Jeffrey Alt - Academia.edu (original) (raw)

Papers by Jeffrey Alt

Proceedings of the Ocean Drilling Program. Scientific results, Aug 1, 1999

In this paper, we present a detailed description of the morphology and microstructure of fibrous ... more In this paper, we present a detailed description of the morphology and microstructure of fibrous and nonfibrous veins crosscutting basaltic cores recovered from Hole 896A. The nonfibrous veins consisting of spheroidal smectite aggregates and blocky carbonate crystals indicate that minerals crystallized in open spaces during single-stage crack opening. Fibrous veins (mainly smectite + carbonate-bearing) indicate that fibers crystallized by a crack-seal mechanism, involving repeated increments of microcrack openings, followed by displacement-controlled crystal growth. We report descriptions of diagnostic features of the crack-seal veins studied. Finally, composite veins consisting of both nonfibrous and fibrous mineral infill are interpreted to result from recrystallization of fibrous minerals into blocky minerals, or to indicate a decreasing crystallization rate with respect to the fracture opening rate.

Gold contents were determined in 13 samples representing the interval from 2000 to 2111 mbsf in H... more Gold contents were determined in 13 samples representing the interval from 2000 to 2111 mbsf in Hole 504B drilled during ODP Leg 148. The values range from 1.6 to 4.4 ppb, with a mean of 3.0 ppb that is similar to that for the basalts and diabases from the upper parts of the hole. A moderate level of correlation between Zn and Au concentrations observed from the upper parts of subsediment rocks is also valid for the samples from Leg 148. However, the lack of notable correlation between the intensity of alteration and Au contents observed in samples from Legs 69 and 140 is not as obvious for the deepest 300 m of the hole where the inverse correlation between Zn contents and percent alteration is strong. The problem is complicated by the discordance between the samples studied for gold concentration and materials used for shipboard geochemical investigations and determinations of intensity of alteration.

Mineralium Deposita, 2019

Proceedings of the IODP, 2006

Proceedings of the Ocean Drilling Program, 148 Scientific Results, 1996

Ophiolites and oceanic crust: new insights from field studies and the Ocean Drilling Program, 2000

Geochimica et Cosmochimica Acta, 2014

The hydrothermal alteration of the oceanic crust at the ridge axis and on the ridge flanks has a ... more The hydrothermal alteration of the oceanic crust at the ridge axis and on the ridge flanks has a profound influence on the oceans and atmosphere and through subduction of altered crust, the chemistry of the upper mantle. Hydrothermal inputs play a fundamental role in controlling seawater composition and influence many of the records (e.g., Sr, S or O) we use to reconstruct past climate, erosion and other major other Earth processes. If possible, quantifying the chemical and isotopic exchange fluxes from the alteration of the ocean crust will provide a solid foundation for assessing changes in the more complex and variable systems draining the continents. At least in terms of seismic structure, ocean crust formed at intermediate to fast spreading rates is relatively uniform and should most closely relate to the ideal "Penrose" stratigraphy. Consequently, observations from deep drill hole into fast spread crust can be reasonably extrapolated to describe the seawater-basalt e...

Scientific Drilling, 2007

The Integrated Ocean Drilling Program’s (IODP) Expeditions 309 and 312 successfully completed the... more The Integrated Ocean Drilling Program’s (IODP) Expeditions 309 and 312 successfully completed the first sampling of an intact section of upper oceanic crust, through lavas and the sheeted dikes into the uppermost gabbros. Hole 1256D, which was initiated on the Ocean Drilling Program’s (ODP) Leg 206, now penetrates to > 1500 mbsf and > 1250 m sub-basement. The first gabbroic rocks were encountered at 1407 mbsf. Below this, the hole penetrates ~100 m into a complex zone of fractionated gabbros intruded into contact metamorphosed dikes. doi:10.2204/iodp.sd.4.01.2007

Science, 2010

Cations in the Veins Major events in Earth's history, from climate change to tectonic activit... more Cations in the Veins Major events in Earth's history, from climate change to tectonic activity, can be revealed by reconstructing past conditions of the oceans. Clues from ancient ocean chemistry can be found in the cation content of fossilized microorganisms, marine carbonates, or salt deposits from old coastal zones. As these proxies are prone to inconsistencies between samples and methodologies, Coggon et al. (p. 1114 , published online 4 February; see the Perspective by Elderfield ) estimated past seawater composition from the geochemistry of resistant carbonate veins precipitated within fresh basalts on the sea floor. The sudden rise to modern-day levels of ocean magnesium:calcium and strontium:calcium ratios occurred about 24 million years ago, and can be explained by a decrease in seafloor hydrothermal activity combined with a decrease in river discharge.

Science, 2013

Under the Sea Floor Microorganisms living in basaltic sea floor buried beneath sediments derive e... more Under the Sea Floor Microorganisms living in basaltic sea floor buried beneath sediments derive energy from inorganic components from the host rocks that interact with infiltrating seawater, which brings dissolved oxygen and other trace nutrients with it. Lever et al. (p. 1305 ) directly sampled the subseafloor community off the eastern flank of the Juan de Fuca Ridge in the Pacific Ocean and found evidence for ongoing microbial sulfate reduction and methanogenesis. Multiyear incubation experiments with samples of host rock confirmed the microbial activities measured in situ.

Geochimica et Cosmochimica Acta, 2006

Geochemistry, Geophysics, Geosystems, 2012