Mantle Enrichment Processes Research Articles

The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region

Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.

Mantle enrichment processes

A review of Sr- and Nd-isotope and selected trace element data in both within-plate basalts and mantle xenoliths suggests that at least two enrichment processes are active within the upper mantle. One is consistent with the migration of small volume silicate melts with ‘basanitic’ trace element distribution patterns, whereas the other is characterized by relatively high eSr, Rb/Sr and K/Ti ratios, as observed in K-richterite-bearing periodotite xenoliths.

Relative contribution of crust and mantle to flood basalt magmatism, Mahabaleshwar area, Deccan Traps

The 1200 m section of flat-lying basalts in the Mahabaleshwar area is divided into three formations on the basis of the trace elements Sr, Ba, Rb, Zr and Nb. The lowermost unit, the Poladpur Formation, is characterized by high Ba, Rb, and Zr/Nb, and low Sr. These features are accompanied by high K and Si, high and variable 87 Sr/ 86 Sr initial ratios (0.7043 - 0.7196), and low and variable e N d values (+ 2.6 to -17.4). The formation is interpreted as having developed by contamination of the overlying Ambenali magma-type with ancient granitic crust, with simultaneous fractionation of a gabbroic mineral assemblage. The more basic members of the formation are found towards the base of the succession and are more contaminated than the upper flows. The succeeding Ambenali Formation, characterized by the Ambenali magma type, has low Ba, Rb, Sr and Zr/Nb, and low and rather uniform 87 Sr/ 86 Sr initial ratios (0.7038-0.7043) coupled with high and relatively uniform e N d (+4.7 to +6.4). It is interpreted as being essentially uncontaminated and derived from a mantle source with a history of slight trace-element enrichment relative to m.o.r.b.-source. The uppermost group of flows, the Mahabaleshwar Formation, is, like the Poladpur, enriched in Ba, Rb, K and Si relative to the Ambenali, but has lower Zr/Nb and higher Sr. 87 Sr/ 86 Sr initial ratios (0.7040-0.7056) are slightly higher than in the Ambenali, and e N d lies in the range +7.1 to -3.0. In this formation Sr correlates positively with the other incompatible elements and with 87 Sr/ 86 Sr initial ratios. This is in strong contrast to the relations observed in the Poladpur, and we believe that the behaviour of Sr may be a simple pointer to the distinction between mantle and crustal contributions. Assuming that late-stage crystal fractionation processes can be allowed for, if Sr correlates positively with elements such as K, Rb and Ba then mantle enrichment processes are clearly implied. Conversely, as for example in the Poladpur, if the correlation is negative, crustal contamination is suspected because Sr is unlikely to behave as an incompatible element in most crustal derived melts or fluids because of buffering by residual plagioclase. Furthermore, the relative uniformity of the Mahabaleshwar Formation, the position on the Sr and Nd isotope diagram close to the ‘mantle array’, the fact that in terms of both incompatible element concentrations and isotopes the rocks are similar to tholeiites from oceanic islands such as Hawaii and Kerguelen, are all factors that reinforce the conclusion that these are mantle derived magmas which have suffered insignificant crustal contamination. They are, however, derived from a mantle which is trace-element enriched relative to the Ambenali source. Thus in the succession as a whole the crustal contribution appears to be small. Maximum amounts of contamination in the Poladpur Formation are difficult to determine but the average amount is probably in the region of 6-12 percentage mass. The whole sequence therefore contains a crustal contribution of about 2-3%.

The role of apatite in mantle enrichment processes and in the petrogenesis of some alkali basalt suites

Analyses of Sr and REE in apatites from a variety of mantle-derived parageneses are used in conjunction with trace element data from the literature to investigate relationships between alkali basalts and apatite-rich materials in upper-mantle source regions. Despite difficulties in interpretation, positive P-anomalies in the hygromagmatophile element abundance patterns of some continental primary alkali basalts suggest either P-enrichment of their source or assimilation of P-rich material, or both. Amphibole- and apatite-rich xenoliths occur in several alkali-basalt provinces, and by virtue of the P and LREE enrichment represent a probable source of the P anomalies and part of the other trace element enrichments of these magmas. Incorporation of such apatite-rich materials by later primary magmas would be enhanced by the high P 2O 5 concentrations required to achieve apatite saturation in basaltic liquids. In the early stages of mantle diapirism an undersaturated magma, produced by slight partial melting of garnet peridotite, might fractionate as it rises to the range of amphibole stability. Hygromagmatophile element patterns of clinopyroxenite xenoliths indicate that clinopyroxene fractionation could produce P-enriched liquids which might subsequently crystallize amphibole- and apatite-rich materials now represented by xenoliths. During generation of later primary magma, apatite-rich materials might preferentially contaminate the liquids, to yield positive P-anomalies. This model requires that magmas undergo prolonged fractionation at considerable depth (~ 100 km), a process which is apparently most probable in subcontinental environments. An apatite- and zircon-bearing mica-clinopyroxenite xenolith from Matsoku provides a link between the S. African MARID suite and amphibole and apatite-rich xenoliths from various alkali basalt provinces. Unusual REE patterns ( La N < Ce N < Nd N , Ce N/Y N −10 ) of apatites in this xenolith suggest a link between the MARID suite xenoliths and postulated pre-Karroo mantle metasomatism.