The East Pacific Rise (EPR) at 11°20′N erupts an unusually high proportion of enriched mid‐ocean ridge basalts (E‐MORB) and thus is ideal for studying the origin of the enriched heterogeneities in the EPR mantle far from mantle plumes. These basalts exhibit large compositional variations (e.g., [La/Sm] N = 0.68–1.47, 87 Sr/ 86 Sr = 0.702508–0.702822, and 143 Nd/ 144 Nd = 0.513053–0.513215). The 87 Sr/ 86 Sr and 143 Nd/ 144 Nd correlate with each other, with ratios of incompatible elements (e.g., Ba/Zr, La/Sm, and Sm/Yb) and with the abundances and ratios of major elements (TiO 2 , Al 2 O 3 , FeO, CaO, Na 2 O, and CaO/Al 2 O 3 ) after correction for fractionation effect. These correlations are interpreted to result from melting of a two‐component mantle with the enriched component residing as physically distinct domains in the ambient depleted matrix. The observation of [Nb/Th] PM > 1 and [Ta/U] PM > 1, plus fractionated Nb/U, Ce/Pb, and Nb/La ratios, in lavas from the northern EPR region suggests that the enriched domains and depleted matrix both are constituents of recycled oceanic lithosphere. The recycled crustal/eclogitic lithologies are the major source of the enriched domains, whereas the recycled mantle/peridotitic residues are the most depleted matrix. On Pb‐Sr isotope plot, the 11°20′N data form a trend orthogonal to the main trend defined by the existing EPR data, indicating that the enriched component has high 87 Sr/ 86 Sr and low 206 Pb/ 204 Pb and 143 Nd/ 144 Nd. This isotopic relationship, together with mantle tomographic studies, suggests that the source material of 11°20′N lavas may have come from the Hawaiian plume. This “distal plume‐ridge interaction” between the EPR and Hawaii contrasts with the “proximal plume‐ridge interactions” seen along the Mid‐Atlantic Ridge. The so‐called “garnet signature” in MORB is interpreted to result from partial melting of the eclogitic lithologies. The positive Na 8 ‐Si 8 /Fe 8 and negative Ca 8 /Al 8 ‐Si 8 /Fe 8 trends defined by EPR lavas result from mantle compositional (vs. temperature) variation.

Young intra-transform lavas erupted as a result of extension within the Garrett Transform Fault on the southern East Pacific Rise, are more porphyritic, less evolved, have lower concentrations of incompatible trace elements, and lower ratios of more incompatible to less incompatible elements (e.g. low K/Ti and La/Sm) compared to lavas from the adjacent East Pacific Rise ridge axis. Sr, Nd and Pb isotope compositions overlap with the depleted end of the field for Pacific mid-ocean ridge basalts, but extend to lower 87Sr/86Sr (0.702137), 206Pb/204Pb (17.462), 207Pb/204Pb (15.331), 208Pb/204Pb (36.831), and higher 143Nd/144Nd (0.513345) than any lavas previously reported from the Pacific. Peridotites from the Garrett Transform have Nd isotope compositions within the range of the intra-transform lavas. The unusual major and trace element compositions of the Garrett lavas appear to be characteristic of other intra-transform lavas from elsewhere in the Pacific. The chemical and isotopic features of the Garrett lavas can be explained by remelting, beneath the transform, a two-component upper mantle which was depleted in incompatible element-enriched heterogeneities during melting beneath the East Pacific Rise ridge axis (within the past 1 Ma). Our data place new constraints on the trace element and isotope composition of the depleted mantle component that contributes to magmatism in the Pacific, and show that this component is heterogeneous, both on the scale of a single transform fault, and on the scale of an ocean basin.

The Archaean gneiss complex of the Saglek-Hebron area in northern Labrador is dominated by polyphase similar to 3.7 Ga old tonalitic-trondhjemitic-granodioritic (TTG) Uivak I Gneiss, that exhibit 3.8-3.9 Ga inheritance. These gneisses are interleaved with a heterogeneous group of supracrustal rocks known as the Nulliak supracrustal assemblage. Early Archaean ultramafic rocks occur either as tectonically-emplaced slivers of lithospheric mantle along structural contacts within the Uivak I Gneiss, or as layers of metakomatiite in Nulliak supracrustal units. The metakomatiites yield a Pb-Pb isochron age of 3845 +/- 160 Ma, reflecting a significant change from an initial Archaean mantle-like mu(1)-value of 7.9 to a range of mu(2)-values of 2.0-4.2. Members of the tectonically-emplaced lithospheric mantle suite have less well correlated Pb-206/Pb-204 and Pb-207/Pb-204 compositions. Both ultramafic suites were collected in an area identified as transitional between granulite and amphibolite facies. However, model calculations show that the data scatter exhibited by the lithospheric mantle cannot be accommodated by post-3.85 Ga U-depletion. Evidence is provided that the lithospheric mantle suite reflects variable degrees of mixing between a depleted mantle and a high-mu reservoir suggesting that the two ultramafic rock suites had different evolutionary histories.Timing of late Archaean reworking during medium- to high-grade metamorphism was determined by mineral whole-rock dating of a garnet pyroxenite from an area dominated by Kiyuktok Gneiss (reworked similar to 3.75 Ga Uivak Gneiss). Sm-Nd mineral whole-rock data for this gneiss yielded an isochron equivalent to an age of 2570 +/- 17 Ma, significantly younger than its Pb-Pb isochron age of 2735 +/- 43 Ma. The negative epsilon(Nd)(t) = - 7.9 reflects a significant crustal residence time, indicating that the Sm-Nd system of the whole rock has been reset. The internal Pb mineral isochron is interpreted to reflect the maximum age of peak metamorphism. The age discrepancy of ca. 165 Ma between the Sm-Nd and the Pb-Pb isochron ages suggests slow cooling at a minimum rate of 1.3 degrees C Ma(-1). This indicates that the Saglek-Hebron segment of the North Atlantic Craton experienced gradual uplift during late Archaean exhumation. (C) 1999 Elsevier Science B.V. All rights reserved.

Samples of volcanic rock, collected from the flanks of the East Pacific Rise at 10°30′N, were used to investigate changes in the geochemistry of magmatism at the ridge axis, over the past 800 ka at this location. We show that there have been large variations in the major element chemistry of the lavas erupted at the spreading axis on this ridge segment over this period. For example, the average MgO content of lavas erupted at the ridge axis increased from about 3.0% at 600 ka, to about 7.0% at 300 ka. Since 300 ka the average MgO content has systematically decreased, and the average MgO content of lavas collected from within the neovolcanic zone at 10°30′N is 6.0%. These temporal changes in major element chemistry are not accompanied by systematic changes in isotope composition or incompatible trace element ratios, and are interpreted to reflect changes in the average rate of supply of melt to the ridge axis during this period. The data support previous arguments that changes in melt supply rate over periods of 100–1000 ka have an important influence on the major element chemistry of the lavas erupted at fast spreading ridges. At 10°30′N, the melt supply rate appears to have been relatively low for much of the past 800 ka. Samples younger than 50 ka, collected from within 3 km of the ridge axis at 10°30′N (inside the neovolcanic zone), have a smaller range in major element chemistry compared to the samples dredged from the ridge flanks. Variations in the chemistry of lavas erupted over periods of less than about 100 ka may be controlled by the geometry of the magma plumbing system beneath the ridge axis.

Lavas from the islands of Tafahi and Niuatoputapu, at the northern end of the active Tonga-Kermadec are in the southwest Pacific, were erupted at a convergent plate margin, get they can be shown to contain a contribution from two different mantle plumes. High concentrations of Nb relative to other high field strength elements in these Lavas, compared to other Tonga lavas, reflect an ocean island basalt component in the mantle wedge derived from the nearby Samoa mantle plume. Pb isotope compositions indicate that most of the Pb in these lavas is derived from the oceanic crust of the plume-generated Louisville Seamount Chain, which is being subducted beneath the Tonga are. These two plume components were thus introduced into the are lavas in very different ways and provide insight into upper-mantle dynamics and magma-generation processes occurring in an active arc-back-arc system.

New trace element and Sr, Nd, and Pb isotope data for lavas from the active Tonga–Kermadec arc in the southwest Pacific, the volcano of Niua fo'ou in the back-arc Lau Basin, and Pacific Ocean sediments from DSDP Sites 204 and 275, and ODP Site 596, are integrated with existing geochemical data for lavas from the Lau Basin, Samoa, the Louisville Ridge Seamount Chain (LR-SMC) and the extinct Lau Ridge arc, giving new insights into the petrogenesis of lavas in an active arc–back-arc system. Geochemical variations in Tonga–Kermadec arc lavas are the result of (1) differences in the amount and composition of the material being subducted along the arc, and (2) pre-existing heterogeneities in the upper mantle. Differences in the material being subducted beneath the arc have an important influence on the chemistry of the arc lavas. At the Kermadec Trench, ∼1 km thick layer of sediment is being subducted beneath the arc, compared with ∼200 m at the Tonga Trench. This results in the high Th/U and more radiogenic Pb isotope compositions of Kermadec lavas compared with Tonga lavas. The latter have Pb isotope compositions intermediate between those of Pacific sediments and Pacific mid-ocean ridge basalt (MORB), suggesting that much of the Pb in these lavas is derived from subducting Pacific Ocean crust. This is supported by the Pb isotope signatures of the subducting LR-SMC, which are also observed in lavas from the northern Tongan islands of Tafahi and Niuatoputapu. High field strength element (HFSE) and heavy rare earth element (HREE) concentrations are generally lower in Tongan lavas (particularly those from northern Tongan islands) than in Kermadec lavas. The Tonga Ridge basement, the proto-Tonga arc lavas (ODP Site 839) and the older Lau Ridge arc lavas are generally less depleted than the modern arc lavas. In the back-arc region, upper-mantle depletion as inferred from HFSE and HREE contents of the lavas broadly increases eastwards across the Lau Basin, whereas the subduction signature and volatile (CO2 and F) contents increase eastwards towards the modern arc. These observations suggest that depletion is due to melt extraction during back-arc extension and volcanism, together with a long ‘residence time’ of mantle material within the mantle wedge. The upper mantle beneath the northernmost end of the Tonga arc and Lau Basin contains an ocean-island basalt (OIB) component derived from the Samoa plume to the north. This is reflected in high concentrations of Nb relative to other HFSE in lavas from Niua fo'ou, and Tafahi and Niuatoputapu islands at the northern end of the Tonga arc. Pb isotopes also suggest an LR-SMC contribution into Tafahi and Niuataputapu. Trace element and isotope modelling is used to investigate the combined effects of varying mantle source depletion and subduction on the geochemistry of the arc lavas. The results suggest that the arc lava geochemistry can be explained largely by the balance between a relatively constant subduction input of Pb, Th, U, Cs, Ba, Sr, Rb, K and Sc [corresponding to 0.001–0.005 weight fraction of the Stolper & Newman (1994, Earth and Planetary Science Letters, 121, 293–325] ‘H2O-rich component’ composition), into the overlying, but variably depleted mantle wedge.

Trace element and Th, Sr and Pb isotope data for young lavas from the Tonga-Kermadec arc in the southwest Pacific suggest that geochemical variations in the lavas along the arc are linked to differences in the material being subducted beneath the arc. Lavas from the southern (Kermadec) segment of the arc have relatively radiogenic Pb isotope compositions, which reflects a contribution from subducted sediment. In contrast, much of the Pb in Tonga lavas is derived from the altered oceanic crust in the subducting Pacific Plate, and lavas from the northernmost Tonga islands of Tafahi and Niuatoputapu contain Pb and Sr derived from the subducted part of the Louisville Seamount Chain. The origin of the Pb in the lavas from these two islands can thus be traced to a point on the subducting slab, and this observation is used to estimate the rate at which trace elements are transported beneath the arc. Our calculations suggest that fluid-soluble elements such as U, Sr and Pb are transported from the subducted slab, across the mantle wedge and back to the surface in lavas over a period of approximately 2–3 Ma, and that magmas are erupted at the surface less than 350 ka after the melts are generated in the mantle wedge.