Dietrich Ackermand

THE FOUNDATION SEAMOUNT CHAIN : A FIRST SURVEY AND SAMPLING

The Foundation Seamounts form a 1400 km-long chain on the Pacific plate from 32 °S, 127 °W to the Pacific-Antarctic spreading axis at 38 °S, 111 °W. Previously only known from sparse single-beam echosoundings and satellite altimetry, we present here the first multibeam bathymetric survey and geological sampling results. We confirm that the submarine topography correlates with the altimetry, and that the chain is volcanic rather than tectonic or microcontinental in origin. The chain can be divided up morphologically and geochemically into three section: (1) west of 125 °W large flat-topped volcanoes composed of incompatible-element depleted lavas ( ≈ 1) of a near-ridge origin with little or no plume influence, (2) between 125 and 115 °W true intraplate volcanoes with incompatible element enrichment ( > 1.9) generated over the Foundation plume, (3) east of 115 °W E-W-trending volcanic ridges with compositions ( 2.0-0.3) suggestive of interaction between the plume and the Pacific-Antarctic spreading axis. On the spreading axis moderate incompatible element enrichments ( ≈0.8, cf. ≈ 0.3 outside the Foundation area) also suggest plume influence. It appears that the activity of the Foundation plume in the last few million years has (1) significantly waned and (2) become wholly channeled towards the spreading axis. The Foundation plume may be in the process of “dying”.

Magmatic sulfides and oxides in volcanic rocks from the Pitcairn hotspot (South Pacific)

SummaryThe Pitcairn hotspot, located about 60 km east of Pitcairn Island (South Pacific), consists of several active volcanoes < 500 m below sea level. The volcanic rocks from these seamounts are classified in four main rock-types: (1) picritic basalt containing Ti-bearing chromite (8–10 wt.% TiO2); (2) alkali basalt (Ti-bearing chromite with 4–6 wt.% TiO2); (3) trachyandesite containing titanomagnetite (18–22 wt.% TiO2); and sulfides, and (4) trachyte (titanomagnetite with 19–23 wt.% TiO2); The metallic oxides are zoned with decreasing Tîl02 contents from core to rim. Crystal fractionation (> 60%) is the main process responsible for differentiating these rock-types from an enriched source.Pyrrhotite and rare chalcopyrite grains in contact with pyrrhotite are observed only in the trachyandesite (3) in disseminated phenocryst clusters, usually in contact with large euhedral titanomagnetite phenocrysts. In addition, large euhedral pyrrhotite flakes, some with hexagonal habit, coat the walls of vesicles. All these pyrrhotite grains show a small range in Fe/S (0.90–0.99). The pyrrhotite in clusters precipitated earlier or simultaneously with titanomagnetite in a magmatic reservoir during crystal-liquid fractionation. Late precipitated vesicle pyrrhotite was formed by diffusion of Fe from the trachyandesitic liquid after the formation of the vesicles. Iron diffused from the glassy groundmass into the vesicle and reacted there with sulfur-bearing volatiles.ZusammenfassungDer Pitcairn Hotspot, ca. 60 km östlich von der Insel Pitcairn, besteht aus mehreren noch aktiven Vulkanen, die bis zu 500m unter dem Meeresspiegel aufragen. Die Hotspot Gesteinsproben können vier Vulkanittypen zugeordnet werden: (1) Pikritbasalt mit Ti-reichem Chromit (8–10 Gew.% TiO2); (2) Alkalibasalt (Ti-reicher Chromit, 4–6 Gew.% TiO2); (3) Trachyandesit mit Titanomagnetit (18–22 Gew.% TiO2); und Sulfiden sowie (4) Trachyt (Titanomagnetit, 19–23 Gew.% TiO2); Die Metalloxyde haben, verbunden mit abnehmendem TiO2-Gehalt, einen Zonarbau vom Kern zum Rand. Eine Kristallfraktionierung (< 60 %) ist Hauptursache für die Differenzierung der vier Vulkanittypen aus einer angereicherten Magmenquelle.Pyrrhotit und sehr wening Chalkopyrit als Kontaktphase zum Pyrrhotit sind nur im Trachyandesit (3) in Clustern mit idiomorphen Kristalleinsprenglingen im Kontakt mit Titanomagnetit gefunden worden. Weiterhin bedecken große idiomorphe Pyrrhotit plättchen, davon einige mit hexagonalem Habitus, die Wände der Gasblasen. Die Variationsbreite des Fe/S aller Pyrrhotite ist mit 0,90-0,99 gering. Die Pyrrhotite in den Clustern sind früher als oder gleichzeitig mit Titanomagnetit im Magmenreservoir während der Kristall-Schmelze Fraktionierung auskristallisiert. Die spät gebildeten Pyrrhotite in den Gasblasen sind durch einen Diffusionsprozeß von Fe aus der trachyandesitischen Schmelze entstanden. Eisen diffundierte aus der glasigen Grundmasse in die Hohlräume und reagierte dort mit Schwefel, der als volatiler Bestandteil vorlag.

Petrology, chemistry and phase relations of borosilicate phases in phlogopite diopsidites and granitic pegmatites from the Tranomaro belt, SE Madagascar; boron-fluid evolution

Abstract The boron-bearing minerals grandidierite, werdingite, serendibite and sinhalite are common in high-grade rocks of the Tranomaro belt in southeastern Madagascar. The mutual occurrence of these phases allows a new understanding of the role of boron-rich fluids in the crustal evolution of Gondwana, and we provide critical borosilicate data to constrain that development. We distinguish two types of grandidierite depending on their B2O3 and Al2O3 contents and on their relations with associated borosilicate phases. (1) At Vohibola the presence of sinhalite and serendibite associated with phlogopite lenses in metasedimentary diopsidites indicates an evaporitic origin from calc-silicate sediments. (2) At Cape Andrahomana borosilicates are associated with pegmatites and granites that were emplaced along shear zones on the boundary of the Tranomaro belt. The shear zones acted as conduits for boron-bearing fluids and for granitic partial melts, which had derived their boron from calc-silicate sedimentary protoliths. Using geothermometry and geobarometry of minerals from associated rocks, we calculate that ambient pressures and temperatures changed in time from 7.5 to 4.0 kbar and from c. 800 °C to 700 °C. Our results confirm the important role of shear zones in channelling the fluid flow of boron-bearing fluids that were derived from crustal melt granites in the same shear zones, but that ultimately derived their boron from early metasediments. We provide new information on the mineralogy, phase assemblages and paragenetic history of multiple borosilicates.