Search

It is well established that mantle plumes are the main conduits for upwelling geochemically enriched material from Earth's deep interior. The fashion and extent to which lateral flow processes at shallow depths may disperse enriched mantle material far (>1,000 km) from vertical plume conduits, however, remain poorly constrained. Here, we report He and C isotope data from 65 hydrothermal fluids from the southern Central America Margin (CAM) which reveal strikingly high 3He/4He (up to 8.9RA) in low-temperature (=50 °C) geothermal springs of central Panama that are not associated with active volcanism. Following radiogenic correction, these data imply a mantle source 3He/4He >10.3RA (and potentially up to 26RA, similar to Galapagos hotspot lavas) markedly greater than the upper mantle range (8 ± 1RA). Lava geochemistry (Pb isotopes, Nb/U, and Ce/Pb) and geophysical constraints show that high 3He/4He values in central Panama are likely derived from the infiltration of a Galapagos plume-like mantle through a slabwindowthat opened ~8 Mya. Two potential transport mechanisms can explain the connection between the Galapagos plume and the slab window: 1) sublithospheric transport of Galapagos plume material channeled by lithosphere thinning along the Panama Fracture Zone or 2) active upwelling of Galapagos plume material blown by a "mantle wind" toward the CAM. We present a model of global mantle flow that supports the second mechanism, whereby most of the eastward transport of Galapagos plume material occurs in the shallow asthenosphere. These findings underscore the potential for lateral mantle flow to transport mantle geochemical heterogeneities thousands of kilometers away from plume conduits. [ABSTRACT FROM AUTHOR]

Abstract: Mantle plume‐derived melts provide a unique glimpse into the chemical heterogeneity of the deep mantle. However, the exact processes involved in forming and sampling lower mantle heterogeneities remain unresolved and thus, require further investigation to understand the cumulative effects of planetary differentiation, crustal recycling, and mantle mixing that lead to their formation. The Galapagos Plume is exceptional in that its lavas sample four distinct isotopic mantle components. However, the origin of each of these end‐members remains debated. In this study, we investigate the spatial and temporal appearance and evolution of these four isotopic end‐members that comprise the present‐day Archipelago by examining ancient Galapagos lava flows preserved in the accreted terranes in Costa Rica and Panama. We discovered that the spatial relationship between each of the classically defined Galapagos Domains has remained relatively constant throughout the evolution of the plume. Our new results extend the time scale of distinct isotopic heterogeneity in the source of the plume to at least 70 Ma for the Southern Domain and 90 Ma for the Central, and Northern Domains, suggesting that geochemically heterogeneous plumes rising through the mantle preserve distinct isotopic heterogeneity on the time scale of tens of millions of years. Additionally, trace element systematics of olivine crystals from the Galapagos‐related Quepos and Azuero terranes (50–70 ma) suggest derivation from sources that included recycled components. Collectively, these results suggest that mantle plumes not only sample large‐scale heterogeneities within the deep mantle, but that these source heterogeneities can remain sampled throughout the lifetime of a plume. [ABSTRACT FROM AUTHOR]

Abstract: Two competing hypotheses have been proposed for garnet-bearing ultramafic rocks in the Dominican Republic: (1) The ultrahigh pressure (UHP) – ultrahigh temperature (UHT) hypothesis involves a magmatic protolith of mantle origin, which was then delivered to, and incorporated into deep-subducted oceanic crust (eclogite) at UHP conditions (Abbott et al., 2005, 2006, 2007; Abbott and Draper, 2010; Gazel et al., 2011). (2) The low-pressure (LP) hypothesis involves a plagioclase-bearing, arc-related protolith of crustal origin, which was then subducted to UHP conditions (De Hoog, 2011; Hattori et al., 2010a,b). In both hypotheses, the rocks were uplifted to the surface by an as yet poorly understood mechanism. Here we respond to concerns regarding the integrity of REE analyses, Cpx-Grt REE partitioning, other matters related to the interpretation of the trace element data, and Grt-Spl major-element thermometry. We show that none of the concerns precludes a UHP magmatic origin. [Copyright &y& Elsevier]

Abstract: The volcanic front in southern Central America is well known for its Galapagos OIB-like geochemical signature. A comprehensive set of geochemical, isotopic and geochronological data collected on volumetrically minor alkaline basalts and adakites were used to better constrain the mantle and subduction magma components and to test the different models that explain this OIB signature in an arc setting. We report a migration of back-arc alkaline volcanism towards the northwest, consistent with arc-parallel mantle flow models, and a migration towards the southeast in the adakites possibly tracking the eastward movement of the triple junction where the Panama Fracture Zone intersects the Middle America Trench. The adakites major and trace element compositions are consistent with magmas produced by melting a mantle-wedge source metasomatized by slab derived melts. The alkaline magmas are restricted to areas that have no seismic evidence of a subducting slab. The geochemical signature of the alkaline magmas is mostly controlled by upwelling asthenosphere with minor contributions from subduction components. Mantle potential temperatures calculated from the alkaline basalt primary magmas increased from close to ambient mantle (~1380–1410°C) in the Pliocene to ~1450°C in the younger units. The calculated initial melting pressures for these primary magmas are in the garnet stability field (3.0–2.7GPa). The average final melting pressures range between 2.7 and 2.5GPa, which is interpreted as the lithosphere–asthenosphere boundary at ~85–90km. We provide a geotectonic model that integrates the diverse observations presented here. The slab detached after the collision of the Galapagos tracks with the arc (~10–8Ma). The detachment allowed hotter asthenosphere to flow into the mantle wedge. This influx of hotter asthenosphere explains the increase in mantle potential temperatures, the northwest migration in the back-arc alkaline lavas that tracks the passage of the hotter asthenosphere, and the presence of a slab melting signature in the volcanic front caused by recycling of Galapagos Hotspot tracks. [Copyright &y& Elsevier]

Geological mapping and geochronological studies have shown much lower eruption rates for ocean island basalts (OIBs) in comparison with those of lavas from large igneous provinces (LIPs) such as oceanic plateaux and continental flood provinces. However, a quantitative petrological comparison has never been made between mantle source temperature and the extent of melting for OIB and LIP sources. Here we show that the MgO and FeO contents of Galapagos-related lavas and their primary magmas have decreased since the Cretaceous period. From petrological modelling, we infer that these changes reflect a cooling of the Galapagos mantle plume from a potential temperature of 1,560–1,620 °C in the Cretaceous to 1,500 °C at present. Iceland also exhibits secular cooling, in agreement with previous studies. Our work provides quantitative petrological evidence that, in general, mantle plumes for LIPs with Palaeocene–Permian ages were hotter and melted more extensively than plumes of more modern ocean islands. We interpret this to reflect episodic flow from lower-mantle domains that are lithologically and geochemically heterogeneous. [ABSTRACT FROM AUTHOR]

Major and trace element abundances, including highly siderophile elements, and 187Os and 182W isotopic compositions were determined for ca. 89 Ma mafic and ultramafic rocks from the islands of Gorgona (Colombia) and Curaçao (Dutch Caribbean). The volcanic systems of both islands were likely associated with a mantle plume that generated the Caribbean Large Igneous Provence. The major and lithophile trace element characteristics of the rocks examined are consistent with the results of prior studies, and indicate derivation from both a chemically highly-depleted mantle component, and an enriched, or less highly-depleted mantle component. Highly siderophile element abundances for these rocks are generally similar to rocks with comparable MgO globally, indicating that the major source components were not substantially enriched or depleted in these elements. Rhenium-Os isotopic systematics of most rocks of both islands indicate derivation from a mantle source with an initial 187Os/188Os ratio between that of the contemporaneous average depleted mid-ocean ridge mantle and bulk silicate Earth. The composition may reflect either an average lower mantle signature, or global-scale Os isotopic heterogeneity in the upper mantle. Some of the basalts, as well as two of the komatiites, are characterized by calculated initial 187Os/188Os ratios 10–15 % higher than the chondritic reference. These more radiogenic Os isotopic compositions do not correlate with major or trace element systematics, and indicate a mantle source component that was most likely produced by either sulfide metasomatism or ancient Re/Os fractionation. Tungsten-182 isotopic compositions measured for rocks from both islands are characterized by variable μ182W values ranging from modern bulk silicate Earth-like to strongly negative values. The μ182W values do not correlate with major/trace element abundances or initial 187Os/188Os compositions. As with some modern ocean island basalt systems, however, the lowest μ182W value (−53) measured, for a Gorgona olivine gabbro, corresponds with the highest 3He/4He previously measured from the suite (15.8 R/R A). Given the lack of correlation with other chemical/isotopic compositions, the mantle component characterized by negative μ182W and possibly high 3He/4He is most parsimoniously explained to have formed as a result of isotopic equilibration between the mantle and core at the core-mantle boundary. [ABSTRACT FROM AUTHOR]