Incompatible element

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

A statistical description of concurrent mixing and crystallization during MORB differentiation: Implications for trace element enrichment

Abstract: The pattern of trace element enrichment and variability found in differentiated suites of basalts is a simple observable, which nonetheless records a wealth of information on processes occurring from the mantle to crustal magma chambers. The incompatible element contents of some mid-ocean ridge basalt (MORB) sample suites show progressive enrichment beyond the predictions of simple models of fractional crystallization of a single primary melt. Explanations for this over-enrichment have focused on the differentiation processes in crustal magma chambers. Here we consider an additional mechanism and focus instead on the deviation from simple fractionation trends that is possible by mixing of diverse mantle-derived melts supplied to magma chambers. A primary observation motivating this strategy is that there is significant chemical diversity in primitive high-MgO basalts, which single liquid parent models cannot match. Models were developed to simulate the compositional effects of concurrent mixing and crystallization (CMC): diverse parental melts were allowed to mix, with a likelihood that is proportional to the extent of fractional crystallization. Using a simple statistical model to explore the effects of concurrent mixing and crystallization on apparent liquid lines of descent, we show how significant departure from Rayleigh fractionation is possible as a function of the diversity of trace elements in the incoming melts, their primary MgO content, and the relative proportion of enriched to depleted melts. The model was used to make predictions of gradients of trace element enrichment in log[trace element]–MgO space. These predictions were compared with observations from a compilation of global MORB and provide a test of the applicability of CMC to natural systems. We find that by considering the trace element variability of primitive MORB, their MgO contents and degree of enrichment, CMC accurately predicts the pattern of trace element over-enrichment seen in global MORB. Importantly, this model shows that the relationship between over-enrichment and incompatibility can result from mantle processes: the fact that during mantle melting maximum variability is generated in those elements with the smallest bulk Kd. Magma chamber processes are therefore filtering the signal of mantle-derived chemical diversity to produce trace element over-enrichment during differentiation. Finally, we interrogate the global MORB dataset for evidence that trace element over-enrichment varies as a function of melt supply. There is no correlation between over-enrichment and melt supply in the global dataset. Trace element over-enrichment occurs at slow-spreading ridges where extensive steady-state axial magma chambers, the most likely environment for repeated episodes of replenishment, tapping and crystallization, are very rarely detected. This supports a model whereby trace element over-enrichment is an inevitable consequence of chemically heterogeneous melts delivered from the mantle, a process that may operate across all rates of melt supply.

Sr-Nd-Pb isotopic and major and trace element compositions of the Yufu-Tsurumi volcanic rocks: implications for the magma genesis of the Yufu-Tsurumi volcanoes, northeast Kyushu, Japan

Abstract: We have characterized the Sr-Nd-Pb isotopes and major and trace element compositions of 10 Quaternary volcanic rocks from the Yufu and Tsurumi volcanoes in northeast Kyushu, Japan. The enriched incompatible elements, negative Nb, and positive Pb and Sr anomalies are generally interpreted to be from island arc affinities of the lavas. However, the LREE/HREE ratio of the lavas (La/Yb = 8.5 ± 1.3) is greater than that from the island arc intermediate volcanics from northeast Japan (La/Yb = 3.8 ± 0.6), suggesting a different origin. One dacite sample had a Sr/Y ratio > 40, and the SiO2, Al2O3, MgO, Y, Yb, Sr, and 87Sr/86Sr compositions fell within the range of typical adakites. Other lavas were classified as normal island arc-type magmas. These results suggest that a partial melting of the subducting Philippine Sea Plate played a role in the genesis of the Yufu-Tsurumi volcanic rocks. The Sr, Nd, and Pb isotopic compositions show evidence of mixing between melts, derived from oceanic basaltic crust and subducted terrigenous sediments for the adakite magma, whereas another source is required to explain the enrichment in Sr-Nd isotopes and depletion in Pb isotopes of the island arc-type magma. Although the island arc-type magma appears to have a different source, a mixing of the adakite magma with this magma is apparent from our observations of the trace elements and isotopes.

A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites

Abstract: Entrainment of antecrystic zircon in early saturated main silicate phases has been recognized as a complicating factor in high-precision U-Pb geochronology of Phanerozoic granitic systems, but has not been demonstrated for Precambrian rocks. We report U-Pb ages (secondary ion mass spectrometry) and trace element (laser ablation–inductively coupled plasma–mass spectrometry) compositions of zircon from three main ferroan granite types of the late Paleoproterozoic Wiborg rapakivi granite batholith of southeastern Finland and vicinity. Zircon was extracted from the interior parts of megacrystic (diameters as large as 15 cm) rapakivi-textured alkali feldspar ovoids using a microdrilling technique. Compared to zircon in ovoid groundmass, zircon in the ovoids has higher incompatible element values (e.g., uranium and rare earth elements) and, for two of the three samples examined, slightly older (by ∼1 m.y.) average U-Pb ages. These observations suggest that the ovoid material of the rapakivi granites entrained zircon from earlier magmas that were compositionally different than the final magmatic host of the ovoids. Groundmass zircon implies similar average U-Pb ages for all the studied samples, ca. 1628 Ma; this is regarded as the final crystallization age of the granites. These observations show that it is viable to measure statistically valid age differences between antecrysts and groundmass of Proterozoic granites. Overall, our sampling technique provides enhanced textural control of antecrystic zircon in a wide range of sample materials.

Petrogenesis of the highly potassic 1.42 Ga Barrel Spring pluton, southeastern California, with implications for mid-Proterozoic magma genesis in the southwestern USA

Abstract: Syenites from the Barrel Spring pluton were emplaced in the Early Proterozoic Mojave crustal provine of southeastern California at 142 Ga All rocks, even the most mafic, are highly enriched in incompatible elements (eg K2O 4–12 wt%, Rb 170–370 ppm, Th 12–120 ppm, La 350–1500xchondrite, La/Ybn 35–100) Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source Trace element models establish two plausible sources for Barrel Spring magmas: (1) LREE enriched garnet websterite with accessory apatite±rutile (enriched lithospheric mantle), and (2) garnet amphibolite or garnet-hornblende granulite with enriched alkali basalt composition, also with accessory apatite±rutile (mafic lower crust) Nd and Pb isotopic ratios do not distinguish a crust vs mantle source, but eliminate local Mojave province crust as the principal one, and indicate that generation of the enriched source occurred several hundred million years before emplacement of the Barrel Spring pluton 140–144 Ga potassic granites are common in southeastern California, suggesting a genetic link between the Barrel Spring pluton and the granites; however, although the same thermal regime was probably responsible for producing both the granitic and syentic magmas, elemental and isotopic compositions preclude a close relationship Isotopic similarity of the Barrel Spring pluton to 140–144 Ga granites emplaced in the Central Arizona crustal province to the east may imply that a common component was present in the lithosphere of these generally distinct regions

Average composition of basic magmas and mantle sources of island arcs and active continental margins estimated from the data on melt inclusions and quenched glasses of rocks

Abstract: Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl, H2O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE) do not correlate with MgO, but correlate with each other and K2O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most compositions are characterized by weak negative correlation between H2O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR). Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas. Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5 kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with positive anomalies of U, Rb, Ba, B, Pb, Cl, H2O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted plate.