“Swapping” Families: Serial Parenting and Economic Support for Children

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71408/1/j.1741-3737.2000.00111.x.pd

Kinetics of the reaction H_2O+O=2OH in rhyolitic and albitic glasses: Preliminary results

The kinetics of homogeneous reactions are important in understanding the cooling history of rocks and in understanding experimental speciation data. We have experimentally studied the kinetics of the interconversion reaction between H_2O molecules and OH groups in natural rhyolitic glasses (0.5-2.3% total water) and a synthetic albitic glass (l% total water) at 400-600°C. The reaction rate increases with temperature and total water content. Equilibrium is not always approached monotonically; the speciation may first depart from equilibrium and then come back to equilibrium. Experimental reaction rates agree with those inferred from previous speciation data of rhyolitic glasses quenched from 850°C. The experimental data are modeled successfully by considering both the reaction and the diffusion of OH that brings OH groups together to react. This study shows that species concentrations in glasses quenched from ≤ 600°C reflect those at experimental temperatures unless the water content is higher than that used in the present study. Species concentrations in glasses with total water contents ≥0.8 wt% and which were rapidly quenched in water from 850°C do not represent their equilibrium concentrations in the melt at 850°C, but record a lower apparent equilibrium temperature that depends on water content and quench rate. Natural rhyolitic glasses and glass inclusions do not record preeruptive melt speciation, though total water content may be conserved. The experimental data are used to infer cooling rates for natural obsidian glasses. Pyroclastic glass fragments from the bb site of Mono Craters have cooling rates similar to air-cooled experimental charges (~3°C/s). Different types of glasses from the Mono Craters have different cooling rates, which cover four orders of magnitude. Some natural obsidians appear to have had complex cooling histories. The wide range of cooling rates and thermal histories is consistent with previous inferences that some obsidian clasts at the Mono Craters formed as glass selvages lining volcanic conduits or dikes that were subsequently caught up in the explosive eruption, which led to variable degrees of transient heating followed by rapid cooling and deposition. These experimental data reveal surprisingly rich detail in water speciation in volcanic glasses and show how, at least in principle, quantitative constraints on thermal histories can be extracted by experimentation and application of kinetic models

New calibration of infrared measurement of dissolved water in rhyolitic glasses

This paper presents a new calibration for infrared analyses of dissolved water and its species concentrations in rhyolitic glasses. The new calibration combines infrared/manometry measurements and infrared study of hydrous rhyolitic glasses heated at different temperatures. The heating experiments show that the ratio of the molar absorptivity of the 5230 cm^(−1) band to that of the 4520 cm^(−1) band varies with water concentration. Therefore, earlier calibrations assuming constant molar absorptivities are not accurate. Using our new calibration, total water concentration, and species concentrations can be calculated as follows: (ƍƍ_0)C_1 = a_(0-523); (ƍƍ_0)C_2 = (b_0 + b_(1-523) + b_(2-452)_452, ardC = C_1 + C_2, where C_1, C_2, and C are the mass fractions of molecular H_2O, H_2O present as OH, and total H_2O, πƍ_0 is the ratio of the density of the hydrous glass to that of the anhydrous glass and is approximately 1 - C, _(523) and _(452) are the absorbances (peak heights) of the 5230 cm^(−1) and 4520 cm^(−1) bands per mm sample thickness and relative to a baseline that was fit by a flexicurve, a_0 = 0.04217 mm, b_0 = 0.04024 mm, _1 = −0.02011 mm^2, and b_2 = 0.0522 mm^2. The new calibration has a high internal reproducibility in calculating H_2O_(total), six times better than the calibration of Newman et al. (1986). We expect the new calibration to be accurate in retrieving H_2O_(total) for H_2O_(total) ≤ 5.5 wt% and in retrieving molecular H_2O and OH concentrations for H_2O_(total) ≤ 2.7 wt%. Using the new calibration, the equilibrium coefficient K for the reaction H_2O + O = 2OH is independent of H_2O_(total) (for H_2O_(total) ≤ 2.4 wt%) at a given temperature and can be expressed as ln K = 1.876 - 3110T, where T is in K. The bulk water diffusivity reported before is not affected by the new calibration, but the molecular H_2O diffusivity will be roughly 4–30% greater

New calibration of infrared measurement of dissolved water in rhyolitic glasses

This paper presents a new calibration for infrared analyses of dissolved water and its species concentrations in rhyolitic glasses. The new calibration combines infrared/manometry measurements and infrared study of hydrous rhyolitic glasses heated at different temperatures. The heating experiments show that the ratio of the molar absorptivity of the 5230 cm^(−1) band to that of the 4520 cm^(−1) band varies with water concentration. Therefore, earlier calibrations assuming constant molar absorptivities are not accurate. Using our new calibration, total water concentration, and species concentrations can be calculated as follows: (ƍƍ_0)C_1 = a_(0-523); (ƍƍ_0)C_2 = (b_0 + b_(1-523) + b_(2-452)_452, ardC = C_1 + C_2, where C_1, C_2, and C are the mass fractions of molecular H_2O, H_2O present as OH, and total H_2O, πƍ_0 is the ratio of the density of the hydrous glass to that of the anhydrous glass and is approximately 1 - C, _(523) and _(452) are the absorbances (peak heights) of the 5230 cm^(−1) and 4520 cm^(−1) bands per mm sample thickness and relative to a baseline that was fit by a flexicurve, a_0 = 0.04217 mm, b_0 = 0.04024 mm, _1 = −0.02011 mm^2, and b_2 = 0.0522 mm^2. The new calibration has a high internal reproducibility in calculating H_2O_(total), six times better than the calibration of Newman et al. (1986). We expect the new calibration to be accurate in retrieving H_2O_(total) for H_2O_(total) ≤ 5.5 wt% and in retrieving molecular H_2O and OH concentrations for H_2O_(total) ≤ 2.7 wt%. Using the new calibration, the equilibrium coefficient K for the reaction H_2O + O = 2OH is independent of H_2O_(total) (for H_2O_(total) ≤ 2.4 wt%) at a given temperature and can be expressed as ln K = 1.876 - 3110T, where T is in K. The bulk water diffusivity reported before is not affected by the new calibration, but the molecular H_2O diffusivity will be roughly 4–30% greater

The permeability evolution of tuffisites and implications for outgassing through dense rhyolitic magma

There is growing evidence that outgassing through transient fracture networks exerts an important control on conduit processes and explosive‐effusive activity during silicic eruptions. Indeed, the first modern observations of rhyolitic eruptions have revealed that degassed lava effusion may depend upon outgassing during simultaneous pyroclastic venting. The outgassing is thought to occur as gas and pyroclastic debris are discharged through shallow fracture networks within otherwise low‐permeability, conduit‐plugging lava domes. However, this discharge is only transient, as these fractures become clogged and eventually blocked by the accumulation and sintering of hot, melt‐rich pyroclastic debris, drastically reducing their permeability and creating particle‐filled tuffisites. In this study we present the first published permeability measurements for rhyolitic tuffisites, using samples from the recent rhyolitic eruptions at Chaitén (2008‐2009) and Cordón Caulle (2011‐2012) in Chile. To place constraints on tuffisite permeability evolution, we combine (1) laboratory measurements of the porosity and permeability of tuffisites that preserve different degrees of sintering, (2) theoretical estimates on grainsize‐ and temperature‐dependent sintering timescales, and (3) H2O diffusion constraints on pressure‐time paths. The inferred timescales of sintering‐driven tuffisite compaction and permeability loss, spanning seconds (in the case of compaction‐driven sintering) to hours (surface tension‐driven sintering), coincide with timescales of diffusive degassing into tuffisites, observed vent pulsations during hybrid rhyolitic activity (extrusive behaviour coincident with intermittent explosions) and, more broadly, timescales of pressurisation accompanying silicic lava dome extrusion. We discuss herein the complex feedbacks between fracture opening, closing, and sintering, and their role in outgassing rhyolite lavas and mediating hybrid explosive‐effusive activity

21. See supporting material on Science Online

melt fraction will be more gradual, reflecting the gradual increase of water solubility in olivine and orthopyroxene. Our results therefore support the concept that the low-velocity zone may be related to partial melting (1, 2, 6). However, even in the absence of melting, the partitioning of water between olivine and orthopyroxene would strongly depend on depth. The high water solubilities in aluminous orthopyroxene at low pressure and temperature will effectively "dry out" olivine, and this may also contribute to a stiffening of the lithosphere. In any case, however, our results imply that the existence of an asthenosphere-and therefore of plate tectonics as we know it-is possible only in a planet with a water-bearing mantle. Stefan Rahmstorf A semi-empirical relation is presented that connects global sea-level rise to global mean surface temperature. It is proposed that, for time scales relevant to anthropogenic warming, the rate of sea-level rise is roughly proportional to the magnitude of warming above the temperatures of the pre-Industrial Age. This holds to good approximation for temperature and sea-level changes during the 20th century, with a proportionality constant of 3.4 millimeters/year per°C. When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level. U nderstanding global sea-level changes is a difficult physical problem, because complex mechanisms with different time scales play a role (1), including thermal expansion of water due to the uptake and penetration of heat into the oceans, input of water into the ocean from glaciers and ice sheets, and changed water storage on land. Ice sheets have the largest potential effect, because their complete melting would result in a global sea-level rise of about 70 m. Yet their dynamics are poorly understood, and the key processes that control the response of ice flow to a warming climate are not included in current ice sheet models [for example, meltwater lubrication of the ice sheet bed (2) or increased ice stream flow after the removal of buttressing ice shelves (3)]. Large uncertainties exist even in the projection of thermal expansion, and estimates of the total volume of ice in mountain glaciers and ice caps that are remote from the continental ice sheets are uncertain by a factor of two (4). Finally, there are as yet no published physically based projections of ice loss from glaciers and ice caps fringing Greenland and Antarctica. For this reason, our capability for calculating future sea-level changes in response to a given surface warming scenario with present physicsbased models is very limited, and models are not able to fully reproduce the sea-level rise of recent decades. Rates of sea-level rise calculated with climate and ice sheet models are generally lower than observed rates. Since 1990, observed sea level has followed the uppermost uncertainty limit of the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (TAR), which was constructed by assuming the highest emission scenario combined with the highest climate sensitivity and adding an ad hoc amount of sea-level rise for "ice sheet uncertainty" (1). While process-based physical models of sealevel rise are not yet mature, semi-empirical models can provide a pragmatic alternative to estimate the sea-level response. This is also the approach taken for predicting tides along coasts (for example, the well-known tide tables), where the driver (tidal forces) is known, but the calculation of the sea-level response from first principles is so complex that semi-empirical relationships perform better. Likewise, with current and future sea-level rise, the driver is known [global warming (1)], but the computation of the link between the driver and the response from first principles remains elusive. Here, we will explore a semiempirical method for estimating sea-level rise. As a driver, we will use the global average near-surface air temperature, which is the standard diagnostic used to describe global warming