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.

Parameters of the near-bottom nepheloid layer off the northwestern Africa coast

Vertical profiles of light scattering at a right angle and turbidity profiles in seawater indicating suspended matter concentration in the near-bottom nepheloid layer (NNL) were measured simultaneously with temperature, salinity, and density profiles at the continental slope off the northwestern Africa. About 100 stations 5' apart in latitude and longitude were carried out over an ocean area of 6100 sq. km. Special features of the NNL variability in the area were analyzed. It was found that some structural parameters of the NNL (maximum transparency depth, that is the upper boundary of NNL; NNL thickness; maximum and total turbidity) correlate with ocean depth. On the average, thickness of the NNL in the area is 20-40% of the ocean depth. At most stations the NNL is fairly strong. In the shelf region NNL turbidity was influenced by the intensive near-shore upwelling. Formation of ''high-energy near-bottom layers'' in the shelf region resulted from passing of a mesoscale cyclonic eddy that caused redistribution of measured quantities within the entire water column.

Geochemistry of interstitial waters of Lau Backarc Basin sediment samples

Interstitial water samples from Sites 834 through 839, drilled during Ocean Drilling Program Leg 135 in the backarc Lau basin (Southwestern Pacific), have been analyzed for major elements, manganese, copper, strontium, barium, vanadium, and 87Sr/86Sr isotopic composition values. The concentration-depth profiles of the major chemical components show almost straight concentration gradients at all sites, and seem to reflect slight alteration of volcanic material. However, in the lower part of the sedimentary cover, where volcanogenic material is abundant and where diagenetic minerals occur, systematic decreases in calcium, strontium, manganese, copper, and vanadium concentrations are observed. A downwelling flow of bottom seawater, which affected the diagenetic chemical signature of the interstitial water, is probably responsible for the recorded chemical features. This hypothesis is supported by strontium isotope data obtained from interstitial water samples at Site 835. It is also in accordance with data from heat flow and physical properties.

Miocene to Pleistocene planktonic foraminifer biostratigraphy of ODP Leg 135 sites

Diverse and well-preserved planktonic foraminifers were recovered from six sites (834-839) drilled in the Lau Basin. Planktonic faunas from the Tongan Platform sites varied from those of the Lau Basin sites by being less well preserved (Site 840) to being very poorly preserved and very sparse (Site 841); at Site 841 most samples were barren. All sites penetrated a volcaniclastic sequence in which thick ash beds were encountered; foraminifer populations within the ash beds were often very small, making it difficult to obtain biostratigraphic data. No hiatuses were encountered in the upper Miocene to Pleistocene sections of the Lau Basin, but a possible break occurs at Site 840 on the Tongan Platform. Site 834 penetrated through a Quaternary-Pliocene sequence overlying basaltic basement, and topmost Miocene (Zone N17B) sediments interbedded within the volcanic sequence. Site 835 penetrated into the lower Pliocene (Zones N19 to N19-20). Site 836 penetrated the shortest section, with Zone N22 {Globorotalia (Truncorotalia) crassaformis hessi Subzone) directly overlying basalts. Site 837 penetrated into the basal part of Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) overlying basalt. Site 838 failed to encounter basalts, with the oldest sediment being from Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone). Site 839, within the same basin as Site 838, located Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) sediments directly overlying igneous basement. Site 840 penetrated into the upper Miocene Zone N17A without encountering any major unconformity. Site 841, studied mainly from core-catcher samples, penetrated a Quaternary to questionable upper Miocene sequence that was in fault contact with middle Miocene (Zones N8 to N9) sediments. For the Lau Basin sites, reworking was encountered only in Sites 834 and 835. Site 834 was drilled adjacent to the Lau Ridge, on which are developed numerous reef al and shallow-water environments, where erosional conditions could have been expected during sea-level lowstands. Site 835 was drilled in a narrow basin that has been remote from these erosional influences; slumping and erosion of material from the adjacent basin slopes appears to have been the source of the reworking. For the Tongan Platform sites, reworking was observed only in the lower part of the upper Miocene section at Site 841, where late Eocene larger foraminifers are present in conglomerates and grits. The presence of Globorotalia (Globorotalia) multicamerata and small specimens of Sphaeroidinellopsis spp. in the Pleistocene of Site 840 may indicate reworking, but this is not clear. Unit I, which marks a reduction in volcanic activity in the Lau Basin, ranges in age from the lower part of Zone N22 (Globigerinoides quadrilobatus fistulosus Subzone) at Sites 834 and 835, to within Zone N22 (Globorotalia crassaformis hessi Subzone) at Sites 836 to 838, and within the upper part of Zone N22 (Bolliella praeadamsi Subzone) at Site 839. Units II and III are generally represented by thick to very thick ash beds, which generally contain low-diversity and often poorly preserved assemblages. Igneous sources seem to have remained important contributors of sediment up to the present day.

Geochemistry of Lau Backarc Basin sediments

Major and minor (Mn, Sr, Ba, V, Cr, Ni, Co, Zn, Cu, Zr, Y, Sc) elements and mineralogic compositions were determined on bulk sediments collected during Ocean Drilling Program Leg 135. Three classes of sediment samples from holes drilled in the Lau Basin are discriminated by mineralogy and major element data. Samples labeled Class 1 are significantly enriched in biogenic calcite and occur predominantly in the northern part of the basin (Sites 834-835), whereas those of Class 3 are mostly enriched in volcanogenic material and are predominant in the central part of the basin (Sites 836-839). The minor element composition records the effects of the hydrothermal activity on the sediments. In the northern area of the basin (Sites 834-835), sedimentation is characterized by higher accumulation rates of the carbonate and hydrothermal fractions. These sediments are probably reworked predominantly, transported in the water column, and then settled locally. Thus, ponded sediments are probably responsible to this high accumulation rates. Diagenetic processes altered the volcanic material to a grade corresponding to the stability of phillipsite. In the central area of the basin (Sites 836-839), sedimentation is characterized by the action of bottom currents preferentially reworking the carbonate and hydrothermal fractions. Volcanogenic accumulation rates are greater at these sites than in the northern Lau Basin. Alteration of volcanic material is more important deeper in the holes and records authigenesis of clay rich in Fe-Mg, most likely smectite. Locally, clay minerals have apparently incorporated Cr and other ore-forming elements.