Geocheminar - fall-2012

Oct. 2, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Patrick Boehnke - ESS
• Jason Kaiser -

A Refined Model of Zircon Saturation in Crustal Magmas Patrick Boehnke (UCLA)

Improvements in experimental, analytical and computation methodologies together with published studies yielding seemingly contradictory results prompted us to return to the determination of zircon stability in the range of felsic to intermediate melts expected in continental environments. We re-analyzed both the run products from the 30 year old zircon crystallization study of Harrison and Watson (1983) and a new style of zircon dissolution experiments (up to 25 kbars) using a large radius ion microprobe to constrain a refined zircon solubility model. The new data yield broadly similar patterns as before when arrayed for temperature and confirm that the parameter M [=(K+Na+2Ca)/(Si Al) as molar abundances] is an appropriate compositional proxy for the mechanism by which zircon is dissolved. We used a Bayesian approach to optimize calculation of the coefficients in the zircon solution model, which is given by: ln DZr = (10108±32)/T(K) – (1.16±0.15) . (M-1) – (1.48±0.09), where DZr is the distribution coefficient of Zr between zircon and melt and the errors are at one sigma. Sensitivity tests indicate that temperature and composition are the two dominant controls on zircon solubility in crustal melts with, surprisingly, no observable pressure effect. Our new data together with literature results suggest a weaker dependence of zircon solubility on water content than previously thought. Comparison of the down-temperature extrapolation with natural examples confirms the validity of the model at ca. 700°C.

Temporal Record of Magma Accumulation and Storage in the Pastos Grandes Caldera Complex Jason Kaiser (Oregon)

The Pastos Grandes Caldera Complex in SW Bolivia is a nested caldera system that has produced 3 regionally extensive ignimbrites as well as small lava flows and domes. Volcanic activity in the complex spanned from 5.45 Ma to 85 ka. The most recent eruptions produced a string of domes in the caldera which contain granodiorite xenoliths. These xenoliths represent the remnant pluton that sourced the youngest ignimbrite of the complex at 2.89 Ma. Zircons were separated from fresh pumice and lavas in order to obtain U-Pb crystallization ages. The combined zircon crystallization age spectra of each volcanic unit reveal a temporal record of magma accumulation and storage. These data support the idea that each ignimbrite eruption was sourced by discrete pulses of magma into the shallow crust. The youngest ignimbrite eruption was also associated with subsequent effusive eruptions. Together, the ignimbrite, post-caldera lavas, and the plutonic xenolith represent a complete magma cycle related to a super-volcanic eruption. Combining the age spectra of these magma stages has allowed us to compare magma residence times before and after a climactic eruption. The U-Pb crystallization data will be compared with in-situ trace element geochemical data to provide a chronologic record of changing melt composition.

Oct. 9, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Paul Warren - ESS

Meteorite Northwest Africa 6693 is a new type of cumulate achondrite, with a unique combination of oxygen isotopic composition (low ?17O) and FeO-rich, low MgO/FeO bulk composition. Cumulus phases include pigeonite (oikocrysts up to 14 mm) and olivine. The parent magma was probably also saturated with feldspar. Mafic silicates are uniformly ferroan (e.g. olivine near Fo49). The feldspar is albitic (near Ab92) with diverse K/Ca ratio: from consistently ~0.46 in one end of the stone, to 5.2 in an enclave of micrographic olivine-feldspar intergrowth. After initial crystallization, minor additional melt formed within, or infiltrated into, the rock, to form discrete overgrowths on some pyroxenes. Final cooling occurred at a moderate intrusive-igneous rate. Olivine, metal, and sulfide phases are all very Ni-rich (e.g., olivine NiO averages 0.77 wt%). Spinel V/(Al+Cr) suggests high oxygen fugacity: fO2 of IW+2. The bulk-rock composition features chalcophile depletions, but nonvolatile lithophile elements are only subtly fractionated from chondritic, and even siderophile concentrations are near-chondritic: Co, Ni, Ir and Os are all at 0.7-1.0×CI chondrites, and Au at 0.55×CI. Apparently the parent melt was not produced by extensive igneous fractionation, and the parent planetesimal was so pervasively oxidizing that FeNi metal did not efficiently sequester the siderophile elements. The high FeO and low MgO/FeO of NWA 6693 were probably in large measure already properties of the original primitive material.

Oct. 16, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Edwin Schauble - ESS

Mass independent isotope fractionations driven by differences in volumes and shapes of nuclei are known in several elements, and are likely to be found in more. Relativistic electronic structure calculations can predict and reproduce these effects, but at present are computationally intensive and limited to modeling small gas-phase molecules and clusters. We would like to be able to predict fractionation in all kinds of geological materials, including liquids and crystals. This talk will focus on possible ways to extend the models we can make to look at more complex materials.

Oct. 23, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Gaojun Li - Nanjing University

Seminar Description coming soon.

Oct. 30, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Pam Hill - ESS

Seminar Description coming soon.

Nov. 6, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Axel Schmitt - ESS

The Taupo Volcanic Zone (New Zealand) is a location of extremely prolific crustal melt generation in an extensional convergent margin setting, and it has produced several large-magnitude eruptions throughout the Quaternary. Recurrent eruptions from the Okataina/Tarawera volcanic complexes between c. 45 ka and 1350 CE provide insights into processes of silicic magma accumulation and storage at depth. Zircon geochronological and geochemical data reveal that (1) the last voluminous caldera-forming eruption (Rotoiti) at 45 ka is 15 ka younger than previously thought, and (2) the silicic magma reservoir cycled through episodes of near-complete solidification and re-melting which occurred likely at time scales <1000 years. The thermo-mechanical conditions within the rhyolite magma storage zone are thus largely controlled by episodic inputs of fresh basaltic magma, which over time have partially erased the crystal record from earlier magmatic phases. Prograde crystal resorption can explain the paradoxically brief zircon crystallization intervals observed in many large-volume eruptions, whereas complex zircon age populations are preserved in rapidly melted and vented small-volume eruptions.

Nov. 13, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Ed Young - ESS

Seminar Description coming soon.

Nov. 20, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Rita Economos - ESS

Seminar Description coming soon.

Nov. 27, 2012
noon - 1 p.m.
Geology 4677

Presented By:

• Rita Economos - ESS

The Tianshan orogen, in the southern Central Asian orogenic belt, consists of continental fragments stitched together during mid- to late Paleozoic arc magmatism generated by the closure of the paleo–Asian Ocean. Controversy persists regarding the timing of final structural amalgamation of the region and therefore whether Permian magmatism was generated in a subduction or intraplate environment. Based on new field mapping, zircon U-Pb geochronology, and isotope data from the 295–290 Ma Gobi-Tianshan intrusive complex in southwestern Mongolia, we argue that this complex is a voluminous intermediate batholith generated by subduction and that it is related to plutons of similar age and character in Tianshan tectono-magmatic belts to the west. In the study area, as well as in Carboniferous and Permian plutons across the Tianshan, mantle isotopic signatures remain consistently primitive. Permian plutons show an increase in radiogenic Sr with no concurrent decrease in radiogenic Nd, which may be due to the influence of subducting continental sediment in the early Permian. This model explains the transitional nature of magmatic compositions and structures in the Gobi-Tianshan intrusive complex.