Magnetic and Petrologic Variations along the Galapagos Spreading Center and Their Relation to the Galapagos Melting Anomaly

Basalts from the Galapagos Spreading Center were examined through chemical and petrographic means. They were found to be generally sparsely Plagioclase, clinopyroxene phyric. Samples of basalts from essentially age 0 to 2.7 m.y. were studied. Fractionation trends comparable to other tholeiitic suites are applicable in relating the basalts of this study group. The higher Fe and Ti contents of Galapagos basalts can be explained by complex Plagioclase, olivine, and clinopyroxene fractionation.Alteration of the basalts occurred post-cooling and was generated by reaction with: (1) a reducing hydrothermal fluid and (2) oxygenated, descending seawater. Alteration is restricted to rinds paralleling cracks and fractures, which allowed egress of mineralizing fluids. The paragenesis of the secondary mineral suite is as follows: pyrite-K-poor smectite-K-rich smectite-Fe-oxides (hydrated) ± calcite/zeolites.

“…Based on FeO* and TiO 2 contents, our samples are quite similar to those from Site D-l7, described by Anderson et al (1975) for the Galapagos Spreading Center.…”

Section: Igneous Geochemistrysupporting

confidence: 52%

Geochemistry and Mineralogy of Fresh and Altered Basalts from the Galapagos Rift

Schrader¹,

Stow²

1983

“…[Melson et al, 1976]) dredged from the region of the Galapagos spreading center (Anderson et al, 1975;Schilling et al, 1976), and drilled from the nearby Peru basin (Kempe, 1976). The low Ni (30 to 90 ppm) and Co (53 to 69 ppm) contents are compatible with the high FeOVMgO ratios (~2.1), both values suggesting substantial fractionation of Mg-rich olivine from a precursor magma.…”

Section: Holes 424 and 424bmentioning

confidence: 57%

“…Rocks having similar FeO* and SiO 2 contents have been dredged (Anderson et al, 1975;Schilling et al, 1976) from the Galapagos region and drilled (Kempe, 1976) from the Peru basin, but none is reported to be so depleted in Na 2 O as those described here. The less-fractionated nature (compared with Site 424 rocks) of Site 425 basalt applies also to very low P 2 O 5 , Sr, and Ba in the three samples, and very low K 2 O in two of them.…”

Section: Site 425mentioning

confidence: 99%

See 1 more Smart Citation

Petrology of Basalt Drilled from the Galapagos Spreading Center, Deep Sea Drilling Project Leg 54

Fodor¹,

Berkley²,

Keil³

et al. 1980

Basalt recovered from Site 424, 22 km south of the Galapagos spreading center, East Pacific Ocean, is FeO*-and TiO 2 -rich (-13.3 and 1.85 wt. %, respectively, FeO* expressing the total Fe content) oceanic tholeiite containing groundmass clinopyroxene (Fs 16 _ 35 ), plagioclase (An 60 _ 70 ), titaniferous magnetite, pyrrhotite, and interstitial material, much of which is altered but SiO 2 -rich where fresh. Trace-element abundances in these ferrobasalts (FeOVMgÕ 2.1) show low Ni contents (30 to 90 ppm), a depletion in light REE, and a negative Eu anomaly. Basalt recovered from Site 425, 62 km north of the spreading center, is high-SiO 2 (51.5 wt %) and lowNa 2 O (-2.0 wt °/o) oceanic tholeiite and is composed of phases similar to those of Site 424 ferrobasalt, except that clinopyroxene is richer in MgO (Fs lo _ 3O ). Basalts at both sites are relatively fresh and free of notable hydrothermal alteration.The ferrobasalts are compositionally similar to those previously reported from the region of the Galapagos spreading center, and are characteristic of a "normal" mid-oceanic ridge environment where substantial clinopyroxene and plagioclase (and minor olivine) fractionation occurred in light REE and LIL element-depleted parental magma. The Site 425 basalt, although fractionated, is substantially closer to primitive oceanic tholeiite than ferrobasalt and is probably the product of largely plagioclase and lesser olivine fractionation from primitive oceanic tholeiite magma.

“…Ferrobasalts similar to those from Site 424 have also been dredged from the Galapagos Rift (Anderson et al, 1975;Schilling et al, 1976), from the Juan de Fuca Ridge (Dietrick and Lynn, 1975), and drilled on the Nazca plate (DSDP Leg 34, Bunch and Laborde, 1976;Kempe, 1976) and on the East Pacific Rise in the Siqueiros area (DSDP Sites 420-423, 427-429; this volume). Examples of the least and most evolved basalts from these areas are given in Table 7.…”

Section: Discussionmentioning

confidence: 99%

Geochemistry and Mineralogy of Basalts from the Galapagos Spreading Center, Deep Sea Drilling Project Leg 54

Mattey¹,

Muir²

1980

Basalts drilled at Site 424 and Hole 425 near the Galapagos Spreading Center at 86°W are moderately evolved basalts and ferrobasalts characteristic of the non-rifted ridges in the eastern Pacific. Three distinct chemical types occur at Hole 425, but only one type, a ferrobasalt, occurs in all four holes at Site 424. Despite strong enrichment of Fe, Ti, Zr, and the rare-earth elements, these basalts are similar to depleted mid-ocean ridge basalts and were therefore generated within a depleted mantle source. Electron-microprobe mineral analyses were obtained on several samples; one from Site 424 was intensively studied. The variations in groundmass pyroxene compositions within the single sample are almost equivalent to the entire variation observed in all the Galapagos basalts. Phenocryst compositions are used to model least-squares crystal fractionation among the basalts. Although satisfactory fits to major-oxide compositions can be obtained by fractionating mainly plagioclase and clinopyroxene plus minor olivine, this fails to account for the levels of certain trace elements in the most evolved basalts of Hole 425. Instead of fractionation, we believe that mixing of ferrobasalts with more primitive basalts explains these variations more satisfactorily.