34. YTTRIUM AND OTHER TRACE ELEMENTS IN BASALTS FROM DSDP LEG 37

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1 34. YTTRIUM AND THER TRACE ELEMENTS IN BASALTS FRM DSDP LEG 37 R. St J. Lambert, Department f Gelgy, Unversty f Alberta, Edmntn, Alberta, Canada and J.G. Hlland, Department f Gelgcal Scences, Unversty f Durham, Suth Rad, Durham, England INTRDUCTIN Ths reprt s based n majr and trace (Ba, Nb, Zr, Y, Sr, Rb, Zn, Cu, C, N) element analyses by XRF fr 2 basalts frm Hle 332A, 25 frm Hle 332B, 2 frm Hle 333A, 2 basalts and 2 perdttes frm Ste 334, and 4 basalts frm Ste 335. In addtn, nfrmatn was avalable frm thn sectns f 27 samples (thse fr whch C data are nt gven) and frm Sr and stpes and Th and U abundances (Gray et al., ths vlume). Data are presented n Chapters 2, 3, 4, and 5 (ths vlume). CLASSIFICATIN BY TRACE ELEMENTS Facets f the gechemstry have been extracted frm the tabulated data and are presented n Fgures -. Lke mst basalt sutes, Leg 37 basalts pssess a hgh pstve crrelatn f T and Zr, whch can be extended t nclude Nb, Y, Rb, Zn, P25, Na2θ, Fe 2 3 /Mg, and U, all f whch crrelate pstvely. The basalts frm Leg 37 can therefre be classed as fllws: () Hgh Zr basalts frm Hles 332A and 332B (Zr > 5 ppm); (2) Lw Zr basalts frm Hles 332A and 332B; and (3) Basalts frm Stes 334 and 335 whch have hgh Y relatve t Zr cmpared wth Hles 332A and B (Fgure ). The tw samples f basalt frm Hle 333A fall nt the frst tw categres lsted abve, but d nt shw a pstve Fe2θ3/Mg versus Zr crrelatn (Fgure 2). The Zr-Y crrelatn s shwn n Fgure, where t can be seen that all samples analyzed fall nt a smple pattern, except 332B-23-, 78-8 cm (6A), frm Unt 5 f the shpbard classfcatn. In the hgh Zr grup, Hle 332A has slghtly hgher Y fr a gven Zr than Hle 332B. Because t s currently wdely assumed that T, Zr, and Y are alteratn-resstant (Cann, 97), t was prvsnally assumed that a classfcatn based n ther abundances culd be genetcally sgnfcant. A gd Zr and Fe2θ3/Mg crrelatn s shwn n Fgure 2, where the same grups appear as n Fgure. The nly hghly aberrant sample s 332A-7-, 4-43 cm (#5), whch has unusually lw Mg. A smlar pattern s bservable n basalts frm Iceland (Sgvaldasn, 974), but all f Sgvaldasn's basalts have Zr >5 ppm. T and Na shw the same srt f pattern (Fgure 3) except that n the Hle 332B hgh Zr grup there s n varatn f T wth Na. T and Na were therefre cmpared wth Nb, Zr, Y, and Rb (halvng Zr t arbtrarly reduce ts sgnfcance) n Fgure 4, where the same pattern s seen, the tw aberrant samples beng frm Unts 2 and 5 f Hle 332B. The clse nterrelatnshp f the elements gven n Fgures -4 suggests that ether alteratn prcesses have been very unfrm r that there s sme systematc genetc cntrl f the bserved varatns. Hgh and varable plagclase cntents, prmary r accumulatve, culd prduce sme f the bserved varatn, but nt the crrelatn f Fe2θ3/Mg wth the ther elements. A mre satsfactry chce fr verall cause s varatn f degree f partal meltng and fractnal crystallzatn, because hgh degrees f meltng and lw degrees f fractnal crystallzatn wuld presumably change majr/trace element rats n bradly the manner bserved n ths sute. YTTRIUM GECHEMISTRY Varatn f Y wth Ca, Nb, and Zn s shwn n Fgures 5-7. The hgh and lw Zr grups are readly dentfed n each fgure, and attentn s drawn t the crrelatn wth Zn, an element nrmally gechemcally asscated wth ferrmagnesan mnerals, rughly equally n pyrxenes and lvnes, but prbably mre hghly cncentrated n magnette. Varatn f Y wth Ca was shwn t be f sme nterest n dfferentated gneus seres by Lambert and Hlland (974). The data frm DSDP Leg 37 shwn n Fgure 5 suggest that cean-flr basalts are nt necessarly relatvely rch n Y (Lambert and Hlland, 974, p. 44), but that crcumstances d exst n basalt generatn whch wll prduce hgh Y (e.g., n Ste 335, Fgure 5). The Leg 37 samples fall nt tw man grups, the lw-zr set f Hles 332A and 332B, whch shw scatter between Y and Ca, and the hgh-zr set and samples frm 333A, 334, and 335, whch tgether appear t shw a "trend" f ncreasng Y wth decreasng Ca. Ths feature s als clearly exhbted by Icelandc basalts, as shwn n Fgure 5, Sgvaldasn's (974) samples lyng between the bundares ndcated. Hs subsets -3 can als be dstngushed by ther Ca/Y rats. The hgh-zr set data can be caused n ether r bth f tw ways: systematc mechancal mxture f cnstant Ca, Y mnerals (e.g., An?s + a cnstant rat f pyrxene and lvne) r fractnatn f a mneral r mnerals lyng alng the trend, such as Ams r Ca-pr pyrpc garnets (see Lambert and Hlland, 974, fg. ). Generatn f ths pattern by secndary alteratn seems ruled ut by the smlar behavr f Icelandc and Leg 37 rcks. The lw-zr set Ca-Y data are less easy t nterpret. The hgh Ca samples may cntan CaCθ3, but n the absence f C2 data n cnclusn s pssble. The samples wth lw Ca and lw Y all have lw 567

2 R. ST J. LAMBERT, J. G. HLLAND /u + 332A 332B α A&B hgh Zr (Ml 332A&B/^^S' wz^t;. 332A hgh Zr ' ] I~Λ 332 B hgh Zr 7^ D - 5me symbls as Fg.. - (3 - l 2 Nc^ wt.% Fgure 3. T2 and Na2 n Leg 37 gneus rcks. 4 Fe2θ3/Mg, ndcatng pssble lvne cntrl f these rck cmpstns. Fgures 6 and 7 shw the hgh degree f lnear crrelatn between Y, Nb, and Zn, wth Zn = 3.2Y and Y =.5Nb + except at Ste 335. The latter may ndcate ccurrence f Nb nly n subsdary phases n these basalts, the ppm Y beng the average cmpnent f Y n the prncpal slcate phases. The Zn-Y crrelatn rases new prblems, hwever. Fgure. Zr and Y n Leg 37 gneus rcks. α α h 332 B lwzr h H / / 7 / r / \ / rr ^^»y_ j k rfm) + [ ^ % \ - &? - ^^332A&B. hgh Zr 334 Same symbls as Fg.. H Fβ2θ3/Mg wt. % Fgure 2. Zr and Fe2θj*/Mg n Leg 37 gneus rcks. ZN, C, AND NI IN LEG 37 IGNEUS RCKS The prblem f Zn s examned further n Fgure 8 and C and N are llustrated n Fgure 9. Usng Y as a key element, N/Y and Zn/Y rats shw systematc behavr. Usng the shpbard classfcatn, t can be seen that unts 332A- and 332B- have smlar rats as d unts 332A- and 332B-7. N s hgh n all lw-zr unts (332A- and 7, 332B-, -4, -5, -7, and -8) als n 332B-9 and -. Further, Unt 3 f 332A and Unt f 332B bth appear t be dvsble nt tw dstnct subunts. Varatns f N prbably reflect lvne cntrl, whereas Zn varatns are due t all ferrmagnesan mnerals and magnette. Bth Hles 332A and 332B therefre penetrated lava ples f wdely varyng rgn, mst clearly shwn by trace elements and t a lesser extent by Fe 2 3 /Mg and Na 2. C, hwever, shws n systematc varatn as seen n Fgure 9. N/C vares wdely and cmparatvely randmly frm t. Wth the nfrmatn n hand, n cause fr ths varatn has been fund. STRNTIUM IN LEG 37 IGNEUS RCKS Lke the elements dscussed prevusly, Sr shws a clear pattern vs-a-vs Zr, but wth sme verlap between the varus grups. When Sr s examned aganst K2 (Fgure ), t s seen that K/Sr vares wdely n these rcks, averagng 2 n the hgh-zr grups, 5 n the lw-zr grups, and 25 at Stes 334 and 335. The lw K/Sr n the lw-zr grup s cnsstent wth the ntn that ths grup has been frmed by a hgh degree f partal meltng wth lmted subsequent fractnal crystallzatn. There are n bservable crrelatns f Sr 87 /Sr 86 (Gray et al., ths vlume) wth Sr, explcable n grups f stpe exchange wth seawater rather than element exchange. 568

3 YTTRIUM AND THER TRACE ELEMENTS 4 : 3-2- CN σ Same symbls as Fg. I Nb+ /2 Zr +Y +Rb ppm Fgure 4. Na 2 + T 2 and Nb + Y 2 Zr + Y + Rb n Leg 37 gneus rcks. 8 DISCUSSIN In ther dscussn f the rgn f basalts recvered frm Leg 6, Rdley et al., (974) qute varus gechemcal parameters whch are beleved t reflect alteratn prcesses, and ther parameters were dscussed by Cann (97). Besdes T, Zr, Y, and Nb, whch are thught t reman cnstant durng alteratn, Mg, Ca, and Sr are cted as decreasng durng alteratn, and K and Rb are cted as ncreasng. n the bass f the abve results, we cnsder that there s lttle drect gechemcal evdence fr alteratn f element abundances n these basalts, even thugh the xygen and strntum stpes d shw cnclusve evdence f exchange. If alteratn s respnsble fr the prpsed dvsn nt lw- and hgh-zr types, t must have perated n such a way as t prduce very lw abundances f the suppsedly nert elements and at the same tme change Fe2θ3/Mg and Na2θ wthut sgnfcant alteratn f the majr mnerals n these rcks.unfrtunately, at the tme f wrtng, nly fur thn sectns f the lw-zr basalts are avalable t us; f these 332A- 4-3, cm (#4B) s a cmparatvely fresh almst hlcrystallne basalt wth abundant plagclase and augte mcrphencrysts, 332B-2-5, 3-6 cm (#) s bradly smlar n appearance, and has nly sme 5% altered brwnsh glassy resduum, and 332B-2-, 27-3 cm (#B) s a very fne graned quench-textured basalt wth very lttle vsble alteratn. These three are nt partcularly dstngushable n ther general appearance n thn sectn frm the scre f sectns avalable frm hgh-zr types. Hwever, 332B-35-2, 5-8 cm (#N) s mderately altered and ts gechemstry culd have been sgnfcantly affected by ths prcess. It has the hghest Sr^/Sr 86 and <5 R f all the samples s far measured. It s therefre cncluded that the majr cause f the varatns n the gechemstry f these rcks les n ther meltng hstry rather than secndary r hghlevel alteratn prcesses. The stpc anmales must therefre be due t exchange rather than mneralgcal reactn prcesses. It s als cncluded that 332A, Unt 3 and 332B, Unt shuld each be subdvded. Fnally, the lw-zr types (Unts and 7 f 332A and Unts,4, 5?, 7, 8, and A f 332B) are very defcent ndeed n Nb, Zr, Y, Sr, Rb, P 2 á, T 2, and Na 2, even fr basalts, and must have rgnated under rather extreme cndtns, presumably very hgh degrees f partal meltng f the upper mantle. TECHNIQUES All analyses were made n a Phlps 22 XRF unt usng neat pwdered brquettes, prepared drectly frm pwders as suppled by Dalhuse Unversty (thse samples wth Ba and C) r as crushed at Edmntn n a tungsten carbde swng-mll. Matrx crrectns were carred ut by an teratve cmputer prcedure, and the standards used were the current nternatnal standards fr majr elements and spked glasses specally 569

4 R. ST J. LAMBERT, J. G. HLLAND Average Ca, Y n all basalts Φ Average Iceland (Sgvaldasn 2-- Same symbls as n Fg.l. [Same symbls - * -An 7 D * α Iceland "trend" Θ IS 2 Fgure 5. Ca and Y n Leg 37 gneus rcks wth the Icelandc pattern fr cmparsn. prepared by Plkngtn Glass fr the trace elements (Brwn et al., 97, p ). ACKNWLEDGMENTS We wsh t acknwledge dscussns wth clleagues at the Unversty f Alberta n the subject matter f ths reprt and fnancal assstance frm the Natural Envrnment Research Cuncl f Great Brtan fr supprt f the X-ray flurescence labratry at the Unversty f Durham. REFERENCES Brwn, G.M., Emeleus, C.H., Hlland, J.G., and Phllps, R., 97. Mneralgcal, chemcal and petrlgcal features f 2 3 Fgure 6. Nb and Y n Leg 37 gneus rcks. 4 5 Apll rcks and ther relatnshp t gneus prcesses. In Levnsn, A.L. (Ed.), Apll Lunar Sc. Cnf. Prc: New Yrk (Pergamn Press), v., p Cann, J.R., 97. Rb, Sr, Y, Zr and Nb n sme cean flr basaltc rcks: Earth Planet. Sc. Lett., v., p. 7-. Lambert, R. St J. and Hlland, J.G., 974. Yttrum gechemstry appled t petrgeness utlzng calcumyttrum relatnshps n mnerals and rcks: Gechm. Csmchm. Acta, v. 38, p Rdley, W.I., Rhdes, J.M., Red, A.M., Jakes, P., Shh, C, and- Bass, M.N Basalts frm Leg 6 f the Deep-Sea Drllng Prject: J. Petrl., v. 5, p Sgvaldasn, G.E., 974. Basalts frm the centre f the assumed Icelandc mantle plume: J. Petrl., v. 5, p

5 YTTRIUM AND THER TRACE ELEMENTS Same symbls as n Fg. I Left hand bar Zn / Y Rght Sandbar N/Y * Zn >N α. 6" 4" lw Zr types = = m < Average Fgure 8. N, Zn, and Y n Leg 37 gneus rcks. 2 Fgure 7. Zn and Y n Leg 37 gneus rcks

6 R. ST J. LAMBERT, J. G. HLLAND. h Same symbls as Fg. I. 6- E Q. Q-4- J s «D + 2- h N ppm Fgure 9. C and N n Leg 37 gneus rcks CNI " Same symblsas n Fg.l. f, \*334 ~^^^ V - f \. Λ 332 B lw Zr 335 ^ \ / V I J / t *) y δ _ J H 332 A & \ hgh X / - ' B Zr N. \ H 6 8 Sr ppm Fgure. K2 and Sr n Leg 37 gneus rcks

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