Enrichment In LIL Elements Research Articles

The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presented and integrated with previous data for this low-K intra-oceanic arc suite. Geochemically, the arc tholeiites and basaltic andesites range from extremely HFS element depleted (northern Tonga) to near N-MORB-like HFS element abundances in L'Esperance (southern Kermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristic selective enrichment generally recognized in arc magmas, and thus indicate decoupling of the HFS and LIL elements. Modelling suggests a compositionally variable source (mantle wedge) along the arc, ranging from restite after remelting an N-type MORB source (northern end), to progressively less depleted, MORB-like sources southwards. Thus, the low HFS/LIL element ratios are interpreted in terms of HFS depletion followed by LIL element enrichment associated with subduction; broad correlations occur between Zr/Ba and Sr/Nd ratios (fractionation corrected) and 87Sr/86Sr and Nd/Nd ratios.Derivation of the arc magmas from depleted peridotote requires superimposed fractional crystallization, which has been modelled thermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentally produced partial melts from depleted lherzolite (Jaques & Green, 1980). It is shown that the arc tholeiites and basaltic andesites (and also high Mg-andesites) are potentially developed at low pressures ≤ 5 kb), from parental magmas also generated at relatively low pressure (≤ 10 kb). These data further suggest that a southward increasing depth of magma segregation (correlating with Benioff Zone geometry) could account for differences in chemistry between the Tonga and Kermadec arc segments.The mechanism of LILE enrichment is still highly problematic, but it is suggested that the model of Tatsumi et al. (1986) may account for much of the geochemical data; this involves relatively shallow release, via fluids, of LIL elements into overlying peridotite beneath the fore-arc region. Induced convection in the mantle wedge moves the metasomatized mantle into the zones of magma generation.The development of the inferred, variably depleted mantle wedge source is here related to active back-arc spreading which is slightly older and more rapid behind the northern region of the arc. The lherzolite restite from this spreading process is interpreted to undergo further partial melting in the metasomatized mantle wedge overlying the subduction zone, involving induced convection.The back-arc island of Niua fo'ou is geochemically quite distinct from the arc magmas, being similar to N-MORB in its trace elements, but to OIB in its isotope ratios.

Corundum, Cr-muscovite rocks at O'Briens, Zimbabwe: the conjunction of hydrothermal desilicification and LIL-element enrichment ? geochemical and isotopic evidence

Monomineralic domains of chlorite, corundum and Cr muscovite coexist over a kilometer scale within ultramafic schists of the Harare greenstone belt (2.73 Ga). This exotic lithological association includes the conjunction of some of the most aluminous (Al2O3∼88 wt%) and potassic (K2O∼10 wt%) rocks known. The paragenetic sequence developed from chlorite→corundum→corundum+ diaspore: Cr muscovite variably overprinted both the corundum and chloritite domains. Terminal stages were marked by sporadic production of andalusite+quartz, and finally margarite.

The Jabel Sayid complex, Arabian Shield: geochemical constraints on the origin of peralkaline and related granites

The Jabel Sayid Complex comprises two principal rock types which require distinct source regions. An earlier intrusion (572 ± 24 Ma) of biotite-hornblende monzogranite displays selective enrichment of LIL elements and depletion of HREE characterizing a calc-alkaline magma derived from a garnet-bearing source within a hydrated mantle wedge. The younger intrusion (534 ± 40 Ma) is an aegirine-arfvedsonite peralkaline granite which can be derived neither by fractionation of a calc-alkaline magma, nor by closed-system anatexis of a crustal source. This granite is enriched in HFS elements and strongly depleted in Sr, Ba and Eu, and requires the contribution of a fluorine-bearing fluid phase. This volatile phase has resulted in a range of metasomatic rock types including an aplitic rim which carries extreme levels of HFS elements and is a potential economic source of U, and a red granite characterized by widely ranging trace element abundances and strong Eu depletion. The peralkaline granite is typical of siliceous melts derived from an intra-plate tectonic environment, with trace elements strongly controlled by volatiles which are probably mantle-derived. The role of crustal contamination on such granites can neither be proved nor excluded.

Lower Palaeozoic convergent plate margin volcanism on Bømlo, southwest Norwegian Caledonides: geochemistry and petrogenesis

AbstractMajor and trace element analyses of a Lower Palaeozoic metavolcanic sequence of convergent plate type from Bømlo, southwest Norwegian Caledonides, are presented and discussed. This sequence ranges in age from the Upper Cambrian through the Lower Silurian. Petrogenetic models for the lavas in terms of partial melting and crystal fractionation are discussed. Two models are presented for the metabasalts in order to explain their different trace element abundances and ratios:(1) REE modelling, assuming a mantle source with REE abundances twice chondritic, suggests progressively more varied sources with time. Thus the metabasalts from the oldest (Upper Cambrian–Lower Ordovician) Geitung Unit of primitive island arc type, and those of the mid-Ordovician Siggjo Complex of ‘Basin and Range’ type can be modelled in terms of high (around 25%) and moderate (around 5%) degrees of partial melting of spinel lherzolite, respectively. The metabasalts of the post-Ashgillian Vikafjord Group of typical continental flood basalts are compatible with moderate (c. 5–10%) degrees of partial melting of spinel- and garnet-lherzolite sources. The supposed Lower Silurian Langevåg Group of calc-alkaline ‘Andean’ type metabasalts, grading into alkaline to tholeiitic metabasalts of early marginal basin (youngest) character, require low (<5%) to moderate degrees of partial melting of amphibole-, garnet- and spinel-lherzolite sources, respectively.(2) Source heterogeneity, produced by subduction zone-derived enrichment of LIL elements, and contemporaneous stabilization of minor phases which accommodate HFS elements. This process, combined with possible continental contamination, may possibly yield the trace element concentrations and ratios of the different metabasalts by partial melting of modally similar mantle sources.

Evolution of the late Archaean lower continental crust in southern West Greenland

Summary Accretion of new continental crust onto the base of pre-existing sialic crust is indicated by field, geochemical and isotopic data from two suites of Archaean gneisses from the Inner Godthåbsfjord region of southern West Greenland. Crustal accretion combined with horizontal tectonic interleaving of existing rock suites thickened the crust resulting in high-grade metamorphism. Enrichment in LIL elements resulted from the depletion in these elements of granulite-facies gneisses below the present level of erosion. Related fluid movements were significant in leading to the anatexis of rocks enriched in LIL elements. The present-day distribution of rock suites is at least in part a function of the juxtaposition of different levels of the late Archaean crust.

Chemical and isotopic effects of late Archaean high-grade metamorphism and granite injection on early Archaean gneisses, Saglek-Hebron, northern Labrador

Summary Early Archaean (3700–3800 Ma) layered tonalitic to granodioritic Uivak gneisses form approximately 60–70% of the Archaean gneiss complex between Saglek and Hebron fiords, northern Labrador. Effects of late Archaean metamorphic and igneous activity can be recognized on the geochemistry and isotope chemistry of the Uivak gneisses: depletion in rocks affected by 2900 Ma granulite facies metamorphism, with a loss of K, Rb and redistribution of REE and a corresponding enrichment of LIL elements in the amphibolite facies area. LIL enrichment is at least in part by addition from intrusive late Archaean granitoid sheets. The depleted granulites (K/Rb 500–2000) show a scatter of data points on a Rb/Sr- 87 Sr/ 86 Sr diagram to the left of a 3560 Ma reference isochron (ISr 0.700) consistent with marked Rb loss and some homogenization of Sr. Amphibolite facies gneisses from a broad transitional zone into granulite facies have K/Rb values of 200–300 and plot on a c. 3560 Ma isochron with a high ISr (0.7038) consistent with a slight loss of Rb at 2900 Ma but without marked Sr homogenization. Amphibolite facies gneisses away from the transition zone show marked enrichment in LIL elements with unusually low K/Rb ratios (100–200). Rb/Sr data plot close to a 3560 Ma reference isochron, ɛSr 3560 values are strongly negative, interpreted as due to Rb addition at 2900 Ma rather than derivation from an unusually depleted mantle source. Pb-Pb isotope data from the depleted granulite show a cluster of unradiogenic data points. Samples from a monolith of gneisses in a transitional zone between amphibolite and granulite facies yields a 3808 Ma isochron (μ 1 8.5) suggesting there was locally little disturbance of the Pb/U system at 2900 Ma. Pb-Pb data from the Rb-enriched amphibolite facies area show a markedly higher proportion of radiogenic Pb compared to the granulite facies rocks which cannot be explained by U movement alone. Local galena mineralization suggests that Pb as well as U was mobile during the late Archaean and we interpret the results as mixtures of > 3600 crustal Pb with younger material. Sm-Nd data points (Collerson & McCulloch 1982) show a scatter about a 3665 Ma isochron with an average ɛNd 3665 of +2.48. Marked LIL + LREE enrichment is suggested and we interpret the scatter of ɛNd 3665 values as being due to late movement of LREE. The late Archaean granitoid sheets have a chemistry and petrology suggesting derivation largely by partial melting of earlier sial with modifications by fluid redistribution of alkalis. New minerals such as allanite and apatite grow in older rocks adjacent to young veins. Sr, Nd and Pb data all suggest mixing between early Archaean crust and a component with mantle-like isotopic character at c. 2900 Ma. The mixing process was complex and took place by ingestion of older rocks in younger granite, mixing of partial melts of different origin and fluid transport of components from younger intrusions into older gneisses.

Geochemical characteristics of basaltic volcanism within back-arc basins

Summary Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB and basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.

Cenozoic volcanic rocks of eastern China — secular and geographic trends in chemistry and strontium isotopic composition

The Cenozoic volcanic rocks of eastern China are subalkalic to alkalic basalts erupted in an early Tertiary back-arc rift environment and from scattered late Tertiary and Quaternary volcanic centers in a continental area crossed by active faults, driven by subduction of the Pacific plate and the collision of India and Eurasia. Immobile trace elements and major elements conform very well to each other in classification of the 59 rocks for which complete data are reported and they correctly identify the tectonic setting. LIL-element enrichments of the basalts lie between those of P-MORB and ocean island alkalic basalts, and show a secular increase. 87Sr/ 86Sr ratios of basalts vary from 0.7029 to 0.7048. Alkalic basalts are systematically less radiogenic than geographically coextensive and contemporaneous tholeiitic basalts. Increase of radiogenic Sr with increasing crustal thickness and crustal age and with silica enrichment of the magmas suggests crustal contamination but this is inadequate to explain the LIL-element enrichment patterns and variable LIL-element enrichments. The preferred hypothesis is that the alkalic magmas come from a deeper source, with long-term LIL-element depletion and low Rb/Sr ratio but relatively recent LIL-element enrichment. Conversely the tholeiitic magmas are melts of subcontinental mantle lithosphere that is more LIL-element depleted than the alkalic source, at the time of magma genesis, but has had an elevated Rb/Sr ratio for much of its post-consolidation history.

Basalts from DSDP Leg 37 and the FAMOUS area: compositional and petrogenetic comparisons

Basalts from the DSDP Leg 37 sites can reasonably be supposed to have originated at or near the present-day spreading center (FAMOUS area) investigated and sampled in detail by surface vessels and submersible vehicles in the median valley. Nevertheless, the Leg 37 samples show a more restricted range of compositions than those presently observed in the median valley. Basalt glasses from DSDP Leg 37 contain 0.6 to 1.3 wt.% TiO2, with FeO*/MgO ranging between 0.8 to 1.5. In contrast, basalt glasses from the median valley range from 0.5 to 2.0 wt.% TiO2, and FeO*/MgO varies from 0.7 to 1.8.The low-TiO2 basalts at site 332B can be compared to similar very young basalts that characterize the central volcanic extrusive hills such as Mt. Pluto and Mt. Venus in the median valley. The more iron-enriched basalts can be compared with certain FAMOUS basalts dredged from the rift mountains south of fracture zone A. The major- and minor-element variation trends among Leg 37 basalt glasses parallel trends within the FAMOUS data set, which are compatible with high level fractional crystallization. Overall, the best parent for site 332 basalts is a 'low-olivine' candidate parent magma type, which has been found in fracture zone B and in the median valley south of fracture zone B, and which resembles basalt at site 334. Compared to some parent–residual pairs tested in the FAMOUS area, the fractionation calculations for site 332 variations show good fits for TiO2 and K2O, and show little excess LIL-element enrichment. In all calculations, pyroxene dominates olivine as the most important mafic phase. This is consistent with the observed slight relative enrichment in light rare-earth elements in the 332A glasses compared to their inferred parents, but cannot be reconciled with light rare-earth depletion at site 335.A possible interpretation of the lava sequence at site 332B is that it represents a rapid accumulation of basalts extruded over a relatively short time span in the center of the median valley. Fractionation may have taken place in a stagnating chamber isolated by collapse of the valley walls. In this chamber there was not sufficient time or space to develop the thermal gradients and marginal enrichment in LIL-elements and volatile components which may have generated the high TiO2 basalts in the FAMOUS area. Geochemically unique basalt at site 335 shows depleted light rare earths combined with LIL-element and TiO2 enrichment. This basalt could have been a flank eruption unrelated to processes in the median valley.

Oldest Atlantic seafloor

Basalt recovered beneath Jurassic sediments in the western Atlantic at Deep Sea Drilling Project sites 100 and 105 of leg 11 has petrographic features characteristic of water-quenched basalt extruded along modern ocean ridges. Site 100 basalt appears to represent two or three massive cooling units, and an extrusive emplacement is probable. Site 105 basalt is less altered and appears to be a compositionally homogeneous pillow lava sequence related to a single eruptive episode. Although the leg 11 basalts are much more closely related in time to the Triassic lavas and intrusives of eastern continental North America, their geochemical features are closely comparable to those of modern Mid-Atlantic Ridge basalts unrelated to postulated “mantle plume” activity. Projection of leg 11 sites back along accepted spreading “flow lines” to their presumed points of origin shows that these origins are also outside the influence of modern “plume” activity. Thus, these oldest Atlantic seafloor basalts provide no information on the time of initiation of these “plumes”. The Triassic continental diabases show north to south compositional variations in Rb, Ba, La, and Sr which lie within the range of “plume”-related basalt on the Mid-Atlantic Ridge (20 °–40 °N). This suggests that these diabases had mantle sources similar in composition to those beneath the present Mid-Atlantic Ridge. “Plumes” related to deep mantle sources may have contributed to the LIL-element enrichment in the Triassic diabase and may alos have been instrumental in initiating the rifting of the North Atlantic. Systematically high values for K and Sr87/Sr86 in the Triassic diabases may reflect superimposed effects of crustal contamination in the Triassic magmas.