Robert D. Francis

Sulfide globules in mid-ocean ridge basalts (MORB), and the effect of oxygen abundance in FeSO liquids on the ability of those liquids to partition metals from MORB and komatiite magmas

The coexistence of sulfide liquids (or more accurately, FeSO liquids) with silicate liquids in a variety of magma systems is well known, as is the ability of immiscible FeSO liquids to segregate Ni, Cu and platinum-group metals into orebodies. Evidence for association of FeSO liquids with magmas that form mid-ocean ridge basalts (MORB) comes from sulfide globules in basalt glass from Siqueiros Fracture Zone, East Pacific Rise; three fracture zones in the Indian Ocean; Lau Basin; and ∼ 23 previously published oceanic localities. Approximately 40% of the sulfur originally present in magma at Siqueiros Fracture Zone exsolves as a sulfide liquid during fractional crystallization. Virtually all of the Cu, and 20% of the Ni, originally in the magma is extracted by the sulfide liquid. Significant variations in the ability of FeSO liquids to partition Ni Cu exist among MORB and other mafic magmas, such as komatiites and layered mafic intrusions. Nernst partition coefficients (sulfide to silicate melt; DSL) are 2–3 times higher for Ni in MORB than in komatiite, and ∼ 5 times higher for Cu. Reasons for this variation are not entirely clear, but a possible contributing factor is that FeSO liquids in MORB appear to have a higher oxygen content than those in komatiites. That FeSO liquids associated with MORB magma are oxygen-rich is suggested by low observed values at Siqueiros Fracture Zone for the NiFe exchange coefficient KD3 [1.21−4.56; KD3 = (FeNi)olivine(FeNi)sulfide]. It has been previously shown that natural magmatic FeSO liquids are rich in O2− [O(S + O) is apparently always greater than ∼ 0.49], and that KD3 is inversely correlated with O(S + O). KD3-values below ∼ 12 would result from FeSO liquids with O(S + O)> 0.49. KD3-value of> 20 would be impossible (values from 12 to 20 would be unlikely) because they would require an FeSO liquid poor in O2−. This argument for low KD3 in magmas is strengthened by the low KD3-values of MORB, whose globules are inarguably of magmatic origin. KD3 in komatiites is higher than in MORB (indicating a lower oxygen fugacity in komatiite magma) and varies from ∼ 5 to > 20, with values of > 20 coming from highly metamorphosed komatiites. Fleet and MacRae argued on the basis of experimental results that KD3-values of 20 could not have resulted from magmatic processes, but might have come from metamorphism under oxygen-deficient conditions.

Decrease in natural marine hydrocarbon seepage near Coal Oil Point, California, associated with offshore oil production

Prolific natural hydrocarbon seepage occurs offshore of Coal Oil Point in the Santa Barbara Channel, California. Within the water column above submarine vents, plumes of hydrocarbon gas bubbles act as acoustic scattering targets. Using 3.5 kHz sonar data, seep distribution offshore of Coal Oil Point was mapped for August 1996, July 1995, and July 1973. Comparison of the seep distributions over time reveals more than 50% decrease in the areal extent of seepage, accompanied by declines in seep emission volume, in a 13 km 2 area above a producing oil reservoir. Declines in reservoir pressure and depletion of seep hydrocarbon sources associated with oil production are the mechanisms inferred to explain the declines in seep area and emission volume.

Enhancing Diversity in the Geosciences.

Abstract An innovative interdisciplinary project at California State University, Long Beach, was designed to increase the attractiveness of the geosciences (physical geography, geology, and archaeology) to underrepresented groups. The goal was to raise awareness of the geosciences by providing summer research opportunities for underrepresented high school and community college students and their faculty. A survey of a larger sample provided insight into strategies for enhancing geoscience awareness. A qualitative evaluation pointed to its success in meeting project goals. This unprecedented level of collaboration has set the groundwork for an institutional shift for inclusion of minorities in the geosciences and warrants replication.

Cenozoic Attenuation Detachment Faulting: A Possible Control on Oil and Gas Accumulation in East-Central Nevada (1)

Current explanations for low-angle, younger over older faults in eastern Nevada are (1) a master detachment extending through the crust, (2) mid-crustal ductile extension, and (3) local gravity sliding. None of these explanations are fully supported by field evidence. Our field evidence indicates that thermally induced ductile extension beneath the White Pine and Schell Creek ranges produced low-angle, normal faults in competent units above and detachment faults along ductile shales. Thin brittle units between shale detachments, fragmented by low-angle normal faulting, were pulled apart into isolated lenticular detachment structures. Duck Creek Valley, a Neogene graben similar to Railroad Valley, may have formed in a zone of weakness caused by Oligocene attenuation, suggesting that potential oil-producing basins in Nevada may contain subsurface detachment structures. Oligocene attenuation may have contributed to fracturing of reservoirs (brittle units), local thinning of source rocks (ductile units), and formation of structural traps on detachment topography. Lateral variation in fracture density, by creating barriers to updip migration, also may contribute to oil accumulation. Other favorable structures could be large, early Miocene gravity slide blocks sealed by Neogene basin fill. In basins similar to Railroad Valley, these structures should be prime exploration targets because oil generation occurred after the structures formed.

The role of attenuation in the formation of the Railroad Valley structural basin, east-central Nevada: Detachment control of petroleum reservoirs

The eastern boundary of Railroad Valley has long been considered a large displacement steep normal fault. By mapping a low-angle fault, called the White Pine detachment (WPD), from the ranges into the valley, we show that steep normal faulting was not a significant process in the formation of Railroad Valley. The WPD or its equivalent extends throughout the White Pine and Grant ranges, as shown by mapping of previous authors. Well data show that it is also present in Railroad Valley. Our structure-contour map of the WPD, based on surface and subsurface data, shows that the WPD dips uniformly from the ranges into Railroad Valley and is not displaced significantly by high-angle faulting. Even if high-angle faulting were assumed, the maximum displacement consistent with the data would be 2000 ft (610 m), much smaller than the total structural relief of at least 16,000 ft (4880 m) between the Grant Range and the adjacent valley. Structural elevations in the valley are low because the Paleozoic section has been greatly attenuated. A related contribution to structural relief is uplifting of the ranges caused by diapiric emplacement of plutons in response to tectonic unloading. The WPD is only one of many detachments in Railroad Valley. We propose that most or all of the oil reservoirs are in attenuated, brittle blocks between detachments. Seals are provided by detachments or by the discontinuous nature of associated fracturing. Attenuation, by juxtaposing hot infrastructure rocks with rocks that normally occur thousands of feet above the granitic and metamorphic basement, may have contributed to maturation of source rocks. (Begin page 1154)

Student Participation in an Offshore Seismic-Reflection Study of the Palos Verdes Fault, California Continental Borderland

We have developed a marine seismic-reflection program that involves students in research and instruction and is being successfully used to train students in quantitative and structural concepts. Over 100 high-school to graduate-level students have participated in the program over the last five years (1993–98). The research target is the Palos Verdes fault, a major seismogenic strike-slip fault in the California Continental Borderland. Students have participated in cruise planning and data acquisition, processing, and interpretation, and they have produced semester reports, senior theses, and oral papers and posters at meetings. Seismic mapping of a 10-km part of the fault south of Los Angeles Harbor shows a small (300 m) restraining segment and a larger (1–2 km) releasing segment in the fault. Two small (4-m-high and 1-km-wide) bulges on the seafloor indicate shortening near the restraining segment. This evidence of Holocene strain provides information on the tectonic history of this important fault. The ...