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Deep Sea Drilling Project Initial Reports Volume 37

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Deep Sea Drilling Project Initial Reports Volume 37 57. GEOCHEMISTRY, NORMATIVE MINERALOGY, AND DIFFERENTIATION TRENDS OF BASALT GLASSES FROM DSDP LEG 37 F. Aumento, Dalhousie University, Halifax, Nova Scotia, Canada and D.R.C. Kermpe, Department of Mineralogy, British Museum (Natural History), London, England The geochemical investigation of basaltic glasses oc- groundmass of the crystalline rocks, in contrast to the curring on the margins of crystalline lavas for which fresh glasses selected for analysis. The relative impor- abundant geochemical data are also available may be a tance of these two factors is evaluated with the data in key factor in evaluating both the differentiation hand. patterns of the magmas involved and their original The main mineral phases found in both the glasses compositional variations. In addition, through the ease and the crystalline rocks include plagioclase, olivine, with which devitrification, and hence alteration, can be clinopyroxene, and spinels. Addition of model concen- detected in these glasses, it is possible to produce trations of these phases to the glasses, thereby ap- analytical data on material for which one may be cer- proximating the crystalline whole-rock compositions, tain that little or no chemical exchange with seawater will cause the +Ve or -Ve shifts shown in Table 1. It has taken place. The latter may not be true for the will be seen that this multiple-phase addition will ex- associated fine-grained rocks, where incipient altera- plain adequately the lower Siθ2 and Tiθ2 content of the tion is not always evident. rocks compared to the glasses, and may explain in part Over 190 basalt glasses from Holes 332A, 332B, the similar concentrations of AI2O3 and CaO found in 332D, 333A and Sites 334 and 335 were analyzed using both media, but not the tendency for AI2O3 to be lower, Cambridge and ARL microprobes at Dalhousie or CaO to be higher in the crystalline rocks. More im- University and the Smithsonian Institution, respective- portant, these mineralogical additions in no way ex- ly. Since most the available glasses came from Holes plain the lower FeOTOT and much higher K2O values 332A and 332B and Site 335, these are dealt with more for the rock; similarly, their addition would disrupt the fully, and some 175 analyses are presented for them in similar MgO and Na2θ concentrations found in both Chapters 2, 3, 4, and 5 (this volume). the crystalline and glassy media. Obviously, therefore, The major element concentrations for the glasses adding modal amounts of the visible mineral phases to from Holes 332A, 332B, and Site 335 are plotted these glasses will not help all the compositional dis- against depth in Figures 1 and 2, where the glass data crepancies to converge; the opposite is true, especially are superimposed on the compositional fields formed in the case of K2O and possible of Na2θ, MgO, and TOT by the associated crystalline whole-rock analyses. Site FeO 335, which has the simplest geochemistry, serves to As mentioned previously, halmyrolysis may be the evaluate the analytical results rather well. Figure 2 other agent affecting the compositional discrepancies. shows two systematic fields of distribution for both Table 1 also shows the direction of chemical variations Tiθ2 and K2O: in each case the lower values were one might expect the glass compositions to take given produced by the ARL probe at the Smithsonian and the exposure to seawater (after Hart, 1970, Aumento et al., higher values by the Cambridge probe at Dalhousie. in press). The effects of halmyrolysis are not completely Other oxide determinations do not show such marked understood, and on occasion opposing trends have analytical discrepancies, which were traced to the use of been reported. Halmyrolysis should enhance the different standards by the two laboratories. In addition mineral addition effects in the cases of Siθ2 and Tiθ2, to these analytical artifacts, a systematic compositional and assist the otherwise nonexistent AI2O3 and CaO shift between glasses and crystalline rocks is also ap- trends. At the same time it will enhance the glass/rock parent. The glasses are always relatively enriched in divergence by increasing the -Ve effect of Na2θ and the Siθ2, Tiθ2, FeOTOT , and in some instances in AI2O3 as -1-Ve one of FeOTOT . However, it will oppose the -Ve well, and depleted in K2O. CaO and Na2θ (and MgO, mineralogical K2O effect, thereby providing a possible which is not plotted) do not show systematic dif- explanation for the lower K2O concentrations found in ferences, although for site 335 CaO in glasses is lower the fresh glass relative to the crystalline whole rock than in their crystalline counterparts. Lower concen- analyses. trations of potassium, a lithophile element, in the glass Halmyrolysis, therefore, has affected the crystalline phase, is one of the more surprising finds revealed by rocks to a considerable extent. We cannot say how these plots. much interaction with seawater has altered all the These compositional shifts could result from: (1) the original oxide concentrations since some of the effects accumulation, in the crystalline rocks, of distinct have the same trend as those of the mineral additions; mineral phases in a groundmass of composition similar other halmyrolysis effects, notably those of FeOTOT to that of the glasses, or (2) the halmyrolysis of the and Na2θ, seem to have trends opposed to those 729 o TOT SiO2 TiO2 AI203 FeO CαO Nα2O K20 44 48 0.3 0.7 15 20 6 8 10 II 13 15 2.0 3.( 0.2 0.4 100 I ' I 100 • « 332 A 200 200 300 300 x Q_ Lü Q 400 400 JJ5 I • r II i 450 450 ü 500 1 500 mi 550 550 Figure 1. Downhole variations of the major elements in Hole 332A and Site 335 glasses superimposed on the whole-rock compositional fields. TOT SiO2 TiO2 AI203 FeO CαO Nα2O K20 44 48 50 0.4 0.8 1.2 10 15 20 25 6 8 10 9 II 13 15 2.0 3.0 0.2 0.4 0.6 ~T—i—I—r i—•—r 332 B 200- 200 300 300 .400 400 E CΛ • H x h- Q_ g Lü Q 500 500 1 W 600 600 • > σ w 700 700 o ss H Figure 2. Downhole variations for the major elements in 332B glasses superimposed on the whole-rock compositional fields. g F. AUMENTO, D. R. C. KEMPE TABLE 1 Summary of Geochemistry, Normative Mineralogy, and Differentiation Trends of Basalt Glasses 1 Whole Rock Modal Postulated Composition Mineral Halmyrolysis Relative to Effect of Mineral Additions to Glass Addition Effect on Glass Plag. 01 CPX SP Effect Glass SiC 2 - A12O3 -or= FeOTOT - +or= CaO +or= MgO = Na2O = + K2O Note: Column 1 gives the relative compositional differences between the whole rock analyses and the associated fresh glasses; columns 2 to 5 give the shifts expected from original glass compositions through the addition of the minerals plagioclase, olivine, clinopyroxene, and spinel; column 6 attempts to show the total overall changes expected in the glass analyses with the addition of modal concentrations of these minerals, and column 7 shows possible halmyrolysis effects on original glass compositions. Aumento et al. (in press) reported for other oceanic basalts, including basalt glasses, which in the past have rocks. The latter is not surprising, however, since, for been recovered only from the upper reaches of the example, the loss of Na2θ to seawater detected by oceanic crust, are quartz normative. Aumento et al. (in press) was thought to be remarkable, Plots of the Ab/(Ab + An) ratios against the Mg and the reversal of this trend, reported here, is more (Mg + Fe* + Mn) ratios (Figure 4) show the basalt acceptable. What is clear, however, is that examination glasses from all holes to be fairly "primitive,"'forming a of these fresh glasses offers us the only opportunity of trend at Site 332 parallel to the general fractionation measuring original compositions free from the effects trend for tholeiites, with Hole 332A basalts being the of halmyrolysis. We should use the glass data and apply most differentiated. No differentiation is apparent at corrections to the whole rock oxide values measured, Site 335, whereas the cores from Sites 333 and 334— the thereby reproducing their original, prehalmyrolysis basalt of the latter closely resembling that from Site compositions. 257, Leg 26—(Kempe, 1974) are too short to be plotted. The Leg 37 shipboard report noted the existence of a If the Mg and Ab enrichment trends and the T1O2 (wt number of repetitive differentiation cycles down the %) content are plotted against normalized normative ol deeper holes. The relative scarcity of fresh glass does content (positive) or Q content (negative), similar not permit us to carry out such detailed studies, but trends are apparent in each case: the three parameters some differentiation trends are still evident. thus reflect the changes in normative mineralogy, and All but 3 of the 33 Hole 332A and the 4 Hole 332D in each other. Even closer correlation is obtained if the glasses (normalized to Fe2θ3 = 1.50%, cofree) are Mg/(Mg + Fe* + Mn) ratio is plotted against Tiθ2 for, quartz tholeiites (PL-tholeiites in the classification of for example, Site 332. Between the different sites, Shido et al., 1971), whereas the 51 of Hole 332B, apart however, there are considerable differences (Figure 5). from the 7 of the upper section down to Core 8, are Holes 332A and 332B show distinct trends towards olivine tholeiites (OL-tholeiites) (Figure 3). If the true Fe/Mg, Ab/An, and Tiθ2 enrichment with increase in levels of the basalt horizons are equated so that the normative quartz. Hole 333A shows a slight trend; Site sediment-basalt contact is 44 meters deeper for Hole 334 glasses form a group; wjiereas Site 335, with a 332B than for Hole 332A, it appears that 332A and similar range to Site 332 in normative olivine and 332D basalts are equivalent to those of the upper part quartz, shows almost constant TiCh and, as already of 332B and are structurally higher than basalts in the stated, little or no Fe/Mg and Ab/An differentiation.
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