Sr-Nd-Pb-Hf isotope results from ODP Leg 187: Evidence for mantle dynamics of the Australian-Antarctic Discordance and origin of the Indian MORB source

New high precision PIMMS Hf and Pb isotope data for 14–28 Ma basalts recovered during ODP Leg 187 are compared with zero-age dredge samples from the Australian-Antarctic Discordance (AAD). These new data show that combined Nd-Hf isotope systematics can be used as an effective discriminant between Indian and Pacific MORB source mantle domains. In particular, Indian mantle is displaced to lower εNd and higher εHf ratios compared to Pacific mantle. As with Pb isotope plots, there is almost no overlap between the two mantle types in Nd-Hf isotope space. On the basis of our new Nd-Hf isotope data, we demonstrate that Pacific MORB-source mantle was present near the eastern margin of the AAD from as early as 28 Ma, its boundary with Indian MORB-source mantle coinciding with the eastern edge of a basin-wide arcuate depth anomaly that is centered on the AAD. This observation rules out models requiring rapid migration of Pacific MORB mantle into the Indian Ocean basin since separation of Australia from Antarctica. Although temporal variations in isotopic composition can be discerned relative to the fracture zone boundary of the modern AAD at 127°E, the distribution of different compositional groups appears to have remained much the same relative to the position of the residual depth anomaly for the past 30 m.y. Thus significant lateral flow of mantle along the ridge axis toward the interface appears unlikely. Instead, the dynamics that maintain both the residual depth anomaly and the isotopic boundary between Indian and Pacific mantle are due to eastward migration of the Australian and Antarctic plates over a stagnated, but slowly upwelling, slab oriented roughly orthogonal to the ridge axis. Temporal and spatial variations in the compositions of Indian MORB basalts within the AAD can be explained by progressive displacement of shallower Indian MORB-source mantle by deeper mantle having a higher εHf composition ascending ahead of the upwelling slab. Models for the origin of the distinctive composition of the Indian MORB-source based on recycling of a heterogeneous enriched component that consist of ancient altered ocean crust plus<10% pelagic sediment are inconsistent with Nd-Hf isotope systematics. Instead, the data can be explained by a model in which Indian mantle includes a significant proportion of material that was processed in the mantle wedge above a subduction zone and was subsequently mixed back into unprocessed upper mantle

Proterozoic magma activities and crustal evolution of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

The Ninth Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 5 Dec. / Entrance Hall (1st floor), National Institute of Polar Researc

Proterozoic magma processes and evolution of continental lithosphere in the Sør Rondane Mountains, East Antarctica

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議室前フロ

Landslide survey : High Lossit, near Machrihanish Bay, Mull of Kintyre

A walkover survey was carried out from the 20th to the 21st of February 2014 to inspect a landslide at High Lossit, Mull of Kintyre, Scotland [NR 62655, 19980]. The subsidence and ground deformation associated with the landslide affected an area of approximately 12 000 sq m of rough grazing land, bordering coastal cliffs, and causing minor damage to stone walls and fencing. The reported landslide occurred within the boundary of a larger, pre-existing landslide that probably occurred following deglaciation of the area during the Late Devensian. The original landslide had an estimated area of 40 000 sq m and likely occurred by rotational failure or sliding of deeply weathered basalt of the Clyde Plateau Volcanic Formation, forming a mass movement deposit comprising angular blocks of bedrock in a matrix of gravelly clay within an area bound by a 5 to 30 m high back-scarp. The recently reported landslide (with an area of approximately 12 000 sq m) is classified as a dominantly translational slide within the older landslide deposit, with rotation at the head of the landslide, developing into a translational slide in the main body and toe. The landslide is developed in a north to north-west facing slope, in gently north-east dipping basalt of the Clyde Plateau Volcanic Formation. The cause of the recent slope failure is likely due to a combination of driving forces including excessive water ingress after prolonged heavy rainfall, and existing slope instability due to the high slope angle and presence of large blocks of heavily weathered and altered basalt. The slope has a history of instability, reflected in the pre-existing scarp of the post-Late Devensian landslide and there is evidence for active soil movement prior to the recent slip recorded by offset of a stone wall at the foot of the recent slip. Large Post-Late Devensian landslips have been identified in numerous coastal locations around the Mull of Kintyre. The causes of these large landslips are poorly understood, but in the High Lossit area, intense weathering of basalts and local faulting in addition to over-steepening of the slope through glacial erosion may have been contributing factors to slope instability soon after deglaciation. Further assessment, including detailed geological mapping, would be required to properly understand the effect of the highly weathered and altered basalt on the ground stability and to develop the ground model. To assess for likelihood of future movement, a hydrogeological study of the site would also be required as the drainage of the affected field has been altered following the landslide

A feldspar-nepheline achondrite clast in Parnallee

A feldspar-nepheline clast (FELINE) has been identified in Parnallee (LL3.6). Plagioclase is An_, Ab_ and nepheline contains 0.24-3.12wt% Cl. The calculated bulk composition is mildly alkaline, with 3.5wt% Na_2O. Plagioclase has heavy REE depletion and a positive Eu anomaly (Eu/Eu^*=65). Nepheline has lower total REE than plagioclase. On a three isotope plot, the oxygen isotope composition of FELINE falls near the Carbonaceous Chondrites Anyhdrous Minerals Line, beneath the Terrestrial Fractionation Line. This suggests that the parental material had carbonaceous chondrite affinities. It was derived from a melt with moderately enriched LREE and Eu (13.5×CI), which probably underwent an influx of Na-, Cl-rich fluids during crystallisation. This LREE-enrichment suggests that Ca-pyroxene crystallised in the parent body residue during a melt extraction event. REE abundances and the oxygen isotope signature are consistent with an origin as a lost plagiophile melt fraction complementary to the ureilites. FELINE provides further evidence that achondritic fragments with an igneous, exotic origin are an important component of chondritic meteorites

Reverse magnetic anomaly controlled by Permian Igneous rocks in the Iberian Chain (N Spain)

Two important reverse dipolar magnetic anomalies in the Iberian Chain (Spain) are located over Permian igneous rocks. A detailed study of one of them, the Loscos magnetic anomaly, where the geological structure is well constrained, reveals that the source of the anomaly must be a reverse remanent magnetisation carried by igneous rocks, acquired during the period of the Kiaman reverse magnetic superchron. Magnetic and gravimetric detailed survey (with 50 new gravimetric measurements and 8 main magnetic profiles -six of them N?S and the rest E?W- with a total length of 40km), together with a study of the petrophysical characteristics of igneous materials, data processing and interpretation (upward continuation, 2.5D modelling, etc.) allowed to characterize qualitatively the anomaly and its source. Two overlapping anomalies with different wavelength were identified, indicating the presence of a shallower strongly altered igneous body with heterogeneous magnetic properties, and a deeper, large igneous body, responsible for the main, long-wavelength anomaly

Application of the Moessbauer technique to the study of probable lunar and planetary surface material and to the study of returned lunar surface samples Final report

Preliminary results from Mossbauer instrumental analysis of Apollo 12 lunar rock and soil sample

The Saint-Georges-sur-Loire olistostrome, a key zone to understand the Gondwana-Armorica boundary in the Variscan belt (Southern Brittany, France).

In the southern part of the French Armorican massif, the Ligerian domain is located along the boundary between Gondwana and Armorica. Lithological, geochemical and structural data on the Saint-Georges-sur-Loire Unit, which is the northern part of the Ligerian domain, allow us to distinguish two sub-units. A southern sub-unit, formed by various blocks (chert, limestone, sandstone, rhyolite, mafic rocks) of Silurian to Middle Devonian age included as olistoliths in a Middle-Late Devonian terrigeneous matrix, overthrusts a sandstone-pelite northern sub-unit. Both units experienced two deformation events. The first one is a top-to-the-NW thrusting and the second one is a left-lateral wrenching. The Saint-Georges-sur-Loire Unit is an accretionary prism formed during the Late Devonian closure of the Layon rift, coeval with the main phase of the Variscan orogeny. The Layon rift, which according to the mafic olistoliths was partly floored by oceanic crust, appears as a buffer structural zone that accounts for the lack in Central Brittany of any tectonic or sedimentary echo of the closure of the Medio-European Ocean. The tectonic evolution of the Saint-Georges-sur-Loire Unit supports a polyorogenic model for this part of the Variscan Belt