13. MAJOR-ELEMENT CHEMISTRY OF BASALTIC GLASSES RECOVERED DURING DEEP SEA DRILLING PROJECT LEG 64 1

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1 . MAJOR-ELEMENT CHEMSTRY OF BASALTC GLASSES RECOVERED DURNG DEEP SEA DRLLNG PROJECT LEG 6 Danel J. Fornar, Department of Geologcal Scences, State Unversty of New York at Albany, Albany, New York Andrew D. Saunders, Department of Geology, Bedford College, Regenfs Park, London, NW NS, Unted Kngdom and Mchael R. Perft, Research School of Earth Scences, Australan Natonal Unversty, Canberra, P.O. Box, Australa ABSTRACT Fourteen mcroprobe analyses of basaltc glasses from three drll stes plus two dredged samples from the seafloor at the mouth of the Gulf of Calforna suggest that magmatc lquds that cooled to form the volcanc crust n the Gulf of Calforna have been fractonated, whereas other, more prmtve submarne basalts from the Md-Oceanc Rdge (MOR) have not. The lmted chemcal dversty among drll stes suggests that magmatc lquds, whch erupted along the accretonary boundary n the Gulf of Calforna, are grossly smlar. Major-element varatons n the glasses from Hole A may, n part, however, be attrbuted to smple crystal-lqud fractonaton n a subrdge magma chamber. Addtonally, the analyss of a glass from Hole 5B, near the eastern contnental slope of southernmost Baja Calforna, suggests that magmatc lquds whch produced submarne basalts assocated wth ntal openng of the Gulf may have been chemcally less evolved than present ones. Trace-element concentratons (Saunders et al., ths volume, Pt. [hereafter "ths volume"]) n whole rocks from Hole 5B suggest that ths magma may have experenced mnor salc contamnaton on ts ascent through lower crustal contnental materal. Whole-rock and glass major-element concentratons, however, suggest that early Plocene(?) basalts that erupted n Hole 5B were derved from a magmatc lqud of (deep) mantle orgn. The lqud quckly rose to the surface wth lttle shallow-level fractonaton. NTRODUCTON Fresh basaltc glass from selvages or crusts of extrusve submarne volcancs provdes an ndrect means of assessng the chemcal characterstcs of magmatc lquds that erupt along the accretonary axs of the MOR. Whole-rock compostons are the result of nteractng magmatc and secondary processes, and they reflect some stage of a crystallzaton sequence. They are often based because of phenocryst concentratons and varable weatherng effects; as such, they provde less quanttatve nformaton about the composton of the lqud from whch they crystallzed. The quantty of data on basaltc seafloor glasses has ncreased steadly over the last fve years, and numerous nvestgators have compled major-element chemcal characterstcs of drlled, dredged, and submersble basalts from the major ocean basns and along the MOR (Melson et al., 96; Bryan, 99; Bence et al., 99; Byerly and Snton, 90). n ths chapter we present major-element analyses of basaltc glasses from holes drlled by Glomar Challenger durng Leg 6. Eght of the glasses were sampled from a typcal oceanc, upper-crustal sequence of extrusve basalts from Hole A. Most of the glass recovered durng Leg 6 occurs n ths sequence; however, small amounts of glass were also recovered n the Guaymas Basn stes and are represented n ths study by three samples from Holes and A. The glass sample from Hole 5B, at the base of the contnental slope and off the southeastern tp of Baja Calforna, was taken from a weathered cobble of olvne basalt that had retaned some glass selvages. Addtonally, glasses from two samples dredged near the mouth of the Gulf are ncluded n these data. One sample s from the Alarcon Seamount ( 6'N; 'W), and the other s from a basement outcrop n the Rse-Mazatlán Basn ( 'N,.5'W). METHOD All glasses were analyzed at the Lamont-Doherty Geologcal Observatory on an ARL-EXM three-spectrometer electron mcroprobe. Glass fragments were mounted n epoxy resn, polshed, and analyzed wth a defocused electron beam (~ 5 µm spot sze). The acceleratng voltage was 0 kv, and the specmen current was approxmately 0. µa for 60 s. Table presents the averages computed from analyses of each glass fragment (samples wth Plot Numbers and average eght ponts). Synthetc and natural mneral standards and wholerock (artfcal) and natural basaltc glasses ensured standardzaton. A more detaled descrpton of the standardzaton procedures and data precson s reported n Fornar et al. (99). RESULTS Fgure depcts the data from Table on MgO-varaton dagrams; decreasng MgO appears to the rght of each plot. To dscuss the chemcal characterstcs of glasses from an ndvdual ste and to compare nterste glass compostons, we wll dscuss each oxde separately. SO The slca content of the glasses vares between and 50.5 wt.%, and only a vague pattern of ncreasng slca wth decreasng MgO emerges. Several groupng and trends, however, do occur, and they become more ob- Curray, J. R., Moore, D. G., et al., nt. Repts. DSDP, 6: Washngton (U.S. Govt. Prntng Offce). We wll refer to the samples by ther plot numbers, whch appear on Table above the offcal DSDP sample numbers. 6

2 D. J. FORNAR, A. D. SAUNDERS, M. R. PERFT Table. Mcroprobe analyses of natural glasses, Leg 6. Plot Number DSDP Sample Number -, - cm 6-, cm -, cm -, 9 cm -, - cm -, cm 9-, - cm 9-, -5 cm 5B- -, -0 cm - -, -6 cm A- -, 0- cm A- -, - cm Alarcon Seamount Staton 9 Rse-Mazatlán Basn Staton 59 Number of Analyses SO A O TO Fe O FeO* MnO MgO CaO Na O K O P O 5 Cr O Total Mg Number CaO/Al θ CPW Norms OR AB AN NE D Wo En Fs HY En Fs OL Fo Fa MG L AP CR _ _ _ _ Note: Analytcal method yelded total ron as FeO; FeO' done usng FeO + = 0.5 FeO. = 0.5 FeO; Fe θ* = FeO ; Fe + used n Mg number calculaton = 0.9 FeO; norm calculatons were vous and dstnct n the other oxde plots. Samples through (Hole A) and,, and (Holes, A) fall between 9 and 50.5 wt.%; they are clearly dstnct from Sample 9 (Hole 5B), whch has the lowest SO content ( wt.%) of all samples and has major-element characterstcs smlar to Samples and. A O All of the glasses except Sample 9 have A O contents that range from 5 to 6.6 wt.% and are wthn the feld determned by Bence et al. (99) for ocean-rdge basaltc glasses (see Perft et al., ths volume, Pt. [hereafter "ths volume"]). The trend among the glasses of decreasng A O wth MgO suggests Plagoclase- and olvne-controlled fractonaton. Sample 9 has the hghest content of A O ( wt.%), whch s.6 wt.% hgher than that found n the correlatve whole-rock analyss reported by Saunders et al. (ths volume) for a sample taken at cm from Secton of Core, Hole 5B. FeO* Total FeO contents exhbt a negatve correlaton wth MgO and range from 9. to. wt.%. Note that the Fe O used n calculatng CPW norms was calculated from mcroprobe analyses, whch yelded total ron as FeO (see Table, note). Even though glasses n Samples through came from one petrographcally dstnct unt (Unt of Saunders et al., ths volume), a sgnfcant degree of fractonaton from Samples through 5 to Samples 6 through s apparent. The trend of ron enrchment wth dfferentaton, however, appears to be greater n Samples,, 6, and than n,, 5, and. The latter group are more lke Samples through, whch also show relatvely low FeO* contents (.0-. wt.%). Although smlar n other respects, Samples and have sgnfcantly dfferent FeO* contents (~l wt.% dfference). Sample 9 s agan separated from the other glasses, because t has the lowest FeO* content (9. wt.%)a consequence of ts relatvely less-fractonated nature (Mg number = 6.). MgO-Mg Number Compared to the basalts recovered durng Leg 6 (see Saunders et al., ths volume; Perft et al., ths volume), the glasses have a much more restrcted range of MgO and Mg number. The average Mg number (5. ±.9) for the glasses reported here s nearly dentcal to the average of 600 basalt glasses from the ocean floor (5.6; Wlknson, n press). Based on ther calculated Mg numbers and MgO contents (~. wt.%) Samples 9,, and are the most prmtve glasses yet recovered from the Gulf of Calforna. Sample, contanng.6 wt.% MgO, s the most prmtve glass from Hole A. t may represent an analogous, parental lqud compos- 6

$5) and appear to be transtonal lqud compostons between the most prmtve and more fractonated glasses wth the lowest MgO contents (6.6-6. wt.%).$

3 BASALTC GLASSES ton from whch the more evolved glasses (Samples and ) n the hole were derved. As wth Sample 6 from Hole A, Samples through from Hole are moderately evolved (Mg numbers = ) and appear to be transtonal lqud compostons between the most prmtve and more fractonated glasses wth the lowest MgO contents ( wt.%). TO There s a broad lnear trend of ncreasng TO wth decreasng MgO n the glasses (Fg. ). Basalt glasses from the MOR characterstcally exhbt ths trend as a consequence of crystal fractonaton (Fg. ; Perft, ths volume). The most prmtve glasses (9,, ) have the lowest TO contents (.-. wt.%), whereas the more evolved samples are slghtly more varable, havng a range from.0 to.5 wt.% TO. The dvergent trends of ncreasng FeO* n Hole A glasses are also apparent n the TO varatons. Samples,, 6, and exhbt a larger degree of TO enrchment than Samples,, 5, and. Furthermore, Samples and have, as do the FeO* contents, more affntes wth the latter group. CaO-CaO/Al O n general, the glass data exhbt a lnear decrease of CaO wth decreasng MgO (Fg. ). Although Samples and have the hghest CaO contents, Sample 9 has a somewhat low content (.6 wt.%) n comparson to the moderately fractonated basalts (.6-.9 wt.%). n detal, Samples,, 5, and show a more rapd decrease n CaO wth MgO than Samples,, 6, and, whch exhbt lttle varaton. Because of decreasng A O contents wth MgO n the latter group, these four samples ncrease lnearly n CaO/Al O ratos wth decreasng Mg number, ths parallels the trend of the crystallne basalts n Hole A (see Perft et al., ths volume) and MOR basalt glasses n general. Compared to the other Leg 6 glasses and crystallne basalts, Samples,, and 5 have relatvely hgh CaO/Al O ratos. Assumng they are related to Sample, as already mentoned, they exhbt a sharp decrease n CaO/Al O rato wth decreasng Mg number. Sample 9 s agan somewhat unusual n havng the lowest CaO/ A O rato. Samples through have CaO/Al O ratos smlar to many of the more evolved Leg 6 basalts (see Perft et al., ths volume; Saunders et al., ths volume). Na O/K O/P O 5 The alkal and P O 5 abundances (Fg. ) ndcate very lttle dstncton among the groups of glasses. All three plots show flat-to-slghtly ncreasng trends and fractonaton for these three oxdes. Alkal contents of the glasses are near the upper range of most oceanc-rdge basaltc glasses (Bence et al., 99). DSCUSSON The chemcal characterstcs of the Leg 6 glasses are generally smlar as a group, but contrast to those of the glasses from other segments of the MOR. n partcular, unlke more prmtve (hgher-mg number) MOR glasses and basalts from Leg 6, they appear to have undergone a reasonable degree of fractonal crystallzaton. On average, ther Mg numbers are lower than those of the crystallne basalts from the same holes. n part, ths may be caused by some accumulaton of ferromagnesan phases n the crystallne basalts. That the glasses from each hole represent some of the most evolved samples, however, suggests that they are products of crystal-lqud fractonal crystallzaton, rather than drect partal melts from the mantle. The Leg 6 glasses plot n a feld parallel to, but dstant from, FAMOUS glasses on a CPW normatve trangle (Fg. ). The Leg 6 glasses fall n nearly the same feld as glasses from the Cayman Trough (Thompson et al., 90), whch have probably undergone smple, sngle-stage fractonaton from an evolved ocean-rdge parental magma. The Leg 6 and the Cayman glasses are hgher n Na O than FAMOUS glasses; ths partally accounts for ther beng further nto the Plagoclase feld than the FAMOUS glasses. Fgure s a plot of FeO*/ MgO versus TO and shows the Leg 6 data fallng n a feld separate from Md-Atlantc Rdge (MAR) glasses and smlar to Cayman glasses and East Pacfc Rse (Batza et al., 9) and sla Tortuga tholetes (Batza, 9). Leg-6 glasses are hgher n TO for a gven FeOVMgO rato than the MAR glasses. Ths feature s apparent n the crystallne basalts as well (Perft et al., ths volume). The major-element characterstcs of the Leg 6 glasses, as presented n Fgure, suggest that there were temporal varatons n the Hole A magmatc lquds. One example of ths s the chemcal dfference between Samples,, 6, and and,, 5, and. The dfferences may be a consequence of dfferent lqud lnes of descent from a sngle, prmtve, parental magma or of dervatons from two or three dfferent parental magmas. Perft et al. (ths volume) have shown that basalts from Unts 6,, and n Hole A can be related by the fractonal crystallzaton of Plag + Ol ± Cpx from the same or smlar parental magmas; but dfferent lqud lnes of descent may be nvolved. For nstance, assumng that Sample represents the least-fractonated lqud, then the evolved glass of Sample could be derved from Plag + Ol + Cpx fractonaton n the wt.% rato :: whereas Sample requres only Plag + 0 (:) fractonaton (Perft et al., ths volume). The rapd fall n CaO and only gradual ncreases n FeO* and TO wth decreasng MgO n Samples,, 5, and (Fg. ) may be a result of sgnfcant Cpx fractonatonunlke the fractonaton controlled solely by Ol + Plag n the other glasses. The dfferences n fractonatng assemblages may be caused by temporal varatons n the depth of magma crystallzaton, changes n phase stabltes as lquds evolve, or mxng of prmtve magmas wth more evolved types. Clearly, the major element varatons n basalts from Hole A suggest a multstage evoluton. Chemcal data strongly suggest that the basalts from Holes and A could have been derved from a common parental magma by varyng degrees of Ol + Plag fractonaton (Saunders et al., ths volume). The hgh CaO/Al O ratos (Table ) and relatvely low Mg 65

4 D. J. FORNAR, A. D. SAUNDERS, M. R. PERFT 0.U 9.θ r h h H.0 5.0^ 5 6 ßn _l.0.0 r.0 % 5.0h 6.0^ 9 6 *.0-- 5#..0h h-.0 h- { 9 5 % 6 f.0 \-.0 h MgO (wt. %) Fgure. MgO varaton dagrams, Leg 6 glasses ".0 r 9 5.^ MgO (wt. n j J

BASALTC GLASSES 0 Plagoclase 0 0 0 Olvne Fgure. CPW normatve trangle (plagoclase-pyroxene-olvne), Leg 6 glass data.

5 BASALTC GLASSES 0 Plagoclase Olvne Fgure. CPW normatve trangle (plagoclase-pyroxene-olvne), Leg 6 glass data. (Felds for FAMOUS and off-rdge glass data are from varous sources n Bryan [99] and Thompson et al. [90]. Plagoclase s total An + Ab, pyroxene s total D + En + Fs, and olvne s total Fo + Fa. Phase boundares are after OsbornandTat [95].).0.5 SS S.o CM.5.0 s \ Cayman Trough,'' / L / Md-Atlantc Rdge FeO /MgO Fgure. TO -FeO*/MgO varaton dagram, Leg 6 glasses. (Sold crcles = Leg 6 sample numbers; open crcles = basaltc lavas from sla Tortugaa young [. Ma] tholetc volcanc sland n the central Gulf of Calforna [Batza, 9]; crosses = oceanfloor basalts dredged from the East Pacfc Rse, near the Squeros Transform [Batza et al., 9]. Cayman Trough and Md- Atlantc Rdge glass felds are from Thompson et al. [90].) numbers of Samples - support ths proposal, but small amounts of Cpx fractonaton may also have been mportant n generatng ntermedate lquds from prmtve parents (Perft et al., ths volume). Sample 9 from Hole 5B exhbts major element characterstcs that dffer from the other Leg 6 samples. On geographc evdence alone t may represent basaltc magma that accompaned the begnnng of oceanc crustal formaton n the Gulf of Calforna. For t to have been derved from a more prmtve magma (.e., smlar n composton to the accompanyng olvne basalt), equal amounts of Plag, Ol, and Cpx would have to be subtracted (Perft et al., ths volume). Ths agrees nether wth the lqudus phases nor wth the trace-element varatons (Saunders et al., ths volume). Alternatvely, t may have experenced mnor, lower-cfustal contamnaton from relct contnental materal and may have orgnated from a deep asthenospherc source, rapdly rsng to the surface wth lttle shallow-level fractonaton. t does reflect a more prmtve basaltc lqud and, wth Samples and, one could suggest that ther average major-element content s a vald estmate for juvenle, accretonary magmatc lquds that erupted n the newly formed Gulf of Calforna. SUMMARY Mcroprobe analyses of basaltc glasses from Leg 6 ndcate that major-element characterstcs of magmatc lquds that erupted along the accretonary boundary n the Gulf of Calforna are grossly smlar. The majorelement varatons that do occur, as exemplfed by the glasses from Hole A, may, n part, be explaned by smple crystal-lqud fractonaton as magma batches evolve wthn the magma chamber or durng ther ascent to the seafloor. Dfferent sources and magma mxng, however, cannot be ruled out on major-element evdence alone. Samples 9,, and are less-fractonated samples and are lkely to represent older types of magmatc lquds that erupted on the newly formed oceanc crust of the Gulf of Calforna. ACKNOWLEDGMENTS Jeff Nemtz kndly suppled the glasses from Alarcon Seamount () and the Mazatlán-Rse Basn (). Donna Kelly, Heather Sloan, Maureen Martuscello, and Debbe Schwartz asssted n typng and preparng tables and fgures; ther help s greatly apprecated. Don Elthon asssted the senor author wth the mcroprobe analyses and provded moral and techncal support n the face of nstrument ntransgence. REFERENCES Batza, R., 9. Geology, petrology and geochemstry of sla Tortuga, a recently formed tholetc sland n the Gulf of Calforna. Geol. Soc. Am. Bull., 9:09-. Batza, R., Rosendahl, B. R., and Fsher, R. L., 9. Evoluton of oceanc crust. Petrology and chemstry of basalts from the East Pacfc Rse and the Squeros Transform Fault. J. Geophys. Res., :65-6. Bence, A. E., Bayls, D. M., Bender, J. F., et al., 99. Controls on the major and mnor element chemstry of md-ocean rdge basalts and glasses. n Talwan, M., Harrson, C. G., and Hayes, D. E., (Eds.), Deep Drllng Results n the Atlantc Ocean: Ocean Crust: Washngton (Amercan Geophyscal Unon), pp. -. Bryan, W. B., 99. Regonal varaton and petrogeness of basalt glasses from the FAMOUS area, Md-Atlantc Rdge. /. Petrol. 0(Pt. ):9-5. Byerly, G. R., and Snton, J. M., 90. Compostonal trends n natural basaltc glasses from Deep Sea Drllng Project Holes D and A. n Donnelly, T., Francheteau, J., Bryan, W., Robnson, P., 6

6 D. J. FORNAR, A. D. SAUNDERS, M. R. PERFT Flower, M., Salsbury, M., et al., nt. Repts. DSDP, 5, 5, 5, Pt. : Washngton (U.S. Govt. Prntng Offce), Fornar, D. J., Peterson, D. W., Lockwood, J. P., et al., 99. Submarne extenson of the southwest rft zone of Mauna Loa Volcano, Hawa: Vsual observatons from U.S. Navy Deep Submergence Vehcle DSV Sea Clff. Geol. Soc. Am. Bull., 90:5-. Melson, W. G., Valuer, T. L., Wrght, J. L., et al., 96. Chemcal dversty of abyssal volcanc glass erupted along Pacfc, Atlantc and ndan Ocean floor spreadng centers. Geophyscs of Pacfc Ocean Basn and ts Margns: Geophyscal Monograph 9: Washngton (Amercan Geophyscal Unon), 5-6. Osborn, E. F., and Tat, D. B., 95. The system dopsde-forsterteanorthoste. Am. J. Sc., Bowen Volume, -. Thompson, G., Bryan, W. B., and Melson, W. G., 90. Geologcal and geophyscal nvestgaton of the Md-Cayman Rse Spreadng Center: Geochemcal varaton and petrogeness of basalt glasses. /. Geol., :-55. Wlknson, J. F. G., n press. The geness of md-ocean rdge basalt. Earth Sc. Rev. 6

Wilbur and Ague 4 WILBUR AND AGUE; APPENDIX DR1. Two-dimensional chemical maps as well as chemical profiles were done at 15 kv using

Wilbur and Ague 4 WILBUR AND AGUE; APPENDIX DR1. Two-dimensional chemical maps as well as chemical profiles were done at 15 kv using DR2006139 Wlbur and Ague 4 WILBUR AND AGUE; APPENDIX DR1 MINERAL ANALYSES Two-dmensonal chemcal maps as well as chemcal profles were done at 15 kv usng the JEOL JXA-8600 electron mcroprobe at Yale Unversty