Cenozoic alkali basaltic magmas from Greene Point and Baker Rocks (Victoria Land, Antarctica) carry abundant ultramafic xenoliths of spinel harzburgite, spinel lherzolite and some composite xenoliths.
In many harzburgitic xenoliths whole rock and trace element patterns reveal the occurrence of cryptic metasomatic enrichments in incompatible elements; pyroxenite veins are also thought to be linked to metasomatic effects (Perinelli et al., 1998). Melt pods are widespread in these material and reveal complex relation with metasomatic events and in situ partial melting. Clinopyroxenes and glasses of these rocks have been analyzed by electron microprobe for major elements and by Secondary Ion Mass Spectrometry for a set of 21 trace elements; 40 trace elements on whole rocks were determined by Inductively Coupled Mass Spectrometry to try to define the nature of metasomatism and unravel the origin of glasses.
Glass compositions are silica rich (SiO2 59-70 wt%), exhibit high Mg# (0.53-0.69) and are Q-normative; in Greene point harzburgite glass pods are crystal free, while at Baker Rocks compositions of mineral coexisting with acidic glasses are in strong contrast with the host paragenesis: their olivine is Fo-rich (~0.92) while clinopyroxene has higher Al, Ca, Cr and Ti contents respect to the host rock. REE chondrite-normalized patterns of glasses in Greene Point harzburgite are characterized by LREE enrichment and show unfractionated HREE abundance, by contrast glasses in Baker Rocks harzburgite show weakly REE fractionated pattern. In coexisting clinopyroxene/glass pairs Nerst distribution coefficients were determined: at Greene Point Cpx/liqDHREE result to be ³ 1 suggesting a that unfractionated HREE of glass are linked to very low degree of partial melting of host lherzolite in accordance with experimental observation of Blundy et al. (1998). At Baker Rocks in situ partial melting is to be excluded on account of textural relations and different Cpx/liqDHREE calculated between glass and their clinopyroxene. On basis of glass and associated clinopyroxenes chemistry, the silicate melts of Baker Rocks are interpreted as product of metasomatic event in which carbonate rich melt reacted with xenolith mineralogy.
Cenozoic alkaline basaltic magmas of northern Victoria Land, Antarctica yield a wide variety of ultramafic xenoliths including clinopyroxene-rich xenoliths and augite and kaersutite megacrysts. Pyroxenites and megacrysts were collected in Mt Melbourne Volcanic Province, respectively at Browning Pass, Baker Rocks (both sites are near the coast of the Ross Sea) and at the base of Mt. Overlord, about 80 km inland. Sixteen samples have been petrographycally examined and chemically analyzed, including trace element analyses of minerals by ion microprobe. The dominant assemblage of pyroxenites consists of clinopyroxene + olivine ± spinel ± amphibole with strongly variable modal proportions of minerals. Rare interstitial plagioclase is found in Browning Pass samples. All pyroxenites belong to Al-augite Group (Frey and Prinz, 1978); the collection includes wehrlites, clinopyroxenites and rare dunites and olivinewebsterite. Moreover, Baker Rocks xenoliths are sometimes composite showing wehrlite and depleted lherzolite lithologies in sharp contact.
Their typical texture is cumulitic and in only few cases deformation fabrics and polygonal texture may be found. Mt Overlord samples show widespread metasomatic effects, evidenced by clinopyroxene-amphibole replacement and by variably re-crystallized melt pools. In the Browning Pass pyroxenites the metasomatic effects are incipient as evidenced by rare amphibole lamellae in substitution of the clinopyroxene. In few samples from Baker Rocks and Mt. Overlord clinopyroxenes show orthopyroxene exsolutions.
Major and trace element compositions of bulk rock and minerals indicate that pyroxenites and megacrysts are related to Cenozoic alkaline magmas and that formed through polybaric fractionation processes affecting McMurdo alkaline magmas.
We evaluated the P and T of segregation of clinopyroxenes from parent basaltic melts. The obtained data depict a P-T gradient of 15°C/kbar for the whole area in contrast with widespread volcanic activity. If we hypothesize that pyroxenite veins represent the walls of the paths followed by adiabatically rising basaltic melts, we can assess the temperature of the ambient mantle, if we know the mantle potential temperature (Tp) at some depth. From the onset of crystallization in primitive melts we get Tp =1300-1350°C at P= 2- 2.5GPa. In this way we obtain, for the present day geothermal gradient of the area, a more realistic value of 30°C/kbar. This value is considerably higher than the 8°C/kbar obtained from the equilibration coarse crystals of lherzolite nodules in the same area and allows to clearly identify the heating of the mantle due to Cenozoic magmatism.
The different thermal profile obtained from the pyroxenites with respect to those obtained by spinel-peridotite xenoliths, thus seems to be linked to the geodynamic evolution in this area. In particular the computed geothermal gradient fits with the heat-flow values of this area that range between 66 e 114 mW/m2 (Blackman et al., 1987; Della Vedova et al., 1992). The gradient that we propose seems to be compatible with a static rift geotherm of 90mW/m2 . See figure.
We discuss the concept of components in the Earth's mantle starting from a petrological and geochemical approach, but adopting a new method of projection of geochemical and isotopic data. This allows the compositional variability of magmatic associations to be evaluated in multi-dimensional space, thus simultaneously accounting for a large number of compositional variables. We demonstrate that ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are derived from a marble-cake mantle, in which different degrees of partial melting of recycled lithosphere, which are heterogeneous in age and composition, contribute to the magma genesis. This view is supported by the variability in the geochemical and isotopic signatures of OIB that are observed on the scale of a single ocean island as well as on that of an ocean, mostly varying between two extreme compositions, that are not strictly related to the commonly accepted mantle components (DMM, EMI, EMII, HIMU). Rather they are a distinctive feature of the mantle source sampled at each ocean island and are strongly dependent on the Pb isotope system. We recommend a change in perspective in studies of MORB–OIB geochemistry from one based on physically distinct mantle components to a model based on the existence of a marble-cake-like upper mantle. Although resembling the statistical upper mantle, this model implies that geochemical homogenization can be attained only within the limits of local mantle composition, so that a world-wide uniform depleted reservoir cannot be sampled by simply extending the volume of the region undergoing partial melting.
Abstract Mount Melbourne and Mount Rittmann are quiescent, although potentially explosive, alkaline volcanoes located 100 km apart in Northern Victoria Land quite close to three stations (Mario Zucchelli Station, Gondwana and Jang Bogo). The earliest investigations on Mount Melbourne started at the end of the 1960s; Mount Rittmann was discovered during the 1988–89 Italian campaign and knowledge of it is more limited due to the extensive ice cover. The first geophysical observations at Mount Melbourne were set up in 1988 by the Italian National Antarctic Research Programme (PNRA), which has recently funded new volcanological, geochemical and geophysical investigations on both volcanoes. Mount Melbourne and Mount Rittmann are active, and are characterized by fumaroles that are fed by volcanic fluid; their seismicity shows typical volcano signals, such as long-period events and tremor. Slow deformative phases have been recognized in the Mount Melbourne summit area. Future implementation of monitoring systems would help to improve our knowledge and enable near-real-time data to be acquired in order to track the evolution of these volcanoes. This would prove extremely useful in volcanic risk mitigation, considering that both Mount Melbourne and Mount Rittmann are potentially capable of producing major explosive activity with a possible risk to large and distant communities.
'/%3e%3cuse xlink:href='%23a' transform='rotate(72 0 0)'/%3e%3cuse xlink:href='%23a' transform='rotate(-72 0 0)'/%3e%3cuse xlink:href='%23a' transform='scale(-1 1) rotate(72)'/%3e%3c/g%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h1200v600H0z'/%3e%3cpath stroke='%23FFF' stroke-width='60' d='m0 0 600 300M0 300 600 0' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='40' d='m0 0 600 300M0 300 600 0' clip-path='url(%23c)'/%3e%3cpath stroke='%23FFF' stroke-width='100' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cpath stroke='%23C8102E' stroke-width='60' d='M300 0v300M0 150h600' clip-path='url(%23b)'/%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(45.4 0 0 45.4 900 120)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(30 0 0 30 900 120)'/%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 519.022 -457.666) scale(40.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 519.022 -457.666) scale(25)'/%3e%3c/g%3e%3cg transform='rotate(82 900 240)'%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='rotate(-82 668.57 -327.666) scale(45.4)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='rotate(-82 668.57 -327.666) scale(30)'/%3e%3c/g%3e%3cuse xlink:href='%23d' fill='%23FFF' transform='matrix(50.4 0 0 50.4 900 480)'/%3e%3cuse xlink:href='%23d' fill='%23C8102E' transform='matrix(35 0 0 35 900 480)'/%3e%3c/svg%3e)
