This paper presents new major and trace-element data and Lu–Hf and Sm–Nd isotopic compositions for representative suites of marine sediment samples from 14 drill sites outboard of the world’s major ...subduction zones. These suites and samples were chosen to represent the global range in lithology, Lu/Hf ratios, and sediment flux in subducting sediments worldwide. The data reported here represent the most comprehensive data set on subducting sediments and define the Hf–Nd isotopic variations that occur in oceanic sediments and constrain the processes that caused them.
Using new marine sediment data presented here, in conjunction with published data, we derive a new Terrestrial Array given by the equation,
ε
Hf
=
1.55
×
ε
Nd
+
1.21. This array was calculated using >3400 present-day Hf and Nd isotope values. The steeper slope and smaller
y-intercept of this array, compared to the original expression (
ε
Hf
=
1.36
×
ε
Nd
+
2.89;
Vervoort et al., 1999) reflects the use of present day values and the unradiogenic Hf of old continental samples included in the array.
In order to examine the Hf–Nd isotopic variations in marine sediments, we have classified our samples into 5 groups based on lithology and major and trace-element geochemical compositions: turbidites, terrigenous clays, and volcaniclastic, hydrothermal and hydrogenetic sediments. Compositions along the Terrestrial Array are largely controlled by terrigenous material derived from the continents and delivered to the ocean basins via turbidites, volcaniclastic sediments, and volcanic inputs from magmatic arcs. Compositions below the Terrestrial Array derive from unradiogenic Hf in zircon-rich turbidites. The anomalous compositions above the Terrestrial Array largely reflect the decoupled behavior of Hf and Nd during continental weathering and delivery to the ocean. Both terrigenous and hydrogenetic clays possess anomalously radiogenic Hf, reflecting terrestrial sedimentary and weathering processes on the one hand and marine inheritance on the other. This probably occurs during complementary processes involving preferential retention of unradiogenic Hf on the continents in the form of zircon and release of radiogenic Hf from the breakdown of easily weathered, high Lu–Hf phases such as apatite.
Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their ...behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales.
Oceanic lithosphere carries volatiles, notably water, into the mantle through subduction at convergent plate boundaries. This subducted water exercises control on the production of magma, ...earthquakes, formation of continental crust and mineral resources. Identifying different potential fluid sources (sediments, crust and mantle lithosphere) and tracing fluids from their release to the surface has proved challenging
. Atlantic subduction zones are a valuable endmember when studying this deep water cycle because hydration in Atlantic lithosphere, produced by slow spreading, is expected to be highly non-uniform
. Here, as part of a multi-disciplinary project in the Lesser Antilles volcanic arc
, we studied boron trace element and isotopic fingerprints of melt inclusions. These reveal that serpentine-that is, hydrated mantle rather than crust or sediments-is a dominant supplier of subducted water to the central arc. This serpentine is most likely to reside in a set of major fracture zones subducted beneath the central arc over approximately the past ten million years. The current dehydration of these fracture zones coincides with the current locations of the highest rates of earthquakes and prominent low shear velocities, whereas the preceding history of dehydration is consistent with the locations of higher volcanic productivity and thicker arc crust. These combined geochemical and geophysical data indicate that the structure and hydration of the subducted plate are directly connected to the evolution of the arc and its associated seismic and volcanic hazards.
The Izu‐Bonin‐Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four ...sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore‐arc basalt (FAB). FABs have mid‐ocean ridge basalt (MORB)‐like compositions, however, FAB are consistently lower in the high‐field strength elements (TiO2, P2O5, Zr) and Ni compared to MORB, with Na2O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off‐axis magmas return to more primitive compositions. Melt models require a two‐stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400 °C–1,480 °C) at relatively low pressures (1–2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle.
Plain Language Summary
This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu‐Bonin subduction zone to recover samples from its earliest history, before formation of Izu‐Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid‐ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it rises toward the surface. Small amounts of melt formed in equilibrium with garnet deeper in the Earth, while later melts formed in equilibrium with spinel at shallower depths in the Earth. The high extent of melting required, and the high calculated temperatures of these lavas, suggests the involvement of the Manus hot spot, which was located above the newly formed subduction zone some 52 million years ago. The lavas found in these drill holes are similar to lavas found in ocean crust that has been emplaced into mountain belts throughout the world and supports the proposition that in most areas, on‐land ocean crust formed above subduction zones, not at mid‐ocean ridges.
Key Points
Fore‐arc basalts (FABs) formed by decompression melting in response to subduction initiation and differ from mid‐ocean ridge basalts
FABs form by two‐stage melting, with early garnet field melts extracted prior to spinel field melting, resulting in LREE/HREE depletion
Highly depleted FAB may reflect both an older depletion event and higher ambient temperatures related to the Manus plume
Hawaiian volcanoes record 6 Ma of potentially deep mantle chemistry and form two parallel volcanic chains that are geochemically unique, named Loa and Kea. Loa volcanoes erupt lavas with isotopically ...enriched compositions thought to reflect the presence of recycled material in the deep mantle source of the Hawaiian plume. Variations in stable thallium (Tl) isotopes have been used to trace recycled pelagic ocean sediment from subduction to eruption in arc and intraplate lavas. Previous work attributed heavy Tl isotopic compositions in eight Loa samples to recycled sediments in their source. We reexamined this hypothesis using a large sample set (n = 34) of shield‐stage, tholeiitic basalt from 13 Hawaiian volcanoes representing the entire range of isotopically enriched and depleted compositions along the Hawaiian chain. Samples were acid‐leached prior to isotopic analysis to remove post‐eruption alteration and resulting ε205Tl values show statistical differences between Loa and Kea volcanoes. Corresponding isotopic data and re‐analyzed trace element concentrations suggest that the ε205Tl values are primary magmatic signatures. Possible co‐variations between heavy ε205Tl and oxygen isotopes in samples from Kea‐trend volcanoes could reflect the presence of ancient, recycled pelagic sediment on the Kea side of the Hawaiian plume, which samples the average deep Pacific mantle. As such, the deep mantle source of both Loa and Kea Hawaiian volcanoes may contain recycled materials of different natures and recycling histories, which supports work from both geophysical and geochemical studies suggesting that the Earth's lower mantle is chemically heterogeneous on multiple spatial scales.
Plain Language Summary
The Hawaiian volcanoes form two parallel geographic and geochemical trends, named Loa and Kea, that are produced by a deep mantle plume originating at the core‐mantle boundary. Volcanoes from the Loa trend have more “enriched” isotopic compositions, indicative of recycled surface materials in their source, whereas volcanoes from the Kea trend tend to have average Pacific mantle compositions. Thallium (Tl) isotopes (205Tl and 203Tl) are unequally distributed across Earth's chemical reservoirs and can show large concentration contrasts, for example between pelagic sediments (>>100 ng/g) and the Earth's mantle (<1 ng/g). We measured the Tl isotopic composition in Hawaiian samples and found that, among other indicators, the heavier Tl isotopic compositions measured in some volcanoes of the Kea geochemical trend might co‐vary with oxygen isotopes, suggesting that their Tl compositions could result from recycled surface materials in their source. This shows that the mantle source of both the Loa and Kea geochemical trends likely contains materials recycled through the mantle, which is significant because thus far the Kea volcanoes have shown fairly uniform isotopic compositions representative of the average, deep Pacific mantle.
Key Points
Thallium isotopic compositions measured in a sample set of Hawaiian shield lavas represent primary magmatic signatures
Heavier isotopic values in some Kea‐trend volcanoes suggest the presence of ancient pelagic sediment in the Kea source of the Hawaiian plume
The deep mantle source of both Loa and Kea Hawaiian volcanoes contains recycled materials of varying lithologies, histories, and ages
•First precise stable isotope measurements of V, Ni and Mo in crude oils.•First order constraints are placed on the magnitude of isotope variability.•Isotope compositions are unaffected by ...generation, expulsion, and migration.•V and Ni stable isotope compositions are likely source dependent.•V, Ni, and Mo isotope compositions are likely affected by paleoredox chemistry.
Crude oils often have high concentrations of transition metals including vanadium (V), nickel (Ni), iron (Fe), and to a lesser extent molybdenum (Mo). Determining the conditions under which these metals enter into crude oil is of interest for the understanding of biogeochemical cycles and the pathways leading to oil formation. This study presents the first high precision measurements of V, Ni, and Mo stable isotopes determined for a set of globally distributed crude oils as a first examination of the magnitude of potential stable isotope fractionation. Vanadium stable isotope compositions are presented for crude oils formed from different source rocks spanning a range of geologic ages (Paleozoic–Tertiary) and are complemented by Ni and Mo stable isotope compositions on a subset of crude oils produced from lacustrine source rocks in the Campos Basin, Brazil. The crude oils span a wide range of V and Mo isotope compositions, and display more restricted Ni stable isotope signatures. Overall, the stable isotope composition of all three systems overlaps with previously determined values for igneous and inorganic sedimentary materials. Comparisons between vanadium concentration and stable isotope composition yield distinct clusters associated with crude oils predominantly derived from terrestrial/lacustrine or marine/carbonate source rocks. The Ni stable isotope signatures of studied crude oils are similar to that of carbonaceous shales. The Mo stable isotope signatures of the lacustrine sourced crude oils are similar to what is observed for rivers. This indicates trace metal stable isotopic composition of crude oils are unlikely to result from mass dependent fractionation associated with the generation, expulsion, and migration of petroleum and are instead primarily inherited from the initial sedimentary organic matter or metal bearing fluids present during metalation. In contrast, although Mo stable isotopes also span a wide range of values, they do not appear to be correlated with source rock lithology, perhaps suggestive of a greater role for secondary processes. Finally, both V and Ni stable isotope compositions co-vary with V/(V+Ni), a commonly used parameter in determining crude oil grade. Since V/(V+Ni) ratios reflect redox conditions, the correlation implies that stable V and Ni isotope compositions may also respond to the redox conditions of the depositional environment. The contrasting behavior of these three isotope systems in this initial dataset provides fundamental guidance for future investigation to fully exploit the potential of these new isotopic tracers. These isotope tracers can be developed to determine a broad range of factors fundamental to the formation and preservation of petroleum source rocks that spans from provenance evaluation to paleoredox chemistry.
Sediment transport from the subducted slab to the mantle wedge is an important process in understanding the chemical and physical conditions of arc magma generation. The Aleutian arc offers an ...excellent opportunity to study sediment transport processes because the subducted sediment flux varies systematically along strike (Kelemen et al., 2003) and many lavas exhibit unambiguous signatures of sediment addition to the sub-arc mantle (Morris et al., 1990). However, the exact sediment contribution to Aleutian lavas and how these sediments are transported from the slab to the surface are still debated. Thallium (Tl) isotope ratios have great potential to distinguish sediment fluxes in subduction zones because pelagic sediments and low-temperature altered oceanic crust are highly enriched in Tl and display heavy and light Tl isotope compositions, respectively, compared with the upper mantle and continental crust. Here, we investigate the Tl isotope composition of lavas covering almost the entire Aleutian arc a well as sediments outboard of both the eastern (DSDP Sites 178 and 183) and central (ODP Hole 886C) portions of the arc. Sediment Tl isotope compositions change systematically from lighter in the Eastern to heavier in the Central Aleutians reflecting a larger proportion of pelagic sediments when distal from the North American continent. Lavas in the Eastern and Central Aleutians mirror this systematic change to heavier Tl isotope compositions to the west, which shows that the subducted sediment composition is directly translated to the arc east of Kanaga Island. Moreover, quantitative mixing models of Tl and Pb, Sr and Nd isotopes reveal that bulk sediment transfer of similar to 0.6-1.0% by weight in the Eastern Aleutians and similar to 0.2-0.6% by weight in the Central Aleutians can account for all four isotope systems. Bulk mixing models, however, require that fractionation of trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd in the Central and Eastern Aleutians occurs after the sediment component was mixed with the mantle wedge. Models of Sr and Nd isotopes that involve sediment melting require either high degrees of sediment melting (>50%), in which case trace element ratios like Ce/Pb, Cs/Tl, and Sr/Nd of Aleutian lavas need to be produced after mixing with the mantle, or significant fluid additions from the underlying oceanic crust with Sr and Nd isotope compositions indistinguishable from the mantle wedge as well as high Sr/Nd ratios similar to that of low (<20%) degree sediment melts. Thallium isotope data from Western Aleutian lavas exhibit compositions slightly lighter than the upper mantle, which implies a negligible sediment flux at this location and probably involvement of low-temperature altered oceanic crust in the generation of these lavas. In general, the lightest Tl isotope compositions are observed for the highest Sr/Y ratios and most unradiogenic Sr and Pb isotope compositions, which is broadly consistent with derivation of these lavas via melting of eclogitized altered oceanic crust.
Sediment is actively being subducted in every convergent margin worldwide. Yet, geochemical data for arc lavas from several subduction zones, such as Northern Tonga and Costa Rica have revealed ...either attenuated or limited evidence for sediment in their mantle source regions. Here we use thallium (Tl) isotopes to trace slab components in lavas from the Tonga-Kermadec and Central American arcs. In general, both arcs display Tl isotope data that are most compatible with addition of sediment to the sub-arc mantle from the subducting slab. This evidence is particular strong in the Tonga-Kermadec arc where pelagic clays dominate the Tl budget along the entire arc. Contributions from altered oceanic crust as well as the Louisville Seamount chain that subducts underneath Northern Tonga are not visible in Tl isotopes, which is likely due to the very high Tl concentrations found in pelagic sediments outboard of the Tonga-Kermadec arc. Lavas from Central America reveal variable and systematic Tl isotope compositions along-strike. In particular, lavas from Nicaragua are dominated by contributions from sediments, whereas Costa Rican samples reveal a significant altered oceanic crust component with little influence from sediments on thallium isotope composition. The absence of a sediment signature in Costa Rica corresponds with the Cocos Ridge and the seamount province subduction, which results in a thinner sediment cover. Furthermore, the subducted sediment is dominated by carbonates with very low Tl concentrations and, therefore, small amounts of carbonate sediment added to the mantle wedge do not contribute significantly to the overall Tl budget.
A review of Tl isotope and concentration data from the Aleutians, Marianas, Tonga-Kermadec and Central American arcs demonstrate that pelagic sediments are detectable in most arcs, whereas altered oceanic crust components only become appreciable when sediment Tl concentrations are very low (e.g. carbonate) or if sediments are no longer a significant component of the subducting slab (e.g. slab melting in Western Aleutians). As such, Tl isotopes are a promising tool to trace sediment subduction although this requires at least some pelagic sediment is present in the subducted sediment package.
We suggest that thallium partitioning between the slab and mantle wedge is most likely controlled by retention in phengite or by partitioning into fluids. Residual phengite likely produces high Cs/Tl ratios because Tl should be more compatible in phengite than is Cs, however, this conclusion needs experimental verification. The stability of phengite is lower at higher fluid contents, which results in hyperbolic relationships between Cs/Tl and possible indicators of fluids such as Sr/Nd and Ba/Th. Thus, combined Tl isotopic and elemental systematics not only provide detailed information about the specific slab components that contribute to arc lavas, but also potentially shed light on the mineralogy and physical conditions of subducting slabs.
•Thallium isotopes in Tonga-Kermadec and Central American arcs exhibit large variation.•Thallium isotopes effectively trace pelagic sediment in subduction zones.•Residual phengite and fluids control the release of thallium to the arc surface.
The Popocatépetl Volcanic Complex (PVC) is an active arc volcano located in central Mexico, 70 km southeast of Mexico City. Current models of the PVC’s plumbing system and magma petrogenesis are ...largely based on studies of isolated Plinian eruptions over the past 23.5 ka and present-day Vulcanian activity, while voluminous interplinian effusive summit and flank eruptions remain underrepresented. Here, we present a detailed petrological characterization focussed on ortho- and clinopyroxene in five effusive flank eruptions and two Plinian eruptions of the PVC during the last ∼14.1 ka. Texturally and compositionally defined pyroxene populations are used to constrain magmatic temperatures and deconvolve crystallization histories. At least two long-lived, inter-connected magmatic environments (ME) are identified in the mid- to upper crust beneath the PVC: (1) a mafic ME crystallizing high-Mg orthopyroxene + clinopyroxene + Cr-spinel ± sulfide at 1000–1115°C, and (2) an evolved, shallower ME crystallizing plagioclase + low-Mg orthopyroxene + clinopyroxene + Fe-Ti oxides + apatite ± sulfide at long-term storage temperatures of ∼970°C. The architecture of the PVC plumbing system has remained stable throughout the last ∼14.1 ka, and both MEs have sustained above-solidus magma storage temperatures fueled by recharge with hydrous, high-Mg basaltic mantle melts that crystallized fosteritic olivine + Cr-spinel + low-Ca clinopyroxene in the lower- to mid-crust at 1080–1220°C. Lavas and pumices show texturally and compositionally diverse crystal cargoes indicative of frequent magma mixing, with ≤67% of pyroxene crystals originating from the mid- to upper crustal mafic ME, of which ≤74% were stored and diffusively overprinted in the evolved ME for centuries to millenia. Pyroxene crystals of different origins, ages and thermal histories are stored in the evolved ME as a heterogeneous crystal mush that is frequently disrupted, reorganized and replenished by mafic injections. Magma recharge causes melt and crystal hybridization over timescales ranging from near-instantaneous to millenia, which produces the diverse crystal cargo and restricted whole-rock compositions typical for the PVC and many other arc volcanoes. We suggest that hot storage conditions and magma dynamics similar to the PVC may be characteristic for many other arc volcanoes of intermediate sizes and compositions.
The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and ...near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of
K
2
O
-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca.
3000
μ
g/g
), whereas globules and the interface are Cl poor (ca.
300
μ
g/g
). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (
87
Sr
/
86
Sr
of
0.70580
±
10
), indicating that they likely formed from the same source. However, globules show high
δ
11
B (ca. + 7
‱
), whereas their host andesites are isotopically lighter (ca. – 1
‱
), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low
δ
11
B
forearc basalt parental magmas (average
87
Sr
/
86
Sr
of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered.
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