Mantle Heterogeneity in the North Atlantic: Evidence from Leg 49 Geochemistry

Chemical compositions and 1-atm. phase relations were determined for basalts drilled from Holes 501, 5O4A, 504B, 505, and 5O5B on Legs 68, 69, and 70 of the Deep Sea Drilling Project. Chemical, experimental, and petrographic data indicate that these basalts are moderately evolved (Mg' values from 0.60 to 0.70), with olivine plus Plagioclase and often clinopyroxene on the liquidus. Chemical stratigraphy was used to infer that sequential influxes of magma into a differentiating magma chamber or separate flows from different magma chambers or both had occurred. Two major types of basalt were found to be inter layered: Group M, a rarely occurring type with major element chemistry and magmaphile element abundances within the range of the majority of ocean-floor basalts (TiO 2 = 1.3%, Na 2 O 2.5%,Zr = 103 ppm, Nb 2.5 ppm, and Y = 31 ppm); and Group D, a highly unusual series of basalt compositions that exhibit much lower magmaphile element abundances (TiO 2 = 0.75-1.2%, Na 2 O = 1.7-2.3%, Zr = 34-60 ppm, Nb = 0.5-1.2 ppm, and Y = 16-27 ppm). The liquidus temperatures of the Group D basalts are high (1230-1260°C) compared with those of other ocean-floor basalts of similar Mg' values. They have high CaO/Na 2 O ratios (5-8) and are calculated to be in equilibrium with unusually calcic Plagioclase (An 78 _g4). The two basalt groups cannot be related by fractionation processes. However, constant Zr/Nb ratios (40) for the two groups suggest a single mantle source, with differences in magmaphile element abundances and other element ratios (e.g., Zr/Ti, Zr/Y, Ce/Yb) arising through sequential melting of the same source. Magmas similar to Group D, if mixed with more typical mid-ocean-ridge basalt (MORB) magmas in shallow magma chambers, could provide a source for the highly calcic Plagioclase phenocrysts that appear in more common (i.e., less depleted) phyric ocean-floor basalts.

Section: Basalt Chemistrysupporting

confidence: 57%

Costa Rica Rift Zone Basalts: Geochemical and Experimental Data from a Possible Example of Multistage Melting

Autio¹

1983

“…We see the East Pacific Rise as having a regional mantle source at one end of the spectrum of Rise-ridge mantle sources; it has a low proportion of such undepleted "pockets." At the other end of the spectrum is the North Atlantic, which seems to have a mantle source which consists of little else but such undepleted pockets (e.g., Bougault et al, 1979;Tarney et al, 1979) with many compositions (Cann et al, 1979). An explanation for the sequence seen on OCP Ridge-where undeniably depleted tholeiites make up much of the ridge whilst less depleted basalts cap its summit-might lie in local elevation of the geotherm as less dense, undepleted mantle-perhaps containing some melt fraction-impinges on the lithosphere from below.…”

Section: Tholeiite-alkalic Basalt Mixing Hypothesismentioning

confidence: 99%

Compositions of Basaltic Glasses from the East Pacific Rise and Siqueiros Fracture Zone, Near 9°N

Natland¹,

Melson²

1980

The compositions of 45 natural basalt glasses from nine dredge stations and six Deep Sea Drilling Project Leg 54 sites near 9°N on the East Pacific Rise have been determined by electron microprobe. These comprise 19 distinct chemical groups. Seventeen of these fall in the range of the eastern Pacific tholeiite suite, which is characterized by marked enrichment in FeO*, TiO 2 , K 2 O, and P 2 O 5 as CaO, MgO, and A1 2 O 3 all decrease. Based on trace elements, an estimated 50-75 per cent fractionation of plagioclase, clinopyroxene, and olivine is required to produce ferrobasalts from parental olivine tholeiites. Additional chemical variations occur which require source heterogeneities, differences in the degree of melting, different courses of shallow fractionation, or magma mixing to explain. Glass compositions from within the Siqueiros fracture zone are mostly less fractionated than those from the flanks of the Rise, and show chemical differences which require variations in the depth of melting or highpressure fractionation to explain. Some of them could not be parental to East Pacific Rise flank ferrobasalts. Two remaining glass groups, from dredge hauls atop a ridge and a seamount, respectively, have distinctly higher K 2 O, P 2 O 5 , and TiO 2 as well as lower CaO/Al 2 O 3 and SiO 2 at corresponding values of MgO than the tholeiite suite. These abundances, and whole-rock Y/Zr, Ce/Y, Nb/Zr, and isotopic abundances indicate that these basalts had a deeper, less depleted mantle source than the Rise tholeiite suite. Trace element abundances preclude the "ridge" basalt type from being a hybrid between the "seamount" basalt type and any East Pacific Rise tholeiite so far analyzed. The East Pacific Rise glasses from 9°N compare very closely to glasses dredged and drilled elsewhere on the East Pacific Rise. However, glass compositions from Site 424 on the Galapagos Rift drilled during Leg 54, as well as glasses and basalts dredged from the Galapagos and Costa Rica rifts, indicate that a greater degree of melting prevailed along much of the Galapagos Spreading Center than anywhere along the East Pacific Rise. PROCEDURES Samples were selected aboard the Glomar Challenger and from dredge hauls archived at the Scripps Institution of Oceanography. The sample selection for the DSDP materials was guided by the availability of glasses, and by identification of distinct petrographic types. The dredge sample selection was guided by published analyses and descriptions (Batiza et al., 1977; Johnson, 1979; Batiza and Johnson, this volume; Schrader et al., this volume). Of particular interest were olivine-spinel phyric oceanites (dredge SD-7, Siqueiros Expedition, Siqueiros fracture zone, described by Schrader et al., this volume), alkalic olivine basalts (dredge SD-8, Siqueiros Expedition, Siqueiros fracture zone, described by Batiza et al., 1977), and various MgO-rich and MgO-poor basalts that have been used elsewhere in fractionation and mixing calculations (Batiza et al., 1977; Batiza and Johnson, this volume). Samples were an...

“…Second, it emphasizes the overall unity of the magmatism, since, although Hole 552 shows only the Zr-enriched type (with very small basement penetration) and Hole 554 only the Zr-poor type, the other two holes (with the greatest basement penetration), show both magma types in the same section. The diversity of magma types can probably, as has been said above, be related to mantle heterogeneity, such as has already been documented on Leg 49 of the DSDP (Cann et al, 1979) and south of the Azores (Bougault and Treuil, 1980). The within-hole variation can be seen from the plots of Figures 2-5 but is also shown in Figure 6, where two variables have been chosen to plot against depth in each hole.…”

Section: Geochemical Variation Within Andmentioning

confidence: 74%

Trace and Major Element Geochemistry of Basalts from Leg 81

Richardson¹,

Oakley²,

Cann³

1984

Self Cite

Fifty-two samples of basalt from the four holes drilled on the Leg 81 transect across the Rockall margin were analyzed by X-ray fluorescence for Rb, Sr, Y, Zr, and Nb. On the basis of these results 13 samples were chosen for major and supplementary trace-element analysis. The results show no progressive change in the character of the volcanism, from Hole 555 in the continental domain through Holes 552 and 553A in the dipping reflector sequence to Hole 554A on the outer high. Two distinct magma types are present, apparently reflecting heterogeneity of the underlying mantle, but both types are present in both Holes 553A and 555, while Hole 552 and Hole 554 are each composed of a single type. Both magma types have a clear ocean-floor basalt signature when examined by discrimination diagrams, as does the basalt from Deep Sea Drilling Project Site 112, which formed at the same time as the Leg 81 basalts slightly farther south along the spreading center. In contrast, the basalts of East Greenland, formed at the same time, are more enriched in incompatible elements and have a within-plate geochemical signature, as is found in some basalts of Iceland today. Clearly the present distinction in geochemistry between the basalts of Iceland and those erupting well south on the Reykjanes Ridge was already established when continental splitting took place.