ALBERT

Description: Mafic to ultramafic intrusions of the Qullinaaraaluk suite (Q-suite) were emplaced into the Ungava craton of the Northeastern Superior Province during an episode of intense igneous activity and crustal reworking from c. 2·74 to 2·70 Ga. Orthopyroxene-rich Q-suite intrusions from the Hudson Bay Terrane and southwestern Rivière Arnaud Terrane, and orthopyroxene-poor Q-suite intrusions from the north–central Rivière Arnaud Terrane indicate the existence of at least two Q-suite magma types: a subalkaline magma parental to the orthopyroxene-rich intrusions and a transitional magma parental to the orthopyroxene-poor intrusions. Both types of intrusions are characterized by light rare earth element (LREE)-enriched, high field strength element (HFSE)-depleted trace element profiles that reflect, in large part, contamination by the tonalite–trondhjemite–granodiorite-dominated crust. Near-chondritic to strongly sub-chondritic initial Nd (2·72 Ga) values (+2 to –10) of the Q-suite intrusions reflect the combined effects of both the amount of crustal contamination and the age-dependent isotopic composition of the contaminant. The inferred trace element profiles of the uncontaminated Q-suite magmas were probably flat to LREE-depleted. The transitional magmas that produced the least evolved dunitic cumulates of the Q-suite were ferropicrites (MgO ~14 wt %, FeO TOT ~17 wt %). In contrast, the magmas parental to the primitive Q-suite harzburgites were Fe-rich, high-Mg basalts (MgO ~11 wt %; FeO ~14 wt %). The high Fe contents of the Q-suite magmas are incompatible with derivation from a pyrolitic mantle [Mg-number ~0·90, Mg/(Mg + Fe TOT )] and require sources significantly enriched in iron (Mg-number ≤0·79). Both magma types are also characterized by relatively low Ni contents suggesting derivation from source regions depleted in Ni relative to pyrolitic mantle peridotite. Differences in the major element compositions of the subalkaline and transitional parental magmas may reflect compositional diversity among the Fe-rich mantle sources. Comparisons with melting experiments on compositions analogous to the Martian mantle suggest that the Q-suite magmas may rather be generated by different degrees of melting of a common source with an Fe content slightly lower than that of the Homestead L5 ordinary chondrite (Mg-number = 0·77). The Fe-rich picritic to high-Mg basaltic magmas last equilibrated with garnet-free harzburgitic to lherzolitic residues at upper mantle pressures (≤5 GPa). The craton-wide occurrence of c. 2·72–2·70 Ga Q-suite mafic to ultramafic plutons suggests that underplating by Fe-rich mantle melts may have had a key role in the c. 2·74–2·70 Ga cratonization of the Northeastern Superior Province.

Description: Mid-ocean ridge basalts (MORB) from the Arctic Ocean have been significantly less studied than those from other oceans. The Arctic ridges (Gakkel Ridge and Lena Trough) are ultraslow-spreading ridges with low melt productivity and are thus the best locations to investigate mantle heterogeneity. We report the major and trace element and Sr–Nd–Pb–Hf isotope compositions of basalts generated along the Lena Trough and the westernmost part of the Gakkel Ridge in the Arctic Ocean. Basalts from the northern Lena Trough and westernmost Gakkel Ridge (NLT–WGR) have compositions close to normal MORB. The geochemical composition of the NLT–WGR lavas confirms a binary mixing model involving melts from a depleted MORB mantle source and a Spitsbergen amphibole-bearing subcontinental lithospheric mantle (SCLM) source. In contrast, in the central part of the Lena Trough (CLT), the basalts are alkalic with relatively high Mg-number (60–65), high SiO 2 (51·0–51·6 wt %), Al 2 O 3 (18·1–18·4 wt %), Na 2 O (4·0–4·2 wt %), K 2 O (1·0–1·6 wt %), K 2 O/TiO 2 (0·6–0·9) and (La/Sm) PM (1·4–1·8), and low FeO (6·5–6·8 wt %) contents. These basalts display isotope variations with 87 Sr/ 86 Sr ranging from 0·70361 to 0·70390, 143 Nd/ 144 Nd from 0·51283 to 0·51290 ( Nd + 3·7 to +5·2), 176 Hf/ 177 Hf from 0·28313 to 0·28322 ( Hf + 11·6 to +14·9) and 206 Pb/ 204 Pb from 17·752 to 17·884, 207 Pb/ 204 Pb from 15·410 to 15·423 and 208 Pb/ 204 Pb from 37·544 to 37·670. These isotope compositions clearly distinguish the CLT lavas from those generated along the Gakkel Ridge. For the CLT lavas, involvement of a phlogopite- or amphibole- and (possibly garnet)-bearing SCLM source component is proposed. Owing to SCLM contamination along the entire length of the Lena Trough, we classify the Lena Trough as an ocean–continent transition boundary. Magmatism similar to that observed in the Lena Trough would be expected to occur wherever ocean spreading initiates.

Description: The Ninetyeast Ridge is an ~5500 km long, north–south-oriented, submarine volcanic ridge in the eastern Indian Ocean that formed from magmatism associated with the deep-seated Kerguelen mantle plume as the Indian plate drifted rapidly northward during the Late Cretaceous. Basalts recovered along the ridge have the characteristic Dupal geochemical signature of Indian Ocean basalts, but debate concerning the nature and number of components in their mantle source persists. New multiple collector inductively coupled plasma mass spectrometry (Pb, Hf) and thermal ionization mass spectrometry (Sr, Nd) isotopic analyses were obtained for tholeiites representative of the ~180 m of basaltic basement recovered from three drill sites (Site 758, 82 Ma; Site 757, 58 Ma; Site 756, 43 Ma) along the Ninetyeast Ridge during Ocean Drilling Program Leg 121. No systematic isotopic variation is observed along the ridge, which is inconsistent with the hypothesis of an aging mantle plume origin for the ridge. The isotopic compositions are generally intermediate between those of the volcanic products of the Kerguelen and Amsterdam–St. Paul hotspots and define mixing trends between components with relatively enriched and depleted signatures. At least three, possibly four, source components are required to explain the observed isotopic variability along the Ninetyeast Ridge. The unradiogenic signatures of some Ninetyeast Ridge basalts (e.g. 87 Sr/ 86 Sr = 0·70381–0·70438) are not related to the source of Indian MORB and indicate the presence of a relatively depleted component in a deep mantle source. A similar source component is also identified in other Indian Ocean island basalts (e.g. Crozet, Réunion) not related to magmatic activity of the Kerguelen hotspot. The Pb–Hf–Sr–Nd isotopic compositions of the Ninetyeast Ridge basalts are consistent with the presence of a mixture of recycled sediments and lower continental crust together with altered oceanic crust in their mantle source, hence supporting a deep origin for the enriched Dupal signature encountered in ocean island basalts.

Description: The South Kaua`i Swell (SKS) volcano was sampled during four JASON dives and three dredge hauls recovering rocks that range from fresh pillow lavas to altered volcanic breccias. Two geochemical groups were identified: shield-stage tholeiites (5·4–3·9 Ma) and rejuvenation-stage alkalic lavas (1·9–0·1 Ma). The young SKS ages and the coeval rejuvenated volcanism along a 400 km segment of the Hawaiian Islands (Maui to Ni`ihau) are inconsistent with the timing and duration predictions by the flexure and secondary plume melting models for renewed volcanism. The SKS tholeiites are geochemically heterogeneous but similar to lavas from nearby Kaua`i, Ni`ihau and Wai`anae volcanoes, indicating that their source regions within the Hawaiian mantle plume sampled a well-mixed zone. Most SKS tholeiitic lavas exhibit radiogenic Pb isotope ratios ( 208 Pb*/ 206 Pb*) that are characteristic of Loa compositions (〉0·9475), consistent with the volcano’s location on the west side of the Hawaiian Islands. These results document the existence of the Loa component within the Hawaiian mantle plume prior to 5 Ma. Loa trend volcanoes are thought to have a major pyroxenite component in their source. Calculations of the pyroxenitic component in the parental melts for SKS tholeiites using high-precision olivine analyses and modeling of trace element ratios indicate a large pyroxenite proportion (≥50%), which was predicted by recent numerical models. Rejuvenation-stage lavas were also found to have a significant pyroxenite component based on olivine analyses (40–60%). The abundance of pyroxenite in the source for SKS lavas may be the cause of this volcano’s extended period of magmatism (〉5 Myr). The broad distribution of the Loa component in the northern Hawaiian Island lavas coincides with the start of a dramatic magma flux increase (300%) along the Hawaiian Chain, which may reflect a major structural change in the source of the Hawaiian mantle plume.