Geochemical discrimination of different magma series and their differentiation products using immobile elements

Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment.

Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks

A method of discriminating between different types of mid-ocean ridge basalts and continental tholeiites with the Nb1bZr1bY diagram

SUMMARY Plate motions relative to the hotspots over the past 4 to 7 Myr are investigated with a goal of determining the shortest time interval over which reliable volcanic propagation rates and segment trends can be estimated. The rate and trend uncertainties are objectively determined from the dispersion of volcano age and of volcano location and are used to test the mutual consistency of the trends and rates. Ten hotspot data sets are constructed from overlapping time intervals with various durations and starting times. Our preferred hotspot data set, HS3, consists of two volcanic propagation rates and eleven segment trends from four plates. It averages plate motion over the past ≈5.8 Myr, which is almost twice the length of time (3.2 Myr) over which the NUVEL-1A global set of relative plate angular velocities is estimated. HS3-NUVEL1A, our preferred set of angular velocities of 15 plates relative to the hotspots, was constructed from the HS3 data set while constraining the relative plate angular velocities to consistency with NUVEL-1A. No hotspots are in significant relative motion, but the 95 per cent confidence limit on motion is typically ±20 to ±40 km Myr −1 and ranges up to ±145 km Myr −1 . The uncertainties of the new angular velocities of plates relative to the hotspots are smaller than those of previously published HS2-NUVEL1 (Gripp & Gordon 1990), while being averaged over a shorter and much more uniform time interval. Nine of the fourteen HS2-NUVEL1 angular velocities lie outside the 95 per cent confidence region of the corresponding HS3NUVEL1A angular velocity, while all fourteen of the HS3-NUVEL1A angular velocities lie inside the 95 per cent confidence region of the corresponding HS2-NUVEL1 angular velocity. The HS2-NUVEL1 Pacific Plate angular velocity lies inside the 95 per cent confidence region of the HS3-NUVEL1A Pacific Plate angular velocity, but the 0 to 3 Ma Pacific Plate angular velocity of Wessel & Kroenke (1997) lies far outside the confidence region. We show that the change in trend of the Hawaiian hotspot over the past 2 to 3 Myr has no counterpart on other chains and therefore provides no basis for inferring a change in Pacific Plate motion relative to global hotspots. The current angular velocity of the Pacific Plate can be shown to differ from the average over the past 47 Myr in rate but not in orientation, with the current rotation being about 50 per cent faster (1.06 ± 0.10 deg Myr −1 ) than the average (0.70 deg Myr −1 ) since the 47-Myr-old bend in the Hawaiian‐Emperor chain.

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Young tracks of hotspots and current plate velocities

Magma type and tectonic setting discrimination using immobile elements

Easter Island has developed around three volcanoes—Poike, an older (3 m.y.) strato-volcano, Rano Kau, a caldera, and the fissure complex of Terevaka and its associated cones. The lavas show a wide compositional spread from tholeiites and olivine tholeiites to hawaiites, mugearites, benmoreites, trachytes and rhyolites (comendites). Hawaiite is by far the most abundant rock type and trachytes and rhyolites are relatively rare. Intermediate and acid rocks are concentrated in the southwestern part of the island on or around Rano Kau. The basaltic rocks, which are plagioclase-phyric or aphyric, are transitional hypersthenenormative types characterized by high contents of Fe, Ti and Zr but low K and Mg. The Poike basalts are marginally lower in Zr, Nb, Y and Zn compared with those of the younger volcanoes, but the trachytes from this centre show anomalously high concentrations of Rb, Zr and Nb. The island's youngest flow, the Roiho basalt, is an olivine tholeiite with distinctly more alkaline affinities: it is olivine-microphyric with relatively high contents of Mg, Ni and K.

Petrology and geochemistry of Easter Island

The Tertiary complex of doleritic cone sheets in Ardnamurchan has been sampled and a geochemical study made on 98 rock specimens by X-ray fluorescence analysis. A range in composition from 48 to 66% (wt.) SiO2 is associated with, for example, an increase in Rb from 2 to 126 ppm, and a decrease in Ni from 86 to < 1 ppm. R-mode factor analysis indicates element groupings that are most easily related to a history of crystal fractionation beneath the central intrusive complex. No chemical basis can be found that would add support to either the two-centre or the spiral-fracture hypotheses relating to the disposition of the cone sheets. Plots of oxide and normative data show the Hebridean cone-sheet complex to have a tholeiitic character, which is compared with the Hawaiian tholeiitic and Hebridean alkalic series. A transitional nature for the two Hebridean series, compared with the two Hawaiian series, is probably related to derivation from an olivine basalt parent rather than from an olivine tholeiite as in Hawaii.

J. G. Holland

Papers

Petrology and geochemistry of Easter Island

Hebridean tholeiitic magmas: A geochemical study of the Ardnamurchan cone sheets