Sr Isotopic Determination Research Articles

Assessment of Nanosecond Laser Ablation Multi‐Collector Inductively Coupled Plasma‐Mass Spectrometry for Pb and Sr Isotopic Determination in Archaeological Glass: Mass Bias Correction Strategies and Results for Corning Glass Reference Materials

This work presents an evaluation of various methods for in situ high‐precision Sr and Pb isotopic determination in archaeological glass (containing 100–500 μg g−1 target element) by nanosecond laser ablation multi‐collector‐inductively coupled plasma‐mass spectrometry (ns‐LA‐MC‐ICP‐MS). A set of four soda‐lime silicate glasses, Corning A–D, mimicking the composition of archaeological glass and produced by the Corning Museum of Glass (Corning, New York, USA), were investigated as candidates for matrix‐matched reference materials for use in the analysis of archaeological glass. Common geological reference materials with known isotopic compositions (USGS basalt glasses BHVO‐2G, GSE‐1G and NKT‐1G, soda‐lime silicate glass NIST SRM 610 and several archaeological glass samples with known Sr isotopic composition) were used to evaluate the ns‐LA‐MC‐ICP‐MS analytical procedures. When available, ns‐LA‐MC‐ICP‐MS results for the Corning glasses are reported. These were found to be in good agreement with results obtained via pneumatic nebulisation (pn) MC‐ICP‐MS after digestion of the glass matrix and target element isolation. The presence of potential spectral interference from doubly charged rare earth element (REE) ions affecting Sr isotopic determination was investigated by admixing Er and Yb aerosols by means of pneumatic nebulisation into the gas flow from the laser ablation system. It was shown that doubly charged REE ions affect the Sr isotope ratios, but that this could be circumvented by operating the instrument at higher mass resolution. Multiple strategies to correct for instrumental mass discrimination in ns‐LA‐MC‐ICP‐MS and the effects of relevant interferences were evaluated. Application of common glass reference materials with basaltic matrices for correction of ns‐LA‐MC‐ICP‐MS isotope data of archaeological glasses results in inaccurate Pb isotope ratios, rendering application of matrix‐matched reference materials indispensable. Correction for instrumental mass discrimination using the exponential law, with the application of Tl as an internal isotopic standard element introduced by pneumatic nebulisation and Corning D as bracketing isotopic calibrator, provided the most accurate results for Pb isotope ratio measurements in archaeological glass. Mass bias correction relying on the power law, combined with intra‐element internal correction, assuming a constant 88Sr/86Sr ratio, yielded the most accurate results for 87Sr/86Sr determination in archaeological glasses

Accurate Determination of Sr Isotopic Compositions in Clinopyroxene and Silicate Glasses by LA‐MC‐ICP‐MS

The low‐Sr content (generally < 100 μg g−1) in clinopyroxene from peridotite makes accurate Sr isotopic determination by LA‐MC‐ICP‐MS a challenge. The effects of adding N2 to the sample gas and using a guard electrode (GE) on instrumental sensitivity for Sr isotopic determination by LA‐MC‐ICP‐MS were investigated. Results revealed no significant sensitivity enhancement of Sr by adding N2 to the ICP. Although using a GE led to a two‐fold sensitivity enhancement, it significantly increased the yield of polyatomic ion interferences of Ca‐related ions and TiAr+ on Sr isotopes. Applying the method established in this work, 87Sr/86Sr ratios (Rb/Sr < 0.14) of natural clinopyroxene from mantle and silicate glasses were accurately measured with similar measurement repeatability (0.0009–0.00006, 2SE) to previous studies but using a smaller spot size of 120 μm and low‐to‐moderate Sr content (30–518 μg g−1). The measurement reproducibility was 0.0004 (2s, n = 33) for a sample with 100 μg g−1 Sr. Destruction of the crystal structure by sample fusion showed no effect on Sr isotopic determination. Synthesised glasses with major element compositions similar to natural clinopyroxene have the potential to be adopted as reference materials for Sr isotopic determination by LA‐MC‐ICP‐MS.

Crustal contamination and crystal entrapment during polybaric magma evolution at Mt. Somma–Vesuvius volcano, Italy: Geochemical and Sr isotope evidence

New major and trace element analyses and Sr-isotope determinations of rocks from Mt. Somma–Vesuvius volcano produced from 25 ky BP to 1944 AD are part of an extensive database documenting the geochemical evolution of this classic region. Volcanic rocks include silica undersaturated, potassic and ultrapotassic lavas and tephras characterized by variable mineralogy and different crystal abundance, as well as by wide ranges of trace element contents and a wide span of initial Sr-isotopic compositions. Both the degree of undersaturation in silica and the crystal content increase through time, being higher in rocks produced after the eruption at 472 AD (Pollena eruption). Compositional variations have been generally thought to reflect contributions from diverse types of mantle and crust. Magma mixing is commonly invoked as a fundamental process affecting the magmas, in addition to crystal fractionation. Our assessment of geochemical and Sr-isotopic data indicates that compositional variability also reflects the influence of crustal contamination during magma evolution during upward migration to shallow crustal levels and/or by entrapment of crystal mush generated during previous magma storage in the crust. Using a variant of the assimilation fractional crystallization model (Energy Conservation–Assimilation Fractional Crystallization; [Spera and Bohrson, 2001. Energy-constrained open-system magmatic processes I: General model and energy-constrained assimilation and fractional crystallization (EC–AFC) formulation. J. Petrol. 999–1018]; [Bohrson, W.A. and Spera, F.J., 2001. Energy-constrained open-system magmatic process II: application of energy-constrained assimilation–fractional crystallization (EC–AFC) model to magmatic systems. J. Petrol. 1019–1041]) we estimated the contributions from the crust and suggest that contamination by carbonate rocks that underlie the volcano (2 km down to 9–10 km) is a fundamental process controlling magma compositions at Mt. Somma–Vesuvius in the last 8 ky BP. Contamination in the mid- to upper crust occurred repeatedly, after the magma chamber waxed with influx of new mantle- and crustal-derived magmas and fluids, and waned as a result of magma withdrawal and production of large and energetic plinian and subplinian eruptions.

A comparison of the geochronology and geochemistry of plagioclase-dominated granitoids across a major terrane boundary in the SW Balitic Shield

To make reconstructions of the Proterozoic palaeogeography meaningful, the terranes making up the individual shields must first be identified and the timing of terrane accretion determined. New chemical and isotopic data confirm the terrane-separating character of a major shear zone, the Mylonite Zone, in the Southwestern Scandinavian Gneiss Complex. These data relate to plagioclase-dominated granitoids to the east of the Mylonite Zone. Two zircon upper-intercept age determinations result in 1674 +24 −19 and 1688 +10 −10 Ma. Two separate crystals from the 1688-Ma rock give two single-zircon evaporation ages: one crystal gives 1646 ± 172 Ma (2σ) and the other 1654 ± 46 Ma (2s). Earlier investigations have pointed to vast amounts of similar looking granitoids to the west of the Mylonite Zone. However, these rocks are younger, approximately 1.60 Ga. The granitoids to the west of the Mylonite Zone are typical calc-alkaline rocks with a pronounced increase in the content of mafic minerals concurrent with a decrease in quartz. The trend of the eastern rocks is different, mainly involving changes in the quartz/total feldspar ratio. At the same SiO 2 content, the eastern rocks are lower in MgO and CaO and higher in (Na 2O + K 2O) and especially in Ba than their western counterparts. Also other elements like Ni show differences between the two suites. We conclude that these similar looking rocks belong to two different suites that are different in age and have different chemical signatures, probably caused by different source-rock chemistries. ϵ Nd1.68-values in the eastern suite range from + 2.0 to 2.5, suggesting that assimilation of large amounts of old continental rocks did not take place. Results from Sr-isotope determinations support this suggestion. The present results suggest, together with older data, that no granitoid rocks older than approximately 0.9 Ga are common to the two terranes separated by the Mylonite Zone. There is thus no compelling evidence suggesting that the two terranes formed one entity before the Sveconorwegian (Grenvillian) orogeny.

A Pb, Sr and Nd isotope study of the basement and mesozoic ring complexes of the Jos Plateau, Nigeria

Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic ( 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement of average Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting. The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr ( 0.5126,18.4 and 0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a well-defined Pb/Pb isotope array with a slope age of1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and granitoids. The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement, probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb. The isotopic evidence supports a model for the Mesozoic anorogenic (“A-type”) granites of Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.

A Pb, Sr and Nd isotope study of the basement and mesozoic ring complexes of the Jos Plateau, Nigeria

Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic (∼ 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement of average Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting. The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr (∼ 0.5126, ∼ 18.4 and ∼ 0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a well-defined Pb/Pb isotope array with a slope age of ∼ 1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and granitoids. The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement, probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb. The isotopic evidence supports a model for the Mesozoic anorogenic (“A-type”) granites of Nigeria in which mantle-derived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.

Crustal accretion in the Pan African: Nd and Sr isotope evidence from the Arabian Shield

Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770−590 Ma and initial 87Sr/ 86Sr ratios of 0.7038−0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial 143Nd/ 144Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (ε Nd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.

Petrology and chemistry of recent lavas in the northern Marianas: Implications for the origin of island arc basalts

Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (87Sr/86Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits.

Santiaguito Volcanic Dome, Guatemala

The Santiaguito volcanic dome insouthwestern Guatemala is a multiple extrusive dome that has shown constant volcanic activity since its birth in 1922. Fourteen extrusive units are mapped. Five of these are volcanic domes; the remaining nine are lava flows which generally cling to the sides of the domes. The volume of material extruded at Santiaguito since 1922 (0.7 km 3 ) is a small fraction of the volume of pyroclastic debris from the 1902 eruption of Santa Maria (5.5 km 3 ), Santiaguito9s parent composite cone. Extrusion of the dome began in the center of the explosion crater created by Santa Maria9s 1902 activity. This crater was volumetrically much smaller (0.5 km 3 ) than the amount of material erupted during the 1902 event, and local slumping near the crater has occurred and is continuing along a series of east-trending faults. The general westward growth of the dome complex and many of the structural featureson Santiaguito are controlled by these near-vertical faults. The domes are studded with Pelean spines. Twenty-five new chemical analyses are presented, showing Santiaguito9s eruptive products to be soda-rich dacite of the calc-alkaline suite. The dome lava has differentiated quite significantly from the overwhelmingly abundant pyroxene andesite magma which makes up Santa Maria and the older volcanic rocks in the area. Trace element analyses of the lavas, along with major element data suggest a differentiation by fractional crystallization under constant or increasing PO 2 . Sr isotope determinations could not detect contamination of the lavas by radiogenic crustal material; the Sr 87 /Sr 86 ratio in Santiaguito rock averages .7043. The Santiaguito lavas are tridymite-bearing hypersthene dacites, characterized by strongly zoned plagioclase phenocrysts and oxyhornblende. Compositional and petrographic comparisons are made with other modern volcanic domes and other Central American lavas.

Petrogenesis of the volcanic rocks of the Central North Island, New Zealand, as indicated by a study of [formula omitted] ratios, and Sr, Rb, K, U and Th abundances

The data cover the Pleistocene to Recent basaltic, andesitic, and especially rhyolitic (including lavas, ignimbrites, and pumice deposits) volcanic rocks of the Taupo Volcanic Zone, and adjacent areas. Analytical data are presented for approximately one hundred volcanic samples, including forty-four new Sr isotope determinations. In addition, limited data are presented on the Triassic-Jurassic eugeosynclinal and miogeosynclinal greywacke-argillite sequences which flank the western and eastern margins of the volcanic zone. The following points are shown by the data: 1. (a) The rhyolitic extrusives have relatively low concentrations of Rb (average 108 ppm), Sr (125 ppm), K (2.69%), U (2.53 ppm), and Th (11.3 ppm), when compared with recent estimates of average “granite” concentrations, suggesting that the rhyolitic magmas have no history involving strong fractional crystallization processes; 2. (b) a complete continuity exists, for the elements studied, through the basaltandesite-dacite-rhyolite compositional range; 3. (c) the initial (i.e. present day) Sr isotopic ratios of the andesitic, dacitic, and rhyolitic extrusives are identical, and exhibit a small but similar range of variation (0.7046–0.7067). This variation is possibly real. The initial Sr isotopic ratios of the basalts are identical within experimental error (0.7041–0.7044), being consistently lower than the Sr isotopic ratios of the intermediate and acidic volcanics; 4. (d) only minor differences exist between the rhyolitic eruptive types in their element abundances. For example, the ignimbrites have slightly higher average Rb, K and Sr abundances than the rhyolite lavas, while the pumices have lower average Rb, K, U, and Th, and higher Sr than the rhyolitic ignimbrites and lavas; 5. (e) the element ratios R Kb , U K , Th K , and Th U exhibit a greater range of variation in the basaltic and andesitic volcanic rocks compared to the rhyolitic rocks; 6. (f) the Triassic-Jurassic eugeosynclinal greywacke-argillite sediments (referred to as the “marginal facies” of the New Zealand geosyncline) have present day Sr isotopic ratios which are closely similar to those of the rhyolitic and andesitic volcanics. The Sr isotopic ratios of these sediments vary according to lithology. The overall composition of the sediments is andesitic to dacitic. It is suggested that the rhyolitic magmas were derived by widespread partial fusion of these eugeosynclinal greywacke-argillite sediments, although this suggestion is based on a number of assumptions concerning the lateral and vertical extent of the sediments. It is shown that such an origin is consistent with the main chemical and mineralogical characteristics shown by the rhyolitic volcanic rocks. The data presented for the andesites are interpreted to be inconsistent with their derivation by fractional crystallisation from the outcropping basalts, or by direct mantle derivation involving a single stage process. The data are believed to be most consistent with crustal hybridisation of basalt, or of a primary andesitic magma (as the data cannot be interpreted to be inconsistent with the existence of a third primary magma type). It is concluded, in the light of the data presented, that the Mesozoic eugeosynclinal greywacke-argillite sediments would constitute the most logical choice of crustal material involved in this process.