Abstract
The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage ...regions located directly above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterized by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e. mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveal that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveals that the Floreana mush zones are located in the upper mantle, at a depth of 23·7 ± 5·1 km. Our barometric results are compared with recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.
The flow of high‐temperature and compositionally enriched material between mantle plumes and nearby spreading centers influences up to 30% of the global mid‐ocean ridge system and represents a ...significant, but currently unconstrained, flux of volatiles out of the mantle. Here, we present new analyses of H2O, F, Cl, and S in basaltic glass chips from an archetypal region of plume‐ridge interaction, the Galápagos Spreading Center (GSC). Our data set includes samples from the eastern GSC, on ridge segments that are strongly influenced by the adjacent Galápagos mantle plume, and complements published analyses of volatiles largely from the western GSC. We use forward models of mantle melting to investigate the role of solid and melt‐phase transport from a lithologically heterogeneous (peridotite‐pyroxenite) mantle in plume‐ridge interaction along approximately 1,000 km of the GSC. Our results indicate that the observed geochemical and geophysical variations cannot be recreated by models that only involve solid‐state transfer of material between the Galápagos mantle plume and the GSC. Instead, we show that the geochemical and geophysical data from the GSC are well‐matched by models that incorporate channelized flow of volatile‐rich melts formed at high‐pressures (>3 GPa) in the Galápagos plume stem to the GSC. In addition, our new models demonstrate that channelized flow of enriched, plume‐derived melt can account for up to ∼60% of the H2O outgassed from regions of the GSC, which are most strongly influenced by the Galápagos mantle plume.
Plain Language Summary
Approximately one‐third of Earth’s global mid‐ocean ridge system is influenced by the transfer of compositionally distinct material from nearby upwellings of anomalously hot mantle. Transfer of this plume material to oceanic spreading centers might represent an important mechanism of volatile loss from Earth’s mantle, but there are limited constraints on the quantities of H2O and other volatiles that degas from these plume‐influenced spreading centers. In this study, we evaluate the mechanism of plume‐ridge interaction between the Galápagos mantle plume and the nearby Galápagos Spreading Center (GSC) using new analyses of volatiles in basalts erupted on the ridge. The results from new numerical models demonstrate that the geochemical and geophysical signatures of plume‐ridge interaction along the GSC are best explained if the transport of deep sourced mantle material between the Galápagos mantle plume and GSC occurs in the melt phase rather than as a solid. In addition, our new analyses enable us to constrain the flux of H2O out of the GSC and demonstrate that melt channelization can account for up to ∼60% of the H2O flux out of plume‐influenced ridges.
Key Points
Basalts erupted on segments of the global ridge system adjacent to the Galapagos mantle plume have high volatile (H2O and F) contents
Channelized melt transport between the Galápagos mantle plume stem and the Galápagos Spreading Center (GSC) causes variations in crustal thickness and geochemistry
Plume‐derived volatile‐rich melts contribute up to 20%–60% of the total H2O outflux at the GSC
The global mid-ocean ridge (MOR) system represents a major site for outgassing of volatiles from Earth's mantle. The amount of H2O released via eruption of mid-ocean ridge basalts varies along the ...global ridge system and greatest at sites of interaction with mantle plumes. These deep-sourced thermal anomalies affect approximately one-third of all MORs – as reflected in enrichment of incompatible trace elements, isotope signatures and elevated ridge topography (excess melting) – but the physical mechanisms involved are controversial. The “standard model” involves solid-state flow interaction, wherein an actively upwelling plume influences the divergent upwelling generated by a mid-ocean ridge so that melting occurs at higher pressures and in greater amounts than at a normal spreading ridge. This model does not explain, however, certain enigmatic features including linear volcanic ridges radiating from the active plume to the nearby MOR. Examples of these are the Wolf–Darwin lineament (Galápagos), Rodrigues Ridge (La Réunion), Discovery Ridge (Discovery), and numerous smaller ridge-like structures associated with the Azores and Easter–Salas y Gómez hot spots. An important observation from our study is that fractionation-corrected MORB with exceptionally-high H2O contents (up to 1.3 wt.%) are found in close proximity to intersections of long-lived plume-related volcanic lineaments with spreading centres.
New algorithms in the rare-earth element inversion melting (INVMEL) program allow us to simulate plume–ridge interactions by mixing the compositions of volatile-bearing melts generated during both active upwelling and passively-driven corner-flow. Our findings from these empirical models suggest that at sites of plume–ridge interaction, moderately-enriched MORBs (with 0.2–0.4 wt.% H2O) result from mixing of melts formed by: (i) active upwelling of plume material to minimum depths of ∼35 km; and (ii) those generated by passive melting at shallower depths beneath the ridge. The most volatile-rich MORB (0.4–1.3 wt.% H2O) may form by the further addition of up to 25% of “deep” small-fraction plume stem melts that contain >3 wt.% H2O. We propose that these volatile-rich melts are transported directly to nearby MOR segments via pressure-induced, highly-channelised flow embedded within a broader “puddle” of mostly solid-state plume material, spreading beneath the plate as a gravity flow. This accounts for the short wavelength variability (over 10s of km) in geochemistry and bathymetry that is superimposed on the much larger (many 100s of km) “waist width” of plume-influenced ridge.
Melt channels may constitute a primary delivery mechanism for volatiles from plume stems to nearby MORs and, in some instances, be expressed at the surface as volcanic lineaments and ridges. The delivery of small-fraction hydrous melts from plume stems to ridges via a two-phase (melt-matrix) regime implies that a parallel, bimodal transport system is involved at sites of plume–ridge interaction. We estimate that the rate of emplacement of deep-sourced volatile-rich melts in channels beneath the volcanic lineaments is high and involves 10s of thousands of km3/Ma. Since mantle plumes account for more than half of the melt production at MORs our findings have important implications for our understanding of deep Earth volatile cycling.
•H2O contents of MORB are highest in those interacting with near axis plumes.•MORB with high H2O coincide with volcanic lineaments radiating from plumes.•REE inversion models can be used to estimate primary melt volatile contents.•Solid-state flow accounts for plume-influenced MORB with moderate H2O contents.•Two-phase flow in melt-rich conduits delivers volatiles from plumes to ridges.
Abstract
Sheared peridotite xenoliths are snapshots of deformation processes that occur in the cratonic mantle shortly before their entrainment by kimberlites. The process of deformation that caused ...the shearing has, however, been highly debated since the 1970s and remains uncertain. To investigate the processes involved in the deformation, we have studied 12 sheared peridotites from Late Cretaceous (90 Ma) kimberlites in northern Lesotho, on the southeast margin of the Kaapvaal craton. Various deformation textures are represented, ranging from porphyroclastic to fluidal mosaic. Our sample suite consists of eleven garnet peridotites, with various amounts of clinopyroxene, and one garnet-free spinel peridotite with a small amount of clinopyroxene. All of the peridotites are depleted in Fe, and the Mg# of olivine and orthopyroxene range from 91 to 94. Three groups of sheared peridotites are present and have been identified primarily on the basis of Ca contents of olivine and orthopyroxene. The porphyroclasts preserve pre-deformation P–T conditions of 3.5 to 4.5 GPa and 900°C to 1100°C (Group I), 5 to 5.5 GPa and 1200°C to 1250°C (Group II) and 6 ± 0.5 GPa and 1400 ± 50°C (Group III). Group III samples lie above the 40 mW/m2 conductive geothermal gradient, indicating thermal perturbation prior to deformation. The sheared peridotites from Lesotho were affected by various metasomatic events. Pre-deformation metasomatism, involving melts and fluids, is recorded in the porphyroclasts. In Group II and III samples, the clinopyroxene porphyroclasts have similar compositions to Cr-rich and Cr-poor clinopyroxene megacrysts, respectively, that have previously described from southern African kimberlites. This suggests a relationship between them. Younger pre-deformation metasomatism is preserved in a zoned garnet from Group II (enrichment in Ti, Zr, Y + HREE) and orthopyroxene in a Group I sample. The latter exhibits a complex zonation, with a highly enriched (Fe, Ti) inner rim and a less-enriched outer rim. These enrichments must have occurred shortly before deformation. Metasomatism during deformation is revealed by the complex chemical changes recorded in olivine neoblasts with, depending on the sample, increasing or decreasing contents of Ti, Ca, Al, Cr, Mn and Na. Crystallographic preferred orientations of olivine neoblasts are consistent with bimodal, B, C, E, AG-type fabrics and indicate the presence of a hydrous metasomatic agent. We suggest that, akin to the shallower sheared peridotites (Group I), Groups II and III were influenced by early (proto-)kimberlite melt pulses and propose the following model: (proto-)kimberlitic melts invaded the lower lithosphere. These melts followed narrow shear zone networks, produced by deformation at the lithosphere–asthenosphere boundary, heated and metasomatized the surrounding peridotites and were responsible for megacryst crystallization. Sheared peridotites from close to the melt conduits (Group III) have compositions comparable to Cr-poor megacrysts, while those located at a greater distance (Group II) resemble Cr-rich megacrysts. Reactive infiltration of volatile-rich proto-kimberlite melts caused rheologically weakening of olivine in the lithospheric mantle. The consequence of this positive feedback mechanism of metasomatism, weakening and deformation—due to the high magmatic and metasomatic activity in the Late Cretaceous—is the progressive perforation of the lower Kaapvaal lithosphere by rheologically weak zones and the destruction of the protecting dry and depleted layer at its base. This could have caused the observed thinning and destabilization of the lower lithosphere below the southern margin of the Kaapvaal craton.
Abstract
Sheared peridotites from the Kaapvaal craton may be broadly divided into two types: (1) high T and refertilized and (2) low T and highly depleted, which equilibrated at conditions lying ...either above or along the Kaapvaal craton conductive geotherm, respectively. Here, we have studied 14 low-T sheared peridotites from Kimberley entrained by several Late Cretaceous (90 Ma) kimberlites in order to constrain the nature and timing of the deformation. The sample suite comprises nine garnet peridotites (GPs) with various amounts of clinopyroxene ± isolated spinel, three garnet-free phlogopite peridotites (PPs) with minor amounts of spinel, one garnet–spinel peridotite (GSP) and one dunite. The peridotites have intense deformation textures, ranging from porphyroclastic to fluidal mosaic. Olivine and orthopyroxene compositions (Mg# = 91–94) indicate varying degrees of depletion, similar to coarse-grained peridotites from the same localities. Pre-deformation conditions of the GPs are preserved in the cores of large (>100 μm–mm diameter) porphyroclasts and give a range in temperature of 930–1000°C at pressures of 4.0 ± 0.4 GPa. The GSP was equilibrated at 840°C and 3.1 GPa. Projected onto a 40-mW/m2 geothermal gradient, the PP samples yield temperatures of 850–870°C at 3.3–3.4 GPa. Trace element measurements by laser ablation inductively coupled plasma mass spectrometry and electron microprobe indicate that the ‘cold’ sheared peridotites were influenced by several metasomatic events, ranging from ‘old’ pre-deformation metasomatism to interactions shortly before or during deformation. The old pre-deformation metasomatism is recorded in garnet, clinopyroxene and orthopyroxene porphyroclasts and implies interactions with phlogopite–ilmenite–clinopyroxene- or muscovite–amphibole–rutile–ilmenite–diopside-related metasomatic agents, which also led to crystallization of phlogopite in the garnet-free peridotites. A ‘young’ metasomatic event caused an enrichment in Fe, Ti, Ca and Y (+heavy rare earth elements) and is evident in zoned orthopyroxene and clinopyroxene and phlogopite, the crystallization of new clinopyroxene porphyroclasts and compositional heterogeneities in garnet. This young event marks the beginning of extensive kimberlite-related metasomatism in the late Cretaceous beneath Kimberley. The metasomatism caused the deformation (triggered by a kimberlite pulse?), resulting in the recrystallization of fine-grained, mainly olivine, neoblasts (down to <10 μm). These record the metasomatic conditions at the time of deformation, revealing an increase in temperature up to 1200°C accompanied by an increase in Ti content up to 300 μg/g. Crystal preferred orientations of olivine neoblasts suggest the presence of elevated concentrations of water (B, C, E type) or the presence of a melt during the deformation (AG type). We suggest that these high water contents led to hydrolytic weakening of the cratonic lithosphere and prepared the pathways for subsequent kimberlite magmas to reach the surface. We propose that the deformation is a byproduct of extensive metasomatism, resulting in a metasomatism–deformation cycle. In times of extensive magmatism and metasomatism, fluids and melts flow along the pathways established by previous metasomatic agents, leading to further hydrolytic weakening of these mantle segments. Later, deformation was initiated by a new pulse of melt/fluid, with one of the later pulses eventually reaching the surface and transporting fragments of sheared and undeformed peridotites with it. The remaining peridotite anneals after the period of extensive metasomatism and recrystallizes to become coarse-grained peridotite again.
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