Evidence for variability of magmatic processes and upper mantle heterogeneity in the axial region of the Mid-Atlantic Ridge near 22° and 36° N

Bougault, Henri; Cambon, Pierre; Corre, Olivier Le; Joron, Jean-Louis; Treuil, M.

doi:10.1016/0040-1951(79)90333-0

Cited by 48 publications

(29 citation statements)

References 12 publications

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“…Comprehensive studies of basalts from the North Atlantic Ocean have illustrated the chemical variability of magmas being erupted along the Mid-Atlantic Ridge (Schilling, 1973(Schilling, , 1975aHart et al, 1973;White et al, 1976;Bougault et al, 1979;Tarney et al, 1978Tarney et al, , 1979Wood et al, 1979). It is therefore important, before any detailed comparisons between back-arc-basin and ocean-ridge basalts are made, that the composition of ocean-ridge basalts be clearly delineated.…”

Section: Introductionmentioning

confidence: 99%

Geochemistry of Basalts from the Shikoku and Daito Basins, Deep Sea Drilling Project Leg 58

Marsh¹,

Saunders²,

Tarney³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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Leg 58 successfully recovered basalt at Sites 442, 443, and 444, in the Shikoku Basin, and at Site 446 in the Daito Basin. Only at Site 442 did penetration reach unequivocal oceanic layer 2; at the other sites, only off-axis sills and flows were sampled. Petrographic observations indicate that back-arc basalts from the Shikoku Basin, with the exception of the kaersutite-bearing upper sill at Site 444, are mineralogically similar to basalts being erupted at normal mid-ocean ridges. However, the Shikoku Basin basalts are commonly very vesicular, indicating a high volatile content in the magmas. Site 446 in the Daito Basin penetrated a succession of 23 sills which include both kaersutite-bearing and kaersutite-free basalt varieties.A total of 187 samples from the four sites has been analyzed for major and trace elements using X-ray-fluorescence techniques. Chemically, the basalts from Sites 442 and 443 and the lower sill of Site 444 are subalkaline tholeiites and resemble N-type ocean-ridge basalts found along the East Pacific Rise and at 22° N on the Mid-Atlantic Ridge (MAR), although they are not quite as depleted in certain hygromagmatophile (HYG) elements. They do not show any chemical affinities with island-arc tholeiites. The basalts from Site 446 and from the upper sill at Site 444 show alkaline and tholeiitic tendencies, and are enriched in the more-HYG elements; they chemically resemble enriched or E-type basalts and their differentiates found along sections of the MAR (e.g., 45°N) and on ocean islands (e.g., Iceland and the Azores).Most of the intra-site variation may be attributed to crystal settling within individual massive flows and sills, to high-level fractional crystallization in sub-ridge magma chambers, or, where there is evidence of a long period of magmatic quiescence between units, to batch partial melting. However, the basalts from Sites 442 and 443 and from the lower sill at Site 444 cannot easily be related to those from Site 446 and the upper sill at Site 444, and it is possible that the different basalt types were derived from chemically distinct mantle sources. From comparison of the Leg 58 data with those already available for other intra-oceanic back-arc basins, it appears that the mantle sources giving rise to back-arc-basin basalts are chemically as diverse as those for mid-ocean ridges. In addition, the high vesicularity of the Shikoku Basin basalts supports previous observations that the mantle source of back-arc-basin basalts may be contaminated by a hydrous component from the adjacent subduction zone. INTRODUCTIONShikoku back-arc or marginal basin, lying between the Iwo Jima and Kyushu-Palau Ridges, sampled basalts During Leg 58, basaltic rocks were recovered at four are no older than 21, 17, and 15 m.y., respectively, from of the five sites occupied in the Shikoku and Daito both sides of the north-south-trending axial zone. Only Basins. At Sites 442, 443, and 444, in the intra-oceanic at Site 442 were basalts penetrated which belong unequivocally to oceanic layer 2; at S...

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Section: Introductionmentioning

confidence: 99%

Geochemistry of Basalts from the Shikoku and Daito Basins, Deep Sea Drilling Project Leg 58

Marsh¹,

Saunders²,

Tarney³

et al. 1980

Initial Reports of the Deep Sea Drilling Project

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“…Examples of these three types of basalt are given in Table 7. N-type MORB is most abundant and is found, for example, along the Mid-Atlantic Ridge at 22°N (Bougault et al, 1979). It is characterized by very low abundances of the incompatible elements and by strongly light-REdepleted REE patterns (Analysis 12, 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

Initial Reports of the Deep Sea Drilling Project

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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.

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“…These distinctive chemical features find their counterpart in clearly different isotopic ratios, which implies the two groups were derived from different mantle sources (Castillo et al, this volume). This is substantiated by markedly different ratios of highly incompatible elements (La, Nb, Ta, Th) that reflect the nature of the source rather than variable mantle melting (Bougault et al, 1979). For example, La/Th and La/Ta ratios are consistently higher in the basalts (about 15 and 18, respectively) than the dolerites (about 10 and 12.5, respectively).…”

Section: Geochemistrymentioning

confidence: 97%

Geochemistry and Petrography of Cretaceous Sills and Lava Flows, Sites 800 and 802

Floyd¹,

Winchester²,

Castillo³

1992

Proceedings of the Ocean Drilling Program

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On the basis of their respective eruptive environments and chemical characteristics, alkalic dolerite sills from the northern Pigafetta Basin (Site 800) and tholeiitic pillow lavas from the Mariana Basin (Site 802) sampled during Ocean Drilling Program Leg 129 are considered to represent examples of the widespread mid-Cretaceous volcanic event in the western Pacific. Both groups of basic rocks feature mild, low-grade, anoxic smectite-celadonite-carbonate-pyrite alteration; late-stage oxidation is very limited in extent, with the exception of the uppermost sill unit at Site 800. The aphyric and nonvesicular Site 800 alkalic dolerite sills are all well-evolved mineralogically and chemically, being mainly of hawaiite composition, and are similar to ocean island basalts. They are characterized by high contents of incompatible elements (for example, 300-400 ppm Zr), well-fractionated rare earth element patterns ([La/Yb] N 18-21) and HIMU isotopic characters. They probably represent deep-sea, lateral, intrusive off-shoots from nearby seamounts of similar age. The olivine-plagioclase ± clinopyroxene phyric tholeiitic pillow lavas and thin flows of Site 802 are nonvesicular and quench-textured throughout. Relative to normal-type mid-ocean ridge basalt, they are enriched in large-ion-lithophile elements, exhibit flat (unfractionated) rare earth element patterns and have distinctive (lower) Zr/Nb, Zr/Ta, La/Ta, and Hf/Th ratios. Overall they are compositionally and isotopically similar to the mid-Cretaceous tholeiites of the Nauru basin and the Ontong-Java and Manihiki plateaus. The Site 802 tholeiites differ from the thickened crustal segments of the oceanic plateaus, however, in apparently representing only a thin veneer over the local basement in an off-axis environment.

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Evidence for variability of magmatic processes and upper mantle heterogeneity in the axial region of the Mid-Atlantic Ridge near 22° and 36° N

Cited by 48 publications

References 12 publications

Geochemistry of Basalts from the Shikoku and Daito Basins, Deep Sea Drilling Project Leg 58

Geochemistry of Basalts from the Shikoku and Daito Basins, Deep Sea Drilling Project Leg 58

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

Geochemistry and Petrography of Cretaceous Sills and Lava Flows, Sites 800 and 802

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