George D. Kamenov

Optimization of mixed Pb–Tl solutions for high precision isotopic analyses by MC-ICP-MS

Measurements of SRM 981 Pb isotope standard mixed with Tl for mass bias corrections were conducted with a MC-ICP-MS (Nu Plasma) using a Nu Instruments desolvating nebulizer. During the initial experiments, Pb and Tl from single-element, concentrated stock solutions were mixed in 2% HNO3 prior to isotope ratio measurements. The results revealed relatively poor precision and accuracy of the Pb isotope measurements, large variations in e205Tl in the same standard (ranging from −3.9 to +30.1), and large variations in the observed Pb/Tl intensity ratios. When analyses were restricted to freshly mixed (<1 hour) Pb–Tl solutions, however, highly precise isotopic ratios were obtained for lead (206Pb/204Pb = 16.9373 (±0.0011, 2σ), 207Pb/204Pb = 15.4907 (±0.0012, 2σ), and 208Pb/204Pb = 36.6935 (±0.0039, 2σ)) and for thallium (e205Tl = 1.5 (±0.8, 2σ)). In addition, Pb/Tl intensity ratios were constant and corresponded to the mixing ratios of the prepared solutions. A series of experiments revealed that the poor precision and accuracy observed for the initial set of isotope ratio measurements resulted from variable photoxidation of Tl+ to Tl3+, which occurs in the presence of Pb and solar UV radiation. This reversible reaction generates Tl3+, which behaves distinctly from Tl+ during desolvation and leads to consistently higher measured Pb/Tl and 205Tl/203Tl ratios. The extent of the interaction between Pb and Tl and the subsequent effect on isotope ratio measurements is sensitive to a combination of factors, including differences in the acid matrix and molarity, desolvation conditions, and UV light exposure. It appears that the observed e205Tl variations in the Pb–Tl3+-bearing solutions dominantly result from mass-dependant differential diffusion of Tl during desolvation. These experiments suggest that great care must be exercised during isotopic analyses of systems utilizing one element for mass-bias correction on another, such as Pb–Tl, Cu–Zn and Mo–Zr, which readily undergo redox reactions under laboratory conditions.

Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N: Implications for ridge crest plumbing and decadal changes in magma chamber compositions

Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.

Crustal Evolution in the Southern Appalachian Orogen: Evidence from Hf Isotopes in Detrital Zircons

Paired U-Pb and Lu-Hf isotopic compositions of individual detrital zircons reveal that Mesoproterozoic lithosphere in the southernmost Appalachian orogen evolved almost continuously from ∼1200 to ∼900 Ma, with a noticeable concentration only at ∼1050 Ma. The evolution of the Lu-Hf system in this crust is characterized by a systematic, linear evolution pattern of 176Hf/177Hf versus age over this interval, which suggests a limited range of mixing ratios between juvenile and older crustal components to create a new Mesoproterozoic lithospheric reservoir. Extrapolation of the Lu-Hf systematics of this reservoir from the Mesoproterozoic to the Paleozoic using typical crustal values shows general coincidence with the lower part of the Hf isotopic array defined by Paleozoic detrital zircons. Because the Paleozoic array extends to both much higher and much lower ϵHf values than are likely to develop from extrapolations of the Mesoproterozoic array, the Paleozoic array is most compatible with an orogen formed largely from the accretion of numerous discrete crustal blocks that may or may not have derived partly from the southern Appalachian Mesoproterozoic lithosphere. These data are most compatible with existing models for Appalachian orogenesis that invoke accretion of exotic terranes, few, if any, of which were true juvenile terranes.

Crustal evolution of southern Laurentia during the Paleoproterozoic: Insights from zircon Hf isotopic studies of ca. 1.75 Ga rocks in central Colorado

Lu-Hf depleted mantle model (TDM) ages, obtained by analysis of zircons previously dated by U-Pb methods, demonstrate that ca. 1.75 Ga bimodal Paleoproterozoic rocks in the Gunnison-Salida region of central Colorado, and by extension in much of the southwestern U.S., were formed by partial melting of preexisting crustal rocks, the ages of which mostly ages greater than 2.0 Ga indicate that range from 1.83 to 1.87 Ga. Some calculated Lu-Hf TDM even older crust was probably involved, consistent with the limited presence of ca. 2.5 Ga xenocrystic zircons in some rocks. These results suggest that rocks related to the Trans-Hudson and Penokean orogens are cryptically present much farther to the south than previously believed. Coupled with the bimodality of the volcanic suite in central Colorado, and indeed in much of the southwestern U.S., these results indicate that, in contrast to current juvenile arc-accretion models, melting of older crust related to extensional tectonics played an important role in the genesis of many magmatic rocks between 1.6 and 1.8 Ga.

The Pb isotopic record of historical to modern human lead exposure

Human teeth and bones incorporate trace amounts of lead (Pb) from the local environment during growth and remodeling. Anthropogenic activities have caused changes in the natural Pb isotopic background since historical times and this is reflected in the Pb isotopes of historical European teeth. Lead mining and use increased exponentially during the last century and the isotopic compositions of modern human teeth reflect the modern anthropogenic Pb. USA teeth show the most radiogenic Pb and Australian teeth show the least radiogenic Pb, a result of different Pb ores used in the two regions. During the last century the Australian Pb was exported to Europe, Asia, South America, and Africa, resulting in swamping of the local environmental Pb signal by the imported Pb. As a result, the modern human teeth in Europe show a significant drop to lower isotopic values compared with historical times. Similarly, modern human teeth in other regions of the world show similar Pb isotopic ratios to modern European teeth reflecting the Pb imports. The specific pattern of human Pb exposure allows us to use the Pb isotopic signal recorded in the skeleton as a geo-referencing tool. As historical European teeth show a distinct Pb signal, we can identify early European skeletal remains in the New World and likely elsewhere. In modern forensic investigations we can discriminate to some extent Eastern Europeans from Western and Northern Europeans. Australians can be identified to some extent in any region in the world, although there is some overlap with Western European individuals. Lead isotopes can be used to easily identify foreigners in the USA, as modern USA teeth are distinct from any other region of the world. By analogy, USA individuals can be identified virtually in any other region of the world.

Origin and significance of ice‐rafted detritus in the Atlantic sector of the Southern Ocean

Piston core TN057-14 from the eastern Atlantic sector of the Southern Ocean contains several layers rich in volcanic tephra that were deposited during the last glacial period. Comparison with nearby cores (TN057-13PC4/ODP Site 1094) indicates that these tephra layers are correlative with the South Atlantic (SA) ice-rafted detritus (IRD) layers identified by Kanfoush et al. (2000, 2002). We analyzed the tephra for major and trace elements as well as isotopic ratios of strontium, neodymium, and lead. The elemental and isotopic composition of all tephra layers examined indicates the South Sandwich Island volcanic arc (SSI) as their dominant source, with minor input from Bouvet Island. Similar analyses of clear mineral grains (mainly feldspars, quartz, and olivine) also point to the SSI as the main source. We conclude that tephra erupted from the SSI was first deposited as air fall on sea ice and multiyear ice and then secondarily rafted to the studied sites. Deposition of the tephra-rich IRD layers was controlled by changes in sea surface temperature and sea-ice conditions in the Polar Frontal Zone of the South Atlantic, rather than Antarctic ice sheet dynamics.

Detrital mineral chronology of the Uinta Mountain Group: Implications for the Grenville flood in southwestern Laurentia

Numerous studies have shown that large quantities of Grenville-age detritus dominate Neoproterozoic to Cambrian arenites in southwest Laurentia (southwestern United States). U-Pb ages and Hf isotopic compositions of zircons and 40 Ar/ 39 Ar ages of white mica from clastic sedimentary rocks of the Neoproterozoic Uinta Mountain Group also indicate significant Mesoproterozoic detritus mixed with a variably abundant Archean component. Zircons with ages representative of the Paleoproterozoic basement in the eastern Uinta Mountains or the younger Paleoproterozoic rocks of the adjacent Yavapai-Mazatzal terranes were not observed. A limited range of initial ϵ Hf (∼90% between –3 and +3) for Mesoproterozoic zircons suggests derivation from a source region (or regions) characterized by mixing between juvenile and reworked older crust during Grenville orogenesis. The enriched Grenville-age basement proposed to underlie much of southeastern North America may be this source based on similarities of Hf isotopic data from Mesoproterozoic zircons in Mississippi River sand and available paleocurrent data. If so, then disruption of this supply in the Cambrian may be related to Iapetan rifting and, perhaps, the separation of the Precordillera terrane from Laurentia.

Variations in the strontium isotope composition of seawater during the Paleocene and early Eocene from ODP Leg 208 (Walvis Ridge)

We refined the strontium isotope seawater curve for the Paleocene and early Eocene by analysis of samples recovered from the Walvis Ridge during Ocean Drilling Project (ODP) Leg 208. The highest Sr-87/Sr-86 values occurred in the earliest Paleocene at similar to 65 Ma and generally decreased throughout the Paleocene, reaching minimum values between 53 and 51 Ma in the early Eocene before beginning to increase again at similar to 50 Ma. A plausible explanation for the Sr-87/Sr-86 decrease between 65 and 51 Ma is increased rates of hydrothermal activity and/or the eruption and weathering of large igneous provinces ( e. g., Deccan Traps and North Atlantic). Strontium isotope variations closely parallel sea level and benthic delta O-18 changes during the late Paleocene and early Eocene, supporting previous studies linking tectonic reorganization and increased volcanism to high sea level, high CO2, and warm global temperatures.

Palaeozoic Lachlan orogen, Australia; accretion and construction of continental crust in a marginal ocean setting: isotopic evidence from Cambrian metavolcanic rocks

Abstract The Lachlan orogen developed as a classic accretionary orogen in an oceanic setting between the palaeo-Pacific subduction zone and the Australian craton. Direct evidence for the composition and age of the lower crust and the basement to the thick Palaeozoic turbidite fan of the Lachlan orogen is limited. Exposures of Cambrian metavolcanic rocks and geophysical data suggest that most of the basement is the mafic oceanic crust along with possible small fragments of older continental crust. The trace element compositions of Cambrian metavolcanic rocks in the western and central Lachlan orogen are similar to those of volcanic rocks formed in modern back-arc and forearc settings. Pb, Nd and Sr isotopic data from these Cambrian rocks suggest a supra-subduction zone setting with little or no influence of continental crust other than subducted sediment.

Evidence for Patterns of Selective Urban Migration in the Greater Indus Valley (2600- 1900 BC): A Lead and Strontium Isotope Mortuary Analysis

Just as modern nation-states struggle to manage the cultural and economic impacts of migration, ancient civilizations dealt with similar external pressures and set policies to regulate people’s movements. In one of the earliest urban societies, the Indus Civilization, mechanisms linking city populations to hinterland groups remain enigmatic in the absence of written documents. However, isotopic data from human tooth enamel associated with Harappa Phase (2600-1900 BC) cemetery burials at Harappa (Pakistan) and Farmana (India) provide individual biogeochemical life histories of migration. Strontium and lead isotope ratios allow us to reinterpret the Indus tradition of cemetery inhumation as part of a specific and highly regulated institution of migration. Intra-individual isotopic shifts are consistent with immigration from resource-rich hinterlands during childhood. Furthermore, mortuary populations formed over hundreds of years and composed almost entirely of first-generation immigrants suggest that inhumation was the final step in a process linking certain urban Indus communities to diverse hinterland groups. Additional multi disciplinary analyses are warranted to confirm inferred patterns of Indus mobility, but the available isotopic data suggest that efforts to classify and regulate human movement in the ancient Indus region likely helped structure socioeconomic integration across an ethnically diverse landscape.