Jason Phipps Morgan

Deep Roots of the Messinian Salinity Crisis

The Messinian salinity crisis—the desiccation of the Mediterranean Sea between 5.96 and 5.33 million years (Myr) ago—was one of the most dramatic events on Earth during the Cenozoic era. It resulted from the closure of marine gateways between the Atlantic Ocean and the Mediterranean Sea, the causes of which remain enigmatic. Here we use the age and composition of volcanic rocks to reconstruct the geodynamic evolution of the westernmost Mediterranean from the Middle Miocene epoch to the Pleistocene epoch (about 12.1–0.65u2009Myr ago). Our data show that a marked shift in the geochemistry of mantle-derived volcanic rocks, reflecting a change from subduction-related to intraplate-type volcanism, occurred between 6.3 and 4.8u2009Myr ago, largely synchronous with the Messinian salinity crisis. Using a thermomechanical model, we show that westward roll back of subducted Tethys oceanic lithosphere and associated asthenospheric upwelling provides a plausible mechanism for producing the shift in magma chemistry and the necessary uplift (∼1u2009km) along the African and Iberian continental margins to close the Miocene marine gateways, thereby causing the Messinian salinity crisis.

The spreading rate dependence of three‐dimensional mid‐ocean ridge gravity structure

In this paper we observe that isomorphism classes of certain metrized vector bundles over P Z −{0,∞} can be parameterized by arithmetic quotients of loop groups. We construct an asymptotic version of theta functions, which are defined on these quotients. Then we prove the convergence and extend the theta functions to loop symplectic groups. We interpret them as sections of line bundles over an infinite dimensional torus, discuss the relations with loop Heisenberg groups, and give an asymptotic multiplication formula.We analyze over 1300 km of high resolution along-axis gravity profiles at ridges with half-spreading rates ranging from 1.2 to 5.5 cm/yr. The results show consistently higher along-axis gradients of mantle Bouguer anomaly at the slow-spreading Mid-Atlantic Ridge (MAR) (0.3–1.2 mgal/km) than at the intermediate- to fast-spreading Cocos-Nazca Ridge and East Pacific Rise (EPR) (0.1–0.2 mgal/km). The regional peak-to-trough amplitude of mantle Bouguer anomaly is also greater along the MAR (30–60 mgal) than the Cocos-Nazca Ridge and the EPR (10–20 mgal). With increasing spreading rate, the regional peak-to-trough amplitude of axial seafloor depth decreases from 1000–1700 m to 200–700 m. 3-D numerical experiments suggest that mantle contributions to the gravity can be significant only near large-offset transforms. At the more commonly observed non-transform offsets, gravity anomalies will reflect crustal thickness variations. n nThe along-axis gravity data thus indicate that the amplitude of along-axis crustal thickness variation decreases with increasing spreading rate. We propose that this spreading rate dependent crustal accretion style may originate in the mantle: finite-amplitude mantle upwelling is intrinsically plume-like (3-D) beneath a slow-spreading ridge but more sheet-like (2-D) beneath a fast-spreading ridge. Such a transition in mantle upwelling may occur if the relative importance of passive upwelling over buoyant upwelling increases with increasing spreading rate. Small amplitude 3-D upwellings may occur at a fast-spreading ridge, but their effects on crustal thickness variations will be significantly reduced by along-axis melt flows along a persistent low-viscosity crustal magma chamber. In contrast, the large crustal thickness variations due to 3-D mantle upwellings will be maintained at a slow-spreading ridge because less along-axis melt flows can occur in the colder and more rigid crust there.

Are the regional variations in Central American arc lavas due to differing basaltic versus peridotitic slab sources of fluids

Central American arc volcanism shows strong regional trends in lava chemistry that result from differing slab contributions to arc melting. However, the mechanism that transfers slab-derived trace elements into the mantle wedge remains largely unknown. By using a dynamic model for mantle flow and fluid release, we model the fate of three different slab-fluid sources: sediment, ocean crust, and serpentinized mantle. In the open subarc system, sediments lose almost all their highly fluid mobile elements by ∼50 km depth, so other fluid sources are necessary to explain the slab signal in arc-lava compositions. The well-documented transition from lavas with a strong geochemical slab signature (i.e., high Ba/La ratios) found in Nicaragua to lavas with a weaker slab signature (i.e., low Ba/La ratios) erupted in Costa Rica seems easiest to produce by a higher fraction of serpentine-hosted fluids released from the deeply faulted, highly serpentinized lithosphere subducting beneath Nicaragua than from the less deeply faulted, thicker, amphibolitic oceanic-crust and oceanic-plateau lithosphere subducting beneath Costa Rica.

Two-stage melting and the geochemical evolution of the mantle: a recipe for mantle plum-pudding

Abstract We explore a geochemical model for mantle evolution where a sequence of hotspot and ridge upwelling has melted the mantle to make hotspot and mid-ocean ridge basalts and their residues, and plate subduction has re-cycled and stirred all of these differentiation products back into the mantle. After billions of years this process has mixed various `plums of incompatible-element rich veins within a matrix made from the residues of melting that have been depleted in incompatible elements. We propose that the mantle flows upward and melts in a two-stage process. During the first stage, plume upwelling and melting creates an enriched ocean island basalt by extracting a low degree melt (∼1–4%) from the rising mantle mixture. The plums are easier to melt, so proportionally more of the incompatible elements are extracted from these components. After melt extraction, the mixture of leftovers is depleted in composition, even though it still contains ∼96–99% of the mass of the original plume upwelling. These depleted leftovers are hot and buoyant so they pond beneath the lithosphere as an asthenosphere layer. When they rise and melt a second time beneath a mid-ocean ridge, a depleted mid-ocean ridge basalt is extracted. The now extremely depleted leftovers, ∼85% of the mass of the original plume upwelling, accrete to oceanic lithosphere which eventually subducts to recycle leftovers, eroded continental crust, and basaltic plums back into the mantle. Observed trace element, rare gas, and isotopic contrasts between oceanic island and mid-ocean ridge basalts can be produced by a recipe which assumes that throughout Earth history these two sequential stages of deep plume and shallower ridge melting have both created and reprocessed the plums and residues that make up the present-day mantle. In this recipe the two-stage melting process does not change through time, but the rate of mantle overturn slows over time in proportion to the decrease in radioactive heat production.

Thermodynamics of pressure release melting of a veined plum pudding mantle

[1]xa0This study explores the thermodynamics of adiabatic decompression melting of peridotitic mantle containing pyroxenite veins that have lower solidi than the peridotite. When a vein of lower solidus-temperature material melts adjacent to more refractory material, additional heat will flow into the melting region to increase its melting productivity. If pyroxenite veins have a solidus-depletion gradient ((∂Tm/∂F)P) like that of olivine or peridotite, then the melting of the veins is enhanced by up to a factor of 4 by this heat. However, the solidus-depletion gradient of pyroxenites is apparently lower than that of peridotites; thus pyroxenite melting would be even more enhanced. If pyroxenite veins have a gentler solidus-pressure (T-P) dependence (i.e., lower (∂Tm/∂P)F) than that of peridotite solidi, then although these veins will experience enhanced melting while they are the only melting assemblage, they will stop melting soon after their peridotite matrix begins to melt. During large-scale peridotite melting the material ascends along a T-P path close to that of the peridotite solidus, so that the mixtures temperature remains lower than the solidus of the residual pyroxenite, and pyroxenite melting ceases throughout the shallower sections of the melting column. If pyroxenitic material makes up a large fraction of the mantle mixture (∼20%), then the heat consumed by deep pyroxenite melting cools the ascending mantle mixture enough so that peridotite melting begins ∼5–10 km shallower than it would in the absence of precurser pyroxenite melting. After recycling into the mantle, the melt extraction residue will again melt if it is reheated to ambient mantle temperatures and rises again to shallow depths.

The generation of a compositional lithosphere by mid-ocean ridge melting and its effect on subsequent off-axis hotspot upwelling and melting

The generation of a ∼50–500 times more viscous and 1–2% less dense depleted restite layer by melting and melt extraction beneath a mid-ocean ridge can be a significant barrier to subsequent off-axis upwelling and melting. This compositional lithosphere is formed by melting at a mid-ocean ridge which results in the “dewatering” and increase in Mg# of olivine crystals in the restite residue from melt-extraction; effects which can produce up to a 1000-fold increase in the viscosity of the restite relative to upwelling asthenosphere [1,2]. Compositional lithosphere may be the principle mechanical barrier to plume ascent and melting beneath young seafloor. The formation of compositional lithosphere at the ridge axis would also lead to uniform horizontal motion beneath the thickening thermal boundary layer — this may be the physical reason underlying the excellent bathymetric fits of age-plate cooling models which assume this simple pattern of flow. The presence of a ∼ 1021 Pa-s restite layer within and above the primary melting generation region of a mid-ocean ridge may also provide a mechanism for viscous pressure gradients to effectively focus migrating melt to a narrow region of axial volcanism and, simultaneously, to restrict ridge upwelling rates to the “passive” (plate separation) upwelling speeds implied by recent measurements of UTh disequilibria in MORB. These last two effects critically depend on whether or not the presence of migrating melt will locally weaken the restite layer beneath the ridge-axis to an “asthenosphere” as opposed to a “compositional lithosphere” viscosity. Finally, melting at off-axis plumes that ceases at the base of a relatively uniform thickness compositional/thermal lithosphere would modulate different temperature plume inputs into a uniform temperature asthenosphere that reflects the solidus temperature near the base of the lithosphere. This may be the origin of the relatively uniform asthenosphere temperature that is sampled by melting and crustal production at the global mid-ocean ridge system.

Isotope topology of individual hotspot basalt arrays: Mixing curves or melt extraction trajectories?

[1]xa0Arrays of basalts from the same hotspot usually plot within an elongate tube-like field in 87Sr/86Sr–143Nd/144Nd–206Pb/204Pb space (Hart et al., 1992). Each hotspot array tube (HART) is commonly interpreted as the result of melting multiple basalt sources that are variably proportioned mixtures of the hotspot source components. I propose instead that a HART is the isotopic trace of the melt extraction trajectory produced by progressive melting of a heterogeneous source mixture characteristic to that hotspot. melt extraction trajectories form when the sources of the individual hotspot basalts differ in the amount of previous melt extraction from the upwelling and melting plume mantle mixture. Sequential melts along a melt extraction trajectory trace a one-dimensional path in through isotope space, independent of the number of distinct isotopic components in the initial source mixture. Subsequent partial mixing of the melts produced along a melt extraction trajectory would still tends to preserve this tube-like isotope topology. The melting physics that leads to a melt extraction trajectory also provides straightforward explanations for the 187Os/188Os contrasts between mid-ocean ridge basalts and their presumed abyssal peridotite source and for the enigmatic trace element and isotopic patterns of pyroxenite veins and peridotite exposed within orogenic lherzolites.

Asthenosphere flow model of hotspot–ridge interactions: a comparison of Iceland and Kerguelen

We develop a numerical model to test the asthenosphere flow paradigm in which hotspots feed the low viscosity asthenosphere, and lithosphere growth consumes the asthenosphere. The dynamics of this flow model are controlled by the relative position of the hotspot to the ridge, absolute plate velocities, lithosphere and asthenosphere rheologies, and tectonic boundary conditions. The model is applied to two distinct regions — the Iceland hotspot centered on the Mid-Atlantic Ridge, and the Kerguelen hotspot located near the Southeast Indian Ridge. The Iceland model generates along-ridge flow rates as high as 30 cm/y for an asthenosphere viscosity of 5×1018 Pa s. These flow rates are consistent with previous interpretations of the origin of southward pointing Vs south of Iceland. The preferred Kerguelen model produces significant hotspot-to-ridge flow followed by along-ridge flow; velocities from the off-axis hotspot to the ridge are 66 cm/y for a model within which asthenosphere viscosity varies from 5×1018 to 5×1019 Pa s. The asthenosphere flow paradigm can explain major features of hotspot-ridge interactions for both on-axis and off-axis hotspots.

Continental geotherm and the evolution of rifted margins

Models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins. The discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect. Work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 °C, ∼100 °C cooler than that beneath the oceans. The continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes. Upwelling of such cool mantle during rifting leads to little melt production, even for rapid extension rates, explaining the formation of amagmatic margins away from mantle plumes. However, the transition to seafloor spreading and the development of normal-thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and/or plume material. This influx may cause a transient thermal uplift, recorded as a breakup unconformity. Conversely, at volcanic margins, the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid-ocean ridge system away from the volcanic margin, leading to a pulse of rapid subsidence.

Evidence for variable upper mantle temperature and crustal thickness in and near the Australian-Antarctic Discordance

Abstract The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the ‘slow’ Mid-Atlantic Ridge and ‘fast’ East Pacific Rise. We used satellite-derived gravity data, in combination with SeaMARC II bathymetry in and near the AAD, to examine regional density variations in the upper mantle beneath the AAD. Through three-dimensional gravity analysis, we found that at least two end-member models satisfy the gravity observations: regional crustal thickness variations of at least 3 km along the SEIR near the AAD or a temperature anomaly of the order of 150°C in the upper mantle beneath the SEIR. These new observations, combined with other geophysical and geochemical characteristics of the Australian-Antarctic Discordance, provide further evidence that the temperature structure of a mid-ocean ridge is a controlling factor, in addition to spreading rate, in the crustal accretionary process. Numerical models of mantle flow beneath mid-ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process. Our results indicate that temperature is important, especially at intermediate and slower spreading rates, where thermal effects can dominate mantle flow beneath a mid-ocean ridge and result in increasing crustal production with decreasing spreading rate. At the constant, intermediate spreading rate of 37 mm/yr, characteristic of the SEIR in and near the AAD, our numerical models show that significant crustal thinning (2–4 km) can occur with relatively small variations in upper mantle temperature, all else being equal. Thus, combined with our end-member gravity models, these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the AAD.