To quote from the 2018 Nature article: "During these cruises, Rare Earth Yttrium-rich mud having a maximum of almost 8,000 ppm of total REY content (ΣREY) was confrmed."

8000 ppm would be 0.8% concentration. Most of the samples are poorer than that.

https://www.nature.com/articles/s41598-018-23948-5.pdf