Normal Mid-ocean Ridge Basalt Source Research Articles

Geochemistry of the Peramora Mélange and Pulo do Lobo schist: geochemical investigation and tectonic interpretation of mafic mélange in the Pangean suture zone, Southern Iberia

The Peramora Melange is part of an accretionary complex between the South Portuguese Zone (a fragment of Laurussia) and the Ossa Morena Zone (para-autochthonous Gondwana) and is an expression of the Pangean suture zone in southwestern Iberia. The suture zone is characterized by fault-bounded units of metasedimentary rocks, melanges, and mafic complexes. Detailed geologic mapping of the Peramora Melange reveals a complex pattern of imbricated schists and mafic block-in-matrix melanges. Geochemical signatures of the Pulo do Lobo schist (PDL) are consistent with derivation from both mafic and continental sources. The mafic block-in-matrix melange displays normal mid-ocean ridge basalt (NMORB) geochemical signature, juvenile Sm–Nd isotopic compositions, and a range of zircon ages similar to those observed in the PDL, suggesting a sedimentary component. Taken together, these data suggest a complex tectonic history characterized by erosion of a NMORB source, melange formation, and imbrication during underplating occurring during the final stages of continent–continent collision.

Combined Trace Element and Pb-Nd-Sr-O Isotope Evidence for Recycled Oceanic Crust (Upper and Lower) in the Iceland Mantle Plume

We present the results of a comprehensive major element, trace element and Sr–Nd–Pb–O isotopic study of post-glacial volcanic rocks from the Neovolcanic zones on Iceland. The rocks studied range in composition from picrites and tholeiites, which dominate in the main rift systems, to transitional and alkalic basalts confined to the off-rift and propagating rift systems. There are good correlations of rock types with geochemical enrichment parameters, such as La/Sm and La/Yb ratios, and with long-term radiogenic tracers, such as Sr–Nd–Pb isotope ratios, indicating a long-lived enrichment/ depletion history of the source region. Sr/Sr vs Nd/Nd defines a negative array. Pb isotopes define well-correlated positive arrays on both Pb/Pb vs Pb/Pb and Pb/Pb diagrams, indicating mixing of at least two major components: an enriched component represented by the alkali basalts and a depleted component represented by the picrites. In combined Sr–Nd–Pb isotopic space the individual rift systems define coherent mixing arrays with slightly different compositions. The enriched component has radiogenic Pb (Pb/Pb > 19 3) and very similar geochemistry to HIMU-type ocean island basalts (OIB). We ascribe this endmember to recycling of hydrothermally altered upper basaltic oceanic crust. The depleted component that is sampled by the picrites has unradiogenic Pb (Pb/Pb < 17 8), but geochemical signatures distinct from that of normal mid-ocean ridge basalt (N-MORB). Highly depleted tholeiites and picrites have positive anomalies in mantle-normalized trace element diagrams for Ba, Sr, and Eu (and in some cases also for K, Ti and P), negative anomalies for Hf and Zr, and low dOolivine values (4 6–5 0‰) below the normal mantle range. All of these features are internally correlated, and we, therefore, interpret them to reflect source characteristics and attribute them to recycled lower gabbroic oceanic crust. Regional compositional differences exist for the depleted component. In SW Iceland it has distinctly higher Nb/U ( 68) and more radiogenic Pb/Pb ratios (18 28–18 88) compared with the NE rift (Nb/U 47; Pb/Pb 1⁄4 18 07–18 47). These geochemical differences suggest that different packages of recycled oceanic lithosphere exist beneath each rift. A third and minor component with relatively high Sr/Sr and Pb/Pb is found in a single volcano in SE Iceland (Oraefajokull volcano), indicating the involvement of recycled sediments in the source locally. The three plume components form an integral part of ancient recycled oceanic lithosphere. The slope in the uranogenic Pb diagram indicates a recycling age of about 1 5 Ga with time-integrated Th/U ratios of 3 01. Surprisingly, there is little evidence for the involvement of North Atlantic N-MORB source mantle, as would be expected from the

Does depleted mantle form an intrinsic part of the Iceland plume?

Icelandic basalt ranges in composition from voluminous tholeiite, erupted in the rift zones, to small‐volume, mildly alkaline basalt erupted off‐axis. In addition, small‐volume flows of primitive basalt, highly depleted in incompatible elements, are sometimes found in the actively spreading rift axes. Relative incompatible‐element depletion or enrichment in Icelandic basalt is correlated with variation in radiogenic isotope ratios, implying that the mantle beneath Iceland is heterogeneous and that the relative contribution of the various mantle components relates to eruption environment (on‐ or off‐axis) and hence to degree of melting. Thus small‐degree off‐axis melting preferentially samples an enriched and more fusible mantle component, whereas more extensive melting beneath the rift axes produces magma that more closely represents the bulk Iceland plume mantle composition. The small‐volume flows of depleted basalt represent melts that have preferentially sampled a depleted and more refractory mantle component. A debate has arisen over the nature of the depleted component in the Iceland plume. Some authors [e.g., Hanan and Schilling, 1997] argue that the depleted component is ambient upper mantle, the source of normal mid‐ocean ridge basalt (NMORB) in this region. Others [e.g., Thirlwall, 1995; Kerr et al., 1995; Fitton et al., 1997], however, have used various lines of evidence to suggest that the plume contains an intrinsic depleted component that is distinct from the NMORB source. Hanan et al. [2000] attempt to refute the existence of a depleted Iceland plume (DIP) component through a critical evaluation of the Nb‐Zr‐Y arguments advanced by Fitton et al. [1997] and the Hf‐Nd‐isotopic evidence presented by Kempton et al. [1998]. In this paper we examine the case presented by Hanan et al. [2000] and conclude that their arguments are flawed. Firstly, their trace‐element data set excludes data from depleted Icelandic basalt samples and so it is not surprising that they find no evidence for a DIP component. Secondly, they present two new Hf‐isotope analyses of a single depleted Icelandic basalt sample and show that the data plot in their NMORB field on an εHf versus εNd diagram. However, new data allow the resolution of distinct NMORB and depleted Icelandic basalt fields on this diagram. We conclude that trace‐element and radiogenic isotope data from Iceland require the existence of a DIP component.

Age and geochemistry of Karoo dolerite dykes from northeast Botswana

The Botswana Dyke Swarm is a prominent 800 km long and 100 km wide feature on aeromagnetic maps of southern Africa, but little has been published on its exact age or geochemical composition. New age, trace element and isotope data for this dyke swarm show that is magmatism is indistinguishable from Karoo continental flood basalts. Ar/Ar dating gives an age of 178.9 ± 1.4 Ma. Both high Ti-Zr and low Ti-Zr dolerites occur, but the high Ti-Zr rocks appear to be the dominant type of magmatism. Low Ti-Zr mafic rocks have isotopic and trace element characteristics similar to a combination of a normal mid-ocean ridge basalt source with sedimentary and fluid-enriched components, which are thought to reside in the sub-continental lithospheric mantle. A lithospheric component seen in the high Ti-Zr mafic rocks is similar to that in nephelinites from Zimbabwe.

Geochemistry of basalts from the Hayes Transform region of the Mid‐Atlantic Ridge

Fresh glassy aphyric and phyric basalts and dolerites have been recovered from the Mid‐Atlantic Ridge spreading segments adjacent to the Hayes Transform (33°40′N) and within the transform valley. Glass and whole rock compositions exhibit Mg numbers that range from 44.3 to 70.6, CaO/Al2O3 ratios of 0.57–0.87, Na2O contents of 1.84–3.49 wt %, (La/Yb)cn ratios of 0.40–1.39, 87Sr/ 86Sr ratios of 0.70270–0.70403, and 206Pb/204Pb ratios of 17.95–18.79. These basalts are some of the most primitive and most fractionated basalts from the Mid‐Atlantic Ridge. The Hayes Transform appears to mark a geochemical boundary between two major mantle provinces based on major, trace, rare earth element, and Sr‐Nd‐Pb isotopic compositions of these basaltic samples. Modeling of partial melting and fractional crystallization from minor, trace, and rare earth elements indicates that the southern Hayes basalts could have been generated by ∼20% melting of a fertile normal mid‐ocean ridge basalt (NMORB) source region, efficient pooling of melts, and low‐pressure fractionation along a magmatically robust spreading segment (HA‐1). The northern Hayes basalts are estimated to have been generated by ∼13 to ∼20% partial melting of a heterogeneous source region (enriched MORB (EMORB) and infertile NMORB source domains), incomplete pooling of melts, and moderate pressure fractionation along a magmatically starved spreading segment (OH‐3). Transform valley basalts show estimated extents of melting from ∼11 to ∼22% of a heterogeneous source region, efficient and inefficient pooling of melts, and moderate pressures of fractionation.

Cross-arc geochemical variations in volcanic fields in Honduras C.A.: progressive changes in source with distance from the volcanic front

A geochemical traverse across Honduras reveals the heterogeneity of the mantle underneath Central America. Alkali basalts from Lake Yojoa (170 km behind the front) have low 87Sr/86Sr but high La/Yb, and elevated incompatible trace element abundances, consistent with derivation from a normal mid-ocean ridge basalt source mantle via low degrees of melting. These lavas lack evidence for an enriched source thought to be intermingled with normal mid-ocean ridge basalt source mantle beneath most of Central America. The amplitude of the subducted slab signature decreases smoothly with distance from the volcanic front. Lavas from Zacate Grande, the area nearest to the volcanic front (17 km behind the arc), display large ion lithophile element enrichment and high field strength element depletion indicating the involvement of subducted material in magma genesis. Components of subducted material are not evident in lavas from Lake Yojoa, the area furthest from the arc. Basalts and basaltic andesites from Tegucigalpa, 102 km behind the volcanic front, are geochemically intermediate between those of Lake Yojoa and Zacate Grande. The lavas from Tegucigalpa show a decreased influence of the subduction component, and are affected by assimilation-fractional crystallization processes at shallow depths. The gradual decrease in the subducted component from the volcanic front to Zacate Grande, Tegucigalpa and finally Lake Yojoa contrasts with the abrupt decrease documented for southeast Guatemala, the only other area in Central America where a cross-arc transect has been studied.

Geochemistry of Cenozoic basaltic and silicic magmas in the central portion of the Loei–Phetchabun volcanic belt, Lop Buri, Thailand

Cenozoic volcanic rocks outcrop in the central portion of the Loei–Phetchabun volcanic belt in central Thailand in the Lop Buri area. The volcanic rocks range in composition from basalt to high-silica rhyolite. In general, the volcanic rocks decrease in age from south to north. The oldest rocks studied are 55–57 Ma rhyolites that are isotopically and geochemically distinct from younger (13–24 Ma) rhyolites that occur farther north. Intermediate rocks (andesite and dacite) are less voluminous than rhyolite. Basalt occurs in the central and northern parts of the area and ranges in composition from olivine tholeiites to nepheline normative alkali basalts. The isotopic, major, and trace element compositions of the andesites, dacites, and younger rhyolites are consistent with an origin for these rocks by variable degrees of partial melting of metabasaltic crustal rocks, themselves derived from a depleted mantle source at approximately 530 ± 100 Ma. The apparent extent of partial melting of metabasalt increases from rhyolite to andesite. The isotopic and trace element systematics of the basalts are consistent with a refertilized depleted mantle source with characteristics of a mixture of normal mid-ocean ridge basalt source mantle and enriched mantle II type mantle.

High field strength and transition element systematics in island arc and back-arc basin basalts: Evidence for multi-phase melt extraction and a depleted mantle wedge

The geochemical character of the mantle wedge in convergent margin settings is investigated using the high field strength and transition elements Ti, Zr, V, Sc and Y in basaltic rocks from island arcs and associated back-arc basins. Systematic differences are observed in the composition of each arc relative to the adjacent back-arc basin. The arcs have lower average abundances of incompatible high field strength elements and Y, and extend to higher ratios of Ti/Zr, V/Ti and Sc/Y. Partial melting of a normal mid-ocean ridge basalt source accounts for the back-arc basin basalt compositions, but cannot explain the higher element ratios observed in many arc basalts. These arc magmas are consistent with derivation from sources which are more depleted in incompatible elements those that of the back-arc basin basalts. Source depletion by melt extraction, prior to arc magma genesis, explains the similar degree of Y depletion and, by analogy, the heavy REE depletion, to that of the high field strength elements in island arc basalts. The observations are not compatible with the presence of residual titanate phases (e.g. rutile). However, amphibole may be required in the source of a subgroup of arc basalts which have relatively high Zr abundances and anomalously low Ti/Zr and high V/Ti values. These results have implications for subduction zone geochemical budgets, as the influence of any slab-derived component may be enhanced in regions of extensive mantle wedge depletion.