52. PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS, LEG 37, DEEP SEA DRILLING PROJECT

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52. PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS, LEG 37, DEEP SEA DRILLING PROJECT Lenid Dmitriev, Institute f Gechemistry, Academy f Sciences f the USSR, Mscw, USSR INTRODUCTION During Leg 37

1 52. PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS, LEG 37, DEEP SEA DRILLING PROJECT Lenid Dmitriev, Institute f Gechemistry, Academy f Sciences f the USSR, Mscw, USSR INTRODUCTION During Leg 37 ceanic basement was drilled at fur sites (332, 333, 334, and 335) n the west flank f the Mid-Atlantic Ridge at apprximately 37 N latitude. This reprt deals with the basaltic rcks recvered frm Hle 332B and with a plutnic sequence encuntered at Site 334. Hle 332B was drilled abut 3 km frm the ridge crest in basement apprximately 3.5 m.y. ld. This hle penetrated 583 meters f acustic basement with apprximately 2% recvery. The basaltic sequence was divided by the shipbard party int 11 lithlgic units largely n the basis f phencryst cntent. At Site 334 a plutnic cmplex was encuntered beneath apprximately 5 meters f basalt. The cmplex is a layered sequence f gabbr, trctlite, and lherzlite abut 67 meters thick. Numerus breccia znes cnsist f plutnic clasts set in a matrix f nannfssil chalk. BASALTIC ROCKS OF HOLE 332B Tw hundred seventy-six analyses f fresh basalt with CO2 less than 1 wt % were selected frm Table 1B, Chapter 2 (this vlume?. These were pltted against depth, and paired crrelatins were calculated fr the majr xides. A special cmputer prgram was used fr the calculatins which divided all analyses int tw principal grups distinguished by different crrelatin trends amng the majr xides. The differences between the tw grups have been demnstrated t be statistically significant by A.V. Garanin (Institute f Gechemistry, USSR Academy f Sciences). Analytical differences between different labratries (see Wright, this vlume) are distinctly less than the differences between the tw grups. The paired crrelatins are shwn in Figure 1 (a-g) where the different analyses are identified by subbttm depth. These diagrams clearly shw the existence f the tw main magma types and prvide evidence that the bserved chemical variatins are due chiefly t differentiatin f these tw types. Magma grup 1 is characterized by high, relatively high ttal irn (FeO), and lw SiO 2 and CaO. TiO 2 and AI2O3 exhibit a psitive crrelatin, but has a negative crrelatin with all ther xides except (FeO). The cncentratin f (FeO) prbably des nt depend n cntent. Chemical variatins in magma grup 1 reflect fractinatin and accumulatin f livine r livine spinel. Petrgraphically, these basalts range frm carsely phyric picrites t aphyric types. Magma grup 2 has relatively high cncentratins f Siθ2 and CaO and lw and (FeO). Wide variatins in AI2O3, CaO, and especially (FeO) and Tiθ2 characterize this grup. In cntrast t grup 1 magmas, Tiθ2 and AI2O3 shw a negative crrelatin and (FeO) and Tiθ2 shw a psitive crrelatin (Figure If, lg). ithe chemical variatins within magma grup 2 are prbably related t plagiclase fractinatin and accumulatin. Tw subgrups (2a and 2b) are recgnized n the basis f chemistry and mineralgy. Grup 2a magmas have higher cncentratins f Siθ2, (FeO), and TiO 2 and lwer AI2O3 and CaO than thse f grup 2b. Rcks f grup 2a are aphyric r sparsely phyric with phencryst assemblages plagiclase livine clinpyrxene. Subgrup 2b cnsists chiefly f carsely plagiclase-phyric basalt. The distributin f grup 1 and grup 2 magmas with depth is shwn schematically in Figure 2, alng with shipbard lithlgic units and palemagnetic units. It can be seen that there is generally a clse crrespndence between chemical type and lithlgic and palemagnetic units. There is als evidence fr cyclic variatins in magma chemistry. All f the rcks are belived t be related t tw main eruptive cycles, I and II. Bth cycles cntain all three recgnized magma grups, and cnsist f several subcycles generally beginning with magmas f grup 1 and ending with magmas f grup 2b. Only the last subcycle is represented cmpletely; the thers nrmally terminate with magmas f grup 2a (Figure 2). The bundary between the tw main cycles is marked by alteratin f the basalts (high CO2 cntents) and by interlayering f basalts and sediments at the end f the first cycle. Generally the thickness and abundance f sedimentary interbeds increase upward nt nly in the sectin as a whle, but als within individual cycles and subcycles. In mst f the cycles and subcycles the different magma types ccur in discrete units. Hwever, in the lwer part f subcycle I2, grup 1 and grup 2a magmas are intimately interlayered. This "interlayered cmplex" is succeeded upward by magmas f grup 2b and appears t be part f a nrmal cycle. In general the thickness f individual flws and the vlume f lava in individual cycles appears t increase upward in the sectin. Experimental petrlgy suggests that magmas f grup 1 shuld frm by partial melting f mantle material at depths f 4 t 6 km and magmas f grup 2 shuld frm at depths f abut 2 km. The frequent eruptin f these tw distinct magma types suggests a 681

2 L. DMITRIEV 52 α Symbl Subbttm depth (m) Number f analyses 12 Magma type u π u 7: OOnJ O] ln θ ' *.. D V a 49 Ml OS λtm' m ; 4 ) A. >v b Oceanic thleiite (2 an.) 48 - O Figure la. Paired crrelatins between sme majr elements in basalts frm Hle 332B. high degree f tectnic activity in the area at the time f frmatin. Because f the tectnic instability, magmas mved rapidly t the surface rather than accumulating in large chambers where they wuld underg extensive crystal fractinatin. Mst prbably the bserved differentiatin in these magmas is due t crystallizatin f superheated magma during its rise t the surface and separatin f crystals by flw prcesses. In general, the thickness f lithlgic units, the abundance f sedimentary interlayers, and the vlume f magma erupted in a given cycle jappears t increase upward in the sectin, suggesting that magmatic and tectnic activity in the area tapered ff gradually rather than ending abruptly. In Figure 3 basalts frm Hle 332B are cmpared with basalts dredged frm 67 statins alng ridge crests in the Atlantic, Pacific, and Indian ceans (Sharaskin et al., in press) and with cntinental thleiites (Mansn, 1967). Cmpared t ther ceanic basalts, thse frm Hle 332B are distinctly lwer in TiC>2, (FeO), and Na2θ and higher in CaO and AI2O3. Cmpared t cntinental livine thleiites, Hle 332B basalts are mre unifrm in cmpsitin and are lwer in (FeO) and K2O and higher in CaO and AI2O3. It is als apparent that sme 332B basalts are significantly enriched in, CaO, and AI2O3, reflecting accumulatin f livine r plagiclase. It is interesting that the unusual character f these rcks is nt reflected in the ther xides. PLUTONIC COMPLEX AT SITE 334 The plutnic cmplex at Site 334 has been studied in detail because it may shed light n the cmpsitin and rigin f ceanic Layer 3. Apprximately 67 meters f interlayered gabbr, trctlite, and lherzlite were penetrated befre the hle was terminated. The average chemical cmpsitin f each lithlgic type is given in Table 1 (clumns 2, 3, and 4). Clumns 1 and 5 give 682

3 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS Figure lb. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. TABLE 1 Average Cmpsitin f Lherzlite, Trctlite, and Gabbr Frm Site 334 (wt %) X 1 S X 2 S X 3 S J 4 S X 5 S SiO 2 TiO 2 A1 2 O 3 Fe 2 3 FeO MnO CaO Na 2 O K 2 O Nte: 1 = Mid-ceanic ridge thleiite (average f 2 analyses, Sharaskin et al., 2 = Gabbr frm Site 334 (8 analyses); 3 = Trctlite frm Site 334 (8 analyses); 4 = Plagiclase Lherzlite frm Site 334 (12 analyses); 5 = Lherzlite frm mid-ceanic ridges (69 analyses, Dmitriev, et al., 1972); The sum in clumns 2, 3, and 4 is less than 1% because Fe 2 θ3 nas been recalculated t FeO. 683

4 L. DMITRIEV ; V Λ. A A.A A y^^ A A A A >W Figure lc. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. 28 average analyses fr ceanic thleiite and ceanic lherzlite, respectively. Figure 4 (a,b) is a graphical cmparisn f the plutnic rcks t average ceanic thleiites and lherzlites. It can be seen that Site 334 lherzlites are quite similar t average ceanic lherzlite but have smewhat lwer cncentratins f SiCh, TiO 2, Na 2 O, and K2O and higher AI2O3 and CaO. Gabbr frm Site 334 is cmpsitinally similar t basalt frm Hle 332B and has smewhat lwer (FeO), TiCh, Na2θ, and K2O and higher AI2O3 and CaO than the average ceanic thleiite. These differences prbably reflect reginal differences in the cean basins. Trctlites are cmpsitinally intermediate between gabbr and lherzlite, and ne might cnclude that all three rck types in this cmplex had a cmmn rigin. Hwever, the chemical relatinship shwn in Figure 4 and the range f cmpsitin f each type suggest that the trctlites are nt related t the ther rcks by a simple fractinatin scheme. Frmatin f the trctlites prbably required metasmatic transfer f Na 2 O, K2O, TiO 2, AI2O3, and CaO. In Table 2 the average cncentratin f selected trace elements in gabbr, trctlite, and lherzlite frm Site 334 are cmpared with cncentratins in average ceanic thleiite and lherzlite. Figure 4 shws that Site 334 trctlites are significantly enriched in Au, Cu, Ba, and Li cmpared t the assciated gabbrs and lherzlites. These data cnfirm the cnclusin that the trctlites were frmed independently f gabbrs in an pen system. 684

5 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS Figure Id. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. TABLE 2 Distributin f Sme Trace Elements in Lherzlites, Trctlites, and Gabbr Frm Site 334 (all elements in ppm, Au-ppb) Cr V Ni C Sr ßa Li Rb Cu Au* 33 (116) 314 (91) 113 (116) 42 (87) 13 (1) 23 (88) 5.6 (32) 1.6 (43) 97 (94).77 (32) 131 (8) 131(5) 379 (7) 66(7) 14(7) 22(7) 5. (5) 2. (5) 86(8) 4.7 (8) 17 (6) 123 (6) 82 (8) 62(6) 1(7) 38(6) 8. (4) 2.3 (8) 17 (7) 3.2 (6) 48 (5) 11(8) 1844 (12) 96(9) 5(9) 29(9) 6.1 (4) 2.1 (9) 45 (12) 6.3 (8) 44 (37) 47 (37) 25 (37) 117(37) 8(16) 4.1 (37).5 (37) 4 (37) 1.4(35) Nte: 1 = Mid-ceanic ridge thleiite (Dmitriev et al., in press); 2 = Gabbr frm Site 334; 3 = Trctlite frm Site 334; 4 = Plagiclase Lherzlites frm Site 334; 5 = Lherzlite frm mid-ceanic ridges (Dmitriev et al., in press). Number f analyses is in parentheses. 685

6 L. DMITRIEV 1 ö 17 O O n O O^ OΛ θ8 O n π π Λ πftadw" v #; π /. V A A> *i? AA:. '".A, D > A» 1 D i i i, i i i I i i i i i i i i i i i i i Figure le. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. 28 REFERENCES Dmitriev, L., Ukhanv, A., and Sharaskin, A., Cmpsitin f the upper mantle: Gechimia, n. 1. (English translatin in Gechem. Internatinal, v. 9, p ). Mansn, V., Gechemistry f basalts. In Hess, H., and Pldervaart, A. Ed., Basalts v. 1, New Yrk (Interscience), in press. Sharaskin, A., Charin, G., and Dmitriev, L., in press. Gechemistry f mid-cean ridge basalts. 686

7 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS v Δ π U, A Φ.6.4 cf.2., A1 2 O 3 Figure If. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. 687

8 L. DMITRIEV * n 1.2- V ö v w l ^ D D ^ D D D v ] v 1.- Λ i.8.6 A ) O A n u i* * A. *.4 θ ^θ.2. i 1, FeO Figure lg. Paired crrelatins between sme majr elements in basalts frm Hle 332B. See Figure la fr legend. 688

9 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS c 2 Cre Brief petrgraphic descriptin 1-4 I-II 2BM 1 Carsely phyric t aphyric plagiclase basalts interlayered with sft sediments (empty space in sectin) fπ A / IIlB 2BM 2 Sparcely phyric t aphyric basalts (pl>l>py). Sedimentary layers IV/V 2BM 3,4 Carsely phyric t aphyric livine-rich basalts. Sediments VI 2BM 4 Sparcely phyric t aphyric basalts (pl>l>py). 29 VI 2BM-5 Olivine-rich basalts VI 2BM Sparcely phyric t aphric basalts (pl>l>py). 35 V1T Olivine-rich basalts VIII 2BM Carsely phyric t aphyric plagiclase 7 basalts. Numerus sedimentary layers IX/X Interlayered,livine-rich basalts and basalts with (pl>l>py) X/XI Sparsely phyric t aphyric basalts. Micrdleritic basalts (pl>l>py). Figure 2. Schematic sectin f Hle 332B shwing the distributin f lithlgic and chemical units. 689

10 / L. DMITRIEV - / / / / A j / CaO / A / V i / I * 1 1 '1 1 ' 1 ^ / /I / * i V' ^ r ' / λ ". TiO 2 - ^ " i i i i' I Oxides (wt. %) Figure 3. Histgrams f the majr elements in ceanic and cntinental basalts. 1 = Basalts frm Hle 332B; 2 = Mid-cean ridge basalts (2 analyses) (Sharaskin et al, in press); 3 = Cntinental livine thleiites (182 analyses) (Mansn, 1967). 69

11 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS A1 2 O 3 ; FeO; CaO O O SiO, 15 H II A1 2 3 FeO A CaO III OL SiO, q wt% Figure 4a. Crrelatin between and ther majr elements in basalts, gabbrs, trctlites, and Iherzlites. Data frm Table 1. I = Gabbr frm Site 334; II = Trclite frm Site 334; HI = Lherzlite frm Site 334; OT = Average ceanic thleite; OL = Average ceanic Iherzlite. Ovals shw the standard deviatin. T 4 691

12 L. DMITRIEV J OT TTO, Na 2 D II L III A 'O. -is,. 1 ~r.5- -fe r 3 -i r Figure 4b. Crrelatin between and ther majr elements in basalts, gabbrs, trctlites, and Iherzlites. Data frm Table 1. I = Gabbr frm Site 334; II = Trctlite frm Site 334; III = Lherzlite frm Site 334; OT = Average ceanic thleiite; OL = Average ceanic Iherzlite. Ovals shw the standard deviatin. vit% 4 692

13 PETROCHEMISTRY OF BASALTS AND PLUTONIC ROCKS Figure 5. Crrelatin between and sme trace elements in basalts, gabbrs, trctlites, and Iherzlites. Data frm Table 2. I = Gabbr frm Site 334; II = Trctlite frm Site 334; III = Lherzlite frm Site 334; OT = Average ceanic thleiite; OL = Average ceanic lherzlite. vt% 693

12. GEOCHEMISTRY OF BASALTS FROM THE NORWEGIAN-GREENLAND SEA, LEG 38, DSDP

12. GEOCHEMISTRY OF BASALTS FROM THE NORWEGIAN-GREENLAND SEA, LEG 38, DSDP 12. GECHEMISTRY F BASALTS FRM THE NRWEGIAN-GREENLAND SEA, LEG 38, DSDP H. Raschka and F.-J. Eckhardt, Bundesanstalt für Gewissenschaften und Rhstffe, Hannver, Germany INTRDUCTIN The main bjectives f these