Trace element geochemistry of peridotites from the Izu-Bonin-Mariana Forearc, Leg 125

Trace element analyses (first-series transition elements, Ti, Rb, Sr, Zr, Y, Nb, and REE) were carried out on whole rocks and minerals from 10 peridotite samples from both Conical Seamount in the Mariana forearc and Torishima Forearc Seamount in the Izu-Bonin forearc using a combination of XRF, ID-MS, ICP-MS, and ion microprobe. The concentrations of incompatible trace elements are generally low, reflecting the highly residual nature of the peridotites and their low clinopyroxene content (n ratios in the range of 0.05-0.25; several samples show possible small positive Eu anomalies. LREE enrichment is common to both seamounts, although the peridotites from Conical Seamount have higher (La/Ce)n ratios on extended chondrite-normalized plots, in which both REEs and other trace elements are organized according to their incompatibility with respect to a harzburgitic mantle. Comparison with abyssal peridotite patterns suggests that the LREEs, Rb, Nb, Sr, Sm, and Eu are all enriched in the Leg 125 peridotites, but Ti and the HREEs exhibit no obvious enrichment. The peridotites also give positive anomalies for Zr and Sr relative to their neighboring REEs. Covariation diagrams based on clinopyroxene data show that Ti and the HREEs plot on an extension of an abyssal peridotite trend to more residual compositions. However, the LREEs, Rb, Sr, Sm, and Eu are displaced off this trend toward higher values, suggesting that these elements were introduced during an enrichment event. The axis of dispersion on these plots further suggests that enrichment took place during or after melting and thus was not a characteristic of the lithosphere before subduction. Compared with boninites sampled from the Izu-Bonin-Mariana forearc, the peridotites are significantly more enriched in LREEs. Modeling of the melting process indicates that if they represent the most depleted residues of the melting events that generated forearc boninites they must have experienced subsolidus enrichment in these elements, as well as in Rb, Sr, Zr, Nb, Sm, and Eu. The lack of any correlation with the degree of serpentinization suggests that low-temperature fluids were not the prime cause of enrichment. The enrichment in the high-field-strength elements also suggests that at least some of this enrichment may have involved melts rather than aqueous fluids. Moreover, the presence of the hydrous minerals magnesio-hornblende and tremolite and the common resorption of orthopyroxene indicate that this high-temperature peridotite-fluid interaction may have taken place in a water-rich environment in the forearc following the melting event that produced the boninites. The peridotites from Leg 125 may therefore contain a record of an important flux of elements into the mantle wedge during the initial formation of forearc lithosphere. Ophiolitic peridotites with these characteristics have not yet been reported, perhaps because the precise equivalents to the serpentinite seamounts have not been analyzed

The ophiolite-related Mersin Melange, southern Turkey: its role in the tectonic–sedimentary setting of Tethys in the Eastern Mediterranean region

WOS: 000222988100001The Mersin Melange underlies the intact Mersin Ophiolite and its metamorphic sole to the south of the Mesozoic Tauride Carbonate Platform in southern Turkey The Melange varies from chaotic melange to broken formation, in which some stratigraphic continuity can be recognized. Based on study of the broken formation, four lithological associations are recognized: (1) shallow-water platform association, dominated by Upper Palaeozoic-Lower Cretaceous neritic carbonates; (2) rift-related volcanogenic- terrigenous-pelagic association, mainly Upper Triassic andesitic-acidic volcanogenic rocks, siliciclastic gravity flows, basinal carbonates and radiolarites; (3) within-plate-type basalt radiolarite-pelagic limestone association, interpreted as Upper Jurassic-Lower Cretaceous seamounts with associated radiolarian sediments and Upper Cretaceous pelagic carbonates; (4) ophiolite-derived association, including fragments of the Upper Cretaceous Mersin Ophiolite and its metamorphic sole. Locally, the ophiolitic melange includes granite that yielded a K/Ar radiometric age of 375.7 +/- 10.5 Ma (Late Devonian). This granite appears to be subduction influenced based on 'immobile' element composition. The Mersin Melange documents the following history: (1) Triassic rifting of the Tauride continent; (2) Jurassic-Cretaceous passive margin subsidence; (3) oceanic seamount genesis; (4) Cretaceous supra-subduction zone ophiolite genesis; (5) Late Cretaceous intra-oceanic convergence/metamorphic sole formation, and (6) latest Cretaceous emplacement onto the Tauride microcontinent and related backthrusting. Regional comparisons show that the restored Mersin Melange is similar to the Beysehir-Hoyran Nappes further northwest and a northerly origin best fits the regional geological picture. These remnants of a North-Neotethys (Inner Tauride Ocean) were formed and emplaced to the north of the Tauride Carbonate Platform. They are dissimilar to melanges and related units in northern Syria, western Cyprus and southwestern Turkey, which are interpreted as remnants of a South-Neotethys. Early high-temperature ductile transport lineations within amphibolites of the metamorphic sole of the Mersin ophiolite are generally orientated E-W, possibly resulting from vertical-axis rotation of the ophiolite while still in an oceanic setting. By contrast, the commonly northward-facing later stage brittle structures are explained by backthrusting of the ophiolite and melange related to exhumation of the partially subducted northern leading edge of the Tauride continent

Four-Hundred-and-Ninety-Million-Year Record of Bacteriogenic Iron Oxide Precipitation at Sea-Floor Hydrothermal Vents

Fe oxide deposits are commonly found at hydrothermal vent sites at mid-ocean ridge and back-arc sea floor spreading centers, seamounts associated with these spreading centers, and intra-plate seamounts, and can cover extensive areas of the seafloor. These deposits can be attributed to several abiogenic processes and commonly contain micron-scale filamentous textures. Some filaments are cylindrical casts of Fe oxyhydroxides formed around bacterial cells and are thus unquestionably biogenic. The filaments have distinctive morphologies very like structures formed by neutrophilic Fe oxidizing bacteria. It is becoming increasingly apparent that Fe oxidizing bacteria have a significant role in the formation of Fe oxide deposits at marine hydrothermal vents. The presence of Fe oxide filaments in Fe oxides is thus of great potential as a biomarker for Fe oxidizing bacteria in modern and ancient marine hydrothermal vent deposits. The ancient analogues of modern deep-sea hydrothermal Fe oxide deposits are jaspers. A number of jaspers, ranging in age from the early Ordovician to late Eocene, contain abundant Fe oxide filamentous textures with a wide variety of morphologies. Some of these filaments are like structures formed by modern Fe oxidizing bacteria. Together with new data from the modern TAG site, we show that there is direct evidence for bacteriogenic Fe oxide precipitation at marine hydrothermal vent sites for at least the last 490 Ma of the Phanerozoic

Chromite and platinum group elements mineralization in the Santa Elena Ultramafic Nappe (Costa Rica): geodynamic implications

Chromitites associated with strongly altered peridotite from six distinct localities in the Santa Elena ultramafic nappe (Costa Rica) have been investigated for the first time. Santa Elena chromitites commonly display a compositional variation from extremely chromiferous (Cr/(Cr+Al)=0.81) to intermediate and aluminous (Cr/(Cr+Al)=0.54). This composition varies along a continuous trend, corresponding to calculated parental liquids which may have been derived from the differentiation of a single batch of boninitic magma with Cr-rich and (Al, Ti)-poor initial composition. Fractional precipitation of chromite probably occurred during differentiation of the boninitic melt and progressive metasomatic reaction with mantle peridotite. The distribution of platinum group elements (PGE) displays the high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio typical of ophiolitic chromitites and, consistently, the platinum group minerals (PGM) encountered are mainly Ru-Os-Ir sulfides and arsenides. Textural relations of most of the platinum group elements suggest crystallization at magmatic temperatures, possibly under relatively high sulfur fugacity as indicated by the apparent lack of primary Os-Ir-Ru alloys. The chemical and mineralogical characteristics of chromitites from the Santa Elena ultramafic nappe have a strong affinity to podiform chromitites in the mantle section of supra-subduction-zone ophiolites. Calculated parental melts of the chromitites are consistent with the differentiation of arc-related magmas, and do not support the oceanic spreading center geodynamic setting previously proposed by some authors

Mineral chemistry of the ophiolitic peridotites and gabbros from the Serow area: Implications for tectonic setting and locating the Neotethys suture in NW Iran

The Serow ophiolite in NW Iran, located at the Iran-Turkey border, is composed of mantle sequence peridotites, predominantly lherzolitic-harzburgite with subordinate amounts of lherzolite and dunite, and a crustal sequence made from gabbros, diabases, pillow lavas and deep marine carbonates and radiolarite sediments. The rocks appear as a tectonic mélange. This ophiolitic complex forms part of the ophiolites marking a branch of Neotethys oceanic crust in NW Iran. The chemistry of olivine, orthopyroxene and clinopyroxene in the lherzolitic-harzburgite and clinopyroxene in the gabbros suggests a supra-subduction setting for the ophiolite. The Serow ophiolite is similar to other ophiolites in NW Iran such as the Piranshahr, Naghadeh and Khoy and NE Turkey ophiolites in terms of the rock units, tectonic setting and age. The Serow ophiolite links the Iranian ophiolites from Baft in the SE through the South Azerbaijan suture to the Izmir-Ankara-Erzincan suture in the NW

Ophiolites in the North Himalayan nappes and Indus Suture Zone in Eastern Ladakh (NW Himalaya, India)

Ophiolites are fragments of ancient oceanic lithosphere, preserved in orogenic belts in a context of plate convergence. They are generated at mid-ocean ridges, in a supra-subduction zone or volcanic arc. Commonly, several magmatic events are recorded, as shown, for instance, in the Oman Ophiolite (Goodenough et al. 2014). The Ophiolitic rocks of Eastern Ladakh are subdivided in two main groups, based on the geodynamic setting during their formation: the supra-subduction zone ophiolite and the ophiolitic “mélanges”, corresonding both to the Indus Suture Zone. Recent detailed studies North-East of the Tso Moriri area revealed a large diversity of ophiolitic rocks and associated sediments. We identified three distinct tectonic units containing ophiolites: The Nidar Ophiolite, the Drakkarpo nappe and the Karzok-Ribil nappe. The Nidar supra-subduction zone Ophiolite represents a complete ophiolitic sequence, from mantle to sediments, which underwent a low greenschist facies metamorphism. This ophiolitic sequence was thrusted towards the South. They record a first magmatic event in a mid-ocean ridge setting, and a second one in a supra-subduction zone at around 130 Ma. The Drakkarpo nappe is a “mélange” unit composed of thick polygenic conglomerates and volcano-sedimentary rocks, mainly composed of tuffs and augite-basalts (OIB), serpentinites, pillow lavas and gabbros. This unit is interpreted as being a part of an accretionary wedge containing slices of oceanic islands arc. This nappe marks the Indus Suture Zone. The Karzok-Ribil nappe is a newly defined tectonic unit involved in the North Himalayan nappe stack. It can be followed at the top of the Tetraogal nappe and around the Tso Morari dome. The Karzok-Ribil nappe is composed of segments of ophiolitic sequence (serpentinites, gabbros, pillow lavas), radiolarites, polygenic conglomerates, agglomeratic slates from the indian margin, augite-basalts (OIB) and limestones. It is interpreted as being originally a seamount, located close to the Indian passive margin in a ocean-continent transition zone. The new lithostratigraphy and structural analyses of the Eastern Ladakh ophiolites and their associated sediments allow us to better constrain the formation and emplacement mechanisms of these tectonic units. It defines or precises the paleogeography and geometry of the north Indian passive margin, prior to the Himalayan collision. REFERENCES Goodenough, Kathryn M., Robert J. Thomas, Michael T. Styles, David I. Schofield, and Christopher J. MacLeod. 2014. “Records of Ocean Growth and Destruction in the Oman–UAE Ophiolite.” Elements 10 (2): 109–114