Ultramafic lamprophyre (UML) dykes from the Ferrar Province (Pensacola Mountains) of Antarctica preserve trace element and isotope signatures similar to Bouvet volcanic rocks, which are considered to ...reflect the palaeo composition of the Bouvet mantle plume. We report Sr, Nd, Pb, and Os isotope compositions for three ultramafic lamprophyre dykes emplaced at 183.2±2.2 Ma, coincident with the main Karoo–Ferrar magmatic event. The ultramafic lamprophyre dykes are characterized by high Ti, Cr, Ni, Nb/La, La
N/Yb
N, and Mg# values, and are the most primitive rocks of the Ferrar Province. The dykes have initial (183 Ma)
87Sr/
86Sr ratios of 0.7044–0.7055,
εNd of 4.6–4.8,
208Pb/
204Pb of 39.6–40.3, and
187Os/
188Os of 0.120–0.146 and contrast markedly with even the most primitive rocks of the Ferrar and Karoo provinces. The trace element and isotope characteristics have affinities to ocean island basalt (OIB) and the highly radiogenic character of
208Pb/
204Pb and
206Pb/
204Pb bear closest resemblance to Bouvet, which has previously been postulated as the plume responsible for the Ferrar Province. The ultramafic lamprophyres are believed to be the result of melting enriched Bouvet mantle plume material and represent one of the mantle end members in the Karoo–Ferrar province.
The Middle Jurassic–Lower Cretaceous Fossil Bluff Group of Alexander Island, Antarctica represents the fill of a fore‐arc basin unconformably overlying an accretionary complex. Like most fore‐arc ...basins, this example had been considered to have a passive origin, as a topographic hollow between the arc and the trench‐slope break. Recent discoveries of igneous rock coeval with sedimentation have altered this view. Oxfordian–Kimmeridgian basaltic and rhyolitic sills and lava flows are found in a restricted area at the north of the basin, within a single formation. Chemically, most basalts are high‐Nb types, which cannot have originated in a supra‐subduction zone setting. Since the age of emplacement of these rocks coincides with a gap in the record of plutonism in the Antarctic Peninsula volcanic arc, it is concluded that a late Jurassic pause in subduction led to active rifting to form the fore‐arc basin.
Rare basaltic volcanic rocks from the northern Antarctic Peninsula (Jason Peninsula) are established as marginally predating a silicic large igneous province, which developed along the proto-Pacific ...margin of Gondwana.
40Ar–
39Ar geochronology reported here has dated the basalts in the interval, 175–168 Ma, which overlaps with the dates previously obtained on the silicic volcanic rocks, 171–168 Ma.
The basalts are evolved with low Mg#, Cr and Ni, indicating they have undergone significant fractional crystallisation from mantle-derived melts. Their trace elements (high Th/Yb) and isotope ratios (positive εSr and low εNd) indicate that they are derived from lithospheric mantle, which has been significantly modified by subduction-derived fluids and sediments.
Cretaceous age (126–106 Ma) primitive mafic dykes from the Antarctic Peninsula are also interpreted as partial melts of the subduction-modified mantle, and together with the Middle Jurassic basalts are used to monitor the Antarctic Peninsula lithosphere. The shift from lithosphere-derived (Jurassic) to asthenosphere-derived (Cretaceous) partial melts in the northern Antarctic Peninsula is attributed to thinning of the lithosphere, which was a consequence of the voluminous silicic volcanism of the Middle Jurassic, coupled with regional extension. The chemistry, chronology and rarity of the basalts are consistent with the model proposed for the voluminous silicic volcanism.
Mafic dykes of the Antarctic Peninsula continental-margin arc are compositionally diverse, comprising calc-alkaline (dominant), shoshonite, tholeiite, and OIB-like varieties. Their compositions give ...information about different mafic magma sources tapped during arc evolution.
The South Sandwich volcanic arc is sited on a young oceanic crust, erupts low-K tholeiitic rocks, is characterized by unexotic pelagic and volcanogenic sediments on the down-going slab, and simple ...tectonic setting, and is ideal for assessing element transport through subduction zones. As a means of quantifying processes attending transfer of subduction-related fluids from the slab to the mantle wedge, boron concentrations and isotopic compositions were determined for representative lavas from along the arc. The samples show variable fluid-mobile/fluid-immobile element ratios and high enrichments of B/Nb (2.7 to 55) and B/Zr (0.12 to 0.57), similar to those observed in western Pacific arcs. δ11B values are among the highest so far reported for mantle-derived lavas; these are highest in the central part of the arc (+15 to +18‰) and decrease toward the southern and northern ends (+12 to +14‰). δ11B is roughly positively correlated with B concentrations and with 87Sr/86Sr ratios, but poorly coupled with other fluid-mobile elements such as Rb, Ba, Sr and U. Peridotites dredged from the forearc trench also have high δ11B (ca. +10‰) and elevated B contents (38–140ppm). Incoming pelagic sediments sampled at ODP Site 701 display a wide range in δ11B (+5 to −13‰; average=−4.1‰), with negative values most common. The unusually high δ11B values inferred for the South Sandwich mantle wedge cannot easily be attributed to direct incorporation of subducting slab materials or fluids derived directly therefrom. Rather, the heavy B isotopic signature of the magma sources is more plausibly explained by ingress of fluids derived from subduction erosion of altered frontal arc mantle wedge materials similar to those in the Marianas forearc. We propose that multi-stage recycling of high-δ11B and high-B serpentinite (possibly embellished by arc crust and volcaniclastic sediments) can produce extremely 11B-rich fluids at slab depths beneath the volcanic arc. Infiltration of such fluids into the mantle wedge likely accounts for the unusual magma sources inferred for this arc.
► Subduction recycling are evaluated considering B systematics for the SSI arc. ► SSI lavas have heaviest 11B values (+12 to +18‰) yet observed for arc-rocks. ► Metasomatized forearc mantle provides high 11B to SSI lavas. ► Multi-stage recycling of fluids at slab depths account for unusual magma source. ► Tectonic erosion appears to be the mechanism for recycling heavy B via the trench.
A small 33±0.8 Ma lamproite pluton is exposed in the midst of a 23-26 Ma basalt-rhyolite province in Middle Park, NW Colorado. It contains abundant phlogopite phenocrysts in a fine-grained groundmass ...of analcime pseudomorphs after leucite, biotite, potassic richterite, apatite, ilmenite and accessory diopside. The phlogopite phenocryst cores contain ∼4 wt.% TiO2, 1% Cr2O3 and 0.2% BaO. The smallest groundmass biotites have normal pleochroism but compositions unlike any previously reported, with ∼2% Al2O3, ∼8% TiO2 and F<1.5%. Apart from those elements affected by leucite alteration, both the elemental and isotopic composition of this lamproite are close to those of the Leucite Hills, Wyoming. Its Nd-isotopic model age (TDM = 1.6 Ga) is outside the Leucite Hills range but within that of other Tertiary strongly potassic magmatism in the region underlain by the Wyoming craton. Evidence from both teleseismic tomography and the mantle xenoliths within other western USA mafic ultrapotassic igneous suites shows that the total lithospheric thickness beneath NW Colorado was probably ∼150-200 km at 33 Ma, when the Middle Park lamproite was emplaced. This is an important constraint on tectonomagmatic models for the Cenozoic evolution of this northernmost part of the Rio Grande rift system.
The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower 206Pb/204Pb for a given εNd and 208Pb/204Pb) could reflect processes that ...occurred within a few tens of millions of years preceding the initial breakup of Gondwana. Alternatively, the Indian Ocean isotopic signature could be a much more ancient upper-mantle feature inherited from the asthenosphere of the eastern Tethyan Ocean, which formerly occupied much of the present Indian Ocean region. Age-corrected Nd, Pb, and Sr isotopic data for 46–150 Ma seafloor lavas from sites in the western Indian Ocean and ocean-ridge-type Tethyan ophiolites (Masirah, Yarlung–Zangpo) reveal the presence of both Indian-Ocean-type compositions and essentially Pacific–North Atlantic-type signatures. In comparison, Jurassic South Pacific ridge basalts from Alexander Island, Antarctica, possess normal Pacific–North Atlantic-type isotopic ratios. Despite the very sparse sampling of old seafloor, the age-corrected εNd(t) values of the old Indian Ocean basalts cover a greater range than seen for the much more thoroughly sampled present-day spreading axes and islands within the Indian Ocean (e.g. 18 εNd units for basalts in the 60–80 Ma range vs 15 εNd units for 0–10 Ma ones). The implications of these results are that the upper mantle in the Indian Ocean region is becoming increasingly well mixed through time, and that the Indian Ocean mantle domain may not greatly pre-date the age of earliest spreading in the Indian Ocean.
For ground penetrating radar (GPR), smaller antennas would provide considerable practical advantages. Some of which are: portability; ease of use; and higher spatial sampling. A theoretical ...comparison of
the fundamental limits of a small electric field antenna and a small magnetic field antenna shows that the minimum Q constraints are identical. Furthermore, it is shown that only the small magnetic
loop antenna can be constructed to approach, arbitrarily closely, the fundamental minimum Q limit. This is achieved with the addition of a high permeability material which reduces energy stored in
the magnetic fields. This is of special interest to some GPR applications. For example, applications requiring synthetic aperture data collection would benefit from the increased spatial sampling offered
by electrically smaller antennas. Low frequency applications may also benefit, in terms of reduced antenna dimensions, by the use of electrically small antennas. Under these circumstances, a magnetic type
antenna should be considered in preference to the typical electric field antenna. Numerical modeling data supports this assertion.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, GIS, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Knowing the spatial scales at which effective management can be implemented is fundamental for conservation planning. This is especially important for mobile species, which can be exposed to threats ...across large areas, but the space use requirements of different species can vary to an extent that might render some management approaches inefficient. Here the space use patterns of seabirds were examined to provide guidance on whether conservation management approaches should be tailored for taxonomic groups with different movement characteristics. Seabird tracking data were synthesised from 5419 adult breeding individuals of 52 species in ten families that were collected in the Atlantic Ocean basin between 1998 and 2017. Two key aspects of spatial distribution were quantified, namely how far seabirds ranged from their colony, and to what extent individuals from the same colony used the same areas at sea. There was evidence for substantial differences in patterns of space-use among the ten studied seabird families, indicating that several alternative conservation management approaches are needed. Several species exhibited large foraging ranges and little aggregation at sea, indicating that area-based conservation solutions would have to be extremely large to adequately protect such species. The results highlight that short-ranging and aggregating species such as cormorants, auks, some penguins, and gulls would benefit from conservation approaches at relatively small spatial scales during their breeding season. However, improved regulation of fisheries, bycatch, pollution and other threats over large spatial scales will be needed for wide-ranging and dispersed species such as albatrosses, petrels, storm petrels and frigatebirds.
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