Showing papers on "Incompatible element published in 1980"

Lead isotopic study of young volcanic rocks from mid-ocean ridges, ocean islands and island arcs

Abstract: Lead isotopic compositions of young volcanic rocks from different tectonic environments have distinctive characteristics Their differences are evaluated within the framework of global tectonics and mantle differentiation Ocean island leads are in general more radiogenic than mid-ocean ridge basalt (morb) leads They form linear trends on lead isotopic ratio plots Many of the trends extend toward the field of morb On plots of 207 P b / 204 Pb against 206 Pb / 204 Pb, their slopes are generally close to 01 Island arc leads in general are confined between sediment and morb type leads with slopes of ca 030 on a plot of 207 P b / 204 Pb against 206 Pb / 204 Pb Pb, Sr and Nd isotopic data of Hawaiian volcanics are closely examined Data from each island support a two-component mixing model However, there is a lack of full range correlation between islands, indicating heterogeneity in the end members This mixing model could also be extended to explain data from the Iceland-Reykjanes ridge, and from 45° N on the Atlantic Ridge The observed chemical and isotopic heterogeneity in young volcanic rocks is considered to be a result of long-term as well as short-term mantle differentiation and mixing Lead isotopic data from ocean islands are interpreted in terms of mantle evolution models that involve long-term (more than 2 Ga) mantle chemical and isotopic heterogeneity Incompatible element enriched ‘plume’-type morb have Th/U ratios ca 30 too low and Rb/Sr ratios ca 004 too high to generate the observed 208 Pb and 87 Sr respectively for long periods of time Elemental fractionation in the mantle must have occurred very recently This conclusion also applies to mantle sources for ocean island alkali basalts and nephelinites Depletion of incompatible elements in morb sources is most probably due to continuous extraction of silicate melt and/or fluid phase from the low-velocity zone throughout geological time Data on Pb isotopes, Sr isotopes and trace elements on volcanic rocks from island arcs are evaluated in terms of mixing models involving three components derived from (1) sub-arc mantle wedge, (2) dehydration or partial melting of subducted ocean crust, and (3) continental crust contamination In contrast to the relation between 87 Sr/ 86 Sr and 143 Nd / 144 Nd ratios of ocean volcanics, there is a general lack of correlation between Pb and Sr isotopic ratios except that samples with very radiogenic Pb ( 206 Pb / 204 Pb > 195) have low 87 Sr/ 87 Sr ratios (07028- 07035) These samples also have inferred source Th/U ratios (30-35) not high enough to support long-term growth of 208 Pb Data suggest that their mantle sources have long-term integrated depletion in Rb, Th, U and light ree High 238 U / 204 Pb (y a)values required by the Pb isotopic data are most probably due to depletion of Pb by separation of a sulphide phase Relations between Pb, Sr and Nd isotopic ratios of young volcanic rocks could be explained by simultaneous upward migration of silicate and/or fluid phase and downward migration of a sulphide phase in a differentiating mantleration of a sulphide phase in a differentiating mantle

Geochemical evidence for the genesis and eruptive setting of lavas from Tethyan ophiolites

Abstract: Most Tethyan ophiolites in the Mediterranean region fall into two categories: 1) fragmentary sequences of late Tri-assic to early Cretaceous age; and 2) fully-developed complexes. probably of late Cretaceous age. Lavas of the first category (from Calabria, Northern and Southern Apennines, Eastern Alps and Greece) are characterized by 'immobile’ trace element ratios, such as Zr/Y, Nb/Zr and Ce/Yb, which vary from slightly lower to significantly greater then chondritic values. These ratios and the absolute abundances of the component incompatible elements are similar to or greater then those of 'typical MORB' although abundances of the compatible element, Cr, are identical. Lavas with these characteristics are usually associated with 'anomalous' areas of oceanic crust, such as the Gulf of Aden near Afar or the North Atlantic near Iceland. ‘Non-ophiolite' lavas of similar age have the composition of within plate alkali basalts. By contrast, lavas of the second category (Cyprus (Troodos Massif), Baer Bassit, Oman (Semail Nappe)) exhibit Zr/Y ratios which are significantly lower thanchondritic and similar to or lower than 'typical MORB'. Compared with 'typical MORB', such lavas contain similar or lower contents of incompatible elements, are strongly depleted in Or and contain much lower Ce/Sr ratios (even when alteration has been taken into account). Lavas with these characteristics have occasionally been recovered from back-arc spreading centres but virtually never from major oceans. 'Non-ophiolite' lavas of this age have the composition of island arc tholeiites. The results of petrogenetic modelling of ophialite lava compositions suggest that, prior to partial melting beneath Tethyan spreading centres, the mantle source regions for the two lava types had undergone different histories. Ophiolites of the first category can be related to mantle which had first been depleted in incompatible elernents relative to 'bulk earth', and had then been enriched in the same elements by migrating fluids or interstitial melts in a 'within-plate' setting. Ophiolites of the second category can be related to mantle which had first been depleted in incompatible elements, but had then been enriched by aqueous fluids and hydrophilic elements driven off subducted oceanic lithosphore in an arc-basin setting. The modelling also requires that ]eyes derived from the latter as compared with the former source were produced by a greater degree of partial melting followed by a greater proportion of olivine crystallization from the segregated magma. These conclusions are consistent with: 1)formation of the older ophiolites in small Red-Sea-type basins related to fragmentation of continental crust during initial opening of the Atlantic; and 2) formation of the younger ophiolites in marginal basins created at a time of major changes in the relative movement of the African and Eurasian plates.

Halogens in the Mantle Beneath the North Atlantic

Abstract: F, Cl and Br contents of tholeiitic volcanic glasses dredged along the Mid-Atlantic Ridge from 53 degrees to 28 degrees N, including the transect over the Azores Plateau, are reported. The halogen variations parallel those of $^{87}$Sr/$^{86}$Sr, La/Sm or other incompatible elements of varying volatility. The latitudinal halogen variation pattern is not obliterated if only Mg-rich lavas are considered. Variations in extent of low-pressure fractional crystallization or partial melting conditions do not appear to be the primary cause of the halogen variations. Instead, mantle-derived heterogeneities in halogens, with major enrichments in the mantle beneath the Azores, are suggested. The Azores platform is not only a 'hotspot' but also a 'wetspot', which may explain the unusually intense Azores volcanic activity. The magnitude of the halogen and incompatible element enrichments beneath the Azores appear strongly dependent on the size of these anions and cations, but independent of relative volatility at low pressure. The large anions Cl and Br behave similarly to large cations Rb, Cs and Ba, and the smaller anion F similarly to Sr and P. Processes involving crystal and liquid (fluid and/or melt), CO$\_{2}$ rahter than H$\_{2}$O dominated, seem to have produced these largescale mantle heterogeneities. Geochemical 'anomalies' beneath the Azores are no longer apparent for coherent element pair ratios of similar ionic size. Values of such 'unfractionated' coherent trace element ratios provide an indication of the mantle composition and its nature before fractionation event(s) which produced the inferred isotopic and trace element heterogeneities apparently present beneath the North Atlantic. The relative trace element composition of this precursor mantle does not resemble that of carbonaceous chondrites except for refractory trace element pairs of similar ionic size. It is strongly depleted in halogens, and to a lesser extent in large alkali ions Rb and Cs relative to refractory Ba. These relative depletions are comparable within a factor of 5 to Ganapathy & Anders's estimates for the bulk Earth, with the exception of Cs. There is also evidence for removal of phosphorus into the iron core during its formation. With the exception of San Miguel, alkali basalts from the Azores Islands appear to have been derived from the same mantle source as tholeiitic basalts from the ridge transect over the Azores Platform but by half as much degree of partial melting. The Azores subaerial basalts seem to have been partly degassed in Cl, Br and F, in decreasing order of intensity. A working model involving metasomatism from release of fluids at phase transformation during convective mantle overturns is proposed to explain the formation of mantle plumes or diapirs enriched in larger relative to smaller halogen and other incompatible trace elements. The model is ad hoc and needs testing. However, any other dynamical model accounting for the 400-1000 km long gradients in incompatible trace elements, halogens and radiogenic isotopes along the Mid-Atlantic Ridge should, at some stage, require either (1) some variable extent of mixing or (2) differential migration of liquid relative to crystals followed by re-equilibration (or both), as a diffusion controlled mechanism over such large distances is clearly ruled out, given the age of the Earth.

Tertiary or Mesozoic komatiites from Gorgona Island, Colombia: Field relations and geochemistry

Abstract: An exceptional occurrence of ultramafic lavas within the volcanic member of the Mesozoic (or younger) Gorgona Igneous Complex represents the first known komatiites of post-Precambrian age Gorgona komatiites are virtually unaltered and display typical spinifex textures, with 7–10 cm long plates of olivine (Fo 88 to 91) surrounded by acicular aluminous augite, subordinate plagioclase (An 56 to 78), basaltic glass, and two spinel phases The MgO contents of the komatiites range from 15 to 22 wt% Sr and Nd isotopic compositions are indicative of depletion of incompatible elements in the mantle source region, as is the case for “normal” mid-ocean ridge basalts The komatiites are low in total REE abundances and extremely depleted in LREE They represent primary melts generated by high degree of partial melting of the mantle Eruption temperatures are estimated at 1,450° to 1,500° C

Chemical Variations in Upper Mantle Nodules from Southern African Kimberlites

Abstract: A summary of the rock and mineral nodules erupted with kimberlites is presented. Garnet-peridotites are separated into various categories according to: their relatively depleted and fertile chemical character; their deformed and undeformed nature; and the Ca/(Ca + Mg) ratio of their clinopyroxenes (with associated temperature characteristics). Some bulk chemical similarities are noted between high-temperature, deformed and less depleted peridotites and the wall rocks to minor intrusions. There is clear evidence of the occurrence of infiltration metasomatism (involving K, Ti and other incompatible elements) in some mantle xenoliths before their incorporation in the kimberlite. These metasomatic effects may be linked with earlier magmatic events. Extreme chemical heterogeneities may be produced by metasomatism, as with restricted partial melting. Attempts to find homogeneous rocks with average or pristine upper mantle compositions are considered unrealistic. Within the Kaapvaal craton there is no overall pattern of lateral variations in the nodule types from kimberlite, though there is evidence of local heterogeneity and regional changes at craton margins. Boyd & Nixon’s models of vertical layering of rocks and minerals in the upper mantle are contrasted with a model in which horizontal variations in temperatures occur after magma intrusion. It is suggested that the high-temperature deformed peridotites and the megacrysts result from the intrusion of high-temperature magmas into overlying cooler mantle, and that xenoliths from kimberlite provide little direct evidence of mantle stratification with depth.

Isotopic and trace element evidence from lavas, bearing on mantle heterogeneity beneath Kenya

Abstract: Nd, Sr and Pb isotope data, together with new major and trace element data are presented for lavas from northern Kenya. A general trend towards silica saturation and decreasing incompatible element contents is observed from the Miocene to the present day. Significantly, the abundances of different incompatible elements decrease at different rates. The Nd, Sr and Pb isotope compositions of the basic lavas are similar to those observed on the Atlantic ocean islands. Comparison of the Sm/Nd ratios required to produce the Nd isotope ratios with those observed in the rocks indicates that light rare earth elements (r.e.e.) have probably been added to the source region of the lavas comparatively recently. A model involving recent metasomatism of the subcontinental mantle beneath Kenya, which could account for the correlated silica undersaturation and incompatible element content of the lavas, is proposed.

Evidence for heterogenes primary MORB and mantle sources, NW Indian Ocean

Abstract: Basalts from 5 Deep Sea Drilling Project (DSDP) sites in the northwest Indian Ocean (Somali Basin and Arabian Sea) have general geochemical features consistent with a spreading origin at the ancient Carlsberg Ridge. However, compared to most MORBS from other oceans they have low normative olivine, TiO2, and Zr contents. There is no evidence that the mantle source of these northwest Indian Ocean basalts was enriched in incompatible elements relative to the Atlantic and Pacific ocean mantles. In detail, incompatible element abundances in these DSDP basalts establish that they evolved from several compositionally distinct parental magmas. In particular, basalts from site 236 in the Somali Basin have relatively high SiO2 and low Na, P, Ti, and Zr contents. These compositional features along with low normative olivine contents are similar to those proposed for melts derived by two-stage (or dynamic) melting. Published data also indicate there is no enrichment in incompatible elements at the southwest Indian Ocean triple junction, although southwest Indian Ocean basalts have slightly higher 87Sr/86Sr than “normal” Atlantic MORB. The data suggest that there are significant subtle geochemical variations in the Indian Ocean mantle sources, but are insufficient to show whether these variations have a systematic temporal or geographic distribution.

Geological and Geophysical Investigation of the Mid-Cayman Rise Spreading Center: Geochemical Variation and Petrogenesis of Basalt Glasses

Abstract: Basalt glasses erupted from the transverse spreading center within the Cayman Trough have higher contents of $TiO_{2}, Na_{2}O, K_{2}O, P_{2}O_{5}$, Zr, Y, Sr, and Ba than typical mid-ocean ridge basalts (MORB). These compositional features are unique and the glasses appear to be transitional between fractionated MORB and incompatible-element enriched basalts from certain oceanic regions such as the mid-Atlantic Ridge near 43°N, the triple junction near Bouvet, and Fracture Zone B at the FAMOUS (MAR 37°N) spreading center. The major element compositional variation defines an apparent cotectic, in the normative plagioclase-pyroxene-olivine triangle, which is systematically offset into the plagioclase field compared to most other MORB trends. Model fits based on the assumption of simple, single-stage fractional crystallization are very good for both major and trace elements, with no apparent "decoupling" of the incompatible elements as observed in the FAMOUS (37°N) spreading center. These calculations indic...

Crystal fractionation in a CUmulate Gabbro, Mid‐Atlantic Ridge, 26°N

Abstract: Gabbros dredged from the Mid-Atlantic Ridge near 26°N have distinct cumulate textures and display phase, rhythmic, and cryptic layering. The gabbros have a tholeiitic iron enrichment trend and a concomitant increase in the incompatible element abundances. The gabbroic phases have large variations in both the Fo content of olivine (64–82) and the An content of the plagioclase (50–80) and the smaller but significant variations in En content of clinopyroxene and orthopyroxene (42–46 and 68–74, respectively). Increasing Fo contents of olivine correlate with increasing iron enrichment and increasing modal abundance of cumulate pyroxene. These chemical variations and observed textures are consistent with a model involving crystal fractionation. Fractionation models linking crystallization of phases in the gabbros to chemical variations in nearby basalts are tested. A model involving clinopyroxene crystallization can, mathematically, produce the chemical variations found in any one dredge site, but none of the basalts contain clinopyroxene, and hence the model is rejected. The observed gabbroic fractionation cannot produce the magmatic source for more evolved basalts; therefore it is likely that basaltic magmas are not greatly influenced by shallow magma chamber processes either by crystal fractionation or by magma mixing with evolved gabbroic magmas during transport to the surface. The chemical variations observed between dredge sites cannot be successfully modeled through fractionation involving any combination of the phases observed in the basalts or the gabbros.

Lateral chemical heterogeneity in the Palaeocene upper mantle beneath the Scottish Hebrides

Abstract: The early major products of Tertiary volcanicity in both Skye and Mull are transitional basic lavas, similar in their major-element chemistry to world-wide alkali basalt series. In contrast, their contents of incompatible trace elements bear more resemblance to those of olivine tholeiites. The Mull basalts have similar ranges of silica saturation, Mg/(Mg+Fe), Y and Yb, but lower overall abundance ranges of strongly incompatible elements than the Skye basalts. The variation of incompatible elements in the Mull and Skye lavas is consistent with a model of a mantle source from which a small amount of melt (no more than 1 % ?) had been extracted, with the pre-Tertiary upper-mantle fusion beneath Mull slightly greater than beneath Skye. Chemical and tectonic considerations suggest that this mantle was neither residual from the formation of the Archaean Lewisian complex, nor emplaced as a result of tension associated with the Gainozoic rifting of the North Atlantic. Data on major and trace elements for a mafic alkalic dyke of the Permian swarms that pass through western Scotland show that these have the requisite geochemical characteristics to have caused this depletion. Such dykes are more abundant in the region of Mull than Skye.

The pliocene volcanism of the voras Mts (Central Macedonia, Greece)

Abstract: Major element chemistry, K/Ar ages and trace element data are reported for volcanic rocks from the Voras volcanic complex of Central Macedonia (Greece). Petrological data show that the Voras volcanic rocks consist essentially of intermediate members of the high-K calc-alkaline and shoshonitic series, the most abundant rock types being high-K andesites and dacites, latites and trachytes. K/Ar ages determined on selected samples show values ranging between 5.0 and 1.8 m.y., indicating that the Voras volcanism was active from Pliocene up to the Lower Pleistocene. The high Th, Hf, Ba and LREE shown by some representative samples together with their strongly fractionated light and heavy REE patterns and the absence of significant negative Eu anomalies indicate a magma genesis by low degrees of partial melting of a source enriched in large ion lithophile elements, leaving a garnet-bearing and plagioclase-free residue. This source is believed to be represented by a mantle garnet peridotite enriched in incompatible elements.

A two-reservoir recycling model for mantle-crust evolution

Abstract: The exact solutions for the isotopic compositions and the concentrations of the two-reservoir model for mantle-crust evolution are given for arbitrary rates of crustal growth and of back flow to the mantle. The critical parameters are the chemical fractionation factors for crustal growth and refluxing and the integrated fractional mass-removal rates from the crust and the mantle. For the case where refluxing is proportional to crustal growth, all the solutions reduce to simple analytic expressions. The expression for the mean age of the mass of the crust with refluxing is given. If refluxing is significant, the model shows that highly incompatible elements have short residence times in the mantle. With plausible concentration values, material balance implies that the continents were derived from only a small fraction of the mantle.

Chemical variation in metabasites from a proterozoic amphibolite-granulite transition zone, South Norway

Abstract: The metabasites were originally minor intrusions which are now characterised by wholly metamorphic textures and mineral assemblages diagnostic of an amphibolite-granulite facies transition. Two forms of chemical variation can be distinguished: (1) an igneous-type differentiation, involving tholeiitic iron enrichment accompanied by increases in some incompatible elements, and decreases in the transition trace elements; (2) metamorphic fractionation effects resulting in deficiencies in K, Rb, Sr, Ba, Zr, and enhancement in Na in the highest grade rocks. These distributions closely parallel those in the host acid-intermediate charnockitic gneisses. The data preclude the possibility that the deficiencies were caused by removal of melts from the granulites. The metamorphism closely followed intrusion, and it is unlikely that the fractionations were caused by secondary dehydration of once hydrous assemblages. The preferred model involves intrusion and crystallisation directly under high-grade conditions, possibly with CO2-rich fluids playing an important role in suppressing the formation of hydrous minerals.

Mantle Heterogeneity Beneath Southern Africa: Evidence from the Volcanic Record

Abstract: The scale, timing and development of mantle heterogeneity beneath southern Africa is assessed by reference to data obtained from mantle derived igneous rocks and, to a lesser extent, from peridotites contained in kimberlite. Sr isotope data for ultrabasic and basic igneous rocks ranging in age from 3.5 Ga komatiites to Tertiary olivine melilitites indicate that heterogeneity existed at 3 Ga and was well established by 2 Ga, and also suggest progressive development of variability in Sr isotope ratios in the mantle source regions involved. Detailed evaluation of Sr isotope and incompatible element and inter-element ratios, together with rare earth element patterns, of the widespread Jurassic Karroo volcanics shows that the overall compositional variability of these volcanics is best explained by (horizontal) mantle heterogeneity. Both depletion and enrichment pre-Karroo processes appear to have affected the parental mantle source regions. Evidence for such enrichment is provided by kimberlite peridotite nodules that have been subjected to mantle metasomatic processes, leading to the development of phlogopite and the amphibole potassic richterite, with consequent enrichment of incompatible elements.

Nonlinear Nature of the Unavoidable Long-Lived Isotopic, Trace and Major Element Contamination of a Developing Magma Chamber

Abstract: Contamination of magmas by crustal materials is a possible alternative explanation of many of the trace element and isotopic features of basalts which have been advanced as evidence of mantle heterogeneity. The space problems associated with the emplacement of large masses of plutonic cumulates in most central volcanoes, and with the formation of the gabbro-dunite portion of the oceanic crust, indicates that at least some digestion of pre-existing crustal rocks has occurred. Contamination is therefore unavoidable. In the ocean basins, contamination is not due directly to incorporation of sea water, but to the digestion of previously erupted basalts which may have been hydrothermally altered by circulating sea water, plus some sediment. In a magma chamber that is periodically replenished with parental magma and periodically tapped to provide lava flows during fractional crystallization, contamination produces changes in the composition of the erupted product which are not, in general, linear between the initial and final states of the system. Moreover, the change in composition produced need not lead towards the composition of the contaminant. Provided that the periodic additions and subtractions of magma are small relative to the size of the magma chamber, the effects of short-term variations in the amount of contamination are smoothed out, while the effects on incompatible element concentrations are persistent long after active contamination has ceased. The effects on major components that enter the crystallizing phases are constrained by the need for the remaining liquid to conform to the relevant low-pressure phase equilibria.

The origin of lavas from the Ninetyeast Ridge, eastern Indian Ocean: An evaluation of fractional crystallization models

Abstract: Ferrobasalts from DSDP sites 214 and 216 on the Ninetyeast Ridge are characterized by high absolute iron (FeO>12.9 wt %), FeO/MgO>1.9, and TiO/sub 2/>2.0 wt %. Their trace element abundances indicate a tholeiitic affinity; however, they are distinct from midocean ridge incompatible element-depleted tholeiites owing to higher contents of Ba, Zr, and Sr and flat to slightly light-REE-enriched, chondrite-normalized REE patterns. Calculations using major and trace element abundances and phase compositions are generally consistent with a model relating most major elements and phase compositions in site 214 and 216 ferrobasalts by fractionation of clinopyroxene and plagio-class. However, some incompatible element abundances for site 216 basalts are not consistent with the fractional crystallization models. Baslats from site 214 can be related to andesitic rocks from the same site by fractionating clinopyroxene, plagioclase and titanomagnetite. Site 254 basalts, at the southern end of the Ninetyeast Ridge, and island tholeiites in the southern Indian Ocean (Amsterdam-St. Paul or Kerguelen-Heard volcanic provinces) possibly represent the most recent activity associated with a hot spot forming the Ninetyeast Ridge. These incompatible-element-enriched tholeiites have major element compositions consistent with those expected for a parental liquid for the site 214 and 216 ferrobasalts. However, differences in the tracemore » element contents of the basalts from the Ninetyeast Ridge sites are not consistent with simple fractional crystallization derivation but require either a complex melting model or a heterogeneous mantle source.« less

Geochemical features of some Archaean and post-Archaean high-magnesian-low-alkali liquids

Abstract: High-magnesian-low-alkali liquids are found as mafic lavas ranging in age from Archaean to Cainozoic. The most magnesian lavas are represented by Archaean spinifex textured peridotitic komatiites, and in this study these liquids are used as a comparative base for younger, less magnesian liquids. The post-Archaean lavas fall into three categories: (1) the Cape Smith (Proterozoic) - Baffin Bay (Cainozoic) group, (2) the low-Ti ophiolitic basalts of Cyprus, which represent remelting of a sequentially depleted source, and (3) the boninite group, which are the products of (wet?) melting of a source that had previously experienced depletion and addition of incompatible element enriched phases. With the use of parameters such as Al$_{2}$O$_{3}$/TiO$_{2}$, Sc/Zr, Ti/V, a comparison of Archaean komatiites with the younger high magnesian lavas indicates that the bulk of the variation seen in these rocks types can be interpreted in terms of the amount of partial melting and nature of residual phases. However, some of the variability that occurs within individual lava provinces (particularly among the light rare earth elements) is best explained by a heterogeneity superimposed on a previously homogeneous source. The abundance of high-magnesian liquids declines sharply after the Archaean as does the maximum MgO content achieved by the lavas.

Geochemical evolution in the Blake River Group, Abitibi Greenstone Belt, Superior Province

Abstract: In well exposed, well developed greenstone belts of the Superior Province there is a clear progression from stratigraphically lower, geochemically primitive volcanic rock types (komatiites, tholeiites) to overlying geochemically evolved calc-alkaline volcanic rock types. In the western Blake River Group of the Abitibi Greenstone Belt the change from tholeiitic to calc-alkaline volcanics represents a geochemical discontinuity defined by an increase in incompatible elements and light/heavy rare-earth element fractionation in the overlying rocks. Quantitative modelling of the parameters of the discontinuity indicates that it can be explained by a change to very small amounts of melting of unmodified mantle lherzolite, although this is not a unique solution. In calc-alkaline suites showing high degrees of rare-earth element fractionation the calculated melt fraction required of unmodified mantle becomes unrealistically low and models involving a geochemically evolved source may have to be considered.

Trace Element Evidence for Mantle Heterogeneity Beneath the Scottish Midland Valley in the Carboniferous and Permian

Abstract: The alkaline rocks of Carboniferous to Permian age in the Midland Valley province range in composition from hypersthene-normative, transitional basalts to strongly undersaturated basanitic and nephelinitic varieties. They were formed by varying degrees of equilibrium partial melting of a phlogopite peridotite mantle. Ba, Ce, Nb, P, Sr and Zr were strongly partitioned into the liquid during melting; K and Rb were retained by residual phlogopite for small degrees of melting only. The composition of the mantle source is inferred to have been broadly similar to that from which oceanic alkaline basalts are currently being generated. It was, however, heterogeneous as regards distribution of the incompatible trace elements, with up to fourfold variations in elemental abundances and ratios. The mantle beneath the province may be divisible into several areas, of some hundreds of square kilometres each, which retained a characteristic incompatible element chemistry for up to 50 Ma and which imparted a distinctive chemistry to all the basic magmas generated within them.