The Maar-Diatreme System in a Mixed "Hard/Soft-Rock" Setting: an Example from the Pali Aike Volcanic Field, Argentina
The eruptive processes in diatremes remain poorly understood compared to those at other volcano types, because these processes occur at depth. Except for maar-diatreme volcanoes formed during kimberlitic eruptions, volcanologists agree that these systems are of phreatomagmatic origin. The origin of kimberlitic diatremes is more contentious, but studying non kimberlitic equivalents can be a good approach to better understand kimberlitic diatremes considering their numerous common characteristics. The geometry of maar-diatreme systems is strongly influenced by their setting in "hard-rock" or "soft-rock" environments (Lorenz, 2003, Geolines 15:72-83). Formation of maar-diatreme systems in "hard-rock" environments, like in the West Eifel Volcanic Field of Germany, is largely described in the literature but emplacement in "soft-rock" environments or mixed settings is not. In the case of "hard-rock" environments external water is provided by fracture aquifers. The eruption products are juvenile clasts and country rock fragments. The inner crater walls of the maar, and the diatreme walls, have steep slopes. In the case of "soft- rock" environments, water is contained in the sediment pores and the walls tend to be at lower angles. We recently conducted field work on maars, cinder cones and spatter rings of the Pali Aike Volcanic Field of southern Argentina as part of the Potrok Aike Maar Lake Sediment Archive Drilling Project (PASADO). These Quaternary monogenetic volcanoes were emplaced in a mixed "hard/soft-rock" environment containing young glacial sediments, basaltic lava flows, partly consolidated fluviatile sediments, and older indurated sedimentary rocks. The mixed environment of emplacement is reflected in a phreatomagmatic deposit on the inner slope of a tephra ring exposing some lapilli-tuff layers. The lapilli fraction comprises approximately 40% lithics on average (visual estimate): at least half of the fraction is composed of basaltic lava derived from a pre- existing lava flow. The rest of the fraction comprises (1) rounded pebbles likely derived from glacial deposits but also (2) angular, well-indurated sedimentary rocks (quartzite, beige siltstone, red shale and pink to orange sandstone) presumably derived from relatively deep formations of the Austral sedimentary basin.
Late Jurassic ultramaphic lamprophyres with kimberlitic affinity in the allochthonous Batain nappes of Eastern Oman
Carbonatite, alkaline volcanic rocks and ultramafic lamprophyres with kimberlitic affinities have been recently discovered in the allochthonous Batain nappes of Eastern Oman. The main bodies of the ultramafic lamprophyres occur in a diatreme at the coast of the Asseelah village, northeastern Oman. The second major outcrops of ultramafic lamprophyres occur as several 6 km long dykes at the Bomethra area. The diatreme consists of heterogeneous deposits dominated by 'diatreme facies' volcaniclastic rocks. These include accretionary and armoured carbonate lapilli, and carbonate-dominated tuffs, all of which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation. The Asseelah ultramafic rocks may be classified as either aillikite and/or carbonatite with kimberlitic affinity. Garnet (G0), chromite, phlogopite, ilmenite, zircon, apatite, rutile, corundum, and sillimanite have been recovered from heavy mineral concentrates. Zircon grains extracted from the diatreme rocks have a mean age of 137 + 1 Ma (95 % confidence, MSWD = 0.49). The trace element patterns of the zircon gains are typical of kimberlitic to carbonatitic rocks and their Hf isotope ratio (176Hf/177Hf = 0.28286 + 1, å Hf = 6.2) is typical of kimberlitic zircons of early Cretaceous. The lamprophyric dyke swarms of the Bomethra area comprise macrocrystic, spinel and phlogopite bearing hybabyssall facies calcite aillikites/damtjernites with pelletal lapilli and globular segregationary textures. The main dyke extends in length up to 6 km and ranges in width between 1 and 30 meters with two main blows 300-500 m in width. The petrography, mineralogy, trace element and isotopic composition of the dyke rocks are comparable to aillikites and damtjernites with kimberlitic affinity. Kimberlite indicator minerals include chromite, Cr-diopside, G4 garnet, and picroilmenite. The Asseelah and the Bomethra ultramafic rocks are enriched in light REE and have a high modal proportion of Ti-Al rich phlogopite, suggesting that they were derived from a source region which has experienced melt-depletion followed by metasomatic enrichment. This enrichment of the source region could be a consequence of the upward percolation of an alkaline melts that penetrated the base of the subcontinental lithosphere during the break-up of Gondwanaland. There is no obvious age difference (137 Ma; U-Pb zircon dating of Asseelah rocks and 150- 162 Ma; Ar-Ar age dating of Bomethra rocks) between these various rocks, so the initial magmas were formed around the same time. These ages correlate with large-scale tectonic events recorded in the early Indian Ocean at 140-160 Ma. The magmatism is probably a distal effect of the breakup of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean. The magmas in the Batain area are petrogenetically related and appear to have originated in a single event, possibly triggered by the arrival of the hot material beneath the Batain lithosphere that had been recently metasomatised and is related spatially and compositionally to mantle upwelling associated with the rifting.
Quantitative Analysis of Trace Element Impurity Levels in Some Gem-Quality Diamonds
Perhaps the most important information required to understand the origin of diamonds is the nature of the fluid that they crystallise from. Constraining the identity of the diamond-forming fluid for high purity gem diamonds is hampered by analytical challenges because of the very low analyte levels involved. Here we use a new ultra- low blank 'off-line' laser ablation method coupled to sector-field ICPMS for the quantitative analysis of fluid-poor gem diamonds. Ten diamonds comprised of both E- and P-type parageneses, from the Premier Mine, South Africa, were analysed for trace element abundances. We assume that the elemental signatures arise from low densities of sub-microscopic fluid inclusions that are analogous to the much higher densities of fluid inclusions commonly found within fluid-rich diamonds exhibiting fibrous growth. Repeatability of multiple (>20) blanks yielded consistently low values so that using the current procedure our limits of quantitation (10-ƒã blank) are <1pg for most trace elements, except for Sr, Zr, Ba, from 2-9pg and Pb ~30pg. Trace element patterns of the Premier diamond suite show enrichment of LREE over HREE. Abundances broadly decrease with increasing elemental compatibility. As a suite the chondrite normalised diamond patterns show negative Sr, Zr, Ti and Y anomalies and positive U, and Pb anomalies. All sample abundances are very depleted relative to chondrites (0.1 to 0.001X ch). HREE range from 0.1 to 1ppb as do Y, Nb, Cs. Other lighter elements vary from 2-30ppb. Pb reaches several ppb and Ti ranges from ppb values up to 2ppm. No significant difference were observed between the trace element systematics of the eclogitic and peridotitic diamonds. Overall, these initial data have inter-element fractionation patterns similar to those evident from fluid-rich fibrous diamonds and can be sued to infer that both types of diamond-forming fluids share a common origin.