Mark B. Boslough

Shock-induced devolatilization of calcite

The amounts of CO_2 and CO evolved upon shock compression and decompression of calcite to 18 GPa (180 kbar) have been determined using a new gas phase shock recovery technique and gas source mass spectrometry. The data demonstrate that from ∼0.03 to 0.3 mole percent of calcite is devolatilized at shock pressures significantly lower than those predicted (30 GPa) for the onset of volatilization by continuum thermodynamic theory and are in qualitative agreement with release adiabat data for calcite and aragonite. Carbon and oxygen isotope ratios in the shock-released CO_2 are the same as those in the unshocked (hydrothermal) calcite, demonstrating that the CO_2 comes from the calcite rather than other potential sources.

Impact cratering and spall failure of gabbro

Both hypervelocity impact and dynamic spall experiments were carried out on a series of well-indurated samples of gabbro to examine the relation between spall strength and maximum spall ejecta thickness. The impact experiments carried out with 0.04- to 0.2-g, 5- to 6-km/sec projectiles produced decimeter- to centimeter-sized craters and demonstrated crater efficiencies of 6 × 10^(−9) g/erg, an order of magnitude greater than in metal and some two to three times that of previous experiments on less strong igneous rocks. Most of the crater volume (some 60 to 80%) is due to spall failure. Distribution of cumulative fragment number, as a function of mass of fragments with masses greater than 0.1 g yield values of b = d(log N_f)/d log(m) −0.5 −0.6, where N is the cumulative number of fragments and m is the mass of fragments. These values are in agreement or slightly higher than those obtained for less strong rocks and indicate that a large fraction of the ejecta resides in a few large fragments. The large fragments are plate-like with mean values of B/A and C/A 0.8 0.2, respectively (A = long, B = intermediate, and C = short fragment axes). The small equant-dimensioned fragments (with mass < 0.1 g and B ∼ 0.1 mm) represent material which has been subjected to shear failure. The dynamic tensile strength of San Marcos gabbro was determined at strain rates of 10^4 to 10^5 sec^(−1) to be 147 ± 9 MPa. This is 3 to 10 times greater than inferred from quasi-static (strain rate 10^0 sec^(−1)) loading experiments. Utilizing these parameters in a continuum fracture model predicts a tensile strength of σ_ m χ^[0.25-0.3], where e is strain rate. It is suggested that the high spall strength of basic igneous rocks gives rise to enhanced cratering efficiencies due to spall in the <10^2-m crater diameter strength-dominated regime. Although the impact spall mechanism can enhance cratering efficiencies it is unclear that resulting spall fragments achieve sufficient velocities such that fragments of basic rocks can escape from the surfaces of planets such as the Moon or Mars.

PARTICLE VELOCITY EXPERIMENTS IN ANORTHOSITE AND GABBRO

Shock wave experiments have been conducted in San Gabriel anorthosite and San Marcos gabbro to 11 GPa using a 40 mm-bore propellant gun. Particle velocities were measured directly at several points in each target by means of electromagnetic gauges. Hugoniot states were calculated by determining shock-transit times from the gauge records. Sound speeds indicate a loss of shear strength upon shock compression for both rocks, with the strength loss persisting upon release to zero stress in the anorthosite. Stress-density release paths in the anorthosite indicate possible transformation of albite to jadeite + (quartz or coesite), with the amount of material transformed increasing as a function of shock stress. Electrical interference effects in the gabbro precluded the determination of accurate release paths for that rock.

Shock wave apparatus for studying minerals at high pressure and impact phenomena on planetary surfaces

Shock wave and experimental impact phenomena research on geological and planetary materials is being carried out using two propellant (18 and 40 mm) guns (up to 2.5 km/sec) and a two‐stage light gas gun (up to 7 km/sec). Equation of state measurements on samples initially at room temperture and at low and high temperatures are being conducted using the 40 mm propellant apparatus in conjunction with Helmholtz coils, and radiative detectors and, in the case of the light gas gun, with streak cameras. The 18 mm propellant gun is used for recovery experiments on minerals, impact on cryogenic targets, and radiative post‐shock temperature measurements.

CRATERING AND SPALL FRACTURE IN GABBRO

Both hypervelocity impact and dynamic spall experiments were carried out on a series of gabbro samples. Craters produced by impact of 0.04 to 0.2g projectiles at 5-6 km/sec velocity demonstrated cratering efficiencies of 6×l0-9 g/erg. Most of the crater volume (~60-80%) is due to spall failure. The dynamic tensile strength of San Marcos gabbro was determined at 104 to 105 sec−1 to be 147 ± 9 MPa, some 3 to 10 times greater than inferred from quasi-static (strain rate 100 sec−1) experiments- Based on a continuum fracturing model for gabbro we predict tensile strengths σm∝ɛ˙[0.25−0.3], where ɛ˙ is strain rate. We correlate the high dynamic strength with the high value of cratering efficiency in the <102m crater diameter, strength-dominated regime.

A method of determining points on the principal isentropes of molecular liquids

We have examined the feasibility of using a large‐diameter, projectile‐target impact to carry out one‐dimensional, isentropic compression experiments on molecular fluids. By employing a three‐layered target geometry, with a thin foam driver layer and a thick, high‐impedance anvil layer, liquid H2O can be compressed to a state within 0.1% of its principal isentrope at pressures up to about 30 GPa. The pressure and density of the state achieved can be determined from electromagnetic particle velocity gauges imbedded on the interfaces bounding the sample.