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Chemical analysis using laser-induced breakdown spectroscopy (LIBS) is well suited for field applications and was applied here for shipboard characterization of a large sample set during the RR1310 rock dredging expedition to the Tuvalu Seamounts. Although recently the most common data treatment for LIBS has consisted of a partial least squares approach to define sample groupings, we show that quantitative data of useful quality can be obtained with a univariate approach. Here, our analysis goal was a quantitative comparison of the total alkali (Na2O+K2O) versus silica (SiO2) contents of 586 representative dredge samples with known ranges in common rock types. Out of those samples, >400 form a single large group of alkalic basalts with minor basanites/tephrites (SiO2: 43-48 wt%, Na2O+K2O: 3-5 wt%), similar to known shieldstage compositions of the Rurutu and Samoa hotspots in the sampling area. In contrast, several dredge hauls contain samples with compositions that do not overlap with the majority of samples. This includes three dredges performed on the northern boundary of the Lau Basin that contain similar SiO2 compositions, but slightly higher total alkali (Na2O+K2O) content. Despite this difference, they classify as basanite/tephrite, similar to a subset of the main group. More importantly, similar compositions were previously reported from the same tectonic boundary, ascribed to hotspot mantle source material mixed into the Lau Basin back-arc. Although the quality of the compositional data suffices to enable sample selection for time-intensive analyses, higher precision is required for more in-depth petrogenetic interpretation. Error analysis based on repeat standard measurements suggests averaging 100 spectra per sample is optimal here, while use of a higher resolution spectrometer, together with better laser control, would improve results and interpretations. [ABSTRACT FROM AUTHOR]

Several magmatic processes contribute to the Fe isotope composition of igneous rocks. Most basalts fall within a limited range of δ 56 Fe ( + 0.10 ± 0.05 ‰ ), although more differentiated lavas trend towards slightly elevated values (up to + 0.3 ‰ ). New data for basalts and olivine crystals from the Samoan Islands show higher δ 56 Fe values than have previously been reported for basalts worldwide. Common magmatic processes – from partial melting of average mantle to subsequent differentiation of melts – cannot sufficiently fractionate the Fe isotopes to explain the elevated δ 56 Fe values ( ∼ + 0.3 ‰ ) in rejuvenated Samoan lavas. Instead, a mantle source with an elevated δ 56 Fe value – in conjunction with effects due to common magmatic processes – is required. The Samoan mantle source is known to be unique in its radiogenic isotope composition and indications that melting of the Samoan mantle source can generate elevated δ 56 Fe values in lavas comes from: (1) High f O 2 values of Samoan lavas and their likely sources affecting Fe isotope fractionation during melting; (2) Metasomatism that caused elevated δ 56 Fe in the Samoan mantle, as observed in xenoliths; and (3) Involvement of a pyroxenite source lithology, based on the Zn/Fe ratios and TiO 2 (and other high field-strength element) abundances of the lavas, that can generate melts with elevated δ 56 Fe values. Two models are presented to explain the elevated δ 56 Fe values in Samoan lavas: a metasomatized source ( ∼ + 0.07 ‰ ) or the presence of a pyroxenite source component ( ∼ + 0.12 ‰ ). Both models subsequently elevate δ 56 Fe values with both partial melting ( ∼ + 0.14 ‰ ) and fractional crystallization ( ∼ + 0.1 ‰ ). These processes may be related to an upwelling mantle plume with a pyroxenite component, or melting of previously metasomatized upper mantle. [ABSTRACT FROM AUTHOR]

Sr isotope measurements by multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) are complicated by Kr interferences at atomic masses 84 and 86. Particularly the interference on 86 Sr is problematic for 87 Sr/ 86 Sr measurements, since it is the normalizing stable isotope, and as such also used in the mass-based fractionation correction using 86 Sr/ 88 Sr. Although on-peak baseline measurements can subtract an average Kr contribution, signal variations and errors within-run are propagated into the final error, and signal-to-noise ratios are lower than combined Sr + Kr signals. Alternatively, intensities of interference-free 82 Kr or 83 Kr isotopes can be monitored to correct the interfering Kr masses, although the errors for these signals are amplified and propagated into the 86 Kr and 84 Kr correction. Instead, our approach depends on the use of the most abundant Kr isotope ( 84 Kr) for correction, which can then be used to estimate interference by 86 Kr on 86 Sr. This is accomplished by solving how much Kr is needed to explain the difference between I 84 /I 88 and I 86 /I 88 versus 84 Sr/ 88 Sr and 86 Sr/ 88 Sr (I 8 × : signal intensity at mass 8 ×). A solution with the traditional isotope dilution equation would require off-line iteration to include an exponential mass fractionation correction. Instead, we formulate an online exponential correction of the Sr data without iterating, relying on a separate Kr correction that estimates Kr mass fractionation with either the linear law or a series expansion of the exponential law. Both methods advance MC-ICP-MS long-term 87 Sr/ 86 Sr precision for NIST SRM 987 to ± 16 ∗ 10 − 6 or 22.5 ppm 2σ, outperforming an 83 Kr-based correction and an on-peak baseline correction applied to the same raw data. Both our methods find identical results for USGS rock standards that agree with published (TIMS) values. [ABSTRACT FROM AUTHOR]

Abstract: Linear chains of intraplate volcanoes and their geochemistry provide a record of mantle melting through geological time. The isotopic compositions of their lavas characterize their mantle sources, and their ages help backtrack these volcanoes to their original, eruptive source regions. Such data may shed light on a much-debated issue in Earth Sciences: the origin of intraplate volcanism and its underlying mantle and lithosphere dynamics. We show here that three major Western Pacific Seamount groups, ∼50–100 million years in age, display distinct Sr, Nd, Hf, and Pb isotopic signatures that can be traced back in time, both geographically and geochemically, to three separate, recently-active intraplate volcanoes in the South Pacific Cook–Austral Islands. Their unique 100 million year history, which shows a persistent geochemical fingerprint, suggests formation from large volumes of laterally fixed, long-lived source regions. Such longevity is unlikely to be attained in the relatively dynamic upper mantle. Therefore, these sources are likely anchored deep in the mantle, isolated from homogenization by mantle convection, and imply a primary origin from deep mantle plumes rather than resulting from lithosphere extension. [Copyright &y& Elsevier]

The Northwestern Hawaiian Ridge is an age- progressive volcanic chain sourced from the Hawaiian mantle plume. Proximal to the Northwestern Hawaiian Ridge are several clusters of smaller seamounts and ridges with limited age constraints and unknown geodynamic origins. This study presents new bathymetric data and 40Ar/39Ar age determinations from lava flow samples recovered by remotely operated vehicle (ROV) from two east–west-trending chains of seamounts that lie north of the Pūhāhonu and Mokumanamana volcanoes. The previously unexplored Naifeh Chain (28°48′N,167°48′W) and Plumeria Chain (25°36′N, 164°35′W) contain five volcanic structures each, including three guyots in the Naifeh Chain. New 40Ar/39Ar age determinations indicate that the Naifeh Chain formed ca. 88 Ma and the Plumeria Chain ca. 85 Ma. The Cretaceous ages, coupled with a perpendicular orientation of the seamounts relative to absolute Pacific plate motion at that time, eliminate either a Miocene Hawaiian volcanic arch or Cretaceous mantle- plume origin. The seamounts lie on oceanic crust that is modeled to be 10–15 Ma older than the corresponding seamounts. Here, two models are put forth to explain the origin of these enigmatic seamount chains as well as the similar nearby Mendelssohn Seamounts. (1) Diffuse lithospheric extension results in the formation of these seamounts until the initiation of the Kula- Pacific spreading center in the north at 84– 79 Ma, which alleviates the tension. (2) Shear-driven upwelling of enriched mantle material beneath young oceanic lithosphere results in an age- progressive seamount track that is approximately perpendicular to the spreading ridge. Here we show that all sampled seamounts proximal to the Northwestern Hawaiian Ridge are intraplate in nature, but their formations can be attributed to both plume and plate processes. [ABSTRACT FROM AUTHOR]

To deconvolve contributions from the four overlapping hotspots that form the "hotspot highway" on the Pacific plate—Samoa, Rarotonga, Arago-Rurutu, and Macdonald—we geochemically characterize and/or date (by the 40Ar/39Ar method) a suite of lavas sampled from the eastern region of the Samoan hotspot and the region "downstream" of the Samoan hotspot track. We find that Papatua seamount, located ~60 km south of the axis of the Samoan hotspot track, has lavas with both a HIMU (high μ = 238U/204Pb) composition (206Pb/204Pb = 20.0), previously linked to one of the Cook-Austral hotspots, and an enriched mantle I (EM1) composition, which we interpret to be rejuvenated and Samoan in origin. We show that these EM1 rejuvenated lavas at Papatua are geochemically similar to rejuvenated volcanism on Samoan volcanoes and suggest that flexural uplift, caused by tectonic forces associated with the nearby Tonga trench, triggered a new episode of melting of Samoan mantle material that had previously flattened and spread laterally along the base of the Pacific plate under Papatua, resulting in volcanism that capped the previous HIMU edifice. We argue that this process generated Samoan rejuvenated volcanism on the older Cook-Austral volcano of Papatua. We also study Waterwitch seamount, located ~820 km WNW of the Samoan hotspot, and provide an age (10.49 ± 0.09 Ma) that places it on the Samoan hotspot trend, showing that it is genetically Samoan and not related to the Cook-Austral hotspots as previously suggested. Consequently, with the possible exception of the HIMU stage of Papatua seamount, there are currently no known Arago-Rurutu plume-derived lava flows sampled along the swath of Pacific seafloor that stretches between Rose seamount (~25 Ma) and East Niulakita seamount (~45 Ma), located 1400 km to the west. The "missing" ~20-million-year segment of the Arago-Rurutu hotspot track may have been subducted into the northern Tonga trench, or perhaps was covered by subsequent volcanism from the overlapping Samoan hotspot, and has thus eluded sampling. Finally, we explore tectonic reactivation as a cause for anomalously young volcanism present within the western end of the Samoan hotspot track. [ABSTRACT FROM AUTHOR]

Water influences geodynamic processes such as melting, deformation and rheology, yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savai'i and two dunite xenoliths from Ta'ū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), and major and trace element concentrations. Clinopyroxenes from the Ta'ū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savai'i peridotites are highly depleted harzburgites (melt depletion ≥17 %). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element enrichments. However, despite metasomatism the H2O concentrations in olivines (0 − 4 ppm H2O) and orthopyroxenes (17 − 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than those of other incompatible trace elements in these samples. Transects across mineral grains show generally homogeneous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai'i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere. [ABSTRACT FROM AUTHOR]

• The Melanesian Border Plateau was constructed from four volcanic sources. • At least two hotspots as well as tectonic drivers sourced MBP volcanism. • We propose the term oceanic mid-plate superstructures for some compound volcanoes. • OMSs appear common on lithosphere that drifted over the pacific LLSVP. The ocean basins contain numerous volcanic ridges, seamounts and large igneous provinces (LIPs). Numerous studies have focused on the origin of seamount chains and LIPs but much less focus has been applied to understanding the genesis of large volcanic structures formed from a combination or series of volcanic drivers. Here we propose the term Oceanic Mid-plate Superstructures (OMS) to describe independent bathymetric swells or volcanic structures that are constructed through superimposing pulses of volcanism, over long time periods and from multiple sources. These sources can represent periods when the lithosphere drifted over different mantle plumes and/or experienced pulses of volcanism associated with shallow tectonic drivers (e.g. plate flexure; lithospheric extension). Here we focus on the Melanesian Border Plateau (MBP), one example of an OMS that has a complex and enigmatic origin. The MBP is a region of shallow Pacific lithosphere consisting of high volumes of volcanic guyots, ridges and seamounts that resides on the northern edge of the Vitiaz Lineament. Here we reconcile recently published constraints to build a comprehensive volcanic history of the MBP. The MBP was built through four distinct episodes: (1) Volcanism associated with the Louisville hotspot likely generating Robbie Ridge and some Cretaceous seamounts near the MBP. (2) Construction of oceanic islands and seamounts during the Eocene when the lithosphere passed over the Rurutu-Arago hotspot. (3) Reactivation of previous oceanic islands/seamounts and construction of new volcanos in the Miocene when the lithosphere passed over the Samoa hotspot. (4) Miocene to modern volcanism driven by lithospheric deformation and/or westward entrainment of enriched plume mantle due to toroidal mantle flow driven by the rollback of the Pacific plate beneath the Tonga trench. The combination of these processes is responsible for ∼222,000 km2 of intraplate volcanism in the MBP and indicates that this OMS was constructed from multiple volcanic drivers. [ABSTRACT FROM AUTHOR]