Geochemistry of Basalts from Holes 519A, 520, 522B, and 524, Deep Sea Drilling Project Leg 73 (South Atlantic)

Abstract

During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 5 1) and 522B (Anomaly 16) have ages of ~ 12 m.y. and ~38 m.y., respectively. The major-and trace-(including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption. In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts. In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it. Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of ~ 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick,-74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks. The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

Calcium Isotope Composition of Meteorites, Earth, and Mars

“…Ages and K/Ca compositions of meteorites come from Jarosewich (1990), Kallemeyn et al (1989), Laul et al (1986), Mittlefehldt (1994), Mittlefehldt & Lindstrom (1990), Shih et al (1993), Stolper & McSween (1979), von Michaelis et al (1969, and Warren et al (1999). Igneous rocks from Earth include the following: DM-40 and DM-10 evolved marginal members of the Precambrian Muskox layered intrusion; 81-AZ-WG-1 a Late Cretaceous peraluminous muscovite-bearing granite from the Santa Catalina Mountains of southern Arizona (Farmer & DePaolo 1984); Tsm-7 a strongly welded vitric crystal-poor rhyolitic Tertiary ignimbrite from the early Sapinero Mesa Tuff collected from its type locality near its source the San Juan and Uncompahgre caldera complex, western San Juan volcanic field, Colorado (Simon 2000); Fish Canyon Tuff a slightly welded plagioclase sanidine hornblende quartz crystal-rich Tertiary dacitic ignimbrite collected in Saguache Creek, Colorado some tens of km from its source the La Garita caldera, central San Juan volcanic field (Whitney & Stormer 1985); 83-IB-1 and 83-IB-12 biotite and hornblende-bearing granodiorites from central and northeastern Oregon, respectively (Marshall & DePaolo 1989); BCR-1 a well characterized U.S. Geological Survey standard of Columbia River Basalt, Oregon (Gladney et al 1983); 519-8-3 and 519-95 massive tholeiitic normal-type mid-ocean ridge basalts, dredged from the South Atlantic ridge during the Deep Sea Drilling Project Leg 73 (Dietrich et al 1984); BMR-10 a relatively coarse-grained plagioclase and clinopyroxene phyric basalt from Mount Bangum a Quaternary volcano in the New Britain island arc of Papua New Guinea (DePaolo & Johnson 1979); NM5087 a plagioclase phyric massive tholeiitic normal-type mid-ocean ridge basalt flow unit dredged from the North Atlantic ridge during the expedition at 22…”