Negative High Field Strength Element Research Articles

The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach

The present-day tectonic setting of the Tyrrhenian Sea is dominated by the eastward migration of the Tyrrhenian–Appenines subduction system and the existence of a contemporaneous and parallel extensional–compressional regime. This complex setting leads to the occurrence of a wide spectrum of magma-types in the South Italy volcanoes. Here, major and trace-element data for primitive melt inclusions preserved in olivine phenocrysts have been obtained in order to add constraints on the origin of the calc-alkaline magmas from the Aeolian arc (Stromboli and Vulcano islands), the potassic magmas from the Campania Province (Vesuvius and Phlegraean Fields) and the Na-alkaline magmas from Ustica Island. The approach used to determine the possible mantle sources of the trapped melts for each population of melt inclusions is based on the determination of the trace-element incompatibility sequence taken as the relative order of increasing bulk partition coefficients, which depends on the mineralogy of the source and gives direct information about minerals residual at the time of melting. Compositional similarities between the melt inclusions and their host lavas suggest that shallow-level magma contamination did not contribute significantly to the geochemical characteristics of the magma-types encountered in the region. Results of the trace-element modelling indicate that the melt inclusions from the Aeolian Islands and Campania Province volcanoes originate from mantle sources strongly affected by subduction-related metasomatic processes. Trace-element relationships of melt inclusions from Vulcano and Stromboli reflect melting of peridotitic sources that have been enriched by a slab-derived, aqueous fluid formed during dehydratation of K-free phases at shallow to intermediate depths. The negative high-field strength elements (HFSE) anomalies of these inclusions were generated in the absence of any residual phase in which HFSE might be compatible. In addition, their major element characteristics require the involvement of a clinopyroxene-rich source component in their genesis. With regard to the Vesuvius and Phlegraean Fields melt inclusions, their calculated incompatibility sequences point to a common phlogopite-bearing mantle source likely to be the result of interaction and hybridisation reactions with K2O and H2O-rich phases released from the slab at larger depths. Finally, trace-element characteristics of melt inclusions from Ustica Island imply the derivation from an upwelling enriched intraplate mantle source with a HIMU-type signature reflecting an ancient basaltic source component. Moreover, the source seems to be contaminated by a slab-derived fluid due to the rollback motion of the Ionian slab, in a process similar to that operating beneath Mount Etna, South of Ustica.

Mechanisms and Sources of Mantle Metasomatism: Major and Trace Element Compositions of Peridotite Xenoliths from Spitsbergen in the Context of Numerical Modelling

Mineral and whole-rock chemical data for peridotite xenoliths in basaltic lavas on Spitsbergen are examined to reassess mechanisms of melt-fluid interaction with peridotites and their relative role versus melt composition in mantle metasomatism. The enrichment patterns in the xenoliths on primitive mantle-normalized diagrams range from Th-La-Ce 'inflections' in weakly metasomatized samples (normally without amphibole) to a continuous increase in abundances from Ho to Ce typical for amphibole-bearing xenoliths. Numerical modelling of interaction between depleted peridotites and enriched melts indicates that these patterns do not result from simple mixing of the two end-members but can be explained by chromatographic fractionation during reactive porous melt flow, which produces a variety of enrichment patterns in a single event. Many metasomatized xenoliths have negative high field strength element and Pb anomalies and Sr spikes relative to rare earth elements of similar compatibility, and highly fractionated Nb/Ta and Zr/Hf. Although amphibole precipitation can produce Nb-Ta anomalies, some of these features cannot be attributed to percolation-related fractionation alone and have to be a signature of the initial melt (possibly carbonate rich). In general, chemical and mineralogical fingerprints of a metasomatic medium are strongest near its source (e.g. a vein) whereas element patterns farther in the metasomatic 'column' are increasingly controlled by fractionation mechanisms.

What Is the "Calc-alkaline Rock Series"?

The "calc-alkaline" series of rocks was originally defined in the early 1930s on the basis of the "alkali-lime index" on a combined SiO2 versus (Na2O + K2O) and SiO2 versus CaO plot. The usage of the term has evolved considerably since, and today it is used variably for the subalkalic basaltandesite-dacite-rhyolite suite, or any rock suite containing andesite, or island-arc rocks, or rocks with high ratios of large-ion-lithophile elements (LILE) to high-field-strength elements (HFSE), or simply rocks with negative HFSE anomalies (e.g., Nb-Ta) in primitive mantle-normalized multielement diagrams. Although such variable usage is normal in science, the use of certain geochemical variation diagrams to define and depict the calc-alkaline series is not strictly appropriate. Two of these widely used diagrams are the total alkalies-silica (TAS) diagram and the (Na2O + K2O)-FeO*-MgO (AFM) triangular diagram, neither of which has calcium as one of the plotting parameters. The TAS diagram can be used to depict "alkaline," "subalkaline," and probably "transitional" rocks, but not "calc-alkaline" or "high-alumina" rocks. Care must be taken while using such diagrams for rock classification, inasmuch as some of them are inherently unsuitable and several together may classify even a single rock into widely different associations. "Association" or "suite" are probably better terms than "series," because they imply neither comagmatic relationships nor linear trends. The calc-alkaline suite of rocks is abundant along destructive plate margins, but calc-alkaline geochemistry is not a 100% foolproof indicator of subduction processes, inasmuch as calc-alkaline rocks are also known from regions undergoing extension, such as the Basin and Range province and the Gulf of California. Therefore, caution must be exercised in interpreting ancient terrains with complicated geology and calc-alkaline rocks as former subduction zones. Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic.

A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet

On the South Karakorum margin, Neogene high-temperature–medium-pressure (HT–MP) gneisses define an east–west trending thermal anomaly. These rocks have been heated from 600 to 750°C during a slight pressure drop from 0.7 to 0.5 GPa. Their retrogressive path cross-cuts the relaxed geotherm of tectonically thickened crust. Such a P–T evolution occurs only if an advective source of heat is involved. Involvement of an advective heat source is also implied by the occurrence of Neogene granitoids and lamprophyres within the HT–MP gneiss area. These rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies. We interpret these geochemical characteristics to be the result of melting of metasomatized Asian lithospheric mantle. The Nd and Sr isotopic compositions of the South Karakorum Neogene magmatic rocks ( ϵ Nd=−12 to −7 and 87Sr/ 86Sr=0.705–0.725) further suggest they could have originated from mixing between Asian variously metasomatized mantle and Precambrian crust. By contrast, the origin of the youngest magmatic rocks (<10 Myr), here exemplified by the Hemasil syenite and associated lamprophyres, requires involvement of a depleted mantle. The combined ϵ Hf– ϵ Nd signature of these rocks ( ϵ Hf=+10.4–+11.5 and ϵ Nd=+3.4–+4.3) suggests that the source of the Hemasil syenite could have been depleted mantle contaminated by oceanic sediments, likely during the earlier subduction of the Tethyan ocean. Neogene magmatic rocks with the same geochemical characteristics and evolution as those of South Karakorum have previously been described in South Tibet. Based on their location and the geochemical evolution of their source region, we here propose that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab breakoff of the subducting Indian continental margin starting at about 25 Ma. This model is supported by available geophysical data from South Karakorum and South Tibet.

Hf-Nd Element and Isotope Perspective on the Nature and Provenance of Mantle and Subduction Components in Western Pacific Arc-Basin Systems

This paper develops methods for using the integrated study of Hf-Nd element and isotope covariations to define the nature and provenance of the mantle and subduction inputs to subduction systems. In particular, it can be demonstrated that (1) Hf-Nd isotope space permits discrimination between mantle of Pacific and Indian provenance, (2) displacements from mantle arrays on Hf-Nd isotope and trace element projections can be related to the magnitude, source and composition of the subduction input, and (3) Hf-Nd isotope and trace element covariations can be used to interpret high field strength element (HFSE) anomalies [specifically, Hf anomalies on extended rare earth element (REE) patterns] in subduction-related magmas. These methods are tested using published volcanic arc data coupled with new data from the many components of the Izu-Bonin-Mariana (IBM) subduction system, namely the pre-subduction marginal basins, the Eocene to Recent volcanic arcs, and the crust, volcanogenic sediments and pelagic sediments of the subducting Pacific plate. The results of the IBM study show that the mantle that fed the IBM system was always of Indian provenance and that Pacific volcanogenic sediments make the most significant, though variable, contribution to the subduction component. Modelling demonstrates that the Nd/Hf ratio of the subduction component probably lay between 40 and infinity and thus was probably the main cause of the negative HFSE anomalies that characterize much of the Recent arc. This result may further indicate that the subducting sediment lost elements to the mantle wedge mostly by dehydration rather than fusion. In contrast, the data also show that the positive Hf anomalies that characterize much of the Protoarc cannot be attributed directly to subduction. One option consistent with Hf-Nd systematics is that the positive Hf anomalies in the Protoarc boninites were caused by fusion of mafic veins in their shallow mantle sources. Comparison with published data on other arcs shows significant inter-arc variations. For example, the subduction components in near-continent arcs (Banda, Lesser Antilles) appear to have lower Nd/Hf ratios more consistent with sediment fusion, and at least one arc (Tonga-Fiji) carries evidence of temporal variations in mantle provenance.

Experimental determination of partition coefficients for rare earth and high-field-strength elements between clinopyroxene, garnet, and basaltic melt at high pressures

Clinopyroxene/melt and garnet/melt partition coefficients have been determined for Ti, Sr, Y, Zr, Nb, Hf, and rare earth elements from 19 doped experiments on 1921 Kilauea basalt. The experiments were carried out from 2.0 to 3.0 GPa and 1310° to 1470 °C. The purpose was to derive a set of partition coefficients for high-field-strength elements (HFSE) and rare earth elements (REE) in a systematic, linked set of experiments at P and T conditions relevant to basalt petrogenesis. These data are used in melting models to understand the development of negative HFSE anomalies observed in many abyssal peridotite clinopyroxenes. It is shown that melting can account for the observed trace element patterns in some residual peridotites, but that other processes may also be needed to account for most residual mantle compositions in mid-ocean ridge systems. It is also shown that REE are more strongly fractionated by garnet at these P-T conditions than previously thought.

Mineral-aqueous fluid partitioning of trace elements at 900–1200°C and 3.0–5.7 GPa: new experimental data for garnet, clinopyroxene, and rutile, and implications for mantle metasomatism

In order to constrain the role of fluid phases during metasomatic processes in the upper mantle, trace element partition coefficients for Ba, Sr, Pb, Nb, Ta, Zr, Hf, Ti, La, Ce, Sm, Tb, and Yb between aqueous fluids and eclogite assemblage minerals (garnet, clinopyroxene, and rutile) have been determined experimentally at 900–1200°C and 3.0–5.7 GPa. Using a new experimental technique in which diamond aggregates are added to the experimental capsule set-up, the fluid was separated from the solid residue so that both quenched solute and residual minerals could be analysed directly. Trace element concentrations were determined in situ by laser ablation microprobe (LAM). The partitioning behaviour is controlled by temperature, pressure, and crystal chemistry; whereas fluid composition is not as crucial. Neither addition of hydrochloric acid nor high silica concentrations in the fluid have strong effects on trace element partitioning. Results indicate that in the presence of garnet or clinopyroxene, Nb and Ta are highly soluble in aqueous fluids, whereas Zr and Hf show variable solubilities. Low field strength elements (LFSE) and light rare earth elements (LREE) are always enriched in the fluid (D (fluid/Min) > 1). Generally, D (fluid/Cpx) is positively correlated with temperature only for high field strength elements (HFSE), but positively correlated with pressure for all other elements. Therefore, the lowest Nb/La is achieved at high pressures and low temperatures. However, even the highest pressures and lowest temperatures examined did not exhibit strong negative HFSE anomalies in the fluid. Garnet retains compatible trace elements at 3 GPa and 1000°C much more effectively (D (fluid/gt)Yb= 0.002) than at 5.7 GPa at the same temperature (D (fluid/gt) Yb = 0.04). Decreasing temperature results in a lowered D (fluid/gt) , particularly for Zr, Hf, and heavy rare earth elements (HREE). At 5 GPa and 900°C a strong intra-REE fractionation is observed (D (fluid/gt)Sm/Yb around 100) and significantly negative anomalies for Hf and Zr, but not for Nb and Ta, are developed. Only residual rutile fractionates all HFSE from all other trace elements. Tantalum and niobium are retained most effectively by rutile, as is the case for rutile/melt partitioning. Fluid/mineral trace element partitioning has important implications for mantle metasomatism in subarc regions. A model is proposed in which HFSE depletions, as observed in island arc volcanic rocks, could originate from a selective enrichment of the mantle wedge in LFSE and LREE by aqueous fluids derived from a rutile-bearing subducted slab. It is shown that melting of the enriched mantle wedge, which had previously been depleted by melt extraction (depleted MORB mantle) can produce magmas with trace element patterns similar to those of subduction-related volcanic rocks.