Ian J. Parkinson

Trace element models for mantle melting: Application to volcanic arc petrogenesis

Abstract Understanding mantle melting above subduction zones requires an evaluation of the behaviour of elements for which the mantle contribution greatly exceeds any subduction contribution. In this paper we present a compilation of partition coefficients for a suite of these elements (Nb, Zr, Y, Yb, Ca, Al, Ga, V, Sc, Fe, Mn, Co, Cr, Mg, Ni) for temperatures of 1200–1300°C, oxygen fugacities of QFM ± 1 and sub-alkaline compositions. These coefficients yield good-fit mantle depletion trends for abyssal, orogenic and trench-wall peridotites. Modelling of pooled melts from mantle melting columns, presented as FMM (fertile MORB mantle) normalized patterns, give signatures of the composition and degree of melting of the mantle wedge that are generally independent of the subduction component. In particular, patterns formed from melting of fertile mantle exhibit normalized element abundances in the order VHI > HI > MI (VHI = very highly incompatible, HI = highly incompatible and MI = moderately incompatible) at low degrees of melting, becoming VHI = HI = MI at high degrees of melting. With derivation from progressively depleted sources, the patterns for moderate degrees of melting change to VHI < HI = MI at moderate degrees of depletion and VHI < HI < MI at high degrees of depletion. The details, but not the principles, can be varied by changing the shape of the melting column, the porosity of the mantle during melting, the potential temperature of the mantle, and the temperature and depth of initiation of melting. Bivariate plots of elements of contrasting compatibilities (Cr-Yb, Sc-Yb, Nb-Yb) can be contoured according to the degree of depletion or enrichment of the mantle and the degree of melting, with selected plots also emphasizing the role of garnet (Ti-Yb) and oxygen fugacity (V-Yb). Evaluation of data from present-day volcanic arcs suggests that: (1) intra-oceanic arcs with associated active backarc basins are derived principally from fertile MORB mantle that has lost up to about 3% melt in a previous melting event; (2) this depletion takes place in spinel lherzolite facies, supporting models that relate it to backarc basin melting events; (3) oceanic arcs with no associated backarc basins are derived principally from fertile MORB mantle, though enriched sources can be important locally; (4) intra-continental arcs are commonly derived from enriched mantle, probably because of the involvement of sub-continental lithosphere; (5) degrees of melting are probably high (in the order of 25–30%) in intra-oceanic arcs on thin crust, decreasing to 150r less in areas of thicker lithosphere; (6) some 10% melting can be explained by volatile addition to the mantle, the remainder by decompression.

The redox state of subduction zones: insights from arc-peridotites

Spinel peridotites from a variety of island arcs have been utilised to calculate the redox state of the mantle wedge above subduction zones. Oxygen fugacities (fO2 values) calculated from the ferric iron content of spinels, measured by Electron Microprobe (EMP) using secondary standards [Wood, B.J., Virgo, D., 1989. Upper mantle oxidation state: ferric iron contents of lherzolite spinels by 57Fe Mossbauer spectroscopy and resultant oxygen fugacities. Geochim. Cosmochim. Acta, 53, 1277–1291.], yield values which range from 0.3 to 2.0 above the fayalite–magnetite–quartz (FMQ) buffer. These data provide further evidence that the mantle wedge is ubiquitously oxidised relative to oceanic and ancient cratonic mantle. There is no correlation between fO2 values and the presence of hydrous phases and, in fact, the most oxidised samples contain no hydrous phases. Within individual suites there is no correlation between fO2 and degree of depletion as indicated by spinel Cr#, except for a suite of reacted forearc-peridotites. However, when the data is viewed as a whole there is broad a positive correlation between fO2 and spinel Cr# suggesting that partial melting processes may influence the redox state of the mantle wedge. We suggest that the ultimate source of the oxygen which oxidises the mantle wedge is from the subducted slab. It is not clear whether this oxidising agent is a solute-rich hydrous fluid or a water-bearing silicate melt. However, our data does indicate that silicate melts are effective oxidisers of the depleted shallow upper mantle. Simple mass balance calculations based on the ferric iron content of primitive subduction zone magmas indicates that the source region must contain 0.6–1.0 wt.% Fe2O3. This amount of Fe2O3 in a fertile spinel peridotite yields an oxygen fugacity of 0.5–1.7 log units above FMQ in the IAB source. If water is the sole oxidising agent in the mantle wedge then 0.030–0.075 wt.% H2O is required which is considerably less than the 0.25% H2O envisaged by Stolper and Newman [Stolper, E.M., Newman, S., 1994. The role of water in the petrogenesis of Mariana trough magmas. Earth Planet. Sci. Lett., 121, 293–325.], suggesting water is not necessarily an efficient oxidising agent. Alternatively, ferric iron may be added to the mantle wedge by addition of a ferric iron-rich sediment melt or more likely as a solute-rich hydrous fluid. This model would produce spinel, orthopyroxene or amphibole in the wedge with only a slight increase in fO2 of the source region. Although it is unclear which model is correct the maximum fO2 of the fertile mantle wedge is unlikely to be above FMQ+2 and therefore some decompression melting in the mantle wedge is required to explain the higher fO2 values of primitive arc lavas than arc-peridotites.

Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites

Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (187Os/188Os(i)=0.1272–0.1371) in terms of global plume related magmatism, and this is coupled with anomalously high Os abundances and radiogenic 143Nd/144Nd(i) isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high 3He/4He, and the non-systematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORB mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting (>25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the ‘Kea’ component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle.

Geochemistry of metasomatism adjacent to amphibole-bearing veins in the Lherz peridotite massif

Abstract The Lherz peridotite massif, in the French Pyrenees, is intruded by a number of hornblendite and garnet-amphibole-pyroxenite (GAP) veins. New, high quality, elemental and isotopic data are presented for veins and their adjacent harzburgite wallrocks in order to evaluate the extent of reaction and the ability of fluids to permeate clinopyroxene-poor peridotites. Hornblendite and GAP veins have convex upward rare earth element (REE) profiles consistent with an origin as crystal segregates from alkali basalts. In all of the traverses Mg# increases away from the veins and MnO, TiO2, Zr, and the REE decrease away from the veins within a zone Ce Sm ratios and decrease in Ce contents away from the veins is consistent with equilibration of the calculated melt from the vein with the preexisting harzburgites. A region with high Ce Sm on the right of one vein may be the result of chromatographic fractionation of melt during percolation from the amphibole-bearing veins. However, this is not observed on the opposite side of the vein and ratios are variable within the zone, so an asymmetrical and irregular chromatographic front would be required. The high Ce Sm ratios, therefore, most likely reflect pre-vein REE heterogeneity in the harzburgites. Data for the Lherz massif suggest that dramatic variations in incompatible element concentrations can develop metasomatically in the continental lithospheric mantle over a relatively short length scale.

Ultramafic lamprophyres of the Ferrar large igneous province: evidence for a HIMU mantle component

Abstract Ultramafic lamprophyre (UML) dykes from the Ferrar Province (Pensacola Mountains) of Antarctica preserve trace element and isotope signatures similar to Bouvet volcanic rocks, which are considered to reflect the palaeo composition of the Bouvet mantle plume. We report Sr, Nd, Pb, and Os isotope compositions for three ultramafic lamprophyre dykes emplaced at 183.2±2.2 Ma, coincident with the main Karoo–Ferrar magmatic event. The ultramafic lamprophyre dykes are characterized by high Ti, Cr, Ni, Nb/La, LaN/YbN, and Mg# values, and are the most primitive rocks of the Ferrar Province. The dykes have initial (183 Ma) 87Sr/86Sr ratios of 0.7044–0.7055, eNd of 4.6–4.8, 208Pb/204Pb of 39.6–40.3, and 187Os/188Os of 0.120–0.146 and contrast markedly with even the most primitive rocks of the Ferrar and Karoo provinces. The trace element and isotope characteristics have affinities to ocean island basalt (OIB) and the highly radiogenic character of 208Pb/204Pb and 206Pb/204Pb bear closest resemblance to Bouvet, which has previously been postulated as the plume responsible for the Ferrar Province. The ultramafic lamprophyres are believed to be the result of melting enriched Bouvet mantle plume material and represent one of the mantle end members in the Karoo–Ferrar province.