Showing papers by "Anders Meibom published in 2004"

H2-rich fluids from serpentinization: Geochemical and biotic implications

Abstract: Metamorphic hydration and oxidation of ultramafic rocks produces serpentinites, composed of serpentine group minerals and varying amounts of brucite, magnetite, and/or FeNi alloys. These minerals buffer metamorphic fluids to extremely reducing conditions that are capable of producing hydrogen gas. Awaruite, FeNi3, forms early in this process when the serpentinite minerals are Fe-rich. Olivine with the current mantle Fe/Mg ratio was oxidized during serpentinization after the Moon-forming impact. This process formed some of the ferric iron in the Earth's mantle. For the rest of Earth's history, serpentinites covered only a small fraction of the Earth's surface but were an important prebiotic and biotic environment. Extant methanogens react H2 with CO2 to form methane. This is a likely habitable environment on large silicate planets. The catalytic properties of FeNi3 allow complex organic compounds to form within serpentinite and, when mixed with atmospherically produced complex organic matter and waters that circulated through basalts, constitutes an attractive prebiotic substrate. Conversely, inorganic catalysis of methane by FeNi3 competes with nascent and extant life.

Distribution of magnesium in coral skeleton

Abstract: Ion micro-probe imaging of the aragonite skeleton of Pavona clavus, a massive reef-building coral, shows that magnesium and strontium are distributed very differently. In contrast to strontium, the distribution of magnesium is strongly correlated with the fine-scale structure of the skeleton and corresponds to the layered organization of aragonite fibers surrounding the centers of calcification, which have up to ten times higher magnesium concentration. This indicates a strong biological control over the magnesium composition of all structural components within the skeleton. Magnesium may be used by the coral to actively control the growth of the different skeletal crystal components.

Zircon U-Pb SHRIMP dating of the Yematan batholith in Dulan, North Qaidam, NW China

Abstract: The Yematan batholith crops out over 120 km2 in the North Qaidam ultrahigh pressure (UHP) metamorphic belt. It consists of granodiorite, monzogranite and biotite granite and forms an irregular intrusion into Neoproterozoic gneiss that has undergone Caledonian UHP metamorphism. Zircons from the Yematan granodiorite yield a SHRIMP U-Pb age of 397±3 Ma. These granitic rocks have geochemical characteristics intermediate between I- and S-type granites, and are post-collisional. We suggest that the Yematan granitic rocks were formed during the last exhumation event of the North Qaidam UHP belt.

Osmium isotopic compositions of Os‐rich platinum group element alloys from the Klamath and Siskiyou Mountains

Abstract: [1] We present new measurements of Os-186/Os-188 and Os-187/Os-188 in 10 Os-rich platinum group element (PGE) alloys from placer deposits formed by the mechanical erosion of peridotite-bearing ophiolites in the Klamath and Siskiyou Mountains in northern California and southwestern Oregon. These data nearly double our database of high-precision Os-186/Os-188 measurements on such samples. Together with previously published data, our new results reinforce the conclusion that the radiogenic Os-186/Os-188 compositions of these PGE alloys are very difficult to reconcile with a derivation of their Os from the outer core. Such a model requires extremely early growth of the inner core to its present size, within several hundred million years after accretion of the Earth, which is geophysically implausible. Collectively, our data suggest instead that partial melting or metasomatic processes in the upper mantle play a primary role in controlling the Os isotopic systematics of these Os-rich PGE alloys and suggest the existence of upper mantle components characterized by radiogenic Os-186/Os-188 ratios. Pyroxene-rich lithologies are possible candidates.

Granitic magmatism on the early Paleozoic UHP belt of northern Qaidam, NW China

Abstract: Three types of granite association were found on the northern margin of the Qadaim Basin, which is an early Paleozoic ultrahigh pressure metamorphic belt with the eclogite ages of 466-495 Ma. The first type of association is quartz monozdiorite+granodiorite+monzogranite, the second is monzogranite+two mica granite +muscovite granite+syenogranite, and the third is granite+monzogranite+biotite granite. The granite bodies from the first association occurred in small stocks while the second and the third in batholiths. Their country-rocks are composed chiefly of Proterozoic and Paleozoic rocks. The Proterozoic sequences of rocks consist of medium- and high-grade metamorphic gneiss and schist, and constitute the basement of this region. The Lower Paleozoic sequences consist mainly of strongly-deformed Ordovician deep-sea deposits and volcanic rocks, while the upper Paleozoic includes the Upper Devonian continental coarse clastic rocks and the Carboniferous shallow-sea deposits. The monzogranite in the first association has a zircon SHRIMP age of 473 Ma, the syenogranite in the second association has a zircon SHRIMP age of 446 Ma, and the granodiorite in the third association has a zircon SHRIMP age of 397 Ma. From the rock-forming minerals of the three types of granite association, the first association has the same minerals as the third one, consisting of plagioclase, hornblends, quartz, alkaline feldspar and biotite, and different from the third one, which consists of potassium feldspar, quartz, muscovite, biotite and plagioclase. Petrochemically, both the first one and the third one belong to the I-type granite, with SiO,=61%-69%, Na O/KO>1, ANK 1, δEu=O.1-0.3. Tectonically, the granite from the first association intruded in island arc environment or active edge of continent, the second formed in syn-collision between Qadaim land and Qilian land and the third formed in post-collision. Combined with regional geology, we suggested that the oceanic plate of the south Qilian Ocean subducted under the Qilian continental plate in the early Ordovician and the ocean closed soon. However, the subduction of the south Qilian Ocean plate dragged the Qaidam continent plate to continue to subduct into the mantle. With the increasing of temperature and pressure, the materials of the plates subjected to metamorphism. The fluids were developed from some mineral dehydration in the course of the matmorphism from the green schist phase through hornblend phase to eclogite phase, and raised in the mantle wedge and induced partial melting between the upper mantle and lower crust, forming I-type granite magma of the first association (the averaged age is about 473 Ma). With the continuing of subduction of the Qaidam plate, the Qilian continental plate thrusted on the Qaidam continental plate, forming continent collision belt, which produced thicker crust and partial melting to form S-type granite magma of the second association (at about 446 Ma). The oceanic plate subducted early into the mantle metamorphosed eclogite with a larger density, which separated from the upper felsic plate, which is called delamination. As a result, heavier mass went down while lighter mass uplifted. Meanwhile, the asthenosphere upwell provided material and energy to the bottom of the lithosphere. the exhumed mass squeezed and abraded with adjacent blocks and produced partial melting, forming the third association magma (at 397 Ma).