ALBERT

Description: Vesteris Seamount is a solitary alkaline volcano in the Greenland Basin some 280 km NW of Jan Mayen. Topographic and geophysical studies have shown no sign of an associated plume trace. Evidence from ash layers in sediment cores around the volcano and dating of dredged samples show that it has been active in Quaternary times. The lavas from Vesteris studied here consist of basanites, tephrites, mugearite, and alkali basalts. Crystal fractionation models are consistent with the generation of the tephrites and mugearite from a basanitic parent. Extensive kaersutite fractionation is required late in the fractionation sequence to produce the extreme mugearite composition. Na-Al-Fe-rich green cores to many clinopyroxene phenocrysts at Vesteris suggest a fractionation history beginning at high pressure in the mantle. Differences between Vesteris and Jan Mayen in the ratios of highly incompatible trace elements such as Ce/Pb and Rb/Cs, which will not normally be fractionated from one another during mantle melting, suggest that the two are not derived from the same source. Relatively unradiogenic Sr isotope ratios (compared with Bulk Earth), and highly incompatible trace element patterns similar to those for St. Helena, suggest that Vesteris magmas are derived from a depleted, asthenospheric source. We propose that the Vesteris basanites are very low degree partial melts (˜1%) of this source, most probably those which give rise to the seismic low-velocity zone (LVZ). Such small-degree melts may preferentially tap small-scale heterogeneities in the asthenosphere. Vesteris lies at the intersection of two major structural trends in the Greenland Basin—(1) a zone of major reorientation of spreading direction on the Mohns Ridge north of Jan Mayen and (2) the extension of the Kolbeinsey Ridge axis. We propose that a combination of the extensional stress fields related to these two lineaments produces sufficient dilation of the lithosphere at Vesteris to allow magmas from the LVZ to reach the surface.

Description: The basalt stratigraphy of the Deccan Trap between Mahabaleshwar Ghat and Belgaum over-steps the basement from north to south. Sr-isotope and Zr/Nb ratios, and Sr, Rb, and Ba concentrations correlate portions of the post-Poladpur stratigraphy over 250 km along the Western Ghats, thereby confirming a southerly component of dip of 0·06°. At the southwestern margin, the stratigraphy extends upwards from the compositionally uniform Ambenali Formation (Cox & Hawkesworth, 1984) into a sequence of grossly heterogeneous flow units which have been allocated to the Mahabaleshwar and Panhala Formations (Lightfoot & Hawkesworth, 1988). The Mahabaleshwar Formation is represented only by a sequence of highly fractionated flows (termed the Kolhapur unit) with similar 87Sr/86Sr0 to the Mahabaleshwar (0·7045), but with Sr〈240 ppm and TiO2〉2·25%. Succeeding the Kolhapur unit are a series of flows with high 87Sr/86Sr0 (0·7045-0·705), Zr/Nb 〉 13, and low Sr (〈 200 ppm), which have been allocated to the Panhala Formation, and a group of flows with high 87Sr/86Sr0 (0·707–0·708) and Sr (〉230), but trace element concentrations similar to the Mahabaleshwar Formation; these have been allocated to the Desur unit of the Panhala. Geochemical variations in flows overlying the Ambenali define two distinct trends: one is attributed to gabbro fractionation, and the other to variations in the compositions of the parental magmas, and arguably their source regions. There is little evidence for significant crustal contamination in these flows, and the degree of fractionation and the composition of the phase extract are shown to vary along strike within the Mahabaleshwar Formation. The high TiO2 content of Kolhapur unit flows is shown to be the result of shallow-level gabbro fractionation, rather than the presence of a primitive high-Ti magma. Mahabaleshwar Formation basalts exhibit a broad negative correlation between the degree of fractionation and Sr-isotopic composition. The endmember with lower 87Sr/86Sr0 has different Zr/Y from the Ambenali basalts, and would appear to have been generated by lower degrees of melting of a similar source. The other endmember has more radiogenic Sr, lower Zr/Nb, similar Zr/Y, but higher mg-number. The simplest interpretation is that these magmas were more primitive and hence hotter and more able to interact with the lithosphere en route to the surface, and that they then mixed to produce the Mahabaleshwar array. The Panhala Formation basalts plot on the Sr-Nd array defined by the Mahabaleshwar Formation, and the Desur unit basalts plot on an extension of this array; this suggests that the source characteristics are also lithospheric. The absolute elemental abundances may then be a function of melting and fractionation. We are impressed by the apparent switch from crustal lithospheric contributions to mantle lithospheric contributions through the stratigraphy, and suggest that this, together with the more protracted fractionation of the magma, reflects a change in the availability of the lithospheric components accompanying the southerly migration of the volcanic edifice.

Description: In many subaerial hydrothermal ore deposits arsenian pyrite is an important host for Au, however, arsenian pyrite is rare on the modern seafloor. During a recent survey for submarine hydrothermal mineralization in the western Woodlark Basin volcanic breccias containing abundant arsenian pyrite were dredged from the flanks of a volcanic seamount in a water depth of 2000 m. This area is particularly interesting because it is located at the transition from continental splitting to oceanic spreading where enhanced heat flow and deep crustal faults may fertilize mineralization. The sulfidic breccia is essentially monomictic and matrix-supported containing altered dacitic clasts. Mineralogical investigation of the breccia reveals silicification and sulfidation as the main alteration types. Quartz occurs in fragments and also constitutes the breccia matrix attesting to silicification as a significant alteration process. Pyrite is the dominant ore mineral with only minor amounts of Fe-oxyhydroxide and goethite. Bulk geochemistry shows a slight enrichment of Au (0.12 ppm) in association with elements such as As-Ag-Hg-Zn-Pb-Sb, key elements indicative of a low sulfidation environment. Three generations of pyrite are recognized on the basis of morphology. Arsenic-free, early framboidal pyrite (py1) is overgrown by arsenian colloform (py2) or massive pyrite (py3) containing up to 3.93 wt% As. Arsenic speciation in the pyrite is in the form of As1- and As3+. The presence of arsenian pyrite in hydrothermal breccias at this seamount indicates the potential for Au mineralization in the area.

Description: Phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the Mid-Atlantic Ridge (MAR) east of Ascension Island (7–11°S). The results indicate that mid-ocean ridge basalt (MORB) magmas beneath different segments of the MAR have crystallized over a wide range of pressures (100–900 MPa). However, each segment seems to have a specific crystallization history. Nearly isobaric crystallization conditions (100–300 MPa) were obtained for the geochemically enriched MORB magmas of the central segments, whereas normal (N)-MORB magmas of the bounding segments are characterized by polybaric crystallization conditions (200–900 MPa). In addition, our results demonstrate close to anhydrous crystallization conditions of N-MORBs, whereas geochemically enriched MORBs were successfully modeled in the presence of 0·4–1 wt% H2O in the parental melts. These estimates are in agreement with direct (Fourier transform IR) measurements of H2O abundances in basaltic glasses and melt inclusions for selected samples. Water contents determined in the parental melts are in the range 0·04–0·09 and 0·30–0·55 wt% H2O for depleted and enriched MORBs, respectively. Our results are in general agreement (within ±200 MPa) with previous approaches used to evaluate pressure estimates in MORB. However, the determination of pre-eruptive conditions of MORBs, including temperature and water content in addition to pressure, requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water.