Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

A new geospeedometer is developed based on the differential closures of Mg and rare earth element (REE) bulk-diffusion between coexisting plagioclase and clinopyroxene. By coupling the two elements with distinct bulk closure temperatures, this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples. As the existing Mg-exchange thermometer was calibrated for a narrow temperature range and strongly relies on model-dependent silica activities, a new thermometer is developed using literature experimental data. When the bulk closure temperatures of Mg and REE are determined, respectively, using this new Mg-exchange thermometer and the existing REE-exchange thermometer, this speedometer can be implemented for a wide range of compositions, mineral modes, and grain sizes. Applications of this new geospeedometer to oceanic gabbros from the fast-spreading East Pacific Rise at Hess Deep reveal that the lower oceanic crust crystallized at temperatures of 998–1353 °C with cooling rates of 0.003–10.2 °C/yr. Stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies. Together with existing petrological, geochemical and geophysical evidence, results from this new speedometry suggest that the lower crust formation at fast-spreading mid-ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal-scale hydrothermal circulations. The replenished melts become chemically and thermally evolved, accumulate as small magma bodies at various depths, feed the shallow axial magma chamber, and may also escape from the mush zone to generate off-axial magma lenses

A reactive porous flow control on mid-ocean ridge magmatic evolution

Mid-ocean ridge basalts (MORB) provide fundamental information about the composition and melting processes in the Earth’s upper mantle. To use MORB to further our understanding of the mantle, is imperative that their crustal evolution is well understood and can thus be accounted for when estimating primary melt compositions. Here, we present the evidence for the occurrence of reactive porous flow, whereby migrating melts react with a crystal mush in mid-ocean ridge magma chambers. This evidence comprises both the textures and mineral major and trace element geochemistry of rocks recovered from the lower oceanic crust, and occurs on a range of scales. Reaction textures include dissolution fronts in minerals, ragged grain boundaries between different phases and clinopyroxene–brown amphibole symplectites. However, an important finding is that reaction, even when pervasive, can equally leave no textural evidence. Geochemically, reactive porous flow leads to shifts in mineral modes (e.g. the net replacement of olivine by clinopyroxene) and compositions (e.g. clinopyroxene Mg–Ti–Cr relationships) away from those predicted by fractional crystallization. Furthermore, clinopyroxene trace elements record a progressive core–rim over-enrichment (relative to fractional crystallization) of more-to-less incompatible elements as a result of reactive porous flow. The fact that this over-enrichment occurs over a distance of up to 8mm, and that clinopyroxenes showing this signature preserve zoning in Fe–Mg, rules out a diffusion control on trace element distributions. Instead, it can be explained by crystal–melt reactions in a crystal mush. The data indicate that reactive flow occurs not only on a grain scale, but also on a sample scale, where it can transform one rock type into another [e.g. troctolite to olivine gabbro, olivine gabbro to (oxide) gabbro], and extends to the scale of the entire lower oceanic crust. Melts undergoing these reactive processes change in composition, which can explain both the major element and trace element arrays of MORB compositions. In particular, reactive porous flow can account for the MORB MgO–CaO–Al2O3 relationships that have previously been interpreted as a result of high-pressure (up to 8 kbar) crystal fractionation, and for over-enrichment in incompatible elements when compared with the effects of fractional crystallization. The finding of a significant role for reactive porous flow in mid-ocean ridge magma chambers fits very well with the geophysical evidence that these magma chambers are dominated by crystal mush even at the fastest spreading rates, and with model predictions of the behaviour of crystal mushes. Together, these observations indicate that reactive porous flow is a common, if not ubiquitous, process inherent to mushy magma chambers, and that it has a significant control on mid-ocean ridge magmatic evolution

A mineral and cumulate perspective to magma differentiation at Nisyros volcano, Aegean arc

Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust

Olivine slurry replenishment and the development of igneous layering in a Franklin sill, Victoria Island, Arctic Canada

The Franklin sills and dykes on Victoria Island in the Canadian Arctic represent the sub-volcanic plumbing system to the Natkusiak flood basalts, which are associated with the late Neoproterozoic (c. 723–716 Ma) break-up of Rodinia. The Lower Pyramid Sill (LPS) is the distal end of a sill complex that may be rooted in the Uhuk Massif, a major fault-guided magmatic feeder system. The LPS is unusual for a thin (c. 21 m), shallow, tholeiitic intrusion because it displays well-developed cumulate layering similar to that seen in large layered intrusions. The LPS has an aphanitic, olivine-phyric (c. 5%) Lower Chilled Margin (LCM), a (<1 m thick) dendritic, olivine-phyric Lower Border Zone (LBZ), a (c. 7 m thick) olivine-dominated (up to c. 55%) melagabbro–feldspathic-peridotite zone (OZ), a thin (c. 1 m) clinopyroxene-rich cumulate gabbro (CPZ) containing sector-zoned euhedral clinopyroxene, a (c. 10 m thick) doleritic gabbro zone (DZ), a (<1 m thick) aphyric, dendritic Upper Border Zone (UBZ) and an aphanitic, olivine-phyric (c. 5%) Upper Chilled Margin (UCM). Distinct compositional groups recognized in olivines from the OZ can be associated with specific crystal morphologies, some showing significant reverse zoning. Melt compositions were calculated through application of the olivine–melt Fe ¼ Mg exchange coefficient. The calculations suggest that phenocrystic and primocrystic olivine (Fo88–82) in the LCM–LBZ and lower OZ formed from melts with c. 13–10 wt % MgO. Modeling implies that reversely zoned olivine primocrysts and chadacrysts have rims in equilibrium with melts of c. 10–8 wt % MgO that were saturated only in olivine (þ minor chromite), whereas some olivine cores formed from melts as evolved as c. 6–5 wt % MgO that would have coexisted with a gabbroic assemblage. The presence of multiple olivine populations in the OZ (some reverse zoned) indicates that the LPS did not crystallize from a single pulse of melt that evolved by closed-system fractional crystallization. We propose that the reverse zoning pattern records incorporation of evolved crystals, most derived from the mushy gabbroic host, when an olivine-charged replenishment under- or intraplated the partly crystallized basaltic magma, now preserved as the DZ. The intervening CPZ may also owe its origin to the emplacement of the olivine slurry, possibly as a result of pore-scale melt mixing at this interface. The DZ shows inward differentiation trends that can be explained by in situ differentiation. The data imply that late emplacement of olivine-ric

Geodynamic setting and origin of the Oman/UAE ophiolite

The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and UAE (henceforth referred to as the Oman ophiolite) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise (EPR), making it our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, as modern fast-spread ocean crust is largely inaccessible

The significance of plagioclase textures in mid-ocean ridge basalt (Gakkel Ridge, Arctic Ocean)

Textures and compositions of minerals can be used to infer the physiochemical conditions present within magmatic systems. Given that plagioclase is an abundant phase in many magmatic systems, understanding the link between texture and process is vital. Here, we present a database of textural and compositional data for > 1800 plagioclase crystals in mid-ocean ridge basalt from the Gakkel Ridge (Arctic Ocean) to investigate the physiochemical conditions and processes that govern the formation of plagioclase textures and compositions. The Gakkel basalts have high modal crystal contents (up to 50%). The crystal cargo is complex, with both individual plagioclase and glomerocrysts showing large variations in crystal habit, zoning and resorption. The most common types of zoning are reverse and patchy; we attribute patchy zoning to infilling following either skeletal growth or resorption. Resorption is abundant, with multiple resorption events commonly present in a single crystal, and results from both magmatic recharge and decompression. Periods of strong undercooling, distinct to quench crystallisation, are indicated by matured skeletal crystals and thin normally zoned melt inclusion-rich bands following resorption. Individual samples often contain diverse textural and compositional plagioclase groups. Furthermore, most plagioclase is not in equilibrium with its host melt. Finally, the porous open structures of some glomerocrysts suggest that they represent pieces of entrained disaggregated mush. We interpret this to indicate that the crystal cargo is not generally phenocrystic in origin. Instead, plagioclase crystals that formed in different parts of a mush-dominated plumbing system were entrained into ascending melts. The textures of individual crystals are a function of their respective histories of (under)cooling, magma mixing and decompression. The morphologies of melt inclusion trapped in the plagioclase crystals are associated with specific host crystal textures, suggesting a link between plagioclase crystallisation processes and melt inclusion entrapment. The database of plagioclase presented herein may serve as a template for the interpretation of plagioclase textures in magmatic systems elsewhere

Brown amphibole as tracer of tectono-magmatic evolution of the Atlantis Bank Oceanic Core Complex (IODP Hole U1473A)

Brown amphibole is a minor but common mineral component in lower oceanic crust. It is generally interpreted as products of migrating SiO2 and H2O-rich fluids or melts, which can be either residual melts from advanced magmatic differentiation of Mid-Ocean Ridge Basalt (MORB), or hydrothermal fluids including a seawater component. Within the lower oceanic crust exhumed at the Atlantis Bank Oceanic Core Complex (OCC), along the ultraslow Southwest Indian Ridge, brown amphibole is ubiquitous in all lithologies from olivine- to oxide-gabbros and diorites, including both undeformed and plastically deformed varieties. We here show the results of a systematic petrological study conceived to unravel the nature of the H2O-rich component recorded in brown amphiboles and document: (i) the evolution of migrating melts during the magmatic stage and (ii) different extents of melt-bearing deformation events recorded throughout the entire crustal transect. The low Cl contents and the light over heavy Rare Earth Elements (LREE/HREE) ratios and high Ti contents in brown amphiboles, indicate they crystallized from melts with a magmatic hydrous component. Consistently, their δ18O values are in equilibrium with MORB composition, except for diorite amphiboles that possibly record the local assimilation of altered minerals. In undeformed olivine gabbros, interstitial pargasite crystallized at hypersolidus conditions (~1000°C) from the melt residual after late stages of MORB differentiation. We speculate that before the olivine gabbro crystal mush reached fully solid state, some aliquots of residual melts were extracted and accumulated within discrete intervals. There, ferrobasaltic melts differentiated through the early crystallization of Fe-Ti oxides and clinopyroxene as liquidus phases, ultimately forming the oxide gabbros. This process promoted rapid Si enrichment and depletion in Fe, Ti, V in the residual melt, later extracted to form the crosscutting diorite veins. The mylonitic olivine gabbros record high-temperature plastic deformation (~900°C ± 50°C) under hypersolidus conditions, involving melts residual from previous crystallization of the gabbroic rock. Further solid-state plastic deformation led to substantial grain-size reduction and, consequently, to an increase in porosity. This created pathways for subsequent melt focussing, which likely represent late-stage differentiated melts migrating throughout the lower crustal section. This study shows that brown amphibole in the Atlantis Bank lower oceanic crust is the crystallization product of melts residual from advanced magmatic differentiation, which are also locally involved in the plastic deformation events during crustal accretion

Consequences of a crystal mush-dominated magma plumbing system: a mid-ocean ridge perspective

Crystal mush is rapidly emerging as a new paradigm for the evolution of igneous systems. Mid-ocean ridges provide a unique opportunity to study mush processes: geophysical data indicate that, even at the most magmatically robust fast-spreading ridges, the magma plumbing system typically comprises crystal mush. In this paper, we describe some of the consequences of crystal mush for the evolution of the mid-ocean ridge magmatic system. One of these is that melt migration by porous flow plays an important role, in addition to rapid, channelized flow. Facilitated by both buoyancy and (deformation-enhanced) compaction, porous flow leads to reactions between the mush and migrating melts. Reactions between melt and the surrounding crystal framework are also likely to occur upon emplacement of primitive melts into the mush. Furthermore, replenishment facilitates mixing between the replenishing melt and interstitial melts of the mush. Hence, crystal mushes facilitate reaction and mixing, which leads to significant homogenization, and which may account for the geochemical systematics of mid-ocean ridge basalt (MORB). A second consequence is cryptic fractionation. At mid-ocean ridges, a plagioclase framework may already have formed when clinopyroxene saturates. As a result, clinopyroxene phenocrysts are rare, despite the fact that the vast majority of MORB records clinopyroxene fractionation. Hence, melts extracted from crystal mush may show a cryptic fractionation signature. Another consequence of a mush-dominated plumbing system is that channelized flow of melts through the crystal mush leads to the occurrence of vertical magmatic fabrics in oceanic gabbros, as well as the entrainment of diverse populations of phenocrysts. Overall, we conclude that the occurrence of crystal mush has a number of fundamental implications for the behaviour and evolution of magmatic systems, and that mid-ocean ridges can serve as a useful template for trans-crustal mush columns elsewher

Reaction between mid-ocean ridge basalt and lower oceanic crust: an experimental study

Reaction between mid‐ocean ridge basalt (MORB) and crystal mush in the lower oceanic crust has been invoked to explain chemical variations of both MORB and minerals in the lower oceanic crust. Nonetheless, such reactions have been little studied experimentally. We conducted experiments to investigate the mechanisms and chemical consequences of melt‐mush interaction by reacting molten MORB with troctolite at 0.5 GPa. Isothermal experiments demonstrate that melt infiltrates into troctolite with dissolution of plagioclase and olivine. The reacted melts have higher MgO and Al2O3 and lower TiO2 and Na2O contents and crystallize more primitive olivine and plagioclase compared to those crystallized from the unreacted melts, suggesting melt‐mush reaction could result in the formation of high‐Al basalt. The melt compositional variations induced by reaction also significantly affect the calculated pressures for MORB fractionation, indicating that major element‐based barometers for MORB fractionation can only be used reliably if reaction can be ruled out. After reaction, the troctolite contains olivine with plagioclase inclusions and poikilitic clinopyroxene with partially resorbed olivine and plagioclase chadacrysts, indicating that melt‐mush interaction occurs through dissolution‐reprecipitation mechanisms. Clinopyroxene has high Mg# (>83) and elevated Na2O and TiO2 contents, and olivine has different Fo versus Ni correlations from fractional crystallization models, which provide testable parameters for the effect of melt‐mush reaction in the rock record. By comparison with samples from lower oceanic crust and layered intrusions, we propose that melt‐mush reaction plays an important role during magma transport in the crystal mush in both oceanic and continental magma systems

Crystallization of superfast‐spreading oceanic crust (ODP Hole 1256D, Pacific Ocean): constraints from zircon geochronology

Studies of oceanic crust, which covers a large proportion of the Earth's surface, have provided significant insight into the dynamics of crustal accretion processes at mid‐ocean ridges. It is now recognized that the nature of oceanic crust varies fundamentally as a function of spreading rate. Ocean Drilling Program (ODP) Hole 1256D (eastern Pacific Ocean) was drilled into the crust formed at a superfast spreading rate, and hence represents a crustal end member. Drilling recovered a section through lava and sheeted dykes and into the plutonic sequence, the study of which has yielded abundant insight into magmatic and hydrothermal processes operating at high spreading rates. Here, we present zircon U‐Pb dates for Hole 1256D, which constrain the age of the section, as well as the duration of crustal accretion. We find that the main pulse of zircon crystallization within plutonic rocks occurred at 15.19 Ma, consistent with magnetic anomalies, and lasted tens of thousands of years. During this episode, the main plutonic body intruded, and partial melts of the base of the sheeted dykes crystallized. One sample appears to postdate this episode by up to 0.25 Myr, and may be an off‐axis intrusion. Overall, the duration of crustal accretion was tens to several hundreds of thousands of years, similar to that found at the fast‐spreading East Pacific Rise and the slow‐spreading Mid‐Atlantic Ridge. This indicates that crustal accretion along slow‐ to superfast‐spreading ridges occurs over similar time scales, with substantially longer periods of accretion occurring at ultraslow‐spreading ridges characterized by thick lithosphere