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The abundance of mantle-derived rocks and lavas, in combination with its tectonic evolution, render Mexico a perfect laboratory to investigate the chemical and the isotopic heterogeneity of the lithospheric mantle. New data on the composition of noble gases and CO2 in Mexican mantle xenoliths and lavas is reported. Our samples consist of six ultramafic nodules from the Durango Volcanic Field (DVF) and the San Quintin Volcanic Field (SQVF), monogenetic complexes belonging to the Mexican Basin and Range province; and four lavas from the Sierra Chichinautzin (SCN), a Quaternary monogenetic volcanic field located in the Mexican volcanic arc. Ne and Ar isotopes in fluid inclusions reveal mixing between atmospheric and MORB-like fluids (e.g., 40Ar/36Ar < 1,200). DVF and SQVF nodules record low 40Ar/36Ar and 4He/20Ne that confirm the existence of recycled atmospheric-derived noble gases in the local mantle. The averages of the Rc/Ra ratios (3He/4He corrected for atmospheric contamination) measured in Mexican localities are within the MORB-like range: DVF= 8.39 ± 0.24 Ra, SQVF = 7.43 ± 0.19 Ra and SCN lavas = 7.15 ± 0.33 Ra (1σ). With the aim of assessing the isotopic variability of the Mexican lithospheric mantle, the above results were compared with similar data previously obtained from ultramafic nodules found in the Ventura Espiritu Santo Volcanic Field (VESVF), another Quaternary monogenetic volcanic complex belonging the Basin and Range. The higher 3He/4He ratios in DVF relative to those reported for the VESVF and the SQVF are explained as reflecting different ages of mantle refertilization, triggered by the retreating of the Farallon slab (∼40 Ma ago) and associated delamination slab processes. We propose that the DVF mantle was refertilized more recently (

Carbonation of oceanic lithosphere occurs widely during seawater alteration, and subducted carbonated oceanic lithosphere may play a key role in carbon recycling in subduction zones. Here we investigate the Ca isotopic composition of arc lavas from the Tonga rear arc and Mariana arc, western Pacific, to explore the effect of fluid‐mediated carbonate transfer in subduction zones. Fresh basalts (0.84 ± 0.01‰, 2sd, n = 3) and dacites (0.84 ± 0.10‰, 2sd, n = 9) from the Tonga rear arc show indistinguishable δ44/40Ca, reflecting negligible Ca isotopic fractionation during differentiation of hydrous arc magmas. More importantly, arc lavas from both the Tonga rear arc (0.84 ± 0.09‰, 2sd, n = 12) and Mariana arc (0.79 ± 0.12‰, 2sd, n = 9) display MORB‐like δ44/40Ca values. The MORB‐like δ44/40Ca of arc lavas indicates that the carbonates released from altered oceanic lithosphere (AOL) do not significantly modify the Ca isotopic composition of the mantle wedge, although extensive volcanic CO2 degassing at both arcs suggests that slab fluids might introduce abundant carbonate into the depleted mantle wedge. These results could be attributed to a limited Ca budget in the slab fluids added to the mantle wedge and/or homogenization effect of variable δ44/40Ca for the slab fluids. At cold subduction zones, a fraction of carbonates from the AOL may survive during slab dehydration and recycle into the deep mantle. Key Points: Negligible Ca isotopic variation occurs during hydrous arc magma differentiation, regardless of the specific water contentsIncorporation of slab components has limited effects on Ca isotopes of arc magmasSlab fluid‐mediated carbonate transfer from altered oceanic lithosphere is insufficient to change the mean δ44/40Ca of the mantle wedge [ABSTRACT FROM AUTHOR]

New U-Th-Ra, major and trace element, and Sr-Nd-Pb isotope data are presented for young lavas from the New Britain and Western Bismarck arcs in Papua New Guinea. New Britain is an oceanic arc, whereas the latter is the site of an arc-continent collision. Building on a recent study of the Manus Basin, contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc-continent collision and possible slab detachment. All three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle. However, the Western Bismarck arc lavas diverge from the Pb isotope mixing array formed by the New Britain and the Manus Basin lavas toward elevated Pb/Pb. We interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision. U and Ra excesses are preserved in all of the lavas and are greatest in the Western Bismarck arc. High-Mg andesites with high Sr/Y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge. Data for two historical rhyolites are also presented. Although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions, both formed from mafic parents within millennia. [ABSTRACT FROM AUTHOR]

Geochemical proxies, especially the trace element ratios of arc lavas from Barren and Narcondam Islands of the Andaman-Nicobar Islands group, display dissimilar characteristics. Narcondam lavas (mostly andesitic) are typically characterized by high Ba/La, Ba/Nb and Th/Nd ratios, reflective of the imprint of substantial subduction component in the form of sediment fluid and melt. On the other hand, Barren lavas (mostly basaltic) show relatively high Ba/Th ratios, indicative of fluid-induced subduction component, mainly signature-inducing fluid component, derived from altered ocean crust. [ABSTRACT FROM AUTHOR]

Examination of an extensive major and trace element database for about 700 whole rocks from the Ecuadorian Andes reveals series of local trends typified by three volcanoes: Iliniza and Pichincha from the Western Cordillera and Tungurahua from the Eastern Cordillera. These local trends are included in a more scattered global trend that reflects typical across-arc chemical variations. The scatter of the global trend is attributed to greater crustal contributions or decreasing melt fractions. Trace element modelling shows that the local trends are consistent with mixing, and not with any fractional crystallization or progressive melting dominated processes. These local trends are extendable to include samples from other Ecuadorian volcanoes, suggesting that mixing processes are dominant throughout the region. Mixing model using trace and major element analyses identifies two end-members: low-silica, basaltic and high-silica, dacitic magmas. It also shows that mixing occurred between magmas after their segregation, rather than earlier mixing between the solid sources prior to melting. As a consequence, there must exist efficient magma-mixing processes that can overcome the obstacles to mixing magmas with contrasting physical properties, and can produce series of hybrid liquids over regional-scale. Model calculations show that estimated silicic end-members are primary magmas and are not co-magmatic derivatives of the corresponding mafic end-members. Lavas of Ecuadorian volcanoes are likely originated from magmas of contrasting origins, such as basaltic magmas generated by fluxed melting of peridotites in the mantle wedge and dacitic, adakite-type magmas originating from the slab or the mafic lower crust. [ABSTRACT FROM AUTHOR]

Abstract: The Pacific Plate is currently being subducted at a rapid rate beneath the Indo-Australian Plate along the Tonga–Kermadec Trench in the southwestern Pacific. It has long been assumed that the lithosphere being subducted is relatively old and homogeneous in composition. Basaltic lavas dredged from the upper crust of the incoming lithosphere along the length of the trench are mid- to late-Cretaceous in age. Although the samples are mainly N-MORB, they range from tholeiitic to alkalic basalts. Concentrations of incompatible trace elements show a high degree of variability (e.g., Ba=8 to 270 ppm, Rb=0.4 to 39 ppm, Sr=60 to 625 ppm, and Nd=2.3 to 25 ppm). Neodymium, Sr and Pb isotopic data also show wide ranges (εNd(T) =5.7–11.2; 87Sr/86Sri =0.70224–0.70311; 206Pb/204Pbi =17.84–20.22). More importantly, the basaltic crust being subducted displays a latitudinal compositional variation that is similar to that shown by the Tonga–Kermadec arc lavas. Previous studies have proposed that the variably depleted sub-arc mantle, which was preconditioned through a backarc melt extraction or displacement process, is mainly responsible for the latitudinal variation in the Tonga–Kermadec arc lavas. However, our new results suggest a greater role of the lithospheric input into the source of arc lavas. The three end-member possibilities linking the latitudinal variation of the lithospheric input to the source of arc lava output are: (1) the mantle wedge beneath the volcanic arc on the west side of the trench, the main source of Tonga–Kermadec arc lavas, is a western extension of the Cretaceous Pacific upper mantle east of the trench; (2) the altered oceanic crust melts and the resultant slab melt modifies the mantle source of arc lavas; and (3) fluids dehydrated from the altered oceanic crust effectively transfer the compositional signature of the subducted slab into the mantle source of arc lavas. [Copyright &y& Elsevier]

Pliocene volcanics on the island of Bequia comprise two interbedded suites of basalts and andesites. The isotopically homogeneous suite (IHS) has a limited range of Sr—Nd—Pb isotopes (87Sr/86Sr 0.7040−0.7046, 143 Nd/144 Nd ∼0.5130 and 206 Pb/204Pb 19.36−19.51), and mantle-like δ18O values (∼5.5 in clinopyroxene). The isotopically diverse suite (IDS) is characterized by much wider ranges of radiogenic isotopes (87 Sr/86Sr 0.7048−0.7077, 143 Nd/144 Nd 0.5128−0.5123 and 206 Pb/204 Pb 19.7−20.2), in which all of the Sr and Pb ratios are higher and Nd ratios are lower than those of the IHS. The IDS is also characterized by high δ18 O values, up to 7.6 in clinopyroxene. The Sr and Pb isotope ratios are too high, and the Nd isotope ratios are too low in the IDS for any of these lavas to be derived from unmodified depleted mantle.Both suites are petrologically very similar and their major element compositions and phenocryst contents suggest that they were formed largely by fractional crystallization of a hydrous tholeiitic melt at pressures <3 kbar. The isotopic ratios and enrichments in large ion lithophile elements (LILE), and to some extent light rare earth elements (LREE), as compared with mid-ocean ridge basalts (MORB), of the IHS lavas suggest that they were derived from a depleted mantle source which had been re-enriched by the addition of 1–4% of a subduction component. This component probably comprised a mixture of dehydration fluids, and perhaps minor siliceous melts, released from subducting sediments and mafic crust. The extreme isotopic ranges, large enrichments in incompatible elements, more fractionated LREE patterns and higher δ18 O values of the IDS lavas are interpreted as resulting from ∼ 10–55% assimilation—fractional crystallization of sediments, derived from the Guyana Shield, which are present in the arc crust, by IHS type melts. [ABSTRACT FROM AUTHOR]