UV Laser Ablation Studies Volcanic Hazard
Peter van Calsteren
Laser ablation technology used by investigators at The Open University in Milton Keynes, UK, offers high-resolution sampling for earth and environmental sciences. One topic of research involves the processes and parameters that enable the modeling of magma generation and volcanic activity.
To understand the length of time between eruptions, scientists are analyzing the chemistry of rocks from eruption sites along ocean ridges and islands, of continental flood basalts such as the Columbia River Basalts, and of specimens from "supervolcano" calderas such as Taupo in New Zealand and Toba on Sumatra.
The total output of volcanic activity on Earth makes it the most energetic -- and sometimes the most violent -- process on the planet's surface. The collapse of an underground magma chamber can result in extremely violent eruptions, causing devastation of the same magnitude as large meteorite impacts.
The eruptions that formed the Yellowstone and Valles calderas in the US produced hundreds of cubic kilometers of lava, and a similar event today would cause social and economic collapse on a continental scale. Fortunately, repose time between these eruptions is on the order of hundreds of thousands of years.
Magma chamber recharge times, however, are only a fraction of that. Lava that erupts from a volcano is produced by partial melting in the upper mantle or lower crust, and the composition of the melt, or magma, is determined by the chemical and mineralogical composition of the source rock and by physical parameters such as temperature and time.
Thermal and chemical erosion of intruding juvenile magma in the upper crust forms the chamber, the contents of which evolve with the interaction of a number of processes, including continuous or episodic replenishment by juvenile magma, partial evacuation of erupting lava, assimilation of the host rock and crystallization of minerals.
Geochemical analysis offers a means of better understanding these processes. The juvenile magma that enters a chamber, for example, has a different ratio of strontium isotopes than the rock into which the chamber expands. Some crystallizing minerals, such as zircon and plagioclase (a type of feldspar), can remain in a magma chamber for long periods of time, and they record the conditions in the chamber in the form of growth zones similar to tree rings. Studying the chemical differences in these growth zones reveals the past dynamics of the site in which the crystallizing minerals were located.
The zonation in plagioclase grains can be very complex, with 10 or more variation cycles that each is approximately 10 to 30 µm thick. Variations in the ratios of 87Sr to 86Sr and among trace elements that constrain the parameters for the various magma chamber processes follow this visual zonation. Analytical techniques for the measurement of these isotope ratios therefore must be sensitive enough to yield meaningful data from small samples that pertain to individual growth zones.
The Open University scientists, together with colleagues from Oxford University and from Nu Instruments Ltd. in Wrexham, both in the UK, have developed an inductively coupled plasma source multicollector mass spectrometer for the measurement of isotope ratios with such sensitivity. The instrument combines the ionization efficiency of an inductively coupled plasma source and the high precision and accuracy offered by a magnetic-sector analyzer.
One consideration in developing the instrument was the type of laser used to ablate small amounts of material from the samples. Quadrupled or quintupled Nd:YAG lasers are not always sufficiently sensitive or precise to yield meaningful data in this application. In contrast, almost all materials effectively absorb the 193-nm wavelength of an ArF excimer laser. The photon energy is absorbed in a submicron-thick surface layer that ablates with virtually no thermal effects in remaining material.
The very high spatial resolution of the homogenized excimer beam facilitates the study of incremental processes in the growth of individual minerals. The ablated material is carried in a helium gas flow and is introduced into the plasma torch of the inductively coupled plasma source multicollector mass spectrometer, where it is ionized.
Three main parts
The instrument comprises three main parts (Figure 1). The first is the mass spectrometer from Nu Instruments, a double-focusing instrument with electrostatic sector energy focusing before a sector magnet mass analyzer. It has 12 fixed Faraday and three ion-counting channels. Another element is the company's desolvating nebulizer.
Figure 1. UV laser ablation contributes to a technique for analysis of crystallizing minerals from magma chambers.
It also incorporates a Universal Platform laser ablation microscope from New Wave Research Inc. of Fremont, Calif., and an ExciStar 193-nm ArF excimer laser from TuiLaser AG of Munich, Germany. The pulsed laser beam is homogenized by two multisegment lenses and a trepan rotating condenser (Figure 2). The radiation from the individual lens segments overlaps at the ablation point, where it is brought into pseudofocus, resulting in flat-bottom ablation pits.
Figure 2. The UV laser path is indicated schematically with attenuation and homogenization optics.
For example, a single-pulse ablation pit in soft glass is approximately 200 nm deep, and the bottom roughness is only about 2 percent (see table, page 36). This enables the very precise excavation of growth zones or diffusion gradients in samples.
During analysis, the stage-mounted ablation chamber with the sample and the helium nozzles move in a given direction (left to right, top to bottom or vice versa) relative to the fixed position of the ablation spot (Figure 3).
Figure 3. The ablation chamber, mounted on an X-Y-Z stage, moves relative to the fixed position of the laser.
The researchers have compared the instrument's performance for conventional analysis of an aspirated solution standard with that for laser ablation of a homogenized feldspar glass by alternating the analytes while keeping all other system parameters constant. First, they aspirated NBS987 87Sr/86Sr standard in 2 percent HNO3 through the desolvating nebulizer, with only a helium flow through the ablation chamber. They collected data 100 times for eight seconds in time-resolved mode.
Second, they turned on the laser and ablated a trench in a sample, collecting data in this mode. They followed this with another trench, ablated in the opposite direction. During laser ablation, clean HNO3 was aspirated. Next, they switched off the laser and collected the isotope ratio standard data. They repeated this cycle and bracketed each pair of ablation data with standard data to enable drift corrections.
The variation in the isotope ratio data in feldspar glass was more than three times larger than for the solution standard results with the instrument parameters untouched. The system can resolve variations in the 87Sr/86Sr isotope ratio of approximately 0.01 percent, offering perspective on the cycle of influx of juvenile melt into a magma chamber and the assimilation of the host rock.
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