Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Geochemical and Sr–Nd isotope variations within Cretaceous continental flood-basalt suites of the Canadian High Arctic, with a focus on the Hassel Formation basalts of northeast Ellesmere Island

Abstract: Early- to mid-Cretaceous flood-basalt suites of the northeast Canadian High Arctic assigned to a High Arctic Large Igneous Province (HALIP) were studied for their whole-rock geochemistry and Sr–Nd isotopes. Data from basalt flows within the upper Albian to lower Cenomanian Hassel Formation of northeast Ellesmere Island are compared with former published data and new inductively coupled plasma mass spectrometry data of the stratigraphic equivalent Strand Fiord basalts and the older, late Hauterivian to Aptian Isachsen basalts from Axel Heiberg Island. The transitional to mildly alkaline aphyric Hassel basalts, with ocean island basalt (OIB)-like geochemical signatures in parts, have an Ar–Ar whole-rock age of on average 96.4 ± 1.6 Ma. They represent two geochemically different flow units without a fractional crystallization relationship: the high-phosphorous (HP) and low-phosphorous (LP) basalts. The Hassel HP basalts differ from the LP basalts by additionally higher Ba, K, Rb, Th and LREE contents, a pronounced positive Eu anomaly (Eu/Eu* = 1.74–1.76), as well as lower Ta, Nb, Zr and Hf concentrations. The Nd and Sr isotope ratios of the Hassel HP basalts [e Nd(t) of −1.3 to −1.4, 87Sr/86Sr(t) of 0.70706–0.70707] and the LP basalts [e Nd(t) of 4.5–4.9, 87Sr/86Sr(t) of 0.7038–0.7040] indicate an origin from different mantle sources. The geochemically similar tholeiitic Isachsen (ca. 130–113 Ma) and Strand Fiord basalts (ca. 105–95 Ma) are also incompatible element enriched relative to the primitive mantle, however, with negative Sr–P anomalies as well as partially negative K, Ta and Nb anomalies. In terms of incompatible element ratios (Zr/Nb, Nb/Th), several mantle components are involved in the formation of the flood-basalt suites: a component with primitive mantle composition, an OIB-like component (probably subducted and recycled oceanic crust) and an enriched lithospheric component. The latter component, probably metasomatized subcontinental lithospheric mantle, has contributed mainly to the Isachsen, Strand Fiord and Hassel HP basalts. In the Sr–Nd isotope system, the enriched lithospheric component is characterized by hypothetical values of e Nd(t) of approximately −4, and 87Sr/86Sr(t) of approximately 0.7083. The HALIP-related flood-basalt suites of the Canadian High Arctic are interpreted to be part of a continental rift zone that was active during the opening of the Amerasia Basin of the Arctic Ocean and the development of the oceanic Alpha-Mendeleev Ridge into a Large Igneous Province.

Trace Element and Isotope Geochemistry of the Northern and Central Tongan Islands with an Emphasis on the Genesis of High Nb/Ta Signatures at the Northern Volcanoes of Tafahi and Niuatoputapu

Abstract: We present a new comprehensive isotope and trace element dataset for north and central Tongan lavas including high-precision High Field Strength Element measurements. The emphasis of our study is on lavas from the northernmost volcanic islands of Tafahi and Niuatoputapu that exhibit unique compositions compared with other Tongan volcanoes, in particular elevated high-precision Nb/Ta ratios as high as similar to 28. These lavas are extremely depleted in incompatible, fluid-immobile elements such as Ti, Zr, Sc and Yb, suggesting about 6% depletion of their mantle wedge source relative to average upper mantle prior to 20-30% partial melting. The extremely depleted mantle wedge was re-enriched in highly incompatible elements including Nb and Ta from the subducting slab. Three slab components-fluids from altered oceanic crust, pelagic sediments, and Louisville Seamount Chain lavas-have contributed to the source region of the magmas. A fluid from the subducted Louisville lavas has been added after the other two slab components and has radiogenic Pb-206/Pb-204. Components from the altered oceanic crust and Louisville sea-mounts exhibit little Nb/Ta fractionation, as demonstrated by the nearly uniform and mid-ocean ridge basalt-like Nb/Ta ratios. We suggest that the apparently large slab fluid flux beneath Tafahi and Niuatoputapu results from focused flow through subducting Louisville Seamount crust that had no or little sediment cover, allowing the fluids to pass through the subducting lithosphere more efficiently. Thus, the high Nb/Ta ratios indicate that the Louisville-derived fluids had probably equilibrated with residual rutile. Our results imply that significant amounts of Nb and Ta can be mobilized from distinct subducting oceanic lithologies-a process that may probably also operate beneath other island arcs. We propose a model in which the appearance of the subducted seamount signatures some 3-4Myr after their actual subduction is explained by storage in shallow metasomatized lithospheric mantle. This relic Louisville seamount signature is subsequently reactivated and added to transiting magmas 4 Myr later. This reactivation may result from slab rollback of the subducting Pacific plate towards the east. Our new temporal and spatial model also provides a viable explanation for the conundrum of the occurrence of Louisville signatures even though the Louisville seamount chain is, at present, being subducted several hundred kilometres further south.

Transitional basalts and tholeiites from the East Pacific Rise, 9°N

Abstract: Basalts from the base of a small seamount on ∼1.5-m.y.-old crust west of the East Pacific Rise (EPR) at 9°N are intermediate in chemical and isotopic composition between light-rare-earth-element-depleted tholeiite (normal midocean ridge basalt (MORB)) and alkali basalt. Like oceanic alkali basalt, these rocks contain significantly more Ba, K, P, Sr, Ti, U, and Zr than normal MORB. Since the absolute abundances of these elements are still well below alkali basalt levels, the label transitional is adopted for these basalts. A series of fractionated MORB also occurs in this area, northwest of the Siqueiros Fracture Zone - Transform Fault. The normal tholeiites are either olivine-plagioclase or plagioclase-clinopyroxene phyric, while the transitional basalts are spinel-olivine phyric. Fractional crystallization quantitatively accounts for the chemical variability of the tholeiitic series but not for the transitional basalts. The tholeiitic series probably evolved in a crustal magma chamber ∼4 km below the crest of the East Pacific Rise. 143Nd/144Nd and other chemical data suggest that the large-ion-lithophile-enriched transitional basalts may represent a hybrid of normal MORB and Siqueiros area alkali basalt. Incompatible element plots of K, P, and U indicate possible derivation of the transitional basalts by magma mixing. Magma mixing of unfractionated normal MORB and Siqueiros alkali basalt has been quantified. Derivation of the transitional basalts from a 1:1 mixture is supported by all available chemical data, including Cr, Cu, Nd, Ni, Sm, Sr, U, and V. This magma mixing apparently occurred at ≲30 km depth within a few tens of kilometers from the EPR axis. These Siqueiros area EPR transitional basalts are compared with Mid-Atlantic Ridge (MAR) transitional basalts from the Iceland and Azores areas. The Siqueiros area basalts reflect a profound chemical and isotopic heterogeneity in the upper mantle, similar to that found along the MAR. Unlike the MAR, the EPR shows no evidence of plumelike bulges and associated large-scale outpourings of nonnormal MORB resulting from these mantle heterogeneities. Siqueiros alkali basalt and MORB, as well as transitional basalt and MORB, were recovered from single dredge hauls. Such close spatial and temporal proximity of the inferred mantle sources places severe constraints on geometric and physicochemical upper mantle models.