ABSTRACT This paper addresses the composition, geochemistry, isotopic characteristics, and age of... more ABSTRACT This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.
We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone th... more We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone that offsets by 320 km the Mid Atlantic Ridge in the central Atlantic. This expedition (S19) was part of PRIMAR (Russian-Italian Mid Atlantic Ridge Project). The field work aimed at obtaining geophysical and petrological data from a prominent transverse ridge that runs on the southern side of the transform valley and constitutes a major topographic anomaly relative to the depth/square root of age relationship. Previous work had shown that a relatively undisturbed section of oceanic lithosphere is exposed on the northern side of the transverse ridge for roughly 270 km along a seafloor spreading flow line. Given an average spreading half rate of 16 mm/y, this length corresponds to over 16 My. One of the objectives of our expedition was to sample at close-spaced (~ 5 km) horizontal intervals the mantle ultramafic basal unit, in order to detect temporal variations of mantle composition and of ...
The analysis of the bottom relief structure investigated with multibeam SIMRAD 12S and sedimentar... more The analysis of the bottom relief structure investigated with multibeam SIMRAD 12S and sedimentary cover of depressions investigeted with PARASAUND in the MAR crest zone near Sierra-Leone Fault (22 Cruise of the RV "Akademik Nikolay Strachov", and 10 Cruise of the RV "Akademik Ioffe") point on the complicated character of the tectonic activity distribution in this region. The left-lateral displacements of the rift velley and the absence of transform faults are typical for this region. Two extremely deep rift depressions (up to 5000 m) are located in the rift valley: one on 5°54'N latitude (Markov depression) and the other on 5°46'N latitude. About 40 m sediments cover their bottom. On contary, in the depression located parallel to the rift valley directly to the west from the two mentioned rift depressions the sedimentary cover is absent and bottom has very dissected, apparently volcanic, relief. In the MAR crest zone in 20 miles to the south-west from ...
Small-scale variations in composition of mantle-derived peridotites have been investigated in the... more Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a ``cold'' upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (~8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogeneous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites.Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a ``hot spot''-type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero-age topographic highs in the 2°-3°N and 14°-15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O-rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.
ABSTRACT This paper addresses the composition, geochemistry, isotopic characteristics, and age of... more ABSTRACT This paper addresses the composition, geochemistry, isotopic characteristics, and age of rocks from the Carter Seamount of the Grimaldi seamount group at the eastern margin of the Central Atlantic. The age of the seamount was estimated as 57–58 Ma. Together with other seamounts of the Grimaldi system and the Nadir Seamount, it forms a “hot line” related to the Guinea Fracture Zone, which was formed during the late Paleocene pulse of volcanism. The Carter Seamount is made up of olivine melilitites, ankaramites, and analcime-bearing nepheline tephrites, which are differentiated products of the fractional crystallization of melts similar to an alkaline ultramafic magma. The volcanics contain xenoliths entrained by melt at different depths from the mantle, layer 3 of the oceanic crust, which was formed at 113–115 Ma, and earlier magma chambers. The rocks were altered by low-temperature hydrothermal solutions. The parental melts of the volcanics of the Carter Seamount were derived at very low degrees of mantle melting in the stability field of garnet lherzolite at depths of no less than 105 km. Anomalously high Th, Nb, Ta, and La contents in the volcanics indicate that a metasomatized mantle reservoir contributed to the formation of their primary melts. The Sr, Pb, and Nd isotopic systematics of the rocks show that the composition of the mantle source lies on the mixing line between two mantle components. One of them is a mixture of prevailing HIMU and the depleted mantle, and the other is an enriched EM2-type mantle reservoir. These data suggest that the formation of the Carter Seamount volcanics was caused by extension-related decompression melting in the Guinea Fracture Zone of either (1) hot mantle plume material (HIMU component) affected by carbonate metasomatism or (2) carbonated basic enclaves (eclogites) ubiquitous in the asthenosphere, whose isotopic characteristics corresponded to the HIMU and EM2 components. In the former case, it is assumed that the melt assimilated during ascent the material of the metasomatized subcontinental mantle (EM2 component), which was incorporated into the oceanic lithospheric mantle during rifting and the breakup of Pangea.
We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone th... more We carried out in January-March 1998 a geological-geophysical cruise to the Vema fracture zone that offsets by 320 km the Mid Atlantic Ridge in the central Atlantic. This expedition (S19) was part of PRIMAR (Russian-Italian Mid Atlantic Ridge Project). The field work aimed at obtaining geophysical and petrological data from a prominent transverse ridge that runs on the southern side of the transform valley and constitutes a major topographic anomaly relative to the depth/square root of age relationship. Previous work had shown that a relatively undisturbed section of oceanic lithosphere is exposed on the northern side of the transverse ridge for roughly 270 km along a seafloor spreading flow line. Given an average spreading half rate of 16 mm/y, this length corresponds to over 16 My. One of the objectives of our expedition was to sample at close-spaced (~ 5 km) horizontal intervals the mantle ultramafic basal unit, in order to detect temporal variations of mantle composition and of ...
The analysis of the bottom relief structure investigated with multibeam SIMRAD 12S and sedimentar... more The analysis of the bottom relief structure investigated with multibeam SIMRAD 12S and sedimentary cover of depressions investigeted with PARASAUND in the MAR crest zone near Sierra-Leone Fault (22 Cruise of the RV "Akademik Nikolay Strachov", and 10 Cruise of the RV "Akademik Ioffe") point on the complicated character of the tectonic activity distribution in this region. The left-lateral displacements of the rift velley and the absence of transform faults are typical for this region. Two extremely deep rift depressions (up to 5000 m) are located in the rift valley: one on 5°54'N latitude (Markov depression) and the other on 5°46'N latitude. About 40 m sediments cover their bottom. On contary, in the depression located parallel to the rift valley directly to the west from the two mentioned rift depressions the sedimentary cover is absent and bottom has very dissected, apparently volcanic, relief. In the MAR crest zone in 20 miles to the south-west from ...
Small-scale variations in composition of mantle-derived peridotites have been investigated in the... more Small-scale variations in composition of mantle-derived peridotites have been investigated in the 0°-15°N portion of the Mid-Atlantic Ridge (MAR), thanks to a relatively close-spaced peridotite sample coverage achieved by combining samples collected by Russian and U.S. expeditions. Areal variations in the composition of mantle-equilibrated minerals olivine, orthopyroxene, clinopyroxene, and spinel have been interpreted as due primarily to regional variations in the initial composition, degree of partial melting, and thermal structure of the upper mantle. Mantle rocks from the eastern part of the Romanche transform frequently contain a trapped fraction of basaltic melt, while undepleted mantle prevails in the western part of the Romanche, suggesting a ``cold'' upper mantle thermal regime in this region, which prevented significant melting. Immediately to the north, the St. Paul Fracture Zone (FZ) upper mantle shows intermediate degrees of melting, except for St. Peter-Paul Island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites. Between St. Paul FZ and 4°N (Strakhov FZ) we have an area of strongly depleted upper mantle. Farther north the Doldrums FZ area (~8°N) appears to be underlain by moderately depleted upper mantle with some melt entrapment. The Vema FZ (11°N) is underlain by relatively homogeneous upper mantle which has undergone a rather low degree of melting. The Mercurius and Marathon transforms (between 12° and 13°N) expose moderately depleted peridotites.Finally, the 15°20' FZ area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the MAR axis and strongly depleted mantle south of the transform. The strongly depleted mantle from the 2°-3°N and 14°-15°N regions is associated spatially with light rare earth element enriched mid-ocean ridge basalt showing a ``hot spot''-type geochemical signature. The areal association of refractory peridotites with enriched basalt and with zero-age topographic highs in the 2°-3°N and 14°-15°N regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized, H2O-rich domains which would cause enhanced melting and provide a source for basalt enrichment. These mantle domains might be relicts of an originally subcontinental mantle.
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