Kathryn M. Gillis

Structure of uppermost fast-spread oceanic crust exposed at the Hess Deep Rift: Implications for subaxial processes at the East Pacific Rise

The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr−1, full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (20°–70°W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (90°–40°E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1–2 km of the EPR that accommodated the thickening of the basaltic lava unit to ∼500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

Near-solidus evolution of oceanic gabbros: Insights from amphibole geochemistry

The near-solidus evolution of plutonic rocks formed at slow-spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the Mid-Atlantic Ridge. These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin. In turn, this allows the gabbro solidus to be constrained to 860 ± 30°C, using amphibole-plagioclase thermometry. This is consistent with temperatures from associated secondary clinopyroxene. Magmatic amphibole, which can be identified in almost all samples, formed during metasomatism of a low-porosity crystal mush by an evolved hydrous silicate melt. These amphiboles are characterised by high F, Nb, and F/Cl and low Cl contents. The amphibole-forming reaction involved melt, plagioclase, and clinopyroxene. Amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals. There is no evidence for a seawater component in the magmatic amphibole, as would be expected if high-temperature seawater ingress leads to flux melting, as has recently been suggested. However, the ingress of seawater-derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases. These amphiboles are characterised by high Cl, B, and Cl/F and low Nb, F, and Nb/La. The fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids.

Cracking at the magma–hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus

Abstract The nature of the magma–hydrothermal transition in oceanic hydrothermal systems is poorly understood, in part because the geological relations in this critical region have rarely been observed in modern ocean crust. Detailed mapping was conducted in the Troodos Ophiolite, Cyprus, where a gabbronorite sequence intrudes the sheeted dyke complex, which is truncated at its base by a thin contact aureole composed of massive hornfels. Geothermometric data for hornblende and pyroxene hornfels show that hydrated sheeted dykes were recrystallized at amphibolite to granulite facies conditions (778–986°C). Quartz diorite veins and apophyses, and monomineralic amphibole veins cross-cut the contact aureole and show no preferred age relationships. Geothermometric data indicate that quartz diorite was injected at 817–919°C and that fractures were filled with amphibole at 575–750°C. Phase relations of quartz-hosted, halite-bearing fluid inclusions in quartz diorite veins constrain minimum entrapment temperatures of 225–520°C (average 402°C) and minimum pressures that span lithostatic and hydrostatic conditions. We believe that these characteristics are indicative of a conductive boundary layer that separates an active hydrothermal system from the heat source that drives it. Field and petrological data indicate that transient fracturing caused oscillations in temperature and pressure conditions within the conductive boundary layer, and mixing of hydrothermal and magmatic fluids at the magma–hydrothermal interface. Cross-cutting relations between magmatic and hydrothermal vein networks show that fracturing occurred prior to the cessation of magmatic activity. We explore plausible models for the causes and consequences of fracturing that consider the role of dyke injection, thermoelastic stresses, and volatile build-up.

Fluid evolution in submarine magma‐hydrothermal systems at the Mid‐Atlantic Ridge

Fluid inclusions in a suite of gabbro, quartz-breccia, and metabasalt samples recovered from the MARK area on the Mid-Atlantic Ridge are the product of a complex hydrothermal history involving late stage magmatic fluids at temperatures >700°C and penetration by modified seawater at 300–400°C. The evolution of volatiles during the early stages of solidification and cooling of magma bodies near the ridge-transform intersection is marked by exsolution of a CO2 fluid, entrapped within primary inclusions in fluorapatites. Attendant with progressive melt fractionation, residual evolved melts reached water saturation, and locally, supercritical CO2+H2O+NaCl±Fe brines (>50 wt % NaCl) and cogenetic H2O+CO2-rich vapors (1–2 wt % NaCl) were exsolved as immiscible phases. Concomitant or subsequent fracturing, perhaps in response to volatile exsolution from the melts, allowed migration of these fluids along microfracture networks at >700°C. Trondhjemitic-hosted inclusions, which homogenize by halite dissolution, indicate that the last fluids exsolved from the melts may have been 35–40 wt % brines. The transition from magmatic to seawater-dominated hydrothermal conditions in the gabbros is marked by initial penetration of lower salinity fluids (1–7 wt % NaCl) at temperatures in excess of 400°C, with the general cessation of fluid flow occurring at minimum temperatures of ≈ 250°C. The relative enrichment and depletion of NaCl with respect to seawater in these fluids may record supercritical phases separation of seawater or boiling of hydrothermal fluids enriched in NaCl. Migration along microfracture networks of CH4-rich, 350°C fluids, may reflect deeper seated hydrothermal processes involving hydration of underlying mantle material in response to fluid flow along deeply penetrating fault systems. In shallow crustal rocks, circulation of seawater-derived fluids occurred at temperatures up to 400°C, with subsequent collapse of the active hydrothermal system at minimum temperatures of 200–250°C. In fault-related upflow zones, multiple hydrothermal pulses involving 180–340°C and 3.5–10 wt % NaCl fluids, pervasively altered bounding wall rocks, forming chlorite-rich, pyrite- and chalcopyrite-bearing breccias. At shallow crustal depths, fluids reached temperatures of 150–300°C and contained salinities of 3.8–6.9 wt % NaCl. Following collapse of the axial-related hydrothermal system, the plutonic and shallow crustal rocks were uplifted and emplaced as allocthonous blocks attending formation of the ridge-transform intersection massif.

A view of the lower crustal component of hydrothermal systems at the Mid-Atlantic Ridge

An extensive suite of hydrothermally altered gabbros was recovered by Alvin and dredging from the Mid-Atlantic Ridge, in the vicinity of the eastern ridge-transform intersection (RTI) of the Kane Fracture Zone (MARK), where tectonic extension has provided a window into the lower crustal component of hydrothermal cells. Four alteration types are distinguished on the basis of metamorphic assemblage, mineral composition, and deformation textures. A conceptual model is presented that places temperature - fluid/rock ratio relationships, in conjunction with styles of deformation, into a spatial and temporal framework. Exsolution of late stage magmatic fluids in the vicinity of the magma-hydrothermal interface at temperatures >700°C marks the onset of fluid-rock interaction in the MARK gabbros (Kelley et al., this issue), with subsequent alteration involving seawater-derived fluids being strongly influenced by deformation mechanisms. In zones of ductile shear, hydration was initiated at temperatures between 550° and 700°C and moderate fluid/rock ratios. Elsewhere in the lower crust, the onset of seawater penetration occurred at temperatures of 400°–550°C and was facilitated by brittle fracturing. Early vein networks indicate very low initial fluid/rock ratios and are interpreted as being related to the solidification of plutons. Later fractures indicate higher water/rock ratios and may be related to the downward propagation of shallow fault systems. Throughout the crust, fluid-rock interaction ceased at temperatures 180°–300°C. Subsequent cataclastic deformation associated with the unroofing and emplacement of crustal blocks in the RTI massif produced moderate fluid/rock ratios within localized zones. Hydrothermal alteration in the crustal section exposed at MARK was initiated at lower temperatures, and water/rock interaction proceeded to lower temperatures than many other plutonic suites. These differences must reflect the temporal and spatial evolution of magmatic and tectonic extension that is particular to each section of sampled crust.

Fluid flow patterns in fast spreading East Pacific Rise crust exposed at Hess Deep

Tectonic exposures of a volcanic sequence and sheeted dike complex over a 4-km-wide region at Hess Deep (equatorial Pacific) reveal significant spatial heterogeneity (10–103 m) in the extent and nature of hydrothermal alteration in young, fast spreading East Pacific Rise crust. The volcanic sequence is fairly uniformly altered, with only minor oxidation and alteration to clay minerals. Sheeted dikes in the eastern part of the field area are highly fractured with narrow intervals of intact dikes that dip up to 60°. Their alteration characteristics show a simple depth trend such that with increasing depth the dominant secondary mafic mineral changes from chlorite to amphibole, clinopyroxene replacement increases ( 40%), whole rock δ18O values decreases (4.4–5.5‰ to 3.5–4.5‰), and calculated peak metamorphic temperatures increase (∼250°C to 450°–700°C). Within the deepest dikes, localized zones up to 400-m-wide are chlorite-rich and have low-δ18O (2.9–4.1‰) and low peak metamorphic temperatures (∼345°C). These alteration patterns likely formed within broad recharge zones whereby the low-δ18O zones developed in the regions with the highest fluid flux. In the west, massive, slightly rotated sheeted dikes near the volcanic-sheeted dike transition are δ18O and Cu depleted and display higher peak temperatures (≥345°C) than elsewhere in the shallow dikes. These characteristics are consistent with formation within a high temperature, hydrothermal discharge zone. We propose that the spreading history of a fast spreading ridge segment can create significant spatial heterogeneity in fluid flow and alteration patterns within sheeted dike complexes, similar to those preserved in many ophiolites.

The roof of an axial magma chamber: A hornfelsic heat exchanger

The heat flux from an axial magma chamber into the convecting hydrothermal system at mid-ocean ridges is governed by the conductive boundary layer separating them. The nature of this critical interface has been modeled and seismically imaged, but its characteristics have hitherto rarely been documented in ocean crust. Here, hornfelsic rocks from the sheeted dike–gabbro transitions at two tectonic exposures of fast-spreading East Pacific Rise crust at the Pito and Hess Deeps and in the Oman and Troodos ophiolites are described. These rocks record thermal metamorphism to hornblende and pyroxene hornfels at 700–1000 °C. These temperatures, in conjunction with their geological relationships, imply that the hornfels are preserved fragments of conductive boundary layers. Their distribution at the base of sheeted dike complexes in narrow contact aureoles and dikes intruding the uppermost gabbros tracks the vertical migration of axial magma chambers over minimum depth ranges of 200–400 m. The heat flux across the hornfelsic aureoles (11–44 MW/km) is comparable to hydrothermal fluxes released along fast-spreading ridges, confirming that the heat driving hydrothermal convection is transferred across conductive boundary layers.

Primitive layered gabbros from fast-spreading lower oceanic crust

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies and ancient analogues (ophiolites) that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle and mid-ocean-ridge basalt differentiation. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

Role of upwelling hydrothermal fluids in the development of alteration patterns at fast spreading ridges: Evidence from the sheeted dike complex at Pito Deep

Alteration of sheeted dikes exposed along submarine escarpments at the Pito Deep Rift (NE edge of the Easter microplate) provides constraints on the crustal component of axial hydrothermal systems at fast spreading mid-ocean ridges. Samples from vertical transects through the upper crust constrain the temporal and spatial scales of hydrothermal fluid flow and fluid-rock reaction. The dikes are relatively fresh (average extent of alteration is 27%), with the extent of alteration ranging from 0 to >80%. Alteration is heterogeneous on scales of tens to hundreds of meters and displays few systematic spatial trends. Background alteration is amphibole-dominated, with chlorite-rich dikes sporadically distributed throughout the dike complex, indicating that peak temperatures ranged from 450°C and did not vary systematically with depth. Dikes locally show substantial metal mobility, with Zn and Cu depletion and Mn enrichment. Amphibole and chlorite fill fractures throughout the dike complex, whereas quartz-filled fractures and faults are only locally present. Regional variability in alteration characteristics is found on a scale of <1–2 km, illustrating the diversity of fluid-rock interaction that can be expected in fast spreading crust. We propose that much of the alteration in sheeted dike complexes develops within broad, hot upwelling zones, as the inferred conditions of alteration cannot be achieved in downwelling zones, particularly in the shallow dikes. Migration of circulating cells along rides axes and local evolution of fluid compositions produce sections of the upper crust with a distinctive character of alteration, on a scale of <1–2 km and <5–20 ka.

Volatile element (B, Cl, F) behaviour in the roof of an axial magma chamber from the East Pacific Rise

Abstract Understanding the behaviour of volatile elements at mid-ocean ridges is important for reasons ranging from their influence on mantle viscosity through to their role as a food source for the deep biosphere. With the aim of constraining what processes control the distribution of volatiles in the ocean crust at fast-spreading ridges, we present a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic sequence at the East Pacific Rise (EPR). These amphiboles are massively enriched in chlorine (by more than an order of magnitude), and moderately enriched in boron, with respect to magmatic amphiboles in cumulates from the Mid-Atlantic Ridge (MAR). Similar enrichments have been reported for basaltic glasses from the EPR and are interpreted as indicative of assimilation. The greater enrichments observed in the plutonic section suggest both that assimilation occurs at the roof of the axial magma chamber (AMC) and that lava compositions may record minimum amounts of exogenic contamination. Amphiboles with compositions indicative of crystallisation from a contaminated magma occur to depths of 800 m beneath the sheeted dyke complex. This is interpreted to indicate that at least this upper portion of the plutonic section forms via crystallisation within the AMC followed by subsidence of a crystal mush. Amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids drives AMC magmas to heavier δ11B values (up to +5.8‰). Subsequent degassing within a solidifying crystal mush leads to a negative trend in δ11B–B with the most degassed magma having δ11B as low as −21.2‰. This degassing was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link with the overlying hydrothermal system.