A preliminary assessment of the symmetry of source composition and melting dynamics across the Azores plume
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In order to undertake a preliminary assessment of the extent of symmetry in source composition and melting dynamics in the Azores plume we present new ICP‐MS trace element data along with Sr, Nd, Pb, and U‐Th‐Ra isotope data for samples from the islands Flores and Corvo west of the Mid‐Atlantic Ridge. We also present data from a picrite from Faial and new ICP‐MS trace element data for 28 basaltic lavas from the eastern Azores Plateau to augment data published previously from these samples. Rare earth element data for primitive lavas (MgO ≥ 5% and Mg # ≥ 60) have La/Yb N ∼ 10 and variable Ce and/or Eu anomalies. Multi‐incompatible trace element patterns normalized to primitive mantle are convex upward with small negative Th and K ± Pb anomalies. While lavas to the east are characterized by low Nb/Zr and generally lower La/Yb ratios (with the notable exception of eastern São Miguel), lavas from the western islands have slightly higher Nb/Zr and La/Yb inferred to reflect smaller degrees of partial melting. The Sr‐Nd‐Pb isotope systematics imply that Corvo and Flores sample components which range from an isotopic source commonly found in the Azores (e.g., at the eastern island of Graciosa) to a more depleted, MORB‐like mantle sampled at the Mid‐Atlantic Ridge. However, in common with uncontaminated samples from São Miguel, the Corvo and Flores samples appear to have slightly lower 230 Th excesses and higher La/Yb, Tb/Yb than the other Azores islands or the Mid‐Atlantic Ridge samples. The trace element and isotope data indicate a relatively symmetric pattern with distance across the MAR, while U‐Th disequilibria, and thus inferred melting dynamics, appear less symmetric. Nevertheless, the data suggest that heterogeneities in source composition do not have a large effect on melting dynamics, at least within the Azores islands.Keywords:
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Mid-Atlantic Ridge
Radiogenic nuclide
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Fractional crystallization (geology)
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Kangqiong serpentinites, exposed in the central-western part of Bangong-Nujiang suture zone, constitute the ultrabasic end member of Kangqiong ophiolitic melange. Serpentinites in group A have relatively high Al2O3(4.23%) and Ca O(3.48%) contents, relatively depleted LREEs, their trace element normalized partition curves are similar to those of the global abyssal peridotites,whereas serpentinites in group B have lower Al2O3(0.61%) and Ca O(0.27%) values and slightly more enriched LREEs with negative Eu and Ce anomalies, and their trace element normalized partition curves resemble those of Izu-Bonin-Mariana forearc mantle peridotites, with both of them enriched in LILE(Rb, Cs) and HFSE(U, Nb). Estimation reveals that protoliths of group A were the mantle residue from partial melting of asthenospheric mantle, and protoliths of group B underwent higher-degree partial melting, indicating that they at least have experienced two stages of partial melting. It is preliminarily held that protoliths of group A were the mantle residue from partial melting of asthenospheric mantle in the forearc environment, characterized by depletion of LREE but enrichment of U and Nb because of MOR-like melt-rock interaction, and that protoliths of group B resulted from partial re-melting of group A in the intra-ocean subduction zone and became more reluctant to melting and more enriched in LREE and HFSE(U, Nb) because of boninitic melt-rock interaction, thus belonging to forearc mantle peridotites. Both protoliths of group A and group B transformed into serpentinites resulting from serpentinization, and eventually remained in the suture zone while the ancient ocean collided and closed. They are thus the rock records of intra-ocean subduction of the Bangong-Nujiang Neo-Tethys Ocean.
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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/Al 2 O 3 ratios of 0.57–0.87, Na 2 O contents of 1.84–3.49 wt %, (La/Yb) cn ratios of 0.40–1.39, 87 Sr/ 86 Sr ratios of 0.70270–0.70403, and 206 Pb/ 204 Pb 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.
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Amphibole
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Two voluminous magma types generated in the mantle underlying the Pacific plate are mid‐ocean ridge tholeiite (MORB) erupted at the East Pacific Rise spreading center and Hawaiian tholeiite (HT) erupted above the Hawaiian hot spot or melting anomaly. MORB has low initial 87 Sr/ 86 Sr ratios and low amounts of all incompatible trace elements including rare earths; chondrite‐normalized patterns are depleted in light rare earths. HT, by contrast, has higher initial 87 Sr/ 86 Sr and higher amounts of incompatible trace elements; chondrite‐nor‐malized patterns are enriched in the middle and light rare earths. HT is generally poorer in CaO and Al 2 O 3 and much richer in total iron and TiO 2 compared with MORB having the same MgO content. Primary magma compositions for the two volcanic systems are calculated in Fe‐Mg equilibrium with residual olivine (Fo 92 ). MORB is generated by partial melting of a trace element depleted Iherzolite source leaving a residual assemblage dominated by olivine and orthopyroxene. The percentage of partial melting for a primary magma containing 15% MgO is calculated to be 35–42% in a source mantle having a heavy rare earth content of 3×chondrite and 33–35% MgO. HT, represented by Kilauea tholeiite, is generated by partial melting of a mixture of unmelted and residual mantle for MORB which has been modified by metasomatic addition of a nephelinitic fluid, amphibole, and minor amounts of apatite and Fe‐bearing phases such as sulfide and magnetite/ilmenite. This model yields a picritic magma in equilbrium with magnesian dunite at high (>40%) degrees of partial melting. The source also has 35% MgO before partial melting. Melting in both systems in polyvariant and not controlled by lower‐temperature invariant equilibria. The low‐velocity zone is considered to be the source of metasomatic fluids that are driven upward into the lowermost lithosphere in response to a thermal plume. Picritic primary magmas are produced by shear melting, localized in the zone of thinned and metasomatized lithosphere beneath Hawaii. Melt extraction is rapid and episodic at intervals of months to decades; magma is not stored in the mantle but passes upward to a plexus of storage reservoirs located 2–6 km beneath the surface of Kilauea. Kilauea primary magmas fractionate olivine during upward transport to reach bulk compositions of 13–14% MgO in storage. Different magma batches erupted to the surface, distinguished by different major and minor element compositons compared at similar MgO content, represent combinations of differing degrees of metasomatic enrichment, differing degrees of partial melting, and some effects of premelting mantle heterogeneity.
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