5 research outputs found
Geochemistry of Cenozoic mafic potassic and sodic volcanic rocks in southwestern Madagascar: Long-lived lithospheric mantle heterogeneities in an extensional tectonic setting
No full textCenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K2O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; 87Sr/86Sri = 0.70529–0.70555, 143Nd/144Ndi = 0.51262–0.51263, 206Pb/204Pbm = 18.415–18.424, 207Pb/204Pbm = 15.576–15.579, 208Pb/204Pbm = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; 87Sr/86Sri = 0.70346–0.7036, 143Nd/144Ndi = 0.51281–0.51282, 206Pb/204Pbm = 19.079–19.374, 207Pb/204Pbm = 15.621–15.645, 208Pb/204Pbm = 39.115–39.424). The relatively low CaO, Sc, V, Fe2O3t, MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region
Bovine TB in livestock and wildlife: what's in the genes?
No full textPlease help populate SUNScholar with the full text of SU research output. Also - should you need this item urgently, please send us the details and we will try to get hold of the full text as quick possible. E-mail to [email protected]. Thank you.Journal Articles (subsidised)Geneeskunde en GesondheidswetenskappeMolekul�re Biologie & Mensgenetik
Petrology and geochemistry of Marion and Prince Edward Islands, Southern Ocean: Magma chamber processes and source region characteristics
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Mineral dissolution in silicate melts
No full textQuartz and orthopyroxene in mafic rocks are commonly observed to be
surrounded by fringes of granular pyroxene, and of olivine and clinopyroxene,
respectively. This study reproduces the conditions of formation of these textures,
and investigates their origins, kinetics and phase relations.
Pieces of silica glass or crystals of orthopyroxene were dissolved into
tholeiitic and slightly alkaline basalts, suspended from wire loops in an atmospheric
pressure quenching furnace, and run for 10 minutes to 32 days at subliquidus
temperatures between 1120° and 1190°C and oxygen fugacities close to the QFM
buffer. Polished sections of charges were examined primarily by backscattered
electron imagery and by microprobe analysis.
The textures developed in silica dissolution experiments consist of fringes
of elongate skeletal pyroxenes radially arranged around the silica. The pyroxenes
first nucleate on the surface of the silica. As dissolution continues, growth
continues mostly on existing crystals, rather than by the nucleation of additional
crystals.
Dissolution rates for silica range from 2.8*10⁻¹¹ to 4.4*10⁻¹⁰ms⁻¹, and are
time-independent until growth of the pyroxene fringe hinders transport processes in
the melt. This causes dissolution to slow down, until it ceases altogether after 3 to
8 days. A silica-rich layer of melt forms around the surface of charges run at higher
temperatures, suggesting that convection driven by variations in surface tension
may operate in the charges.
The textures developed in orthopyroxene dissolution experiments consist of
granular olivines, some of which nucleate on the pyroxene surface, whereas
others nucleate within the pyroxene as a result of the decomposition of included
phases. With time, olivine crystals become connected and form complex grain
shapes.
Dissolution rates for orthopyroxene range from 1.7*10⁻¹¹ to 1.2*10⁻⁹ ms⁻¹.
At higher temperatures dissolution rates are constant, but at lower temperatures
dissolution is time-dependent. Unlike silica dissolution, orthopyroxene dissolution
does not cease as a result of continued neocryst growth hindering melt transport,
indicating that the fringe remains permeable.
For both systems, the neocryst compositions are strongly dependent on the
chemistry of the melt formed at the interface between the dissolving crystal and the
bulk melt, and the neocrysts may be metastable with respect to the bulk melt.
Chemical equilibration of olivine neocrysts with time is observed for longer
experiments. Textural equilibration of olivine grains occurs by the processes of
liquid-phase sintering in runs longer than 12 hours.
Subliquidus dissolution data are applied to textures from natural samples
collected from dykes, lava flows and lava lakes, to estimate the residence time of
reacted crystals; values range from 0.6 to 208 days for reacted quartz, and from
0.7 to 462 days for reacted orthopyroxenes. The rates of cooling of the magma and
the size of the magma body in which the reaction occurred are also estimated
Composition and thermal structure of the lithosphere beneath the Ethiopian plateau: evidence from mantle xenoliths in basanites, Injibara, Lake Tana Province
No full textPetrographic, minerochemical, and geothermobarometric data are reported for a suite of
19 spinel-lherzolite pargasite xenoliths hosted in a Quaternary basanitic lava flow from
20 the North-Western Ethiopian Plateau (Injibara, Lake Tana Province). Protogranular to
21 porphyroclastic (deformed) rocks show evidence of a modal metasomatism, represented
22 by a Cl-rich pargasitic amphibole, coupled with criptic enrichment in Fe and Al.
23 Equigranular rocks (granular) record a further cryptic metasomatism, represented by
24 enrichment in Fe, Al, Na, and depletion in Ni, Cr and Cl. Some xenoliths (transitional)
25 show intermediate textural and compositional characters, indicating that the granular
26 samples represent an evolution of the deformed ones. All xenoliths give the same P–T
27 equilibration conditions for Opx-Cpx pairs (947–1015 C and 1.3–2 GPa), but in
28 granular samples, recrystallised olivine and spinel record T about 100 C higher. Two
29 distinct metasomatic processes, probably connected with the emplacement of the Afar plume, are proposed. The first one is a pervasive modal metasomatism produced by
2 water-rich fluids. The latter is a non-pervasive cryptic metasomatism, probably con-
3 nected to migration of melts. The comparison among the mantle beneath the Ethiopi-
4 an Volcanic Plateau, the southern Main Ethiopian Rift and the central Main Ethiopian
5 Rift suggests a spatial heterogeneity in mantle processes, during asthenospheric
6 upwelling.In press1–323.3. Geodinamica e struttura dell'interno della TerraJCR Journalreserve
