Alan D. Smith

Critical evaluation of Re-Os and Pt-Os isotopic evidence on the origin of intraplate volcanism

Evaluation of Re–Os and Pt–Os isotope systematics indicates that the mantle plume model does not provide a unique explanation of the 187Os/188Os–186Os/188Os isotopic variation in intraplate volcanic rocks. The low Os contents of ocean floor basalt and sediment compositions results in the amount of recycled crust (up to 55%) required in a plume to explain the range in 187Os/188Os, being unrealistically high unless non-equilibrium melting conditions are invoked. The low Pt content of MORB also limits the Pt/Os ratio of recycled crust, requiring plume models to resort to interaction with the outer core to explain suprachondritic 186Os/188Os ratios. Generation of the sources of intraplate volcanism by mixing of sediment into the mantle similarly requires high amounts (30–50%) of crustal materials. Non-equilibrium melting regimes can be avoided when the sources for intraplate volcanism are formed by metasomatic processes at shallow level in the mantle. Wetspot compositions formed by fluid metasomatism at convergent margins could produce the range of 186Os/188Os–187Os/188Os signatures in intraplate volcanism over 2 Gyrs if the fluid was derived principally from the slab sediment component and the Pt/Os and Re/Os ratios of the hydrous peridotite remained unmodified. However, the most feasible method of generating suprachondritic 186Os/188Os–187Os/188Os is from partitioning of platinum group elements into pyroxenites precipitated from Mg-rich melts. Isotopic compositions of 186Os/188Os=0.119850, γOs=+10 could be generated from depleted mantle compositions in 150 Myrs in an oceanic perisphere domain containing 60–70% pyroxenite veins, or in a continental mantle section containing as little as 15% pyroxenite over 2 Gyrs. Osmium isotope systematics therefore do not prove the mantle plume over alternative models, as illustrated for the Columbia River Basalt Group, where 187Os/188Os ratios of up to 0.144 in the least crustally contaminated basalts can be explained with a source containing 15–35% Mesozoic pyroxenite.

Mobilization of REE during crustal aging in the Troodos Ophiolite, Cyprus

REE data for secondary minerals from low- and high-temperature alteration environments in the Troodos Ophiolite have been determined using a combined mass spectrometry-neutron activation technique. Smectite- and analcime-REE mimic fresh rock profiles and have concentrations ranging from < 1–5 times chondritic values. Celadonite is depleted in LREE and total REE relative to the rocks. Groundmass-replacing chlorite and epidote are similar to, or enriched in, REE relative to fresh rocks. In contrast, fault-related chlorite and void-filling epidote are LREE-enriched with large positive Eu anomalies and yield comparable REE profiles to those of hydrothermal fluids venting at mid-ocean ridges. The zeolites, clinoptilolite and laumontite, are also LREE-enriched with REE contents significantly lower than rock values. Carbonate is enriched in HREE relative to LREE and concentrations vary from 0.2 to 18 times chondritic values. REE data for these minerals indicate that seawater-REE contents are modified by seawater-rock interaction in volcanic sequences and sheeted dykes in oceanic crust during axial hydrothermal alternation and subsequent crustal aging. The absolute and relative abundances of REE in seawater-derived solutions evolve in response to the precipitation of secondary minerals and the prevailing water/rock ratios. Rocks altered at low temperatures will show no change or a decrease in REE content; the LREE/HREE ratio will reflect the proportion of smectite, celadonite and carbonate in the groundmass assemblage. In contrast, rocks altered at high temperatures will show no change or an increase in REE; the LREE/HREE ratio will reflect the proportion of void-filling and groundmass-replacing chlorite and epidote in the rock.

A Reappraisal of Stress Field and Convective Roll Models for the Origin and Distribution of Cretaceous to Recent Intraplate Volcanism in the Pacific Basin

Correlations between volcanic output along the Hawaiian and Cook-Austral-Marquesas chains and basin-wide plate reorganizations at 25 and 5 Ma support the contention of Jackson and Shaw (1975) that intraplate volcanism throughout the Pacific basin is controlled by the transmitted stress field. Reconstruction of the Pacific Basin for the Early Cretaceous through Eocene based on a paleomagnetic model demonstrates that oceanic plateaus (Shatsky Rise, Mid-Pacific Mountains, Magellan Rise, Ontong Java) were generated in zones of tension in the wake of retreating triple junctions, and that ocean island chains may be divided on the basis of propagating- or leaky-fracture origin. The latter, including the Louisville, Marshall-Gilbert, Line Island, and Cook-Austral-Marquesas chains show nonlinear age progressions, and followed pre-existing NNW-SSE-trending fracture zones initiated by transform faulting during the early history of the Pacific plate. Volcanism attributed to propagating fractures includes the Sala y Gomez, Juan Fernandez, and Caroline chains, which extrapolate to breaks in nearby subducting slabs, suggesting stressing of the plate by convergent margin geometry. The Emperor Chain is unique in representing volcanism along a propagating fracture induced at a divergent margin. The location and orientation of this chain is attributed to the geometry of the Kula-Pacific Ridge, following plate reorganizations at 82 Ma that prematurely halted triple-junction volcanism on Meiji seamount. Subsequent volcanism along the Hawaiian Chain marks reorientation of the stress field to control by convergent-margin geometry following abandonment of the Pacific-Kula Ridge, and does not require a change in Pacific plate motion at 43 Ma. The distribution of Pacific intraplate volcanism is more regular than would be expected from plumes, and can be explained as a result of shallow volatile-bearing sources tapped under developing hotcell conditions, with the change from plateau to island chain volcanism reflecting changes in the transmitted stress field induced by large-scale plate interactions.

Sr, Nd and Pb isotopic and geochemical constraints for the origin of magmas in Popocatépetl volcano (central Mexico) and their relationship with the adjacent volcanic fields

We present new geochemical data (major- and trace-elements, as well as Sr and Nd isotopic compositions) of volcanic rocks erupted from Popocatépetl volcano during the volcanic event from December 2000 to January 2001. These data along with an exhaustive compilation of geochemical and Sr, Nd, and Pb isotope data reported for Popocatépetl rocks and nearby volcanic areas are used to examine the origin and geochemical evolution of the magmas in the central Mexican volcanic belt (CMVB). During this period of volcanic eruptions Popocatépetl produced ash columns as high as 7 km. Pyroclastic flows and lahars were observed after the completion of the activity. Samples of banded pumice and a bomb fragment transported by the lahar were chemically analysed for this work. Rocks show an andesitic composition with 58.5–61.7 wt.% SiO2 and 5.9–4.0 wt.% MgO. Contents of large ion lithophile elements (LILE), rare-earth elements (REE) and Zr are nearly constant through the compositional range. No significant Eu anomaly is present, but the samples show Nb-anomaly relative to LILE and high-field strength elements (HFSE). Nd- and Sr-isotopic compositions of these samples range from 143Nd/144Nd = 0.51291 to 0.51287 and 87Sr/86Sr = 0.70399 to 0.70422. Comparison of Popocatépetl products with volcanic rocks from the nearby areas shows that the magmas in CMVB were generated in a heterogeneously veined-mantle source enriched in LILE, HFSE, and REE. Additional crustal assimilation as well as fractional crystallization could account for the great chemical variability of rocks in the CMVB. Statistical comparison of the geochemical compositions of the volcanic products ejected from 1994 to 2000 to those ejected during the 2001 event shows that most geochemical parameters (major- and trace-elements, normative minerals, Sr and Nd isotopic composition, as well some elemental ratios) present no statistically significant differences. Statistically significant differences in the mean only were computed for the major-elements SiO2, FeO, MgO, CaO, and K2O, as well as for the rare-earth elements Nd, Sm, Eu, Gd, Dy, Ho, Tm, and Yb.

A Pre-Irradiation group separation (PIGS) technique for the analysis of rare-earth elements during Nd-isotopic analysis of geological samples☆

Abstract PIGS is a pre-irradiation group separation technique for the analysis of REE by neutron activation which is used in conjunction with Nd isotopic studies. Removal of interfering and background-producing matrix elements allows the analysis of up to 12 REE within 12 days of irradiation by use of their shorter-lived radioisotopes. Small samples and samples with high Fe, Th or U, may be analysed with superior precision and detection limits compared to conventional INAA.