The large low shear‐wave velocity provinces (LLSVP) are thermochemical anomalies in the deep Earth's mantle, thousands of km wide and ∼1800 km high. This study explores the hypothesis that the LLSVPs ...are compositionally subdivided into two domains: a primordial bottom domain near the core‐mantle boundary and a basaltic shallow domain that extends from 1100 to 2300 km depth. This hypothesis reconciles published observations in that it predicts that the two domains have different physical properties (bulk‐sound versus shear‐wave speed versus density anomalies), the transition in seismic velocities separating them is abrupt, and both domains remain seismically distinct from the ambient mantle. We here report underside reflections from the top of the LLSVP shallow domain, supporting a compositional origin. By exploring a suite of two‐dimensional geodynamic models, we constrain the conditions under which well‐separated “double‐layered” piles with realistic geometry can persist for billions of years. Results show that long‐term separation requires density differences of ∼100 kg/m3 between LLSVP materials, providing a constraint for origin and composition. The models further predict short‐lived “secondary” plumelets to rise from LLSVP roofs and to entrain basaltic material that has evolved in the lower mantle. Long‐lived, vigorous “primary” plumes instead rise from LLSVP margins and entrain a mix of materials, including small fractions of primordial material. These predictions are consistent with the locations of hot spots relative to LLSVPs, and address the geochemical and geochronological record of (oceanic) hot spot volcanism. The study of large‐scale heterogeneity within LLSVPs has important implications for our understanding of the evolution and composition of the mantle.
Key Points
LLSVPs may be compositionally subdivided into a basaltic shallow domain and a primordial deep domain, consistent with seismic observations
Geodynamic simulations predict different types of plumes to rise from the flanks and roofs of such compositionally “double‐layered” piles
Underside reflections from LLSVP roofs support a basaltic origin for LLSVP shallow domains, and thus a chemical origin for the whole LLSVPs
SUMMARY
We examine the rheology and thermal structure of the oceanic lithosphere, expressed in situ by plate flexure beneath the Hawaiian Ridge, where volcanoes of variable sizes have loaded seafloor ...of approximately the same age, and thus where the lithosphere is expected to have had an approximately uniform age-dependent thermal structure at the time of loading. Shipboard and satellite-derived gravity, as well as multibeam bathymetry data are used in models of plate flexure with curvature-dependent flexural rigidity, the strength of which is limited, in the shallow lithosphere, by brittle failure, and in the deeper lithosphere, by low-temperature plasticity (LTP). We compute relative likelihoods and posterior probabilities for four model parameters: average crustal density ρc, friction coefficient for brittle failure ${\mu _f}$, a pre-exponential weakening factor F controlling the strength of LTP and lithospheric geotherm age t. Results show that if the lithosphere temperatures were as is expected for normal (t = ) 90-Myr-old seafloor at the time of volcano loading, the rheology must be significantly weaker than expected. Specifically, weak brittle strengths (μf ≤ 0.3) show relatively high probabilities for three of the six published LTP flow laws examined. Alternatively, moderate-to-large brittle strengths (μf ≥ 0.5) require all LTP flow laws to be substantially weakened with F = 102 to > 108 or, equivalently, activation energy reduced by 10–35 per cent. In contrast, if the lithosphere has been moderately reheated by the Hawaiian hotspot, represented by geotherms for t = 50–70 Myr, then the flow laws of Evans & Goetze, Raterron et al. and Krancj et al. require little or no weakening. Such modest thermal rejuvenation is allowed by heatflow constraints, supported by regional mantle seismic tomography imaging as well as compositions of mantle xenoliths, and reconciles previously noted discrepancies between the LTP strengths of lithosphere beneath Hawaii versus that entering the Pacific subduction zones.
According to classical plume theory, purely thermal upwellings rise through the mantle, pond in a thin layer beneath the lithosphere, and generate hotspot volcanism. Neglected by this theory, ...however, are the dynamical effects of compositional heterogeneity carried by mantle plumes even though this heterogeneity has been commonly identified in sources of hotspot magmas. Numerical models predict that a hot, compositionally heterogeneous mantle plume containing a denser eclogite component tends to pool at ∼300–410 km depth before rising to feed a shallower sublithospheric layer. This double-layered structure of a thermochemical plume is more consistent with seismic tomographic images at Hawaii than the classical plume model. The thermochemical structure as well as time dependence of plume material rising from the deeper into the shallower layer can further account for long-term fluctuations in volcanic activity and asymmetry in bathymetry, seismic structure, and magma chemistry across the hotspot track, as are observed.
•We simulate thermochemical plume dynamics in the upper mantle in 3D geodynamic models.•The interaction of eclogitic material with phase changes complicates plume dynamics.•An eclogite-bearing plume is therefore expected to pool in the mid upper mantle.•This behavior can explain seismic observations from regional tomography in Hawaii.•Pulsations of eclogite-bearing plumes can also produce variations in hotspot activity.
Oceanic transform faults play an essential role in plate tectonics. Yet to date, there is no unifying explanation for the global trend in broad-scale transform fault topography, ranging from deep ...valleys to shallow topographic highs. Using three-dimensional numerical models, we find that spreading-rate dependent magmatism within the transform domain exerts a first-order control on the observed spectrum of transform fault depths. Low-rate magmatism results in deep transform valleys caused by transform-parallel tectonic stretching; intermediate-rate magmatism fully accommodates far-field stretching, but strike-slip motion induces across-transform tension, producing transform strength dependent shallow valleys; high-rate magmatism produces elevated transform zones due to local compression. Our models also address the observation that fracture zones are consistently shallower than their adjacent transform fault zones. These results suggest that plate motion change is not a necessary condition for reproducing oceanic transform topography and that oceanic transform faults are not simple conservative strike-slip plate boundaries.
An ongoing challenge in studies of the oceanic upper mantle is how intraplate hotspots impact the thermal structure of the lithosphere. To address this issue at the Hawaiian hotspot, we analyze ...mineral compositions for a petrographically diverse suite of garnet pyroxenite xenoliths from the Salt Lake Crater (SLC) rejuvenation stage, volcanic tuff ring in Honolulu. Garnet‐clinopyroxene geobarometry and two‐pyroxene geothermometry indicate equilibrium pressures of 13–18 kbar and temperatures of 1000°C–1100°C. These pressures place the xenoliths at mid‐lithospheric depths of 45–55 km, with temperatures 200°C–300°C hotter than expected for normal 90‐Myr‐old oceanic lithosphere. Garnet and clinopyroxene occur as discrete primary grains, as well as exsolution blebs and lamellae, with lateral dimensions up to several hundred microns. Compositions within garnet and pyroxene grains are remarkably uniform and display no systematic variation with distance to grain boundaries. Together, these observations indicate that the calculated pressures and temperatures reflect the thermal state of the lithosphere under which the xenoliths last equilibrated. We attribute the elevated lithospheric temperatures under Honolulu primarily to the heating by magma as it penetrated the lithosphere during rejuvenation magmatism and the voluminous shield magmatic stage. We anticipate such magmatic heating to be common among all Hawaiian volcanoes, supporting conclusions of a recent study of earthquakes beneath Hawai‘i Island. This local lithospheric thermal anomaly may also contribute to the enigmatically weak flexural response of the lithosphere due to volcano loading along the Hawaiian hotspot chain.
Plain Language Summary
The Hawaiian‐Emperor volcano chain in the Pacific Ocean is one of the most well‐known surface expressions of a mantle hotspot, having persisted over the past 82 Myrs. This continued influx of magma and heat has impacted the thermal structure of the Pacific tectonic plate to an unknown degree. This study directly addresses this issue using rocks (xenoliths) that originated in the lithospheric plate, but became engulfed in magma as it passed through the plate and eventually erupted at Salt Lake Crater, a late‐stage volcanic ring tuff on the island of O‘ahu. We use the chemical compositions of the minerals, as well as their modes of occurrence found in the mantle xenoliths to calculate the temperatures and pressures (i.e., depths) at which the xenoliths equilibrated. We find that the xenoliths equilibrated at depths where temperature was significantly higher than expected for normal oceanic lithosphere having the same age as the Hawaiian lithosphere. We infer that these anomalous temperatures were introduced by magma that rose from the hot mantle through the lithospheric plate during the volcanic construction of the island of O‘ahu. Such magmatic heating may impact how the plate flexes and faults due to the enormous weight of this and the other Hawaiian volcanoes.
Key Points
Revised geothermometer and new geobarometer calculations place the xenoliths at mid‐lithospheric depths of 45–55 km
The temperatures at these depths are 200°C–300°C warmer than expected for normal lithosphere beneath 90‐Myr‐old seafloor
The adjacent lithosphere is inferred to have been reheated rapidly during the voluminous Ko'olau shield volcano magmatism
Tectonic-plate material is generally thought to be neither created nor destroyed at plate boundaries called oceanic transform faults. An analysis of sea-floor topography suggests that this assumption ...is incorrect.
We use 2‐D numerical models to explore the thermal and mechanical effects of magma intrusion on fault initiation and growth at slow and intermediate spreading ridges. Magma intrusion is simulated by ...widening a vertical column of model elements located within the lithosphere at a rate equal to a fraction, M, of the total spreading rate (i.e., M = 1 for fully magmatic spreading). Heat is added in proportion to the rate of intrusion to simulate the thermal effects of magma crystallization and the injection of hot magma into the crust. We examine a range of intrusion rates and axial thermal structures by varying M, spreading rate, and the efficiency of crustal cooling by conduction and hydrothermal circulation. Fault development proceeds in a sequential manner, with deformation focused on a single active normal fault whose location alternates between the two sides of the ridge axis. Fault spacing and heave are primarily sensitive to M and secondarily sensitive to axial lithosphere thickness and the rate that the lithosphere thickens with distance from the axis. Contrary to what is often cited in the literature, but consistent with prior results of mechanical modeling, we find that thicker axial lithosphere tends to reduce fault spacing and heave. In addition, fault spacing and heave are predicted to increase with decreasing rates of off‐axis lithospheric thickening. The combination of low M, particularly when M approaches 0.5, as well as a reduced rate of off‐axis lithospheric thickening produces long‐lived, large‐offset faults, similar to oceanic core complexes. Such long‐lived faults produce a highly asymmetric axial thermal structure, with thinner lithosphere on the side with the active fault. This across‐axis variation in thermal structure may tend to stabilize the active fault for longer periods of time and could concentrate hydrothermal circulation in the footwall of oceanic core complexes.
Oceanic detachment faults represent an end-member form of seafloor creation, associated with relatively weak magmatism at slow-spreading mid-ocean ridges. We use 3-D numerical models to investigate ...the underlying mechanisms for why detachment faults predominantly form on the transform side (inside corner) of a ridge-transform intersection as opposed to the fracture zone side (outside corner). One hypothesis for this behavior is that the slipping, and hence weaker, transform fault allows for the detachment fault to form on the inside corner, and a stronger fracture zone prevents the detachment fault from forming on the outside corner. However, the results of our numerical models, which simulate different frictional strengths in the transform and fracture zone, do not support the first hypothesis. Instead, the model results, combined with evidence from rock physics experiments, suggest that shear-stress on transform fault generates excess lithospheric tension that promotes detachment faulting on the inside corner.
Understanding the partial melting process is key to our ability to relate geochemical characteristics of hotspot and mid-ocean ridge lavas to the dynamics and chemical structure of the mantle. We ...present a method of computing the trace-element and isotopic compositions of magmas generated by melting a heterogeneous source in mantle plumes and beneath mid-ocean ridges. The method simulates fractional melting with thermodynamically consistent melting functions of three mantle lithologies, each with a distinct trace-element and isotopic composition. A key assumption is that enriched mantle peridotite (EM) and pyroxenite (PX) both begin melting deeper than depleted mantle peridotite (DM). Model calculations can explain many features observed in a compilation of ocean island basalt (OIB) and mid-ocean ridge basalt (MORB) data. In particular, La/Sm and Sr, Nd, and Pb isotope ratios are most variable and extend to compositions most distinct from average MORB compositions in islands formed on relatively old seafloor (>∼16 m.y.). The compositions of hotspot islands erupting on younger seafloor are less variable and overlap more appreciably with common MORB compositions. Models predict these observations to result from the effect of lithospheric thickness in limiting the extent of partial melting. Beneath old seafloor where the lithosphere is thick, models predict the overall extent of partial melting and the extent of DM melting to be small; therefore, magma compositions are most sensitive to changes in the relative proportions of EM and PX, as well as to mantle temperature. Beneath young seafloor where the lithosphere is thin, models predict more extensive melting of all lithologies. Progressive extraction of the different components during melting can also explain the general topology of isotopic arrays defined by OIBs globally. Our models not only predict testable variations in trace-element and isotopic ratios with mean extent of partial melting, but also show that pyroxenite can complicate such correlations because it can melt to very high extents, in many cases, completely. If the thickness of the mantle plume layer beneath the lithosphere exceeds that of the DM melting zone, an increase in the mantle flow rate with depth, which is predicted for mantle plumes, can enhance the flux of incompatible elements and isotopes derived from PX and EM, relative to the case of the more uniform flow profile expected beneath mid-ocean ridges. Thus, differences in both mantle flow and lithospheric thickness between mid-ocean ridges and hotspots can lead to important distinctions in the isotopic and trace-element characteristics of MORBs and OIBs, independent of any compositional differences between their respective mantle sources. In addition, variations in mantle flow alone can contribute to geographic variations in hotspot geochemistry observed both in intraplate settings and where hotspots interact with mid-ocean ridges.
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