Geology, Mineralogy and Geochemistry of the Late Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica

<p>Rocks of the McMurdo Volcanic Group occur as stratovolcanoes, shield volcanoes, scoria cones, plugs, flows and volcanic piles up to 4000 m high along the Ross Sea margin of the Transantarctic Mountains and make up the Balleny Islands 300 km north of the Antarctic continental margin. The rocks are predominantly undersaturated and range from alkali basalt and basanite to trachyte and phonolite. Four volcanic provinces are recognised; Balleny, Hallett, Melbourne and Erebus. The Balleny volcanic province is situated along a transform fault in the South Pacific Ocean. The rocks are predominantly basanite. Hallett volcanic province occurs along the coast of northern Victoria Land as four elongate piles formed extensive of hyaloclastites, tuffs, breccias and capped by subaerial eruptive products. The lavas are a basanite/alkali basalt-trachyte-quartz trachyte association, and were extruded over the last 7 m.y. Melbourne volcanic province stretches across the Transantarctic Mountains in northern Victoria Land and ranges in age from 0 to 7 m.y. A Central Suite of intermediate and trachytic lavas form stratovolcanoes, cones and plugs, while many small basanite outcrops constitute a Local Suite. Three lava lineages, resulting from differentiation, are recognised. 1) Lavas at The Pleiades and Mt Overlord consist of a mildly potassic trachyandesite-tristanite-K-trachyte-peralkaline K-trachyte lineage. Major, trace and rare earth element (REE) data suggest evolution by fractional crystallization of olivine, clinopyroxene, magnetite, apatite and feldspar. 2) A basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite lineage, found at The Pleiades is believed to result from fractional crystallization of olivine, clinopyroxene, kaersutite, magnetite, apatite and feldspar. 3) An oversaturated (Q = 0 to 18%) strongly potassic quartz trachyandesite-quartz tristanite-quartz trachyte lineage occurs at only Mt Melbourne. The Erebus volcanic province covers all McMurdo Volcanic Group rocks in south Victoria Land. Mt Erebus itself is still active, but the province includes rocks as old as 15 m.y. Two lava lineages very similar chemically are recognised: 1) The Erebus lineage consists of strongly porphyritic nepheline hawaiite-nepheline benmoreite-anorthoclase phonolite. Phenocrysts of feldspar, clinopyroxene, olivine, magnetite and apatite are characteristic. The chemistry of the lineage is compatible with fractional crystallization of the phenocryst phases. 2) A kaersutite lineage consists of basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite-kaersutite phonolite-pyroxene phonolite. Clinopyroxene (Wo44-48 En40-48 Fs7-14) is ubiquitous, kaersutite is common in all intermediate lavas and primary olivine (Fa12 to Fa26) is confined to the basanites. Major element mass balance models for lavas from Hut Point Peninsula suggest formation by fractional crystallization of olivine, clinopyroxene, spinel (includes magnetite and ilmenite), kaersutite, feldspar and apatite. Middle REE show a marked depletion consistent with kaersutite fractionation. REE abundances were evaluated using the mass balance models and published partition coefficients. Calculated REE abundances show excellent agreement with the measured values. Abundances of "incompatible" elements Pb, Rb, Cs, Th and U are not consistent with the models and "volatile enrichment" processes are invoked to explain their abundances. Intermediate lavas of the kaersutite lineage are rare in the Erebus volcanic province, occurring only at Hut Point Peninsula and Brown Peninsula. At other areas basanite and phonolite lavas predominate. However these are considered to form by fractional crystallization processes similar to Hut Point Peninsula lavas. Erebus lineage lavas differentiated at higher temperatures and, lower PH2O than those of the kaersutite lineage, which characterize the periphery of Ross Island. REE abundances and comparison with experimental melting studies indicate DVDP basanite originated by a low degree of partial melting (1-5%) of a hydrous garnet peridotite mantle at pressures of 25-30 kbars. These data suggest that Ross Island is the site of a mantle plume with a diameter of, about 100 km and centred on Mt Erebus.</p>

Geology, Mineralogy and Geochemistry of the Late Cenozoic McMurdo Volcanic Group, Victoria Land, Antarctica

<p>Rocks of the McMurdo Volcanic Group occur as stratovolcanoes, shield volcanoes, scoria cones, plugs, flows and volcanic piles up to 4000 m high along the Ross Sea margin of the Transantarctic Mountains and make up the Balleny Islands 300 km north of the Antarctic continental margin. The rocks are predominantly undersaturated and range from alkali basalt and basanite to trachyte and phonolite. Four volcanic provinces are recognised; Balleny, Hallett, Melbourne and Erebus. The Balleny volcanic province is situated along a transform fault in the South Pacific Ocean. The rocks are predominantly basanite. Hallett volcanic province occurs along the coast of northern Victoria Land as four elongate piles formed extensive of hyaloclastites, tuffs, breccias and capped by subaerial eruptive products. The lavas are a basanite/alkali basalt-trachyte-quartz trachyte association, and were extruded over the last 7 m.y. Melbourne volcanic province stretches across the Transantarctic Mountains in northern Victoria Land and ranges in age from 0 to 7 m.y. A Central Suite of intermediate and trachytic lavas form stratovolcanoes, cones and plugs, while many small basanite outcrops constitute a Local Suite. Three lava lineages, resulting from differentiation, are recognised. 1) Lavas at The Pleiades and Mt Overlord consist of a mildly potassic trachyandesite-tristanite-K-trachyte-peralkaline K-trachyte lineage. Major, trace and rare earth element (REE) data suggest evolution by fractional crystallization of olivine, clinopyroxene, magnetite, apatite and feldspar. 2) A basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite lineage, found at The Pleiades is believed to result from fractional crystallization of olivine, clinopyroxene, kaersutite, magnetite, apatite and feldspar. 3) An oversaturated (Q = 0 to 18%) strongly potassic quartz trachyandesite-quartz tristanite-quartz trachyte lineage occurs at only Mt Melbourne. The Erebus volcanic province covers all McMurdo Volcanic Group rocks in south Victoria Land. Mt Erebus itself is still active, but the province includes rocks as old as 15 m.y. Two lava lineages very similar chemically are recognised: 1) The Erebus lineage consists of strongly porphyritic nepheline hawaiite-nepheline benmoreite-anorthoclase phonolite. Phenocrysts of feldspar, clinopyroxene, olivine, magnetite and apatite are characteristic. The chemistry of the lineage is compatible with fractional crystallization of the phenocryst phases. 2) A kaersutite lineage consists of basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite-kaersutite phonolite-pyroxene phonolite. Clinopyroxene (Wo44-48 En40-48 Fs7-14) is ubiquitous, kaersutite is common in all intermediate lavas and primary olivine (Fa12 to Fa26) is confined to the basanites. Major element mass balance models for lavas from Hut Point Peninsula suggest formation by fractional crystallization of olivine, clinopyroxene, spinel (includes magnetite and ilmenite), kaersutite, feldspar and apatite. Middle REE show a marked depletion consistent with kaersutite fractionation. REE abundances were evaluated using the mass balance models and published partition coefficients. Calculated REE abundances show excellent agreement with the measured values. Abundances of "incompatible" elements Pb, Rb, Cs, Th and U are not consistent with the models and "volatile enrichment" processes are invoked to explain their abundances. Intermediate lavas of the kaersutite lineage are rare in the Erebus volcanic province, occurring only at Hut Point Peninsula and Brown Peninsula. At other areas basanite and phonolite lavas predominate. However these are considered to form by fractional crystallization processes similar to Hut Point Peninsula lavas. Erebus lineage lavas differentiated at higher temperatures and, lower PH2O than those of the kaersutite lineage, which characterize the periphery of Ross Island. REE abundances and comparison with experimental melting studies indicate DVDP basanite originated by a low degree of partial melting (1-5%) of a hydrous garnet peridotite mantle at pressures of 25-30 kbars. These data suggest that Ross Island is the site of a mantle plume with a diameter of, about 100 km and centred on Mt Erebus.</p>

Providencia Island: A Miocene Stratovolcano on the Lower Nicaraguan Rise, Western Caribbean—A Geological Enigma Resolved

ABSTRACT The Providencia island group comprises an extinct Miocene stratovolcano located on a shallow submarine bank astride the Lower Nicaraguan Rise in the western Caribbean. We report here on the geology, geochemistry, petrology, and isotopic ages of the rocks within the Providencia island group, using newly collected as well as previously published results to unravel the complex history of Providencia. The volcano is made up of eight stratigraphic units, including three major units: (1) the Mafic unit, (2) the Breccia unit, (3) the Felsic unit, and five minor units: (4) the Trachyandesite unit, (5) the Conglomerate unit, (6) the Pumice unit, (7) the Intrusive unit, and (8) the Limestone unit. The Mafic unit is the oldest and forms the foundation of the island, consisting of both subaerial and subaqueous lava flows and pyroclastic deposits of alkali basalt and trachybasalt. Overlying the Mafic unit, there is a thin, minor unit of trachyandesite lava flows (Trachyandesite unit). The Breccia unit unconformably overlies the older rocks and consists of crudely stratified breccias block flows/block-and-ash flows) of vitrophyric dacite, which represent subaerial near-vent facies formed by gravitational and/or explosive dome collapse. The breccias commonly contain clasts of alkali basalt, indicating the nature of the underlying substrate. The Felsic unit comprises the central part of the island, composed of rhyolite lava flows and domes, separated from the rocks of the Breccia unit by a flat-lying unconformity. Following a quiescent period, limited felsic pyroclastic activity produced minor valley-fill ignimbrites (Pumice unit). The rocks of Providencia can be geochemically and stratigraphically subdivided into an older alkaline suite of alkali basalts, trachybasalts, and trachyandesites, and a younger subalkaline suite composed dominantly of dacites and rhyolites. Isotopically, the alkali basalts together with the proposed tholeiitic parent magmas for the dacites and rhyolites indicate an origin by varying degrees of partial melting of a metasomatized ocean-island basalt–type mantle that had been modified by interaction with the Galapagos plume. The dacites are the only phenocryst-rich rocks on the island and have a very small compositional range. We infer that they formed by the mixing of basalt and rhyolite magmas in a lower oceanic crustal “hot zone.” The rhyolites of the Felsic unit, as well as the rhyolitic magmas contributing to dacite formation, are interpreted as being the products of partial melting of the thickened lower oceanic crust beneath Providencia. U-Pb dating of zircons in the Providencia volcanic rocks has yielded Oligocene and Miocene ages, corresponding to the ages of the volcanism. In addition, some zircon crystals in the same rocks have yielded both Proterozoic and Paleozoic ages ranging between 1661 and 454 Ma. The lack of any evidence of continental crust beneath Providencia suggests that these old zircons are xenocrysts from the upper mantle beneath the Lower Nicaraguan Rise. A comparison of the volcanic rocks from Providencia with similar rocks that comprise the Western Caribbean alkaline province indicates that while the Providencia alkaline suite is similar to other alkaline suites previously defined within this province, the Providencia subalkaline suite is unique, having no equivalent rocks within the Western Caribbean alkaline province.

The role of lithospheric thickness in the formation of ocean islands and seamounts: contrasts between the Louisville and Emperor-Hawaiian hotspot trails

The Hawaii-Emperor and Louisville seamounts form the two most prominent time-progressive hotspot trails on Earth. Both formed over a similar time interval on lithosphere with a similar range of ages and thickness. The Hawaii-Emperor seamounts are large and magma productivity appears to be increasing at present. The Louisville seamounts, by contrast, are smaller and the trail appears to be waning. We present new major- and trace element data from five of the older (74–50 Ma) Louisville seamounts drilled during International Ocean Drilling Program (IODP) Expedition 330 and compare these to published data from the Emperor seamounts of the same age. Despite drilling deep into the shield-forming volcanic rocks at three of the Louisville seamounts, our data confirm the results of earlier studies based on dredge samples that the Louisville seamounts are composed of remarkably uniform alkali basalt. The basalt composition can be modelled by ∼1.5–3% partial melting of a dominantly garnet lherzolite mantle with a composition similar to that of the Ontong Java Plateau mantle source. Rock samples recovered by dredging and drilling on the Emperor Seamounts range in composition from tholeiitic to alkali basalt and require larger degrees of melting (2–10%) and spinel- to garnet lherzolite mantle sources. We use a simple decompression melting model to show that melting of mantle with a potential temperature of 1500ºC under lithosphere of varying thickness can account for the composition of the shield-forming tholeiitic basalts from the Emperor seamounts, while post-shield alkali basalt requires a lower temperature (1300–1400ºC). This is consistent with the derivation of Hawaii-Emperor shield-forming magmas from the hotter axis of a mantle plume and the post-shield magmas from the cooler plume sheath as the seamount drifts away from the plume axis. The composition of basalt from the Louisville seamounts shows no significant variation with lithosphere thickness at the time of seamount formation, contrary to the predictions of our decompression melting model. This lack of influence of lithospheric thickness is characteristic of basalt from most ocean islands. The problem can be resolved if the Louisville seamounts were formed by dehydration melting of mantle containing a small amount of water in a cooler plume. Hydrous melting in a relatively cool mantle plume (Tp = 1350–1400 °C) could produce a small amount of melt and then be inhibited by increasing viscosity from reaching the dry mantle solidus and melting further. The failure of the plume to reach the dry mantle solidus or the base of the lithosphere means that the resulting magmas would have the same composition irrespective of lithosphere thickness. A hotter mantle plume (Tp ≈ 1500 °C) beneath the Emperor seamounts and the Hawaiian Islands would have lower viscosity before the onset of melting, melt to a larger extent, and decompress to the base of the lithosphere. Thus our decompression melting model could potentially explain the composition of both the Emperor and Louisville seamounts. The absence of a significant lithospheric control on the composition of basalt from nearly all ocean islands suggests that dehydration melting is the rule and the Hawaiian islands the exception. Alternatively, many ocean islands may not be the product of mantle plumes but instead be formed by decompression melting of heterogeneous mantle sources composed of peridotite containing discrete bodies of carbonated and silica-oversaturated eclogite within the general upper mantle convective flow.

Alkali basalt from the Seifu Seamount in the Sea of Japan: post-spreading magmatism in a back-arc setting

We present geochemical and 40Ar∕39Ar age data for a peridotite xenolith-bearing basalt dredged from the Seifu Seamount (SSM basalt) in the northeast Tsushima Basin, southwest Sea of Japan. An 40Ar∕39Ar plateau age of 8.33±0.15 Ma (2σ) was obtained for the SSM basalt, indicating that it erupted shortly after the termination of back-arc spreading in the Sea of Japan. The SSM basalt is a high-K to shoshonitic alkali basalt that is characterized by light rare earth element enrichment. The trace element features of the basalt are similar to those of ocean island basalt, although the Yb content is much higher, indicating formation by the low-degree partial melting of spinel peridotite. The Nd, Sr, and Pb isotopic compositions of the SSM basalt differ from those of back-arc basin basalts in the Sea of Japan. The Sr–Nd isotopic composition of the SSM basalt suggests its source was depleted mid-ocean ridge mantle containing an enriched mantle (EM1) component. The SSM basalt was formed in a post-back-arc extension setting by the low-degree partial melting of an upwelling asthenosphere that had previously been associated with the main phase of back-arc magmatism.

Synthesis of X-Zeolite from Waste Basalt Powder and its Influencing Factors and Synthesis Mechanism

Traditional hydrothermal method (TH) and alkali fusion-assisted hydrothermal method (AFH) were evaluated for the preparation of zeolites from waste basalt powder by using NaOH as the activation reagent in this study. The synthesized products were characterized by BET, XRD, FTIR and SEM. The effects of acid treatment, alkali/basalt ratio, calcination temperature and crystallization temperature on the synthesis process were studied. The results showed that AFH successfully synthesized zeolite X with higher crystallinity and no zeolite was formed by TH. The specific surface area of synthetic zeolite X was 486.46 m2·g−1, which was much larger than that of original basalt powder (12.12 m2·g−1). Acid treatment and calcination temperature had no effect on zeolite types, but acid treatment improved the yield and quality of zeolite. Alkali/basalt ratio and crystallization temperature not only affected the crystallinity of synthesized zeolites but also affected its type. The optimum synthesis condition of zeolite X are as follows: acid treatment of 5 wt% HCl solution, NaOH/basalt ratio of 1:1, a calcination temperature of 650 °C and crystallization temperature of 120 °C. The work shows that basalt can be used as a raw material to prepare zeolite.

Evidence for a Carbonatite-Influenced Source Assemblage for Intraplate Basalts from the Buckland Volcanic Province, Queensland, Australia

Eastern Australia contains a widespread suite of primitive (MgO ≥ 7.5 wt.%) intraplate basaltic provinces, including those sited along the longest continental hotspot track on Earth (≈2000 km), the Cosgrove track. The Buckland volcanic province is the most southerly basaltic province on the Cosgrove track before a >1600 km stretch that contains only sparse leucitite volcanism. Buckland is also situated just northeast of the edge of thick cratonic lithosphere where it transitions to a thinner continental lithosphere (<110 km) to the east, which may influence the production of plume-derived melts. Here, analysis of minor and trace elements in olivines in alkali basalts and basanites from the Buckland Province are combined with whole-rock compositions to elucidate the mantle source assemblages, and to calibrate minor and trace element indicators in olivine for application to source mineralogy. Olivine xenocrysts show element concentration ranges typical for peridotites; Mn and Al concentrations indicate that the ambient mantle is spinel, rather than garnet, peridotite. High modal pyroxene content is indicated by high Ni, Zn/Fe, and Fe/Mn in olivines, while high Ti/Sc is consistent with amphibole in the source. Residual phlogopite in the source of the basanites is indicated by low K/Nb in whole rocks, while apatite contains high P2O5 and low Rb/Sr (≥0.015) and Sr/La (≥13). The basanite source assemblage probably contains apatite, phlogopite, olivine, clinopyroxene and orthopyroxene, whereas the alkali basalt source assemblage is probably amphibole, olivine, orthopyroxene and clinopyroxene ± phlogopite ± apatite. Both source assemblages correspond broadly to olivine websterite, with the basanite source lying deeper than that for alkali basalt, explaining the occurrence of phlogopite in the source. This mineralogy, along with whole-rock Ti/Eu, Zr/Hf and P2O5/TiO2 values approaching those of natural carbonatites, provide evidence showing that the Buckland source consists of a peridotite that has interacted with a carbonate-rich melt whose origin may be in the deep lithosphere or asthenosphere beneath the craton. Similar enrichment processes are probably common throughout eastern Australia, controlling trace element characteristics in basaltic provinces. The topography of the underside of the lithosphere may play a significant role in determining mantle source assemblages by diverting and concentrating melt flow, and thus influence the location of basaltic provinces.