Incompatible element

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

Cretaceous to Mid-Eocene pelagic sediment budget in Puerto Rico and the Virgin Islands (northeast Antilles Island arc)

Abstract: Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS).

Enriched mantle – Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)

Abstract: The major, trace (including rare earth) element abundances, and Sr-Nd-Pb isotopic compositions, have been analysed for andesitic basalt and andesitic sills and lavas of the Jurassic Ferrar Magmatic Province, Prince Albert Mountains, Antarctica. The typical “crustal signature” of the Ferrar magmatism, characterized by relatively high SiO2, LREE and LILE contents in these samples, is associated with high 87Sr/86Sr and low 143Nd/144Nd. Systematic correlations of major and trace elements indicate that fractional crystallization was important. However, increases in incompatible elements are positively correlated with initial 87Sr/86Sr, suggestive of crustal assimilation processes. The observed correlations between initial 87Sr/86Sr and LREE enrichments have been modelled by an AFC process, starting from the least evolved sample and assuming the compositions of the orthogranulites of Victoria Land as contaminants. The REE patterns of the least evolved Ferrar rocks approach those of E-type MORB, differing only by higher LREE/IREE. The enrichment in LREE, accompanying high initial 87Sr/86Sr, 207Pb/204Pb and low 143Nd/144Nd compared with E-type MORB, can be explained by interaction of “primary Ferrar basalt” with crystalline basement. We propose a petrological model whereby Ferrar magmas were generated through high degrees of melting of an E-type MORB mantle source, and subsequently these “primary” melts underwent AFC processes inheriting a crustal signature. The Sr-Nd-Pb isotopic compositions required by the AFC model for the primary Ferrar basalt are similar to those of the Dupal signature of the oceanic basalts of the Southern Hemisphere (Hart 1984). Transantarctic Mountains would have been located inside the Dupal anomaly in pre-Gondwana dispersion times.

Evidence for Residual Melt Extraction in the Takidani Pluton, Central Japan

Abstract: The Takidani pluton represents one of a few locations where melt extraction from a crystal mush is preserved in the natural rock record, making it an extremely good case study for investigating the generation of evolved melt reservoirs in the upper crust. Located in the Japan Alps, the Takidani pluton shows a clear vertical zonation consisting of granite and granodiorite in the lower and mid- dle section, a fine-grained porphyritic granitic unit in the upper section and a marginal granodiorite at the roof contact with the host-rock. We present a detailed petrographic and geochemical study using samples collected along a section that traverses the entire vertical section of the pluton. No sharp contacts are found between units. Instead, gradual changes in rock fabric and mineralogy are observed between the lower granodiorite and overlying porphyritic unit. Major and trace elem- ent bulk-rock compositions show sigmoidal variations from the bottom to top of the pluton. Incompatible elements and silica contents increase roofwards within the porphyritic unit. Plagioclase chemistry reveals three main crystal populations (P1, P2 and P3) with Fe contents increasing towards the base of the pluton. Comparison with existing crystallization experiments, thermobarometry and hygrometry indicate that the magmas were emplaced at around 200 MPa, 850–900 C and bulk water contents of 3–4wt %. Whole-rock major and trace element analyses to- gether with mineral chemistry and textural observations suggest that the fine-grained porphyritic unit was extracted from the underlying granodiorite at temperatures between 800 and 740 C and crystallinities of 45–65 wt %. Radiogenic isotopes indicate only minor assimilation (2–6 wt %) and support melt evolution through crystal fractionation. The fine-grained matrix of the porphyritic unit may have been the result of pressure quenching associated with a volcanic eruption.

Mid‐Pleistocene basalt from the Seguam Volcanic Center, central Aleutian Arc, Alaska: Local lithospheric structures and source variability in the Aleutian Arc

Abstract: Recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern Aleutian Arc and the sources and processes that produce the broad spectrum of Aleutian basaltic lavas. Mid-Pleistocene basalt from the Seguam Island volcanic center in the central Aleutian Arc erupted through strongly extended arc crust, contains 4–8% MgO, is low in K2O ( 18%), and is distinguished from Aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements (REE) and low Sr and Pb content. The Ba/La (47–72) and Pb/La (0.7–1.5) ratios are high, incompatible element ratios are variable, and REE patterns cross one another. The 87Sr/86Sr ratio (0.70365 ± 0.00003) is the highest value obtained from the Aleutian islands, also 206Pb/204Pb (18.91 ±0.02), 207Pb/204Pb (15.60 ±0.02), and 208Pb/204Pb (38.51 ±0.04) ratios are among the most radiogenic values known, whereas the 143Nd/144Nd ratio (0.51297 ±0.00003) is among the lowest measured. Low abundances of incompatible elements, high Ba/La and Pb/La ratios, uniform Sr, Nd, and Pb isotope ratios, and high eruptive temperatures (∼1160° C) suggest that basalt erupted at Seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust. Partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics. Trace element abundances rule out an eclogite source and suggest instead a peridotite-dominated source with ∼5–10% of a subducted sediment component. This peridotite may reside in the mantle wedge or in the Amlia Fracture Zone (AFZ) of the Pacific plate that was subducted beneath Seguam ∼1 Ma. In addition, the AFZ possibly enhanced the subduction of terrigenous sediment into the source region beneath Seguam and this extra terrigenous component may explain the Ba/La, Pb/La and Sr, Nd, and Pb isotope ratios. The distinctive composition of Seguam basalt reflects a complex set of physical conditions, including fracture zone subduction and intraarc extension, that affected both the source region and thermo-chemical history of magma ascent. Local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes. Models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the Aleutian Arc.