The recent discovery of two diamonds formed deep within the Earth's mantle has sparked a fascinating conversation about the planet's role in sustaining life. These diamonds, acting as time capsules from the depths, have revealed a unique connection between plate tectonics and the cycling of phosphorus, an essential element for life as we know it.
The Phosphorus Mystery
For years, scientists have studied phosphorus cycling in the Earth's shallow layers, but a puzzle remained: why hasn't this vital element been permanently locked away in the deepest parts of the mantle over billions of years of tectonic activity?
Unraveling the Mystery
New research, led by Qiwei Zhang and colleagues, has provided an intriguing answer. It turns out that the descending oceanic plates, which drive plate tectonics, are simply too hot to allow phosphorus to be subducted into the lower mantle. This heat keeps phosphorus near the surface, where it can continue to support life. As Pearson puts it, "If 90% of phosphorus got subducted, we'd have a real crisis." Instead, it's "torched back off" the plates and remains accessible.
Super-Deep Diamonds: Nature's Probes
To explore this further, Zhang analyzed two super-deep diamonds, one from Brazil and the other from Canada's Northwest Territories. These diamonds, formed at depths between 410 and 700 kilometers, act as natural probes, offering a glimpse into the deep mantle. Initially, Zhang thought the inclusions were common minerals, but Raman spectroscopy revealed something extraordinary.
The Rare Find: Tuite
Zhang discovered tuite, an ultra-rare mineral previously found only in highly shocked meteorites. Tuite is a high-pressure transformation of apatite, the most common phosphate mineral in Earth's crust. Finding tuite inside these diamonds proves that phosphorus can make it to the lower mantle, but Zhang's modeling shows this is an inefficient process.
The Challenge of Deep Cycling
Transporting phosphorus to such depths requires a rare event: a "cool" subduction zone, with temperatures around 1,100 C, compared to the typical 1,700 C at those depths. This anomaly allows phosphorus to be preserved and transported, but it's an exceptional circumstance.
Implications Beyond Phosphorus
The research has broader implications. When a tectonic slab is cold enough to preserve phosphorus, it transforms into a new, highly dense mantle rock type. This unique density likely influences how deeply these cold slabs can penetrate into the lower mantle. Beyond planetary evolution, this research has practical applications for the diamond industry, as super-deep diamonds are often the most valuable.
A Deeper Understanding
Zhang's work not only helps us understand the deep Earth but also highlights the importance of phosphorus cycling for the origin of life. As Zhang says, "We now know why this process is important for life's origins." This research reminds us of the intricate connections and delicate balance that make our planet habitable.
