An official website of the United States government
Abstract The architecture of lower oceanic crust at slow- and ultraslow-spreading ridge is diverse, yet the mechanisms that produce this diversity are not well understood. Particularly, the 660-km2 gabbroic massif at Atlantis Bank (Southwest Indian Ridge) exhibits significant compositional zonation, representing a high magma supply end member for accretion of the lower ocean crust at slow and ultraslow-spreading ridges. We present the petrographic and geochemical data of olivine gabbros from the 809-meter IODP Hole U1473A at Atlantis Bank gabbroic massif. Structurally, the upper portion of U1473A consists of a ∼600-meter shear zone; below this, the hole is relatively undeformed, with several minor shear zones. Olivine gabbros away from the shear zones have mineral trace element compositions indicative of high-temperature reaction with an oxide-undersaturated melt. By contrast, olivine gabbros within shear zones display petrographic and chemical features indicative of reaction with a relatively low-temperature, oxide-saturated melt. These features indicate an early stage of primitive to moderately evolved melt migration, followed by deformation-driven transport of highly evolved Fe-Ti-rich melts to high levels in this gabbroic massif. The close relationship between shear zones and the reaction with oxide-saturated melts suggests that syn-magmatic shear zones provide a conduit for late-stage, Fe-Ti-rich melt transport through Atlantis Bank lower crust. This process is critical to generate the compositional zonation observed. Thus, the degree of syn-magmatic deformation, which is fundamentally related to magma supply, plays a dominant role in developing the diversity of lower ocean crust observed at slow- and ultraslow-spreading ridges.
more »
« less
Trans‐Lithospheric Ascent Processes of the Deep‐Rooted Magma Plumbing System Underneath the Ultraslow‐Spreading SW Indian Ridge
Abstract Processes of magma generation and transportation in global mid‐ocean ridges are key to understanding lithospheric architecture at divergent plate boundaries. These magma dynamics are dependent on spreading rate and melt flux, where the SW Indian Ridge represents an end‐member. The vertical extent of ridge magmatic systems and the depth of axial magma chambers (AMCs) are greatly debated, in particular at ultraslow‐spreading ridges. Here we present detailed mineralogical studies of high‐Mg and low‐Mg basalts from a single dredge on Southwest Indian Ridge (SWIR) at 45°E. High‐Mg basalts (MgO = ∼7.1 wt.%) contain high Mg# olivine (Ol, Fo = 85–89) and high‐An plagioclase (Pl, An = 66–83) as phenocrysts, whereas low‐Mg basalts contain low‐Mg# Ol and low‐An Pl (Fo = 75–78, An = 50–62) as phenocrysts or glomerocrysts. One low‐Mg basalt also contains normally zoned Ol and Pl, the core and rim of which are compositionally similar to those in high‐Mg and low‐Mg basalts, respectively. Mineral barometers and MELTS simulation indicate that the high‐Mg melts started to crystallize at ∼32 ± 7.8 km, close to the base of the lithosphere. The low‐Mg melts may have evolved from the high‐Mg melts in an AMC at a depth of ∼13 ± 7.8 km. Such great depths of magma crystallization and the AMC are likely the result of enhanced conductive cooling at ultraslow‐spreading ridges. Combined with diffusion chronometers, the basaltic melts could have ascended from the AMC to seafloor within 2 weeks to 3 months at average rates of ∼0.002–0.01 m/s, which are the slowest reported to date among global ridge systems and may characterize mantle melt transport at the slow end of the ridge spreading spectrum.
more »
« less
- Award ID(s):
- 1657983
- PAR ID:
- 10563186
- Publisher / Repository:
- American Geophysical Union
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 129
- Issue:
- 1
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The discovery of systematic differences in the trace element composition of forsteritic olivines in primitive magmas from within-plate, arc and mid-ocean ridge volcanoes engendered much debate about a causal link to the recycling of oceanic crust into the mantle sources of within-plate and arc magmas. Here we address this problem using Cr-spinel bearing, forsteritic (~Fo80–91) olivines from high-Mg# = 50 = 73 [Mg# = molar ratio of Mg/(Mg + Fe2+)*100] arc magmas from the Trans-Mexican Volcanic Belt (TMVB). The TMVB arc front olivines have similar high Ni, low MnO, and low Mn/Fe as forsteritic olivines from within-plate basalts erupting through thick lithosphere (= WPB-thick). However, the olivines in TMVB arc front primary melts crystallize at much lower temperatures of $${T}_{\mathrm{cryst}}^{\mathrm{oliv}}$$~1119 ± 38 °C (calculated with olivine–spinel aluminum exchange thermometry) in hydrous (~4–9 wt % H2O), silicic, less magnesian (≤10 wt % MgO) mantle melts from mostly garnet-free mantle sources. Model calculations suggest that the primary arc front melts last equilibrated in the mantle at pressures of ~1.4 to ~1.9 GPa (~51–69 km depth) and low temperatures (Tsource = 1150 ± 45 °C) that are only slightly higher than the olivine crystallization temperatures. While the $${\mathrm{Kd}}_{\mathrm{oliv}/\mathrm{melt}}^{\mathrm{Ni}}$$ increases in the cooler and silicic melts, such modulation cannot account for the full range of Ni concentration in TMVB magmatic olivines. A small population of very high-Ni olivines (>4000–5500 μg/g Ni) is best explained by crystallization in Ni-rich components melt that formed by melt rock reaction processes in the mantle wedge. Unlike Ni, olivine MnO is not sensitive to melt temperature and only moderately to melt composition, and thus retains mantle source characteristics. In the TMVB, olivine Fo-MnO-Mn/Fe systematics record an ambient mantle wedge (= mantle without slab component) that is similar to WPB sources and that is variably depleted by slab flux-driven melt extraction. Overall, the olivine Fo-Ni-MnO systematics confirm with greater detail than possible by bulk rock studies that the TMVB primary melts are hydrous and silicic and originate from a mantle wedge that is strongly and variably modified by the slab flux. These results reaffirm a strong genetic link between slab recycling and the genesis of silicic arc magmas.more » « less
-
Abstract Trace element concentrations in abyssal peridotite olivine provide insights into the formation and evolution of the oceanic lithosphere. We present olivine trace element compositions (Al, Ca, Ti, V, Cr, Mn, Co, Ni, Zn, Y, Yb) from abyssal peridotites to investigate partial melting, melt–rock interaction, and subsolidus cooling at mid-ocean ridges and intra-oceanic forearcs. We targeted 44 peridotites from fast (Hess Deep, East Pacific Rise) and ultraslow (Gakkel and Southwest Indian Ridges) spreading ridges and the Tonga trench, including 5 peridotites that contain melt veins. We found that the abundances of Ti, Mn, Co, and Zn increase, while Ni decreases in melt-veined samples relative to unveined samples, suggesting that these elements are useful tracers of melt infiltration. The abundances of Al, Ca, Cr, and V in olivine are temperature sensitive. Thermometers utilizing Al and Ca in olivine indicate temperatures of 650–1000 °C, with variations corresponding to the contrasting cooling rates the peridotites experienced in different tectonic environments. Finally, we demonstrate with a two-stage model that olivine Y and Yb abundances reflect both partial melting and subsolidus re-equilibration. Samples that record lower Al- and Ca-in-olivine temperatures experienced higher extents of diffusive Y and Yb loss during cooling. Altogether, we demonstrate that olivine trace elements document both high-temperature melting and melt–rock interaction events, as well as subsolidus cooling related to their exhumation and emplacement onto the seafloor. This makes them useful tools to study processes associated with seafloor spreading and mid-ocean ridge tectonics.more » « less
-
Abstract Arc magmas are produced from the mantle wedge, with possible addition of fluids and melts derived from serpentinites and sediments in the subducting slab. Identification of various sources and their relevant contributions to such magmas is challenging; in particular, at continental arcs where crustal assimilation may overprint initial geochemical signatures. This study presents oxygen isotopic compositions of zoned olivine grains from post-caldera basalts and boron contents and isotopes of these basalts and glassy melt inclusions hosted in quartz and clinopyroxene of silicic tuffs in the Toba volcanic system, Indonesia. High-magnesian (≥87 mol% Fo [forsterite]) cores of olivine in the basalts have δ18O values ranging from 5.12‰ to 6.14‰, indicating that the mantle source underneath Toba is variably enriched in 18O. Olivine with <87 mol% Fo has highly variable (4.8–7.2‰), but overall increased, δ18O values, interpreted to reflect assimilation of high δ18O crustal materials during fractional crystallization. Mass balance calculations constrain the overall volume of crustal assimilation for the basalts as ≤13%. The processes responsible for the 18O-enriched basaltic melts are further constrained by boron data that indicate the addition of <0.1 wt% fluids to the mantle, >40% of the fluids being derived from serpentinites and others from altered oceanic crust and sediments. This amount of fluids can increase δ18O of the magma by only ~0.02‰. Approximately 6–9% sediment-derived melt hybridization in the mantle wedge is further needed to yield basaltic melts with δ18O values in equilibrium with those of the high-Fo olivine cores. The cogenetic silicic tuffs, on the other hand, seem to record a higher proportion of fluid addition dominated by sediment-derived fluids to the mantle source, in addition to crustal assimilation. Our reconnaissance study therefore demonstrates the application of combined B and O isotopes to differentiate between melts and fluids derived from serpentinites and sediments in the subducted slab—an application that can be applied to arc magmas worldwide.more » « less
-
Abstract Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1029/2023JB027224
