Description: The Mg- and Si-rich nature of the sub-cratonic lithospheric mantle (SCLM) beneath the Kaapvaal Craton indicates extensive melt depletion, followed by a Si-enrichment process. Six highly silica enriched peridotites from Kimberley containing high amounts of orthopyroxene (Opx) or garnet (Grt) that are locally concentrated in clots, were investigated to constrain the timing and nature of the Si-enrichment process. A clinopyroxene-bearing lherzolite containing an Opx-clot was studied to quantify the effects of recent metasomatism on the Si-enriched samples. Minerals from the lherzolite, together with Opx from harzburgites and Opx- and Grt-clots have Hf–Nd isotope ratios at the time of kimberlite eruption, 90 Ma, comparable to group I kimberlites and are close to trace element equilibrium with kimberlitic melts. This implies the xenoliths underwent major interaction with kimberlitic melts close to the time of kimberlite eruption. Harzburgites and mineral clots record equilibration pressures and temperatures of, respectively, between 3.5–4.3 GPa and 930–1060 °C. The garnets in Opx-clots have low Lu/Hf and εHf(t) −15, whereas garnets from Grt-clots have high Lu/Hf and εHf(t) +10. In contrast, Grt from both Grt- and Opx-clots have low Sm/Nd and εNd −10. The whole rock platinum group element (PGE) concentrations are an order of magnitude higher in the Grt-clot than the Opx-clot. Measured 187Os/188Os range from 0.1085 to 0.1222. The Grt-clot bearing sample yields Nd–Hf–Os isotope model ages that suggest formation in the Neoproterozoic (∼650 Ma). In contrast, an Opx-clot yields TRD ages of 2.8 Ga, which is interpreted as the time of formation of the host harzburgite. The Opx-clots and host harzburgites have comparable Lu–Hf isotope systematics that imply Opx growth at ∼1.3 Ga and hence their formation is not related to the Grt-clots. Garnets from Opx- and Grt-clots have elevated high-field strength element (HFSE) concentrations, and lack HFSE depletion relative to other trace elements with comparable degrees of incompatibility in the mantle (La/Nb 〈 0.5). In addition, calculated melts in equilibrium with Grt have strongly fractionated REE (Nd/Yb 〉 300) and HREE depletion (YbN 〈 0.1) suggesting equilibration with a hydrous melt that is more HREE depleted than a kimberlitic melt. Previous models that related Si-enrichment to subduction are inconsistent with the lack of HFSE depletion (La/Nb 〈 0.5). Therefore the favoured model for Opx- and Grt-clot formation is infiltration of a hydrous melt in a within plate geodynamical environment associated with volcanism in the Mid-proterozoic and Neoproterozoic, respectively. This implies that Si-enrichment of the Kaapvaal SCLM may be a consequence of numerous localised magmatic events rather than a single craton-wide process.

Description: Highlights • The Red Sea Rift (RSR) comprises the typical terrain of slow MOR axes seen elsewhere. • Submarine salt glaciers occur extensively along the RSR and blanket parts of the RSR. • Inter-trough zones are not continental, but oceanic crust covered by evaporite flows. • We see a global mechanism for spreading initiation and no need for a “multi node” model. • We see prospects for large mineral deposits at passive margins that host evaporites. Abstract The transition from continental rifting to seafloor spreading is presently occurring at only a few places on Earth, such as the Red Sea or the Woodlark Basin. Competing theories for how spreading begins (either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete “nodes” separated by thinned continental lithosphere) have been put forward. The major evidence for the nodes theory comes from the Red Sea and geophysical surveys carried out there in the “multi-deeps region” during the 1970's and 1980's. We present new high-resolution multibeam bathymetric information over the same region, which, when combined with acoustic backscatter data, seafloor sampling and magmatic geochemical information appears to provide no support for the nodes model. We show that, although the discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent “windows” onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments and evaporites from the rift flanks. The geophysical data that was previously used to support the presence of continental crust between the “nodes” can be equally well explained by processes related to the sedimentary blanketing and sub-sedimentary hydrothermal alteration. A single, “quasi-instantaneous segment formation” model would appear to be all that is required to explain observations from present-day rifting/spreading transitions globally.

Description: There is a 2–3 order of magnitude difference between the chlorine concentrations of seawater and the magmas making up the oceanic crust. For this reason, chlorine is a key element for tracing present and past water–rock interactions. In order to trace processes such as assimilation of hydrothermally altered oceanic crust by mid-ocean ridge magmas, a high precision method with low detection limits is required, since conventional methods are not able to reveal variations in the intrinsically low chlorine content of these basalts. We present a new method to measure chlorine in basaltic glass by electron microprobe, using a combination of mapping and standard-sample bracketing techniques. Due to the high environmental background of chlorine, special emphasis is put on the influence of different preparation and cleaning procedures, as well as other possible effects on the measurements (e.g. sample size). Calibration of the method is performed by informational values of international standards and by minerals with a fixed chlorine content. Finally, new information on chlorine concentration values for commonly used standard reference materials (SRM) is provided. With this new method chlorine can be measured at very low detection limits (10’s of ppm’s) with a precision of 1–2 ppm standard deviation for low chlorine concentrations.

Description: Mt. Merapi in Central Java is one of the most active stratovolcanoes on Earth and is underlain by a multistage plumbing system. Crystal size distribution analyses (CSD) were carried out on recent Merapi basaltic-andesites and co-eruptive magmatic and plutonic inclusions to characterise the crystallisation processes that operate during storage and ascent and to obtain information on respective time scales. The basaltic-andesites exhibit log-linear, kinked-upwards CSD curves for plagioclase and clinopyroxene that can be separated into two main textural populations. Large plagioclase phenocrysts (≥1.6 mm) make up one population, but correspond to crystals with variable geochemical composition and reflect a period of crystal growth at deep to mid-crustal levels. This population was subsequently influenced by crystal accumulation and the onset of crustal assimilation, including the incorporation of high-Ca skarn-derived xenocrysts. Textural re-equilibration is required for these crystals to form a single population in CSD. A second episode of crystal growth at shallower levels is represented by chemically homogenous plagioclase crystals 〈1.6 mm in size. Crustal assimilation is indicated by, for example, oxygen isotopes and based on the CSD data, crystallisation combined with contamination is likely semi-continuous in these upper crustal storage chambers. The CSD data observed in the basaltic-andesite samples are remarkably consistent and require a large-volume steady state magmatic system beneath Merapi in which late textural equilibration plays a significant role. Plagioclase CSDs of co-eruptive magmatic and plutonic inclusions may contain a third crystal population (〈1 mm) not found in the lavas. This third population has probably formed from enhanced degassing of portions of basaltic-andesite magma at shallow crustal levels which resulted in increased crystallinity and basaltic-andesite mush inclusions. A suite of coarse plutonic inclusions is also present that reflects crystallisation and accumulation of crystals in the deep Merapi plumbing system, as deduced from CSD patterns and mineral assemblages.

Description: Recent basaltic-andesite lavas from Merapi volcano contain abundant and varied igneous inclusions suggesting a complex sub-volcanic magmatic system for Merapi volcano. In order to better understand the processes occurring beneath Merapi, we have studied this suite of inclusions by petrography, geochemistry and geobarometric calculations. The inclusions may be classified into four main suites: (1) highly crystalline basaltic-andesite inclusions, (2) co-magmatic enclaves, (3) plutonic crystalline inclusions and (4) amphibole megacrysts. Highly crystalline basaltic-andesite inclusions and co-magmatic enclaves typically display liquid–liquid relationships with their host rocks, indicating mixing and mingling of distinct magmas. Co-magmatic enclaves are basaltic in composition and occasionally display chilled margins, whereas highly crystalline basaltic-andesite inclusions usually lack chilling. Plutonic inclusions have variable grain sizes and occasionally possess crystal layering with a spectrum of compositions spanning from gabbro to diorite. Plagioclase, pyroxene and amphibole are the dominant phases present in both the inclusions and the host lavas. Mineral compositions of the inclusions largely overlap with compositions of minerals in recent and historic basaltic-andesites and the enclaves they contain, indicating a cognate or ‘antelithic’ nature for most of the plutonic inclusions. Many of the plutonic inclusions plot together with the host basaltic-andesites along fractional crystallisation trends from parental basalt to andesite compositions. Results for mineral geobarometry on the inclusions suggest a crystallisation history for the plutonic inclusions and the recent and historic Merapi magmas that spans the full depth of the crust, indicating a multi-chamber magma system with high amounts of semi-molten crystalline mush. There, crystallisation, crystal accumulation, magma mixing and mafic recharge take place. Comparison of the barometric results with whole rock Sr, Nd, and Pb isotope data for the inclusions suggests input of crustal material as magma ascends from depth, with a significant late addition of sedimentary material from the uppermost crust. The type of multi-chamber plumbing system envisaged contains large portions of crystal mush and provides ample opportunity to recycle the magmatic crystalline roots as well as interact with the surrounding host lithologies.