Michael C. Rowe

Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system

Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These fore-arc basalts (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.

Origin of cross-chain geochemical variation in Quaternary lavas from the northern Izu arc: Using a quantitative mass balance approach to identify mantle sources and mantle wedge processes

[1]xa0We present major, trace element, and Pb-Sr-Nd-Hf isotope data for Quaternary basalt and basaltic andesite lavas from cross-chain volcanoes in the northern Izu (N-Izu) arc. Lavas from Izu-Oshima, Toshima, Udonejima, and Niijima islands show consistent chemical changes with depth to the Wadati-Benioff zone, from 120 km beneath Izu-Oshima to 180 km beneath Niijima. Lavas from Izu-Oshima at the volcanic front (VF) have elevated concentrations of large ion lithophile elements (LILEs), whereas rear-arc (RA) lavas are rich in light rare earth elements (LREEs) and high field strength elements (HFSEs). VF lavas also have more radiogenic Pb, Nd, Sr, and Hf isotopic compositions. We have used the Arc Basalt Simulator version 3 (ABS3) to examine the mass balance of slab dehydration and melting and slab fluid/melt-fluxed mantle melting and to quantitatively evaluate magma genesis beneath N-Izu. The results suggest that the slab-derived fluids/melts are derived from ∼20% sediment and ∼80% altered oceanic crust, the slab fluid is generated by slab dehydration for the VF magmas at 3.3–3.5 GPa/660°C–700°C, and slab melt for RA magmas is supplied at 3.4–4.4 GPa/830°C–890°C. The degree of fluxed melting of the mantle wedge varies between 17% and 28% (VF) and 6% and 22% (RA), with a slab flux fraction of 2%–4.5% (VF fluid) to 1%–1.5% (RA melt), and at melting depths 1–2.5 GPa (VF) and 2.4–2.8 GPa (RA). These conditions are consistent with a model whereby shallow, relatively low temperature slab fluids contribute to VF basalt genesis, whereas deeper and hotter slab melts control formation of RA basalts. The low-temperature slab dehydration is the cause of elevated Ba/Th in VF basalt due mainly to breakdown of lawsonite, whereas deeper breakdown of phengite by slab melting is the cause of elevated K and Rb in RA basalts. Melting in the garnet stability field, and at lower degrees of partial melting, is required for the elevated LILEs, LREEs, and HFSEs observed in the RA basalts. Less radiogenic Sr, Nd, Hf, and Pb in RA basalts are all attributable to lesser slab flux additions. The low H2O predicted for RA basalt magmas (<1.5 wt %) relative to that in VF basalt magmas (5–8 wt %) is also due to melt addition rather than fluid. All these conclusions are broadly consistent with existing models; however, in this study they are quantitatively confirmed by the geochemical mass balance deduced from petrological ABS3 model. Overall, the P-T-X(H2O) structure of the slab and the mantle wedge exert the primary controls on arc basalt genesis.

Quantifying crystallization and devitrification of rhyolites by means of X-ray diffraction and electron microprobe analysis

Abstract Devitrification of silicic volcanic rocks is a relatively common process, resulting in the production of microcrystalline silica and feldspar components. Here we investigate how the products of pervasive devitrification may be characterized using the combined techniques of X-ray powder diffraction, electron microprobe analysis, and X-ray fluorescence analysis to provide a new calibrated approach to calculating the crystallinity and mineral modes in both glassy vitrophyre and devitrified volcanics. Using the integrated areas of the X-ray diffraction peaks associated with both the crystalline and amorphous components, the relative proportions of groundmass crystallites and amorphous material from both glassy and devitrified material can be calculated. A detailed calibration indicates a linear relationship among the ratio of the integrated counts and bulk crystallinity. Mineral proportions are also calculated from X-ray fluorescence measurements of whole-rock and groundmass separates and are well correlated to crystallinities calculated from both X-ray diffraction and electron microprobe image analysis for vitrophyre samples. Devitrification products in a pervasively devitrified sample are tridymite, quartz, sanidine, and a Ca-rich aluminosilicate component. Mineral analysis and X-ray mapping by electron microprobe analysis indicates that the Ca-rich aluminosilicate component appears to be the dominant metastable or amorphous phase in the devitrified sample with proportions calculated from X-ray mapping (~32%) in reasonable agreement with the calculated proportion of amorphous material determined by means of X-ray diffraction (~38%). These results demonstrate the robustness of this combined X-ray diffraction and electron microprobe imagery technique for quantifying and characterizing crystallization in complex samples.

Magmatic–hydrothermal fluids and volatile metals in the Spirit Lake pluton and Margaret Cu–Mo porphyry system, SW Washington, USA

The halogen-bearing minerals tourmaline, amphibole, and biotite formed during magmatic–hydrothermal processes associated with the late-stage cooling of the Spirit Lake granitoid pluton (Mt. St. Helens, WA) and with the younger sulphide-mineralised rocks of the Margaret Cu–Mo porphyry deposit located entirely within the pluton. Major- and trace-element discrimination suggests that one tourmaline population crystallised from fractionated late-stage melt pockets in granodiorite–monzogranitic dykes of the pluton. These coarse, euhedral, oscillatory, and complexly sector-zoned uvite tourmalines span a limited range in Mg/(Mgxa0+xa0Fe) [Mg#] space (0.4–0.7xa0apfu) and show the highest Ti, Ca, F, Nb, and Ta contents, and low X-site vacancies (<0.1xa0apfu), suggesting slow, ordered crystallisation. Conversely, smaller, microcrystalline, pluton-related vein tourmalines show higher X-site vacancies (>0.6 apfu), lower Ca and F contents, and the highest Li, As, and HREE contents (>80xa0ppm Li, >1200xa0ppm As). This population appears to record direct, rapid crystallisation from magmaticxa0±xa0meteoric fluid(s) bearing the signature of the breakdown of primary feldspars and pyroxenes, with fluid exsolution from fractionated melt patches likely triggered by the formation of the previous generation of tourmaline. Mineralised porphyry deposit tourmaline compositions from the stockwork span a much larger range in Mg# space (0.05–0.9xa0apfu) and are almost entirely Ca-free. X-sites of these schorl tourmalines are dominated by Na or vacancies, and the Y-sites are strongly Fe enriched. The highest Mn and Zn concentrations (>4000 and >1000xa0ppm, respectively) potentially reflect the composition of mineralising fluids during ore deposition. A number of boron isotopic analyses yield predominantly heavy boron, but δ11B values range from −5.2 to 6.2xa0‰ and average 1.4xa0‰. Whilst most plutonic tourmalines conform to reported a- and c-sector element partitioning models, those from the mineralised porphyry show large and variable sector fractionation differences, suggesting that external controls may also be important. Wider evidence for late-stage pervasive metasomatism by halogen-bearing exsolved fluid(s) is provided by the high Mg# (>70) secondary amphiboles and biotites from within the Spirit Lake pluton, where the amphiboles are clear replacement products of primary pyroxenes. Fluid halogen fugacity ratios calculated from the biotite compositions overlap with other global mineralised porphyry systems, despite not being immediately associated with sulphide ores. The evidence suggests complex fluid processes and the coincidental development of the mineralised porphyry system within the pluton. Heat, fluids, and metals were therefore likely supplied by a later phase of magmatism, unrelated to the consolidation of the main Spirit Lake granitoid. These new constraints on magmatic–hydrothermal fluid signatures have wider applicability to potentially tracing proximal barren and mineralised processes, and for distinguishing between formation mechanisms for primary and secondary halogen-bearing minerals.