Ophiolite genesis and global tectonics: Geochemical and tectonic fingerprinting of ancient oceanic lithosphere

Overview of ophiolites and related units in the Late Palaeozoic–Early Cenozoic magmatic and tectonic development of Tethys in the northern part of the Balkan region

Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments. Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(Mg + Fe) in equilibrium with Fo90 olivine. Both sets of corrected compositions show large variations in Na2O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al90 and Fe90. These new data are not in agreement with the presentation of Niu & O'Hara (Journal of Petrology 49, 633–664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for high- and low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 200°C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slow-spreading ridges display a characteristic chemical signature of elevated Na90 and Al90 and lowered Si90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle–melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K90. Such variations do not correlate with the other major elements, showing that major element and trace element (and isotope) heterogeneity reflect different processes. This supports the model of movement of low-degree melts for the creation of trace element and isotope mantle heterogeneity, and is inconsistent with large variations in the amount of recycled crust in most ocean ridge mantle sources.

New major and trace element and Sr, Nd, and Pb isotope data, together with 39Ar‐40Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid‐ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid‐ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, 87Sr/86Sr, and lower 143Nd/144Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid‐ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger “garnet signature” inferred from Sm/Nd and 143Nd/144Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation‐corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more “depleted” than is generally inferred from the compositions of MORB.

Recent geological investigations of the Isua Supracrustal Belt (3.8 Ga), southern West Greenland, have suggested that it is the oldest accretionary complex on earth, defined by an oceanic plate‐type stratigraphy and a duplex structure. Plate history from mid‐oceanic ridge through plume magmatism to subduction zone has been postulated from analysis of the reconstructed oceanic plate stratigraphy in the accretionary complex. Comparison between field occurrence of greenstones in modern and ancient accretionary complexes reveals that two types of tholeiitic basalt from different tectonic settings, mid‐oceanic ridge basalt (MORB) and oceanic island basalt (OIB), occur. This work presents major, trace and rare earth element (REE) compositions of greenstones derived from Isua MORB and OIB, and of extremely rare relict igneous clinopyroxene in Isua MORB. The Isua clinopyroxenes (Cpx) have compositional variations equivalent to those of Cpx in modern MORB; in particular, low TiO2 and Na2O contents. The Isua Cpx show slightly light (L)REE‐depleted REE patterns, and the calculated REE pattern of the host magma is in agreement with that of Isua MORB. Analyses of 49 least‐altered greenstones carefully selected from approximately 1200 samples indicate that Isua MORB are enriched in Al2O3, and depleted in TiO2, FeO*, Y and Zr at the given MgO content, compared with Isua OIB. In addition, Isua MORB show an LREE‐depleted pattern, whereas Isua OIB forms a flat REE pattern. Such differences suggest that the Early Archean mantle had already become heterogeneous, depending on the tectonic environment. Isua MORB are enriched in FeO compared with modern MORB. Comparison of Isua MORB with recent melting experiments shows that the source mantle had 85–87 in Mg♯ and was enriched in FeO. Potential mantle temperature is estimated to be approximately 1480°C, indicating that the Early Archean mantle was hotter by at most approximately 150°C than the modern mantle.

The present set of 22 papers stems from the 7th International Symposium on Eastern Mediterranean Geology that was held in Adana, Turkey, 18–22 October 2010. After its initiation in l992, in Adana this international conference has been held successively in Jerusalem (Israel) in l995, Nicosia (Cyprus) in 1998, Isparta (Turkey) in 2001, Thessalonica (Greece) in 2004 and Amman (Jordan) in 2007. The Cyprus and Thessalonica conferences were followed by substantive publications, including one with a focus on Cyprus (Panayides et al. 2000) and another mainly concerned with the Balkan region (Robertson & Mountrakis 2006). A subset of the papers that were presented at the 7th Adana meeting, together with some others, have been prepared and edited for the present volume. Anatolia and the surrounding region provide an excellent opportunity for the study of fundamental geological processes, including rifting, seafloor spreading, ophiolite genesis and emplacement, collision, continental assembly and neotectonics. This volume should interest a wide cross-section of international researchers, including those concerned with hydrocarbons, mineral deposits and seismic risk, and also postgraduate students and advanced undergraduates. The papers highlight the role of fieldwork, the multidisciplinary nature of much of the current research in the region, the role of teamwork and the strong contribution being made by young scientists. Following an introductory chapter, the volume is divided into four sections covering different aspects of the region as a whole. The area discussed mainly lies within Turkey, Cyprus and Syria. Section 1 is made up of a small number of papers that are mainly concerned with the Pontide belt of northern Turkey. Section 2 is concerned with the geological development of the Tauride and Anatolide belts of central and southern Anatolia, especially the Triassic–Jurassic period of rifting and passive margin development and the Late Cretaceous period of ophiolite genesis and emplacement. Section 3 is mainly concerned with the formation of sedimentary basins during closure of several Mesozoic ocean basins and the related structural development during Late Cretaceous to Pliocene time. Finally, Section 4 is devoted to aspects of the structural development of the region, mainly during the Pliocene–Quaternary (i.e. neotectonics) when the plate configuration was essentially as it is today. The area covered by each paper is shown in Figure 1. The introduction by Robertson et al. covers the southern part of Anatolia and the adjacent easternmost Mediterranean region that was the main subject of the international conference. The main focus is on Late Permian–Recent time. A review of the Late Precambrian–Recent geological development of the easternmost Mediterranean region is published elsewhere (Robertson et al. 2012). The authors discuss alternative interpretations of the Mesozoic–Cenozoic inter-relations of the various crustal units that make up the region. In particular, they consider whether these should be interpreted as individual microcontinents separated by Mesozoic small ocean basins or as parts of larger continental units (i.e. microcontinents). The Anatolides in the north are generally interpreted as the metamorphosed equivalents of the Taurides, although different reconstructions exist. The Anatolides are commonly seen as the northern, leading edge of the Mesozoic Tauride–Anatolide continent that subducted and underwent high-pressure/lowtemperature (HP/LT) metamorphism during Late Cretaceous–Early Cenozoic time. The Anatolides are divided into two parts, namely the HP/LTmetamorphosed Afyon–Bolkar Dag zone in the south, which can be closely correlated with the Taurides, and the very HP/LT Tavsanli Zone further north, which also shows some affinities with the Taurides but is less well understood. The Kirsehir Massif is interpreted as a rifted continental block that was separated from a larger Tauride continent to the south by a Mesozoic oceanic basin known as the Inner-Tauride Ocean. However, uncertainties remain, including the reconstruction

Continental deep subduction after the closure of large oceanic basins is commonly ascribed to the gravitational pull of the subducting oceanic slab. However, it is not clear how continental lithosphere adjacent to small oceanic basins was subducted to mantle depths. The Sesia Zone in the Western Alps provides an excellent target for exploration of subduction dynamics in such a tectonic setting. Here we report the first finding of coesite in a jadeite-bearing orthogneiss from the Sesia Zone, providing the first evidence for deep subduction of the continental crust to mantle depths for ultrahigh-pressure (UHP) metamorphism in this zone. Three coesite inclusions were identified by laser Raman spectroscopy in two garnet grains. Based on zircon U-Pb dating and trace element analysis, the UHP metamorphic age was constrained to be 76.0±1.0 Ma. The phase equilibrium modeling yields peak metamorphic pressures of 2.8-3.3GPa, demonstrating the continental deep subduction to mantle depths of >80km. The subducted continental crust was a rifted hyperextended continental margin, which was converted to the passive continental margin during seafloor spreading and then deeply subducted during the oblique convergence between the Adria microplate and Eurasian plate in the Late Cretaceous. Because the slab pull could only play a limited role in closing small oceanic basins for continental collision, the distal push of either continental breakup or seafloor spreading is suggested as the major driving force for the deep subduction of continental crust in the Western Alps. Therefore, deep subduction of the continental crust bordering small oceanic basins would have been induced by the far-field stress of compression, whereas that bordering large oceanic basins was spontaneous due to the oceanic slab pull. This provides a new insight into the geodynamic mechanism of continental deep subduction.

The term ‘hot spot’ emerged in the 1960s from speculations that Hawaii might have its origins in an unusually hot source region in the mantle. It subsequently became widely used to refer to volcanic regions considered to be anomalous in the then-new plate tectonic paradigm. It carried with it the implication that volcanism (a) is emplaced by a single, spatially restricted, mongenetic melt-delivery system, assumed to be a mantle plume, and (b) that the source is unusually hot. This model has tended to be assumed a priori to be correct. Nevertheless, there are many geological ways of testing it, and a great deal of work has recently been done to do so. Two fundamental problems challenge this work. First is the difficulty of deciding a ‘normal’ mantle temperature against which to compare estimates. This is usually taken to be the source temperature of mid-ocean ridge basalts (MORBs). However, Earth's surface conduction layer is ∼200km thick, and such a norm is not appropriate if the lavas under investigation formed deeper than the 40–50km source depth of MORB. Second, methods for estimating temperature suffer from ambiguity of interpretation with composition and partial melt, controversy regarding how they should be applied, lack of repeatability between studies using the same data, and insufficient precision to detect the 200–300°C temperature variations postulated. Available methods include multiple seismological and petrological approaches, modelling bathymetry and topography, and measuring heat flow. Investigations have been carried out in many areas postulated to represent either (hot) plume heads or (hotter) tails. These include sections of the mid-ocean spreading ridge postulated to include ridge-centred plumes, the North Atlantic Igneous Province, Iceland, Hawaii, oceanic plateaus, and high-standing continental areas such as the Hoggar swell. Most volcanic regions that may reasonably be considered anomalous in the simple plate-tectonic paradigm have been built by volcanism distributed throughout hundreds, even thousand of kilometres, and as yet no unequivocal evidence has been produced that any of them have high temperature anomalies compared with average mantle temperature for the same (usually unknown) depth elsewhere. Critical investigation of the genesis processes of ‘anomalous’ volcanic regions would be encouraged if use of the term ‘hot spot’ were discontinued in favour of one that does not assume a postulated origin, but is a description of unequivocal, observed characteristics.

The geochemical characteristics of the mantle during continental breakup and the initial spreading of marginal sea basins remain poorly understood. Mid-ocean ridge basalt (MORB) samples from Hole U1500B and Hole U1503A in the northern margin of the South China Sea (N-SCS), obtained during IODP Expeditions 367 and 368X, provide crucial insights into mantle evolution of the nascent oceanic basin subsequent to continental breakup. This study analyzes major and trace elements, as well as Mo–Sr–Nd–Hf isotopes, in these MORB samples to explore variations in their mantle sources. MORB samples from Hole U1500B, closer to the continent, exhibit higher 87Sr/86Sr ratios, along with lower εNd and εHf values compared to the depleted mantle. Additionally, their δ98/95Mo values correlate positively with Mo/Ce and Mo/Nb ratios, indicating the influence of recycled oceanic crust (ROC) melts in the mantle source. In contrast, MORB samples from Hole U1503A, nearer to the oldest fossil ridge, show a broader range of δ98/95Mo values, reflecting varying extents of contribution of terrigenous sediment melts alongside ROC melts. The differing trace element and Mo–Sr–Nd–Hf isotope compositions of MORBs from the two sites highlight a significant transition in the mantle beneath the nascent mid-ocean ridge of the SCS. During the initial stages of seafloor spreading in the SCS, the mantle source experienced continuous replenishment from enriched components derived from shallow recycling of metasomatized SCLM. This process significantly contributed to the rapid transition from continental rifting to seafloor spreading in the SCS. The enrichment of the asthenospheric mantle, likely induced by previous subduction processes, facilitated rapid rifting and extensive magmatism in the SCS, distinguishing it from magma-poor margin basins. This research provides critical geochemical insights into the mantle evolution beneath nascent mid-ocean ridges, enhancing our understanding of the early processes in marginal sea basins.

Syn-rift exhumation of mantle rocks in a continental breakup zone was highlighted along the present-day west Iberian passive margin [e.g. Boillot et al., 1988, 1995; Whitmarsh et al., 1995, 2001; Beslier et al., 1996; Brun and Beslier, 1996; Boillot and Coulon, 1998; Krawczyk et al., 1996; Girardeau et al., 1998] and along the fossil Tethyan margins [e.g. Froitzheim and Manatschal, 1996; Manatschal and Bernoulli, 1996; Marroni et al., 1998; Muntener et al., 2000; Desmurs et al., 2001]. Along the west Iberian margin, serpentinized peridotite and scarce gabbro and basalt lay directly under the sediments, over a 30 to 130 km-wide transition between the thinned continental crust and the first oceanic crust [Girardeau et al., 1988, 1998; Kornprobst and Tabit, 1988; Boillot et al., 1989; Beslier et al., 1990, 1996; Cornen et al., 1999]. The formation of a wide ocean-continent transition (OCT), mostly controlled by tectonics and associated with an exhumation of deep lithospheric levels, would be an essential stage of continental breakup and a characteristic of magma-poor passive margins. The southwest Australian margin provides an opportunity to test and to generalize the models proposed for the west Iberian margin, as both margins present many analogies. The south Australian margin formed during the Gondwana breakup in the Mesozoic, along a NW-SE oblique extension direction [Willcox and Stagg, 1990]. From north to south, the continental slope is bounded by (1) a magnetic quiet zone (MQZ) where the nature of the basement is ambiguous [Talwani et al., 1979; Tikku and Cande, 1999; Sayers et al., 2001], (2) a zone where the basement shows a rough topography associated with poorly expressed magnetic anomalies [Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999; Sayers et al., 2001], and which is the eastward prolongation of the Diamantina Zone, and (3) an Eocene oceanic domain. The continental breakup zone is believed to be located near or at the southern edge of the MQZ [Cande and Mutter, 1982; Veevers et al., 1990; Sayers et al., 2001]. Breakup is dated at 125 Ma [Stagg and Willcox, 1992], 95 ± 5 Ma [Veevers, 1986] or at 83 Ma [Sayers et al., 2001], and followed by ultra-slow seafloor spreading until the Eocene (43 Ma), and fast spreading afterwards [Weissel and Hayes, 1972; Cande and Mutter, 1982; Veevers et al., 1990; Tikku and Cande, 1999]. The western end of the margin (fig. 1) is starved and bounded in the OCT by basement ridges where peridotite, gabbro and basalt were previously dredged [Nicholls et al., 1981]. Altimetry data [Sandwell and Smith, 1997] show that some of these ridges are continuous over 1500 km along the OCT of the south Australian margin and of the conjugate Antarctic margin. The objectives of the MARGAU/MD110 cruise (May-June 1998; [Royer et al., 1998]; fig. 2) were to define the morpho-structure and the nature and evolution of the basement in the SW Australian OCT. An area of 180 000 km2 was explored with swath bathymetry. Gravimetric data (11382 km) were simultaneously recorded whereas few single channel seismic (1353 km) and magnetic (5387 km) data were obtained due to technical difficulties. Crystalline basement rocks, made of varied and locally well-preserved lithologies, were dredged at 11 sites located on structural highs.

Arc-continent collision, followed by subduction polarity flip, occurs during closure of oceanic basins and contributes to the growth of continental crust. Such a setting may lead to a highly unusual association of ultrapotassic and mid-ocean ridge basalt (MORB)-type volcanic rocks as documented here from an Ordovician succession of the Scandinavian Caledonides. Interbedded with deep-marine turbidites, pillow basalts evolve from depleted-MORB (εNdt 9.4) to enriched-MORB (εNdt 4.8) stratigraphically upward, reflecting increasingly deeper melting of asthenospheric mantle. Intercalated intermediate to felsic lava and pyroclastic units, dated at ca. 474−469 Ma, are extremely enriched in incompatible trace elements (e.g., Th) and have low εNdt (−8.0 to −6.6) and high Sri (0.7089−0.7175). These are interpreted as ultrapotassic magmas derived from lithospheric mantle domains metasomatized by late Paleoproterozoic to Neoproterozoic crust-derived material (isotopic model ages 1.7−1.3 Ga). Detrital zircon spectra reveal a composite source for the interbedded turbidites, including Archean, Paleo-, to Neoproterozoic, and Cambro-Ordovician elements; clasts of Hølonda Porphyrite provide a link to the Hølonda terrane of Laurentian affinity. The entire volcano-sedimentary succession is interpreted to have formed in a rift basin that opened along the Laurentian margin as a result of slab rollback subsequent to arc-continent collision, ophiolite obduction and subduction polarity flip. The association of MORBs and ultrapotassic rocks is apparently a unique feature along the Caledonian-Appalachian orogen. Near-analogous modern settings include northern Taiwan and the Tyrrhenian region of the Mediterranean, but other examples of strictly concurrent MORB and ultrapotassic volcanism remain to be documented.

ABSTRACTEarth history is punctuated by events during which large volumes of predominantly mafic magmas were generated and emplaced by processes that are generally accepted as being, unrelated to ‘normal’ sea-floor spreading and subduction processes. These events form large igneous provinces (LIPs) which are best preserved in the Mesozoic and Cenozoic where they occur as continental and ocean basin flood basalts, giant radiating dyke swarms, volcanic rifted margins, oceanic plateaus, submarine ridges, and seamount chains. The Mesozoic history of Antarctica is no exception in that a number of different igneous provinces were emplaced during the initial break-up and continued disintegration of Gondwana, leading to the isolation of Antarctica in a polar position. The link between the emplacement of the igneous rocks and continental break-up processes remains controversial. The environmental impact of large igneous province formation on the Earth System is equally debated. Large igneous province eruptions are coeval with, and may drive environmental and climatic effects including global warming, oceanic anoxia and/or increased oceanic fertilisation, calcification crises, mass extinction and release of gas hydrates.This review explores the links between the emplacement of large igneous provinces in Antarctica, the isolation of Antarctica from other Gondwana continents, and possibly related environmental and climatic changes during the Mesozoic and Cenozoic.

Observations suggest that global (plate) tectonics operates on the Earth. The most characteristic features of the global tectonics are ocean floor spreading in mid-ocean ridges and subduction in deep-sea trenches. These processes imply the existence of mantle flow. However, within the framework of the plate tectonics, it is impossible to build a consistent quantitative theory of mantle convection because one cannot answer the question of where the tectonic plate “terminates”. From a mathematical point of view, the difficulty of global tectonics is that there are no boundary and initial conditions that would allow one to consider the evolution of some isolated part of the planet (e. g., the upper mantle). Therefore, to obtain a physically justified answer to the questions about the causes and energy sources of mantle motions, it is necessary to consider an evolution of the planet as a single whole. This formulation of the problem leads to the global geodynamics. Unlike the global tectonics, which in fact ignores the existence of the Earth’s core, for the global geodynamics the liquid outer and solid inner core, as well as the processes at the boundary between them and at the boundary between the core and the mantle, which decisively influence the mantle dynamics, are the main objects of the study. In this review, we confine ourselves to the global heat balance of the Earth. In the coming years, the results of the geoneutrino experiment will make it possible to obtain a reliable estimate of the total rate of radiogenic heat production in the Earth and to estimate the heat flow from the core to the mantle. Even this alone will significantly narrow the choice of models describing processes in the core. An ascertainment of the temperature at the inner/outer core interface and an elucidation of the mixing nature in the outer core will allow one to reduce an uncertainty of the temperature at the base of the mantle and to formulate a boundary condition problem for the mantle flow dynamics. Thus, a bridge from global geodynamics to global tectonics will be thrown and the conceptions of the latter will be put on a firm physical basis.

Geodynamic processes, continental growth, and mantle evolution recorded in late Archean greenstone belts of the southern Superior Province, Canada

The West Philippine Basin (WPB), formed by seafloor spreading between 60 and 35 Ma, provides an excellent case study of relationships between basin tectonics and magma chemistry. At 48 Ma, the Izu-Bonin-Mariana (IBM) arc formed along the basin edge, orthogonal to the active spreading center; thus, WPB development is a key issue for this Margins Subduction Factory focus area. WPB basalts from the main spreading stage are normal to enriched mid-ocean ridge basalt (MORB) with an Indian Ocean MORB isotopic signature. Basalts from the Benham Rise and locations near the western Central Basin Spreading Center (CBSC) at 50-35 Ma are geochemically identical to oceanic island basalts. Late-stage CBSC basalts (35-26 Ma) are isotopically like main spreading-stage MORB, with widely varying and decoupled trace element enrichments. Based on basalt geochemistry, the WPB could be a trapped fragment of ancient Indian/Tethyan ocean ridge, as proposed in some models for the initiation of the IBM arc, or it could be a back-arc basin, provided plate configurations allowed replenishment of sub-Indian Ocean asthenosphere. Ocean island basalts were formed by decompression melting of an enriched source beneath the western CBSC, mixing with normal MORB sources to form enriched MORB. This was a transitory feature (15 Ma) related to spreading, rather than a deep-seated plume, and probably did not affect the early IBM arc. Magma formed in small, deep-seated batches as the extension waned. That CBSC activity continued for 22 Ma after the initiation of the IBM arc indicates that forces related to an additional subduction system influenced the WPB.

We present a new method for calculating the major element compositions of primary melts parental to mid‐ocean ridge basalt (MORB). This model is based on the experimental data of Jaques and Green (1980), Falloon et al. (1988), and Falloon and Green (1987, 1988) which are ideal for this purpose. Our method is empirical and employs solid‐liquid partition coefficients (Di) from the experiments. We empirically determine Di = ƒ(P,F) and use this to calculate melt compositions produced by decompression‐induced melting along an adiabat (column melting). Results indicate that most MORBs can be generated by 10–20% partial melting at initial pressures (P0) of 12–21 kbar. Our primary MORB melts have MgO = 10–12 wt %. We fractionate these at low pressure to an MgO content of 8.0 wt % in order to interpret natural MORB liquids. This model allows us to calculate Po, Pƒ, To, Tƒ, and F for natural MORB melts. We apply the model to interpret MORB compositions and mantle upwelling patterns beneath a fast ridge (East Pacific Rise (EPR)8°N to 14°N), a slow ridge (mid‐Atlantic Ridge (MAR) at 26°S), and seamounts near the EPR (Lament seamount chain). We find mantle temperature differences of up to 50°–60°C over distances of 30–50 km both across axis and along axis at the EPR. We propose that these are due to upward mantle flow in a weakly conductive (versus adiabatic) temperature gradient. We suggest that the EPR is fed by a wide (−100 km) zone of upwelling due to plate separation but has a central core of faster buoyant flow. An along‐axis thermal dome between the Siqueiros transform and the 11°45′ Overlapping Spreading center (OSC) may represent such an upwelling; however, in general there is a poor correlation between mantle temperature, topography, and the segmentation pattern at the EPR. For the Lament seamounts we find regular across‐axis changes in Po and F suggesting that the melt zone pinches out off axis. This observation supports the idea that the EPR is fed by a broad upwelling which diminishes in vigor off axis. In contrast with the EPR axis, mantle temperature correlates well with topography at the MAR, and there is less melting under offsets. The data are consistent with weaker upwelling under offsets and an adiabatic temperature gradient in the sub axial mantle away from offsets. The MAR at 26°S exhibits the so‐called local trend of Klein and Langmuir (1989). Our model indicates that the local trend cannot be due solely to intracolumn melting processes. The local trend seems to be genetically associated with slow‐spreading ridges, and we suggest it is due to melting of multiple individual domains that differ in initial and final melting pressure within segments fed by buoyant focused mantle flow.