Benita Putlitz

Incremental growth of the Patagonian Torres del Paine laccolith over 90 k.y.

The Miocene Paine Granite in the Torres del Paine Intrusive Complex, southern Chile, is an extraordinary example of an upper crustal mafic and granitic intrusion. The granite intruded as a series of three sheets, each one underplating the previous sheet along the top of the basal Paine Mafic Complex. High-precision U/Pb geochronology on single zircons using isotope dilution–thermal ionization mass spectrometry yields distinct ages of 12.59 ± 0.02 Ma and 12.50 ± 0.02 Ma, respectively, for the first and last sheet of the laccolith. This age relationship is consistent with field observations. The zircon ages define a time frame of 90 ± 40 k.y. for the emplacement of a >2000-m-thick granite laccolith. These precise U-Pb zircon ages permit identification of the pulses in a 20 k.y. range. The data obtained for the Paine Granite fill the gap between 100 k.y. and 100–1000 yr pulses described in the literature for crustal magma chambers.

Strain‐induced outgassing of three‐phase magmas during simple shear

A major factor determining the explosivity of silicic eruptions is the removal of volatiles from magma through permeability-controlled outgassing. We studied the microstructural development of permeability during deformation of highly viscous magma by performing simple shear experiments on bubble (0.12–0.36 volume fraction) and crystal-bearing (0–0.42 volume fraction) silicate melts. Experiments were performed under torsion, at high temperature and pressure (723–873 K and 150–200 MPa) in a Paterson deformation apparatus at bulk shear strains between 0 and 10. The experimental setup allows for gas escape if bubble connectivity is reached on the sample periphery. Three-dimensional imaging and analysis of deformed bubbles was performed using X-ray tomography. The development of localized deformation in all samples, enhanced by crystal content, leads to brittle fracture at bulk strains > 2 and sample-wide fracturing in samples deformed to strains > 5. A decrease in both bubble fraction and dissolved volatile content with increasing strain, along with strain-hardening rheological behavior, suggests significant shear-induced outgassing through the fracture networks, applicable to shallow conduit degassing in magmas containing crystal fractions of 0–0.42. This study contributes to our understanding of highly viscous magma outgassing and processes governing the effusive-explosive transition.

Transport-controlled hydrothermal replacement of calcite by Mg-carbonates

Dolomitization is one of the most important diagenetic processes, but the reaction rate and time scale of dolomitization remain a topic of controversy. We conducted experiments in which the reaction of single calcite crystals with a Mg-rich fluid at 200 °C leads to the formation of a zoned reaction rim consisting of magnesite and, for intermediate times, Ca-rich dolomite. From detailed documentation of the evolution of the microtexture and chemical composition of the reaction rim, we infer a kinetic model for calcite replacement by Mg-carbonates. The chemical gradient for the structure-forming elements Mg and Ca in the reaction rim and the evolution of the rim thickness strongly indicate that the overall reaction rate is controlled by diffusive transport through the porous reaction rim. The composition of the product phases is kinetically controlled and records the local composition of the interfacial fluid without requiring oversaturation of the reservoir fluid. Reactive transport models on dolomitization processes assume that the rate of dolomitization depends on the rate of dolomite precipitation, which is contradictory to our experimental evidence. Modeling carbonate replacement in natural systems requires detailed knowledge on the evolution of the microstructure controlling the physicochemical transport properties of elements in the pore fluid.

Short magmatic residence times of quartz phenocrysts in Patagonian rhyolites associated with Gondwana breakup

A key parameter in the study of magma evolution is the time scale on which magmatic processes occur. Using nanoscale secondary ion mass spectrometry (NanoSIMS), SIMS, and cathodoluminescence (CL) analyses, we have measured titanium (Ti) diffusion profiles in quartz phenocrysts from a Jurassic rhyolite of the El Quemado Complex (Patagonia, Argentina), providing new insights into the time scales of the associated volcanic processes. CL imaging of quartz phenocrysts reveals oscillatory magmatic zoning. We determined Ti concentrations with SIMS and acquired multiple NanoSIMS profiles across growth zones from core to rim. All transects show sharp changes in the Ti-48/Si-29 ratio, which correlate reasonably well with changes in CL intensity. Diffusion modeling of Ti in quartz yields a surprisingly short time scale for quartz crystallization of 5.6 +/- 2.2 yr and a rapid crystal growth rate of 2.3 x 10(-12) m/s. Based on the observed quartz textures, we suggest that the rhyolite erupted shortly after initial onset of crystallization, followed by decompression-driven quartz dissolution during fast magma ascent. We further argue that the observed oscillatory zoning and the variation of the Ti concentration of the quartz phenocryst does not reflect temperature, pressure, or titanium activity (a(Ti)) changes of the magmatic system, but rather is the result of growth kinetics, which has important implications for the Ti-in-quartz thermometry.

Tectonic controls and Cenozoic magmatism at the Torres del Paine, southern Andes (Chile, 51°10'S)

Cinco complejos intrusivos se situan a lo largo del Lineamiento San Lorenzo-Balmaceda, que es paralelo a la fosa oceanica de la Patagonia meridional. Estos complejos se caracterizan por tener un magmatismo levemente alcalino a calcoalcalino. Se estudiaron la deformacion y la cinematica en el antepais del lacolito Torres del Paine dominado por granitoides (12±2Ma) para evaluar la influencia del campo de esfuerzos corticales pre- y synintrusivo en el emplazamiento del magma. La compresion y la transpresion produjeron un plegamiento en gran escala, con pliegue de longitud de onda de hasta 10 km, y un fallamiento del flysch cretacico y de filones de gabro alcalino. La edad (29,4±0,8 Ma) de los gabros apuntan a una edad maxima oligocena para el plegamiento regional. Ocurren fallas inversas y fallas de rumbo convergentes y sinistrales asi como tambien se conjugan las fallas inversas. El ascenso del magma fue paralelo a la zona de falla Lago Grey de rumbo NNW-SSE,y probablemente coincide con la interseccion fosil de la falla transformante oceanica Madre de Dios. La intrusion se correlaciona en el tiempo con un cambio de dinamica transpresional a transtensional documentada por un cambio cinematico de falla de rumbo sinistral convergente a divergente y, en el espacio, con una reorganizacion de la situacion tectonica de placas durante el Mioceno. Despues de la colision de la dorsal de Chile que separa las placa de Nazca y Antartica, la parte suroccidental de Sur America cambio desde una convergencia rapida oblicua de direccion ENE-WSW entre la placa de Nazca y la placa Sudamerica a una convergencia frontal lenta de direccion este-oeste entre las placas Antartica y Sudamerica. La divergencia observada entre el lineamiento San Lorenzo-Balmaceda y el aparente control del ascenso de magmas a traves de fallas de la corteza superior puede ser explicada intepretando al SLB como un posible sistema de rumbo de la corteza inferior a gran escala, y por la particion del cizalle entre la corteza inferior y la superior.

A NanoSIMS Investigation on Timescales Recorded in Volcanic Quartz From the Silicic Chon Aike Province (Patagonia)

We compare quartz zonation and diffusion timescales of crystal-rich rhyolitic ignimbrites and crystal-poor rhyolitic lava flows from the Jurassic Chon Aike Province as exposed in Patagonia (Argentina). The timescales are assessed by using diffusion modelling based on nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of titanium (Ti) concentration profiles in quartz crystals oriented by image analysis using micro-tomography. Quantitative Ti-data were acquired by SIMS to estimate crystallization temperatures. The textural and geochemical analysis revealed clear differences between crystal-poor rhyolitic lava flows and crystal-rich rhyolitic ignimbrites. Quartz crystals from rhyolitic lava flows display simple oscillatory cathodoluminescence (CL) zoning interpreted to be magmatic and diffusion chronometry suggest a short timescale for quartz crystallization from 5.6 ± 2.2 yr to 41.6 ± 9.8 yr. Resorption textures are rare, and hence crystals in rhyolitic lava flows recorded a simple, rapid extraction, transport and eruption history for these crystal-poor melts. Rhyolitic ignimbrites, in contrast, reveal complex zoning patterns, reflecting several episodes of partial resorption and growth throughout their crystallization history. The complex quartz zoning textures together with longer diffusion times (< 350 yr), rather suggest a storage in a mush with fluctuating pressure and temperature conditions leading to intermittent resorption. Yet, a final quartz overgrowth rim occurred over a much shorter timescale in the order of years (< 3 yr), which implies that crystal-rich ignimbrites can be re-mobilized very fast.