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• W2033995087 startingPage "379" @default.
• W2033995087 abstract "Variations in the abundances of Zn, Cu, and Pb are found to be useful in identifying tectonic regimes and separating oceanisland basalts into enriched- and depleted-source categories. The average Zn, Cu, and Pb contents of normal mid-ocean ridge basalts (N-MORB) are 84, 70, and 0.35 ppm, respectively. Differences in average Zn contents for various ridges reflect more the varying degrees of differentiation than variations of Zn content in the source rocks. At a Mg# of 70, or Mg#70, which is taken to represent primitive MORB, many MORB sequences converge at a Zn content of 58 ± 6 ppm, which is close to the value for primitive mantle (50 ppm) and ordinary chondrites (∼55 ppm). Values of 0.1 to 0.15 ppm Pb in MORB at Mg#70, best defined at the superfast-spreading Southern East Pacific Rise, are similar to estimates of Pb in the primitive mantle (0.12 to 0.18 ppm). They also are near the lower end of the range for ordinary chondrites. The very slow spreading Southwest Indian Ocean Ridge has a sequence with higher Pb contents, in addition to a more normal sequence, which has a visual best value of 0.4 ppm Pb at Mg#70. With the exception of the Walvis Ridge, Zn and Cu appear to be little affected by proximity to hotspots (i.e., E-MORB); however, Pb contents are higher and average about 0.6 ppm. Both Zn and Pb in MORB are incompatible elements (i.e., favor the melt), but Cu is a compatible element. At Mg#70, there is the suggestion of a value of 100 ppm for Cu, with lower values possibly representing partial removal of sulfides and their associated Cu from the source. Nonetheless, Cu contents of primitive MORB tend to be much higher than even high estimates for the primitive mantle (28 ppm), and are closer to ordinary chondrites (∼90 ppm). Therefore, Zn, Cu, and Pb all approximate chondritic values in the primitive MORB melt. Average contents of Zn, Cu, and Pb in oceanic island basalts (OIB) are 115, 62, and 3.2 ppm, respectively. At Mg#70, values of Zn and Cu are similar to the respective averages for OIB, with Zn higher and Cu lower than MORB. At a Mg# of ∼40, however, OIB and MORB tend to have similar Zn contents. With further differentiation, OIB trachytes can contain >200 ppm Zn. Unlike MORB, OIB can differentiate to high Cu contents of 200 ppm at Mg#s of 40 to 60. In contrast to Zn and Cu, Pb regresses to a value of 0.83 ppm at Mg#70 for Hawaiian and Reunion volcanics, which is much less than the average value for Pb in OIB volcanics, but higher than for MORB. Average Zn, Cu, and Pb contents of magmatic-arc basalts are 77, 108, and 1.9 ppm, respectively. In basalts, Zn tends to be incompatible, but a dual incompatible and compatible behavior can occur at high SiO2 contents. Dacites may average near 55 ppm Zn, but peralkalic rhyolite can contain >300 ppm Zn. A dual compatible and incompatible nature occurs for Cu. Most common, particularly in submarine volcanics, is a compatible trend, with a Cu content of around 80 ppm at a Mg# of 60, which decreases to less than 40 ppm at a Mg# of 30. The incompatible trend of increasing Cu can achieve >200 ppm at a Mg# of 30, leaving a gap approaching 100 ppm at that Mg#. The gap is less obvious on a plot of Cu vs. SiO2, but is still there. The compatible trend is proposed to result from sulfur-saturated magmas, whereas the incompatible trend is believed to result from sulfur-deficient magmas. Support for this hypothesis is found in sparse sulfur-isotope data. Zn and Cu both can be incompatible over an extended range of Mg#s or silica content. When Zn and Cu are both compatible, Cu decreases more than twice as rapidly as Zn. Primitive magmas at Mg#70 average about 50 ppm Zn for submarine Mariana arc basalts and 58 ppm for forearc boninites, contents close to MORB values. Mariana arc basalts have a Zn content of ∼45 ppm estimated at Mg#70. Cu varies more widely than Zn in primitive magmas, being about 50 ppm Cu for Mariana Islands volcanics and 120 ppm for Kermadec Islands volcanics, a range broadly around MORB values. Average Pb contents are 1.9 ppm for island-arc tholeiites, 5.6 ppm for high-Al basalt, and 3.2 ppm for alkali basalt with average boninite of approximately 1.8 ppm. Back-arc-basin basalts in the deepest parts of the Mariana trough have Pb contents of 0.45 ppm, but more shallow parts may exceed 1.0 ppm Pb. Although the lower contents are similar to MORB values, the 208Pb/204Pb values are greater than Pacific Ocean MORB. At Mg#70 for rocks from the Tonga and Kermadec island arcs, the Pb content is about 0.1 ppm, similar to MORB." @default.
• W2033995087 created "2016-06-24" @default.
• W2033995087 creator A5047777766 @default.
• W2033995087 date "1995-05-01" @default.
• W2033995087 modified "2023-10-16" @default.
• W2033995087 title "Zinc, Copper, and Lead Geochemistry of Oceanic Igneous Rocks—Ridges, Islands, and Arcs" @default.
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• W2033995087 doi "https://doi.org/10.1080/00206819509465410" @default.
• W2033995087 hasPublicationYear "1995" @default.
• W2033995087 type Work @default.

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