Igneous & Metamorphic Petrology I LECTURE 11

Transcription

1 Igneous & Metamorphic Petrology I LECTURE 11 The Earth s Mantle 1. Structure of the Earth A Reminder The velocities of seismic waves differ with the elastic properties and densities of rocks and allow the internal structure of the Earth to be investigated. Primary waves (P-waves) are compression waves originating through fluctuations in pressure and will propagate through solids and liquids. Secondary waves (S-waves) are shear waves originating through back and forth shearing motion and do not propagate through liquids. Sudden changes (discontinuities) in seismic wave velocities through the Earth identify boundaries between fundamentally different materials. Several significant boundaries and layers are recognised: Crust variable thickness. Continental 10-70km, composition andesitic. Oceanic 8-10km, composition basaltic. The base of the crust is the Moho (the Moho discontinuity). Lithospheric mantle variable thickness. Oceanic lithosphere is ~100 km thick. Continental lithosphere may reach 200 km thick below stable cratons (old crustal units). The entire mantle has a more or less peridotite composition (olivine, opx, cpx ultrabasic rock). The lithospheric mantle is solid and brittle and together with the crust forms the lithosphere the combined layer makes up the plates. Asthenosphere a layer of the mantle about 250 km thick solid rock but flows easily (ductile). The upper layer of the asthenosphere is the low velocity zone where seismic velocities are less than in the overlying lithosphere. This layer is partially molten (no more than 10% by volume) and is variable in thickness and absent in some areas. Mesosphere the lower layer of the mantle, about 2500 km thick, solid but capable of flow. It contains two discontinuities. The 400 km discontinuity where olivine transforms into a spinel structure mineral and the 670 km discontinuity where pyroxene transforms into a perovskite structure. Outer core km thick, completely molten Fe-Ni metal (+lesser amounts of unknown light elements). Inner core 1230 km thick, solid Fe-Ni metal metal (+lesser amounts of unknown light elements). 2. The Origins of Magmas Using simple considerations we can easily see where are the main sources of magmas. In ocean basins, magmas are not likely to come from oceanic crust, since most magmas in ocean basins are basaltic. To generate basaltic magmas by melting of the basaltic oceanic crust would require nearly 100% melting which is highly unlikely. In the continents, both basaltic, intermediate and acidic magmas are erupted and intruded. The continental crust has an andesitic composition and could be melted to produce siliceous magmas. Basaltic magmas must, however, come from the underlying mantle. With the exception of the continents, most magmas are most likely to originate from the melting of ultrabasic mantle peridotite. Not only has melting of the upper mantle, underlying the crust, been responsible for magmatism throughout at least the last 3 bn yrs. Magmatism has also had a profound effect on the composition and nature of the upper mantle. 3. Mantle Rocks Above the 400 km discontinuity the mantle consists of peridotites, olivine + orthopyroxene (Opx) + clinopyroxene (Cpx) rocks with a non-essential Al-rich mineral phase. The peridotites are sub-divided on the basis of the abundances of their essential constituents according to the ternary shown opposite. Orthopyroxene is generally enstatite and clinopyroxene Cr-diopside. The form of the Al-rich phase in mantle peridotites varies with pressure due to phase changes. These arise because of increases in the coordination of Al. At pressures less than 10 kb plagioclase is the stable phase, at pressures of kb plagioclase transforms to spinel (Mg,Fe +2 )(Cr,Al,Fe +3 )2O4 and at pressures >30 kbar garnet is the stable Al-rich mineral. This gives rise to plagioclase peridotites, spinel peridotites and garnet peridotites (found at the largest depths) Mantle Xenoliths Xenoliths (foreign nodules/clasts) found in mantle-derived magmas provide a sample of the oceanic and continental lithosphere. Mantle xenoliths (and single crystals known as megacrysts) are found in a wide range of basic and ultrabasic magmas and are fragments of the lithosphere that were included in magmas as they rose through the mantle. The presence of mantle xenoliths within magmas suggests they have risen rapidly since these dense objects have not settled out but have been transported to the surface. Estimates suggest that basaltic magmas may have risen at velocities of up to 5 m/s, where as those of kimberlite magmas may be as high as 50 m/s. The rate at which xenoliths have been transported to the surface is important in preserving their original mineralogy. Xenolith s rapidly transported are less likely to have equilibrated to lower pressures and to have reacted with the host magma. Many dunite xenoliths seen in basalts, however, are not mantle xenoliths but cumulate xenoliths formed by the accumulation of olivine within shallower magma chambers. M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 1 of 8

2 Igneous & Metamorphic Petrology I LECTURE 11 Xenoliths are most common in basalts, alkaline series magmas (e.g. nephelinites, phonolites etc) and kimberlites. 3.1.a. Types of Mantle Xenolith The three commonest varieties of mantle xenolith are: (1) harzburgites, these are cpx-poor peridotites that are often spinelbearing suggesting they are more commonly found at lower pressures, (2) lherzolites, spinel lherzolites or garnet lherzolites, and (3) eclogites. Eclogite are compositionally similar to basalt but contain sodium-rich pyroxene (omphacite) and pale pink, magnesium rich garnet (almandite and pyrope); kyanite and rutile are also common. Most eclogites shows signs of retrogressive metamorphism, with the pyroxene and garnet crystals separated by rims of hornblende and plagioclase that have developed from the reaction between these crystals. Common minerals are: garnet, quartz, omphacite pyroxene, and sometimes kyanite, paragonite, zoisite, dolomite, and corundum. Eclogite forms by transformation from basaltic rocks at P >15kbar, T>350 C. The phase transition is gradational. Pyroxenites (clino-, orthopyroxenites and websterites) and dunites have been found within lherzolite xenoliths as veins and dykes and may represent the products of magmas that intruded and cooled within the lithosphere. Oceanic lherzolite xenoliths also can contain gabbro veins. A rarer, but highly significant, group of mantle-xenoliths are known as the metasomites. These are peridotites that have unusual accessory mineral phases that are often rich in incompatible elements (i.e. elements that preferentially partition into a magma on melting such as Na, K, Ti, S etc). In basaltic host magmas metasomites are amphibole-bearing (kaersutite) or mica-bearing (paragasite). In kimberlites metasomites are either ilmenite-rutile-phlogopite-sulphide (IRPS) bearing or K-richterite, phlogopite-bearing. Phlogophite-rich xenoliths are sometimes known as glimmerites. Metasomites in kimberlites that are dominated by mica-amphibole-rutile-ilmenite-diopside are known as MARID xenoliths and are sometimes found as veins with lherzolite xenoliths. The textures of mantle-xenoliths vary, however, many (in particular harzburgites) have equilibrated equigranular textures with 60 triple junctions. A proportion of garnet lherzolites, however, have sheared textures with aligned deformed elongate crystals and recrystallisation at the margins of crystals due to deformation. Metasomites differ significantly since many of these have disequilibrium textures with, for example, incompatible-element rich minerals growing as reaction-rims. 3.2.b. Distribution of Mantle Xenoliths Mantle xenoliths vary with magma type and tectonic settling. Mid Oceanic Ridge Mantle xenoliths are rare, mainly cumulate xenoliths. Ocean Islands (e.g. Hawaii) Mantle xenoliths present in alkaline magmas, such as alkali basalts and basanites. These are mainly spinel lherzolites and harzburgites. Metasomites are rare. Convergent plate margins (mainly back arcs) spinel lherzolites and harzburgites in alkali basalts and basanites. Continental rifts spinel and spinel+garnet xenoliths within basanites and more alkaline rocks (nephelinites, phonolites etc). Garnet megacrysts are also found. Cratons kimberlites contain spinel, spinel+garnet, and garnet lherzolites. Eclogites, Metasomites. Xenoliths, can be diamond-bearing Ophiolite Mantle Sequences Ophiolites are slices of the oceanic crust that have been tectonically obducted into the continental crust. Alpine ophiolites preserve only the mantle sequence, but contain eclogite which may be transformed crust. Although most ophiolite sequences have been highly deformed and hydrothermally altered during obduction, they can, however, be reconstructed to examine the structure of the oceanic lithosphere. The generalised upper mantle sequence of ophiolites occurs below layered gabbros that form the base of the oceanic crust (see next lecture). The uppermost layer of the mantle sequence is dominated by harzburgites which grade downwards into lherzolites (plag- or spinel-bearing). Pyroxenite, dunite and gabbro intrusions are found in the mantle sequence and in places contain chromite-rich podiform segregations. The interface between the layered gabbros and the underlying mantlesequence is known as the petrological Moho. Most of the mantle sequence of ophiolites has been altered to serpentinite. Serpentine group minerals, however, often pseudomorph pre-existing minerals. Veining in serpentinites, often by carbonates, is common due to volume reductions associated with serpentinisation. Reduction (i.e. decreases in oxygen fugacity) can lead to iron and nickel within silicates becoming metallic (Fe 0 rather than Fe 2+, Fe 3+ ) resulting in the presence of Ni-bearing metal. 4. Mantle Melting and Composition Mantle Melting. Temperature increases with depth in the Earth along the geothermal gradient. The normal geothermal gradient is somewhat higher below the oceans than the continents at least at shallow depths. However, the normal geothermal gradient is everywhere less than the melting temperature of peridotite. In order to melt the mantle to generate magmas, we must either increase the geotherm or reduce the melting temperature of peridotite. M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 2 of 8

3 Igneous & Metamorphic Petrology I LECTURE 11 Frictional heat, generated by a zone in which shearing is occurring, is one way in which the geotherm can be increased. If the heat cannot conduct away it will cause a local increase in temperature which could lead to partial melting. Such shearing occurs at the base of the lithosphere in the asthenosphere and may explain the presence of the LVZ and sheared peridotites found as xenoliths. Convection in the mantle causes hot, less dense mantle to rise and cooler, denser mantle to sink forming convection cells. The rise of hot mantle raises the geotherm. Melting occurs where the decrease in pressure (due to the decrease in depth) means the temperature of the mantle rocks is higher than the solidus of peridotite. This is, therefore, known as decompression melting. Melting in the mantle can also occur due to the addition of incompatibles to mantle rocks. Volatiles, such as H2O and CO2 in particular cause depression of the solidus and result in melting The Effects of Melting on Mantle Composition. The mantle can be considered to have an overall composition similar to that of the Earth as a whole, albeit depleted in the siderophile (iron-loving) elements such as Fe, Ni, Co, Ir that were removed during early differentiation of the Earth by the separation of our planet s core. Since basaltic magmas are produced by partial melting of peridotite the mantle that remains after melting and separation of the magmas (i.e. after magma extraction) must be depleted in those components within the basaltic melt. These components are often termed incompatible elements since they are those that are not easily incorporated into the atomic structures of common rock forming minerals (those that are easily incorporated are compatible elements). On melting the most incompatible elements are partitioned into the partial melt. Partial melting will, therefore, lead to residues (solids left behind) that are depleted in incompatible elements relative to bulk Earth. Light rare earth elements (LREE) are more incompatible than heavy rare earths (HREE). Depletions in LREE relative to HREE are, therefore, characteristic of mantle rocks that have been depleted by extraction of partial melts. Many mantle xenoliths show depleted trace element patterns relative to bulk Earth that indicate extraction of basaltic magmas. Some xenoliths, however, have trace element patterns similar to bulk Earth that suggest magmas have not been removed. These are described as fertile mantle and often consist of sheared, high temperature garnet lherzolites that have evidently come from the largest depths and are probably samples of the asthenospheric mantle. In general, therefore, the lithosphere is considered depleted and the asthenosphere fertile and undepleted. Some mantle xenoliths, however, show enrichments in incompatible elements relative to bulk Earth suggesting they have been enriched by the addition of components from a magma. These enriched mantle samples include metasomites in which minerals rich in incompatibles such as K, Na, Ti and S are found replacing normal peridotite phases. These mantle rocks have been metasomatised by fluids associated with magmas that have intruded the lithosphere. Mantle metasomatism can occur by the growth of new minerals, known as modal metasomatism, or simply by enrichment of trace elements (LREE), known as cryptic metasomatism The Effects of Melting on Mantle Mineralogy. The mineralogy of mantle peridotite that is the residue after melting is different from peridotite that has not experienced melt extraction. The effect of melting on the mineralogy of peridotite can be considered using the projected phase relations in Cpx-Olivine-Quartz. The first formed liquid generated on partial melting of lherzolite at high pressure (>10 kbar) is generated at a eutectic outside the Cpx-Olivine-Quartz M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 3 of 8

4 Igneous & Metamorphic Petrology I LECTURE 11 system and is alkali-rich (nepheline normative). With increasing temperature more melting occurs and the magma composition moves along the Ol+Cpx cotectic by melting of olivine and Cpx. Once it reaches the Cpx+Ol+Opx eutectic the Cpx disappears completely with further melting. The residual peridotite has now become a harzburgite. If more melting occurs Ol and Opx melt and the magma moves up the Ol+Opx peritectic until all the Opx is exhausted by melting. The residual peridotite is now a dunite. 5. Structure and Composition of the Upper Mantle Mantle xenoliths and ophiolites suggest that the lithospheric mantle is a layer of mantle depleted in basaltic components due to the generation of magmas. High temperature, sheared xenoliths found in kimberlites are thought to be samples of the asthenosphere and suggest this has a fertile composition similar to the bulk mantle. The presence of diamonds within depleted garnet lherzolites from kimberlites within cratons indicates that the lithosphere may be as much as 220 km thick under stable cratons and shelves to less than 130 km adjacent to cratons. Eclogites xenoliths within kimberlites also imply that slices if subducted oceanic crust are present within the continental lithosphere. Metasomites found within alkali basalts (kaersutite and paragasite) and IRPS metasomites in kimberlites have sometimes contained pyroxenite veins and are thought to originate from metasomatic enrichment of the lithosphere by fluids associated with intruded magmas. K-rich metasomites of the MARID suite found in kimberlites are thought to relate to metasomatic enrichment by subductionrelated fluids released from oceanic crust and sediments. The distribution of xenoliths suggest magmas originate due to melting at increasing depth in the series tholeiites, alkaline magmas (alkali basalt, basanites), highly alkaline magmas (nephelinites, phonolites), kimberlites. M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 4 of 8

5 Igneous & Metamorphic Petrology I LECTURE 12 Mantle-Derived Magmas: The Ocean Basins 1. Basalts and Basaltic Magmas Most volcanism (but not all) in the ocean basins is dominated by basaltic magmas. Basalts and their magmas are classified into three groups on the basis of their normative mineralogy: (1) Alkali Basalts, which are silica-undersaturated and alkali-rich, (2) Olivine Tholeiites, which are silica-saturated, and (3) Quartz Tholeiites, which are silicaoversaturated. These three types of basalts occupy different volumes of the quaternary Clinopyroxene-Quartz-Olivine-Nepheline (see opposite). The behaviour of olivine tholeiite melts and quartz tholeiite melts can be illustrated by the ternary Olivine-Quartz-Plagioclase (see lecture 6 notes). At equilibrium the mineralogy of a tholeiitic magma will depend on whether its initial composition was in the olivine tholeiite or quartz tholeiite field. In the diagram opposite only the quartz tholeiite magmas will contain quartz in their groundmass at equilibrium. The notes from lecture 6 describe why. An important feature of the phase relations of basalts is that the critical plane of silica undersaturation (cpx-ol-plag) represents a barrier at low pressures. The diagram below shows two back to back ternary diagrams and illustrate in which direction magma compositions evolve with decreasing temperature across this boundary. Note that these are projections of the phase relations (imagine viewing the ternaries from plagioclase in the quaternary diagram). The important point is that on the Nepheline-rich side of the critical plane of silica undersaturation alkali basalt magmas crystallising down the cotectic become more Nepheline-rich. On the opposite (quartz-rich) side of the plane tholeiite magmas crystallise down the cotectic to become more quartz-rich (more quartz normative). Think of the critical plane of silica undersaturation as the crest of a hill or ridge in terms of temperature. During cooling the magma can only move down hill, therefore, the plane acts as a barrier or a thermal divide. Alkali basalt magmas cannot become more quartz-rich and turn into olivine tholeiites through crystal fractionation because the thermal divide is in the way and they evolve in the opposite direction becoming more Nepheline-rich. Likewise tholeiite magmas cannot evolve into alkali basalt magmas by crystal fractionation Melting of Peridotite to produce Basaltic Magmas The ternary Olivine-Quartz-Nepheline below is a variation diagram and shows the compositions of basaltic liquids produced by increasing partial melting of peridotite at different pressures. This is equivalent to the base of the quaternary shown above. The composition at the furthest point from peridotite is that of the first partial melt produced on melting. The fields extend along a cotectic with increasing melting of olivine and px until they reach the solid light grey field. At this point all the px has been melted and only olivine remains. Further melting of olivine thus causes the magma to become more olivine rich (i.e. it moves directly towards olivine). pressures. The important features to notice are that (1) at low pressures (<10 kbar) all the melts are oversaturated or saturated in silica (i.e. they are olivine and quartz tholeiites), (2) small degrees of melting at high pressure generates alkali basalt magmas which become more silica-rich at with increasing melting (the thermal divide does not exist at high pressure). Alkali basalt magmas are, therefore, produced by small degrees of melting at high pressure and tholeiite magmas are produced at large degrees of melting at high pressure OR by melting at low M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 5 of 8

6 Igneous & Metamorphic Petrology I LECTURE Mid Oceanic Ridge Volcanism Oceanic crust is generated by basaltic volcanism at the mid-oceanic ridges where the convergent motion of the plates causes extension. The structure of oceanic crust can be determined from the upper sequences of ophiolites Oceanic crust in ophiolites Ophiolites are found in numerous localities including Cyprus, New Guinea, Newfoundland, California, and Oman. The S ophiolite in southeastern Oman has probably been studied in the greatest detail (sequence is shown opposite). The rocks probably formed in the Cretaceous not far from what is now the Persian Gulf. The rocks were later thrust westward onto the Arabian shield. The oceanic crust sequence in ophiolites consist of upper pillow lavas, which were erupted onto the ocean floor, underlain by sub-parallel sheeted dykes many of which fed the extrusive activity. Below the sheeted dyke complex the rest of the oceanic crust sequence consists of gabbro and represents the crystallisation products of a magma chamber. At its base the gabbro is layered due to crystal settling within the chamber and overlies the petrological Moho and the mantle peridotites (which are mainly harzburgites) Structure of the Mid Ocean Ridge The mid ocean ridges are linear topographic highs on the ocean floor, transected by transform faults, along which basaltic volcanism is concentrated. The ridge has a 35- km wide central rift originating through extensional tectonics. Ophiolite sequences indicate that the oceanic crust consists of an upper layer of pillow lavas underlain by a sheeted dyke complex. The dykes act as feeders from a basaltic magma chamber that underlies the ridge. As the plates diverge the lower section of the oceanic crust is formed by crystallisation in the magma chamber. The higher topography of the rift is caused by the rise of hot asthenospheric mantle below the ridge as the uppermost expression of mantle rising due to convection. Decompression melting within the rising mantle produces basaltic melts that intrude the overlying mantle to continually resupply the magma chamber Mid Ocean Ridge Basalts Mid Ocean Ridge Basalts (or MORBs) are olivine and quartz tholeiites. The compositions of MORB glasses (e.g. quenched magma) shown in the ternary diagram opposite provides us with information on their origins. On diagram the compositions of the glasses is shown as the shaded region and in part extends along the projections olivine + cpx cotectic of the system at low pressure (1 atmosphere). Glasses plotting along this cotectic represent magmas that have evolved from each other by crystal fractionation of cpx and olivine at low pressure with cooling towards more quartz normative compositions. Silica-poor MORB glasses, however, extend away from the low-pressure cotectic towards more olivine-rich compositions. This suggests the magma crystallised olivine first (before it cooled to the cotectic shown as the dotted arrow). Despite the crystal fractionation MORB does, however, have a very limited range of compositions. The origin of the magma is also constrained by this ternary. The projected phase relations at pressures up to 20kb are also shown on the diagram. During melting of peridotite the first partial melts will have a composition at the eutectic (which at high pressure is at silica-undersaturated compositions off the edge of the diagram. With increasing melting of mantle peridotite the composition of the melt generated will move along the ol+cpx cotectic towards the ol+cpx+opx eutectic where all the cpx will become consumed. From the phase relations we can see that the parent magmas of MORB (the original magma before crystal fractionation) must have evolved along the dashed line by the crystal fractionation of olivine. The original magma composition must lie along this line and be more olivine-rich than MORB glasses. Note this is only possible at pressures >15 kbar. Also at pressures >20 kbar the ternary eutectic enters the silica-undersaturated alkali M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 6 of 8

7 Igneous & Metamorphic Petrology I LECTURE 12 basalt field. Alkali basalts are not found as part of MORB. Melting of peridotite to form MORB magmas must, therefore, occur at between 20 and 15 kbars equivalent to a depth of around 50 to 60 km. The trace elements in MORB provide us with details on the nature of the source peridotite and the degree of partial melting. MORB magmas are enriched relative to bulk Earth, however, MORB has a depleted pattern with compatible elements more abundant than incompatible elements. This must indicate that MORB is derived by melting of depleted lithospheric mantle rather than fertile asthenosphere. Small degrees of partial melting, however, will tend to flatten-out because incompatibles will enter the partial melt first. The depleted pattern of MORB indicates high degrees of melting of 20 to 40% by volume. 3. Enriched Oceanic Basalts Not all oceanic basalts share the depleted compositions of the majority of MORBs, some have enriched trace element patterns which indicate they were partially derived from a fertile or enriched mantle source Enriched MORBs Although most MORB erupted at the Mid Ocean Ridges are depleted (and known as N-MORBs) some have enriched patterns (see diagram below) and are known as E-MORBs. These tholeiites are typical found in regions of the Mid Oceanic Ridges where large volumes of basaltic magma have been erupted. Iceland is an excellent example. Although most E-MORB basalts erupted at such localities are tholeiitic, alkali basalts, intermediate lavas and even rhyolites (e.g. Iceland) can be erupted. The trace element patterns of E-MORBs clearly indicate that more enriched (fertile) material is present in their source regions. Intermediate and acid magmas erupted in Iceland are generated by crystal fractionation of the E- MORBs. The ternary (known as an AFM for alkali, Fe and Mg) opposite shows the evolution of E-MORB magma with increasing crystal fractionation. At first magmas become increasingly Fe-rich (and silica-rich) since Mg-rich olivines and pyroxenes (which have the highest crystallisation temperatures) crystallise and are removed from the magmas by settling. Once the Fe-content of the magmas becomes high enough an iron-oxide mineral (such as magnetite) starts to crystallise. Crystallising and removing magnetite from the magma then causes it to evolve towards more silica and alkalirich compositions of rhyolite. The rhyolites on Iceland tend to erupt after the basalts and intermediate icelandites Ocean Island Basalts The basalts of oceanic islands (Oceanic Island Basalts OIBs), such as the Hawaiian Islands, have incompatible element patterns that are even more enriched than E-MORBs and indicate they are not derived simply by melting of depleted oceanic lithosphere like the N-MORBs. Like Iceland, magmas erupted at Oceanic Islands include alkali basalts, intermediate magmas and rhyolites, however, highly alkaline magmas such as nephelinites and phonolites also occur. The origins of this variety of magmas lies both their sources in the underlying mantle and their evolution by crystal fractionation. 3.1.a. Hot Spots and Oceanic Islands Intraplate oceanic islands form linear arcs in which active volcanoes are located at one end and are thought to form above a plume of hot mantle (Hot Spot) rising up below the lithosphere. Chains of volcanoes are produced as the oceanic plate is dragged across the top of the plume. The change in the orientation of the Hawaiian-Emporer chain of islands and seamounts is the most dramatic evidence for the existence of hot spots since it corresponds with a change in direction of the Pacific plate 40 Ma. The enrichment of OIBs and the timing and type of magmas erupted are all related to proximity to the mantle plume. M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 7 of 8

8 Igneous & Metamorphic Petrology I LECTURE a. Magma Series of Ocean Islands Oceanic islands have any or all of three distinct series of related magmas: (1) tholeiites, (2) alkali basalts to trachytes (alkaline series), and (3) highly alkaline series of basanites, phonolites and nephelinites. On the Hawaiian Islands the shield building phase of the volcanism has been dominated by the eruption of olivine tholeiites. Currently tholeiitic basalts are being erupted on Kilauea and Mauna Loa. Older volcanoes such as Mauna Kea, on the Big Island, and Haleakala, on Maui further to the NW, are likewise mainly composed of tholeiite but are capped by steeper-sided composite cones that consist of alkali basalt, hawaiites and trachytes. These alkaline caps are thought to have existed on top of the tholeiitic shields of all the older islands but have now been removed by erosion. Recent activity has, however, occurred on Oahu island over the eroded tholeiite shield and consists of highly alkaline basanites and nephelinites. The Hawaiian islands, therefore, appear to form by the eruption of large volumes of tholeiite that builds the shield volcano followed by eruption of smaller amounts of alkali basalt to trachyte. Finally some volcanoes then erupt highly alkaline basanites and nephelinites. 3.3.b. Origin of Magma Series The evolution of magma types within each of the series results from crystal fractionation like in the case of icelandic magmas. The AFM variation diagram for the alkaline series is shown opposite. The different series of magmas, tholeiite, alkaline and highly alkaline, however, cannot be derived from each other by crystal fractionation. Alkali basalt, the parental magma for the alkaline series of magmas on Hawaii, cannot be obtained from olivine tholeiite at low pressure by crystal fractionation because of the thermal divide. At high pressures we could, however, generate alkali basalts by low degrees of partial melting of peridotite and then generate tholeiites later with further melting. This is not, however, consistent with the observation that alkali basalts are extruded after the tholeiites. One way to explain the origin of the different magma series and their relative timing would be due to differences in the degree of partial melting due to proximity to the hot spot. Olivine tholeiite magma is generated due to large degrees of partial melting at higher temperatures over the hot spot. Once plate motion has moved the volcano away from the hot spot lower degrees of partial melting occur due to the lower temperatures and alkali basalt magmas are generated. Finally more plate movement occurs before highly alkaline magmas are generated at very low (and perhaps higher pressure) partial melting. The presence of the Loihi seamount to the SE of Hawaii hot spot seems to confirm this explanation since it erupts alkali basalts. M. Genge (room 3.47, ext 46499, m.genge@ic.ac.uk) Page 8 of 8