Terry Plank

Bio: Terry Plank is an academic researcher from Lamont–Doherty Earth Observatory. The author has contributed to research in topics: Mantle (geology) & Subduction. The author has an hindex of 51, co-authored 145 publications receiving 16244 citations. Previous affiliations of Terry Plank include Cornell University & Boston University.

Element transport from slab to volcanic front at the Mariana arc

Abstract: We present a comprehensive geochemical data set for the most recent volcanics from the Mariana Islands, which provides new constraints on the timing and nature of fluxes from the subducting slab. The lavas display many features typical of island arc volcanics, with all samples showing large negative niobium anomalies and enrichments in alkaline earth elements and lead (e.g., high Ba/La and Pb/Ce). Importantly, many of these key ratios correlate with a large range in 238U excesses, (238U/230Th) = 0.97–1.56. Geochemical features show island to island variations; lavas from Guguan have the largest 238U-excesses, Pb/Ce and Ba/La ratios, while Agrigan lavas have small 238U excesses, the least radiogenic 143Nd/144Nd, and the largest negative cerium and niobium anomalies. These highly systematic variations enable two discrete slab additions to the subarc mantle to be identified. The geochemical features of the Agrigan lavas are most consistent with a dominant subducted sediment contribution. The added sedimentary component is not identical to bulk subducted sediment and notably shows a marked enrichment of Th relative to Nb. This is most readily explained by melt fractionation of the sediment with residual rutile and transfer of sedimentary material as a melt phase. For most of the highly incompatible elements, the sedimentary contribution dominates the total elemental budgets of the lavas. The characteristics best exemplified by the Guguan lavas are attributed to a slab-derived aqueous fluid phase, and Pb and Sr isotope compositions point toward the subducted, altered oceanic crust as a source of this fluid. Variable addition of the sedimentary component, but near-constant aqueous fluid flux along arc strike, can create the compositional trends observed in the Mariana lavas. High field strength element ratios (Ta/Nb and Zr/Nb) of the sediment poor Guguan lavas are higher than those of most mid-oceanic ridge basalts and suggest a highly depleted subarc mantle prior to any slab additions. The 238U-230Th systematics indicate >350 kyr between sediment and mantle melting but <30 kyr between slab dehydration and eruption of the lavas. This necessitates rapid magma migration rates and suggests that the aqueous fluid itself may trigger major mantle melting.

Petrological systematics of mid-ocean ridge basalts: Constraints on melt generation beneath ocean ridges

Abstract: Mid-ocean ridge basalts (MORB) are a consequence of pressure-release melting beneath ocean ridges, and contain much information concerning melt formation, melt migration and heterogeneity within the upper mantle. MORB major element chemical systematics can be divided into global and local aspects, once they have been corrected for low pressure fractionation and interlaboratory biases. Regional average compositions for ridges unaffected by hot spots ("normal" ridges) can be used to define the global correlations among normalized Na20, FeO, TiO2 and Si02 contents, CaO/Al 203 ratios, axial depth and crustal thickness. Back-arc basins show similar correlations, but are offset to lower FeO and TiO2 contents. Some hot spots, such as the Azores and Galapagos, disrupt the systematics of nearby ridges and have the opposite relationships between FeO, Na 20 and depth over distances of 1000 km. Local variations in basalt chemistry from slowand fast-spreading ridges are distinct from one another. On slow-spreading ridges, correlations among the elements cross the global vector of variability at a high angle. On the fast-spreading East Pacific Rise (EPR), correlations among the elements are distinct from both global and slow-spreading compositional vectors, and involve two components of variation. Spreading rate does not control the global correlations, but influences the standard deviations of axial depth, crustal thickness, and MgO contents of basalts. Global correlations are not found in very incompatible trace elements, even for samples far from hot spots. Moderately compatible trace elements for normal ridges, however, correlate with the major elements. Trace element systematics are significantly different for the EPR and the mid-Atlantic Ridge (MAR). Normal portions of the MAR are very depleted in REE, with little variability; hot spots cause large long wavelength variations in REE abundances. Normal EPR basalts are significantly more enriched than MAR basalts from normal ridges, and still more enriched basalts can erupt sporadically along the entire length of the EPR. This leads to very different histograms of distribution for the data sets as a whole, and a very different distribution of chemistry along strike for the two ridges. Despite these differences, the mean Ce/Sm ratios from the two ridges are identical. Existing methods for calculating the major element compositions of mantle melts [Klein and Langmuir, 1987; McKenzie and Bickle, 1988; Niu and Batiza, 1991] are critically examined. New quantitative methods for mantle melting and high pressure fractionation are developed to evaluate the chemical consequences of melting and fractionation processes and mantle heterogeneity. The new methods rely on new equations for partition coefficients for the major elements between mantle minerals and melts. The melting calculations can be used to investigate the chemical compositions produced by small extents of melting or high pressures of melting that cannot yet be determined experimentally. Application of the new models to the observations described above leads to two major conclusions: (1) The global correlations for normal ridges are caused by variations in mantle temperature, as suggested by Klein and Langmuir [1987] and not by mantle heterogeneity. (2) Local variations are caused by melting processes, but are not yet quantitatively accounted for. On slower spreading ridges, local variations are controlled by the melting regime in the mantle. On the EPR, local variations are predominantly controlled by ubiquitous, small scale heterogeneites. Volatile content may be an important and as yet undetermined factor in affecting the observed variations in major elements. We propose a hypothesis, similar to one proposed by Allegre et al [1984] for isotopic data, to explain the differences between the Atlantic and Pacific local trends, and the trace element systematics of the two ocean basins, as consequences of spreading rate and a different distribution of enriched components from hot spots in the two ocean basins. In the Atlantic, the hot spot influence is in discrete areas, and produces clear depth and chemical anomalies. Ridge segments far from hot spots do not contain enriched

Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents

Abstract: Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This question is explored here using Th/La, which is low in oceanic basalts ( 0·25) and varies in arc basalts and marine sediments (0·09–0·34). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractionation in Th/La within the continental crust, caused by the preferential partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of ‘fractionation & foundering’, whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60% of its mafic residues to foundering.

Composition of the Continental Crust

Abstract: This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.