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Magmatic accretion at fast spreading ridges

 Magmatic accretion at fast spreading ridges

fast spreading ridge

Cross axis view of a ridge accreted at fast spreading centers (Modified from Sinton and Detrick, 1992, Nicolas and Boudier, 1995, Boudier et al., 1996 ; MacLeod and Yaouancq, 2000 ; and Nedimovic et al., 2005). A melt lens is present at the bottom of the upper crust and at the moho level. Gabbros are vertically foliated to the top and horizontally layered to the bottom. The mush underneath the upper melt lens is intruded by sills.

magma chamber processes

Schematic cross-axis section showing the melt lens and the root of the sheeted dike complex (not to scale). a) General organization, lithologies and temperatures. After Nicolas et al. (2008). For details see Nicolas et al. (2008). b) Magma drained out during an eruption. c) Melt crystallization at the melt lens margins. d) New magma injection leading to partial or total replenishment of the melt lens. e) Hydrothermal circulation in the crust overlying the melt lens ; fluids can be incorporated in the melt lens through assimilation of either hydrothermally altered rocks or brine.

 *Expedition IODP 335 (Hole 1256D) : Une petite vue…

 *Interactions between magma and hydrothermal system in Oman ophiolite and in IODP Hole 1256D : Fossilization of a dynamic melt lens at fast spreading ridges

France, L., B. Ildefonse, and J. Koepke

Geochem. Geophys. Geosyst., (2009) 10, Q10O19, doi:10.1029/2009GC002652.

ABSTRACT The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D.We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.

magma chamber roof ; fast spreading ridge

General schematic model for the dynamics of the melt lens. (a) Schematic cross section at the axis of a fast spreading ridge (modified after Sinton and Detrick [1992]). The red rectangle indicates the location of the axial melt lens. (b) Steady state stage with injection of dikes that have at their base microgranular margins (protodikes). Hydrous partial melting is proposed to occur in the root zone of the sheeted dike complex as a result of hydrothermal fluid intrusion [Nicolas et al., 2008]. © Upward migration of the top of the melt lens resulting in reheating and recrystallization of the base of the dikes (red dots) to granoblastic dikes and in assimilation of xenoliths in the melt lens. Hydrous partial melting of the hydrothermally altered base of the dikes can also occur. (d) Downward migration of the top of the melt lens resulting in the crystallization of the isotropic ophitic gabbros. New dikes can be injected laterally or from below ; their root is typical of protodikes, with microgranular margins, and they grade upward to ‘‘classical’’ dikes with chilled margins.

hornfels

Outcrops in the root zone of the sheeted dikes. (a) Recrystallized sheeted dike (granoblastic dike) intruded by gabbro. Recrystallized chilled margins are crosscut by the intrusive gabbro. A late dike (‘‘dike 2’’) crosscuts the gabbro and recrystallized dikes (‘‘dike 1’’). Photomicrographs and BSE images of samples from the Wadi Gideah area. (a) Fine-grained granoblastic texture with a high oxide concentration in a recrystallized dike margin (plane-polarized light) ; (b) Isotropic gabbro crosscutting a recrystallized granoblastic dike. In the recrystallized dike, the truncated clinopyroxene vein is believed to derivate from a former amphibole bearing hydrothermal vein (cross-polarized light).

Temperature evolution for different generations of amphiboles in sample 08OLC6 : 1) large brown amphibole grains (B-Amp) crystallize, 2) following a retrograde evolution (Amp 1), large green amphibole (G-Amp) grains form. 3) Following a prograde event (reheating), granoblastic brown amphibole assemblages crystallize before 4) a second retrograde evolution (Amp 2) that leads to the crystallization of the granoblastic green assemblages (replacing the brown ones).

 *Hydrous partial melting in the sheeted dike complex at fast spreading ridges : experimental and natural observations

L. France, J. Koepke, B. Ildefonse, S.B. Cichy, F. Deschamps

Contrib Mineral Petrol (2010) 160:683–704 DOI 10.1007/s00410-010-0502-6

oceanic plagiogranite

ABSTRACT

In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calcalkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites ; however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850°C. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated.

PHASES RELATIONS :

hydrous partial melting ; oceanic magma chamber roof

a) Phases present in the products of partial melting and subsolidus experiments as a function of temperature. Minerals abbreviations are the same as in Table 2. b) Phase proportions in the partly molten system were calculated with a least square model according to Albarède and Provost (1977). Standard deviation <1 for all values. Values obtained for experiments at temperatures <950°C are less accurate. Incoherent values are obtained at 850°C.

EXPERIMENTAL HYDROUS PARTIAL MELTS :

hydrous partial melting ; fast spreading ridges

a) FeO*/MgO versus SiO2 diagram from Miyashiro (1974). FeO* FeOtotal, TH tholeiitic field, CA calc-alkaline field. b) Alkaline (Na2O + K2O)-FeOtotal-MgO discriminating diagram from Irvine and Baragar (1971)

NEW THERMOMETER :

Al in Cpx thermometer

Clinopyroxene composition : Al2O3 content (wt%) in clinopyroxene from our experiments as a function of temperature. Standard deviations of analyses are shown. The dashed line is the linear regression with the equation y = 93.145x ? 742 (R2 = 0.976)

 *Subsidence in magma chamber and the development of magmatic foliation in Oman ophiolite gabbros

Adolphe Nicolas, Françoise Boudier, Lydéric France

Earth and Planetary Science Letters 284 (2009) 76–87

ABSTRACT In the Oman ophiolite, the horizon where the melt lens pinched during drifting away from the ocean ridge axis has been identified. Starting in the Root Zone of the Sheeted Dike Complex (RZSDC) located above this horizon, 18 sections down to the upper gabbros unit have been mapped in great detail, in selected areas of the southern massifs of this ophiolite. They are complemented by 133 sites, located throughout the entire ophiolite, where the transition from the RZSDC to the uppermost foliated gabbros is well exposed. Altogether, half the sites and 11 cross sections display, within a few tens of meters beneath the RZSDC, a magmatic foliation which is parallel to the overlying sheeted dikes. In the other sites and cross sections, the gabbro foliation is either flat-lying or steep but not parallel to the sheeted dikes. Compared to the RZSDC isotropic ophitic gabbros where clinopyroxene is interstitial between plagioclase laths, in the topmost steeply foliated gabbros, clinopyroxene is idiomorphic, becoming rapidly granular and tabular down section by recrystallization and peripheral alteration to hornblende. Moving down from these uppermost gabbros and over one hundred meters, the steep foliation becomes stronger and the poorly recovered top gabbros grade into the recrystallized, granular gabbros of the gabbro unit. These repeated observations indicate to ascribe these gabbros to subsidence of a compacting mush from the floor of the melt lens into the underlying, main magma chamber. The topmost gabbros beneath RZSDC, which were expelled from the melt lens by drifting very soon after settling on the melt lens floor, display in plagioclase a spectacular zoning pointing to a fast cooling. Moving downwards, stronger foliation, increased compaction and recovery–recrystallization are explained by the time spent by the subsiding mush inside the magma chamber increasing by one order of magnitude ( 50 to 500 yr) over this vertical distance. This field-based study brings compelling field evidence supporting the former models of subsidence which were based on the assumption that the mush that settled onto the floor of the melt lens is sucked downwards during drifting of the crust away from the ridge axis

foliated gabbros

Progressive evolution of gabbro textures from the RZSDC to the top of the main gabbro unit. When foliation is present, thin sections are presented in their original attitude. (a) Isotropic ophitic gabbro from the RZSDC. (b) Gabbro from the floor of the melt with idiomorphic clinopyroxene and extreme zoning in plagioclase thin laths, also illustrating the development of a steep foliation ; © Uppermost foliated gabbro with a good foliation, thicker and less zoned plagioclase, and recrystallized clinopyroxene in large tabular grains. (d) Foliated granular gabbro, 100 m below floor of melt lens, recording a strong foliation and recrystallization. Width field of view is 12 mm for low magnification, 3 mm for high magnification. Samples (a) 89OA12b ; (b) 06OA20i and 890A34 ; © 90OA97 ; (d) 06OA3.

foliated gabbros

Crystallographic orientations of plagioclase in samples selected alongWadi Farah, at depths between 10 and 365 m below the RZSDC, represented with their position in the upper gabbro section. Crystallographic preferred orientations are Electron BackScattering Diffraction (EBSD) measurements ; lower hemisphere, non polar data, stereoplot in the geographical reference frame with north marked ; contours at 1, 1.5, 2, 2.5…times uniform distribution. pfJ indexes of Mainprice and Silver (1993) measure strengths of crystallographic axes preferred orientation. Samples : 07OA20a, c1, d, e and 072OA13.

foliated gabbros

Models of subsidence, (a) sketch of lid and top kilometer in gabbro unit and (b) detailed view on the triple junction at the closure of melt lens. (a) Origin of layered and foliated gabbros by subsidence of the mush crystallized on the floor of the lens. Left side : magmatic flow surface, internal to the magma chamber wall, with imprint of the plunging flow lineations. Right side : frozen foliations in gabbros outside the magma chamber (bright blue). Roof pendants (light blue), falling from the RZSDC into the melt lens, recrystallize during subsidence, in micronorites which are stretched in lenses parallel to the gabbro foliation. (b) Closure of the melt lens at a triple junction which is stationary in a drifting crust. At the triple junction, the RZSDC isotropic gabbros from the roof of the melt lens join the gabbros from its floor. Just below, the steep foliated gabbros were deposited near the triple junction, the top gabbros subsiding on a shorter distance than deeper gabbros which are issued from the floor further away from the triple junction. Inside the magma chamber, dotted lines in (a) are isochrons tracing the subsidence of a layer from the lens. Dashes in (b) trace the nascent magmatic foliation.  

 *Gabbros from IODP Site 1256, equatorial Pacific : Insight into axial magma chamber processes at fast spreading ocean ridges

Koepke, J., L. France, T. Müller, F. Faure, N. Goetze, W. Dziony, and B. Ildefonse

Geochem. Geophys. Geosyst., (2011) 12, Q09014, doi:10.1029/2011GC003655

ABSTRACT The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts : (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high‐temperature end‐member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of ∼80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256 : relatively primitive MORB melts under near‐liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens–sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well‐connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near‐solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to “granoblastic dikes” by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so‐called “varitextured gabbro” unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.

fast spreading ridge

Schematic cross section of the magmatic system at fast spreading ridges (modified after France et al. [2009b]). The main magma chamber is composed of a mush containing less than 20% of melt, and the upper melt lens is nearly 100% liquid. The yellow lens at the bottom corresponds to sill‐like intrusions [e.g., Boudier et al., 1996]. The dashed blue curves stand for possible hydrothermal cooling paths. The dark blue lines in the bottom part correspond to the layering in the gabbros.

Mushy zone ; fast spreading ridge

Schematic model describing the upper part of the magmatic system present at IODP Hole 1256D. (a) Melt lens position at the base of the sheeted dike complex. The upper melt lens feeds the upper crust (dikes and lavas) and is fed from below. Small white dots at the melt lens margins highlight the mushy zone where the melt lens passes from ∼100% of melt to a fully crystallized gabbro. The black square represents the zoomed area represented in Figure 8b. The white area within the sheeted dike complex highlights a cut in the thickness. (b) Details of the mushy zone present at the melt lens margins. The crystallization progression is highlighted : first, the subophitic domains crystallize from MORB‐like melts (yellow) forming a framework that traps the remaining evolved melt, and second, the Opx‐bearing granular domains crystallize from these trapped fractionated melts (blue). © Corresponding REE patterns for calculated equilibrium melts based on clinopyroxene from the subophitic domains (yellow) and orthopyroxene from the granular domains (blue ; normalized to the 1256D site upper crust composition) (for details see Figure 6).

 *Hydrous magmatism triggered by assimilation of hydrothermally altered rocks in fossil oceanic crust (Northern Oman ophiolite)

L. France, B. Ildefonse, J. Koepke

Geochem. Geophys. Geosyst., (2013) 14, 2598-2614, doi:10.1002/ggge.20137

ABSTRACT Mid-ocean ridges magmatism is by and large considered to be mostly dry. Nevertheless, numerous works in the last decade have shown that a hydrous component is likely to be involved in ocean ridges magmas genesis and / or evolution. The petrology and geochemistry of peculiar coarse grained gabbros sampled in the upper part of the gabbroic sequence from the Northern Oman ophiolite (Wadi Rajmi) provide information on the origin and fate of hydrous melts in fast spreading oceanic settings. Uncommon crystallization sequences for oceanic settings (clinopyroxene crystallizing before plagioclase), extreme mineral compositions (plagioclase An% up to 99, and clinopyroxene Mg# up to 96), and the presence of magmatic amphibole, imply the presence of a high water activity during crystallization. Various petrological and geochemical constraints point to hydration resulting from the recycling of hydrothermal fluids. This recycling event may have occurred at the top of the axial magma chamber where assimilation of anatectic hydrous melts is recurrent along mid-ocean ridges, or close to segments ends where fresh magma intrudes previously hydrothermally altered crust. In ophiolitic settings, hydration and remelting of hydrothermally altered rocks producing hydrous melts may also occur during the obduction process. Although dry magmatism dominates oceanic magmatism, the dynamic behavior of fast spreading ocean ridge magma chambers has the potential to produce the observed hydrous melts (either in ophiolites or at spreading centers), which are thus part of the general mid-ocean ridges lineage.

fast spreading ridge ABOVE : Schematic cross axis view of the magmatic system present at fast spreading ridges (modified after France et al., 2009). From top to bottom, crust is composed of extrusives and dikes (forming the upper crust), and isotropic, vertically foliated, and horizontally layered gabbros (forming the lower crust). The main magma chamber is composed of a mush containing less than 20% of melt, topped by an upper melt lens that is mostly filled with near pure liquid. Dashed blue curves identify hydrothermal circulation. Red triangle shows the level from where the investigated gabbros in the Wadi Rajmi are originated.

ABOVE : Microphotographs of sample 07OL36 ; plane-polarized light for a and cross-polarized light for b. 07OL36 gabbronorite contains two domains, ‘P domain’ contains poikilitic plagioclase grains that include several individual granular clinopyroxene chadacrysts that are devoid of oxide ; and ‘G domains’ exclusively composed of clinopyroxenes containing tiny oxide inclusions and subordinated amphiboles.

ABOVE : Back-scattered images of sample 07OL36. a) ‘P domain’ displaying poikilitic plagioclase containing chadacrysts of granular oxide-free clinopyroxene grains ; b) contact between ‘P domain’ (to the left), and ‘G domain’ (to the right) ; ‘P domain’ is composed of poikilitic plagioclase containing granular Al-Ti-rich and oxide-free clinopyroxene grains, and ‘G domain’ is nearly exclusively composed of oxide-bearing low-Al-Ti-clinopyroxenes and subordinated amphiboles.

MORB contamination ; assimilation ABOVE : Compositional image (Al+Ca+Mg) of sample 07OL36 (image width : 1.5cm). Blue : plagioclase ; yellow : granular oxide-free clinopyroxene (‘P domains’) ; green : clinopyroxene containing tiny oxide inclusions (‘G domains’) ; red : orthopyroxene ; purple : amphibole. Note the patchy texture with roundish ‘P’ and ‘G’ domains. Poikilitic amphibole, plagioclase and orthopyroxene grains are observed. The white box indicates the location of the b) BSE image from above.

hydrous magma ; high An plagioclase ; high Mg pyroxene ABOVE : Mg# in clinopyroxene vs. An content of plagioclase (after Kvassnes et al., 2004) ; the dry and wet fractionation trends are calculated using MELTS (Ghiorso and Sack, 1995) ; both fractionation trends are calculated for different initial compositions. The studied samples (07OL34 and 07OL36) are typical for the wet fractionation trend. For 07OL34 and 07OL36, plotted are densities of analytical points.

hydrous magma ; MORB ; contamination ABOVE : General schematic model for the genesis of the studied hydrous-melt derived coarse-grained gabbros. Stage 1 : Fine-grained isotropic gabbro crystallization from a MORB-like melt ; Stage 2 : Hydrothermal circulation and partial alteration of the stage 1 gabbro ; Stage 3 : Melt intrusion within the crystallized fine-grained isotropic gabbro, this new melt can either represent an upwelling or growing up of the upper melt lens (e.g., France et al., 2009), or a propagation of a ridge segment close to the end of ridge segment (e.g., Wanless et al., 2010), or to a second magmatic stage related to the early obduction during ophiolite emplacement (e.g., Koepke et al., 2009). Stage 3’ : The melt intrusion within the previously hydrothermally altered gabbro triggers the reheating of gabbro and results in its hydrous partial melting ; products are a hydrous partial melt, and a residual reheated-gabbroic assemblage (observed close to the intrusive contact or as enclaves). The formed hydrous melt can mix with the ‘fresh melt’ of Stage 3 to form a hybridized melt. Stage 4 : Hybridized hydrous melt crystallize the coarse-grained gabbros, and dry melt crystallize the fine-grained gabbros. Some residual assemblages are trapped within the crystallizing gabbros. Stage 5 : Hydrothermal circulation and partial alteration of all the lithologies. The triangle and the star highlight the possible location of samples 07OL36, and 07OL34, respectively.

 *Contamination of MORB by anatexis of magma chamber roof rocks : constraints from a geochemical study of experimental melts and associated residues

L. France, J. Koepke, C.J. MacLeod, B. Ildefonse, M. Godard, and E. Deloule

Lithos, (2014) 202-203, 120-137, doi:10.1016/j.lithos.2014.05.018

HIGHLIGHTS :

  • MORBs are contaminated at crustal levels
  • Contamination is related to anatexis of the hydrothermally altered magma chamber roof
  • Natural processes occurring at magma chamber roofs are reproduced experimentally
  • Chemical composition of the contaminating melts is characterized
  • Keys are given to identify contamination in MORB melts

MORB contamination ; oceanic magma chamber roof

ABSTRACT Mid-ocean ridge basalts (MORB) are the most abundant magmas produced on Earth. They are widely studied to infer mantle compositions and melting processes. However, MORB liquids are also the complex end-product of a variety of intra-crustal processes such as partial or fractional crystallization, melt-rock interaction, and contamination. Deciphering the relative contribution of these different processes is of first-order importance. Contamination at ocean crustal levels is likely, and may occur at magma chamber margins where fresh magmas can interact with previously hydrothermally altered rocks. Characterizing the composition of this crustal contaminant component is critical if we are to understand the relative importance of each component in the resulting MORB liquid. Here we present the results of experiments designed to reproduce the processes occurring at oceanic magma chamber roofs, where crustal contamination should be most extensive, by melting a representative sample of the sheeted dike complex. Anatectic melts thus produced are likely to represent the principal crustal contaminant in MORB. These melts were characterized for major and trace elements, showing B, Zr, Hf, U enrichment, and Sr, Ti, V depletion relative to original MORB liquids. In comparison to the starting material, relative element fractionations are observed in the anatectic melts, with enrichments of : U relative to Ba, Nb, and Th ; LREE and MREE relative to Sr ; and Zr-Hf relative to LREE. Bulk partition coefficients for element partitioning during magma chamber roof anatexis are derived, and proposed as valuable tools for tracking MORB contamination. Comparison with natural samples from the East Pacific Rise and the Oman ophiolite shows that anatectic melts can crystallize in situ to form oceanic plagiogranite intrusions, and that residual assemblages associated with the hydrous partial melting stage are represented by hornfelsic dikes and enclaves (also named granoblastic basalts). We now recognize these as commonplace at the root of the sheeted dike complex both at present-day and fossil oceanic spreading centers.

Here are the KD0 determined in this study : partition coefficient ; MORB contamination ; hydrous partial melting

hydrous partial melting ; MORB contamination ; plagiogranite ; hornfels ; granoblastic dike ABOVE : Modal proportions of experiments, and major element composition (in wt %) of experimental melt and residue as a function of the degree of partial melting (F% from 0 to 100%) ; data from France et al. (2010). Shaded area represents the compositional range of primitive MORB (variation is related to spreading rate), from Rubin and Sinton (2007).

MORB contamination ; assimilation ; oceanic plagiogranite ; granoblastic dike ; hornfels ABOVE : N-MORB normalized REE and trace elements contents of experimental melt formed at 955°C (averaged value ; yellow), and related calculated residue composition (see below ; red). Starting material used in experiments is also shown (blue). Normalization values are from Gale et al. (2013). In all graphs dashed lines represent extrapolated values.

MORB contamination ; assimilation ; oceanic plagiogranite ; granoblastic dike ; hornfels ABOVE : REE and trace element compositions of (a) experimental products (955°C) compared (normalized) to the starting material composition, and (b and c) natural plagiogranites and hornfelses compared (normalized) to the regional dikes. Samples from Oman are presented in (b), and samples from IODP Hole 1256D are presented in c). In (b) and c), experimental melts and residue compositions are reported for comparison (same symbols as in (a)). In all graphs dashed lines represent extrapolated values.