(Table 1) Isotopic composition of Sm and Nd in mantle restites of the Central Atlantic and in associated plutonic and volcanic rocks

Ultrabasic rock samples collected from two areas of the crustal zone of the Mid-Atlantic Ridge (MAR): (1) 13-17°N (near the intersection of the ridge axis with the 15°20'N prime fracture zone), and (2) 33°40'N prime (the western intersection of the MAR crest with the Heis fracture zone) were objects of this study. Samples of peridotite and of plutonic and volcanic rocks associated with it were used to measure their Sm/Nd, 143Nd/144Nd, and 147Sm/144Nd ratios, which allowed to test time and genetic relationships between evolution of mantle material under the ridge crest and products of its magmatic activity. Results of this work proved ubiquitous discrepancy between melting degree values of extremely depleted mantle peridotites in the MAR area between 14°N and 16°N, obtained using petrologic and geochemical methods. This discrepancy suggests large-scale interaction between mantle material and magmatic melts and fluids enriched in incompatible elements or fluids. The results obtained suggest that repeated melting of the mantle under the axial MAR zone is an universal characteristic of magmatism in low-velocity spreading centers. The results of this study also proved the crestal MAR zone in the Central Atlantic region show distinct indications of isotope-geochemical segmentation of the mantle. It is suggested that the geochemically anomalous MAR mantle peridotite in the zone of the MAR intersection with the 15°20'N prime fracture zone can be interpreted as fragments of mantle substrate, foreign for the Atlantic mantle north of the equator.

(Table 1) Oxygen isotopes in minerals at DSDP Hole 83-504B basalts

The D/H, 18O/16O and 87Sr/86Sr ratios of the basaltic basement from the Leg 83 section of DSDP Hole 504B show that in that area the oceanic crust has experienced intensive but not pervasive alteration. Isotope ratios of the basalts are very heterogeneous because of an input of oxygen, hydrogen, and strontium from seawater. The hydrogen isotopic composition of many samples displays the complete thermal history of the water-rock interactions. High-temperature mineral formations (actinolites, epidotes, and chlorites) were overgrown by a mineralization at lower temperatures (mixedlayer smectites, iddingsites, and smectites) during successive stages of cooling of the oceanic crust by cold seawater. From 87Sr/86Sr data bulk water/rock ratios up to 5:1 have been calculated. There is evidence that some primary minerals like high-An plagioclases contain oxygen from altered basalts. We have discussed the probability that there existed a seawater/crust interface, now at a depth of 620 m sub-basement, during the high-temperature water/rock interactions. This interface was covered during later magmatism by thick flows, pillow lavas, and intrusives.

(Table 1) 40Ar-39Ar incremental heating ages for ODP Hole 163-988A basalt and plagioclase

Results of 40Ar-39Ar Ar dating constrain the age of the submerged volcanic succession, part of the seaward-dipping reflector sequence of the Southeast Greenland volcanic rifted margin, recovered during Leg 163. At the 63ºN drilling transect, the fully normally magnetized volcanic units at Holes 989B (Unit 1) and 990A (Units 1 and 2) are dated at 57.1 ± 1.3 Ma and 55.6 ± 0.6 Ma, respectively. This correlates with a common magnetochron, C25n. The underlying, reversely magnetized lavas at Hole 990A (Units 3-13) yield an average age of 55.8 ± 0.7 Ma and may correlate with C25r. The argon data, however, are also consistent with eruption of the lavas at Site 990 during the very earliest portion of C24. If so, the normally polarized units have to be correlated to a cryptochron (e.g., C24r-11 at ~55.57 Ma). The lavas at Holes 989B and 990A have typical oceanic compositions, implying that final plate separation between Greenland and northwest Europe took place at ~56 Ma. The age for Hole 989B lava is younger than expected from the seismic interpretations, posing questions about the structural evolution of the margin. An age of 49.6 ± 0.2 Ma for the basaltic lava at Site 988 (~66ºN) points to the importance of postbreakup tholeiitic magmatism at the rifted margin. Together with results from Leg 152, a virtually complete time frame for ~12 m.y. of pre-, syn-, and postbreakup volcanism during rifted margin evolution in Southeast Greenland can now be assembled. This time frame includes continental type volcanism at ~61-60 Ma, synbreakup volcanism beginning at ~57 Ma, and postbreakup volcanism at ~49.6 Ma. These discrete time windows coincide with distinct periods of tholeiitic magmatism from the onshore East Greenland Tertiary Igneous Province and is consistent with discrete mantle-melting events triggered by plume arrival (~61-60 Ma) under central Greenland, continental breakup (~57-54 Ma), and passage of the plume axis beneath the East Greenland rifted margin after breakup (~50-49 Ma), respectively.

Geochemistry on the Kekesai composite intrusion

The late Carboniferous to Permian is a critical period for final amalgamation of the Central Asian Orogenic Belt (CAOB), which is characterized by voluminous igneous rocks, particularly granitoids. The Kekesai composite granitoid porphyry intrusion, situated in the Chinese western Tianshan (southwest part of CAOB) includes two intrusive phases, a monzogranite phase, intruded by a granodiorite phase. LA-ICPMS U-Pb zircon analyses suggest that the monzogranitic rocks formed at 305.5±1.1 Ma, with a wide age range of inherited zircons (358-488 Ma and 1208-1391 Ma), whereas the granodioritic rocks formed at 288.7±1.5 Ma. The monzogranitic and granodioritic phases have similar geochemical features and Sr-Nd-Hf isotopic compositions. They exhibit high and variable SiO2 (66-71 wt.%) and MgO (0.41-2.14 wt.%) contents with some arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements and negative anomalies of Nb, Ta and Ti) and relatively high initial 87Sr/86Sr ratios (ISr=0.7055-0.7059), low positive eNd(t) (+0.84 to +1.03) as well as a large variation in Hf isotopic compositions with eHf(t) between +3.43 to +14.8, implying both of them were derived from similar source materials. These geochemical characteristics suggest that they might be mainly derived from the partial melting of arc-derived Mesoproterozoic mafic lower crust with involvement of a mantle-derived component in variable proportions by mantle-derived magma underplating. The presence of late-Ordovician to earliest early Carboniferous inherited zircons and the Hf isotopic compositions in the monzogranitic sample, similar to that of the widespread juvenile arc rocks, indicates some crust contamination during magma emplacement. Our new data, combined with previous studies, imply that extensive post-collisional magmatism due to underplating of mantle-derived magma, could plausibly be explained by slab break-off regime.

(Table 2) Chemical composition of quenched glasses from the Mohns and Knipovich Ridges

The aim of this project was a petrogeochemical study of igneous rocks in the areas of the Mohns and Knipovich Ridges, both being the northern extensions of the Mid-Atlantic Ridge (MAR), using data available for quenching glass samples collected during Cruises 36 and 38 of R/V Akademic Mstislav Keldysh and during Cruise 15 of R/V Professor Logachev. Results of igneous rock studying from the Mohns and Knipovich Ridges at the background of evolution of the total North Atlantic Province, which had been identified earlier from tectonic and geophysical data, showed that igneous rocks of the Knipovich Ridge can be ranked as shallow tholeiites, primary melts of which were relatively rich in Na and Si and poor in Fe. This type of magma is characteristic of colder regions of the oceanic lithosphere. Its occurrence in the Knipovich Ridge and its potential propagation up to the Gakkel Ridge suggest that igneous rocks of this region originated under conditions of passive spreading in contrast to the MAR region in vicinity of Iceland and Azores, where substantial contribution of hotter material of a rising plume contributed to formation of the oceanic crust. The North Atlantic Ocean is the youngest province in terms of ocean-floor opening. Geologically and geophysically it is one of well studied regions of the World Ocean. Nevertheless some basic key items of its origin still remain to be clarified. In 1975 Scatler et al. proved specifics of this region manifested in growth of the gravity field, and also in relative height of the ocean floor in the region of 33-70°N, which was associated by them with rise of the hotter mantle, as compared with common regions of the Mid-Atlantic Ridge. Later this view was confirmed by character of magmatism, which differed in depth of generation and by melting degree of the resulting primary magma. Uniqueness of the North Atlantic region was also proved by the fact that this region was marked by extensive geochemical anomalies associated with Azores, Iceland, and Jan Mayen. All of these data allow to consider the northern part of the MAR (north of 33°N) as an united global geotectonic province. The Mohns and Knipovich Ridges located north of Iceland locate at the northern end of this province. This is the least known region. Therefore, new data for ridge areas of 73-77°N are needed for more complete geologic history of the Arctic Basin. The aim of this study was to carry out a complex comparison of magmatism at the Mohns and Knipovich Ridges with magmatism at large segments of the MAR northern province and to reconstruct mechanisms of primary magma formation, as well as conditions of their fractionation. This paper was based on results of studying quenched glasses, which reflect evolution of melt in the course of its formation.

Chemical composition of basalts from ODP holes at the Kerguelen Plateau

Petrographic and geochemical study of basalts in the Kerguelen Plateau basement revealed changes in composition and character of volcanism during development of this tectonovolcanic structure. The Kerguelen Plateau is one of the largest intraplate rises in the World Ocean. It started to form about 120 Ma ago. Age of basalts and overlying sediments shows that plateau formation was in the northwest direction. Basalts of the Kerguelen Plateau basement are products of tholeiitic melts in terms of geochemistry, but differ from mid-ocean ridge basalt (MORB). They are enriched in incompatible trace elements and rare earth elements (REE) relative to MORB, and degree of enrichment varies in basalts from different segments of the plateau. Composition of basalts does not directly depend on their age. Specific features of plateau magmatism are commonly explained in terms of a long-living deep magma plume, which variously interacted with a depleted upper mantle source at different stages of plateau formation. However, taking into account block morphology and deep structure of the plateau, one can suggest that plateau volcanism was initiated by a large fault. As the volcanism prograded to the northwest, depth of fault penetration into the mantle changed. Composition of basalts in the plateau basement was also governed by formation depth of primary melts.

Geochemistry at DSDP Leg 82 Holes

DSDP Leg 82 drilled nine sites to the southwest of the Azores Islands on the west flank of the Mid-Atlantic Ridge (MAR) in an attempt to determine the temporal and spatial evolution of the Azores "hot-spot" activity. The chemistry of the basalts recovered during Leg 82 is extremely varied: in Holes 558 and 561, both enriched (E-type: CeN/YbN = 1.5 to 2.7; Zr/Nb = 4.5 to 9.6) and depleted (or normal-N-type: CeN/YbN = 0.6 to 0.8; Zr/Nb > 20) mid-ocean ridge basalts (MORB) occur as intercalated lava flows. To the north of the Hayes Fracture Zone, there is little apparent systematic relationship between basalt chemistry and geographic position. However, to the south of the Hayes Fracture Zone, the chemical character of the basalts (N-type MORB) is more uniform. The coexistence of both E-type and N-type MORB in one hole may be explicable in terms of either complex melting/ fractionation processes during basalt genesis or chemically heterogeneous mantle sources. Significant variation in the ratios of strongly incompatible trace elements (e.g., La/Ta; Th/Ta) in the basalts of Holes 558 and 561 are not easily explicable by processes such as dynamic partial melting or open system crystal fractionation. Rather, the trace element data require that the basalts are ultimately derived from at least two chemically distinct mantle sources. The results from Leg 82 are equivocal in terms of the evolution of the Azores "hot spot," but would appear not to be compatible with a simple model of E-type MORB magmatism associated with upwelling mantle "blobs." Models that invoke a locally chemically heterogeneous mantle are best able to account for the small-scale variation in basalt chemistry.

Stable isotope ratios of foraminifera from sediment cores off northeastern Brazil

The western South Atlantic boundary currents represent a sensitive system within the global thermohaline circulation (THC). We investigated the impact of deglacial THC changes on the western tropical Atlantic studied in six high resolution sediment cores from the upper continental slope of Brazil. The stratigraphy of the cores is mainly based on 14C AMS dating of monospecific foraminiferal samples. Changes in the upper layer tropical ocean during the deglaciation are inferred from stable oxygen isotope measurements on planktic and benthic foraminifera. Variations in the delta18O residuals are assumed to be mainly temperature related. During the Oldest and Younger Dryas cooling periods, two major deglacial THC disturbances are reported from North Atlantic sediment cores. Concomitant to the repeated THC slowdown, we observe an upper layer warming in the tropical ocean. A reduced northward heat export from the tropical areas during these periods (weak North Brazil Current) is additionally reflected by low meridional gradients in the stable oxygen records. This generally agrees with results from coupled ocean atmosphere models.

Isotope geochemistry of Ninetyeast Ridge basalts

The Ninetyeast Ridge lavas have Sr and Nd isotopic ratios intermediate between those of Indian Ocean MORBs and those of the very enriched Kerguelen hot spot. In an Nd-Sr isotope diagram, they also plot close to the fields of St. Paul Island lavas and of the early magmatism on Kerguelen Archipelago. The Ninetyeast Ridge lavas were generated by mixing among at least three components: a depleted, MORB-type component, such as the one erupted today on the Southeast Indian Ridge; a very enriched, high- Sr/ Sr, low-epsilon-Nd, OIB-type component (the Kerguelen hot spot); and an OIB-type component comparable to that sampled from the St. Paul (and Amsterdam) lavas. The Ninetyeast Ridge lavas show a typical Dupal anomaly signature and Pb, Sr, and Nd isotopic systematics indicate that the Kerguelen hot spot was involved in the ridge's formation as the Indian plate moved northward. The different sites cored during ODP Leg 121 show a trend in their isotopic compositions, from less radiogenic Pb/ Pb ratios and intermediate 87Sr/86Sr and 143Nd/**Nd ratios in the oldest lavas (Site 758) toward more radiogenic 206Pb/204Pb, higher epsilon-Nd, and lower 87Sr/86Sr values in the youngest lavas (Site 756). The lavas from Site 757 have 206Pb/204Pb ratios intermediate between those of the lavas from Sites 756 and 758 and higher 87Sr/86Sr and lower epsilon-Nd values. The relative proportions of the hot spot(s) and MORB component have evolved with time, reflecting differences of tectonic setting: the relative proportion of the Kerguelen hot spot component appears lower in the younger Site 756 lavas than in the older lavas from Sites 757 and 758. Site 756 coincides with the beginning of rifting at the Southeast Indian Ridge, about 43 Ma ago. The formation of the early Kerguelen Archipelago lavas may have drained most of the plume-derived material toward the Antarctic plate. Alternatively, the proximity of the spreading-ridge axis may account for the isotopic similarity of the Site 756 lavas to young lavas erupted on the Southeast Indian Ridge, from 33? to 37?S. The older lavas of Ninetyeast Ridge may have formed when the hot spot and ridge axis did not exactly coincide. The involvement of the third component, a St. Paul hot spot, in the genesis of the Ninetyeast Ridge lavas, especially for the Site 756 lavas, is clearly indicated by Sr, Pb, and Nd isotope systematics and also by trace element ratios. These data, together with those from the Kerguelen Plateau, indicate that the Kerguelen hot spot has been active more or less continuously in the South Indian Ocean for at least 115 Ma. This could indicate that the plume, and by inference the Dupal anomaly, is deep seated in origin.

Geochemistry of lavas from the Lau-Tonga arc and back-arc system

Detailed comparison of mineralogy, and major and trace geochemistry are presented for the modern Lau Basin spreading centers, the Sites 834-839 lavas, the modern Tonga-Kermadec arc volcanics, the northern Tongan boninites, and the Lau Ridge volcanics. The data clearly confirm the variations from near normal mid-ocean-ridge basalt (N-MORB) chemistries (e.g., Site 834, Central Lau Spreading Center) to strongly arc-like (e.g., Site 839, Valu Fa), the latter closely comparable to the modern arc volcanoes. Sites 835 and 836 and the East Lau Spreading Center represent transitional chemistries. Bulk compositions range from andesitic to basaltic, but lavas from Sites 834 and 836 and the Central Lau Spreading Center extend toward more silica-undersaturated compositions. The Valu Fa and modern Tonga-Kermadec arc lavas, in contrast, are dominated by basaltic andesites. The phenocryst and groundmass mineralogies show the strong arc-like affinities of the Site 839 lavas, which are also characterized by the existence of very magnesian olivines (up to Fo90-92) and Cr-rich spinels in Units 3 and 6, and highly anorthitic plagioclases in Units 2 and 9. The regional patterns of mineralogical and geochemical variations are interpreted in terms of two competing processes affecting the inferred magma sources: (1) mantle depletion processes, caused by previous melt extractions linked to backarc magmatism, and (2) enrichment in large-ion-lithophile elements, caused by a subduction contribution. A general trend of increasing depletion is inferred both eastward across the Lau Basin toward the modern arc, and northward along the Tongan (and Kermadec) Arc. Numerical modeling suggests that multistage magma extraction can explain the low abundances (relative to N-MORB) of elements such as Nb, Ta, and Ti, known to be characteristic of island arc magmas. It is further suggested that a subduction jump following prolonged slab rollback could account for the initiation of the Lau Basin opening, plausibly allowing a later influx of new mantle, as required by the recognition of a two-stage opening of the Lau Basin.