagioclaspeh enocrysts
(<0.1 vol %). Comparedt o other Hawaiiant holeiitics hield
lavasL, oihit holeiitehs avet hel owesSt iO2( averagea bout4 9.2)
and highestC aO ½.upto 12.4 wt %). The K20 and FeO
contentsa lsoa re somewhaht igher.
The alkalicl avasa re the most evolveda mongo ur suite of
samples. They range from about 4.6 to 5.5 wt % MgO but
their total alkali contentsa re lesst han 5 wt %; thus, none are
hawaiires(b asedo n the classificatiosnc hemeo f Le Bas et al.
[1986]). We found two distinctt ypeso f alkalicl avas. The
south rift lavas have higher SiO2, and lower AI203 and CaO
than the other alkalicl avas( Figure5 ). However,a t the same
MgO value, both have the same AIzO3/CaO ratios. No
plagioclasep henocrystasr e presenti n any of thesel avasb ut
plagioclasme icrophenocrys(t0s.1 -0.5m m) are morea bundant
in the southr ift lavas( 3.8-7.3v ol %) than in the other alkalic
lavas( 1.0-2.4v ol %).
' TRACE ELEMENTS
A wide range of both compatiblea nd incompatibletr ace
element abundances was determined for the Alvin suite of
Loihi lavas( Tables 2 and 3). Among the most incompatible
elements, Cs and Rb have the largest enrichmentf actors
(-3.1), followedi n order by Ba, Th, Nb, Ta, U, La, and Ce
(with factorso f 2.9 to 2.2). Cr, Ni, and Sc are compatible
elementsi n the Loihi lavasa nd vary widely at a givenM gO
content.T he highC r contentsfo r the picriticg lasse(s1 270a nd
1186p pm;s amples1 804-10a nd 1804-12)a re partlyd ue to the
presenceo f Cr-spinelm icrophenocrysitns the analyzedg lass.
Sr and V, which can be compatiblee lements,e speciallyin
evolvedH awaiian lavas,a re incompatiblein all of the Loihi
lavas. Well-definedli nearv ariationsth at projectt hrought he
origin are present for the strongly incompatiblee lements
(Figure 6). Ratios of these(cid:127)e lements also yield good linear
correlationsw ith the evolveda lkalicl avash avingt he highest
ratios( Figure7 ).
Both ICP-ES and ICP-MS analysesw ere made of selected
Loihi glassesto obtain a thoroughc haracterizatioonf these
samples. This resulted in duplicate REE, Y, and Ba
measurementsfo r somes amples.T he ICP-ES Ce data are not
reliableb ecauseo f instrumentp roblemsa nd were not included
in Table 2. For the other elements, most are within 10%
(exceptf or Dy). To evaluatew hichv aluesa re more accurate,
we compared the results for a standard (BHVO-1) that is
similari n compositionto the Loihi glasseasn d wasa nalyzeda s
an unknownw ith both setso f ICP analyses.A comparisono f
these results shows that both techniquesp roduced results
similar (within 10%) to the consensusv alues for BHVO-1,
althought he ICP-MS value for Dy and Er are about 10% high
and the valuesf or Pr andT a are similarlylo w (Tables2 and 3).
The REE patternss howni n Figure8 combinet he ICP-ES data
with the Ce and Pr obtainedb y ICP-MS. The ICP-MS Tb, Er,
and Tm were not includedi n the patternsb ecauseth eyc aused
small positivea nomaliest hat are analyticali n origin (i.e.,
patternsc onstructedu singo nly the ICP-MS data do not have
anomaliesf or thesee lements).
GARCIAE TA L.: TEMPORALG EOCHEMICALE VOLLrHONO F LolltI SUMMITL AVA 543
TABLE 4. Sr, Nd, and Pb IsotopicD ata for Loihi Glasses
Sample 143Nd/144Nde nd 87Sr/O6Sr 2ø6pb/2øqp2bø 7pb/204P2bø spb/204Pb
1801-5 Thol 0.512934 +5.7 0.70356 18.252 15.433 38.001
18.264 15.440 38.026
1801-19 Thol 0.512966 +6.4 0.70353 18.367 15.479 38.127
1801-22 Thol 0.512984 +6.7 0.70353 18.359 15.480 38.133
0.512988 +6.8
1804-1 Thol 0.512950 +6.1 0.70355 18.338 15.458 38.079
1804-10 Thol 0.512955 +6.! 0.70361 I8.416 15.469 38.154
0.512953 +6.1 18.413 15.466 38. I22
1804-21 Trans 0.512947 +6.0 0.70356 18.245 15.437 37.986
1802-4B Alk 0.512961 +6.3 0.70358 18.474 15.492 38.205
0.512961 + 6.3
1804-19 Alk 0.512948 +6.0 0.70358 18.378 15.468 38.116
0.512947 +6.0
Isotopfirca ctionaticoonr rectiofonr N di s1 48NdO/lnqN= d0O.2 4243(164 5Nd/(cid:127)44=N 0d. 241572fo);r S ri t is8 6Sr/(cid:127)SSr
= 0.1194. Pb valuesa re adjustedr elativet o the NBS 981 valueso f Todt et al. [1983],c orrectedf or fractionationu sing
a 0.84%0 per amu factor. Nd and Sr isotopicd ata are reportedr elativet o our measuredv aluesf or La Jolla Nd
043Nd/ln4=N 0d.5 1185+5 0 .00001to2t arla ngeo;r + 0.2e ndu nitsa),n dfo rN BS9 87( 87Sr/8=6 S0r.7 102_5+0 .00002
total range). Within-runu ncertaintieosn sampleN d andS r isotopicra tiosa re lesst hant otal rangesm easuredo n La
JollaN d andN BS9 87s tandard(cid:127)Ess. timateudn certaintoiensP bi sotopeasre 2 ø6pb/2ø=q P+b 0 .0102, ø7pb/2ø=4 pb
-+0.00290, 8pb/2ø=4 +p b0 .032b,a seodn t otarla ngemse asufroerdN BS9 87.T otabl lankasre P b= 2-40p icograms,
Nd < 20 picogramsa,n d Sr < 120 picogramsA. ll are negligiblefo r the purposeo f thiss tudy. DuplicateP b analyses
for 1801-5a nd 1804-10w ere run on separates plitso f glass.
TABLE 5. OxygenIs otopeD ata for Loihi Glasses
Sample RocTk ype MgOw, t% 6(cid:127)80V alues 6(cid:127)80 Average
1804-10 Thol 8.0 4.78, 4.86 4.82 -+ .04
1804-1 Thol 7.3 4.63, 4.71 4.67 -+ .04
1801-09 Thol 7.0 4.71, 4.91 4.81 -+ .10
1801-5 Thol 6.9 4.95, 4.97 4.96 _+..01
1801-1 Thol 6.8 4.92, 5.14, 5.14 5.07 _+..10
1801-22 Thol 6.8 5.03, 5.13 5.08 -+ .05
1801-14 Thol 6.8 5.14, 5.17, 5.22 5.18 _+. 03
1801-19 Thol 6.7 5.09, 5.09 5.09 -+ .06
1801-4 Thol 6.2 4.85, 4.93, 4.98, 5.0!, 5.08 4.97 q- .06
1804-21 Trans 7.0 4.58, 4.65, 4.69, 4.69, 4.81 4.68 + .06
1803-14 Alk 5.5 5.21, 5.27 5.24 +_. 03
1804-19 Pdk 5.2 5.10, 5.16 5.13 -+ .03
1802-4B Alk 5.0 5.13, 5.22 5.18 + .04
Ama!ystE . Ito.
544 GARCL(cid:127) ET AL.: TEMPORALG EOCHEMICALE VOLUTIONO F LOIHI SUMMIT LAVA
(Table 3). The overallr angeso f Nd and Sr isotopicv aluesf or
theses ampleasr e restrictedS;7 Sr/(cid:127)r variesb etwee0n. 70353
and 0.70361,a nd the total spreadin end is only 1.0 (+6.7 to
+5.7; Figure 9a). Both the lowesta nd highestr atiosa re for
tholelites. These values define a small field within the much
larger array for dredgeh aul samplesfr om Loihi, althought he
four dredgeh aul samplesf rom the summita rea analyzedb y
Staudigeel t al. [1984]h avet he samel imitedS r isotoper ange
ON (cid:127)X,,X((cid:127)'(cid:127)_ .(cid:127)_._ THOLElITbEut a somewhatl arger range in Nd isotopes. For a different
groupo f dredgeds amplesT, atsumotoe t al. [1987] reporteda
Z 5 largers preadin 87Sr/(cid:127)Srr atios( 0.70344t o 0.70376)b ut a
relativelyn arrowr angef or Nd isotoped ata (+5.1 to +6.6).
In contrastt o our Sr and Nd isotopicd ata, our Pb isotopic
resultssp ana sw idea rangein 2 ø6pbflø4raPtbio ( 18.25-18.4a7s)
that previouslyre portedf or the entirev olcano( Figures9 b and
9c). Our Pb isotoped ata exhibitl inear trendsw ith positive
46 48 50
slopes,u nlike previousr esultsw hich have a larger analytical
SiO2
uncertaintTyh. ereis n oc orrelatioonf P tisotopewsi thS ro r
Fig. 4. SiO2v ersusto tal alkalis( all in weightp ercent)f or Loihi Nd isotopicra tiosn or of rock typew ith any isotopicra tio,a s
glassesP. reviougsl assd ataf romL oihi[ Mooree ta L, 1982]a re shown wasn otedi n earliers tudies[S taudigeelt al., 1984;T atsumoteot
by the solidd ots. TheA lvin-collectesdu iteo f samplesis subdivided
intot hreer ockt ypesb asedo n thec lassificatiosnc hemoe f Mooree ta l.
[1982]: tholeiitic( squares);t ransitiona(ld iamonds)a; nd alkalic
basalts( triangles). The heavyd iagonalli ne separatesth e tholeiitic 15.0
anda lkalicf ields[ fromM acdonald,1 968].
The REE patternsfo r Loihig lasseasr e generallsym ootha nd o N 14.0 ,-, %
parallel (Figure 8). They are similar to those of typical
Hawaiian lavas (e.g., the U.S. GeologicaSl urveyH awaiian
basalts tandardB, HVO-1)and thosep reviouslpyu blishedfo r
Loihi whole rock samples[ Frey and Clague,1 983]. REE 15.0 I
patternsf or the Loihi tholeiiteg lassesfo rm a tightg roupw ith
crossinpga tternse, specialliyn the middlet o heavyR EE range 00 n
(Figure 8). The glassesw ith the lowest middle REE 12.0
abundancehs aves lightlyp ositiveE u anomalies.T his hasa lso
been noted for some lavas with low REE abundances from
o
Kilauea[ Hoj9nanne t al., 1984]a nd Lanai [Weste t al., 1992] c(cid:127) I1.0
(D
volcanoes. The anomaliesa re greatestf or Lanai lavas,w hich
are an isotopice nd-memberfo r Hawaiiant holeiites[W este t al.,
1987]. Thus this feature may be characteristiocf one of the IO.O
sourcec omponentfso r Hawaiianm agmas.
The Loihi alkalic glassesh ave similar patternst o the
tholeiitesb, ut theyh aveh igherR EE concentration(Fsi gure8 )
1.00
ands omewhast teeperp atterns( La/Yb ratioso f about9 to 11
versus6 to 8 for the tholeiitesF; igure 7). Sample1 804-19,
0.80
from a youngh ydrothermallayc tivec one,h asa flatter pattern
than the other alkalicl avas( Figure 8). Althought hesel avas 0.60
n (cid:127)n½ 0 0 -
are fractionstea(li .e., MgO contentso f 5.0-5.5w t %), none
have negative Eu anomalies,w hich is consistentw ith the 0.40
absenceo f plagioclaspeh enocrystisn thesel avas.H owever,t he
young alkalicl avasf rom the Pele'sV ents area (a prominent 0.20 I I I I I I I I
topographich igh at the southern end of the summit area; 1.40
samples1 804-16 to 1804-19) define a nearly perpendicular
brancht o the overalln egativet rend on the MgO versusA l203
plot (Figure 5), so some plagioclasfer actionationm ust have
occurred. This interpretationis supportedb y (1) their lower
Sr/Cer atior elativet o the othera lkalicla vas(7 versus1 0);a nd O(cid:127)0r0r(cid:127) n ( (cid:127)0 --
o
(2) least squares crystal, fractionationc alculationsw hich
o
indicatet hat the fractionationa ssemblagfeo r these lavasi s I.lO i
dominatedb yp lagioclas(e4 8%, with 40% cpxa nd 12% olivine;
52R2 = 0.04). 5.00 6.00 7.00 8.00
MgO
Sr, Nd, ANDP b ISOTOPREA TIOS
Fig. 5. MgO variationd iagramsfo r major elementsin theA lvin suite
of glassesfr om Loihi. Note the two distinctg roupso f alkalicl avaso n
Eight summita rea glasses(f ive tholeiitict, wo alkalic,o ne the h1203 and CaO panels. Symbolsa s in Figure 4. All valuesi n
transitionalw) ere analyzedfo r Pb, St, and Nd isotopicra tios weightp ercent.
GARCIAE TA L.: TEMPORALG EOCHEMICAELV OLUTIONO FL OIHI SUMMHL' AVA 545
inversec orrelationw ith MgO (Table 5). However,t he three
5O i i
evolveda lkalicg lassesa re the controllingf actor in this trend.
LQ There is no correlationo f MgO with 6(cid:127)80 for the tholeiites
despitet he larger angei n MgO (1.8 wt %). The glassews ith
MgO contents >6.5 wt % (i.e., those with little or no
2O
clinopyroxenoe r plagioclasfer actionation)c an be subdivided
into two groupsb asedo n their oxygenis otopea nd FeO/CaO
ratios( Figure1 0). There is no apparenct orrelationo f oxygen
isotopew ith the other isotopicr atios,e xceptp erhapsw ith
10
2ø(cid:127)pb/2ønpfobr the highF eO/CaOs ample(si. e., >0.96, Figure
10). Uncontaminatebda saltsf rom the CameroonL ine also
haves omel ow 6(cid:127)sO values( 4.5 to 5.5%0) and exhibitp oor
correlationosf O with Pb, Sr, andN d isotoper atios[ Hallidaye t
0
al., 19881.
_ Rb
2O
TEMPORAL EVOLUTION OF LOIHI LAVAS
There is no systematicc ompositionavl ariation in Loihi's
summit lavas; alkalic and tholeiitic lavas are coeval. For
10
examplet,h e mosth ydrothermallayc tivea reao n Loihi, Pele's
Vents [Karl et al., 1988], is composedo f evolveda lkalicl avas
that overliea djacentt holeiites.A nothery ounga lkalicl avaw as
found on the easternf lank of the summit (1803-14). The
oldestl ava sampled( 1802-!, whichi s identicatl o 1802-4B)i s
from near the baseo f the westernp it cratera nd is alsoa lkalic.
Tholeiitesd ominatem ost of the summit platform and the pit
Th
crater sectionso f Loihi. Thus, althougha lkalicl avasa re not
commoni n the summita rea, their eruptionsc learlya re closely
1.5
relatedi n time and spacet o thoseo f the tholeiiticla vas.I f the
summita rea is the regiono f mostr ecentv olcanismo n Loihi
[Fornari et al., 1988] and its earliestv olcanismw as alkalic
[Mooree t al., 1982],t henL oihi may be in a transitionagl rowth
1.0
period between early alkalic volcanisma nd later tholeiitic
volcanism[M ooree t aL, 1982]..The rarityo f tholeiiteso n the
flankso f Loihi [Jorgensoent al., 1988] is strongc ircumstantial
evidence that only the summit area has experiencedt his
0.5
transitional stage of volcanism. }towever, it must be
emphasizedt hat the evidencef or an early phase of alkalic
volcanismi s equivocal. Although some alkalic lavas have
thicker palagoniter ims, this may be a result of their higher
0o
2.0 vesicularity and alkali content. The results of our
stratigraphicallyc ontrolled study do not support the early
alkalicv olcanismh ypothesisb,u t we havee xaminedo nly about
Fig.6 . Variationd iagramfso r highlyin compatibltera cee lementsT: a
10-15% of Loihi's summit section.
versus Rb, Th, and La in Loihi glasses. Note the good linear
correlationsth at projectt hrought he origin. Symbolsa s in Figure4 .
All dataa re in ppma nda re fromT able3 (ICP-MS).
al., 1987]. Comparedto tholeiiticla vasf rom otherH awaiian
0.6
shieldv olcanoesL, oihi'st holelitesh ave some of the lowestS r
andh ighesNt d andP b isotopera tios( Figure9 ). OnlyK ilauea
lavash aveh igher2 ø(cid:127)pbflønPrabt iosa, lthoughn ewP b isotope
dataf or olderK ilaueal avas[ Chene t al., 1991]o verlapw ith the 0.5
Loihif ield. The trendo f our2 ø7pb/2øndpatba i sa t a 35ø a nglet o
the overallf ield for Hawaiiant holelites( Figure 9b). This
featureh asa lsob eenn otedf or shielda nd postshiellda vasf rom
0.4
otherH awaiianv olcan.o es[ e.g.K, ennedeyt al., 1991].K ennedy
et al. [199!] suggestethda tt hisc omponencto uldn ot be from
oceanicli thospheraen dm ustb e a minorp lumec omponent.
0.5
OXYGEN ISOTOPER ^(cid:127)OS
6.0 7.0 8.0 9.0 I0.0
Thirteen samples were analyzed for oxygen isotopes,
L/Yb
includingt he eight samplesa nalyzedf or Pb, Sr, and Nd
isotopesa nd the mostf ractionareathl oleiitics ample( 1801-4). Fig. 7. La/Yb ratiov ersusR b/Y ratiosf or Loihig lassesN. ote good
The 6(cid:127)80 valuesr angef rom +4.7 to 5.2%0a nde xhibiat rough linearc orrelation.S ymbolsa si n Figure4 . All datai n ppm.
546 GARCIA ET AL.: TEMPORALG EOCHEMICALE VOLUTIONO F LOIHI SUMMIT LAVA
80 -ø..Z.. o
-- ' ALKALIC
6o -'.%'.,.,, LAVAS
--% o. ... o 18o2 -4B
40L .-----_(cid:127).. (cid:127) .. ., ...... 1804-19 _
+ '\I......
(cid:127) -(cid:127)..-..
(cid:127){(cid:127)-'.-. (cid:127)
30-
-'.%?.,.
'-',,%-,,,
20-
_ : :,s ---.... _
--...
...... ----..,
o .....
I0-
oo B HKViOlo-uI eo (cid:127)
8
Lo Ce Pr Nd $m Eu Gd Dy Ho Yb Lu
Fig. 8. Chondrite-normalize(dC I chondritev alueso f Bqvnton[ 1984]), rare earth elementp lot of Loihi glassesa nd U.S.
GeologicaSl urveyH awaiianb asalts tandardfr om KilaueaV olcano,B HVO-I. Data are from Table 2 for all elementse xceptC e
and Pr (Table 3). The patternf or BHVO-I is basedo n our analyses.T he patternsf or the alkalicg lasseasr e dashedt;h e
patternsf or the tholeiitics amplesa re solidl ines. Note the crossingp atternsf or thesei sotopicallys imilart holeiitics amples,
includingB HVO-I.
The presenceo f large pit craters at the summit of Loihi I I
indicatest hat magma bodiesh ave underlaint he summit area. 15t .I5(cid:127) I 0I ! I , ' A
However, the intercalated nature of its tholeiitic and alkalic
lavas are strong circumstantiael videncet hat Loihi's magma (cid:127):(cid:127) 15.48
chamber(s) are ephemeral. Perhaps this is a result of the (cid:127) (cid:127) KO0
relativelyl ow magmas upplyr ate for Loihi (comparedt o other o KILAUEA -
Hawaiians hieldv olcanoes[)F reye t al., 1990]. Similarf eatures % 15.46 -
are observeda mong the lavasf rom tl(cid:127)e postshields tage of
Hawaiianv olcanismw, hich is characterizedb y relativelyl ow (cid:127) 15.44 .(cid:127)[.(cid:127) L_OIHI
magmas upplyr ates [Frey et al., 1990]. In contrast,m ature (cid:127) (cid:127), L-(cid:127)(cid:127)ALN AI(cid:127)
Hawaiian volcanoesp roduce relatively homogeneousla va 15.42 MAUNA LOA
PACIFI'C(cid:127)L OHII
0.51M30(cid:127) (cid:127)1(cid:127) KILAUEA
38.1
'Z -0 0(cid:127) .7001I .574-0' (cid:127)(cid:127)'1(cid:127)M5AU N6A L OA
(cid:127) O,5 129-
-ZO 'o.5(cid:127):(cid:127)o KOOLAU 3Z9 MAUNA LOA
_Z 0.5128
._.(cid:127)(cid:127)(cid:127)(cid:127)KKLUOA 0
37.7
0.5127 (cid:127) LANAI LANAI
i I I0 '5129 I
i i i
0.703 0.704 17.8 18.O 18.2 18.4 18.6
87Sr/86Sr
2ø6pb/ 2ø4pb
Fig. 9. Sr, Nd, and Pb isotopicr atios in Loihi glasses.S ymbolsa s in
Figure 4. The Loihi field is for dredgeh aul samples[ Staudigcelt al., Fig. 9. (continued)
1984]. The inset in Figure 9a showst he rangeo f Sr and Nd isotope
ratios in the Alvin suite of samples. The crossr epresentsth e total compositionosv ert ime. For exampleM, auna Loa hase rupted
rangeo btainedi n the Universityo f Hawaiii sotopel ab for the La Jolla
tho!eiiticl avasw ith remarkablys imilarc ompositionast a given
Nd, NBS 987 Sr, and NBS 981 Pb standardsw, hichi s greatert han the
MgO contento vert he last3 0,000y ears[ Rhodes1, 987]. This
two-sigmaa nalyticale rror for our analyses. Fields for some other
Hawaiians hieldv olcanoesa re alsos hown[ from Weste t al., 1987;K urz feature and the generallyh omogeneousc ompositiono f lavas
and Karomet,1 991]. from a singlee ruption( i.e., they lie on an olivinec ontroll ine)