13. PETROLOGY OF BASALTS FROM DEEP SEA DRILLING PROJECT, LEG 38

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1 . PETROLOGY OF BASALTS FROM DEEP SEA DRILLING PROJECT, LEG W.I. Ridley, M.R. Perfit, and ML. Adams, LamontDoherty Geological Observatory, Columbia University, Palisades, New York INTRODUCTION We have determined the bulk composition of basalt samples drilled during Leg. These data provide a basis for discussion of trace element abundances, Sr isotopic compositions, and mineral chemistry. Basaltic basement was collected at tes,,,,,,, 0. Most of the basalts examined have textures that indicate rapid cooling, evidenced by the presence of glass and skeletalshaped olivine and Plagioclase grains. Samples from te are finegrained, subophitic basalts with microphenocrysts of Plagioclase and olivine. Variolitic, amygdaloidal basalt occurs at te containing sparse phenocrysts of altered olivine. Basalt at te is uniformly subophitic with occasional phenocrysts. At least two textural varieties were observed at te : an upper basalt with subophitic to holocrystalline texture, and a lower basalt with a coarse diabase texture. This texture also characterizes the diabase samples examined from te. Finegrained basalts from te are highly altered compared to fresher, aphyric samples from tes and 0. MAJOR ELEMENT CHEMISTRY Major elements were determined on fused glass samples by electron microprobe. θ values were computed so that θ = 0. total iron as. Analyses are computed on an Hθfree basis. Two basalts (GAST, KNIPPA) were run as secondary standards during the analysis of the Leg samples. Data are shown in Table. Basalts at te are quartz tholeiites with low O, K O, and /+ (*) ratios of. Chemically similar basalts were analyzed from te, except for slightly higher AIO values (%%) and higher * ratios (). Plagioclase tholeiites occur at te with greater than % AIO, low total, and high * ratios (). In contrast, samples from te are more alkalic, with % θ, somewhat higher KO, low total and low, but variable, * ratios (). Chemically similar basalt occurs at te. The diabase samples at te are titaniapoor olivine tholeiites, but are enriched in KO relative to tholeiites from other sites. KO enrichment may be due to deuteric alteration. These diabase samples also have high * ratios. One sample () is extremely enriched in and θ, with a lesser enrichment in KO and depletion in. We interpret this as a strongly fractionated facies of the diabase. Two samples from te are tholeiites with above average values and high * ratios (0). Typical lowtitania tholeiites were analyzed from tes and 0 with * ratios between. The overall sample population contains both quartz tholeiites and olivine tholeiites; the latter are close to being silica oversaturated. Samples from tes,,,, 0 have bulk compositions similar to many other oceanic tholeiites, and their variations in AIO, KO, and * ratio probably reflect minor removal or addition of Plagioclase and olivine together with the effects of halmyrolysis. However, those samples with high * ratios (,, ) show no signs of excess olivine and appear to be the most primitive basalts sampled. Several samples are titania rich, but are essentially tholeiites rather than alkalic basalts. These samples also have the lowest * ratios, low, and high KO and may represent basalt magmas that have undergone low pressure fractionation. TRACE ELEMENT CHEMISTRY The abundance of Rb, Sr, Zr, and Y were determined by Xray fluorescence techniques and are shown in Table. Tholeiites from tes,, and have ppm Rb, which, together with low KO contents, suggests that halmyrolysis has had little effect on basalts at these sites. Y contents ( ppm) and Zr (0 ppm) are uniformly low, and together with the low Sr values for te and basalts confirm that these samples are typical oceanic tholeiites. The Sr and AIO contents of samples from te may result from minor Plagioclase accumulation. Samples from te show fold enrichment in Zr and.fold enrichment in Sr over typical oceanic tholeiites, but Rb and Y are not strikingly enriched. Higher than normal Zr was also observed in te basalt, but Rb remained low. Diabases at te show marked enrichment in Rb ( ppm), but this is not accompanied by enrichment in Zr ( ppm) or Y ( ppm), although Sr values are rather high ( ppm). High Rb and Sr contents also characterize te samples, but Y remains low ( ppm) and Zr is variable but not exceptionally high. The trace element composition of te basalts are typical of oceanic tholeiites; Rb is low ( ppm) as are Sr ( ppm) and Zr ( ppm), whereas Y is more varied ( ppm). At te 0, trace element contents show no internal consistency, low Rb ( ppm) and Y ( ppm) are associated with high Zr ( ppm) and Sr ( ppm). Generally the behavior of the lithophile (LIL) trace elements is consistent with the bulk chemistry in that samples designated as typical oceanic tholeiites show appropriate low concentrations of LIL elements. In contrast, those samples with high θ contents show consistently high concentrations of Zr and Y and, in some cases, Sr. Even under the most favorable conditions of fractional crystallization, inspection of the

2 J to TABLE Major Element Composition and CIPW Norms of Leg Samples C O O K O as IeO loo + O = I QZ OR AB AN DI HY OL MT IL CO ;OX te (CoreSection)

3 PETROLOGY OF BASALTS TABLE Trace Element Abundances in Leg Sample (Interval in cm),, 0,, 0,,,, 0,,,, 0, 0,,,,, 0, 0 Rb Sr 0 Y 0 Zr 0 0 Basalts Rb/Sr Zr/Y trace element abundances and within the constraints afforded by the major element data, it would not be possible to derive the titaniarich and LIL elementrich basalts by fractional crystallization of a typical oceanic tholeiite. Rather, it seems that these magmas may be derivatives of a compositionally distinct (and unsampled) parent magma whose mantle source was chemically different from that of typical oceanic tholeiites. MINERAL CHEMISTRY We have determined the range of compositions of the major mineral phases in representative samples of each basalt type from each site. Mineral determinations utilized an ARLEMX microprobe with appropriate application of instrumental and Bencebee matrix corrections. Data are tabulated in Table (Plagioclase), Table (pyroxenes), and Table (irontitanium oxides). w samples contained fresh olivine, and no useful data could be determined for this mineral. Generally the minerals show a limited range of compositions as a consequence of relatively rapid cooling. As might be expected, the most extensive mineral zoning was measured from te and samples which are coarser grained than samples from other sites. Variations in major endmember components of pyroxenes are shown in Figure. The pyroxenes are dominantly calcic diopsidic augites, no lowcalcium pyroxenes were detected. The zoning trends result in depletion in the wollastonite component which is not invariably associated with iron enrichment. These trends are in contrast to the ironenrichment trends at relatively constant wollastonite content observed for many volcanic and plutonic pyroxenes. It is also noted that some of the pyroxenes fall within the twopyroxene field boundary as defined by the trend for calciumrich pyroxenes from the Skaergaard intrusion. In the Leg samples, it appears that relatively rapid cooling initiated rapid crystal growth and consequently metastable compositional trends developed. gnificant deviations from stable crystallization are also indicated by the contents of minor elements,,. Rapidly cooled pyroxenes are highly aluminous and contrast TABLE A Plagioclase in Basalts at te θ A O 0 K K An 0 0 / Note:, = microlites; = (edge), = (core) phenocryst; = microlites; = weakly zoned phenocrysts; = microlites.

W. I. RIDLEY, M. R. PERFIT, ML. ADAMS TABLE B Plagioclases in Basalts at te S Oi O A O N»θ K O Tofil.. 0...0 0..0.. 0..0. 0.... 0. 0..0. 0. 0.. 0. 0.0.0. 0..0. 0. 0.. 0. 0... 0....0. S.0 0. 0... 0.....0 0...0 0...0. 0... 0.0..0 0..0. 0..0.. 0...0 0.0...0 0..0 0.... 0.0 0... 0.. 0...... 0...0.. 0.0 0. 0. 0... 0....0..0 0.... 0... 0. Ml K.

4 W. I. RIDLEY, M. R. PERFIT, ML. ADAMS TABLE B Plagioclases in Basalts at te S Oi O A O N»θ K O Tofil S Ml K An / Note: = (core) phenocryst; = zoned phenocryst; = (core) phenocryst; = (core) phenocryst; = (core) phenocryst;, = (core) phenocryst; (edge) (core) = zoned phenocryst; (edge) (core) = phenocryst; (edge), (core) = phenocryst; (edge) (core) = phenocryst;, = (core) microphenocryst; (edge) (core) = phenocryst; (core) (edge) = phenocryst; (core), (edge) = phenocryst;, = (core) phenocryst. with the lower contents of more slowly cooled pyroxenes. nce the basalts show little variation in bulk AIO content, and there is no obvious relationship between pyroxene minor element content and bulk composition, it can be concluded that the latter is not the dominant factor in determining pyroxene compositions. A brief survey was made of Plagioclase compositions in selected basalts. Results are shown in Table. Plagioclase phenocrysts in te basalts have calcic cores (Anβo zoned to AΠÓO). Groundmass microlites range from Amo to An and probably range down to at least AΠÓO. Variations in phenocryst composition in te basalts suggests a complex history of Plagioclase crystallization. There are two compositional groups of phenocrysts, one with Amo core composition and a second with AΠÓO core composition. Zoning is quite pronounced with rims of Am composition. At te, Plagioclase phenocrysts have An composition and may be continuously zoned to An. This represents the most extreme compositional variability in any of the basalts analyzed. l of the plagioclases contain minor amounts of,, and. Generally there is a positive correlation between An content and and abundances. In addition, the / ratio decreases with decreasing An content. The highest / ratios are associated with quenched Plagioclase in te basalts. Probably the partitioning of and into the Plagioclase is not an equilibrium process for these plagioclases. We have also briefly examined the oxides in these basalts. Surprisingly, most contain primary ilmenite with minor or no titanomagnetite. Analysis of coexisting ilmenite and titanomagnetite could only be accomplished in the coarse diabase at te, indicating crystallization parameters of C and F0 = I0. " atm. SUMMARY l basaltic rocks are either quartz or olivine tholeiites which are best divided on AIO and Ch contents. In terms of major elements and limited trace element abundances, samples from tes, and are abyssal tholeiites typical of basalts dredged from active midoceanic ridges. Samples from tes,, and are highalumina tholeiites, but some have high / ratios that serve to distinguish them from highalumina abyssal tholeiites. Basalts with distinctly higher KO, θ, and overall greater enrichment in lithophile trace elements, occur at tes,, and possibly 0. Diabase at te appears to contain patches of high Ch material which may represent local segregations of in situ fractionated liquid. Phase chemical studies indicate the ubiquity of Plagioclase and pyroxene, general lack of olivine, and rarity of titanomagnetite relative to ilmenite. Compositional zoning is most pronounced in Plagioclase phenocrysts which may have grown largely prior to eruption. Compositional zoning is not extensive in the pyroxenes which are all diopsidic augites, and trends are indicative of rather fast, metastable crystallization. Minor element abundances in both Plagioclase (,, ) and pyroxene (,, ) indicate an influence of bulk mineral chemistry, but also cooling rate has had a large effect.

5 PETROLOGY OF BASALTS , , , TABLE B Continued , , , , , TABLE A Pyroxenes in Basalts at tes and 0 0 O O O Ai A VI ZTet rtions Note:, = Phenocrysts;, = groundmass grains; = center to edge of phenocrysts; = center of phenocryst; = center to edge of phenocrysts.

6 W. I. RIDLEY, M. R. PERFIT, ML. ADAMS TABLE B Pyroxenes in Basalts at te and O A O O A IV VI ZTet Ltions Note: Edge to center of phenocryst. TABLE C Pyroxenes in Basalts at te O O O VI STet Stions , Note:, = Edge and center of phenocryst; = center of small phenocryst;, = edge and center of phenocryst;, groundmass grains; = phenocrysts.

7 PETROLOGY OF BASALTS TABLE D Pyroxenes in Basalts at te O O O A IV VI ETet Etions = , O O A IV A VI ETet Etions Note: = Edge to center, phenocryst;, = edge to center, phenocryst; = edge to center, phenocryst;, = groundmass microlites;, = edge and center of phenocryst;, = center and edge of phenocryst; = inclusion in Plagioclase; = microphenocrysts.

8 W. I. RIDLEY, M. R. PERFIT, ML. ADAMS TABLE E Pyroxenes in Basalts at te O O O VI Tet Ztions O O O IV VI ZTet Etions Note: = Edgecenter of phenocrysts, = center of phenocryst; = centeredge of phenocryst;, = center of phenocrysts; = individual phenocrysts; = center of phenocrysts; = center to edge of phenocryst; = center to edge of phenocryst.

9 PETROLOGY OF BASALTS TABLE Spinels in Leg ;\ Basalts te θ A O O O % Ulv. % RO..... Ilmenite Basis θ log fθ..0 T C 0 Λ Vio% v Figure. Pyroxene compositions in Leg basalts in terms of quadrilateral components diopside (Dl)enstatite (EN)ferrosuite (FS)hedenbergite (HD). The size of the triangles is an indication of the amount of components other than the above present in the pyroxene. These "other" components are due to solid solutions of,, and in the pyroxene structure.

Igneous and Metamorphic Rock Forming Minerals. Department of Geology Mr. Victor Tibane SGM 210_2013

Igneous and Metamorphic Rock Forming Minerals Department of Geology Mr. Victor Tibane 1 SGM 210_2013 Grotzinger Jordan Understanding Earth Sixth Edition Chapter 4: IGNEOUS ROCKS Solids from Melts 2011