Geochemistry of Miocene Alborán Basin volcanism

We present new major and trace element and O-Sr-Nd-isotope data for igneous rocks from the western Mediterranean Alborán Sea, collected during the METEOR 51/1 cruise, and for high-grade schists and gneisses from the continental Alborán basement, drilled during the Ocean Drilling Programme (ODP Leg 161, Site 976). The geochemical data allow a detailed examination of crustal and mantle processes involved in the petrogenesis of the lavas and for the first time reveal a zonation of the Miocene Alborán Sea volcanism: (1) a keel-shaped area of LREE-depleted (mainly tholeiitic series) lavas in the central Alborán Sea, generated by high degrees of partial melting of a depleted mantle source and involving hydrous fluids from subducted marine sediments, that is surrounded by (2) a horseshoe-shaped zone with LREE-enriched (mainly calc-alkaline series) lavas subparallel to the arcuate Betic-Gibraltar-Rif mountain belt. We propose that the geochemical zonation of the Miocene Alborán Basin volcanism results from eastward subduction of Tethys oceanic lithosphere coupled with increasing lithospheric thickness between the central Alborán Sea and the continental margins of Iberia and Africa.

Geochemistry of ocean floor and forearc serpentinites

We provide new insights into the geochemistry of serpentinites from mid-ocean ridges (Mid-Atlantic Ridge and Hess Deep), passive margins (Iberia Abyssal Plain and Newfoundland) and fore-arcs (Mariana and Guatemala) based on bulk-rock and in situ mineral major and trace element compositional data collected on drill cores from the Deep Sea Drilling Project and Ocean Drilling Program. These data are important for constraining the serpentinite-hosted trace element inventory of subduction zones. Bulk serpentinites show up to several orders of magnitude enrichments in Cl, B, Sr, U, Sb, Pb, Rb, Cs and Li relative to elements of similar compatibility during mantle melting, which correspond to the highest primitive mantle-normalized B/Nb, B/Th, U/Th, Sb/Ce, Sr/Nd and Li/Y among subducted lithologies of the oceanic lithosphere (serpentinites, sediments and altered igneous oceanic crust). Among the elements showing relative enrichment, Cl and B are by far the most abundant with bulk concentrations mostly above 1000 µg/g and 30 µg/g, respectively. All other trace elements showing relative enrichments are generally present in low concentrations (µg/g level), except Sr in carbonate-bearing serpentinites (thousands of µg/g). In situ data indicate that concentrations of Cl, B, Sr, U, Sb, Rb and Cs are, and that of Li can be, increased by serpentinization. These elements are largely hosted in serpentine (lizardite and chrysotile, but not antigorite). Aragonite precipitation leads to significant enrichments in Sr, U and B, whereas calcite is important only as an Sr host. Commonly observed brucite is trace element-poor. The overall enrichment patterns are comparable among serpentinites from mid-ocean ridges, passive margins and fore-arcs, whereas the extents of enrichments are often specific to the geodynamic setting. Variability in relative trace element enrichments within a specific setting (and locality) can be several orders of magnitude. Mid-ocean ridge serpentinites often show pronounced bulk-rock U enrichment in addition to ubiquitous Cl, B and Sr enrichment. They also exhibit positive Eu anomalies on chondrite-normalized rare earth element plots. Passive margin serpentinites tend to have higher overall incompatible trace element contents than mid-ocean ridge and fore-arc serpentinites and show the highest B enrichment among all the studied serpentinites. Fore-arc serpentinites are characterized by low overall trace element contents and show the lowest Cl, but the highest Rb, Cs and Sr enrichments. Based on our data, subducted dehydrating serpentinites are likely to release fluids with high B/Nb, B/Th, U/Th, Sb/Ce and Sr/Nd, rendering them one of the potential sources of some of the characteristic trace element fingerprints of arc magmas (e.g. high B/Nb, high Sr/Nd, high Sb/Ce). However, although serpentinites are a substantial part of global subduction zone chemical cycling, owing to their low overall trace element contents (except for B and Cl) their geochemical imprint on arc magma sources (apart from addition of H2O, B and Cl) can be masked considerably by the trace element signal from subducted crustal components.

Sedimentary and subsidence history of ODP Sites 135-840 and 135-841

Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region. Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.

Petrology and origin of basalts at DSDP Hole 66-487

The volcanism of Central America, according to current theory (Pichler and Weyl, 1973; Stoiber and Carr, 1974; Hey, 1977), is related to the subduction of the Cocos Plate under the North American lithospheric plate and the melting of ocean crust material in the subduction zone (Green and Ringwood, 1968; Dickinson, 1970, Fitton, 1971). Since Cocos Plate subduction occurs at the rate of more than 7 cm/y. (Hey et al., 1977), basalts underlying upper Miocene sediments of the Middle America Trench outer slope, penetrated in Hole 487 (Fig. 1) during Leg 66 (Moore et al., 1979), should have formed far from their present position if current theory is accurate. Present manifestations of basaltic magmatism in adjacent areas of the Pacific derive from the axial part of the East Pacific Rise, the Galapagos spreading center, and transform fracture zones. The question arises: Are there analogs of the Middle America Trench basalts among magmatic cock associated with these modern features, or do the trench basalts have some other origin?

(Table 1) Platinum group element abundanves in ODP Leg 135 samples

Ocean Drilling Program Leg 135 backarc basin lavas are characterized by anomalously high Au contents (1.0-11.4 ppb) and strongly fractionated relative platinum group element (PGE) abundances (Pd/Ir ratio, approximately 100). The Rh and Ir contents are very low, ranging from below detection (approximately 0.02 ppb) to 0.08 ppb. The Pd and Pt contents range from <0.3 to 4 ppb. Rh, Pd, and Pt values are consistently and significantly higher in Site 836 and 839 samples relative to those from Sites 834 and 835. Major, trace, and rare earth element (REE) data suggest Sites 836 and 839 have a more pronounced arc signature than Sites 834 and 835. No correlation exists between noble metal abundance and indices of alteration or fractionation (e.g., loss on ignition (LOI), Mg#, and Cr or Ni contents), suggesting that measured values and ratios are primary and reflect characteristics of the mantle source. The evaluation of Leg 135 noble metal data with respect to potential mantle-source components is hindered by the lack of data on magmas derived from such sources. However, analyses of the limited available data for the different magma types suggest that the characteristic enrichment of Leg 135 lavas in Au, relative to Pd and Cu, cannot be derived solely from simple MORB-type or ocean-island-type mantle, or mantle depleted by a previous melt extraction event. The Au-enriched signature of the Lau basin lavas could, however, be produced through the addition of a sedimentary component from the downgoing slab. Separation of Au from the PGE occurs within oceanic hydrothermal systems and gold values of the resultant precipitates are 2-3 orders of magnitude higher than other oceanic crustal components. Even small additions of this component from the downgoing oceanic crust to a supra-subduction zone mantle melt could account for the high mean Au/Pd ratios of the Leg 135 samples (Sites 834 and 835, Au/Pd = 5.04; Sites 836 and 839, Au/Pd = 2.26).

Petrology, geochemistry, and geodynamics at DSDP Hole 66-493

At Site 493, DSDP Leg 66, dioritic basement was reached below lower Miocene (NN1 Zone, 22-24 Ma) terrigenous sediments. Petrographical, mineralogical (including microprobe analyses), and chemical features of the dioritic rocks reveal their magmatic affinity with the calc-alkaline series. Furthermore, their radiometric age (35.3 m.y.) links the basement to the Sierra Madre Occidental in Mexico and to mid-Tertiary volcanic arcs in Central America. The presence of Oligocene diorite 50 km from the trench axis confirms the truncation of the south Mexico margin, which we explain as the result of a 650 to 800 km left-lateral displacement of Central America relative to North America. Truncation must have occurred in the late Miocene, after the diorite intrusion and prior to the present subduction.

Chemical composition of sediment core GeoB12309-5 from the northern Arabian Sea, its pore water profile and age depth relationship as well as CTD data obtained during cruise M74/3

The Arabian Sea off the Pakistan continental margin is characterized by one of the world's largest oxygen minimum zones (OMZ). The lithology and geochemistry of a 5.3 m long gravity core retrieved from the lower boundary of the modern OMZ (956 m water depth) were used to identify late Holocene changes in oceanographic conditions and the vertical extent of the OMZ. While the lower part of the core (535 - 465 cm, 5.04 - 4.45 cal kyr BP, Unit 3) is strongly bioturbated indicating oxic bottom water conditions, the upper part of the core (284 - 0 cm, 2.87 cal kyr BP to present, Unit 1) shows distinct and well-preserved lamination, suggesting anoxic bottom waters. The transitional interval from 465 to 284 cm (4.45 - 2.87 cal kyr BP, Unit 2) contains relicts of lamination which are in part intensely bioturbated. These fluctuations in bioturbation intensity suggest repetitive changes between anoxic and oxic/suboxic bottom-water conditions between 4.45 - 2.87 cal kyr BP. Barium excess (Baex) and total organic carbon (TOC) contents do not explain whether the increased TOC contents found in Unit 1 are the result of better preservation due to low BWO concentrations or if the decreased BWO concentration is a result of increased productivity. Changes in salinity and temperature of the outflowing water from the Red Sea during the Holocene influenced the water column stratification and probably affected the depth of the lower boundary of the OMZ in the northern Arabian Sea. Even if we cannot prove certain scenarios, we propose that the observed downward shift of the lower boundary of the OMZ was also impacted by a weakened Somali Current and a reduced transport of oxygen-rich Indian Central Water into the Arabian Sea, both as a response to decreased summer insolation and the continuous southward shift of the Intertropical Convergence Zone during the late Holocene.

Physiography of the Orca Seamount in the Bransfield Strait, Antarctic Peninsula

Extract from related chapter 5.5.2 in reference: The Orca Seamount was discovered in the central basin of the Bransfield Strait around the posit 62°26'S and 58°24'W on the west side of the Antarctic Peninsula, the most western area of the south polar continent. Through the discovery was made known in 1987, it was only during three bathymetric surveys with high resolution fan echosounders between 1993 and 1995 that the character and complete shape of a remarkable volcano seamount became evident. The data acquisition and processing revealed a spectacular crater of 350 m depth. The relative hight of this 3 km wide "caldera" rim is 550 m with a basal diameter of the seamount cone of 11 km. Its flanks are about 15° steep but in some places the slope reaches up to 36°. The nearly circular shape of the Orca edifice spreads outh with several pronounced spurs, trending parallel to the basin axis in a northeast-southwest direction. The Bransfield Strait is a trough-shaped basin of 400 km length and 2 km depth between the South Shetland Island Arc and the Antarctic Peninsula, formed by rifting behind the islands. The separation of the South Shetland island chain from the peninsula began possibly several million years ago. The active rifting is still going on however, and has caused recent earthquakes and volcanism along the Bransfield Strait. The Strait hosts a chain of submerged seamounts of volcanic origin with the presently inactive Ora Seamount as the most spectacular one. The South Shelfand Island owe their existence to a subduction related volcanism which is perhaps 5-10 times older than the age of Orca and the other seamounts along the central basin of the Bransfield Strait.