Bringing a novel approach to a classic problem, researchers have revealed how changes in ocean chemistry over the past 2 billion years have left an imprint on volcanic rocks formed in island arcs. Island arcs, which arise from volcanic activity along subduction zones where one tectonic plate dives beneath another, play a crucial role in the formation of the continental crust. The study sheds light on how these arcs have interacted with the ocean's chemistry through deep-time connections that are only now being uncovered.
The research is described in a paper appearing in the journal Proceedings of the National Academy of Sciences on October 18.
The research, resulting from a collaboration between the laboratory of Professor of Geology Claire Bucholz and UC Berkeley, has shown that the strontium isotopic compositions of island arc magmas have varied in tandem with seawater strontium composition over geologic time. Specifically, the study proposes a model for this covariation involving transfer of seawater strontium to island arcs through hydrothermal alteration of the ocean floor, which is ultimately subducted into the mantle source regions of island arc magmas and then incorporated into the resulting island arc basalts.
Bucholz studies how different reservoirs on Earth, from the surface to the mantle, interact and how these interactions have changed over time. Subduction zones are an excellent place to ask such questions because they represent an interface between major reservoirs: the ocean, the crust, and the mantle, and because subduction processes have been operating on Earth for billions of years.
The subducting plate is composed of oceanic crust, which is imprinted with the chemistry of seawater through hydrothermal alteration. When the subducting plate descends into the mantle, it heats up and releases water-rich fluids and melts that carry this seawater-derived chemical signature into the overlying mantle. In turn, this influx of fluids causes the mantle to melt, producing magmas that ultimately ascend and erupt on the surface of the Earth. This process creates island arc volcanoes like those found in the Aleutians or elsewhere in the Pacific Ring of Fire. The magmatic rocks found in these island arcs carry the original fingerprint of seawater in their chemical makeup and thus, by studying them, researchers can learn about their connection to the ocean.
When it comes to the modern Earth, geochemists generally accept that there is a connection between seawater and island arc basalts. For example, a well-known characteristic of modern island arc basalts is their enrichment in radiogenic strontium relative to basalts formed at mid-ocean ridges, which are spreading apart rather than subducting. The typical explanation for this enrichment is that the "extra" radiogenic strontium is supplied by the subducting oceanic crust, which incorporated radiogenic seawater strontium before it subducted.
The use of radiogenic isotopes (those produced through radioactive decay) to study arc magmas has a rich history that was pioneered by co-author Donald DePaolo, who received his PhD at Caltech in 1978 and is now on the faculty at UC Berkeley. During his time at Caltech, DePaolo developed the methodology to make the first neodymium isotope measurements on terrestrial rocks and explored their variations in island arc magmas in conjunction with that of strontium isotopes.
This work was foundational for the development of radiogenic isotopic studies of rocks and has been a cornerstone of geochemical studies since. The new study, led by current Caltech graduate student Amanda Bednarick, builds on this foundation and connects researchers who have worked at Caltech over four decades, including co-author Daniel Stolper (PhD '14).
The new work required meticulous combing of past literature data of strontium isotopes in island arc rocks. Although this data has existed for decades, no one had undertaken the task of compiling and analyzing it because the original strontium ratios in magmatic rocks are easily altered and often obscured. However, with careful assessment of existing datasets to select only the most reliable measurements, Bednarick was able to document a clear signal.
"The literature review and compilation that Amanda undertook was time and labor intensive," says Bucholz. "However, it is only through these efforts that the highest-quality datasets emerge and we are able to say something consequential about the geologic past."
"We understand the connections between seawater strontium and island arc volcanic rocks only in a general way, but the correlations found by carefully assessing the data are undeniable," adds DePaolo.
Bednarick showed that the isotopic ratios of strontium in island basalts correlate well with those of seawater through the geologic past, which have varied due to changes in the amount of strontium put into the oceans from the continents (radiogenic) versus the mantle (unradiogenic). Most strikingly, a significant increase in strontium isotope ratios in island arcs coincides with a known shift in ocean chemistry during the late Neoproterozoic era, around 600 million years ago. To understand this relationship, Bednarick and co-authors developed a model incorporating the known record of seawater chemistry and strontium input into island arc magmas. These models indicate that the strontium isotope ratios in island arc basalts can be explained solely by changes in seawater chemistry, which in turn reflects broader geologic and climatic shifts.
"This study highlights how changes at Earth's surface, particularly in ocean chemistry, are recorded in the deep Earth through the volcanic rocks formed in subduction zones. It's a great example of how interconnected Earth's systems are, even over billions of years," explains Bednarick. Understanding the ancient geochemical processes that govern island arc formation is critical for reconstructing the history of Earth's tectonics and oceans. The study suggests that the patterns observed today have deep roots and that the strontium isotope signatures of ancient island arcs can provide valuable clues about the planet's long-term evolution.
Next, Bednarick aims to examine sequences of rock called ophiolites, which represent fragments of hydrothermally altered oceanic crust now preserved on continents, to further understand the chemistry of seawater from over 1 billion years ago.
The paper is titled "Temporal covariation of island arc Sr isotopes and seawater chemistry over the past 2 billion years." In addition to Bednarick and Bucholz, co-authors are DePaolo and Stolper, both of UC and Lawrence Berkeley National Laboratory.
