Jay Miller: Research Projects

My current research program can be summarized under four major topics:

• What is the relationship between the petrology and geochemistry of the Earth's crust and its seismic properties?
• What does the geologic record tell us about the potential of climatic, cultural, and economic impacts of effects of volcanic eruptions?
• What is the nature of sulfide mineralization in seafloor hydrothermal sulfide systems, and how can these active systems be used as analogues of ancient massive sulfide deposits?
• What are the effects low-temperature alteration of the ocean crust, and how does microbial activity effect this alteration?

Seismic properties and petrology

Thus far, results from this area of study have documented the similarity between the Kohistan accreted terrain in the Pakistani Himalayas and other accreted arc environments. Additionally, we have shown that serpentinized peridotite cannot contribute much to most of the subseafloor seismic records we collect, but that we can estimate the water content in the mantle from seismic data. From Alvin dive transects at Hess Deep we also recognize that the remarkable consistency in the seismic signature we recover from fast-spreading ocean crust is not reflected in the thickness of the volcanic units in this area.

Most recently I began investigating whether we can use downhole resistivity and density logging data to correlate structural features between two sections over 1.2 km apart recovered from the Atlantis Bank on the Southwest Indian Ridge. The first step in this investigation was to determine the response characteristics of logging tools in gabbroic rock. Because there are no orientation mechanisms for coring basement rocks, no structural data can be placed in anything but a core reference frame. Correlation between these two cored intervals might allow more than 20,000 structural measurement to be oriented in three dimensions and allow tectonic reconstruction of the uplift of this bank.

Impacts of volcanic eruptions

Most of the focus of this research has been investigating the nature of the 934 AD eruption of Eldgja in south Iceland by comparison to the 1783 Skaftarfires eruption. We have determined the volume of both of these eruptions was in excess of previous estimates, that the 934 AD Eldgja event alone generated in excess of 19 km3 of lava and tephra, and that the volume of aerosols injected into the atmosphere by the Eldgja event probably exceeded that of the 1783 Laki eruption that cast a pale haze over Europe.

I have recently begun a project with cores from the Peru margin, examining the tephrachronologic record archived in the offshore sediments. A large eruption in 1600 AD from Volcan Huaynaputina blanketed the west coast of Peru with ~20 km3 of ash. More than 100 km away, the ash layer is still in excess of 10 cm thick. A core from ~100 km offshore documents an even bigger eruption where a layer in excess of 10 cm thick indicates an eruption of more than twice the volume of Huaynaputina has been dated at 1.4 Ma. Tying the incomplete and eroded onshore tephrachronologic record to ash layers in offshore drill cores will address the frequency and cyclicity of northern Andean volcanism, which has been particularly newsworthy in light of recent activity.

Seafloor sulfide mineralization

Of the four economically important categories of ancient massive sulfide deposits that are known to have modern seafloor analogues, three have been drilled during the Ocean Drilling Program. Scientific ocean drilling has progressed from sampling the simplest system, where only basalt is present for fluid interaction (TAG hydrothermal area), to a more complex system, where terrigenous sediments and basalts are reacted with hydrothermal fluid (Juan de Fuca Ridge). Last year we investigated fluids interacting with felsic rocks on the seafloor at Manus Basin.

An enigmatic result of drilling at the Manus Basin was our discovery of virtually no subseafloor mineralization despite potentially economic-grade seafloor surface mineralization. My investigation into the magmatic activity providing the heat source for the system suggests that sporadic, small volume eruptions intermittently shut down the mineralizing system and do not allow long-term localization of hydrothermal activity and consequent development of economic-sized massive sulfide deposits.

Low temperature alteration of the upper ocean crust

In 1999, I participated on ODP Leg 187 to the Southeast Indian Ridge. Although the primary scientific objective of Leg 187 was igneous geochemistry, the success of our drilling program resulted in recovery of a unique set of samples that allowed us to evaluate the effects of alteration on seafloor basalts in multiple dimensions. Transects of sites sampling crust formed in different magmatic-tectonic environments allow spatial comparisons of the degree of alteration relative to segment-scale processes, while samples collected along these transects, perpendicular to the ridge crest, allow us to determine temporal controls on the alteration of the ocean crust. The impetus for this study was my observation that the oldest rocks recovered showed the lowest alteration intensity, implying that there is no correlation between the degree of alteration and the age of the oceanic basement. This, in turn, would suggest that heat flow models indicating long-term circulation through the ocean crust may not be valid for this part of the mid-ocean-ridge system.

The goal of the current investigation is to determine, at least for this region of the global mid-ocean ridge system, whether the alteration character of the upper ocean crust is fixed at or very near the ridge crest or if this character changes as the ocean crust ages. We have accumulated evidence evidence that the degree of low-temperature alteration does increase with age of the crust but that permeability of the crust is a more important factor than age in the low-temperature alteration process.

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