Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated ²³⁸U/²⁰⁴Pb, ²³²Th/²³⁸U and ²³²Th/²⁰⁴Pb in continental mantle roots and (c) the ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM).Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU.U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between ²⁰⁶Pb/²⁰⁴Pb = 19.93-20.25, ²⁰⁷Pb/²⁰⁴Pb = 15.63-15.66 and ²⁰⁸Pb/²⁰⁴Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle.The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental mantle root. Instead, the enrichment is thought to be associated with the Quaternary intra-plate volcanism in the Middle Atlas. However, the erupted mafic melts have unradiogenic Pb isotopes and lower ²³⁸U/²⁰⁴Pb, ²³²Th/²⁰⁴Pb and ²³²Th/²³⁸U relative to the clinopyroxene and do not seem to have equilibrated with the clinopyroxenes. The high Th abundances and the high ²³²Th/²³⁸U also suggest that the metasomatism was due to carbonatitic liquids.When literature data for Pb isotopes in mantle minerals are considered, the Pb isotope range of Archean, Proterozoic and Phanerozoic continental mantle roots is remarkable in that they are similar to the convecting mantle. This observation does not support the existence of sub-continental lithospheric mantle with high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb for long periods of time. Consequently, the narrow range of Pb isotopes in SCLM worldwide suggests that only the youngest metasomatic events are recorded by incompatible elements such as U, Th and Pb. Numerical modelling of putative magmas generated from Middle Atlas SCLM by fractional, non-modal melting calculations yield extremely high ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb ratios. For example, pure SCLM magmas generated from 0.5% to 10% melting are anticipated to have ²³²Th/²⁰⁴Pb ratios exceeding those known from terrestrial basalts.     

The U, Th and Pb elemental and isotope compositions of mantle clinopyroxenes and their grain boundary contamination derived from leaching and digestion experiments

Minerals from peridotites are known to be affected by trace element contamination on their grain boundaries. In this contribution we investigate the extent and origin of exogenous contamination associated with mantle clinopyroxenes from various localities (Middle Atlas, Beni Bousera [Morocco], Pyrenean Massif, Massif Central [France]) and test the efficacy of different leaching methodologies used to remove this contamination. In doing so we present new U-Th-Pb (double-spike) isotope and trace elemental data of clinopyroxenes and their leachates from spinel-facies sub-continental lithospheric mantle (SCLM, n = 18).Sequential leaching and dissolution of one clinopyroxene separate shows that multiple and short leaching attacks with dilute HCl at moderate temperatures (e.g., 120 °C) interspersed with rigorous ultra-pure water washes do not induce elemental fractionation and are sufficient to remove grain-boundary contamination. Short attacks with very dilute mixtures of HF and HCl induced strong parent/daughter elemental fractionation and significant elemental loss prior to the clinopyroxene digestion with HF/HNO₃. Such leaching is not suitable for studies investigating parent/daughter elemental or isotope ratios of mantle clinopyroxenes. Fluoride co-precipitates that formed in the presence of moderately dilute HF, used during leaching and typical HF/HNO₃ digestions, are an important sink for all trace elements studied here and lock up at least 60% of the trace elements considered.A suite of clinopyroxene-leachate pairs from Moroccan peridotites confirm extreme grain-boundary contamination and show that up to 65% of U, 82% of Th and 91% of Pb, respectively, are of exogenous origin. Pb isotopes of all leachates considered and nearly all reconstructed unleached clinopyroxenes have highly positive Δ7/4 and ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb systematics reminiscent of enriched mantle (particularly EM II), whereas the corresponding extensively leached clinopyroxenes have very different Pb isotope systematics (verified by replicate digestions). Furthermore, the leachates are often marked by remarkably high ²³²Th/²³⁸U (leachate ? bulk silicate Earth) in addition to ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb that are seemingly consistent with depleted mantle (DM) and high ²³⁸U/²⁰⁴Pb (HIMU) mantle mixing. After leaching, clinopyroxenes may have highly elevated ²³⁸U/²⁰⁴Pb and ²³²Th/²⁰⁴Pb that are distinct from typical DM- and HIMU mantle, whereas corresponding leachates bear strong resemblance to anthropogenic Pb recovered from rainwater, snow and aerosols.In the light of the great potential of Pb isotope contamination associated with mantle clinopyroxene we have compiled literature data of mantle peridotites. These data seem to suggest that the SCLM is nearly devoid of HIMU signatures but ostensibly represents an important repository for enriched mantle signatures. However, the dominance of enriched mantle is not verified when ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios for the same database are considered. We highlight that the scarcity of HIMU-like SCLM and the alleged dominance of EM in continental roots may be reconciled if the current Pb isotope database of SCLM contains samples that were insufficiently leached prior to digestion and thus are contaminated by anthropogenic Pb. We stress the importance of replicate digestions, adequate leaching and leachate analysis to verify the U-Th-Pb elemental and isotope systematics of any given mantle (-derived) sample.     

Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from U-Th-Ra and Sr-Nd-Pb isotope systematics

Precise measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr-Nd-Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess ²³⁸U-²³⁰Th-²²⁶Ra disequilibria with ²³⁰Th (18-24%) and ²²⁶Ra (9-21%) excesses, and there exists a positive correlation in a (²²⁶Ra/²³⁰Th)-(²³⁰Th/²³⁸U) diagram. The extent of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria is markedly different in lavas of individual eruption ages, although the (²³⁰Th/²³²Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (²³⁰Th/²³⁸U)-(²⁰⁶Pb/²⁰⁴Pb) and (²²⁶Ra/²³⁰Th)-(²⁰⁶Pb/²⁰⁴Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D_U and D_U/D_Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb > 20.47) due to late Mesozoic metasomatism.     

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria, Sr-Nd-Pb-Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr-Nd-Pb-Hf isotopes (⁸⁷Sr/⁸⁶Sr = 0.70319-0.70322; ¹⁴³Nd/¹⁴⁴Nd = 0.51302-0.51305; ²⁰⁶Pb/²⁰⁴Pb = 19.04-19.06; ²⁰⁷Pb/²⁰⁴Pb = 15.53-15.54; ²⁰⁸Pb/²⁰⁴Pb = 38.61-38.65; ¹⁷⁶Hf/¹⁷⁷Hf = 0.28311-0.28312). All the samples possess (²³⁰Th/²³⁸U) disequilibrium with ²³⁰Th excesses, and they show systematic variations in (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) ratios. The highest ²²⁶Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have (²²⁶Ra/²³⁰Th) ratio that are close to equilibrium. The ²³⁸U-²³⁰Th-²²⁶Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of ²³⁰Th and ²²⁶Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent-daughter nuclides in ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.     

Cretaceous seamounts along the continent-ocean transition of the Iberian margin: U-Pb ages and Pb-Sr-Hf isotopes

To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the “Tore Madeira” cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for ²⁰⁶Pb/²⁰⁴Pb, 15.544-15.828 for ²⁰⁷Pb/²⁰⁴Pb, 38.750-39.936 for ²⁰⁸Pb/²⁰⁴Pb, 0.70231-0.70340 for ⁸⁷Sr/⁸⁶Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high ²⁰⁷Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time, including an about 30° anti-clockwise rotation of Iberia.     

Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

We report new Os-Pb-Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). ¹⁸⁷Os/¹⁸⁸Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260-0.1451; 58.9-168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6-15.1 ppt Os). Radiogenic Os is associated with high ε_Hf and ε_Nd, low ⁸⁷Sr/⁸⁶Sr and does not correlate with Mg^∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic ¹⁸⁷Os/¹⁸⁸Os (0.4344, 10.3 ppt Os), lower ε_Hf and ε_Nd, higher ⁸⁷Sr/⁸⁶Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ε_Hf: +8.9 to +5.0) and Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb: 19.10-19.61; ²⁰⁷Pb/²⁰⁴Pb: 15.56-15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse ¹⁸⁷Os/¹⁸⁸Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3-2.0 ppt). Low-¹⁸⁷Os/¹⁸⁸Os tholeiites show Pb-Hf isotope ratios (²⁰⁶Pb/²⁰⁴Pb:18.81; ²⁰⁷Pb/²⁰⁴Pb: 15.61; ε_Hf: +2.7) that are distinct from those in alkaline lavas with similar ¹⁸⁷Os/¹⁸⁸Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low ²⁰⁶Pb/²⁰⁴Pb and ε_Hf in the other tholeiites probably reflects crustal contamination.The association at Vogelsberg of primitive alkaline and tholeiitic lavas with a range of MORB- to OIB-like Os-Pb-Hf-Nd-Sr isotopic characteristics requires at least two asthenospheric magma sources. This is consistent with trace element modelling which suggests that the alkaline and tholeiitic parent magmas represent mixtures of melts from garnet and spinel peridotite sources (both with amphibole), implying an origin of the magmas in the garnet peridotite-spinel peridotite transition zone, probably at the asthenosphere-lithosphere interface. We propose that uncontaminated Vogelsberg lavas originated in ‘metasomatized’ mantle, involving a 3-stage model: (1) early carbonatite metasomatism several 10-100 Ma before the melting event (2) deposition of low-degree asthenospheric melts from carbonated peridotite at the lithosphere-asthenosphere thermal boundary produces hydrous amphibole-bearing veins or patches, and (3) remobilization of this modified lithospheric mantle into other asthenospheric melts passing through the same area later. In keeping with ‘metasomatized’ mantle models for other continental basalt provinces, we envisage that stage (2) is short-lived (few Ma), thus producing a prominent lithospheric trace element signature without changing the asthenospheric isotopic signatures. Models of this type can explain the peculiar mix of lithospheric (prominent depletions of Rb and K) and asthenospheric (OIB-like high ¹⁸⁷Os/¹⁸⁸Os, ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf) signatures observed in the Vogelsberg and many other continental basalt suites.     

Post-collisional Plio-Pleistocene shoshonitic volcanism in the western Kunlun Mountains, NW China: Geochemical constraints on mantle source characteristics and petrogenesis

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO₂, Al₂O₃ and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low ε_Nd (−1.8 to −8.7) at more restricted ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of ²⁰⁷Pb/²⁰⁴Pb (15.48–15.74) and ²⁰⁸Pb/²⁰⁴Pb (38.30–39.12) ratios at relatively invariant ²⁰⁶Pb/²⁰⁴Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high ⁸⁷Sr/⁸⁶Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet.     

Nd,Pb, and Sr isotope composition of Late Mesozoic to Quaternary intra-plate magmatism in NE-Africa (Sudan,Egypt): high-μ signatures from the mantle lithosphere

The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-μ (μ = ²³⁸U/²⁰⁴Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within ε Nd ca. 3–6, ²⁰⁶Pb/²⁰⁴Pb ca. 19.5–20.5, ²⁰⁷Pb/²⁰⁴Pb ca. 15.63–15.73, ²⁰⁸Pb/²⁰⁴Pb ca. 39–40 and ⁸⁷Sr/⁸⁶Sr ca. 0.7028–0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa–Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa; high-μ lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia; moderate μ lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd–Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

华北克拉通西部陆块北缘新生代岩石圈地幔特征的初步研究

凉城、四子王旗、三义堂和大同的地幔包体的岩石学和矿物的主、微量元素成分显示华北克拉通中、西部北缘总体为过渡型岩石圈地幔,为原始地幔经过不同程度的熔体抽取和后期交代富集作用的残留.凉城岩石圈地幔经历的熔体抽取程度最低,后期交代富集作用比较强烈,这与其橄榄岩中尖晶石的Cr#较低,并且共存的单斜辉石的Mg#较低一致.三义堂岩石圈地幔经历的熔体抽取程度最高,后期交代富集作用最强烈,不同样品甚至同一样品中不同单斜辉石颗粒的La/Yb和LREE配分模式变化范围非常大,且矿物边部比核部更富集微量元素.大同、四子王旗岩石圈地幔经历的熔体抽取程度介于凉城和三义堂之间,但后期的交代富集作用明显不如凉城和三义堂强烈.研究区的岩石圈地幔主要受硅酸盐熔体交代作用的影响,只有三义堂岩石圈地幔还可能也受到了碳酸盐交代作用的影响.     

A 5 million year record of compositional variations in mantle sources to magmatism on Santiago, southern Cape Verde archipelago

High-precision Pb isotope data and Sr–Nd–Hf isotope data are presented together with major and trace element data for samples spanning the 4.6 Ma history of volcanism at Santiago, in the southern Cape Verde islands. Pb isotope data confirm the positive Δ8/4 signature of the southern islands indicating that the north–south compositional heterogeneity in the Cape Verde archipelago has persisted for at least 4.6 Ma. The Santiago volcanics show distinct compositional differences between the old, intermediate and young volcanics, and suggest greater involvement of an enriched mantle (EM1)-like source over time. Isotopic variations in the Santiago volcanics indicate convergence towards a homogeneous EM1-like end-member and distinct temporal variations in the FOZO-like end-member. Santiago and Santo Antão (a northern island, Holm et al. 2006), show a simultaneous decrease in ²⁰⁸Pb/²⁰⁴Pb of the high ²⁰⁶Pb/²⁰⁴Pb FOZO-like source with time. Such systematic archipelago-wide variations in the FOZO-like component suggest that this component is more likely to be present as a coherent package of recycled ocean crust rather than as multiple small heterogeneities dispersed in the upwelling mantle. The temporal variations in ²⁰⁸Pb/²⁰⁴Pb reflect minor lateral variations in Th/U of this recycled ocean crust package entering the melting zone beneath the islands. The location of the EM1-like component is more equivocal. A shallow lithospheric location is possible, but this would require a coincidence between spatial compositional variations in the lithosphere (EM1 is spatially restricted to the southern islands) and flow lines in the upwelling mantle revealed by seismic anisotropy. Therefore, we favour a deeper asthenospheric mantle source for the EM1-like source.     

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using U, Th, and Pa excesses

To examine the petrogenesis and sources of basalts from the Kolbeinsey Ridge, one of the shallowest locations along the global ridge system, we present new measurements of Nd, Sr, Hf, and Pb isotopes and U-series disequilibria on 32 axial basalts. Young Kolbeinsey basalts (full-spreading rate = 1.8 cm/yr; 67°05′-70°26′N) display (²³⁰Th/²³⁸U) < 1 and (²³⁰Th/²³⁸U) > 1 with (²³⁰Th/²³⁸U) from 0.95 to 1.30 and have low U (11.3-65.6 ppb) and Th (33.0 ppb-2.40 ppm) concentrations. Except for characteristic isotopic enrichment near the Jan Mayen region, the otherwise depleted Kolbeinsey basalts (e.g. ⁸⁷Sr/⁸⁶Sr = 0.70272-0.70301, ε_Nd = 8.4-10.5, ε_Hf = 15.4-19.6 (La/Yb)_N = 0.28-0.84) encompass a narrow range of (²³⁰Th/²³²Th) (1.20-1.32) over a large range in (²³⁸U/²³²Th) (0.94-1.32), producing a horizontal array on a (²³⁰Th/²³²Th) vs. (²³⁸U/²³²Th) diagram and a large variation in (²³⁰Th/²³⁸U). However, the (²³⁰Th/²³⁸U) of the Kolbeinsey Ridge basalts (0.96-1.30) are inversely correlated with (²³⁴U/²³⁸U) (1.001-1.031). Samples with low (²³⁰Th/²³⁸U) and elevated (²³⁴U/²³⁸U) reflect alteration by seawater or seawater-derived materials. The unaltered Kolbeinsey lavas with equilibrium ²³⁴U/²³⁸U have high (²³⁰Th/²³⁸U) values (?1.2), which are consistent with melting in the presence of garnet. This is in keeping with the thick crust and anomalously shallow axial depth for the Kolbeinsey Ridge, which is thought to be the product of large degrees of melting in a long melt column. A time-dependent, dynamic melting scenario involving a long, slowly upwelling melting column that initiates well within the garnet peridotite stability zone can, in general, reproduce the (²³⁰Th/²³⁸U) and (²³¹Pa/²³⁵U) ratios in uncontaminated Kolbeinsey lavas, but low (²³¹Pa/²³⁵U) ratios in Eggvin Bank samples suggest eclogite involvement in the source for that ridge segment.     

Isotopic (Sr, Nd, Pb) characteristics of the archean metasomatized mantle—source of the Panozero massif, Karelia

This work addresses the isotopic heterogeneity of the Archean lithospheric mantle by the example of high-Mg, moderate-alkali, LILE and LREE-enriched Panozero sanukitoid massif, which is located in the Central Karelia, southeastern Baltic Shield. Initial Nd and Sr isotope characteristics were determined for mantle source of the massif. ?_Nd(T) in the source are plotted below DM line, varying from + 0.7 to + 1.4 (+1.1, on average). The ⁸⁷Sr/⁸⁶Sr(T) ～ 0.7017 significantly exceeds that of depleted mantle and reflects change of Rb/Sr ratio in the source due to metasomatic reworking of mantle. Data on Pb isotope composition of potassium feldspars indicate that source of monzonites of the Panozero Massif had lower μ (²³⁸U/²⁰⁴Pb) = 8 and higher (²³⁸Th/²³²U) = 4.05 relative to primitive mantle.     

Variable initial Pb isotopic compositions of rocks associated with the UG2 chromitite, eastern Bushveld Complex

An investigation of the Pb isotopic compositions of plagioclase and sulfide in a stratigraphic interval including the UG2 chromitite of the eastern Bushveld Complex has been conducted to determine the Pb isotopic composition(s) of the magma(s) that crystallized to form this part of the intrusion, gain a better understanding of why coexisting plagioclase and sulfide commonly exhibit widely different Pb isotopic compositions, and explore the use of Pb isotopes in deducing post-accumulation history. Analyses were obtained in situ with a NuPlasma multicollector ICP-MS coupled with 193 nm Excimer or 213 nm lasers.Most plagioclase compositions fall on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb geochron of 2.06 Ga, which is the solidification age of the intrusion. The measured ratios have not been affected by radiogenic ingrowth, and plagioclase generally remained closed to Pb exchange after initial cooling. The array of plagioclase compositions on the geochron is significantly larger than that defined by analytical error. This indicates that in terms of Pb at least two different magma compositions were present. The composition of the least radiogenic magma was approximated by that of the contemporaneous BSE with μ (²³⁸U/²⁰⁴Pb) and ω (²³²Th/²⁰⁴Pb) values of ≈9.0 and 35, respectively, suggesting a mantle derivation with little or no involvement of the continental crust, while the second magma possessed a Pb isotopic composition similar to the upper crust with μ ≈ 9.6.Compared to plagioclase, sulfides generally possess slightly higher ²⁰⁶Pb/²⁰⁴Pb ratios for equivalent ²⁰⁷Pb/²⁰⁴Pb ratios such that their compositions fall between the 2.06 and 1.86 Ga geochrons. The latter age is much younger than the cooling age. The data are interpreted to mean that the Bushveld Complex remained buried in the crust at temperatures of several hundred °C for about 200 Ma after solidification, and that any sulfides accessible to fluid continued to re-equilibrated during this time with more radiogenic Pb. The sulfide Pb may have been transported into the Bushveld Complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable. Alternatively, the sulfide Pb may have originated mainly from radiogenic decay of U and Th present in minerals other than the sulfides in the immediately surrounding Bushveld rocks, followed by local redistribution of Pb by whatever fluid was present. Indeed, some sulfides are characterized by ²⁰⁸Pb/²⁰⁴Pb ratios sufficiently high that an external source is unlikely. This observation and the fact that the sulfides display small-scale heterogeneity suggest that most, if not all, of the radiogenic sulfide Pb was locally derived. It also implies that during the post-solidification, re-equilibration period there was no large-scale fluid-flow through the microfracture network because otherwise the isotopic heterogeneities would not have been preserved. The minerals in 2 of the 19 samples studied contain young Pb, presumably introduced by meteoric waters that permeated the macrofracture network.     

Intraplate volcanism in New Zealand: the role of fossil plume material and variable lithospheric properties

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (²⁰⁶Pb/²⁰⁴Pb: 20.57, ²⁰⁷Pb/²⁰⁴Pb: 15.77, ⁸⁷Sr/⁸⁶Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (²⁰⁶Pb/²⁰⁴Pb: 19.01, ²⁰⁷Pb/²⁰⁴Pb: 15.62, ⁸⁷Sr/⁸⁶Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (²⁰⁶Pb/²⁰⁴Pb: 18.99, ²⁰⁷Pb/²⁰⁴Pb: 15.67, ⁸⁷Sr/⁸⁶Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic ²⁰⁶Pb/²⁰⁴Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pb-Sr-Nd isotopic compositions and trace element geochemistry of megacrysts and melilitites from the Tertiary Urach volcanic field: source composition of small volume melts under SW Germany

 The Urach volcanic field is unique within the Tertiary–Quaternary European volcanic province (EVP) due to more than 350 tuffaceous diatremes and only sixteen localities with extremely undersaturated olivine melilitite. We report representative Pb-Sr-Nd isotopic compositions and incompatible trace element data for twenty-two pristine augite, Cr-diopside, hornblende, and phlogopite megacryst samples from the diatremes, and seven melilitite whole rocks. The Pb isotopic compositions for melilitites and comagmatic megacrysts have very radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios of 19.4 to 19.9 and plot on the northern hemisphere mantle reference line (NHRL). The data indicate absence of an old crustal component as reflected in the high ²⁰⁷Pb/²⁰⁴Pb ratios of many basalts from the EVP. This inference is supported by ²⁰⁶Pb/²⁰⁴Pb ratios of ∼17.6 to 18.3 and ɛ_Nd of ∼−7.8 to +1.6 for five phlogopite xenocryst samples reflecting a distinct and variably rejuvenated lower Hercynian basement. The ⁸⁷Sr/⁸⁶Sr ratios of 0.7033 to 0.7035 in the comagmatic megacrysts are low relative to their moderately radiogenic Nd isotopic compositions (ɛ_Nd +2.2 to +5.1) and consistent with a long-term source evolution with a low Rb/Sr ratio and depletion in light rare-earth elements (LREE). The melilitite whole-rock data show a similar range in Nd isotopic ratios as determined for the megacrysts but their Sr isotopic compositions are often much more radiogenic due to surface alteration. The REE patterns and incompatible trace element ratios of the melilitites (e.g. Nb/Th, Nb/U, Sr/Nd, P/Nd, Ba/Th, Zr/Hf) are similar to those in ocean island basalts (OIB); negative anomalies for normalized K and Rb concentrations support a concept of melt evolution in the lithospheric mantle. Highly variable Ce/Pb ratios of 29 to 66 are positively correlated with La/Lu, La/K₂O, and Ba/Nd and interpreted to reflect melting in the presence of residual amphibole and phlogopite. The data suggest an origin of the melilitites from a chemical boundary layer very recently enriched by melts from old OIB sources. We suggest that the OIB-like mantle domains represent low-temperature melting heterogeneities in an upwelling asthenosphere under western Europe. Received: 9 March 1995/Accepted: 24 July 1995     

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotopecompositions for clinopyroxenes from anhydrous spinel peridotiteand garnet ± spinel pyroxenite xenoliths of Pan-Africanlithospheric mantle from Jordan, including the first high-precisiondouble-spike Pb isotope measurements of mantle clinopyroxene.Clinopyroxenes from the peridotites are variably Th–U–LILE–LREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREE–HREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 5–10% melt, whereas theenrichments in Th–U–LILE–LREE require a Pan-Africanor later metasomatic event. The large range of Nd, Sr, Pb andHf isotope ratios in both peridotites and pyroxenites (e.g.Nd 1·4–17·5; ²⁰⁶Pb/²⁰⁴Pb 17·2–20·4;Hf 0·6–164·6) encompasses compositionsmore radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopescover almost the entire range of oceanic basalt values. Hf valuesare some of the highest ever recorded in mantle samples andare decoupled from Nd in the same samples. Marked correlationsbetween Sr–Nd–Pb isotopes, LILE–LREE enrichmentsand HFSE depletion suggest that the metasomatizing agent wasa carbonatitic-rich melt and isotopic data suggest that metasomatismmay have been related to Pan-African subduction. The metasomaticmelt permeated depleted upper mantle (<16 kbar) during Pan-Africansubduction at 600–900 Ma, and the variably metasomatizedmaterial was then incorporated into the Arabian lithosphericmantle. There is no evidence for recent metasomatism (<30Ma) related to the Afar plume like that postulated to have affectedsouthern Arabian lithospheric mantle. Hf isotopes in the mantleclinopyroxenes are unaffected by metasomatism, and even somestrongly overprinted lithologies record ancient (>1·2Ga) pre-metasomatic Lu–Hf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The ‘resistance’ of the Lu–Hf isotopic systemto later metasomatic events resulted in the development of extremelyheterogeneous Hf isotopic signatures over time that are decoupledfrom other isotopic systems. No mantle sample in this studyexactly matches the chemical and isotopic signature of the sourceof Jordanian intraplate basalts. However, the xenolith compositionsare broadly similar to those of the source of Arabian intraplatebasalts, suggesting that the numerous Cenozoic intraplate volcanicfields throughout Arabia may be the product of melting uppermantle wedge material fertilized during Pan-African subductionand incorporated into the Arabian lithospheric mantle. We proposea model whereby the proto-Arabian lithospheric mantle underwenta major melting event in early Proterozoic–late Archeantimes (at the earliest at 1·2 Ga). Island-arc volcanismand major crust formation occurred during the Pan-African orogeny,which liberated fluids and possibly small-degree melts thatmigrated through the mantle creating zones of enrichment forcertain elements depending upon their compatibility. Immobileelements, such as Nb, were concentrated near the base of themantle wedge providing the source of the Nb-rich Jordanian volcanicrocks. More mobile elements, such as LILE and LREE, were transportedup through the mantle and fertilized the shallow mantle sourceof the Jordanian xenoliths. Following subduction, the mantlewedge became fossilized and preserved distinct enriched anddepleted zones. Lithospheric rifting in the Miocene triggeredpartial melting of spinel-facies mantle in the lower lithosphere,which mixed with deeper asthenospheric garnet-facies melts asrifting evolved. These melts entrained segments of variablycarbonatite-metasomatized shallow lithospheric mantle en routeto the surface. KEY WORDS: Arabian lithospheric mantle; Jordan; mantle xenoliths; Sr–Nd–Hf–Pb isotopes     

U-Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of ²⁰⁶Pb/²³⁸U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ¹⁸O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ε_Hf (t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between ²⁰⁶Pb/²³⁸U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high-pressure eclogite-facies regime.     

Nd–Hf–Sr–Pb isotopes and trace element geochemistry of Proterozoic lamproites from southern India: Subducted komatiite in the source

The probable sources of some of the famous Indian diamonds are the 1.2 Ga old Krishna lamproites of Southern India, a rare Proterozoic occurrence of lamproites which are usually Cretaceous or younger in age. In this study we report Nd, Sr, Pb and Hf isotopes and multiple trace element concentrations of the Krishna lamproites. The goals are to evaluate mantle-processes and the petrogenesis of these ultrapotassic rocks of extreme chemical composition in light of these geochemical data, including their major element compositions.The Krishna lamproites show nearly uniform, parallel rare earth element (REE) distribution patterns with high concentrations and extreme light-REE enrichment (La/Yb_(N) = 41–88), high average concentrations of Ba (∼ 1200 ppm), Sr (∼ 1200 ppm), Zr (∼ 930 ppm), La (∼ 230 ppm), high U/Pb and Th/U ratios with notable absence of any Eu-anomaly. These rocks are typically porphyritic without any evidence of crystal accumulation, and have moderately high Mg-numbers (59–73) along with high Ni (average ∼ 301 ppm, highest 819 ppm) and Cr (average ∼ 183 ppm, highest 515 ppm) concentrations that show a positive correlation with MgO (wt.%), implying a role of olivine in the melt source. The low SiO₂ content (lowest 37.8%, average 49%) and high Nb (average 147 ppm), Zr, Sr, as well as Ni and Cr in these rocks indicate lack of upper continental crustal contribution in the genesis of these rocks. The initial Pb-isotopic composition of these lamproites is unusual in that in a ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb plot, these rocks plot to the left of the 1.2 Ga geochron (age of emplacement), unlike most mantle-derived rocks. This Pb-isotopic signature and the superchondritic Nb/Ta ratios (average 23.6) of these rocks rule out their derivation from a metasomatized sub-continental lithospheric mantle. The high ²⁰⁷Pb/²⁰⁴Pb at low ²⁰⁶Pb/²⁰⁴Pb indicates an Archean component in the source of these rocks. We argue that this Archean crustal component, which produced the low-SiO₂ lamproites along with the high Ni and Cr must have been ultrabasic, and we propose a model in which these lamproites formed by partial melting of metasomatized, subducted Archean komatiite in a peridotite mantle-source assemblage. In addition, these rocks display initial Hf isotopic compositions similar to Al-depleted komatiites, and high Nb/U, Nb/Th, and TiO₂ as well as low Al₂O₃/TiO₂ ratios (1.1–4.2) and average CaO/Al₂O₃ of ∼ 1.6 that are also similar to Archean komatiites. This is also supported by the initial Pb isotopic composition of the Krishna lamproites, requiring evolution in a variably high U/Pb, Th/Pb reservoir early in earth history, possibly resulting from preferential segregation of Pb relative to U and Th in the sulfides of the komatiite.The Al-depleted subducted komatiitic component was enriched by carbonate metasomatism in the peridotitic mantle. This metasomatism was responsible for the observed Nd–Hf isotope characteristics, specifically variable ε_Nd(T) at relatively constant ε_Hf(T) in the lamproites. This Nd–Hf-isotopic characteristic seems to be common in global lamproites of all ages. Our proposed model for the genesis of the Krishna lamproites involving a subducted komatiitic source may also be applicable for other global lamproites from cratonic settings, as older komatiite-bearing subducted crustal components were possibly ubiquitous in the architecture of ancient cratonic mantle.     

Miocene to recent alkaline volcanism between Al Haruj and Waw an Namous (southern Libya)

Unspiked K–Ar ages, petrological, geochemical and isotopic data are reported on samples from southern Libya (Wan an Namous—Al Haruj area). The Wan an Namous intracaldera cone dated at 0.2?Ma consists of unusually undersaturated foidite, representing the most extreme compositions among Libyan and Tibestian lavas. A basanitic and a basaltic lava flow located north-west of Wan an Namous, and probably belonging to the Al Haruj volcanic field, were dated at 5.1?Ma and 8.1?Ma. These data extend the range of ages previously reported for Al Haruj lavas. REE and multi-element patterns are typical of alkaline intraplate magmas. Sr ratios range from 0.70314 to 0.70812, whereas Nd ratios are very homogeneous (0.51290–0.51293). Pb ratios (19.231?<?²⁰⁶Pb/²⁰⁴Pb?<?19.547, 15.607?<?²⁰⁷Pb/²⁰⁴Pb?<?15.641 and 38.859?<?²⁰⁸Pb/²⁰⁴Pb?<?39.242) are typical of HIMU-FOZO compositions. Such isotope characteristics are very similar to those available on two Gharyan (northern Libya) lavas and largely overlap those of Hoggar and Cameroon Line alkaline rocks. These lavas were produced by low and variable degrees of partial melting of a garnet- and amphibole-bearing mantle source, constraining the depth of melting between 80 and 150?km. Crustal contamination was also probably involved for the oldest sample. Mineral compositions of a dunite–harzburgite xenolith clearly indicate that the lithospheric mantle was affected by partial melting and metasomatic processes by magmatic liquids, probably associated with the genesis of Cenozoic lavas. Lithospheric delamination and asthenospheric upwelling, due to the reactivation of lithospheric megastructures induced by the Africa-Europe convergence, could represent a model for the genesis of Libyan lavas, as in Hoggar.     

Subduction and melting processes inferred from U-Series, Sr-Nd-Pb isotope, and trace element data, Bicol and Bataan arcs, Philippines

We present U-series, Sr-Nd-Pb isotope, and trace element data from the two principal volcanic chains on Luzon Island, developed over oppositely dipping subduction zones, to explore melting and mass transfer processes beneath arcs. The Bataan (western) and Bicol (eastern) arcs are currently subducting terrigenous and pelagic sediments, respectively, which have different trace element and isotopic compositions. The range of (²³⁰Th/²³⁸U) disequilibria for both arcs is 0.85-1.15; only lavas from Mt. Mayon (Bicol arc) have ²³⁰Th activity excesses. Bataan lavas have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd than Bicol lavas (⁸⁷Sr/⁸⁶Sr = 0.7042-0.7046, ¹⁴³Nd/¹⁴⁴Nd = 0.51281-0.51290 vs. ⁸⁷Sr/⁸⁶Sr = 0.70371-0.70391, ¹⁴³Nd/¹⁴⁴Nd = 0.51295-0.51301) and both arcs show steep linear arrays towards sediment values on ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagrams. Analysis of incompatible element and isotopic data allows identification of a sediment component that, at least in part, was transferred as a partial melt to the mantle wedge peridotite. Between 1% and 5% sediment melt addition can explain the isotopic and trace element variability in the rocks from both arcs despite the differences in sediment supply. We therefore propose that sediment transfer to the mantle wedge is likely mechanically or thermally limited. It follows that most sediments are either accreted, reside in the sub-arc lithosphere, or are recycled into the convecting mantle. However, whole-sale sediment recycling into the upper mantle is unlikely in light of the global mid-ocean ridge basalt data. Fluid involvement is more difficult to characterize, but overall the Bicol arc appears to have more fluid influence than the Bataan arc. Rock suites from each arc can be related by a dynamic melting process that allows for ²³⁰Th ingrowth, either by dynamic or continuous flux melting, provided the initial (²³⁰Th/²³²Th) of the source is ∼0.6-0.7. The implication of either model is that inclined arrays on the U-Th equiline diagram may not have chronologic significance. Modeling also suggests that U-series disequilibria are influenced by the tectonic convergence rate, which dictates mantle matrix flow. Thus with slower matrix flow there is a greater degree of ²³⁰Th ingrowth. While other factors such as prior mantle depletion and addition of a subducted component may explain some aspects of U-series data, an overall global correlation between tectonic convergence rate and the extent of U-Th disequilibria may originate from melting processes.